Final Report: Ground Assessment of Electronic Conspicuity Devices

aircraft are equipped with ADS-B Out, aircraft that typically operate at low altitudes in uncontrolled airspace, such as agricultural aircraft, certain helicopters, and banner towing aircraft, may not be equipped, as the equipment can be costly to install. To address this issue, the ARC envisioned potential use of a transmitting device that is standalone, portable, and would not incur installation costs. A class of broadcast transmitters referred to as Electronic Conspicuity (EC) devices were identified as potential candidates. These EC devices are currently in use in Australia, New Zealand, and the United Kingdom but have not been authorized by the FAA or approved by the Federal Communications Commission (FCC) for operation in the United States. The FAA’s Office of Aviation Safety (AVS) UAS Integration Office, Research, Engineering, and Analysis Division (AUS-300) initiated a research effort to evaluate the potential use of EC technology to report a non-ADS-B-equipped aircraft’s position in low-altitude (below 500 feet) airspace. The FAA’s Airport Technology Research and Development Branch (ATRD) along with the FAA’s Office of NextGen’s (ANG) Engineering Development Services - Surveillance Branch (ANG-C33) (Research Team) were tasked with conducting testing on two models of EC devices to determine the surveillance coverage that may be expected from the use of this type of position reporting device. The Research Team evaluated one in-production EC device that transmits on 1090 megahertz (MHz) and a prototype EC device that transmits on 978 MHz as part of this research effort. The evaluation, which was performed at the FAA’s William J. Hughes Technical Center (WJHTC) and the co-located Atlantic City International Airport (ACY), consisted of ground testing using two EC devices and four GA aircraft: Air Tractor AT-802A, Robinson R44 Raven II Rotorcraft (R44), Piper PA-18 Super Cub, and Cessna 172N. These aircraft are representative of the types of aircraft that are likely to operate at low altitudes. For each aircraft, the Research Team conducted a series of tests to assess three parameters: EC device orientation, azimuth, and range. The orientation test assessed the impact of EC device orientation (vertical, horizontal, and “worst case”) and antenna polarization on transmission strength. The primary purpose of the azimuth test was to assess the impact of the aircraft’s fuselage on transmission strength. This was accomplished by recording the strength of the signal at 45-degree increments around each aircraft at distances of 10-meters and 50 meters from the cockpit. Range tests were conducted as a means of direct measurement of signal attenuation over long distances. Distance measurements were taken at three linear ranges from the aircraft location: 0.5 statute miles (SM), 1 SM, and 2 SM. The results from this testing showed that there are three major compounding factors that will impact the signal strength of a position reporting device when it is operated within an aircraft viii cockpit: 1) Location of the mounting point within the aircraft; 2) Orientation in which the transmitting device is mounted; 3) Aircraft structure and configuration. Any one of these factors will cause a variable degree of attenuation to the signal but, together, can have a multiplicative effect. In addition to these installation impacts, there are also RF impacts that can affect signal propagation. Proximity to metal aircraft structural components can induce distortions, reflections and grounding effects that can constructively or destructively impact the transmitted signal as well, adding to the variability in the transmitted radiation pattern. Analysis of the measured signal strengths captured during this testing revealed a unique radiation pattern for each aircraft that further changed based on the mounting location and orientation, the overall result being inconsistent and unpredictable signal propagation. Variability in signal propagation translates to variability in the distance over which the reported position can be acquired. Measurements showed peaks and nulls in the signal strength at different azimuths which could negatively impact operational track continuity. Testing showed that alignment of the polarization of the transmitting and receiving antennas had a significant impact on the signal strength as well. Even when optimally installed with the transmitting antenna in a vertical orientation, maneuvering of the aircraft during flight will induce misalignment between a transmitting and receiving aircraft. This operational degradation in signal strength will add a dynamic element to the variability and is an additional consideration for aircraft operating at low altitudes that may make more directional changes than higher altitude operations. 1 1. INTRODUCTION At the request of the Federal Aviation Administration’s (FAA) Office of Aviation Safety (AVS) Unmanned Aircraft Systems (UAS) Integration Office, Research, Engineering, and Analysis Division (AUS-300), the Airport Technology Research and Development Branch (ATRD) and Office of NextGen’s (ANG) Engineering Development Services Surveillance Branch (ANG-C33) were tasked with testing portable non-certified position reporting devices. Standard position reporting equipment has one or more antennas mounted in an optimal orientation on the outside of the airframe. This provides the best possible surveillance coverage – both for transmission to, and reception from aircraft and ground stations. In contrast, portable transmitting devices are intended to be mounted inside of an aircraft, in an orientation that is constrained by the layout of the cockpit. To assess the impact to the surveillance coverage due to installation location and orientation, ANG- C33 and ATRD collaborated to perform ground testing of portable devices installed in various aircraft. Testing was conducted at the FAA William J. Hughes Technical Center in Atlantic City, New Jersey between May 28th and June 6th , 2024. Results from these tests are intended to inform potential rulemaking to further enable UAS integration into the National Airspace System (NAS). 1.1 BACKGROUND Safe and effective integration of Unmanned Aircraft Systems (UAS) into the NAS is an active focus of research efforts within the FAA. While the operation of UAS within the NAS have the potential to bring societal and economic benefits, integration must be done in a manner that adheres to regulations that have been put in place to protect the flying public. In June 2021, a UAS Beyond Visual Line of Sight (BVLOS) Aviation Rulemaking Committee (ARC) was convened to provide the FAA with input on how to successfully accomplish the task of low altitude UAS integration into the NAS. The UAS BVLOS ARC presented a final report to the FAA in March 2022. That report included a proposal for operator Right of Way (ROW) in low altitude airspace. Specifically, this proposal called for UAS that are equipped with an FAA approved Detect and Avoid system to have ROW over General Aviation (GA) aircraft that are not equipped with a position reporting capability. However, if a GA aircraft is equipped with an Automatic Dependent Surveillance – Broadcast (ADS-B) Out reporting device, it would retain ROW over any UAS. The FAA’s Office of Aviation Safety (AVS) UAS Integration Office – Research, Engineering and Analysis Division (AUS-300) has undertaken investigations into this proposal. Research was initiated by AUS-300 to evaluate the potential for permitting aircraft operators in low-altitude (below 500 feet) airspace to use portable ADS-B transmitting devices for position reporting. A class of broadcast transmitters referred to as Electronic Conspicuity (EC) devices were identified as potential candidates1. The United Kingdom (UK) has implemented a program called CAP1391, and Australia has implemented an equivalent program called CASA Civil Aviation Order 20.18 Amendment which permits the use of uncertified, portable, low-power, ADS-B transmitting devices for use by pilots as an aid to visual acquisition. These devices have a small form factor, are mounted inside the aircraft cockpit, and are designed to transmit a standard set of position, velocity, and identification information to aid GA pilots to “see and be seen.” 1 Note: Electronic Conspicuity devices are not currently permitted for use within the United States and temporary spectrum approval was received for the testing detailed in this report. 