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COUPLED
OPTICALLY
ALTITUDE ENCODER
DIGITAL
AVIATION ALTIMETERS
FOR GENERAL
R.
Bryant
Floyd
Center
Lanyley Research
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DECEMBER 1975
SPACE ADMINISTRATION WASHINGTON, D. C.
AERONAUTICS AND NATIONAL I NM LIBRARY KAFB, TECH
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D133130 1. Report Government No. 2. Accession No. 3. Recipient’s Catalog No.
NASA TN D-8118 4. Title and Subtitle Report 5. Date December OPTICALLY DIGITAL 1975_________ COUPLED ALTITUDE ENCODER 6- Organization Code wwmn" FOR GENERAL AVIATION ALTIMETERS 7. Author(s) 8. Performing Organization Report No.
Floyd R. Bryant L-10556 10. Work Unit No.
9. Performing Organization Name and Address 513-54-02-03 NASA Langley Research Center 11. Contract Grant No.
Hampton, Va. 23665 13. Type of Report and Period Covered 12. Sponsoring Agency Name and Address Technical Note National Aeronautics and Space Administration 14. Sponsoring Agency Code D.C. 20546 Washington, 15. Supplementary Notes 16. Abstract An coupled altitude encoder which optically pressure can be incorporated into commer- cially available inexpensive aviation general altimeters has been successfully developed. The of pressure altitude is in 100-ft encoding accomplished (30.48-m) increments from -1000 to 20 000 ft (-304.8 to 6096 The prototype encoders m). were retrofitted into two different in- ternal altimeter configurations. A prototype encoder was checked for accuracy of transition points and environmental effects. Each altimeter configuration, with the encoder incorporated, was laboratory tested for performance and was subsequently flight-tested over the specified altitude range.
With few the assembled exceptions, altimeter-encoder met aeronautical standards for altimeters and encoders. Design changes are suggested to improve performance to meet re- quired standards consistently.
17. Key Words (Suggested by Author(s)) 18. Distribution Statement Encoder-altimeter Unclassified Unlimited Air traffic control Subject Category 19. Security Classif. (of this report) 20. Security Classif. (of this 21. of Price* page) No. Pages 22.
Unclassified Unclassified 27 $3.75 For sale by the National Technical Information Service, Springfield, Virginia 22161 &.
COUPLED DIGITAL ALTITUDE ENCODER OPTICALLY FOR GENERAL AVIATION ALTIMETERS Floyd R. Bryant Langley Research Center SUMMARY be into com- An coupled pressure altitude encoder which can incorporated optically devel- aviation altimeters has been successfully mercially available inexpensive general is in 100-ft (30.48-m) increments The encoding of pressure altitude accomplished oped.
encoders were retrofitted into from -1000 to 20 000 ft (-304.8 to 6096 m). The prototype A encoder was checked for different internal altimeter configurations. prototype two Each altimeter of transition points and environmental effects. configuration, accuracy was subse- incorporated, was laboratory tested for performance and with the encoder over the specified altitude range.
quently flight-tested met aeronautical standards With few the assembled altimeter-encoder exceptions, to to altimeters and encoders. Design changes are suggested improve performance for standards meet required consistently.
INTRODUCTION 100 air terminals and 20 Federal Aviation Administration (FAA) More than major centers are to interrogate and track aircraft enroute air traffic control being equipped altitude capability. Most automatically by using radar beacon transponders with reporting and controlled aircraft density controlled high density corridors, using high terminals, codes) 12 500 ft (3810 m) must have the new improved transponders (4096 airspace above current altitude not to the but also to report automatically equipped only identify aircraft, C of increments (ref. 1). This process is known as mode operation in 100-ft (30.48-m) the altitude information be digi- the Mode C operation requires that analog transponder.
for to the station. Out of the tized and furnished to the transponder transmittal ground 000 aviation aircraft in approximately per- 133 general registered 1973, approximately C percent had mode altitude report- were transponder equipped and approximately 1.5 cent (ref. 2).
ing capability are shown in 1. The The components of an altitude reporting system figure primary The pressure altimeter senses the outside pressure through the static-pressure source.
of is an evacuated pressure capsule (bellows) which expands with in- sensor the altimeter pressure) and which drives the pilot’s indicator with mechan- creasing altitude (decreasing mechanical motion into the ical The encoder converts this International Civil linkages.
special digital code used for altitude reporting. The ICAO Aviation Organization (ICAO) from -1000 to 20 000 ft (-304.8 to 6096 nine bits: code for encoding altitude m) required bits of code and three bits of cyclic binary code. The six Gray-code bits encode six Gray The three bits of are the altitude in 500-ft (152.4-m) increments. cyclic binary code information into 100-ft added to resolve the altitude (30.48-m) increments. This coded information is available to the transponder at all times. When interrogated by the ground the altitude information the encoder to the the transponder transmits supplied by radar, relative and ground display. Identification number, altitude, position, airspeed are shown on the radar Position symbol gives distance and azimuth thus pro- display. information, information to the air traffic controllers.
