302(a)(2) Ground equipment [ETM 3.2.1.2] The four beacons should be located on either side of the aircraft ground track to permit an optimum layout. For example, a helicopter should be covered with angles between 30 and 150 (90 being the ideal angle). Two beacons can be located on the axis of the noise measurement points at distances of ±1640 ft (±500 m) from the central microphone, while another two beacons can be located on the track at ±1969 ft (±600 m) from the central microphone.
302(b) Kine-theodolite System [ETM 3.2.2] It is possible to obtain aircraft position data with classical kine-theodolites, but it is also possible to make use of a system composed of two simplified theodolites including a motorized photo-camera on a moving platform, which reports azimuth and elevation. These parameters are sync hronized with coded time and the identification number of every photograph recorded.
Each 0.1-second azimuth and elevation data measurement is sent to a central computer which calculates the aircraft position (X, Y and Z) versus time for each trajectory.
For example, for helicopter testing, photographic stations should be located at sideline positions about 984 ft (300 m) from the track, and at 656 ft (200 m) on either side of the three noise measurement points.
The accuracy of such a system can be ±4.9 ft (±1.5 m) in X, Y and Z over the working area.
302(c) Radar/theodolite Triangulation [ETM 3.2.3] The opto-electronic system shown diagrammatically in Figure 4 uses a single optical theodolite to provide azimuth and elevation while range data are obtained from a radar tracking system using a single transponder. Data from these two sources are transferred to a desktop calculator at a rate of 20 samples/second from which three-dimensional position fixes can be derived. The system also provides tape start and stop times to the measuring sites, synchronizing all tape-recording times. The accuracy of the system is approx imately ±6.6 ft (±2.0 m), ±3.3 ft (±1.0 m) and ±6.6 ft (±2.0 m) for horizontal range (X), cross-track (Y) and he ight (Z) respectively. Uncert ainties associated with the determination of the visual glide slope indicator and ground speed are ±0.1 and ±0.5 kt (±0.9 km/h) 302(d) Photographic Scaling [ETM 3.2.4] The flight path of an aircraft during a noise certification demonstration may be determined by using a combination of ground-based cameras and height data supplied as a function of time from the on-board radio or pressure altimeters.
For example, using this method for a helicopter, three camer as are placed along the intended track, such that one is sited close to the center microphone position and the othe r two are sited close to each of the 10 dB-down points, typically 1640 ft (500 m) either side of the microphone, depending upon the flight procedure being used. The cameras are mounted vertically and ar e calibrated so that the image size, obtained as the helicopter passes overhead, can be used to determine the height of the aircraft. It is important that the time at which each camera fires is synchronized with the on-board data acquisition system so that the height of the aircraft as it passes over each of the cameras can be correlated with the heights obtained from the photographs. The flight path of the aircraft as a function of distance may be obtained by fitting the aircraft data to the camera heights.
Generator
1.2 kW 240 V Interface Intercom Computer Dual floppydisc Radar transmitter/ receiver Optical tracking equipment Threshold detector Aircraft transponder Chart recorder Figure 4. Radar/optical position tracking system The aircraft reference dimension should be as large as possible in order to maximize photograph image size but should be chosen and used with care if errors in aircraft position are to be avoided. For a helicopter, foreshortening of the image due to factors such as main rotor coning (bending of the blades), disc tilt or fuselage pitch attitude, if not accounted for, will result in errors in the measurement of height and/or lateral and longitudinal position.
By erecting a line above each of the camer as at right angles to the intended tr ack, at a sufficient height above the camera in order to provide a clear photographic image of both the line and the aircraft, the applicant may obtain the lateral offset of the aircraft as it passes over each of the cameras. This can be done by attaching marks to the line showing the angular distances from overhead at 5 intervals on either side of the vertical. For example, for helicopters, this method may be used to confirm that the helicopter follows a 6 ± 0.5 glide slope within 10 of the overhead of the center microphone as required by H36.107 of part 36.
Furthermore, from the synchronized times of the aircraft passing over the three camera positions, the ground speed can be determined for later use in the duration adjustment.
Overall accuracy of the system is ±1.0 percent of height and ±1.3 percent of longitudinal and lateral displacements.
Mean approach/climb angles and mean ground speed can be determined within ±0.25 and ±0.7 percent respectively.