2 However, as the conceptualized use in the United States would be for separation purposes, it is important to more thoroughly assess the surveillance coverage that such non-standard devices might be capable of providing. The FAA’s Airport Technology Research and Development Branch (ATRD) along with the FAA’s Office of NextGen’s (ANG) Engineering Development Services - Surveillance Branch (ANG-C33), herein referred to as the “research team”, were tasked with conducting testing on two models of EC devices to determine the surveillance coverage that may be expected from the use of this type of position reporting device. 1.2 PURPOSE This study was undertaken to aid in the assessment of transmitter coverage that might be expected from a GA aircraft if it were to use an EC device to broadcast its position, velocity, and identification information. The transmitter coverage is used here to describe a horizontal pattern of radio frequency (RF) transmission, centered at the aircraft, and radiating outward. While transmission is only one half of a communication link, characterizing transmitter performance through test and measurement will provide valuable input into understanding the potential surveillance environment. This is important information that is needed to determine the feasibility of any potential concept of operation. In a standard ADS-B installation, the radiating antenna element is positioned unobstructed outside of the aircraft. The use of an ADS-B transmitter like an EC device from within an aircraft is non- standard. Compounding factors such as unknown aircraft type and unknown installation orientation make the modelling of signal transmission performance extremely complex. This testing was designed to gather data on effective transmitted signal strength from these portable devices, from a representative set of aircraft. The aircraft were drawn from a population of GA aircraft that often operate below 500 feet in uncontrolled airspace, which is airspace that is also envisioned for BVLOS UAS operations. Low altitude GA aircraft may not have electrical systems and may not be equipped with an ADS-B transmitter. It is in light of this that EC devices were chosen for consideration. Two EC devices were tested: one 1090 megahertz (MHz) Extended Squitter capable production device and one 978 MHz Universal Access Transceiver (UAT) capable prototype. These are representative of both ADS-B data links in use in the United States. 1.3 OBJECTIVES The primary objectives of this testing were to assess the operational impact to the transmitted signal strength of a portable EC device using the following measures: 1. Measure the impact to signal strength due to operation of the device inside of an aircraft. Operation inside of an aircraft can negatively affect signal transmission due to impacts such as distortion and scattering from proximity to metal airframe structures, and blockage or reflections from the body of the airframe. 2. Measure the impact to signal strength due to changes in installation orientation. The orientation in which the device is mounted inside of an aircraft can vary due to physical constraints of the aircraft cockpit and operator preference. 3 Tests were also conducted to investigate the impact to signal strength based on the operational state of the aircraft engine and the distance between the transmitter and receiver. Notes: 1. This study focused on characterizing attenuation of the transmitted signal strength. Factors such as distortion and reflections can also impact the quality of a transmitted signal, further degrading the ability to successfully receive the signal. Effects due to these types of signal quality impairments were not analyzed in this study. 2. Radiated electromagnetic interference (EMI) tests to investigate the impact upon existing avionics/electronics caused by an EC device operating in an aircraft cockpit were not conducted. 2. TEST OVERVIEW This evaluation, which was performed in New Jersey at the FAA’s William J. Hughes Technical Center (WJHTC) and the co-located Atlantic City International Airport (ACY), consisted of ground testing using two EC devices and four GA aircraft in various configurations. The testing involved measuring the impact to signal strength of a transmitting device operating from the inside of an aircraft cockpit. 2.1 EQUIPMENT 2.1.1 EC Transmitting Devices Two EC devices were used to test the impact to signal strength from the operation of a transmitter within the cockpit of an aircraft. A normal, certified installation will have the radio mounted inside of the aircraft while the radiating antenna elements are mounted on the outside of the aircraft for optimal signal radiation. With small, portable, devices such as an EC device, the transmitting antenna is collocated with the radio in a single package and is, therefore, also inside of the aircraft. Two different models were used: one production EC device that transmitted Mode S Extended Squitter (ES) ADS-B signals on 1090 MHz and one prototype EC device that transmitted UAT ADS-B signals on 978 MHz. Both devices are battery operated and are intended to connect to an aircraft windshield via a suction mount. The EC device and suction cup mount can be seen in Figure 1. 4 Figure 1. EC Device with Suction Cup Mount 2.1.2 Aircraft The research team selected the four GA aircraft below as a representative set of aircraft for the research effort: • Air Tractor (AT)-802A (Agricultural sprayer) • Robinson R44 Raven II Rotorcraft (Tourism helicopter) • Piper PA-18 Super Cub (Banner plane) • Cessna 172N (Banner plane). These are aircraft that may regularly operate below 500 feet and might not be equipped with a certified position reporting device2. A twin-engine British Aerospace (BAe) Jetstream 31 turboprop airplane was used for preliminary “dry run” testing. However, results are not included in this report as that aircraft is not considered representative of aircraft that may utilize an EC device. 2.1.3 Data Collection Devices The ANG-C33 Surveillance Engineering Team has developed RF data collection and decoding systems which provide detailed characterization of a given RF environment at multiple frequencies. The configuration for this effort included both 1090 MHz and 978 MHz RF environment data recording suites. These systems consist of specialized multi-channel data acquisition cards and synchronized GPS input which digitize analog input from independent RF front-end receivers. This digitized data is captured using high speed and capacity data storage systems. The resulting data is then post-processed to analyze characteristics of the transmitted signals such as signal strength, message counts, message timing, and error rates. Figure 2 illustrates the RF data collection and decoding systems. 2 All of these aircraft were equipped with certified transponders; however, the installed transponders were disabled for the duration of the testing. 5 Figure 2. Radio Frequency Data Collection and Decoding System The data collection systems were mounted in a vehicle via a custom fabricated mobile equipment rack. An adjustable antenna mast was also fabricated, which attached to the towing hitch of the mobile data recording van and was used for longer range measurements. In this configuration, the antenna was 9 feet, 3 inches above the ground. For the close-range measurements, a tripod was used as an antenna mast. The height was adjusted to a level where the receiving and transmitting antennas were line of sight, and the tripod was manually moved between azimuths. The same vertically polarized and vertically oriented antenna was used for all testing. Figure 3 shows the equipment in place. Figure 3. Mobile data recording van equipped with Data Collection Equipment 2.2 TEST CASES Sections 2.2.1 and 2.2.3 detail the testing methodology that was executed for this study. 6 2.2.1 Device Tests 2.2.1.1 Preliminary Test Prior to conducting ground testing with aircraft, laboratory bench testing of two EC devices was conducted. The aim of bench testing was to determine the device orientation which would deliver the optimal signal strength. ADS-B signals are intended to be transmitted and received with a vertical polarization. Figure 4 presents a simple diagram to show the difference between an antenna transmitting with vertical polarization and one transmitting with horizontal polarization. Optimal signal transmission between two antennas is achieved when they are aligned and have the same polarity. Figure 4. Vertical vs. Horizontal Polarization As the EC devices are stand-alone boxes with no external antenna, it was necessary to characterize their transmitting polarity through inspection to ensure consistent alignment when installed in an aircraft. This testing not only validated the antenna polarization, but also provided insight into the performance at different orientations. The results of this testing guided the placement of the transmitting devices within each aircraft. Spectral characteristics of the devices were also measured. This was an additional step that was required as part of the application process to permit these non-certified devices to be used for this testing. 