Remote Sensing Platforms

gimbal as seen from below --------------------------- 16. Cessna 180 Skywagon ---------------------------------- 17. Beechcraft Bonanza A36 ------------------------------- 18. Cessna 337 Turbo-System Super Skymaster -------------- 19. Beechcraft Queen Air B80 ----------------------------- 20. Grumman OV-1 Mohawk observation aircraft ------------ 21. Lockheed Y0-3A quiet reconnaissance aircraft __________ _ 22. Rockwell Standard Jet Commander 1121 ---------------- 23. Gates Learjet 24 -------------------------------------- 24. Fairchild Hiller Porter -------------------------------- 25. Diagram of NASA 927 Lockheed NP3A Orion flight con- figuration ------------------------------------------- Photographs: 26. Douglas Skywarrior RA-3B --------------------------- 27. Lockheed NC130B (RC130) Earth Resources Aircraft (NASA 929) --------------------------------------- 28. Boeing RC135A --------------------------------------- Diagram of isometric view of Boeing RC135A showing the USQ-28 aerial electrophotomapping system --------------------------- Photograph of E-Systems L450F reconnaissance aircraft in piloted configuration ----------------------------------------------- Photograph of North American Rockwell X15 research aircraft __ _ Diagram of General Dynamics/Martin RB57F ------------------ Photograph of McDonnell Douglas (Phantom II) RF-4C ________ _ Diagram of McDonnell Douglas (Phantom II) RF-4C flight con- figuration -------------------------------------------------- Photograph of Lockheed U-2 ---------------------------------- Photograph of Lockheed SR71A strategic reconnaissance aircraft__ Diagram of USDA Forest Service helicopter fire spotter, Bell 47G- 3B-1 ------------------------------------------------------- IV Page 38 39 40 40 41 42 42 43 44 44 45 45 46 47 47 48 48 48 49 49 50 51 51 51 52 52 53 53 53 54 54 54 55 55 56 56 57 Page 38-41. Photographs: 38. Northrop Falconer surveillance drone ------------------- 57 39. Goodyear blimp Columbia ------------------------------ 58 40. Television camera mounted on blimp ;.l'Jayfiower --------- 58 41. Schweizer sailplane SGS 2-32 -------------------------- 59 42. Diagram of proposed film return Earth-sensing space vehicle (USGS) --------------------------------------------------- 59 43. Photograph of aircraft recovering a capsule such as might contain film from an Earth-sensing spacecraft ----------------------- 59 44. Diagram of Nimbus D satellite -------------------------------- 60 45. Diagram of TIROS M spacecraft ------------------------------ 60 46. Artist's conception of Earth Resources Technology Satellite (ERTS) --------------------------------------------------- 61 47-49. Diagrams: 47. ERTS system ----------------------------------------- 61 48. ATS-B systems and interior experiments ---------------- 62 49. ATS-B exterior experiments --------------------------- 62 50. Artist's conception of ATS-F and ATS-G ---------------------- 62 51. Artist's conception of Synchronous Meteorological Satellite (SMS) 63 52. Diagram of a 6-satellite high-resolution geosynchronous Earth- sensing system --------------------------------------------- 63 53. Photograph of Ranger lunar probe ----------------------------- 64 54. Artist's conception of Lunar Orbiter spacecraft ----------------- 64 55. Photograph of Lunar Orbiter E, showing camera lens ------------ 64 56. Diagram of Surveyor lunar lander ----------------------------- 65 57. Photograph of Mariner 4 spacecraft (flyby version) ------------ 65 58. Artist's conception of Mariner 9 spacecraft (Mars Orbiter) ______ 65 59. Diagram of Viking spacecraft --------------------------------- 66 60. Artist's conception of Pioneer spacecraft ----------------------- 67 61. Artist's conception of Mercury, Gemini, and Apollo spacecraft ____ 67 62. Photograph of Apollo service module, showing Scientific Instrument Module (SIM) ---------------------------------------------- 68 63-66. Diagrams: 63. Sketch of instruments carried in Apollo SIM bay -------- 68 64. Sketch of Lunar Rover Vehicle, showing instruments ---- 68 65. Skylab-A --------------------------------------------- 69 66. Skylab-A, activation and operation-periods of occupancy__ 70 67. Artist's conception of Space Shuttle --------------------------- 71 68. Diagram of evolution of TIROS, TOS, ESSA satellites___________ 72 69. Photograph of TIROS Operational Satellites (TOS) ------------ 73 70. Photograph of interior of the Environmental Survey Satellite, ESSA 3 ---------------------------------------------------- 73 71. Artist's conception of NOAA .Satellite ITOS -------------------- 74 72. Diagram of NOAA Satellite ITOS-D -------------------------- 75 v AFCRL APT ARPA ATS AVCS bps dm3 EREP ERTS FAA f.l. FM GOES Hz inm ITOS kbps kn LST MAC ABBREVIATIONS AND ACRONYMS Air Force Cambridge Research Laboratories Automatic Picture Transmission Advanced Research Projects Agency Application Technology Satellite Advanced Vidicon Camera System Bits per second Cubic decimeter Earth Resources Experiment Package Earth Resources Technology Satellite Federal Aviation Agency Focal length Frequency Modulation Geostationary Operational Environmental Satellite Hertz International nautical mile Improved TIROS Operational System Kilobits per second Knot Large Space Telescope Military Airlift Command MATS ~bps MHz NASA NOAA PCM PM SAM OS SEOS SIM SLAR SMS SPM TIROS TOS TV VHF VTPR VI Military Air Transport Service Megabits per second Megahertz National Aeronautics and Space Administra- tion National Oceanographic and Atmospheric Administration Pulse Code Modulation Phase Modulation Satellite and Missile Observation System Synchronous Earth Observation Satellite Scientific Instrument Module Side-looking Airborne Radar Synchronous Meteorological Satellite Solar Proton Monitor Television Infrared Observation Satellite TIROS Operational Satellite Television Very High Frequency Vertical Temperature Profile Radiometer Rem~ote Sensing Platforms By Alden P. Colvocoresses The author of this report is actually the author-editor with major contributions by others as follows : George Nolan of AFCRL (balloons), David Landen of USGS (aircraft), Stanley Addess of USGS (NASA spacecraft), and Arthur Schwalb and Lewis J. Allison of NOAA (NOAA spacecraft). This report was initiated by the American Society of Photogrammetry as a chapter in the forthcoming "Manual of Remote Sensing." A preliminary version titled "Platforms," dated January 1972, has already been given limited distribution for reviewing purposes. Typical vehicles which carry remote sensors into the atmosphere or beyond into space are described and illustrated, and their performance characteristics are listed. No attempt is made to completely catalog or to cover the history of remote-sensing vehicles. Airborne platforms and spacecraft in use or defined as of 1971-73 were selected from the vehicles which have gained widest acceptance, demonstrated a unique capability, or have been· defined for fu- ture remote-sensing missions. A wide variety of nonorbiting rockets have carried cameras and other sensors into space, but since current United States remote-sensing programs do not include rockets, except for astronomic observa- tions, rockets are not included. However, as of 1972, British industry was building a variety of rockets specifically designed as remote-sensing platforms. Except for a unique British kite bal- loon, only American platforms are covered, and, therefore, this circular could be viewed as a contribution to any international effort to cover the subject. Remote sensing of the Earth is the prime 1 consideration, but sensing of the other planets and moons is also considered. Astronomy, ex- cept to the extent indicated above, is not in- cluded nor is the measurement of the force fields such as gravity or magnetics. AIRBORNE PLATFORMS BALLOONS GENERAL CON SID ERA TIONS The objective of this section on free and tethered balloon platforms is to provide suffi- cient technical and descriptive information to the experimenter so that he can judge whether a balloon is an appropriate platform for his sensor. He can obtain an indepth understanding of balloon capabilities, limitations, and new developments by referring to the literature in the bibliography and contacting the organiza- tions involved in balloon activities. Although balloons were used as camera plat- forms during the Civil War, it was not until the late 1950's that a serious program was initiated to utilize balloons for a wide variety of remote- sensing experiments. By 1972, balloons had risen to above 49 km (160,000 ft) and carried a wide variety of sensors that looked at the Earth's surface, the atmosphere, and celestial bodies. Practically all remote-sensing balloon flights are unmanned, but manned balloon flights, particularly by astronomers, have also been successful (Stehling, 1971). The balloon is a remote-sensing platform which permits the experiment or operation to define the shape, size, and performance required of the carrier; however, the use of balloons is restricted by meteorological factors, especially by wind influences. FREE BALLOONS PAYLOAD-ALTITUDE CAP ABILITIES The current range of altitude and associated payload capabilities for free balloons are shown in figures 1 and 2. In figure 1, each curve repre- sents a single, flight-proven balloon made from a superior quality, lightweight polyethylene film. (Not all of the existing balloon designs are included on the charts.) Scrim balloons are made from Mylar film laminated with a rein- forcing scrim, or mesh, of Dacron thread. The high-strength scrim materials are used for carrying payloads much heavier than is possi- ble with polyethylene balloons. Figure 2 indi- cates the range of payload-altitude capabilities for designated volumes of scrim balloons. Although it is convenient to designate the balloons shown by their volumes, it is important to note that each curve represents an individual design and may be very different from the others in gore pattern, material thickness, type and placement of reinforcements, and location of ducts, valves, etc. In addition to the weight of the sensor, the payload includes the weight of parachute, in- strumentation for balloon command and con- trol, tracking beacons or radar reflectors, ballast and rigging, and other structures. A gas valve located at the top of the balloon is a highly desirable component of a balloon platform. It gives the operator the flexibility of controlling ascent and descent rates to conform to a desired flight profile, or of changing alti- tude to take advantage of more favorable wind conditions. Furthermore, a valve provides an added means of bringing the balloon to Earth after payload separation. Figure 3 shows the inflation of a 23,000-m 3 polyethylene balloon shortly after sunrise-the time of day when surface winds are normally most favorable for launches. Operational requirements sometimes will necessitate the use of a tandem balloon system as shown in figure 4. In this system all of the lifting gas is contained in the top or launch bal- loon before release. As the system ascends, the expanding gas transfers into the main or bot- tom balloon. FLIGHT DURATION Zero-pressure balloons usually carry ballast, which may be released either to increase the ascent rate or to compensate for a descent caused by thermal effects. The quantity of ballast that can be carried determines the flight duration of a zero-pressure balloon. This weight factor is an important