302(e) DGPS-based Time- Space-Position Information Tracking Systems [ETM 3.2.5] 302(e)(1) General [ETM 3.2.5.1] The use of conventional global positi oning system (GPS) receivers on-board aircraft to obtain time-space-position information (TSPI) is not considered to be accurate enough for noise certification testing. However, by using data Page 68
from a second localized fixed-position GPS receiver, a substantial improvement in accuracy can be achieved. Such an arrangement is referred to as a differential GPS (DGPS) system.
FAA may approve the use of such DGPS systems, based on the particular characteristics of the hardware, related software, installation and operational specifics proposed by the applicant. This section summarizes recommended requirements for DGPS systems proposed for use during noise certification testing.
Typically the hardware components of these systems are GPS receivers and antennas on the ground and in the aircraft, the data link transmitter and antenna on the ground and the corresponding receiver and antenna in the aircraft, a laptop computer in the aircraft and batteries and electronic power supplies (see Figure 5). Software running on the laptop computer in the aircraft provides the user control/display function and performs data logging. A personal computer is generally needed to initialize the GPS receiver on the ground, but is not necessary for continuous operation.
Personal computer Personal computer Applicant software Differential correction
CDI/GDI
A I R C R A F T
G R O U N D GPS receiver GPS receiver Data link receiver Data link transmitter Figure 5. DGPS TSPI system basic architecture In addition to generating flight reference data for post-p rocessing, some applicants’ systems provide the pilot with information to navigate the aircraft. Measured aircraft position is compared to a desired reference flight path, and steering commands are sent to a course/g lide slope deviation indicator (CDI/GDI) installed specifically for use with the DGPS system.
Variations on the basic architecture shown in Figure 5 are possible. For example, it is possible to eliminate the data link elements by collecting and storing data from both GPS receivers during a flight and post-processing these data in a single computer after the flight is complete. However, without a data link, DGPS data cannot be used for aircraft guidance, nor can an aircraft-based operator obtain “qui ck-look” information regarding the DGPS solution quality.
Another possible variation on the basic architecture in Figure 5 involves the use of a two-way data link. Typically, identical transceivers would be used on the ground and in the aircraft. This enables gro und tracking of the aircraft during testing.
302(e)(2) System design issues [ETM 3.2.5.2] This section discusses DGPS system design issues including configuration, airport survey, DGPS receiver output data, and sources of error in DGPS systems.
302(e)(2)(A) Coordinate frames and waypoint navigation [ETM 3.2.5.2.1] The native coordinate system for GPS (i.e., the one in which its computations are performed) is the World Geodetic Survey of 1984 (WGS-84). Most GPS receivers provide output position information (latitude, longitude and altitude) in a variety of geodetic coordinate systems by transforming the WGS-84 position data.
Aircraft noise certification tests typically involve the use of a rectangular coordinate frame whose definition is based upon the array of microphones or the centerline of an airport runway. Typically, the frame’s x-axis is established from two points on the ground that are nominally aligned with the runway centerline; the y-axis is orthogonal to the x-axis and also level, and the z-axis is vertical. Some GPS receivers can furnish data in a rectangular coordinate system based on waypoints. These are user-defined reference points intended to facilitate navigation along a route or in a local area.
If a receiver supports waypoint navigatio n then two such points, defined in te rms of latitude, longitude and altitude, can be entered into the receiver. 1 The receiver will subsequently provide airc raft position relative to the coordinate frame implicitly defined by the points (i.e., the distance from the line connecting the two points and the distance to one point).
If waypoint navigation is to be used for noise testing, then the initial survey performed to determine the position of the two waypoints is critical to the accuracy of the TSPI results (see 302(e)(2)(b)). If waypoint navigation is not available, or is not to be used, then the geodetic position solution (i.e., latitude, longitude and altitude) must be transformed to a local coordinate system through post-processing by the applicant prior to noise data processing.