2.2.1.2 Baseline Test Baseline measurements were taken for both the 978 MHz and 1090 MHz transmitters. The baseline measurements were free space measurements, where the transmitting device was placed vertically and horizontally in an open-air, unobstructed location. The location of the transmitting device for this test was the same position that was used for each aircraft in the Azimuth tests (see Figure 6). Each EC transmitting device was tested independently. Measurements were recorded at all azimuth points and ranges. This data was captured as a best-possible transmitter configuration for all scenarios, and as a means of comparison to the later performance of the devices when operating within an aircraft. 2.2.2 Aircraft Tests The testing was broken into three primary categories that evaluated signal strength based upon: orientation, azimuth, and range. 7 2.2.2.1 Orientation The EC devices were mounted in each aircraft in three different orientations: vertical, horizontal, and worst-case3. As portable, non-certified devices, EC transmitters may be mounted in any number of ways based both on physical constraints of an aircraft and on pilot discretion. It was therefore important to test the devices in multiple orientations to better assess potential performance in a range of possible mounting locations. Vertical orientation of the 1090MHz unit, and horizontal orientation of the 978MHz unit maintained the internal antenna perpendicular to the ground. Placement on a vertically oriented cockpit window such that the internal antenna is oriented vertically is, operationally, the best-case installation. Both ADS-B variants, 1090 MHz and 978 MHz, are designed to operate with vertical polarization. Therefore, a vertically oriented cockpit window should provide the best signal propagation. Figure 5 shows the antenna orientation for both devices. While in this testing, the device mounting locations were selected to provide a maximally vertical or horizontal orientation, many aircraft have windows which are sloped. Mounting the transmitting device onto a sloped window will misalign the polarity of the antenna and will result in a degraded radiation pattern. Figure 5. Internal antenna orientation differences between the 1090MHz and 978MHz devices. Horizontal orientation of the 1090 MHz unit, and vertical orientation of the 978 MHz unit, positioned the internal antenna parallel to the ground (horizontally). Horizontal placement in the cockpit window is a sub-optimal configuration. A horizontal placement can impact signal strength due to cross-polarization and pointing loss. Cross-polarization is a reduction in signal strength due to misalignment of the polarities of the transmitting and receiving antennas. Pointing loss is due to a misalignment in the direction of radiation. This orientation may be driven by the architecture of the aircraft or by operator discretion and is considered a sub-optimal installation. 3 Vertical and horizontal refer, here, to the orientation of the radiating antenna element. In the case of the two EC devices, their antennas had different orientations internal to the device. For the 978 MHz device, this meant that a vertical antenna orientation was with the device horizontal on its side. 8 The worst-case location (determined in this testing) was chosen to be the orientation and location with the most signal degradation. In all but one case, this consisted of the device placed horizontally on the floor of the aircraft. In one case, based on input from the pilot, it was determined that operation on the dash of the aircraft was both more likely to, and did, provide a poorer performance. For all test points, both in azimuth and range, measurements were taken with the transmitter in each of the three device orientations4. Section 3 includes images of all installed orientations for each aircraft tested. 2.2.2.2 Azimuth Azimuth testing was another prime focus of this study and was designed to measure the impact to the signal strength due to obstruction from the airframe itself. Operation of a transmitting device from within a cockpit will undoubtedly incur signal blockage. However, the various design and construction of GA aircraft means that the amount of blockage, and the direction in which the signal is blocked by the airframe, will vary with each aircraft. The azimuth tests were also performed at closer ranges. This allowed the measurements to be taken with the least amount of impact from the local environment. That is, that changes in the signal strength are due primarily to the impact from the operational configuration of the device and aircraft alone, and not due to other external causes like multipath reflections. The azimuth testing consisted of taking signal strength measurements at eight (8) equally spaced points (between 0 and 360 degrees) around the aircraft in which the device was installed. At each point, measurements were taken with the device in each of three different installed positions: vertical, horizontal and worst-case. Both 10m and 50m azimuth ranges were tested. Technicians marked out the center point of a 10-meter (m) radius circle and a 50 m radius circle at a designated position on the ramp in front of the WJHTC hangar building. Along each circle, angles were marked at 45-degree intervals for a total of 16 positions (8 at 10 meters and 8 at 50 meters). A 10m radius from the cockpit of the aircraft was the closest circle that could be mapped around all aircraft and provides the best measurement of signal strength. The close distance to the location of the transmitter was intended to minimize external environmental impact to the measured signal strength, such as surface reflections. The 10m measurements are, therefore, the primary focus of this testing. A 50m radius around the cockpit of the aircraft provided enough distance from the aircraft to safely perform engine-on testing. Engine-on testing was included to investigate the potential for additional interference from the normal operation of the aircraft. While the larger radius circle permitted engine-on testing, it also added measurement uncertainty due to surface reflections of the transmitted signal. Therefore, the measurements taken at 50m were only used for a direct engine on/off comparison. Figure 6 shows the layout of both azimuth rings with the aircraft location in the center and the measurement locations marked in yellow. 4 As each aircraft cockpit is unique, the mounting location of the EC device in each aircraft was also unique. However, actual mounting orientations were tailored to each aircraft in order to get the positions as close to vertical and/or horizontal as possible. Worst case was less precise, and generally consisted of placing the device on the floor of the cockpit. 9 Figure 6. Azimuth Test Area Figure 7 shows the tripod at the 315-degree, 10m position (left) and vehicle mount at the 0-degree, 50m position (right) for the AT-802A. Figure 7. Azimuth test examples: AT-802A 2.2.2.3 Range Distance measurements were taken at three linear ranges (in statute miles) from the aircraft location: 0.5 miles, 1 mile, and 2 miles. Each aircraft was parked on the ramp in front of the WJHTC hangar building with the front of the aircraft pointed in the direction of the 0-degree angle mark. The distance measurements were taken as a means of direct measurement of the signal attenuation over longer distances. At each of the three distances, measurements were taken with the transmitting devices in each of the three installation orientations. No azimuth or engine-on measurements were taken for the distance measurements. Due to the potential for surface interference to the signal at these distances, results from this test are included but are not covered at length in this report. Figure 8 shows the test locations. 