viding three-dimensional space 000 aviation aircraft are with Since the approximately 133 general equipped inexpen- the Research Center effort was directed toward the sive nonencoding Langley altimeters, of development of techniques and devices having the potential low cost. These low-cost be incorporated into altimeters to provide automatic altitude re- encoders could existing of capability. Investigations of several possible methods encoding altitude using porting that of aviation altimeters led to the conclusion the simplest and existing types general least expensive method was to encode the motion of the altimeter rocking perhaps the a shaft encoder. Of the various types of digital shaft the shaft using digital encoders, Contact out optically coupled type was selected. type encoders were ruled because they to wear and of the disk segments. Brush contacts on the segmented are subject bridging on the altimeter shaft encoders are disk cause prohibitive drag mechanism. Magnetic limited in resolution and require an expensive and complicated readout system. Capaci- several are and have tive shaft encoders require logic processing circuits, bulky, complex relating components. Optical encoders are not subject to wear problems inher- mechnical have accuracy and and can operate effi- ent in contact devices, extremely high resolution, an ciently at high speed and duty cycle. Readout is accomplished by array of carefully transducers facing an altitude encoding disk having transparent and alined photoelectric A source behind the disk provides As the disk opaque segments. light array excitation.
on rotates in response to the input opaque areas the disk pass between the variable, the and the and the transducer output in source sensors, interrupting light beam, modulating and a function of the accordance with the selected code as input variable. Solid-state opti- cal encoders are best applied where extended service accuracy, and resolution are life, for mandatory, thus making optical encoders an appropriate choice altitude encoders. The coupled encoding system was selected primarily as a means for load optically minimizing on Unlike the more of altimeters effects the altimeter mechanism. expensive types used in commercial the aviation altimeters are not servo driven. Direct aircraft, general
r
of an encoder to gearing the altimeter mechanism has the disadvantage of increased load- of ing the driving aneroid capsule, especially at the higher altitudes where the pressure change per change in altitude is less.
Two differently both used in aviation configured altimeters, widely general aircraft, were selected to demonstrate that the optical encoder could be One retrofitted. configu- ration was chosen for those altimeters requiring a more complex encoder attachment where direct coupling was impossible because of space limitations. The second configu- ration allowed the optical encoder to be mounted directly to the altimeter rocking shaft.
ENCODER DESIGN The digital altitude encoder was to meet the designed minimum performance stan- dards for automatic pressure altitude code reporting generating equipment, (ref. 3). In addition to the minimum other performance standards, requirements were placed upon the design. A of these and summary standards requirements is given in the appendix.
A the selected diagram illustrating altitude encoding technique is shown in figure 2.
A view of a unit is shown in prototype figure 3. As shown in figure the encoder con- 3, sists of four components: a GaAs source a light array, slotted mask, a silicon photode- tector array (parts of the detector and an head), encoding disk. The encoding to be disk, attached to the of rocking shaft a general aviation must encode altitude in 100-ft altimeter, (30.48-m) increments from -1000 to 20 000 ft (-304.8 to 6096 m) or 210 increments in approximately 26 "otation of the shaft. Only the coded segment of the disk is used be- cause of space The width of the limitations. smallest code which determined the slit, 100-ft (30.48-m) resolutions, measures approximately 0.00520 in. (0.132 mm) with 0.00347-in. (0.09-mm) spacing. To minimize optical interference between tracks on the code disk, the various tracks were offset from one another. A total of nine tracks was re- for of quired the range the encoder. The disk consists of 0.010-in. (0.254-mm) thick with a thick iron oxide beryllium copper 0.001-in. (0.0254-mm) deposit on each side. The thickness of the such that beryllium copper was a sharp, accurately coded slit of the width necessary to obtain the resolution difficult to required was fabricate. slits Therefore, were etched through the beryllium copper then a 0.001-in. (0.0254-mm) iron oxide first, coating was deposited on each side of the disk. A second process then produced etching sharply defined slits in the thin oxide layer. The sources consisted of an of light array gallium arsenide solid-state light offset to match the offset in the encoder sources, tracks.
The photodetectors were solid-state detectors positioned the The opposite light sources.
solid-state light sources and silicon detectors were chosen because of their long life, small low power and shock and vibration size, requirements, high resistance. They are readily and available, inexpensive, spectrally compatible. The relative spectral charac- teristics are shown in 4.
figure disk was etched to form a very narrow mask of the same material as the The slotted in, mm) to measured approximately 0.0013 (0.033 The width of the beam light beam.
into the disk.
designed encoding maintain the resolution TO ALTIMETERS RETROFIT OF ENCODER MECHANICAL aviation altimeters the encoders and the two different general The interface between the electrical allowing motion between for the detector head, wiring, required brackets extension of the altimeter parts for mounting the encoder disk, interframe and outer case, for the to the transponder. During the of a connector routing wiring and mounting case, of mercury (95 to 105 kPa) barometric pressure from 28.1 to 31.0 in.
of the adjustment Because the encoder 900 within the altimeter case.
rotated approximately the altimeter of altim- it was mounted on the rotating part the of this adjustment, was to be independent a reel was between the fixed and rotating parts, electrical continuity eters. To maintain the ribbon cable.
used to wind up A were more complex than for the first configuration The interface requirements of limitations in the rocking-arm- because space those for the second configuration B, back of and in line with fabricated to mount an auxiliary shaft A bracket was shaft area.
on to mount the encoder detector head the shaft. Provisions were made the rocking arm were mounted on disk segment and an antibacklash spring same bracket. The encoding hub low friction The disk segment and which was set in jewel pivots.
the auxiliary shaft on the hub. The auxiliary means of three adjustable balance screws were balanced by to arm shaft a pin-and-yoke system attached the rocking was driven directly through shaft without the case is shown in figure 5.