302(e)(2)(B) Test site survey [ETM 3.2.5.2.2] A careful survey of the airport and near by areas where noise testing is to be conducted is critical to the success of a measurement program. The following steps are involved in a survey:
a) An initial reference location, including numerical va lues for its latitude, longitude and altitude, is selected and its coordinates are stored in a permanent file for record keeping. Normally the initial reference location will be a surveyed monument on th e airport upon which latitude and longitude are stamped. Often the monument will have been derived from a third-order survey, in which case geodetic position errors of the order of hundreds of meters are not uncommon. However, such errors have virtually no effect on the measurement of positions relative to that point or another point derived from it. The published airport reference altitude can be assigned to the monument. Although this altitude typically is applicable to the base of the tower, the altitude difference between the mo nument and tower will not degrade the accuracy of differential measurements relative to the reference lo cation. Many GPS receivers have a “survey” mode whereby they average position measurements over a user-selected period of time (e.g., 24 hours) to generate a surveyed position estimate. Typical resulting absolute accuracies are 3 to 10 ft (0.9 to 3 m), which are more than adequate if the DGPS-based TSPI system meas urements will not be related to measurements from another system.2
1. For noise certification testing it is recommended that the GPS receiver read the waypoints from a printable data file. Alternatively, the waypoints could be keyed into the receiver and then written to a data file.
2. Prior to the advent of satellite-based techniques in the 1990s, land surveys were performed using an optical theodolite (to measure angles) and a chain (to measure linear distance). Networks of interlocking tria ngles were surveyed, with measur ements collected at each ver tex. The accuracy of such a survey was classified by the amount that the sum of the interior angles of a triangle deviated from 180º (af ter accounting for the earth’s curvature). A first-order su rvey was the most accurate; the vertices were typically 16 to 64 km (10 to 40 miles ) apart, and the angular error 1 arc second or less. Also, fo r a first-order survey, the latitude/longit ude of one point was measured by astrono mical means (accuracy approximately 15 m (50 ft)). A sec ond-order survey had vertices 8 to 16 km (5 to 10 miles) apart and maximum angular error of 5 arc minutes. A third-order survey had vertices 1.6 to 3.2 km (1 to 2 miles) apart and angular error not exceeding 15 arc minutes.
b) The DGPS-based TSPI system, with the ground-statio n antenna at the initial reference location, is used to measure the coordinates of the location where the ground-station antenna will be installed for the remainder of the test series. The latitude, longitude and altitude of this second location is stored in a permanent file for record keeping. If convenient, the ground-station antenna may be installed at the initial reference location for the duration of the test series.
c) If waypoint navigation is to be used for the meas urement program, the DGPS-based TSPI system, with the ground station at the second (i.e., normal) location, is used to measure the latitude, longitude and altitude of the FROM and TO waypoints which will be used to esta blish the test program coordinate frame. At least three measurements should be made to guard against errors. The resulting locations should be stored in a permanent file for record keeping.
d) The DGPS-based TSPI system, with the ground station at its normal location, is used to measure the microphone positions. The measured positions are stored in a permanent file for record keeping. If waypoint navigation is to be used for the measurement program then microphone positions should be recorded in test coordinates; otherwise latitude, longitude and altitude should be used.
e) If it is not feasible to use the DGPS-based TSPI sy stem to survey the micropho ne locations, then direct measurements of at least three common points should be performed in order that the relationship between the two surveys can be determined. For example, if the microphones are surveyed us ing classical techniques, then a DGPS-based TSPI survey of the two microphones at the ends of a microphone line and one other microphone, as far removed from the first two as possi ble, will be sufficient. The surveys should agree to within 1 ft (30 cm) at each common point. If they differ by more than 1 ft (30 cm) and the difference can be expressed in terms of an offset and a rotation, then it may be possible to adjust the results of one survey to agree with the other. Such adjustments should be approved by the FAA prior to testing.
The above tests should be performed, as a minimum, befo re and after each measurement program. Post-test data analysis should include a comparison of the two surveys.
302(e)(2)(C) Recei ver output data [ETM 3.2.5.2.3] This section addresses the GPS receiver messages3 (output data) which are of interest. All data are typically furnished via RS-232 serial ports (acceptable GPS receivers generally have multiple RS-232 ports).
Three kinds of GPS receiver output data are of interest:
a) data stored during flight testing for use during post-t est processing of noise data , collected from either the aircraft receiver when a real-time data link is used or from both receivers when a real-time data link is not used;
b) differential correction data output by the ground-statio n receiver, transmitted to th e aircraft via a real-time data link, and input to the aircraft receiver. These data are not stored, but directly influence the accuracy of the stored data addressed in a); and
c) data collected from the ground-station GPS receiver during multi-path verification tests prior to flight testing.