consideration in selecting the proper balloon to meet special flight-profile requirements or to achieve a flight longer than 1 day. In contrast to the zero-pressure balloons, superpressure balloons are completely sealed and require no ballast. They float at a constant- density altitude and are capable of flight dura- tions of many months. The superpressure balloons, however, can carry payloads of less than 40 kg and are relatively expensive. There- fore, their use is generally restricted to special applications that involve long duration flights with lightweight sensors. COST Rough estimates of the costs of free balloon platforms made of polyethylene or scrim ma- terial may be made from figure 5. Costs for items such as helium, operations, flight-control instrumentation, and travel are not included. For payloads greater than 1,000 kg, polyethy- lene balloons are normally reinforced with load tapes and a cap on the top. 2 STABILITY The inherent stability of the free-floating balloon is one of its most attractive assets for scientific applications. If reasonable care is taken in mounting and packaging the instru- ments, camera systems, spectrometers, mag- netometers, telescopes, and interferometers ·can be flown without difficulty. The motion of the payload is dependent upon the mechanical properties of the payload sus- pension system. As the partially inflated balloon ascends through the atmosphere, the loadline tends to wind and unwind in random fashion, causing the payload to rotate at rates that are usually much less than 2 r/min. At the same time, the payload may behave as a simple or compound pendulum. Angular displacements from the vertical of 2° or less are generally ob- served. Mter the system has reached float equilibrium, however, payload motions need not be considered for most types of experiments. Gradual changes in payload orientations do oc- cur. Torsional rotation rates- are generally a few revolutions per hour with a maximum rate of 12 r/h having been observed. Displacements from the vertical due to pendulum motion have been reported to be no greater than 0.5° or a hori- zontal displacement of the payload about 1.4 x 10- 3 times the length of the loadline. SOME METEOROLOGICAL CONSIDERATIONS FOR FREE BALLOON FLIGHTS The horizontal trajectory of a free balloon is primarily determined by the winds it encoun- ters while airborne. However, a considerable degree of trajectory control is possible by selecting periods of the year, as determined from climatology, when repeatable wind pat- terns exist and by controlling the balloon's alti- tude using ballast drops and gas valving. Three basic flight profiles may have applica- tion to the use of balloons as platforms for re- mote sensing of the Earth's surface: precision ballooning, station-keeping, and boomerang. Fundamental to these profiles is the occurrence during the summer of stratospheric easterlies, that is, winds blowing from the east. This phenomenon occurs annually and thus can be safely used for planning flight operations. A typical example of the summertime strato- spheric easterlies is given in figure 6, which shows streamline analysis of rocketsonde wind observations at the 5 mb surface (approximate- ly 36.6 km or 120,000 ft) on July 22, 1964. The lower boundary of these easterl~es is approxi- mately 18.3 km (60,000 ft), and they extend up- ward with their strength increasing with alti- tude and also toward lower latitudes at the same altitude. Peak values occur during the latter part of July with minimum values at the begin- ning and end of summer. The stratospheric easterlies have been used for precision ballooning-operations in which a balloon overflies a specific geographical location, such as an instrumented area on a test range. One might also envision the utilization of this wind phenomenon for cross-country traverses of balloons carrying remote sensors. Station-keeping and boomerang balloon flight profiles are also associated with the summertime occurrence of the stratospheric easterlies. How- ever, to achieve these profiles a layer of wester- lies (winds blowing from the west) must exist below the stratospheric easterlies. In this situ- ation there are opposite flows of air in adjacent layers of the atmosphere, providing the setting for a balloon to drift back and forth across an area of interest merely by changing altitude. Between these layers of easterlies and wester- lies a minimum wind layer exists, which pro- vides the opportunity to hover or station keep a balloon over a relatively small area for an ex- tended period. Figure 7 gives information on locations in the Northern Hemisphere where balloon station-keeping and boomerang profiles can be flown. Figure 8 shows the horizontal and vertical trajectories of a station-keeping balloon flight which remained over an area having a radius of approximately 75 km (40 internation- al nautical miles) for 72 hours. g PROJECT STRATOSCOPE By the late 1950's, astronomic experimenters found that when only one or two functions in the balloon were to be performed it was cheaper and more convenient to launch a variety of un- manned balloons rather than a more complex manned flight. Simple experiments could be sent aloft unmanned, with radio signals used to point the equipment, control the flight, and record the data. An unmanned project, Stratoscope, achieved a series of outstanding successes. Stratoscope I, proposed by Martin Schwarzchild, an astrono- mer at Princeton University, and funded by the Office of Naval Research was originally launched in 1957 and was quickly followed by several flights. The results included a series of very de- tailed pictures of solar surface granulations. By 1963, a double-balloon system had taken a 3,150 kg (7,000 lb) payload to an altitude of about 24,400 m (80,000 ft). That year new infrared spectra were obtained of the Moon, Jupiter, and seven stars. Stratoscope II was sponsored by the National Science Foundation, the Office of Naval Re- search, and NASA. In 1970, this balloon lifted a radio-controlled 92-cm (36-in.) diameter tele- scope and brought back extremely high resolu- tion photographs of Jupiter, Uranus, two kinds of nebulas, and Seyfert galaxies whose bright nuclei resemble quasars (figs. 9 and 10). The performance characteristics of Strato- scope II were as follows : Ceiling: 24,400 m (80,000 ft). Size : Length, 200 m ( 660 ft) ; volume, 148,600 m 3 • Payload: 3,150 kg (7,000 lb). Special equipment: A 64-channel radio tele- metry system provided a high degree of remote control. A guidance system for the remotely controlled telescope made by RCA Laboratories, Princeton, N.J., provided a pointing accuracy of 0.02 sec- ond of arc. A key element of Stratoscope II was a precise optical system with a 180-kg (400-lb), 92-cm (36-in.) diame- ter primary mirror. The Stratoscope tele- scope was designed by Perkin-Elmer Corp., Norwalk, Conn. It had a 5.5-m (18-ft) main tube assembly with a 3.3- m (11-ft) side arm forming an L-shaped telescope. The side arm held the guidance equipment, the camera equipment, and the high-resolution TV camera. A low- resolution TV camera was boresighted with the main telescope. After each flight, the system and the exposed film were returned to Earth. POWERED BALLOONS During 1972 the U.S. Air Force launched at least one powered balloon, Pobal, which has been reported by "Aviation Week and Space Tech- nology" (1973). The balloon of 20,200 m 3 (710,000 ft 3 ) rose to a height of 18,300 m (60,000 ft) and was powered by a battery- driven slowly rotating propeller of 9 m (30 ft) diameter. The experiment had a successful dura- tion of 3 hours after which the rudder became dislodged due to winds. A powered balloon as compared with the passive type has the advan- tage of being remotely controlled and, within limitations, being guided along a path or hover- ing over a given area. If successfully developed, this mode has considerable potential application as a remote-sensing platform. TETHERED BALLOONS The basic sources of the material that follows are listed in the references. Particular attention is directed to the "Tethered Balloon Handbook" prepared for the AFCRL by Myers (1968). This work is an authoritative compilation of the his- tory, development, applications, and listings of available modern equipment for tethered bal- loon activities. A paper by Young (1968) pre- sents an excellent overview of tethered balloon technology and. offers predictions for future capabilities. A report by Battelle Columbus Laboratories (1971) sponsored by ARPA pro- vides both a survey and technical assessment of tethered balloon systems. When considering the use of tethered bal- loons, it is essential that the initial planning be concerned with the atmosphere and the air space in which the balloon will be flying. Are wind conditions and other meteorological factors favorable, and are there air-safety constraints imposed on the location of the desired opera- tion? The restrictions imposed by meteorologi- cal factors will depend in large part upon the type of tethered balloon system being consid- ered. The use of air space must be coordinated with the controlling agency. In the United States this is either the FAA or the responsible agency in the case of use of a restricted air space. FAA regulations pertaining to tethered balloons can be found in "Federal Aviation Regulations," pt. 101, v. 6. 4 Although tethered balloons have been associ- ated historically with wartime, serving as both a passive means of defense against aerial at- tacks and as observation platforms for artillery spotters, present-day usage is much broader, encompassing a variety of scientific disciplines. Tethered balloons are being used to carry aloft various types of sensors, meteorological measur- ing equipment, communication relays, and antennas. DESCRIPTIONS AND CAPABILITIES Tethered balloon systems are usually custom tailored for a particular operation. The three main types of tethered balloons in use are the sphere, the natural shape, and the single-hull aerodynamic or streamlined shapes in a variety of finenes~ ratios and fin designs. The hull de- sign chosen for a system depends on such re- quirements as downwind displacement, altitude, payload, windspeed, flight duration, and antici- pated life. The sphere has been used since the inception of tethered balloons and is far from obsolete. Although spheres are generally limited to winds below. 