A view of the completed assembly of the altimeter.
shown in figure 6.
The interfacing parts are for altimeter B were much less complex interface configuration The requirements disk was The configuration was such that the encoder segment than for configuration A.
of the altimeter means of a split hub (See fig. 7).
to the rocking shaft by attached directly ribbon and wiring coil of wire, The encoder detector-head mounting bracket, bracket, are in figure 7.
windup reel shown AND RESULTS TEST PROGRAMS Performance Tests Laboratory of transition a pre- encoder was tested for accuracy the points by using A prototype of +/-0.0170. All a resolution of 0.010 and a certified accuracy dividing head with cision AS 8003 the +/-75-ft (+/-22.86-m) tolerance specified in (ref. 3).
fell within transition points of the allowed tolerance. These was such as to take most the deviation spread However, to more tolerance in the process of should be small in order allow retrofitting deviations A of all the transition points showed the encoder to an altimeter. careful investigation accomplished and that the largest that the artwork and the etching process were accurately misalinement of the detector-head com- deviations were caused by the relative geometric More alinement of these (light source and photodetectors). precise ponents source, mask, +/-50 components would bring the deviations to within ft (+/-15.24 m). The encoder operated at at the specified voltages; the power consumed when it was operated 15 satisfactorily volts was 0.75 watts.
tests were conducted on one each of the two different configurations of assem- Other bled encoder-altimeters from -1000 to 20 000 ft (-304.8 to 6096 m). These tests included measurements of scale hysteresis, friction errors, position effects, barometric error, scale error, and correspondence between altimeter dial reading and encoder digital dis- Minimum performance standards for pressure-operated altimeters are given in play.
reference Minimum performance standards for automatic reporting code generating 4.
are in reference 3. The test apparatus used in conducting these tests equipment given pressure monitor with a resolution of 0.0001 in. of mercury was a Kollsman precision of of (3.4 Pa). The pressure-sensing ele- (0.34 Pa) and an accuracy 0.001 in. mercury aneroid capsule which oscillates at its ment of the monitor is an electron-beam-welded The is evacuated and the natural frequency in the air gap of a magnetic circuit. capsule on the capsule re- outside is exposed to the test pressure. The pressure change acting The sults in a change of its natural frequency; this frequency is the output signal. pres- Kollsman controller with a resolution exceeding 0.001 in.
sure is controlled by a precision Vernier controls connected to a set of precision valves permit the of mercury (3.4 Pa).
of the controller. Self-regulating valves detect any change in the sensitive adjustment and maintain the pressure at the set value.
output through pressure-sensing diaphragms of the tests on this are Altimeter-encoder, configuration A.- Results configuration given in tables I and II.
from 5 to 40 ft (1.52 to (a) Friction errors were out of tolerance in some instances 12.2 the larger errors occurring at the higher altitudes. These errors, especially m), with for a at the altitudes where a smaller change in pressure obtained given altitude higher on the altimeter the aux- change, were probably caused by friction imposed mechanism by shaft on which the code disk segment is attached. In this configuration (as discussed iliary a the auxiliary shaft mounted in jewel bearings was driven through connecting pin before), and yoke by the rocking shaft which is attached to the aneroid capsule through mechanical the code disk linkages. The lack of space of this configuration prevented mounting directly on the rocking shaft.
10 ft at -1000 ft (-304.8 m) and at error was out of tolerance by (3.05 m) (b) Scale of a result of a small error in the zero adjustment the 0 ft (0 m). These errors are of the zero and can be corrected by readjustments position.
altimeter +/-75 ft was within the allowed tolerance of (c) As shown in table I, hysteresis (+/-22.86 m).
correct for altitudes that were checked.
Encoder readings were the (d) at and 270 at sea level conditions.
Position effects were checked 0, 90, 180, (e) at When the instrument was tapped of 100 ft (30.48 m) was noted 180.
An encoder error its zero transition it could be observed that the encoder was near point; lightly, it could was +/-75 ft the allowed error in the transition points have been within tolerance since increment) was 100 ft (30.48 m). When and the resolution of the encoder (one (+/-22.86 m), no position effects to a point half way between transition points, the altimeter was adjusted +/-20 ft on the altimeter indication were within the specified Position effects were obtained.
(+/-6.096 m).
table barometric scale (from ref. 4) are given in n.