10 Figure 8. Range Test Locations 3. RESULTS This section includes all results from tests using the 978 MHz prototype EC device as a transmitter. The 978 MHz transmitter is the primary focus of this testing, as any potential use of portable position reporting devices would likely be on the UAT link. A production 1090 MHz ADS-B EC transmitting device was also tested. Results from azimuth tests using the 1090 MHz EC device are included in Appendix A. 3.1 AZIMUTH TESTS Signal strength measurements were taken at eight different azimuths and two different radii from the center of the aircraft location. A 10m radius circle was the primary focus of this testing. Close proximity to the transmitter was important to minimize the impact due to environment effects such as surface reflections and multipath. Baseline measurements for both 10m and 50m were compared with initial anechoic chamber measurements of the transmit power output from the device. The signal strength measured at a radius of 10m was comparable to the chamber measurements, while the signal strength measured at 50m showed an average gain of 2.7 decibels (dB)5. This equates to an approximate doubling in signal strength, and can largely be attributed to an additive effect from surface reflections. The following plot shows a normalized comparison of the effective transmit power for both 10m and 50m. The difference in signal strength varies between the 10m and 50m measurements, from a minimum of 0.33dB to a maximum of 6dB. 5 Decibels (dB) is a logarithmic scale used in signal strength measurements. Every 10 dB increase or decrease corresponds to an order of magnitude increase or decrease. Example dB to linear conversions: 3dB=2x increase; 6dB=4x; 10dB=10x; 20dB=100x; 30dB=1000x. Negative dB values indicate signal attenuation or loss. -3dB = (1/2)x, -6dB = (1/4)x, -10dB = (1/10)x, -20dB = (1/100)x, -30dB = (1/1000)x. 11 Figure 9. Comparison of effective transmit power, as measured at 10m and 50m. In order to minimize inconsistencies and inaccuracies in the results, the data collected at 10m is the primary focus of this analysis. Unless otherwise specified, results that are discussed are relative to the signal strength measured at azimuth points with a radius of 10m. 3.1.1 Baseline The first test conducted was a baseline study to characterize the RF performance of the transmitter in the test environment with no obstructions. The baseline testing consisted of signal strength measurements at all eight azimuth points for both the vertical and horizontal antenna orientations. The intent of the baseline testing was twofold: 1) as a controlled measurement against which all installation tests can be compared, and 2) to provide a means of identifying the impact to antenna orientation alone. Figure 10 shows a comparison of the baseline normalized signal strength measurements at 10m for the vertical and horizontal antenna orientations. The results in the vertical orientation show a fairly uniform transmission pattern at all azimuth points, as would be expected when no obstructions are present. Some variability in the received signal strength can be attributed to impacts from the physical design of the transmitting device and the prototype antenna. 12 Figure 10. Baseline 10m Signal Strength Comparison: Vertical vs Horizontal (Normalized, in dB) Note: The prototype 978 MHz transmitting device did not have a perfectly polarized antenna. Rather the antenna had a somewhat curved shape that must be considered when reviewing these plots. Specifically, the shape of the antenna will have a tendency to reduce performance in the vertical orientation, and make both orientations appear more similar than they otherwise would be. When the orientation of the device is changed to a horizontal position, an immediate impact to the received signal strength is apparent. The impact ranges from a 50% reduction in power at 90 degrees to over 10 times reduction in power at 270 degrees. Both 978 MHz ADS-B and 1090 MHz ADS-B utilize a vertical polarized antenna as a standard. For both of these tests, the receiving antenna was fixed in a vertical orientation, while the orientation of the transmitting antenna changed. These results demonstrate the importance of proper alignment between the transmitting and receiving antennas. A transmitting device that is not mounted in the proper orientation will have a reduced performance. A reduction in the power that is transmitted from an EC device has a direct impact on the distance that the signal reaches and, therefore, the effective surveillance coverage. While operational surveillance coverage is dependent upon the performance of both the transmitter and receiver, baseline signal strength measurements can be used to calculate an effective transmission distance for the EC device that was used for this test. This distance calculation is based solely on free space path loss (FSPL) and, therefore, only serves to provide a theoretical maximum transmission distance. Actual operational range would be influenced by other factors such as flight pattern, spectrum congestion, and multipath interference. The pitch and bank angle of a maneuvering aircraft is also a contributing factor, as it will cause misalignment between transmitting and receiving antennas, further degrading signal transmission. However, a distance calculation can be 13 useful as an alternative means of assessing the impact due to suboptimal antenna orientation. The following steps were taken to arrive at a theoretical distance of transmission: 1. Start with the measured signal strength at the antenna end of the receiver. 2. Add the FSPL for a 978 MHz signal at a distance of 10m. This is the effective transmit power. 3. Assuming a receiver with a nominal sensitivity of -93dBm6, take the difference between the effective transmit power and the receiver sensitivity. This is the signal attenuation through the air. 4. Rearranging the equation for FSPL, calculate the distance for the effective FSPL at a frequency of 978 MHz. Figure 11 shows the calculated maximum theoretical transmission distances for both vertical and horizontal orientations based on the measured signal strength that was shown in Figure 10. The red bars in the figure represent uncertainty due to variations in the received signal strength. Note: This plot, and all subsequent distance plots, are scaled according to their maximum values in order to show the most detail. Therefore, differences in scale should be expected. Figure 11. Baseline – Maximum theoretical transmission distances in nautical miles. The transmission distances in Figure 9 show a fairly uniform distance of just over 40 nautical miles (nmi) for the unobstructed vertical orientation. However, upon moving the device to a horizontal orientation, the polarization of the transmitter and receiver no longer match, resulting in a transmission distance that is reduced by anywhere from 15 to 30 nmi. 6 A receiver sensitivity of -93dBm is based on the specifications for an airborne transceiver defined in RTCA DO-282C Minimum Operational Performance Standards (MOPS) for UAT ADS-B. 14 The distances in the plot are theoretical maximum free space ranges. In the operational scenarios under consideration, i.e., operations under 500 feet AGL, range is constrained by radio line of sight due to the curvature of the earth. For example, for two aircraft at 500 feet AGL, the radio line of sight range limitation is approximately 55 nmi. 3.1.2 Air Tractor (AT)-802A The AT-802A, pictured in Figure 12, is a single seat agricultural crop-duster aircraft that is 36 feet long, with a 59-foot wingspan (Air Tractor, 2024). This was the largest aircraft tested and the aircraft with the smallest cockpit windows. In contrast to other aircraft under test, the wings of the AT-802A are low mounted. This aircraft also has a large tank between the cockpit and the engine that is used to hold liquid for crop spraying. The tank was empty for this test. Figure 12. AT-802A Agricultural Aircraft Table 1 shows the mounting locations for both EC devices in the aircraft. While the vertical and horizontal mounting locations were in the aircraft windows for this testing, pilot input indicated that this would not be possible in operation as it would obstruct their already limited view. It was due to this same pilot feedback that the dash location was investigated (and eventually used) as the worst case. Table 1. EC Device Installation Orientations: AT-802A Orientation Location 978 MHz 1090 MHz Vertical Right Side Cockpit Window 15 Horizontal Center Cockpit Window Worst Case Shelf Below Window Framing and Behind Tank Figure 13 shows a comparison between the measured signal strength from the device mounted vertically in the AT-802A versus the baseline vertical orientation. The difference shows a significant reduction in signal strength across 75% of the radiation pattern. In the rightward direction (where the device was mounted) the signal is strongest, while the opposite directions show a decrease in signal strength of 12 to15dB (roughly 16 to 30 times reduction). This indicates that, within a single aircraft, a change in the location in which a device is mounted can cause a large change in the transmission pattern. Figure 13. AT-802A Vertical vs Baseline Vertical (normalized, dB). 