55 km/h (30 kn), there are sev- eral areas of the world where tethered systems utilizing a spherical balloon could be used sever- al months per year to fly at altitudes up to 3,000 m (10,000 ft) above sea level. The principal ad- vantage of the sphere is its relatively low ·cost compared with the streamlined shapes. Figure 11 shows a 710-m 3 spherical balloon readied for launch. This system employed three tether lines to assure control of the balloon in maintaining its position over a relatively small area. This particular balloon has been used re- peatedly as a platform for a variety of experi- ments. It has, among other combinations of pay- load and altitude, the capability of carrying a 200-kg (440-lb) payload to 1 km (3,300 ft) with a maximum wind of 35 km/h (20 kn). The natural shape, mainstay of free balloon- ing, also provides a suitable tethered balloon. Its principal advantage is relatively low cost. How- ever, its use is limited to winds of less than 30 km/h (15kn). Streamlined balloons have much higher wind- speed limitations. They also tend to fly nearer to the tether point than spheres in the same wind fields on a single tether. Figure 12 is a picture of a 2,800 m 3 single-hull A-shape kite balloon, designed and manufactured by Airborne Industries, Ltd., Leigh-on-the-Sea, Essex, Eng- land, previously called Lea Bridge Industries. This type of balloon was used in World War II. Figure 13 is a payload-altitude graph for the three most popular models. These curves were developed for 135 kg winch tension on the tether cable, 30°F superheat temperature of the bal- loon gas above ambient standard temperature, and a cable weight of 134 kg/km. Costs range from $15,000 to $35,000 and are subject to change. The envelopes of these balloons have safely withstood windspeeds in excess of 110 km/h (60 kn). Modern tools, both theoretical (computerized models) (Myers and Vorachek, 1971; Doyle and 5 Vorachek, 1971 ; and Doyle and others, 1973) and experimental (instrumented flights) are be- ing utilized to improve tethered balloons and also to provide accurate stability data for exist- ing systems. The recently developed ARPA Family 2 balloons, for example, fly in 140-km/h (75-kn) winds. Measurements directly applica- ble to sensors have been obtained by cinetheo- dolite and telemetry at the U.S. Air Force tethered balloon facility in New Mexico. The data include position, velocity, accelerations, and pitch and roll of the payload on a 2,800-m 3 (100,000-ft 3 ) A-shape (modified BJ barrage) balloon with cable lengths up to 2,750 m (9,000 ft) in winds to 50 km/h (27 kn) . Similar data were obtained using an 800-m 3 (28,000-ft 3 ) natural-shaped balloon on a tri-tether at alti- tudes up to 610 m (2,000 ft) in winds to 25 km/h (13 kn). WINCHES Winches for tethered balloon operations are used to raise and lower balloons to the desired altitude and to make the necessary operational adjustments of the balloon altitude during flight operations. Relatively inexpensive hand-oper- ated winches, or even fishing reels, may be used for small, low-altitude balloons. Power-driven winches come with a multitude of available fea- tures and can range in price from several hun- dred dollars to several hundred thousand dollars. Some of the features of power-driven winches for tethered balloon applications, which add sig- nificantly to the cost of the winch, are variable speed drives, capstan or traction drives, level wind mechanisms, and various line speed, ten- sion, and footage-measuring instruments. The winch system installation may be permanent, mobile, or portable. Tethered balloon winches are rarely off-the- shelf items and are usually custom tailored for a specific application. Many winch manufac- turers have standard components from which special winches can be assembled, thereby elimi- nating much of the design time and expense which would otherwise be required. TETHERS All tethers require a suitably high tensile strength, a high strength-to-weight ratio, low drag, low stretch, torque stability, high flexi- bility, abrasion resistance, and easy splicing. Typical balloon-tethering configurations are the single, dual, or tri-tether arrangements. The different configurations offer the various de- grees of freedom which are dictated by the intended balloon operations. In many instances the tether line is designed to serve additional purposes: to carry the antenna, to encase the power line running from ground to sensors, to support an array of sensors along its length or the tubing for transferring helium to the bal- loon, etc. Tapered or stepped-diameter tethers may be desired for high-altitude flights where the weight of a constant-diameter tether can- not be safely borne. Most tether manufacturers have the capabilities for producing a unique tether to fit a specific need. TETHERED BALLOONS FOR ARCHEOLOGICAL STUDIES Tethered balloons have been used successfully by Julian H. Whittlesey to support aerial cam- eras for recording and mapping archeological sites (figs. 14 and 15) in Turkey, Greece, Italy, and Cyprus. Camera altitudes ranged from 10 to 610 m (30 to 2,000 ft), with Linhof, Graflex, and Hasselblad cameras in single and multiple (multiband) form. Aerodynamic airfoil balloons were also utilized in the presence of high winds. Various schemes were used to obtain stereo- scopic coverage. Some of the sites photographed were in shallow water. The performance of the tethered balloons used by Whittlesey was as follows: Ceiling: From 10 to 610 m (30 to 2,000 ft). Windspeed: With parafoil or ovoid shape, flown in winds from 25 to 40 km/h (15 to 20 kn). Spherical balloons were tethered in relatively calm winds up to 10 km/h 5 kn). Size: Up to 20m 3 (700 ft 3 ). Payload: From 1.4 kg to 2.8 kg (3 to 6lb). In one arrangement, an electrically advanced, radio-controlled Hasselblad EL 500 camera was hung in a gimbal from the tethered balloon by means of four lines (Whittlesey, 1970). This arrangement provided for an unusual degree of platform stability. The balloon was filled with hydrogen. The weight of the entire payload, con- sisting of calibrated camera, lens motor, power- pack, radio receiver, antenna, and magnesium gimbal mount, was 2.8 kg ( 6 lb) . The gross weight of the balloon and payload was 10 kg (23 lb) plus tether lines that weighed several kilograms. Remote-sensing equipment included a variety of small photographic cameras, which were either pneumatically or radio controlled from the ground. As a result of Whittlesey's work, the U.S. Geological Survey is investigating the use of tethered airfoil balloons as platforms for photo- graphically recording paneled control points in support of photogrammetric mapping. TETHERED AND FREE BALLOON MANUFACTURERS The names and addresses of manufacturers of tethered and free balloons are listed below for the reader who wishes to obtain more cur- rent information, including the cost of a par- ticular balloon. (This list may include companies no longer in the business and may omit new companies.) 6 Airborne Industries Ltd. Airborne Works Arteria~ Road Leigh-on-the-Sea Essex, England E. Bollay and Assoc., Inc. Boulder, Colorado 80302 Dewey & Almy Chemical Division 62 Whittemore A venue Cambridge, Massachusetts 02140 Robert Fulton Company Old Ridgebury Road Danbury, Connecticut 06810 Goodyear Aerospace Corp. 1210 Massillon Road Akron, Ohio 44315 ILC Industries, Inc. 350 Pear Street Dover, Delaware 19901 Jalbert Aerology Laboratory 170 N.W. 20th Street Boca Raton, Florida 33432 Raven Industries, Inc. Box 1007, 205 E. 6th Street Sioux Falls, South Dakota 57101 G. T. Schjeldahl Company P.O. Box 170 Northfield, Minnesota 55057 Semco Balloons 2002 N. 11th Street Coeur d'Alene, Idaho 83814 Winzen Research, Inc. 401 W. 84th Street Minneapolis, Minnesota 55420 Some of these manufacturers provide com- plete operational balloon planning and field sup- port. The National Center for Atmospheric Research, Boulder, Colorado 80302, is an excel- lent source of information on all phases of balloon operations. This organization also fur- nishes eomplete flight services. The balloon group at the Air Force Cambridge Research Laboratories, Bedford, Massachusetts 01730, provides complete balloon services for U.S. Air Force and Department of Defense relevant activities. AIRCRAFT GENERAL CO:"J"SIDERATIONS Praetically every type of aircraft built has been used, at one time or another, as a re- mote-sensor platform. From the aircraft types listed in "Jane's All the World's Aircraft" (Taylor) and others, a representative selection has been made from those currently utilized as remote-sensor platforms. A sizable number of conventionally powered aircraft are described, whereas only single examples have been se- lected for unconventional types, which include helicopters, drones (unmanned aircraft) , blimps, and sailplanes. Selected aircraft are described in abbreviated terms and additional data are available from Jane's, the aircraft manufacturer, or the organization using the aircraft. Conventional aircraft currently used for re- mote sensing vary in size from small single- engine piston-powered craft to the sophisticated 4-engine jet military version of the Boeing 707 (RC135) and the Lockheed SR71. The remote- sensing application of widest use is in obtain- ing mapping photographs. Figures 16 to 19 illustrate some of the smaller aircraft fre- quently used by private aerial mapping con- tractors in the United States. Small aircraft 7 of this class are used for most of the aerial photography in the United States for private and governmental organizations. Such planes are adequate for lower altitude aerial photog- raphy and other types of remote sensing where instrumentation is light and simple. A single-engine or light twin-engine aircraft is generally more economical and maneuverable than a large 2- or 4-motored aircraft; however, the selection of the aircraft should be based on the operator's intended use, the geography and meteorology of the operating area, the capital and operating cost, maneuverability, safety, performance, and ease with which the aircraft can be converted to remote-sensing operations. Planes used for remote sensing are usually either commercial or military types that are modified. Modification can be very expensive, and it is generally more economical to select an aircraft that requires minimum change. Technical details relative to the problems asso- ciated with selection and modification of air- craft for aerial photography can be found in the "Manual of Photogrammetry," (1966) ; the "Manual of Photographic Interpretation," ( 1960) ; and the "Manual of Color Aerial Pho- tography," (1968), all published by the Ameri- can Society of Photogrammetry. Adapting air- craft for any other type of remote sensing is generally a highly s~pecialized operation which must be considered on a case-by-case basis. However, many nonphotographic sensors are designed to fit standard mapping-camera ports, and thus aircraft designed for mapping opera- tions may be suitable, with minimum modifica- tion, for other remote-sensing operations. Remote sensors of the active, wave propagating form require considerable power, which may be a major consideration in the choice of aircraft. Jet aircraft have a high potential for remote sensing because of their excellent rate of climb, high operating ceiling, and maximum speed, but the cost of acquisition and conversion from commercial or military types restrict their wider use. High-altitude jet photography is be- ing used increasingly for making photomaps and orthophotomaps where a single exposure may cover an entire map quadrangle. Most jet aircraft applications to remote sensing, how- ever, are still limited to NASA experimental flights, government operations, and a few com- mercial firms. Very high altitude remote-sensing aircraft have to some extent become available to the civilian community. The X15 (fig. 31), which was strictly a research platform, demonstrated remote sensing (photography) from an altitude of more than 107 km (354,000 ft). The X15 was a hybrid aircraft/ spacecraft since its flight trajectory carried it well above the atmosphere. Two other high-altitude aircraft types, the RB57F (fig. 32) and the U-2 (fig. 35), are now available for nonmilitary remote-sensing applications. These provide platforms at serv- ice ceilings of 18.3 km (60,000 ft) and 21.3 km (70,000 ft). Military reconnaissance air- craft, such as the RF4C (fig. 33) and the SR71 (fig. 36) have reported service ceilings of 15.2 km (50,000 ft) and over 24.4 km (80,000 ft), but as yet