3. Standards organizations and manufacturers employ different terminology for pre-defined groups of data parameters available f rom receiver output ports. For example, in the United States, the National Marine Electronics Association (NMEA) uses the term “sentences,” the Radio Technical Commission for Maritime Services (RTCM) uses “message s,” Novatel Communications uses “logs” and Trimble Navigation Lt d.
uses “cycle printouts.”
302(e)(2)(C)(i) Data stored du ring aircraft noise testing when a real-time data link is used [ETM 3.2.5.2.3.1] GPS receivers provide TSPI data in a variety of formats, both industry-standard and proprietary. In the United States, the National Marine Electronics Association (NMEA) has issued standards (References T7 and T8) which are intended to facilitate user communications with GPS receivers and other navigation de vices. Some GPS manufacturers have adopted NMEA standards, some use pr oprietary formats, and some use both . Manufacturers that provide NMEA outputs generally implement only a subset of the full set of messages set forth in the standards, and some follow the older Version 1.5 (Reference T7) rather than Version 2.0 (Reference T8), upon which this guidance was based.
GPS receiver manufacturers have chosen different parameters to indicate the quality or status of the TSPI data. DGPS- based TSPI systems considered for noise certification tests, using a real-time data link, should save data from the aircraft GPS receiver in the receiver’s na tive raw format, in permanent files, for record keeping. Stored data should include time (e.g., Coordinated Universal Time (UTC) or GPS time with or without a local offset), aircraft latitude, longitude and altitude, or equivalently, aircraft position relative to a pre-defined waypoint, together with a status or quality flag indicating the reliability of the DGPS solution.
Typically the applicant will employ post-processing software which will read the raw data, parse and format these data, perform any necessary transformations, and generate a file which will be used for noise data processing. Storage of raw data allows the FAA to verify the validity of the post-processed results.
302(e)(2)(C)(ii) Data st ored during aircraft noise testing when a real-time data link is not used [ETM 3.2.5.2.3.2] DGPS-based TSPI systems considered for noise certification tests which do not use a real-time data link should save data from both the ground and aircraft GPS receivers in raw (i.e., the receiver’s native) format, in permanent files, for record keeping. Manufacturers’ proprietary formats should be used since NMEA standard messages do not support this application.
For post-processing, stored data should include time (e.g., UTC or GPS time) with or without a local offset, satellite ephemeris (see 302(e)(2)(F)(iv)) for a discussion of satellite ephemeris/clock data), pseudo-ranges,4 signal-to-noise ratios,5 and carrier phase.6 Applicants using dual-frequency (L1/L2) receivers will typically also save L2 carrier phase data.7 Typically, post-processing of the ground-based and airb orne GPS data will be performed using manufacturersupplied software. If this is not the case, then any applicant-developed software should be approved by the FAA.
302(e)(2)(C)(iii) Real-time DGPS messages [ETM 3.2.5.2.3.3] GPS receiver manufacturers have implemented both industry-standard and proprietary messages for use on real-time DGPS data links. The Radio Technical Commission for Maritime Services (RTCM), Special Committee 104 (SC -
104), has issued a standard (Reference T10) that is followed by most manufacturers. Manufacturers usually implement
4. Pseudo-range is the receiver’s measured distance to a satellite and is derived from the Coarse/Acquisition (C/A) code. It in cludes a receiver clock bias error and may be quantified in units of time or distance.
5. Signal-to-noise ratio (also called carrier-to-noise ratio) is derived from the receiver’s tracking loop circuits and is a me asure of the received signal strength. It is usually quantified in dB-Hz and varies from approximately 33 to 50.
6. Carrier phase is the amount of carrier cycles (at 1 575.42 MHz) which have accumulated since logging of this parameter was begun. It may be quantified in radians, degrees, cycles or feet (to convert to cycles, divide by the wavelength, 0.6247 ft).