16 In the same manner as the comparison of the “best-case” orientation with the baseline, a comparison can be made between the various orientations of the transmitting device within the aircraft. Figure 14 shows a comparison of the normalized measured signal strength for all mounting orientations within the AT-802A, including the worst-case dash mount. While these measurements show large variability between the various mounting orientations, all instances exhibited strong attenuation toward the front and rear of the aircraft that is likely attributable to obstruction from the aircraft engine and airframe. Figure 14. AT-802A Vertical, Horizontal and Worst-Case orientations (normalized, dB). Using the measured signal strength for the vertical and horizontal orientations, a calculation of the maximum theoretical transmission distance can be made. The following plot, Figure 15, shows the result of this distance calculation for the AT-802A. As shown, the vertical orientation shows the greatest distance in the direction of the mounting location. While the distances based on the horizontal orientation are more evenly distributed, both cases show significantly reduced range in the forward (direction of flight) and aft directions. 17 Figure 15. AT-802A transmission distance in nautical miles. 3.1.3 Robinson R44 Raven II Rotorcraft The R44 rotorcraft, pictured in Figure 16, is 38 feet long with a rotor diameter of 33 feet (Robinson, 2024). This aircraft has a large Plexiglas bubble windshield that provided the least obstruction in the forward direction of all aircraft tested. Figure 16. R44 Helicopter Table 2 shows the mounting locations for both devices in the R44. 18 Table 2. EC Device Installation Orientations: R44 Orientation Location 978 MHz 1090 MHz Vertical Left Forward Cockpit Window Horizontal Top Center Cockpit Window Worst Case Center Right Floorboard Figure 17 shows a comparison of the results from a vertical installation in the R44 versus the baseline measurements for the same orientation. While these measurements show a more uniform radiation pattern than the AT-802A, there is a slight increase in signal strength in the forward direction and a stronger attenuation toward the rear. The bias in the forward direction could be attributable to multipath reflections off the rear of the aircraft cabin. The drop in signal strength towards the rear of the aircraft is likely due to obstruction from the gearbox, engine, and tail boom. 19 Figure 17. R44 Vertical vs Baseline Vertical (normalized, dB). As shown in Figure 18 below, overlaying the results from the vertical, horizontal, and worst-case (floor) mounting positions shows a significant difference in performance based on the orientation of the transmitting device within the R44. Signal attenuation due to blockage from the engine and tail of the aircraft is visible in the data from all orientations. The signal attenuation towards the rear is arguably more significant for this aircraft as it is the one aircraft in this study that is capable of flying backwards. Figure 18. R44 Vertical, Horizontal and Worst-Case orientations (normalized, dB). 20 A theoretical effective transmission distance was calculated for the R44 and is shown in Figure 19 below. The greatest distance in the forward direction is due to minimal obstructions. Toward the rear of the aircraft, the transmission distance is significantly reduced for both the vertical and horizontal orientations. Figure 19. R44 transmission distance in nautical miles. 3.1.4 Piper PA-18 Super Cub The Piper PA-18 Super Cub, or “Banner” plane, is a fixed wing single engine aircraft that is 22 feet long, with a 35-foot wingspan (Smithsonian, 2024). The wings are high mounted as shown in Figure 20. This aircraft is used to haul advertising banners at low altitude up and down the beachline during summer. Figure 20. PA-18 Super Cub 21 Table 3 shows the mounting locations of the EC devices in the PA-18. While the forward cockpit windows were used for the test, the PA-18 is another instance where feedback from a pilot indicated that the cockpit window would not be a practical location operationally. In this case, the pilot would have placed the transmitter in one of the rear windows to prevent obstruction of their view out of the aircraft. Table 3. EC Device Installation Orientations: PA-18 Orientation Location 978 MHz 1090 MHz Vertical Left Forward Cockpit Window Horizontal Top Center Cockpit Window Worst Case Center Floorboard Figure 21 shows a comparison between measurements of the vertically mounted transmitter and the baseline vertical measurements. A large reduction in the measured signal strength at an azimuth of 315 degrees was the result of signal distortion. This may have been caused by coupling with and/or reflections from the aircraft structure. Toward the rear of the aircraft, attenuation due to the aircraft body is visible. 22 Figure 21. PA-18 Vertical vs Baseline Vertical (normalized, dB). Figure 22 shows the measured signal strength of all device orientations in the PA-18. Measurements from operation of the transmitting device in the worst-case (floor) orientation showed the greatest attenuation. Figure 22. PA-18 Vertical, Horizontal and Worst-Case orientations (normalized, dB). Using the measured signal strength from the vertical and horizontal device orientations, a theoretical maximum effective transmit distance was calculated for all azimuth points. Figure 23 shows a plot of those results. Both orientations showed a similar transmission distance toward the 23 front of the aircraft, while the largest distance measured was the horizontal orientation at 135 degrees. The reduced distance in the direction of the device mounting location indicates a potential for additional variability in signal propagation. Figure 23. PA-18 transmission distance in nautical miles. 3.1.5 Cessna 172N The Cessna 172N is a fixed wing, single engine aircraft that is 27 feet long with a 36-foot wingspan (Global Air, 2024). As shown in Figure 24, this aircraft also has high mounted wing. Arguably the most common general aviation aircraft, this specific aircraft was modified for operation as a banner towing aircraft. Figure 24. Cessna 172N 24 Table 4 shows the mounting locations for the EC devices in the Cessna 172N. Table 4. EC Device Installation Orientations: Cessna 172N Orientation Location 978 MHz 1090 MHz Vertical Right Forward Cockpit Window Horizontal Top Center Cockpit Window Worst Case Center Right Floorboard As with other aircraft tested, when comparing the results of the vertical installation in the Cessna to the baseline there was a drop in signal in the opposite direction of the mounting location that is attributable to blockage by the aircraft body, shown in Figure 25 below. 25 Figure 25. Cessna Vertical vs Baseline Vertical (normalized, dB). Figure 26 below shows an aggregate view of measurements from all device orientations. The vertical and horizontal orientations show similar performance, with the most attenuation toward the rear of the aircraft due to blockage by the airframe itself. The worst-case (floor) resulted in a significant attenuation of the signal in all directions. Figure 26. Cessna Vertical, Horizontal and Worst-Case orientations (normalized, dB). A theoretical maximum effective transmission distance was calculated for the measurements from the vertical and horizontal orientations of operation within the Cessna. The following plot, Figure 27, shows the furthest transmission distance in the forward direction of the aircraft, with significant reductions in distance towards the rear. 26 Figure 27. Cessna transmission distance in nautical miles. 