The results of the adjustment (f) +/-25 were within the specified ft (+/-7.62 m). The en- the altimeter indications As shown, four of the baroset knob.
or 100 ft (30.48 m), at settings coder was off one increment, head of the encoder the the detector Since the altimeter was within specified tolerance, to coded disk segment. The ribbon cable been displaced with reference the must have this could have had to be rotated within the case during adjustment allowing the altimeter bracket to a point where the distortion to distort the detector-head mounting enough drag attached to the detector-head mount- this The cable is directly could cause displacement.
could also of the fixed and sections of the cable windup reel bracket. Binding rotating ing is also attached to the detector mounting since the rotating section cause the displacement to the case. a more flexible cable, adjusting bracket and the fixed section Using smaller, or the bracket the two sections of the windup reel, making mounting the clearance between more would correct this problem.
rigid altimeter dial indication and the defined as the difference in the (g) Correspondence, over the range of the encoder, output was checked at each transition point encoder digital of the 420 data points to 6096 m). Approximately 1.7 percent -1000 to 20 000 ft (-304.8 +/-125 ft a correspondence tolerance of (+/-38.1 m) by small outside the allowable were a nonlinearity with result- to 25 ft (1.52 to 7.62 m). The data indicated altitude, amount, 50 ft (15.24 m) over the transition points of approximately in an additional spread in ing of the individual code tracks showed that all tracks were of the encoder. A check that to a same amount. The nonlinearity is attributed slight displace- nonlinear by about the of axis from the axis. A displacement 0.008 in.
of the auxiliary shaft rocking-shaft ment obtained over the range of for the magnitude of the nonlinearity (0.2 mm) would account for alinement of the auxiliary shaft axis would correct the the encoder. More precise data and would bring all correspondence points within specified limits.
nonlinearity of on altimeter-encoder con- configuration B.- Results the test Altimeter-encoder, B are in table III.
figuration given pressure).
Frictional errors were out of tolerance at the higher altitudes (lower (a) on be- These errors were probably caused by additional friction the rocking-shaft pivots of cause of the added of the encoder hub and disk assembly. The use higher quality weight arm could and and proper adjustment of the rocking shaft help jewels pivots for mounting minimize this problem.
a two Scale factor errors were outside the allowed tolerance by small amount at (b) at -1000-ft altitude and 20 ft (6.1 m) at a 500-ft altitudes: 10 ft (3.048 m) a (-304.8-m) of the altimeter zero would eliminate this offset.
(152.4-m) altitude. Readjustment (c) Hysteresis was within specification.
Encoder were correct at the altitudes checked.
(d) readings normal and at and (e) Position effects were checked in operating position 90, 180, +/-20 was observed. Altimeter was within the ft 270. No change in encoder reading in reference 4.
(+/-6.096 m) tolerance specified in was obtained while varying the barometric scale (f) No change encoder reading to from 28.1 to 31.0 in. Hg (95 105 kPa).
in the altimeter dial indi- (g) Correspondence, previously defined as the difference at transition point over the cation and the encoder digital output, was checked each range attached of the -1000 to 20 000 ft (-304.8 to 6096 m). Since the encoder was encoder, the to the shaft of the altimeter in this configuration, nonlinearity experi- directly rocking in A was not present. Five of the 420 data points were outside the ence configuration to be- allowable limits of +/-125 ft (+/-38.1 m) by a small 5 to 45 ft (1.52 13.716 m), amount, to the altimeter scale factor was off zero at zero altitude. As discussed earlier, cause correct the scale factor of the altimeter requires a shift of the altimeter zero adjustment This correction would also bring the correspondence data within in the positive direction.
of +/-125 ft m).
the allowable tolerance (+/-38.1 Tests Qualification to the Encoder-altimeter configuration A was environmentally tested specifications ex- of the altimeter (ref. 4). These tests included high- and low-temperature operation, and vibration. Because of the simplicity of con- treme temperature exposure, humidity, it was tested for vibration effects only.
figuration B, C was tested at -30 operation.- The altimeter-encoder and low-temperature High- for 3 hours be- maintained at each of these temperatures assembly was and 500 C. The C the altimeter to be within specified per- Calibration at 50 showed fore calibration.
transition in were noted in the encoder points No significant effects formance standards.
room of 20 C.
to those at a temperature points comparison within and f-riction of the altimeter were C showed that hysteresis Calibration at -30 at the out of tolerance by 10 ft (3.048 m) the scale factor was However, specifications.
20 000-ft at 0 and 40 ft (12.192 m) at the 20 ft (6.096 m) level, 500-ft (152-m) level, level.