7. The highest accuracy DGPS systems employ the signal carrier (L2 = 1 575.42 MHz), rather than the code (L1 = 1.023 MHz), which modulates the carrier, as the basic measurement observable. These technique s require that the number of full carrier cycles, i.e., 8-inch wavelengths, between the ground station and aircraft be determined once during a test. After the cycle count is established, the ground-station/aircraft separation is tracked to fractions of a wavelength, provided that the receiver carrier tracking loops (circuits) maintain phase lock.
only a subset of the RTCM/SC-104 messages, and some follow the older Version 2.0 of Reference T10 rather than Version 2.1, upon which this guidance was developed. Some manufacturers have also implemented proprietary DGPS messages which frequently bear a close resemblance to the RTCM/SC-104 messages.
For applicants implementing a real-time DGPS data link, it is preferred that RTCM/SC-104 messages be employed for this purpose. Type-1 or Type-9 messages, each of which contains the actual DGPS corrections, should be selected and transmitted at a rate of 0.5 Hz or higher. Other message types (e.g., Type-3 ground-station location and Type-5 satellite health) may be used but should be sent at a rate of once per minute or slower. There is no recommended requirement for storing real-time DGPS correction data. The da ta status or quality flag (see 302(e)(2)(C)(i)) should however provide an indication that the correction data have been properly received and processed by the aircraft.
302(e)(2)(C)(iv) Messages for multi-path testing [ETM 3.2.5.2.3.4] Applicant-designed systems using code-based DGPS processi ng should collect and save data from dedicated multipath tests to be conducted prior to aircraft noise testin g (see 302(e)(2)(E)). Data co llected during multi-path tests should include individual satellite pseudo-ranges and signal-to -noise ratios. These parameters are provided only by receiver manufacturers’ proprietary messages. It is not necessary for applicants to conduct a dedicated test for systems using carrier-based DGPS processing.
302(e)(2)(D) System accuracy and sources of DGPS error [ETM 3.2.5.2.4] If only divergence (spherical spreading) of the noise is considered, and atmospheri c absorption mechanisms are ignored, then a 0.1 dB change in the noise level corresponds to a change of approximately 1.1 percent of the distance between the aircraft noise source and th e measurement microphone. Thus, for an aircraft altitude of 400 ft (122 m), the approximate minimum altitude during noise certification tests, a position error of 4.3 ft (1.3 m) along the line-ofsight vector connecting the microphone and aircraft can be expected to introduce a 0.1 dB error in the processed noise data. A position error of 10 ft (3.0 m) along the line-of-sight vector can be expected to introduce a 0.23 dB error in the processed noise data.
For most DGPS systems, the most important error sources are, in decreasing order of importance, multi-path, correction latency and tropospheric delay. When these error sources are properly controlled, DGPS systems can provide accuracies between a few centimeters and approximately 15.1 ft (4.6 m) for an aircraft in low-dynamics flight regimes. Even the poorest of these accuracies is superior to that achieved by other conventional TSPI systems used for aircraft noise tests, including micr owave and photo-scaling. The best accuraci es are superior to those of a laser tracker.
DGPS systems suitable for consideration by noise certification applicants can be expected to achieve an accuracy of a few inches to 5 ft (1.5 m). The highest accuracy is achieved using carrier-based techniques and post-flight processing of data collected from both the aircraft and ground-sta tion computers. Code-based solutions which use carrier smoothing (e.g., Novatel RT-20) achieve accuracies of 3.0 ft to 4.9 ft (0.9 m to 1.5 m), provided that the error sources discussed in this section are addressed properly. Consequently it is expected that the DGPS systems used for noise certification tests will introduce less than a 0.2 dB error into the noise data in a worst-case scenario (i.e., a noise certification approach measurement). Typical errors will be less than 0.1 dB for noise certification flyover and lateral measurements.
In addition to the three error sources cited above, increases in DGPS position errors can also occur when the manufacturer and model of the ground station and aircraft GPS receivers are not the same, or when the ground station and aircraft receivers use different satellite ephemeris/clock data to specify the satellite orbital parameters.
Sections 302(e)(2)(E)and 302(e)(2)(F) address all of the above errors and include methods for minimizing these errors or eliminating them entirely.
302(e)(2)(E) Multi-path errors [ETM 3.2.5.2.5] 302(e)(2)(E)(i) Characteristics [ETM 3.2.5.2.5.1] Multi-path refers to signals from GPS satellites which are reflected from objects (e.g., the ground, buildings and aircraft structural elements) before reaching the GPS antenna. Multi-path signals add algebraically to the desired lineof-sight signal and thereby decrease the accuracy of meas urements made with the latter. Multi-path conditions can occur independently at the aircraft and ground station antennas. Thus the differential correction data from the ground are not directly useful for correction for multi-path errors at the aircraft antenna. Rather, the broadcast corrections can contain ground-station multi-path errors which, in a statistical sense, add to those in the aircraft.