3.2 ENGINE ON/OFF TEST A comparison of measured received signal strength was made for all aircraft at 8 azimuth points and a distance of 50m with the aircraft engine powered on at idle speed. The size of the circular test pattern, and the corresponding separation from the aircraft, permitted the safe capture of signal measurements while the engine was in an operational state. This same distance from the aircraft added uncertainty to the measured amplitude due to additional environmental impacts such as ground reflections and interactions with multipath from nearby surfaces. For this reason, the results of the engine on/off testing are presented only as a comparison between the two different states. While additional variability may be present in the received signal strength, the test conditions for both the engine on and engine off scenarios were identical. It is therefore assumed that a similar impact to the signal strength was present for both scenarios, providing the ability to perform a relative comparison. It should also be noted that, as the measurements were not taken at the same time, there exists potential for differences due to separation in time. These engine on/off tests were conducted due to a concern that a position reporting device operating within a cockpit may experience additional signal impacts from radiated emissions. While the operation of the aircraft engine cannot be ruled out as a contributing factor to degradations of the transmitted position signal, the results of this comparative analysis proved inconclusive in this regard. Results from the engine on/off testing for each aircraft are provided in sections 3.2.1 through 3.2.4, with a brief discussion on the findings for both the vertical and horizontal test scenarios. 3.2.1 Air Tractor (AT)-802A Figure 28 shows the engine on/off comparison for both the vertical and horizontal installation orientations. While the horizontal orientation compares well between the engine on and engine off cases, the vertical orientation shows differences in the signal strength in the forward and rearward 27 directions. Upon further review of the measurement data, it was observed that there was significant variability in the signal itself in these directions for both the engine on and off conditions. The poor signal quality, combined with the occurrence in both engine on and off states, indicate that the difference is most likely due to distortion and attenuation from the airframe and not due to the operation of the engine. Figure 28. AT-802A – comparison Engine on/off (normalized, dB) (a) Vertical; (b) Horizontal 3.2.2 Robinson R44 Raven II Rotorcraft The results for the R44 engine on/off testing are shown in Figure 29. Differences in measured signal strength are present in both the vertical and horizontal orientations. Significant variability is visible in the comparison of the vertically installed devices. A noticeable attenuation of up to 10dB in the signal strength is visible in the rear and right rear quadrants with engines on. However, as with the AT-802A results, inspection of the data showed instability in both the engine on and off results, particularly toward the rear of the aircraft. While it appears that the operation of the tail rotor contributed to signal distortion in the rearward direction, the results seem to indicate that the engine and airframe obstructions in that direction are the largest contributor to signal impairment. 28 Figure 29. R44 – comparison Engine on/off (normalized, dB): (a) Vertical; (b) Horizontal 3.2.3 Piper PA-18 Super Cub The PA-18 showed variations between the engine-on and off states as seen in Figure 30. However, as with the previous aircraft tested, variability in the measured signal strength was present in both the engine on and off states. Larger fluctuations in the signal strength in the opposite direction of the mounting point of the transmitting device would seem to indicate that the airframe itself is the main contributor to signal degradations. Figure 30. PA-18: Comparison Engine on/off (normalized, dB) – (a) Vertical; (b) Horizontal 29 3.2.4 Cessna 172N In the engine on/off testing of the Cessna aircraft, the vertical installation provided the best performance with the engine off. With the engine on, there was more variation in the measured signal strength which is evident in Figure 31 below. However, measured signal strength in the direction of the mounting location was consistent. While the horizontal orientation did not perform as well as the vertical orientation, the measured signal strength and variability in that signal strength was equivalent for both the engine on and off tests. It is, therefore, difficult to conclude that the operation of the engine was a significant contributing factor degrading the signal strength. Figure 31. Cessna – comparison Engine on/off (normalized, dB) (a) Vertical; (b) Horizontal 3.3 RANGE TEST Range tests were conducted where signal strength measurements were taken at points 0.5 miles, 1 mile, and 2 miles from the transmitting device. These range measurements were collected for each aircraft under test, as well as for a baseline open-air scenario. When operating on the surface, with low relative antenna heights, signals are impacted by losses due to the proximity to the ground. As the signal travels over the airport surface, it can experience peaks and nulls, as well as an overall signal attenuation greater than what would be experienced through free space alone. This surface wave propagation loss is visible in the following plot, which compares the signal strength measurements to the FSPL for the same distance. The plot in Figure 32 shows that, as the transmission distance increases, the signal is attenuated at a faster rate than it would be due to FSPL alone. While attenuation due to signal propagation over the ground is an important point to consider for surface operations, it may also have the potential to degrade the effectiveness of devices operating at low altitudes. 30 Figure 32. Received signal level (dBm) -vs- range (nmi): Comparison to FSPL, 0.1 to 10nmi. 4. CONCLUSION This test evaluated the impact to transmitted signal strength that might be experienced due to the operation of a portable position reporting device from inside of an aircraft cockpit. While it is known that operation within an aircraft will incur a loss of signal strength, there were significant unknown factors regarding how the structure of the aircraft and the orientation of the device might also impact the transmitted signal. The results from this testing showed that there are three major compounding factors that will impact the signal strength of a position reporting device when it is operated within an aircraft cockpit: 1. Location of the mounting point within the aircraft. 2. Orientation in which the transmitting device is mounted. 3. Aircraft structure and configuration. Any one of these factors will cause a variable degree of attenuation to the signal but, together, can have a multiplicative effect. In addition to these installation impacts, there are also RF impacts that can affect signal propagation. Proximity to metal aircraft structural components can induce distortions, reflections and grounding effects that can constructively or destructively impact the transmitted signal as well, adding to the variability in the transmitted radiation pattern. Analysis of the measured signal strengths captured during this testing revealed a unique radiation pattern for each aircraft that further changed based on the mounting location and orientation, with the overall result being inconsistent and unpredictable signal propagation. Variability in signal propagation translates to variability in the distance over which the broadcast signal with the reported position can be acquired. Measurements showed peaks and nulls in the signal strength at different azimuths which could negatively impact operational track continuity. 