(6096-m) varied from 15 to to transition point of the encoder from transition point The span with the exception of one point from room temperature readings ft (4.57 to 7.62 m) error of 15 to 25 ft (4.57 to 7.62 m) 40 ft (12.19 m). The measured differed by which at room of the transition-point readings same order as the repeatability is of the temperature.
assembly was exposed to The altimeter-encoder Extreme temperature operation.- hours Three hours C and 700 C for periods of 24 each.
temperatures of -65 ambient no adverse effects on the altim- been completed, calibrations showed after exposure had or encoder.
eter normal in a humidity chamber in its The instrument was mounted Humidity test.- connected to the static- 10-ft coil of copper tubing with a (3.048-m) operating position was positioned so that moisture to simulate installation conditions. Tubing pressure port at a of 70 C and The chamber was maintained temperature drain out the open end.
would for a of 6 hours. The heat was then shut off and of 95 percent period a relative humidity of 18 hours in this atmosphere. During allowed to cool for a period the instrument was temperature decreased to 38 C. Cali- rose to 100 percent as the the humidity this time, calibration made at room temperature and compared with then made at room bration was and no effects The altimeter was within specification before humidity tests.
temperature noted on the encoder.
were were one of each configuration, The assembled altimeter-encoders, Vibration tests.- from 5 to 500 Hz.
A resonant survey was made for vibration effects. frequency tested of to 500 of 5 to 50 Hz was 1.5 g; in the range 50 Hz, acceleration in the range Maximum A was noted at 50 Hz for each config- was 0.5 g. slight resonance maximum acceleration The altimeters vibrated at 50 Hz in three axes for 2 hours each.
Both units were uration.
In configuration B, where the encoder recalibrated and were within specifications.
were of the encoder showed no sig- directly to the rocking shaft the altimeter, disk was attached attached to an where the encoder disk was auxiliary nificant effects. In configuration A, the rocking shaft with a mechanical link- line with the rocking shaft and driven by shaft in in the direction. It had deviated approximately 40 ft (12.19 m) negative the encoder age, the drive shaft to the altimeter was found that the clamping device connecting auxiliary device was and the rocking shaft was not properly tightened. The clamping tightened to 30 hours of aircraft structure vibration the flight tests.
assembly was subjected during Tests Flight mounted in a small twin- One each of the two altimeter-encoder configurations was to aircraft and was flight-tested. The tests were conducted determine the perform- engine ance of the The performance was evaluated through measurements of the cor- encoders.
at various altitudes.
respondence in 8. The pressure source Pg of the altim- The flight-test setup is shown figure a connected to the copilot’s static-pressure line. The altitude (ICAO) code eter was line to coded decimal (BCD) and displayed on the output of the encoder was converted binary the observer read and recorded the two altimeter dial indi- digital light panel. As shown, encoder outputs. Figure 9 shows the flight package cations and their corresponding digital as it was installed in the aircraft for the flight tests.
0 to 20 000 ft to 6096 correspondence was measured at On one flight from (0 m), The six most bits 1100-ft (335.28-m) levels during both ascent and descent. significant of the code once for 500-ft of the code are in the Gray code and the position changes every 100-ft increment. The three least significant bits encode the altitude in (152.4-m) rather than 500- or 1000-ft (30.48-m) increments. The 1100-ft (335.28-m) increments, to utilize all nine bits of the Four or 304.8-m) were chosen encoder.
(152.4- increments, 0 ft to 2743 Ten to data other data were made from to 9000 (0 m). thirty points flights to 30 hours of time were taken per flight. The altimeter-encoders were subjected flight over a 27-day period.
spread the 0- to 20 000-ft (0- to 6096-m) Plots of the correspondence data obtained during are shown in figures 10 and 11. Some data points were outside the allowable toler- flight of +/-125 ft in both ance (+/-38.1 m) configurations.
A data and transition-point data taken after the flight tests Configuration flight from that obtained before the tests. Exam- showed a shift in the encoder zero point flight a gradual shift over the period during ination of all the flight data indicated that there was of the altimeter before and after which the flights were made. A scale factor calibration in it was concluded that a rela- the flights showed no zero shift the altimeter. Therefore, of detector head and encoder disk had taken place; this displacement tive displacement the A more means of to that obtained during laboratory vibration tests. positive was similar this shift in en- the clamp on the altimeter rocking shaft would prevent gradual securing coder zero caused by vibration.
data taken after the tests also B data and transition-point flight Configuration flight A shift in the encoder from that point obtained before the flight tests.
indicated a zero and after the calibration of the altimeter scale factor before flights indicated an altimeter zero shift of the same magnitude and direction as with the encoder. The unit was vibrated as reference 4 after the flight no further zero shift was to the specifications given in test; it that a shock installation or vibration obtained. Therefore, was concluded during during may have introduced the zero shift. The instruments were mounted on flight testing attached to the airframe and possibly encountered vibrations greater brackets directly would than those received during laboratory testing. Rezeroing the altimeter bring the data within the allowable limits.
flight CONCLUDING REMARKS coupled pressure altitude encoders were designed, and retro- Optically fabricated, aviation altimeters for the indication of the fitted to existing general encoding analog altimeters into the international code used in automatic altitude reporting. The encoders for from -1000 to 20 000 ft (-304.8 to 6096 m).
were designed operation and tests were conducted on two of the prototype encoders installed Laboratory flight aviation With few the in two differently configured general altimeters. exceptions, proto- type encoders installed in the altimeters operated satisfactorily and within specified toler- Comments con- Most of the problems occurred during the retrofitting process.
ances.
in the altimeter-encoder combinations so that production units cerning improvements standards follow: could meet required in should be fabricated in one for 1. Mounting brackets (as configuration A) piece greater rigidity.
of axis with 2. Care must be exercised in the alinement the auxiliary shaft the axis of the altimeter (as in configuration A) to minimize nonlinearity rocking-arm-shaft in the encoder output.