Measurements have consistently shown that the presence of multi-path conditions at the ground station is significantly more deleterious than at the aircraft. This is because ground-station multi-path conditions vary slowly, acting like a bias over a test run of a few minutes, whereas the more dyna mic motion of the aircraft causes the effects of airborne multi-path conditions to behave like noise, which can be reduced somewhat by processing techniques such as filtering and averaging.
For code-based processing, ground station multi-path error is typically between 1.0 ft and 10 ft (0.3 m and 3 m). Under very adverse conditions (e.g., GPS antenna near the side of and well below the top of a large building) multi-path errors can be several hundred meters. Multi-path errors associated with carrier-based processing techniques are significantly less than those for code-based methods and are usually of the order of centimeters.
The extent of the multi-path error primarily depends on two factors: the capability of the ground station antenna and the location of the ground station antenna relative to reflec ting objects such as paved runways, buildings and parked aircraft. Receiver processing (e.g., the us e of narrow correlators available in most Novatel receivers) and/or carrier smoothing, available from several manufacturers, can reduce multi-path errors.
302(e)(2)(E)(ii) Code-based system ground station [ETM 3.2.5.2.5.2] To mitigate the effects of multi-path conditions on DGPS-based TSPI performance, the applicant’s ground station installation should meet the following requirements:
a) the ground station should employ a multi-path-limiting antenna, such as one with a choke ring or an absorbing ground plane; and
b) the ground station antenna should be mounted on a pole or tower, with unobstructed visibility of the sky.
A minimum height of 10 ft (3 m) above ground level is recommended for the ground-station antenna.
Additionally, to ensure that significant undetected multi-path errors do not corrupt the TSPI data collected during aircraft noise testing, the applicant’s ground-station inst allation should be tested for adequate multi-path condition performance prior to commencing the flight test. This can be done by collecting GPS receiver data during the same hours of the day that the system will be used for noise tests, with additional one-hour buffers on either side of this period. The data are then examined on a per-satellite basis, rather than navigation solution basis, for multi-path signatures. This examination should include at least pseudo-ranges and signal-to-noise ratios.
If multiple periods of significant multi-path errors (i.e., several feet) are found, then a new location for the groundstation antenna should be selected and te sted. If only one or two isolated, brief multi-path incidents are found, then the antenna location can be retained but aircraft testing should not be conducted during these periods.
Note.— The satellite-user geometry repeats over a cycle of approximately 23 hours 56 minutes. Thus if a ground Page 74
station multi-path incident is observed one day, it is expected that a similar incident will occur 4 minutes earlier the following day.
These procedures are similar to those utilized by the United States Coast Guard in checking out a marine DGPS station installation (References T10 and T19).8 After establishing a ground-station antenna site/configuration that satisfies the multi-path conditions criterion, the ground-station antenna should not be moved without performing another multipath test. The ground-station GPS receiver and any computer used in conjunction with the receiver may be removed and reinstalled without repeating the multi-path test. The multi-path verification test data should be saved as part of the permanent test-series data archive and should be made available for inspection by the FAA.
302(e)(2)(E)(iii) Carrier-bas ed system ground station [ETM 3.2.5.2.5.3] To mitigate the effects of multi-path conditions on DGPS-b ased TSPI performance, the applicant’s ground-station installation should meet the following recommended specifications:
a) the ground station should employ a multi-path-limiting antenna, such as one with a choke ring or an absorbing ground plane; and
b) the ground station antenna should be mounted on a pole or tower, with unobstructed visibility of the sky. A minimum height of (9.8 ft (3.0 m) above ground level is recommended for the ground-station antenna.
There is no recommended requirement for collecting data to assess multi-path errors when carrier-based processing is employed.
302(e)(2)(E)(iv) Ai rcraft installation [ETM 3.2.5.2.5.4] It is expected that aircraft manufacturers will select a location on each aircraft model that minimizes multi-path effects.