they have not been used for civilian projects. Another consideration is the role of man in the aircraft. With balloons and spacecraft, man may or may not be aboard the platform, but aircraft are generally manned. However, re- mote-control systems are rapidly being devel- oped to the point where unmanned aircraft may become practical. Completely automated air- craft have monitored atomic explosion tests, and drones are an accepted form of military reconnaissance. Even if the aircraft is manned with a pilot, the number of other people that actually fly is a prime consideration relative to the type of aircraft and cost of the flight. As operational systems are defined, they tend to become more automated; there appears to be no real reason why remote-sensing missions, once defined and tested, need a flight crew of more than one or two. The opposite extreme is to perform maximum onboard processing and evaluation with a sizable crew; however, this approach has not yet been shown to be prac- tical. PLATFORM STABILITY In addition to the fundamental consideration of having an aircraft at the right place at the right time, the problem of platform stability is paramount. In general, a highly stable plat- form, free of both vibrations and long-period 8 oscillations, is desired. Constant motion of the platform relative to the scene is an expected condition and is, in fact, required for many remote-sensing missions. Compensation for the resulting image motion, if needed, is normally included in the design of a sensor system or its mount rather than the platform. Vibration, oscillation, or any other anomalies in the pre- determined relative motion of the platform to the scene result in instability which has an overall degrading effect. Aircraft in the atmosphere are inevitably subject to two types of instability: (1) vibra- tions created by the engines or other parts of the aircraft, and (2) rotations (pitch, roll, and yaw) which occur principally because both the aircraft and the atmosphere are dynamic. There are two basic ways of eliminating or at least reducing instability. The first is to provide a mount which isolates the sensor from the air- craft vibration and, by gyros or similar devices, maintains the sensor attitudes with respect to the scene independent of aircraft rotations. Mounts of this type have been designed for general-purpose use by the U.S. Air Force, but they are normally coupled to a specific sensor, such as a mapping camera. The second ap- proach to reducing instability is in the selection and design of equipment and the aircraft it- self. A reciprocating engine creates consider- able vibration, a turbine somewhat less, and a jet still less. Propellers and reciprocating engines create torque as well as vibrations, but torque is normally neutralized by differ- entially adjusting wing lift and rudder, or it may be eliminated by the use of counterrotating pairs of engines. Vibration is reduced and isolated by the use of appropriate engine mounts. If the remote sensing can be limited to specific relatively small areas, propellers may be feathered or the engines even shut down for the critical periods of time if the aircraft is so designed. A high-lift aircraft, such as the Porter (fig. 24), with the engine shut off can actually hover or glide over a given scene for at least half an hour provided that there is a 75 km/h (40 kn) wind in which to head. The craft will, of course, be constantly losing alti- tude while the engine is shut down. Thus sta- bility is affected by the type, number, and use of engines. The design of the airframe itself as well as the sensor location with respect to the aircraft center of gravity are other param- eters that affect stability. Equipping the plane with autopilots, damping, and self-leveling de- vices may also reduce the amplitude as well as the rates of rotational motion. Heavily loaded, a plane flies at a steeper pitch than when the load is reduced; thus the sensor, if fixed to the aircraft, will change in attitude with the use of fuel. A cross wind will cause an aircraft to drift, and if the sensor is not adjusted accord- ingly, the pertinent sensor axis and the ground path of the aircraft will not be alined. A most important factor affecting aircraft stability is the local atmosphere. Winds, tur- bulence, and other aspects of atmospheric dy- namics cause most platform instability. At any given point atmospheric dynamics will vary hourly and daily as well as seasonally. As a general rule the atmosphere becomes more stable with height, and at the operational alti- tudes of U-2's and SR71's atmospheric dynamic effects are almost negligible. Obviously the sta- bility of a twin-engine jet at 20 or 25 km alti- tude is far better than that of a single-recipro- cating engine aircraft at 5 km altitude. DESCRIPTIONS OF AIRCRAFT LOW ALTITUDE, UP TO 9 KM (29,500 FT) SERVICE CEILING, PROPELLER DRIVEN Cessna 180 Skywagon.-The Cessna 180 Sky- wagon (fig. 16) is a single-engine (230 hp), propeller-driven, high-wing aircraft capable of carrying as many as six passengers or equiva- lent. Gross weight is 1,270 kg (2,800 lb). First marketed in 1968, it has had considerable use as a mapping camera platform and is well adapted for sensor equipment up to 180 kg (400 lb) in weight. Performance : Service ceiling: 5,940 m (19,500 ft). Speed: Maximum cruising (75% power) at 1,980 m (6,500 ft) is 260 km/h (160 mi/h). Minimum or stalling is 93 km/h (58 mi/h). Range: From 1,120 to 1,920 km (700 to 1,200 mi) with long-range tanks. Special equipment: Optional electronics for aerial survey projects include navi- gation aids, blind-flying instruments, autopilot, wing-leveling stability aug- mentation, and turn coordinator. Beechcraft Bonanza.-The Beechcraft Bo- nanza (fig. 17) in the standard Model A36 is a single-engine, propeller-driven, low-wing air- craft with retractable landing gear. It is pow- ered by a 285-hp Continental engine. The Model A36, marketed in 1968, was developed from the Model 35, which was introduced in 1946. Maxi- mum gross weight of the A36 is 1,620 kg (3,600 lb). Sensor (payload) capacity is estimated at 360 kg (800 lb). 9 Performance: Service ceiling: 4,870 m (16,000 ft). Speed: Maximum cruising (75% power) at 1,980 m (6,500 ft) is 315 km/h (195 mi/h). Minimum or stalling, gear down, flaps 30° is 103 km/h (64 mi/h) ; gear and flaps up is 120 km/h (75 mi/h). Range: From 1,290 km (800 mi) (75% power) at 1,980 m (6,500 ft) to 1,430 km (890 mi) (55% power) at 3,050 m (10,000 ft). Special equipment: Optional equipment includes autopilot, dual controls, and marker beacon receiver. Cessna Super Skymaster.-The Cessna Super Skymaster (fig. 18) is a twin (tandem) turbo- prop, high-wing craft with twin tail booms, capable of carrying up to six passengers or equivalent. Gross weight is 2,000 kg ( 4,400 lb). The aircraft, marketed in 1965, has been adapted for mapping and remote-sensing mis- sions. Remote Sensing, Inc., Houston, Tex., reports carrying the following load of remote sensors on this aircraft: a Texas Instruments RS-310 infrared scanner, an RC-8 or RC-9 mapping camera, and four gang-mounted Has- selhlad 500 EL multispectral cameras. The 4-seat version of the aircraft has space for equipment weighing 165 kg (365 lb). Performance : Service ceiling: 8,930 m (29,300 ft). Speed: Maximum cruising (75% power) is 305 km/h (190 mi/h). Minimum or stalling is 113 km/h (70 mi/h). Range: At maximum cruising speed from 1,900 to 2,500 km (1,190 to 1,550 mi) with long-range tanks. Special equipment: Optional avionics in- clude blind-flying instruments, auto- pilot, navigation instruments, and an oxygen system. Beechcraft Queen A,ir.-The Beechcraft Queen Air B80 (fig. I9) is a twin-engine, light executive or business transport of conventional low-wing monoplane configuration. The cabin accommodates 6 to II passengers or a crew of two on the flight deck with space for sensor and electronic equipment. Gross weight is 4,000 kg (8,800 lb). It is powered by two 380-hp Lycoming 6-cylinder horizontally opposed air- cooled and supercharged engines. Sensor ca- pacity is 820 kg (I,800 lb). Performance : Service ceiling: 8,I70 m (26,800 ft). Speed: Maximum cruising (70% power) is 360 km/h (225 mi/h) at 4,570 m (I5,000 ft). Range: With maximum fuel and 45-min fuel reserve, 2,500 km (I,500 mi). Special equipment: The aircraft has a deicing system on the leading edge of the wings, optional auxiliary fuel tanks, a pressurization system, and full ground and in-flight air condi- tioning. All Beechcrafts may be equipped with a full range of avionics, including dual VHF, radio, and elaborate positioning and navigation aids, and they may be fitted with vertical mapping camera installations. The twin-engine Beech- crafts carry weather-avoidance radar. The King Air and Queen Air models may be equipped with a wide variety of remote sensors; in I972 Beech announced the RS99 ("Aviation Week and Space Technology," I972) which is a re- mote-sensing version of the B99 series. Grum,man Mohawk.-The Grumman OV-I Mohawk (fig. 20) is a high-performance 2-seat observation and surveillance aircraft developed by Grumman for the U.S. Army. More than 365 Mohawks have been placed in service in five models. The principal difference in the models is in the navigation and remote-sensing equip- ment that has been installed. Some models carry a variety of advanced remote-sensing equipment, such as the KA-30 high-resolution optical photographic system (in model OV- IA) and integrated flight systems for day and night reconnaissance. The aircraft is powered by two I, I60-hp Lycoming turboprop engines. Gross weight for the OV-IA is 6,820 kg (I5,000 lb). Maximum load available for equipment is 2,250 kg ( 5,000 lb). In I97I the U.S. Geological Survey obtained a Mohawk Model OV-I for research concerning the quality of water and the environment. The sensing equipment includes SLAR, a thermal scanner, a 230-mm (9-in.) focal-length camera, and a cluster of Hasselblad cameras for mul- tispectral photography. This model has a ceil- ing of about 7,620 m (25,000 ft). Performance: Service ceiling: (OV-IC) 9,000 m (29,500 ft). Speed: Maximum cruising (OV-IA) is 490 km/h (305 mi/h). Minimum or stalling (OV-IA) is IIO km/h (68 mi/h.) Range: (OV-IA) 2,260 km (I,400 mi) with external tanks, IO% reserve. Special equipment: Mohawk models OV- IB, OV-IC, and OV-ID carry inter- changeable equipment, which includes SLAR and an in-flight processor. One model, the OV-ID, can be converted from infrared reconnaisasnce to SLAR surveillance in about 1 hour. Lockheed Y0-3A.-Lockheed began studies and development on quiet aircraft late in I966. The first Lockheed quiet aircraft was the QT- 2 (Quiet Thruster, 2 seat). The first two pro- totypes were fitted with night sensors. It is reported that reconnaissance flights as low as 30 m (IOO ft) above ground were made with- out detection. Potential of the quiet reconnaissance type of aircraft was recognized, and the U.S. Army Aviation Systems Command undertook fur- ther development with Lockheed in July I968. This resulted in a more refined quiet recon- naissance aircraft, designated Y0-3A (fig. 2I). Basis of the design is the Schweizer SGS 2- 32 sailplane, but the new aicraft has a low- wing configuration. The fuselage has been modi- fied extensively. Perhaps the greatest single reduction in noise has come from using specially developed pro- pellers with three, four, or six blades that rotate at speeds as low as 500 r/min. A modified 2IO- hp air-cooled Continental 6-cylinder engine pro- vides power. IO Infrared equipment is carried in the Y0- 3A for night reconnaissance operations. No de- tails on performance are available. MEDIUM ALTITUDE, 9 TO 15 KM (29,500-49,000 FT) SERVICE CEILING, PROPELLER OR JET Rockwell Standard Jet Commander 1121.