A means of the clamp on the rocking shaft (as in configuration 3. positive securing to a in encoder zero A) should be provided prevent shift during vibrations.
used for the 4. A separate mounting bracket should be mounting wiring connector in The wiring connector board is now attached directly to the board (as configuration B).
ribbon and a of the reel are attached to encoder detector head. The cable part windup this board and during adjustment of the baroset setting could put a strain on the connector a strain would result in a displacement of the detector head relative to the en- board; such coder disk.
alinement of the detector-head parts would lessen the 5. More accurate geometric spread in the transition points and would permit greater tolerance in the retrofitting of the to in of the two encoders the altimeters each configurations.
6. A means of externally zeroing the encoders should be provided in each of the altimeter configurations.
7. Care should be exercised in jewels and pivots to minimize frictional selecting effects.
Langley Research Center National Aeronautics and Space Administration 23665 Hampton, Va.
December 1975 12, APPENDIX SPECIFICATIONS ENCODER for the encoder are as follows: The specifications with the minimum performance standard (from ref. 3) sum- The encoder must comply (1) marized below.
Civil Aviation (ICAO) altitude transmission Code: International Organization (a) code increments Resolution: 100-ft (30.48-m) (b) Accuracy: (c) +/-125 (correspondence between the encoder out- Correspondence: ft (+/-38.1 m) to maintain altitude when referenced put and the altimeter reading used flight kPa)) to 29.921 in. of mercury (101.31 +/-75 a transition from one encoded out- Transition points: ft (+/-22.86 m) (When when to the next encoded output the displayed pressure altitude, put occurs, +/-75 to in. of mercury (101.31 must be within ft referenced 29.921 kPa), nominal altitude for that transition point.)
(+/-22.86 m) of the pressure Zero-foot altitude reference: 29.921 in. of mercury (101.31 kPa) (d) In accordance with Standard Pressure Altitude (e) Encoded output: U.S. Tables, Environmental Same as altimeter of which encoder is a part (f) specifications: (ref. 4) (2) Special requirements.
of optical with the capability of being mechanically (a) The encoder must be design attached to aviation altimeters.
general a for low cost fabrication and retrofitting to (b) The encoder must have potential aviation altimeters.
general a of -1000 to 20 000 ft (-304.8 to 6096 m).
The encoder must have range (c) to +/-1.5 volts must have a operating range of 13.5 dc.
(d) The encoder voltage REFERENCES 1. of Designation Federal Airways, Area Low Routes, Controlled and Airspace, Reporting Points. Federal Register, vol. 38, no. 106, June 4, 1973.
2. Paul Drouilhet, R., Jr.: The Development of the ATC Radar Beacon System: Past, and IEEE Present, Future. Trans. vol.
Communications, Corn-21, no. 5, May 1973, pp.
408-421.
3. Minimum Performance Standard for Automatic Pressure Altitude Code Reporting Gen- AS erating Equipment. Soc. Automot. Eng., 8003, July 1974.
Pressure AS 4. Altimeter, Actuated Sensitive Type. Soc.
392C, Automot. Eng., Feb. 1959.
1-’ ht^.
TABLE I.- RESULTS OF LABORATORY PERFORMANCE TEST ON
ALTIMETER-ENCODER, CONFIGURATION A
[Allowed errors are specified in
reference
4]
tole
(A^ Encoder
Altitude
A^ed
^o"al
error
^