In this regard no recommended specifications have been developed. For most smaller aircraft (e.g., 10 seats or fewer), it has been found that the roof area directly behind the windshield is most advantageous. Manufacturers of larger aircraft have found forward positions on the roof to be desirable, although some have mounted the GPS antenna on the tail structure. Selecting a location for the GPS antenna on a helicopter may be more challenging since the main rotor will momentarily obscure most areas on the airframe.
302(e)(2)(F) Other sources of DGPS error [ETM 3.2.5.2.6] 302(e)(2)(F)(i) Correction latency [ETM 3.2.5.2.6.1] Correction latency, also called staleness, refers to the delay between the time of validity of a differential correction at the ground station and the time that the correction is applied in the aircraft. Delays in pro cessing at both ends of the ground-to-air data link can cause stale corrections to introduce unacceptably large errors.
A second form of latency, solution latency, refers to the delay between the time at which a GPS receiver’s measurement is valid and the time when it is available at the output of the receiver. Solution delays are inherently smaller than correction
8. Coast Guard DGPS ground stations employ two GPS receiver/antenna pairs. The “additiona l” receiver/antenna pair (termed the i ntegrity monitor) provide a real-time continuous ch eck on the validity of the differential corrections generated by the “basic” receiver /antenna pair (termed the reference station). DGPS ground-station architectures be ing investigated for the FAA LAAS program employ between 2 and 4 receiver/antenna pairs to verify the corrections sent to the aircraft. No requirement for redundant ground-station equipment is recommended for DGPS-based TSPI systems used in noise certification tests.
delays and, in this context, are of concern only for aircraft guidance.
For a system with a real-time data link which employs code-based DGPS solutions, it is strongly recommended that ground-to-aircraft messages conform to the RTCM/SC-104 standards used by the Coast Guard DGPS system.9 These messages contain pseudo-range rates-of-change, as well as th e correction at an identified time, to allow the user to correct for most of the latency-induced er ror. It is also preferred that the corr ections be computed and transmitted at least at a 0.5 Hz rate.
302(e)(2)(F)(ii) Tropospheric delay [ETM 3.2.5.2.6.2] The troposphere is that portion of the atmosphere between the earth’s surface and an altitude of approximately 20 miles (32 km). Differences in meteorological conditions between the ground station and the aircraft can cause dissimilar changes in the propagation times of signals from a satellite to these two locations. The effect is most pronounced for low-elevation-angle satellites. Since these changes are no t common to the two locations, they are not removed by differential corrections. Such tropospheric effects can contribute up to 66 ft (20 m) of ranging error on GPS signals, which can translate into as much as 33 ft to 39 ft (10 m to 12 m) of positioning error if not modeled and corrected. In differential mode this positioning error is typically less than 6.6 ft (2 m). Approximately 90 percent of these tropospheric propagation-related errors are due to the hydrostatic, or dry, component of tropospheric delay.
Experiments performed for the FAA LAAS program have found tropospheric differences to introduce DGPS errors between 1 ft and 3 ft (0.3 m and 0.9 m) when the aircraft was at 3000 ft (914 m) altitude, but only a few centimeters when the receiver antennas were at the same altitude. To reduce the effects of tropos pheric errors on DGPS-based TSPI systems used in noise certification tests, it is recommended that use of these systems be limited to the aircraft being within a lateral distance of 20 nm (37 km) and a height of 5000 ft (1524 m) relative to the ground station.
If desired, the hydrostatic component of the tropospheric delay can be effectively removed with the application of the tropospheric delay model (Reference T9) developed by the Radio Technical Commission for Aeronautics (RTCA) as per ICAO Annex 10 Standards and Recommended Practices, al ong with local meteorological measurements at the ground station. The relevant portion of this model is driven by local barometric pressure and satellite geometry (i.e., elevation angle). Reference T18 provides a functional overview of the RTCA model, as well as comparisons with other tropospheric propagation delay models.
302(e)(2)(F)(iii) Mismatched GPS receivers [ETM 3.2.5.2.6.3] Experiments have shown that DGPS errors are increased when the GPS recei vers at the ground station and in the aircraft are not “matched” in terms of manufacturer and model. With mismatched receivers, errors are increased moderately (e.g., 1.5 to 3 times) compared to those when the receivers are matched and when the satellites are operating normally. When a rare, soft satellite-failure, or signa l degradation, occurs, errors of several thousand feet have been observed.10 It is required that applicants’ systems have the same manufacturer/model GPS receiver on the ground and in the aircraft.