- This plane (fig. 22) is a high-speed twin-jet executive transport with standard accommoda- tion for a pilot and as many as eight passengers. It is powered by two General Electric turbo- jet engines. Gross weight is 7,640 kg (16,800 lb) . A stretched version of the Commander is known as the Commodore. Production rights were taken over by Israeli Aircraft Industries in 1967. Sensor capacity is 725 kg (1,600 lb). Performance : Service ceiling: 13,700 m ( 45,000 ft). Speed: Maximum cruising is 805 km/ h (500 mi/h) at 10,670 m (35,000 ft.) Minimum or stalling is 160 km/ h (100 mi/h). Range: 2,965 km (1,840 mi) with maxi- mum fuel and maximum payload at economy cruising speed 760 km/h ( 470 mi/h) at 11,900 m (39,000 ft), no reserve. Special equipment: Blind-flying instru- mentation is standard. Provision is made for full range of radio and radar communications, navigation, storm warning, glideslope and localizer, and Automatic Direction Finding equip- ment. Gates Learjet.-This twin-engine business jet (fig. 23) is powered by two General Electric CJ610 turbine engines. Designed to carry eight passengers, the jet has a gross takeoff weight of 5,900 kg (13,000 lb). The business jet has been modified into a remote-sensing aircraft operating for altitudes well above 12,200 m (40,000 ft). Sensor capacity is 1,250 kg (2,760 lb). First marketed in 1966, the Learjet 24 re- placed the earlier Model 23. Performance (Gates Learjet 24) : Service ceiling: 13,700 m ( 45,000 ft). Speed : Maximum airspeed, Mach 0.81 at 9,450 m (31,000 ft) or 885 km/ h ( 550 mi/h) . Normal cruise speed, Mach 0.77 at 12,500 m (41,000 ft) or 820 km/h (510 mi/h). Stalling speed at sea level with 4,090 kg (9,000 lb) , landing configuration, is 155 km/h (96 mi/h). Range : 2,400 km ( 1,500 mi) with 8 passengers, 45-min fuel reserve. Special equipment: Figure 23 shows a Gates Learj et 24, the model most ex- tensively used in remote sensing, with the special door open to show the camera installation. Several domestic and foreign companies have built modifications to the Learjet door to allow cameras inside the pressurized cabin to photo- graph through an optical glass plate in the floor of the special door. A quick-change modifica- tion to the door allows the jet to be taken off charter service for camera work and returned to passenger hauling. The conversion takes about 3 hours. An earlier Learjet Model 23 was used by astronomers of Rice University, Houston, Tex., in the research project "Flying Infrared Tele- scope," operating on top of the troposphere, at altitudes of 15,200 m ( 50,000 ft) to photograph the stars, planets, and planetary nebulas. The Learj et was provided by NASA Ames Research Center (Hudson, 1968 and 1971). Fairchild Hiller Porter.-This is a versatile airplane (fig. 24) designed to fill a wide variety of aerial sensing needs. It has an extremely short take off of 90 m (300 ft) and a landing distance of less than 45 m (150ft). The Porter carries as many as 11 passengers, or it can carry a remote-sensor package weighing 910 kg (2,000 lb). It has a small but powerful turbine engine and reversible propeller. Addi- tional fuel may be carried in external wing- mounted pylons and would extend the plane's range to a thousand miles. Its gross weight is 2,200 kg ( 4,850 lb). 11 Performance : Service ceiling: 7,770 m (25,500 ft). Speed: Maximum cruising at 3,050-3,650 m (10,000-12,000 ft) is 280 km/h (175 mi/h). Range: 970 km ( 600 mi) with normal configuration and maximum fuel; 1,600 km (1,000 mi) with external wing-mounted tanks. Special equipment: The Porter can be adapted for aerial photography and other remote sensing. A military ver- sion carries a complete radio com- munications s~stem, a pod containing three P-2 70-mm cameras, and 40 flares of 2 million candlepower for night photographs. It has a floor hatch and large cargo doors for installing special sensing equipment. Lockheed NP3A Orion.-This aircraft (fig. 25) , which was derived from the Electra, is a 4-engine turboprop low-wing plane capable of carrying a sensor payload of as much as 9,000 kg (20,000 lb). Maximum gross weight is 51,400 kg (113,000 lb). NASA 927 is such a craft adapted for remote-sensor research. The "Annual Report, 1971," of NASA Manned Spacecraft Center, Earth Observation Aircraft Program, describes this aircraft and the sen- sors in considerable detail. The total sensor- related equipment weighs 2,500 kg (5,500 lb) as configured in this aircraft. Performance : Service ceiling: 9,100 m (30,000 ft). Speed: Maximum cruising is 610 km/h (330 kn). Minimum or stalling is 280 km/h (150 kn). Range: 3,700 to 3,900 km (2,300 to 2,400 mi). Endurance : 7 h. Stability: Extent of average deviations from prescribed attitude is ±2° in pitch and roll and ±8° in yaw. Special equipment: Precise altimeter, Loran navigation system, inertial navigation unit, gyro-stabilized plat- form. Douglas Skywarrior RA-3B.-Production of the twin-jet RA-3B (fig. 26) carrier-based air- craft ended in January 1961, and only four remain in service with the U.S. Navy late in 1973. Nevertheless, they are a major element of the U.S. Navy reconnaissance and mapping effort and are effective, versatile, and durable. The RA-3B photographic reconnaissance air- craft are versions of the standard McDonnell Douglas A-3 airframe. Weight empty is 19,800 kg ( 43,500 lb) ; normal loaded weight is 35,500 kg (78,000 lb) ; and overload weight is 37,300 kg (82,000 lb). Performance : Service ceiling: 13,700 m (45,000 ft). Speed: Maximum cruising 1,020 km/h (630 mi/h) at 9,150 km (30,000 ft) altitude. Landing speed 269 km/h (167 mi/h). Normal range: More than 4,650 km (2,880 mi). Endurance: 5 h, plus 15 h with in-flight refueling capability. Special equipment: This aircraft has 16 camera windows and sufficient space and lifting capacity to carry a wide array of sensors (Barton, 1972). As many as 12 cameras ranging from 45 to 910 mm (1.75 to 36 in.) are car- ried simultaneously. Night photog- raphy supported by flares, infrared scanning, and radar imaging can also be carried out. A dual viewfinder sys- tem also supports the remote-sensing mission. Additional information on special equipment is given in "RA-3B Photographic Capabilities Handbook," published by the U.S. Navy, Fleet Air Reconnaissance Squadron ONE. Lockheed NC130B (RC130) .-This plane (fig. 27) is used as a NASA sensor system fa- cility, which carries both imaging and nonimag- ing remote sensors. The data acquisition sys- tem includes various onboard remote sensors, signal processing, and a control and recording system. The Lockheed NC130B is an all-metal high-wing long-range land-based monoplane powered by four Allison turboprop constant- speed engines. The fuselage can be pressurized and airconditioned in flight and on the ground. The military photomapping version has the principal mission of executing electronic aerial geodetic surveying and photogrammetric map- ping in an integrated system. Gross weight is 61,400 kg (135,000 lb). Weight available for sensor payload is 9,230 kg (20,300 lb). Performance : Service ceiling: 10,700 m (35,000 ft). Speed: Maximum cruising is 590 km/h (320 kn). Range: 4,000 km (2,500 mi). Endurance: 9 h. Stability: Extent of average deviations 12 from prescribed attitude is ±21f2 o in pitch, +7° in roll, and +5° in yaw. Special equipment: In NASA's version, each of the four camera windows ac- commodates the field of view of an R.C-8 metric camera plus allowance for roll stabilization of up to +6°. The TV subsystem is used by the pilot in following a ground track. N aviga- tion subsystems include an inertial navigation system, a radar altimeter set, a navigational computer, and re- mote navigation indicators. Aircraft position in latitude and longitude co- ordinates is displayed at the naviga- tor's station. SLAR is boresighted with a cam.era window. Military photomapping versions carry a similar array of special equipment which sup- ports basic mapping functions. Boeing Stratolifter (RC135A).-This plane is a long-range jet transport developed from the KC135A Stratotanker. The KC135A air- craft were powered by Pratt and Whitney tur- bojet engines and became the first strategic jet transports in the Military Air Transport Service (MATS). All KC135 aircraft are simi- lar to but smaller than the Boeing 707 airliner, both derived from the 707 prototype (figs. 28, 29). Four RC135A aircraft were built. They were similar to the KC135A but specially equipped for multiple cameras and electronic equipment for photomapping and electronic surveying of the terrain, using SHIRAN, an electronic sys- tem for establishing geodetic control. However, the cost of upkeep for photomapping could not be justified, and the four RC135A photomap- ping aircraft were converted from photomap- ping to other uses during 1972. Ten aircraft designated WC135B (same as C135·B) were modified for long-range weather reconnaissance missions for the Military Airlift Command (MAC), Air Weather Service. Other Air Force reconnaissance versions include the RC135B and RC135D. Gross weight (C135B) is 125,000 kg (275,500 lb). Performance : Service ceiling: 9,100 to 13,700 m (30,000 to 45,000 ft). Speed: Average cruising speed 855 km/ h (530 mi/h) at 10,700 m (35,000 ft). Maximum level speed 970 km/h (600 mi/h). Range: 7,400 km ( 4,600 mi) with 24,500 kg (54,000 lb) payload. Maximum ferry range is 14,485 km (9,000 mi). Special equipment: The RC135A car- ries special navigation equipment for improved mapping accuracy. The air- craft were assigned to the 1370th Photo Mapping Wing of the MAC. The mapping system was known as the AN /USQ 28 and was probably the most sophisticated ever built. E-Systems (formerly LTV) L450F recon- naissance aircraft.-This is a motorized ver- sion of the Schweizer sailplane built to meet military aircraft requirements for high-alti- tude and endurance (fig. 30). It is powered by a modified Pratt and Whitney turboprop en- gine. The new design provides the capability of carrying data-gathering or electronic relay equipment similar to that of a communications satellite. The relay equipment can be carried to an altitude of 13,700 m (45,000 ft) and could be operated up to 24 h at a time. Alter- natively, the aircraft can be flown unmanned by remote control over a radius of 400 km (250 mi). For operation in the unmanned mode, auto- matic stabilization equipment is installed, and complete ground control and monitoring by telemetry allows control of the aircraft in a variety of modes. Gross weight is 2,140 kg (4,700 lb). Payload is 160 to 320 kg (350 to 700lb). 13 Performance: Service ceiling : 13,700 to 15,200 m (45,000 to 50,000 ft). Speed: Maximum cruising at 13,700 m (45,000 ft) is 170 km/h (105 mi/h). Best glide ratio: 28:1. Range (piloted version) : 9,700 km (6,000 mi). Endurance: 24 'to 30 h. HIGH ALTITUDE, SERVICE CEILING ABOVE 15 KM (49,000 FT) North American Rockwell X15.