hysteresis reading error error ft m ft m ft m ft m ft m ft m ft m ft m -1 000 -304.8 -70 70 -21.34 21.34 -30 +/-20 -9.14 +/-6.10 30 9.14 75 22.86 -1 000 -304.8 0 0 -70 -21.34 70 21.34 -30 +/-20 -9.14 +/-6.10 50 15.24 75 0 22.86 0 152.4 -80 70 -24.38 21.34 -20 +/-20 -6.10 +/-6.10 40 12.19 75 22.86 500 152.4 1 000 304.8 -90 -27.43 70 21.34 -10 +/-20 -3.05 +/-6.10 40 12.19 75 22.86 1 000 304.8 3 914.4 -75 -22.86 70 21.34 10 3.05 +/-30 +/-9.14 16.76 75 22.86 3 000 914.4 5 -80 1524.0 -24.38 70 21.34 20 6.10 +/-80 +/-24.38 60 18.29 75 22.86 5 000 1524.0 10 000 3048.0 -100 -30.48 80 24.38 20 +/-80 6.10 +/-24.38 60 18.29 75 22.86 10 000 3048.0 15 000 4572.0 -100 -30.48 90 27.43 0 0 +/-105 +/-32.00 15 000 4572.0 20 000 6096.0 -140 100 -42.67 30.48 -10 +/-130 -3.05 +/-39.62 20 000 6096.0 SCALE ERROR ALTIMETER-ENCODER, TABLE II.- BAROMETRIC A CONFIGURATION Altimeter Encoder Correct Tolerance Display indication reading differences ft m ft m kPa ft m ft m in. Hg 0 0 +/-25 +/-7.62 -1730 -527.3 95.5 -1727 -526.4 28.10 0 0 +/-25 +/-7.62 -1335 -406.9 -1340 -408.4 28.50 96.5 +/-25 +/-7.62 -880 -268.2 100 30.48 -863 -263.0 29.00 98.2 +/-25 +/-7.62 -390 -118.9 0 -392 -119.2 29.50 99.9 0 0 -100 -30.48 0 0 0 0 29.92 101.3 +/-25 +/-7.62 110 33.5 -100 -30.48 129 39.3 30.06 101.8 +/-25 0 0 531 161.8 +/-7.62 530 161.5 30.50 103.3 +/-25 +/-7.62 900 274.3 0 893 272.2 30.90 104.6 +/-25 -100 -30.48 974 296.9 +/-7.62 965 294.1 30.99 104.9 s( / 1- Oi TABLE RESULTS ni.- OF LABORATORY PERFORMANCE TESTS ON CONFIGURATION B ALTIMETER-ENCODER, errors
[Allowed are specified in reference
4]
Encoded
Altitude f^Sl
V Hysteresis
F^’ ^
error error hysteresis reading error error
^
ft m ft m ft m ft m ft m ft ft m m ft m -1 000 -304.8 -50 70 -30 +/-20 +/-6.10 -15.24 21.34 -9.14 50 15.24 75 -1 000 22.86 -304.8 0 -40 -12.19 70 -20 +/-20 21.34 -6.10 +/-6.10 10 75 0 0 3.05 22.86 500 152.4 -30 70 +/-20 -9.14 21.34 -40 -12.19 +/-6.10 20 6.10 75 22.86 500 152.4 1 304.8 -50 -15.24 70 +/-20 21.34 -15 -4.57 +/-6.10 15 4.57 75 22.86 1 000 304.8 3 000 914.4 -30 70 +/-30 -9.14 21.34 -10 -3.05 +/-9.14 20 6.10 75 22.86 3 000 914.4 5 000 1524.0 -50 70 -15.24 21.34 -10 -3.05 +/-37 +/-11.28 20 6.10 75 22.86 5 000 1524.0 10 000 3048.0 -90 +/-80 -27.43 80 24.38 -30 -9.14 +/-24.38 20 6.10 75 22.86 10 000 3048.0 15 4572.0 -120 -36.58 90 -70 +/-105 27.43 -21.34 +/-32.00 20 6.10 75 22.86 15 000 4572.0 20 000 +/-130 6096.0 -175 -53.34 100 30.48 -45 -13.72 +/-39.62 20 000 6096.0 L_ A/D (ICAO) ENCODER AIRCRAFT STATIC-PRESSURE PROBE
___________ ANTENNA
OR ORIFICE ^^___^^^ BEACON -\\<
0/<"^^^?
TRANSPONDER
/"n<^
J
^~
^^------
^^^
PILOT’S ALTIMETER
^/
/N/ yf MV GROUND RADAR l^^>~r--Z--,--^,J
t^^
^^-L-L-1-1^^ / \ / \ \ \ ____/_______I____ ~~-------7
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A_\--jL------- AA 194 ARTS III DISPLAY FORMAT
f *
y7 < 023 15
/
/’---^-^
*ARTS III TARGET SYMBOLS INDICATE AIRCRAFT ^________________^~ IDENTIFICATION-ALTITUDE (IN HUNDREDS OF RADAR DISPLAY FEET)-GROUNDSPEED (IN TENS OF KNOTS) PNEUMATIC ELECTRICAL of an automatic altitude system.
Figure 1.- Primary components reporting -’ -~i PHOTODETECTOR-AMPLIFIERS
^^^---
^^
>4^S\-^
PATTERN ETCHED CODE CHEMICALLY
/^>.^^ ’C\ ^^"
l^l/y^ I1" THIN ^TAL
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^^^
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^^
/ \
<^^^0
O^^0^^^
^
SOURCES-1 GALLIUM ARSENIDE LIGHT
\^
N^
\
ALTIIffiTER ROCKING SHAFT of an Figure 2.- Schematic diagram altitude-encoder.
\
w-?
ff
SILICON PHOTO DETECTOR AKRM
^^^^I^^HBp1!
.^V/s^ii^’tw^^.
\
Eilil’
’:;^^<; jtTi’tfiw
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^
MASK / ..l,::,:. ’’^^^^.r.^lAlf;!’^
SLO^rED ^,
("/ l -^S^-
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ENCODING DISK
^^
L-74-7057.1
K Figure Photograph of
3.- altitude-encoder components.