31 Testing showed that alignment of the polarization of the transmitting and receiving antennas had a significant impact on the signal strength as well. Even when antennas are optimally installed in a vertical orientation, maneuvering of the aircraft during flight will induce misalignment between the antennas of a transmitting and receiving aircraft. The resulting operational degradation in signal strength will add a dynamic element to the variability and is an additional consideration for aircraft operating at low altitudes that may make more frequent and/or rapid directional changes than higher altitude operations. The impact that the mounting location has on the ability of the devices to obtain a quality GNSS signal was also observed during testing. ADS-B stands for Automatic Dependent Surveillance – Broadcast. The “dependence” is upon a valid position source such as GPS. During testing, it was found that in many instances when the device was moved out of the window of the aircraft, the position source was lost. Upon loss of a position source, the ADS-B transmitting device will no longer be capable of reporting a valid position - resulting in a loss of surveillance. The receivers used in this testing are laboratory receivers that were designed for test and measurement and do not necessarily represent the performance that may be observed from a receiver that may be used operationally in a UAS or GA aircraft. Operational surveillance coverage is dependent upon the link between both a transmitter and receiver. Further variability, beyond what was measured in this test, may be experienced due to differences in receiver performance. While the overall observed loss of signal strength is relatively similar between the 978MHz and 1090MHz systems when taken in total, the predicted operational range for 978MHz does show an order of magnitude better than 1090MHz. The primary reason for this, however, is most likely the higher sensitivity of the modeled 978MHz receiver as compared to the 1090MHz receiver. Note: Radiated electromagnetic interference (EMI) tests to investigate the impact upon existing avionics/electronics caused by an EC device operating in an aircraft cockpit were not conducted. 5. RECOMMENDATIONS Given the findings and results from this study, the following potential next steps are suggested: • Examine representative receiver performance o The study detailed in this report focused on the performance of EC devices transmitting from within an aircraft. However, as a communication link is dependent upon both a transmitter and receiver, one potential next step could involve investigating the sensitivity of a representative set of ADS-B receivers that may be used on UAS. o The ability to get specific measurements such as signal strength and raw message data from a UAS mounted receiver may not possible, as receiver output is generally provided as a one second track update. Therefore, an additional subcomponent would be to investigate if a tracking/coasting mechanism is required to provide a consistent track and define time durations for the same. 32 o In the same way that transmitting antenna orientation impacts signal strength, receiver antenna orientation is also important to the communication link. A market survey could be conducted to determine the mounting location and orientation of ADS-B receivers that are used on representative UAS. • Examine effects of degraded GPS reception of the EC devices o ADS-B requires a position source such as GPS to provide location as well as accuracy and integrity of the reported position. During the testing which this report covers, it was observed that aircraft obstructions had the potential to degrade the quality of the GPS signal. In the worst-case locations, there were multiple times that the GPS signal was lost completely by the EC device. A degraded GPS position would lead to reduction in the accuracy of the reported position, while the loss of a GPS signal would result in no position being reported. A potential follow-on study could investigate the impact to the concept of operations when such a degraded or lost GPS signal is experienced. • Develop representative encounter geometries and closure rates which would assist in determining alerting times and timeframe/distance required for any preventative and/or corrective guidance or action o Taking into account the operations, speeds, and maneuvers of both UAS and GA aircraft in the airspace below 500 feet, a representative set of encounter trajectories could be another potential step. o These geometries would aid in determining the alerting time required for the appropriate preventative and/or corrective guidance to be issued, as well as for pilot response. Additionally, it would assist in understanding the required level of surveillance performance needed to meet these requirements. 6. REFERENCES Air Tractor (2024). Air Tractor AT-802A. https://airtractor.com/aircraft/at-802a/#specifications Global Air (2024). Cessna 172N. https://www.globalair.com/aircraft-for-sale/specifications?specid=1237 Robinson (2022). R44 RAVEN II and CLIPPER II. Dimensions. https://shop.robinsonheli.com/r44-raven-ii-clipper-ii-helicopters/#dimensions RTCA, Inc. (2020). DO-260C - Minimum Operational Performance Standards for 1090 MHz Extended Squitter Automatic Dependent Surveillance – Broadcast (ADS-B) and Traffic Information Services – Broadcast (TIS-B) https://products.rtca.org/21disbb/ RTCA, Inc. (2022). DO-282C "Minimum Operational Performance Standards for Universal Access Transceiver (UAT) Automatic Dependent Surveillance - Broadcast (ADS-B)" https://products.rtca.org/21dj8e5/ Smithsonian (2024). National Air and Space Museum. Piper PA-18 Super Cub. 33 https://airandspace.si.edu/collection-objects/piper-pa-18-super-cub/nasm_A19761155000 United Kingdom Civil Aviation Authority (2021). CAP1391: Electronic Conspicuity devices. https://www.caa.co.uk/our-work/publications/documents/content/cap1391/ Unmanned Aircraft Systems Beyond Visual Line of Sight Aviation Rulemaking Committee (2022). Final Report. https://www.faa.gov/regulations_policies/rulemaking/committees/documents/media/UAS_ BVLOS_ARC_FINAL_REPORT_03102022.pdf A-1 APPENDIX A—1090 MHZ RESULTS A. RESULTS This appendix repeats the structure of the main body azimuth test results but addresses the results from use of a 1090 MHz transmitting device rather than 978 MHz The test results contained in this Appendix are based on signal measurements at eight azimuth points around a circle of 10-meter radius. At each azimuth point, all antenna orientations were measured as well: vertical, horizontal, and worst-case. The ADS-B link under consideration for use by portable transmitting devices is UAT, which operates on 978 MHz However, there is another ADS-B link that is used in both the United States and abroad that operates on 1090 MHz This 1090 MHz variant has existing, production, portable transmitting devices that are in use in the United Kingdom under the CAP1391 eConspicuity program. While these devices are not permitted for use in the United States, the FAA was able to acquire some representative samples to include in the testing. This permitted a signal strength comparison between 978 MHz and 1090 MHz ADS-B variants, as well as a comparison between production and prototype equipment. Both ADS-B variants utilize vertically polarized antennas. The production 1090 MHz EC device had a vertically polarized antenna, while the 978 MHz device’s antenna was not perfectly polarized owing to it being a prototype. The results from tests utilizing the 1090 MHz device, therefore, permit additional insight into expected antenna performance as well. Note: Signal strength comparisons do not consider differences in modulation type, error correction or bandwidth that exist between the two ADS-B variants. A.1 BASELINE An ADS-B signal is intended to be transmitted with a vertical polarization. That is, the antennas of both the transmitting device and the receiving device need to be oriented perpendicular to the ground for the maximum signal strength to be achieved. Figure A.1 shows a comparison of the Baseline measurements for the 1090 MHz transmitter at 10 meters in free space, with no obstructions. The blue trace shows a uniform signal from a vertically polarized transmitting antenna to the vertically polarized receiving antenna. However, when the transmitting antenna (the device itself in this case) is changed to a horizontal orientation, it no longer matches the polarity of the receiving antenna. This causes a significant drop in signal strength at all azimuths, as is visible in the orange trace of Figure A.1. This attenuation is solely due to the orientation of the transmitting device and does not include any additional signal impact due to potential airframe obstruction. A-2 Figure A.1: Baseline 10m Signal Strength Comparison: Vertical vs Horizontal (Normalized, in dB) Using the measured signal strength from the vertical and horizontal orientations, an effective, baseline, transmission distance was calculated and is shown in Figure A.2. This distance calculation assumes that the receiver is a standard 1090 MHz ADS-B MOPS compliant receiver with a sensitivity of -79 dBm. Note: A receiver sensitivity of -79 dBm is based on the specifications for an airborne 1090ES airborne receiver as defined in RTCA DO-260C MOPS for 1090 MHz ES ADS-B and TIS-B. The calculation uses an effective transmit power (based on measured signal strength) and using the standard receiver sensitivity, works the FSPL equation backwards to determine a theoretical