-This plane (fig. 31) was a highly specialized research ve- hicle jointly funded by the Air Force, the Navy, and the National Aeronautics and Space Ad- ministration (Aerospace Industries Assoc., 1970). Three aircraft were built, with the first flight on June 8, 1959. Tests conducted on this unique platform contributed to research, par- ticularly in studies related to hypersonic and hyperaltitude flight regimes. It attained speeds greater than Mach 6 and altitudes above 107,- 000 m (350,000 ft). The No. 1 plane, now in the Smithsonian Institution, was used for re- search in high-altitude aerial photography, ultraviolet stellar photography, atmospheric density measurements, horizon scanning and definition measurements, and advanced inte- grated data systems. It was also used in re- search to determine the effects of flight regimes covering a wide range of speeds, altitudes, and environmental conditions on photographic film resolution and distortion. The No. 2 plane, shown in figure 31, was fitted with twin drop- pable fuel tanks, coated with ablative mate- rial, and modified as a ramjet test bed. It was the plane which set an unofficial speed record of Mach 6. 7 or 7,290 km/h ( 4,520 mi/h) . The No. 3 aircraft was destroyed in an accident on Nov. 11, 1967, in which Major M. J. Adams lost his life. The three X15's made 197 flights in 10 years. Performance: Maximum altitude: 108,000 m (354,200 ft or 67.1 mi) reached by Joseph Walker, chief test pilot, NASA, on Aug. 22, 1963. Speed: An official speed record was achieved by Joseph Walker, on June 27, 1962, at 6,620 km/h ( 4,104 mi/h). An unofficial speed was recorded at 7,290 km/h (4,520 mi/h) on Nov. 18, 1966, by Major Pete Knight, USAF. Total flight life of aircraft: Approxi- mately 24 h. Maximum launching weight: 23,140 kg (50,900 lb). Maximum landing weight: 7,780 kg (17,120 lb). Payload: Instrumentation payload de- signed for the X15A was 590 kg (1,300 lb). Special equipment: Research was under- taken to determine the effects of high- speed shock waves and friction-heated air turbulence on aerial photographs taken from a camera mounted in the nose of the aircraft. Among the cameras flown was an experimental type KC-1 camera body fitted with a prototype lens, forerunner of the 152- mm (6-in.) focal length f/5.6 Geocon I designed by Dr. James Baker. This camera was installed in an ART-15 gyrostabilized mount in the pressur- ized instrument compartment just aft of the cockpit. A special quartz glass, nearly optically flat and 460 mm ( 18 in.) in diameter, was used for the window. Six flights were made to de- termine geometric image deforma- tions produced by high-altitude super- sonic flight. Another series of flights was conducted to determine image-resolution degradation using an experimental KS-25 camera with a 610-mm (24-in.) focal length lens developed by the Air Force Avionics Laboratory at Wright-Patter- son AFB, Ohio. This camera was capable of resolving better than 115 lines per millimeter on fine-grain photographic film. It was installed in the same ART-15 mount that has been used to carry the KC-1 mapping camera. One ob- jective of this test was to obtain photographic images that would, as nearly as possible, simu- late those that were expected to be obtained later from space. Photographs obtained with the X15 thus permitted comparative studies and interpretation which paved the way for analysis and evaluation of high-altitude air- craft and space photography which was to follow. In another experiment, a gimbal-mounted 4- camera platform was used for research in ul- traviolet steller photography and for photog- raphing the stars from altitudes above 65 km (40 mi). A horizon scanner was also used to study the light spectrum of the sky and to obtain data on the accuracy of sensing the horizon. These data were later used to improve attitude and guid- ance references for Earth-orbiting spacecraft. 14 General Dynamics/Martin RB57F (WB57F). -A General Dynamics/Martin WB57F air- craft is flown and maintained for the NASA Earth Observations Aircraft Program Office. The WB57F shown in figure 32 is a midwing 4-engine aircraft powered by two Pratt and Whitney turbofan engines and two auxiliary re- movable Pratt and Whitney turbojets carried in underwing pods. The data acquisition system comprises an airborne sensor platform, various sensors, and ancillary control and data-recording equipment. The sensor equipment is described in detail in the "Annual Report, 1971," of the NASA Manned Spacecraft Center, Earth Observation Aircraft Program. Gross weight is 28,600 kg ( 63,000 lb) . Estimated weight available for remote-sensor payload is 2,160 kg (4,750 lb). Performance: Service ceiling: 18,900 m (62,000 ft). Speed : Maximum cruising at 18,300 m (60,000 ft) is 740 km/h (460 mi/h). Minimum or stalling is 145 km/h (90 mi/h). Range: 5,320 km (3,300 mi). Endurance: +4 hat service ceiling. Stability: Extent of average deviations from prescribed attitude is approxi- mately +0.5° in pitch and roll and +8° in yaw. Special equipment: Considerable special equipment is c a r r i e d , including weather radar in the fuselage nose, an autopilot, and a navigation system comprising a C-12 Gyrosyn compass, an APN-102 Doppler radar, and an ASN-41A navigator computer. A Litton LTN-51 inertial navigation system was added in 1972. The sys- tem measures the heading, ground speed, and drift angle of the aircraft, and computes present latitude and longitude, distance to destination, ground track, and relative bearing. An APN-159 pulse-type radar altimeter indicates absolute clearance of the air- craft above the terrain. An optical viewfinder enables the equipment op- erator to view the terrain ahead of the aircraft and to measure drift. A multispectral scanning imaging spec- troradiometer was installed in 1972. McDonnell Douglas RF-4C.-This high-per- formance fighter-type aircraft (F-4) (figs. 33, 34) is modified for multiple-sensor reconnais- sance. It incorporates optical, infrared, and electronic sensors for reconnaissance missions day or night and in any kind of weather. Its op- tical systems include a num.ber of cameras of various focal lengths, forward- and side-look- ing cameras, and panoramic cameras that scan the terrain from horizon to horizon. Films can be processed in flight and ejected over the ground command station. The RF -4C also car- ries side-looking radar for mapping on each side of the flight path and a third radar for forward-looking ground mapping. It also car- ries an infrared sensor for day or night use. Gross weight is 24,800 kg (54,600 lb). Performance : Service ceiling: 15,200 m (50,000 ft). However, the F-4B version has tem- porarily reached 30,000 m (98,400 ft) in a time-to-height test which took slightly over 6 min to achieve. Speed: Maximum cruising 2,240 km/h (1,390 mi/h) at 12,200 m (40,000 ft). Range: Ferry range, 3,700 km (2,300 mi). Special equipment: Navigation radar including terrain avoidance, central air data computer, navigational com- puter, and electronic altimeter. Lockheed U-2.-This unique aircraft (fig. 35) is capable of sustained flight at very high altitudes. Although basically a manned aircraft, recent reports ("Aviation Week and Space Technology," 1971) indicate the U-2 has also been converted into a drone. In April 1971, NASA acquired on loan from the U.S. Air Force two U-2 aircraft for high-altitude photography and research. Gross weight is 7,850 kg (17,270 lb) with external tanks. The primary job of the NASA U-2 aircraft is to complement two Earth resources research space flights: the Earth Resources Technology Satellite (ERTS) and the manned orbital work- shop, Skylab. Test sites were photographed and analyzed before the launch of the spacecraft, and the U-2's photographed sites simultaneous- ly with the passes of both ERTS and Skylab in 1972 and 1973. 15 Performance : Service ceiling: 21,300 m (70,000 ft). Speed : Maxim urn cruising is 790 km/h (490 mi/h). Range: Internal fuel only, 100 U.S. gal reserve, 3,550 km (2,200 mi); with external tanks, 100 U.S. gal reserve, 4,200 km ( 2, 600 mi) . Endurance: Approximately 5 h. Stability: The U-2 at high altitude is highly stable as demonstrated by con- tinuous strips of near-vertical map- ping photographs which show no signs of vibration or longer term oscilla- tions even though a stabilizing mount was not used. Special equipment: The Air Force car- ried a variety of frame and panoramic cameras of up to 610 mm (24 in.) focal length. These include mapping cameras in gyrostabilized mounts. NASA uses a gang of small multi- spectral cameras to simulate space imagery and also carries a mapping camera of 152 mm (6 in.) focal length. A ventral periscopic sight or view- finder is included, and an autopilot has also been installed on occasions. In late 1973, NASA plans to install at least one inertial guidance system. Lockheed SR71 (YF-12) .-The Lockheed SR71 (fig. 36) is an Air Force long-range ad- vanced strategic reconnaissance aircraft. One of the fastest and highest flying aircraft in the world, it is capable of worldwide reconnaissance operations including aerial photographic mis- sions and multiple forms of remote sensing. It is powered by two Pratt and Whitney J58 tur- bojets and has a speed of Mach 3, or more than 3,400 km/h (2,100 mi/h). The SR71 is the reconnaissance version of the general YF -12 series of aircraft. However, the reconnaissance version is the only one in operational use today. Gross weight is 76,500 kg (170,000 lb). Performance: Service ceiling: Over 24,400 m (80,000 ft) ; maximum ceiling reported at 36,- 600 m (120,000 ft). Speed: Maximum 3,340 km/h (2,070 mil h) at 24,463 m (80,258 ft) a world record, May 1, 1965. Minimum or stall- ing not given. Range: More than 3,220 km (2,000 mi) ; with aerial refueling it has global range. Special equipment: Equipment ranges from simple surveillance to multiple- sensor high-performance reconnais- sance systems. BELL HELICOPTER 47G-3B-l The Bell 47G-3B-1 helicopter is utilized by the U.S. Department of Agriculture, Forest Service, Northern Forest Fire Laboratory, Mis- soula, Mont., as a platform for a simple infra- red line scanner. The Forest Service Fire 8pot- ter can be used to locate forest fires that are too small to be readily visible to optical detec- tion system. Nominal altitude is 610 m (2,000 ft). Gross weight for the model 47G-3B-:1 shown in figure 37 is 1,340 kg (2,950 lb). Th1s model is a 3-seater with high-altitude perform- ance over a wide range of temperatures. This helicopter is powered by a supercharged Ly- coming engine uprated to 280 hp. During 1973, the Forest Service was experi- menting with a telemetry near-real-time system as compared with the conventional infrared line scanner from which the data must be physically returned and processed before evaluation. With the telemetry system the imagery data are transmitted from the helicopter to the ground station where evaluation can start immediately. Performance: Service ceiling: 4,270 m (14,000 ft). Speed: Maximum 170 km/h (105 mi/h) at sea level. Cruising 135 km/h (83 mi/h) at 1,520 m (5,000 ft). Range: 400 km (250 mi) with maximum fuel at 1,520 m (5,000 ft) and no reserves. Endurance: Approximately 3 h. Maximum payload: 540 kg (1,190 lb). NORTHROP FALCONER DRONE This surveillance drone (see also E-Systems, p. 13) (fig. 38) was developed originally for the U.S. Army Signal Corps. Launched from a zero- length launcher, it is controlled remotely by 16 radio and tracked by radar. It carries still or TV cameras for surveillance over hostile terri- tory. On reaching a predetermined recovery area the controller commands parachute re- covery. The camera is removed, the film is processed, and the prints are delivered within minutes after recovery. Models are available for day or night photointelligence