-~~]
7iT\
---^V
---------------------p----.--1------
-^’1 SILICON PHOTOSENSOR
\
----\ 80 ---------------------T----- --____------
/____
\_______________ M /
\
~-~~r
g
---
\___
^ /
-+--
|
------------------^-------------
g
_/___
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\ 50 ------------,^--------------
--------
^
M /
\
----<1---------;---\----’
7^-----------------------.^-
DIOOE--^""^ GALLIUM ARSENIDE
---\------\--
11000 12000 9000 10000 7000 8000 5000 6000 WAVELENGTH, ANGSTROMS arsenide diode.
and gallium of silicon photosensor 4.- Spectral response Figure CABLE WINDUP REEL
I^^H
BALANCCTe ASSEMBLY
M^H
Figure 5.- of Photograph configuration A.
altimeter-encoder,
^^^1
ENCODER ASSEMBLY MOUNTING BRACKET I
1:.:
^
A. I
6.- Photograph of retrofit components for altimeter-encoder, configuration Figure Figure 7. Photograph of altimeter-encoder, configuration B.
H^^l
INS >J&i \ ALTIMETER ENCODER CONFIGURATION A D J I / \ S
--
\
\
J L
L ICAO/BCD i
p ’<^_
^^
OBSERVER G OBSERVER
?
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y
Y^
p A ^-.
A
----------------^
^
L ALTIMETER ENCODER N _.
CONFIGURATION B ____L
~r\ i-~’"~
^-------.
CORRESPONDENCE: (38.1 m) ^125’ OBSERVER Figure 8.- Block diagram of flight-test instrumentation.
;3 ENCODING ALTIMETERS.
^j^
^^^B
of Figure 9.- Photograph altimeter-encoder flight-test assembly.
H^^l
M CTl
ini’IHIJililllii^^^^^^^^^^^pS^^^^^^gE^Si^^^^^
61.0
ZOO^^^^^^^^^.^ilL^^^^^^^^E^^^EE^^^^^^^^^^^^^^^^^
45.7’- 150r--^^^|^^:^^:^^^^^y^^|^^|^ 30.5
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~’-^~---_-"-Tl--^--j^-V-^’"^^--^^" ---j^^^ ..---j-- 15.2- g
Sot-^^^^^^^p^^^^^^^ii^^^^S^^^^^^^^^^^^^^^^^^
| 0- S -15.2^ -30.5- -100
’^^^^^^^^l^l^^^^.^^^^^’^^^^^^^^^l^^^^^^^^^^^^^
-45.7’- -150i^-S^|^^^^^^^^|^^^^^^^^^?^^^^^^^^^^^^^^^^^^^
-200 imil^^
-61.0J- ^^^^^^’^^^^^^^^^^^^E^^^^^^^^^^^^^^^^^
Increasing altitude ^^^^^^^^^^^^^^^^^^^^^^^i^^Ei
^-^^^^
Decreasing altitude :-;;^^^
^^=^^^^^^^^^^^^^^^^^^^^^^^^
0 2 10 12
’T4 1C
18" Tox lO’ft
0 6 l3 1.8 2.4 3.0
3.7 4.3 1^9-----ii1!;-----6^
km Altitude Figure 10.- Correspondence of altimeter reading and encoder output during flight tests, configuration A.
2; [-’ 1.-^^^----- ig, :----r^r-_^==r=^:r_--^r =---^===^:^==:^^;^r--:::::::.^:::---L_:^_--rrrrr;:^---:r^i^r^r:;:::r^;r::::r^;:^:=^^ j-^^^^-,..--.- =====:--:^::::::::::::’:";:: :;:::::::::::::;:: :::;r^::::::::^:-^:^;:.-;;::::::.::^r:^.:^E-=-^-^z^r:^^^:^E=E-E^^r: o cji m ft ^EE^^-^^’’’"’^^-^--^-’"""" oi ^!j;::^:::!::=::;!;:;E^^^^ 6i.o-
2oo^^^^^^^=^^:^:j:;L!^!^j
45.7 ^^^^^^^^p^.^^^;:;;::::: ^:^g^^^!!H^!^::::!:!!:::::::::^^^^^^^^^=^^^^|
., 1.
-50
(3 -15.2 ^^^^^^^^^^^^^^TT|^T;p=^^J^::::::!::^^||^^^^^^^^^^^^p^^
-30.5 -100 ^^^^^^^^?||p|J^?ii|p-|^^=^=|==T^=^|====^=^^^^===- ..^ Increasing altitude :^Zl^r^:=r=^_._=== ^^^:=z==------- -200 ---===;-=--=--=:-=:===::=::==:--====::==:=:=--=, -:::^: Decreasing altitude -61.0[-
^^^^^-
(4^4^^^mm^il!^li.M4^4^Ml^lll]lllllllllllll^lllNllllllllllll^lll^lllli^ ^3
0 ^lllllllllllllllll^lllllllllllimTffl^WI
^
0 U km 1.3 1.8 2.4 3.0 4,3 4.9 5^5 3.7
^
Altitude B.
and encoder output during flight configuration of altimeter reading tests, 11.- Correspondence Figure to -i
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