maximum effective distance for this transmitting device in this aircraft. The plot shows a fairly uniform transmission distance with a significantly reduced range when the device is not oriented with its antenna aligned with the receiving antenna. Variations in the radiation pattern can be attributed to the physical characteristics of the EC device. A-3 Figure A.2: Baseline – Maximum theoretical transmission distances in nautical miles. A.2 AIR TRACTOR (AT)-802A The AT-802A was a crop duster aircraft that was the largest aircraft under test. In addition to being the largest aircraft, it presented the largest obstruction to signal propagation in both the forward and aft directions due to its large engine and large body. The cockpit is configured with a small canopy that has two parallel windows on each side of the pilot’s head. Figure A.3 presents a comparison between the baseline, free-space, and the measurements from a vertical mounting location in the right cockpit window. The orange trace shows the signal had the best range in direction of the window in which it was mounted and transmitted equally well through both side windows. However, signal propagation was seriously inhibited towards the front and rear of the aircraft due to obstruction from the aircraft’s body and engine. Figure A.3: AT-802A Vertical vs Baseline Vertical (normalized, dB). A-4 The following plot, Figure A.4, shows an overlay comparison of the normalized receive signal strength for all transmitting device mounting orientations, including a worst-case location on the dash of the aircraft. This is an important consideration as, in this case, the pilot indicated that a portable device could not be placed in the window, as it would obstruct their view – causing unsafe flying conditions. Therefore, the pilot would either place the device horizontally on the dash or, for a more permanent installation, mount it horizontally under the dash. The worst-case orientation had the worst performance for this specific aircraft but, in terms of practicality, was also the most likely orientation. Figure A.4: AT-802A Vertical, Horizontal and Worst-Case orientations (normalized, dB). Using the measured signal strength from the vertical and horizontal orientations, an effective transmission distance was calculated for the AT-802A. Figure A.5 shows that the largest transmitted distances are outward from the sides of the aircraft, with significantly reduced distance forward (in direction of flight) and minimal coverage towards the rear. A-5 Figure A.5: AT-802A transmit distance in nautical miles. A.3 ROBINSON R44 RAVEN II ROTORCRAFT The Robinson R44 helicopter had the least obstructed view of any of the aircraft under test. This aircraft has a large plexiglass bubble window covering most of its forward extremity. The transmitting device was placed in the left forward cockpit window for the vertical orientation, and the center forward window for horizontal. As seen in Figure A.6, the comparison between the vertical mounting orientation and the baseline shows a more uniform radiation pattern than the AT-802A. The signal strength is biased in the forward direction. There is a stronger than baseline transmission out the front, and a corresponding weaker signal toward the rear – 180 degrees from the mounting location. The increased signal toward the front could likely be due to an additive effect from reflections off the surface and the rear of the cockpit. Blockage from the location of the engine and the bulk of the metallic aircraft structure in the rear is apparent in the measurements. The rearward attenuation is as significant a reduction as the worst attenuation in the AT-802A. A-6 Figure A.6: R44 Vertical vs Baseline Vertical (normalized, dB). Figure A.7 below shows an overlay of all measured signal strength for all mounting orientations: vertical, horizontal and worst-case (floor). The gap in the data for the horizontal orientation at 180 degrees azimuth is caused by a compounding effect of improper polarization and airframe blocking. For this azimuth and orientation, while the signal is visible in the captured waveform data, it is attenuated and distorted to the point that it is no longer decodable. Figure A.7: R44 Vertical, Horizontal and Worst-Case orientations (normalized, dB). As with the AT-802A, a theoretical effective transmission distance was calculated for the R44 based on the measurements taken. The longest transmission distance is, as expected, directly out A-7 the front of the large cockpit window. Significant reductions in transmission distance are caused by suboptimal mounting orientation and obstruction by the airframe. Figure A.8: R44 transmit distance in nautical miles. A.4 PIPER PA-18 SUPER CUB The Piper PA-18 Super Cub, or “Banner” plane, was perhaps the most basic aircraft that was tested. It is used for hauling advertising banners at low altitude up and down the beachline during summer. The 1090 MHz transmitter was mounted in the front left cockpit window for the vertical orientation, and the center forward cockpit window for the horizontal orientation. While the test used the forward cockpit windows, the pilot indicated they could not place the device in the window due to obstructing their view and would place it in a rear window instead. Figure A.9 shows a comparison between measurements from the best-case, vertical installation and the baseline free-space vertical measurements. As with the previous aircraft, the location in which the 1090 MHz device was mounted is apparent, as the forward left direction provided the strongest signal strength. The significant reduction in signal strength at 270 degrees is possibly due to obstruction or distortion from the window frame which was just to the rear of the mounted location. The impact of the aircraft body is visible in the plot as well, with attenuation in the azimuths towards the rear. A-8 Figure A.9: PA-18 Vertical vs Baseline Vertical (normalized, dB). The following plot, Figure A.10, shows measurements from all mounting orientations. The vertical is the best performing orientation, followed by the horizontal, and the worst-case (floor) confirming its expected performance. Figure A.10: PA-18 Vertical, Horizontal and Worst-Case orientations (normalized, dB). Using the measured signal strengths for the vertical and horizontal orientations, the theoretical maximum effective transmission distance was calculated. In the following plot, Figure A.11, the location in which the device was mounted is clear, with the largest transmit distance. This shows that where the device is mounted in the cockpit is a significant contributing factor to the performance of the position reporting device. A-9 Figure A.11: PA-18 transmit distance in nautical miles. A.5 CESSNA 172N The Cessna 172 is perhaps the most representative GA aircraft. Perhaps not as common as an AT-802A at altitudes below 500 feet, but it is arguably the most common general aviation aircraft in operation. This specific aircraft was modified to support banner towing. The 1090 MHz transmitting device was placed in the front right cockpit window for the vertical orientation, and the center forward cockpit window for horizontal. Figure A.12 below, shows the measured signal strength matches the baseline best in the direction of the mounting location. As with the other aircraft, the bulk of the aircraft body that is situated toward the rear of the aircraft caused a significant attenuation of the signal strength in that direction. Figure A.12: Cessna Vertical vs Baseline Vertical (normalized, dB). A-10 Figure A.13 below, shows an overlay of the measured signal strengths for all mounting orientations within the Cessna. The horizontal orientation experienced the most attenuation in the forward direction, which is likely due to a combination of pointing loss and cross-polarization. The location on the floor again demonstrates the worst-case performance. Figure A.13: Cessna Vertical, Horizontal and Worst-Case orientations (normalized, dB). A theoretical maximum effective transmit distance was calculated for each radial based on the signal strength measurements taken. The plot of these distances in Figure A.14 shows the longest distances in the direction of the mounting location and out the opposite cockpit side window for the vertical orientation. While less than the vertical, the horizontal orientation provided the best transmit distance out the sides of the aircraft but had almost no transmitted signal in the forward direction. Figure A.14: Cessna transmit distance in nautical miles.