and recon- naissance duties. The cameras can be replaced by other sensor systems. This propeller-driven platform is powered by a 9-2-hp McCulloch air- cooled engine. Maximum launching weight is 200 kg (440 lb). Empty weight is 160 kg (350 lb). Performance: Service ceiling: 4,570 m ( 15,000 ft). Speed: Maximum level speed at sea level is 320 km/h (201 mi/h). Endurance at sea level: 35 min. Special equipment: Recovery is by a single-stage 13.4 m ( 44 ft) extended- skirt parachute deployed on command or automatically. The radio-command flight-control system includes a re- ceiver autopilot. Special equipment in- cludes two 75-mm cameras, a flare ejector, a radar beacon or passive radar reflector pods, and a night light identification system. Flares are car- ried for night photography. GOODYEAR BLIMPS The blimps Columbia, America, and May- flower, owned and operated by the Goodyear Tire and Rubber Company, Akron, Ohio, are the only lighter-than-air craft known to be operating in the U.S. today. They were built during the period 1968-1971. The three air- craft are the 297th, 298th, and 299th blimps that the company has built since 1917, mostly for the U.S. Army or Navy. Power is supplied by twin 6-cylinder pusher-type 210- hp Conti- nental engines (figs. 39, 40). The three blimps are in demand by the news media, particularly television networks, for use as aerial and television camera platforms for viewing special events. Their aerial photo- graphs have appeared in virtually every news- paper and magazine. Performance: Service ceiling: Maximum 2,290 m (7,500 ft). Normal operational alti- tude 300 to 910 m (1,000 to 3,000 ft). Speed: Maximum 80 km/h (50 mi/h). Cruising 55 km/h ( 35 mi/h) . Size: Columbia 58.5 m ( 192 ft) long, 18 m (59.5 ft) high, 15m (50ft) wide. Volume: Columbia, 5,800 m 3 (202,700 fV). Weight: Maximum gross 5,600 kg (12,320 lb) ; empty 4,100 kg (9,040 lb). Maximum lift or payload: 1,490 kg (3,280 lb) or six passengers plus pilot. Range : 805 km ( 500 mi) . Endurance: 20 h. Special equipment: The blimp carries special TV orthicon viewing cameras for aerial scenes and transmits the data to a ground-crew bus which serves as radio headquarters. It also carries frame and movie cameras and broadcasting equipment for reporting special :events. A recent report by the Aero-Mechanical Pro- grams of the Goodyear Aerospace Corp. (1971) describes the use of airborne sensing equipment flown aboard a Goodyear blimp, including iso- lated nose-probe acoustic sensor, steerable acoustic sensor, stereo acoustic sensors, direc- tional acoustic sensor, remotely controlled TV camera, infrared covert illuminator, acoustic gradient array, low-light-level and daytime TV, remote trailing sensor, and an aft-looking sensor. Test equipment can be located virtually any- where on the airship. Acoustic and infrared sensors can be isolated from engine noise and heat. Sensors can be lowered or dunked from the airship into hard-to-reach places. Acceleration in the cabin of the aircraft dur- ing powered flight is less than l/40th g (accel- eration of gravity) with a vibration frequency range of 10 to 1,000 Hz. There is no indication that blimps are being used for remote sensing except for their news media role. SCHWEIZER SAILPLANE SGS 2-32 The Schweizer sailplane SGS 2-32 has an unusually large airplane-type cabin capable of 17 carrying two average-sized passengers in addi- tion to the pilot (fig. 41). It has an all-metal monocoque structure, and the airframe has been used as a basis for several powered aircraft. The SGS 2-32 was chosen by Lockheed Missiles and Space Company to form the basic airframe of its Y0-3A quiet reconnaissance aircraft and by Ling-Temco-Vought (now E-Systems) for its L450F. The sailplane's maximum takeoff gross weight is 610 kg (1,340 lb). Performance: Speed: Maximum (smooth air) 225 km/ h (140 mi/h). Minimum or stalling 77 km/h ( 48 mi/h). Minimum sinking speed 0.7 m/s (2.4 ft/s) at 80 km/h (50 mi/h). Best glide ratio: 34: 1 at 97 km/h ( 60 mi/h). Maximum aero-tow speed: 180 km/h (110 mi/h). Special equipment: Optional equipment includes electrical and oxygen systems, radio communications, and such spe- cial instrumentation as might be speci- fied, including provisions for aerial cameras or other remote sensors. SPACECRAFT With the advent of space systems, a new dimension has been added to remote sensing. Platforms are no longer limited by the atmos- phere, and orbits can be defined which provide the following: (1) Any altitude desired as long as it is effectively above that of atmospheric drag effects (about 150 km) and still within the dominant gravity field of the Earth. (2) Con- figuration so that the entire Earth or any desig- nated portion can be covered at specified intervals. (3) Constant position relative to the Earth's surface through the geostationary mode, thus permitting continuous sensing of a given section of the Earth's surface. ( 4) Remote sensing at relatively close range of extrater- restrial bodies such as the Moon and planets and without the interference of the Earth's atmosphere. Remote sensing of the entire Earth on a periodic basis and sizable sections on a continu- ous basis thus becomes feasible. Lighting and atmospheric conditions will limit the effective- ness of the system particularly where sensing of the actual surface with reflected solar ener- gy is involved. The meteorological satellites demonstrate the Earth-sensing electronic trans- mission space modes even though they do not carry sensors that resolve the surface to the extent desired for many applications associated with Earth studies. Satellites designed for remote sensing on an operational basis are generally unmanned though manned space stations provide similar capabilities. Manned extraterrestrial missions, such as Apollo, can provide opportunities for remote sensing of the Moon or planet con- cerned; however, to date manned missions have not been defined with remote sensing as their primary objective. GENERAL CONSIDERATIONS No attempt is made to cover either the theory or the state-of-the-art of space flights. The problems of achieving and maintaining the proper orbit and providing the power and en- vironment for remote sensing are beyond the scope of this circular. However, the problems of data transmission and spacecraft platform stability are unique to remote-sensing missions and are therefore briefly covered. Information in this section has been obtained from "Jane's All the World's Aircraft" unless otherwise noted. DATA TRANSMISSION All space flights require a network of ground control stations for data transmission and com- mand of the spacecraft. These networks vary with the type of mission. The NASA network for Earth and lunar missions is described in the "Space Tracking and Data Acquisition Network Manual" (NASA, 1967). The transmission and control network for planetary flights is de- scribed in "The Deep Space Network" (Calif. Inst. of Tech., Jet Propulsion Laboratory, 1971). The NOAA system for its operational meteorological satellites is described in the "Instructions and Operating Handbook for the Improved TIROS Operational System (IT OS) and the TIROS Operational System (TOS), Part 4." 18 Except for the film return mode, as exempli- fied by manned missions, sensor response must be in electronic signal form and transmitted to Earth. If this is to be done globally on a real- time basis, from Earth-sensing systems such as the meteorological satellites, there must be a ground station network capable of continuously receiving from the satellite. Since the wave- lengths involved require line-of-sight transmis- sion, several ground stations, depending on the orbit, are needed to acquire the data. The use of geostationary communications satellites as re- lay stations is a recent development which promises to simplify the data-transmission problem and reduce the number of ground sta- tions required. Three U.S. ground stations are employed for the Earth Resources Technology Satellite . (ERTS-1) data acquisition. Canadi- ans installed a ground station prior to launch, Brazil installed a ground station during 1973; and other countries have asked for advice in setting up similar installations. For areas out- side the ground stations' range, a wide-band video tape recorder is included on ERTS-1 which stores the data on board and then plays it back when the flight is within range of a ground station. This procedure precludes real- time transmission, but the delay involved in transmission will generally be less than 1 hour. It is important to note, however, that data processing of ERTS is centralized, and there- fore additional delay generally occurs prior to actual receipt of the data by the processing center at Greenbelt, Md. Early in 1973, ERTS-1 lost part of the use of its second tape recorder (the first failed shortly after launch), thus reducing its coverage beyond the range of the ground stations. The problem of recovering hard copy, such as film, is discussed briefly un- der film return (p. 20). During 1973 this mode was being demonstrated by Skylab. PLATFORM STABILITY In theory, a spacecraft might be free of all relative motion except for its designed orbital vector, and thus be a perfectly stabilized plat- form. In practice, spacecraft must be controlled to maintain proper attitude and to dampen in- evitable oscillations, including vibrations that may develop from onboard mechanisms. Atti- tude must he sensed and determined before it can be adjusted. Horizon sensors, particularly in the infrared region, can relate spacecraft attitude to the Earth or another planet as a whole, but there are limits to this procedure, which is accurate only to the order of a tenth of a degree. Truly precise attitude can he ob- tained by stellar observations. This approach is used on extraterrestrial flights and has also been defined for film-return Earth-sensing mis- sions. The control of attitude can be accom- plished by reaction jets, gim·baled inertial sys- tems (including gyros), torquing with respect to the Earth's magnetic field, or even a gravity gradient boom which works on the principle that a linear-shaped mass will aline itself in a gravitational field. To keep from constantly cor- recting for attitude, some limits or deadbands are imposed before corrective action is intro- duced. The fewer the corrections, the more stable and longer lived the platform's attitude control system, and thus reduction of rotational rates (damping) is a desirable objective. Some figures relative to deadbands and rotational rates are quoted for described spacecraft. Station keeping, which involves adjustment by jet thrusters, is another source of instability. On remote-sensing missions it is more desirable to allow the orbit to change somewhat rather than pay the price of constant station keeping. Although there is no such thing as a fully stable Earth orbit, some spacecraft have remained in the same nominal Earth orbit for several years without station-keeping adjustment. DESCRIPTIONS OF SPACECRAFT The spacecraft section is divided into two parts. The first covers experimental spacecraft, which are flown by NASA, and the second covers operational remote-sensing· spaceGraft, which are flown by NOAA. EXPERIMENTAL, UNMANNED, EARTH VIEWING Three basic unmanned space modes for sens- ing the Earth have been defined : (1) Film return, short-lived, 150-500 km altitude, sun-synchronous. (2) Electronic data transmission, long-lived, 300-1,500 km altitude, sun-synchronous, except 19 for TIROS 1-8: (TIROS, ITOS, Nimbus, ERTS-A&B) 1 (3) Electronic data transmission, long-lived, 36,000 km altitude,· geostationary or geo- synchronous : Advanced Technology Satellite (A TS) , Synchronous Meteorological Satellite ( SMS) , and the proposed Synchronous Earth Observation Satellite (SEOS). FILM RETURN The NASA manned spacecraft program has repeatedly returned exposed film to Earth ; however, an unmanned film-return mission has not been flown by NASA except as incident to the manned program (Apollo 6). Unmanned film-return missions have been defined and re- quested of NASA by the U.S. Department of the Interior (1970). Newspapers, periodicals such as "Space Busi- ness Daily" (1971) and "Science News" (Dris- coll, 1971), and a recent book by Klass (1971) refer to so-called reconnaissance/ surveillance satellites of the film-return type. Jane's de- scribes United States SAMOS and U.S.S.R. COSMOS surveillance satellites in general terms, but official information relative to the remote-sensing capabilities of such satellites is not available. The "Aeronautics and Space Re- port of the President for 1970" (1971) tabu- lates basic data on all United States space flights. The semiannual publication "Rockets and Satellites Catalogue of Data" by the World Data Center A ( 1971) tabulates worldwide data on space flights. Surveillance space flights would be tabulated in these publications. Although no unmanned platform designed for film return is specially described, figure 42 illus- trates the configuration that such a vehicle might take. General Electric Company· ( 1967b) and Lockheed Missiles and Space Co. ( 1967 and 1969) have both described film-return missions. The U.S. Department of the Interior proposal ( 1970) of such a mission to NASA did not designate the vehicle but did include the follow- ing specifications: Target date: 1972. Orbit: 232 km altitude, circular, 83° inclina- 1 Designation of NASA programs that exemplify the mode. tion to Equator, sun synchronous, 10:00 a.m. local time for photographic limb. Design life: 30 to 40 days. Cameras: 2 metric, 305-mm focal length with 240-mm (9.5-in.) film width. A pair of stellar cameras are included for attitude determination, and the entire payload weight would be in the order of 1,000 kg. NASA has sponsored the design of a laser altimeter for such a satellite, and Doppler transponders which would provide precise location are cur- rently used on navigation satellites such as the Navy's Tranet. On film-return flights, exposed film is stored on board, and at a prescribed interval or at the end of the mission a reentry vehicle, with the film, is detached on a signaled command. A pro- pulsion unit reduces the velocity of the reentry vehicle so that it reenters the atmosphere, where a parachute further reduces its rate of fall. Aircraft with grappling lines snag the chute lines, and the reentry vehicle is brought on board the aircr~Jt. NASA has demonstrated this procedure by recovery of its biosatellite (TRW, 1969). ThE recovery operation is illus- trated in figure 43. For such remote-sensing missions, onboard data recording to correlate images with precise time, and thus position, is essential. Attitude controls and determination are also needed. The cameras must be command controlled, with flexibility such as exposure variations being highly desirable. Since film will fog due to radi- ation, it must be shielded. Indications are that the practical lifetime is something less than a year. Obviously the film-return mode involves high cost since cameras are not recovered from their limited-life mission, as now defined, and the film recovery operation requires elaborate logistical support. ELECTRONIC DATA TRANSMISSION, GENERALLY SUN SYNCHRONOUS This mode includes NASA and NOAA mete- orological satellites and also the first Earth Resources Technology Satellite (ER.TS-1), which was launched July 23, 1972. The relation- ship of experimental to operational meteoro- logical satellites is shown in figures 68, 69. Such 20 spacecraft, except for the early TIROS (1-8), are launched into near-polar circular orbits, which retain a fixed relationship with respect to the Sun-Earth axis. At any given position on the orbit, local Sun time will be the same, thus creating the sun-synchronous condition. As the Sun latitude changes with the seasons the Sun elevation on any given earth surface scene will also change, but the Sun longitude (time) will be basically fixed whenever the spacecraft passes over the same scene. Altitudes of such orbits are in the order to 300 to 1,500 km. Since the flight is long lived, the orbit must be completely above atmospheric drag effects. The field of view of the sensors and the density and capability of the ground sta- tions to which all data must be transmitted fur- ther define the orbit. Meteorological spacecraft, in near polar orbit, can effectively sense the entire Earth every 24 hours during daylight and every 12 hours when equipped with both day and night sensors. In a high-resolution sys- tem, such as ERTS, the field of view of the sensor is small and even at its 920-km altitude it will take 18 days to completely image the Earth during daylight hours, except for the poles, which are riot covered due to the orbital inclination. To date, 5 Nimbus, 10 TIROS, 9 TOS, 1 ITOS, and 1 ERTS have been success- fully orbited. These are described in chrono- logical order, and TOS and ITOS are described as operational (NOAA) satellites. Nimbus.-The Nimbus program, started in 1958 and still continuing, was conceived as a test bed for spaceborne remote-sensing instru- mentation. Six launches have been made; five were successful. Although primarily meteoro- logical, Nimbus has been utilized successfully to test application of instrumentation on a broad scale of Earth-observing usage, and a further launch is scheduled for 1974 (fig. 44). Description (Nimbus D) : Size (overall in-orbit dimensions) 2.9 m (9.5 ft) high, 2.9 m (9.5 ft) wide, sensor ring 1.47 m (4.82 ft) x 0.33 m (1.08 ft). Weight: 570 kg (1,260 lb). Typical payload: Infrared interferome- ter spectrometer, temperature/humidi- ty infrared radiometer, image-dis- sector camera system, filter wedge spectrometer, selective chopper radi- ometer, satellite infrared spectrome- ter, backscatter ultraviolet spectrome- ter, cloud-top altitude radiometer, monitor of ultraviolet solar energy, and interrogation, recording, and loca- tion system. Payload capacity: 135 kg (300 lb) weight; 100 W available power, 1 m 3 ( 35 ft 3 ) volume. Stabilization: Three-axis active control utilizing flywheels, rate gyros,1horizon scanners, and reaction jets ; to +1 o all axes. Active control is augmented by gravity gradient torquing. N aminal orbit: Circular, high noon, sun synchronous, 80° retrograde, 1,110 km altitude, 107-min period. Design life: 1 yr. On board storage: High data-rate stor- age on two 5-channel tape recorders. Transmission: S-band communication, narrow bands ; PCM 4 kbps real time, 128 kbps playback. Launch vehicle: Thorad/ Agena-D TIROS.-Ten spacecraft in the TIROS series of satellites (fig. 45), launched between 1960 and 1965, were the precursors of the TOS (TIROS Operational Satellite) system. The spacecraft were utilized to test instrumentation and operational system concepts. TIROS design proved successful and formed the basis for the TOS configuration, which was used until re- placed by the ITOS system. TIROS satellites returned 650,000 TV images during the pro- gram, which ended during 1973. 21 Description: Size: Pillbox shape, 18-sided polygon, 56 em (22 in.) high, 107 em (42 in.) diameter. Weight: In orbit, 122-152 kg (270-338 lb). Typical payload (not all instruments were carried on all flights) : Narrow- angle TV, 5-channel scanning radi- ometer, 40° Hanel radiometer, Univer- sity of Wisconsin bolometers, and au- tomatic picture transmission (APT) system. Payload capacity: 6.9 dm 3 (11.9 ft 3 ) volume, 75 W total spacecraft power. Stabilization: TIROS 1-8 spin stabiliza- tion maintained by solid fueled spin- up rockets. Mechanical nutation damp- er provided Vt o damping. TIROS 9- 10 had liquid dampers providing damping to < 0.1 o. Horizon infrared sensors provided input for attitude control maintained to ± 1 o in pitch, roll, and yaw. Orbit: TIROS 1-8 were programed for 740-1,300 km altitude, circular ovbits with a 48°-60° inclination. TIROS 9- 10 were programed for sun-synchron- ous polar circular orbits. Design life: 3 mo. Onboard storage: Video tape recorders capable of storing 32 images/ orbit. Transmission : PCM telemetry system ; data rate for TV, 0.5 Mbps, for APT, 2 kbps. Launch vehicle: Thor Delta. Earth Resources Technology Satellite (ERTS) .-The Earth Resources Technology Satellite (ERTS) is designed to demonstrate the feasibility of mapping and monitoring Earth surface features from space. It therefore carries multispectral imaging systems as well as auxiliary equipment to collect information needed to utilize data returned from these in- struments. At least two spacecraft are planned in this series with the first one being launched into a near-polar circular orbit on July 23, 1972. During its first year of operation, performance of the ERTS spacecraft was literally flawless. Although the vidicons were used for only a relatively short period, the multispectral scan- ner was in continuous use, and the ERTS sys- tem in 1 year accomplished the following: (1) complete cloud-free coverage of the United States; (2) cloud-free coverage of a sizable percentage of the remaining land surface, polar, and coastal areas of the Earth ; and ( 3) repeti- tive coverage, based on the 18-day coverage cycle of ERTS, which shows significant tem- poral changes in the United States and other land areas of the world. Due to the spacecraft stability, scanner per- formance and system corrections made by NASA in processing the initial coverage, the imagery had the following characteristics: (1) multispectral ( 4-band) response with high radiometric fidelity, (2) high geometric fidelity in the form of a defined continuous map proj ec- tion thus providing the potential for an auto- mated mapping system, and (3) spatial fre- quency (resolution) based on picture elements of 80-m side -dimension at ground scale (figs. 46, 47). 22 Description (ERTS-1) : Size: 3 m (10 ft) high x 4 m (13 ft) wide with solar panels extended; sen- sor ring, 1.5 m (5 ft) x 0.3 m (1ft). Weight: 815 kg (1,800 lb). Payload: Three return-beam vidicon cameras, 2 data-colleetion system re- ceivers, 1 multispectral scanner ( 4 bands), 2 video tape recorders. By late 1973, the ERTS-1 return-beam vidi- con cameras had had very limited use, and tape recording was only a fraction of original capability. Payload capacity: 240 kg (530 lb) weight, between 350 and 400 W avail- able power, and 1m 3 (35 ft3 ) volume. Stabilization: Three-axis active control utilizing flywheels, rate gyros, horizon scanners, and reaction jets ; to + 0.4 o in pitch and roll and ±0.6° in yaw. Nominal orbit: Near circular, sun syn- chronous with local sun time of 0942 at descending mode, 99° inclination, 900 to 950 km altitude. This orbit gives full Earth coverage except for polar regions every 18 days. Design life: 1 yr, but ERTS-1 may be operational for more than 2 yrs. On board storage: Two wide-band video tape recorders each having 30-min capacity at 15 Mbps PCM or 3.5 MHz video recording rate. Command stor- age for 30 commands. Transmission: S-band communication, wide band (vide