302(e)(2)(F)(iv) Mismatched sa tellite ephemeris/clock data [ETM 3.2.5.2.6.4]
9. The United States Coast Guard DGPS system’s broadcast messages (as well as marine systems of other nations) include the rate-of-change of each pseudo-range error, in addition to the pseudo-range error at a reference time. The user’s receiver is required to apply an adjusted correction consisting of the broadcast pseudo-range error, plus its rate-of-change multiplied by the time elapsed between the time the adjusted correction is applied, and the validity time for the pseudo-range correction.
10. Beginning on or before 21 October 1993, some differential users with mismatched ground and aircraft receivers experienced position errors of thousands of feet. The GPS Joint Program O ffice (JPO) of the United States Department of Defense attr ibuted the cause to a “def iciency” in Coarse/Acquisition code broadcast by satellite SVN19. It announced that the problem was corrected on 10 January 1994. Official statements are found in Notice Advisory to NAVSTAR User (NANU) 343-93294, 396-93337 and 006-94010.
GPS satellite broadcasts include a navigation message in the form of 50 bits per second modulation superimposed on the pseudo-random codes used for ranging. Within the navigation message are data sets that describe the satellite orbit
(i.e., ephemeris information) and clock. These data sets are transmitted every 30 seconds. The GPS Control Segment uploads multiple ephemeris and clock data sets to the satellites, typically once per day.
Note.— The Control Segment is the ground-based portion of the total GPS system. It includes the Operational Control facility in Colorado Springs, Colorado, United States, where the satellite ephemeris and clock data are calculated, five worldwide sites which collect satellite broadcast signals and provide data to the operational control facility, and three locations from which new ephemeris/clock data are uploaded to the satellites.
Satellites typically change their broadcast ephemeris and clock message every four hours. The ephemeris/clock data sets are used by a receiver to compute its own position and, in the case of a reference station, differential corrections for use by other receivers. For a DGPS system to achieve full accuracy, both the ground station and aircraft receiver must use the same ephemeris and clock data sets. Internal receiver logic ensures that the ephemeris and clock data sets used by a given receiver are consistent for each satellite. However, occasionally the ground and aircraft receivers may use different ephemeris/clock data sets unless measures are taken by the user to ensure that the sets match. Mismatched ephemeris/clock data sets can occur for several reasons, e.g., a receiver is too busy performing other tasks when the data sets change, or a receiver encounters an error while decoding new data and continues to use an old data set.
The RTCM/SC-104 messages used by the Coast Guard DGPS system guard against mismatched ephemeris/clock data sets by including the issue of data (I OD), an eight-bit data set label broa dcast by each satellite, in the broadcast messages (References T10 and T19). User receivers which conform to the RTCM/SC-104 standards will not apply differential corrections unless the IOD from the satellite and the DGPS correction message agree. The applicant should ensure that the ground station and aircra ft use the same ephemeris and clock data sets during testing. One way is to purchase GPS receivers and select DG PS messages which cause this check to be performed automatically. Another way to ensure agreement between the ground and aircraft ephemeris/clock data sets is to store in a permanent file for record keeping, at a rate of once each 30 seconds, the IOD used by each receiver and compare the IODs during posttest processing.
302(e)(3) System approval recommendations [ETM 3.2.5.3] This section summarizes approval recommendations for DGPS-based TSPI systems proposed for use during noise certification tests.
302(e)(3)(A) Design issues [ETM 3.2.5.3.1] Each applicant’s TSPI system design should address the issues identified in Table 4. The applicant’s documentation (see 302(e)(3)(C)) should address each item in the table.
Table 4. TPSI systems design development issues Number Issue Major considerations 1 Selection of processing method (real-time versus post-test) Need for aircraft guidance, ability to check testrun quality 2 Selection of solution method (carrier versus code) Accuracy (favors carrier), robustness (favors code), cost (favors code) 3 Use of geodetic or waypoint coordinates Waypoints can simplify post-processing but are not available for all receivers 4 Selection of GPS receiver and antenna Items 1, 2, 3 and others (antenna multi-path control, data messages, solution latency, matched air/ground receivers, and IOD capability) 5 Selection of data link equipment (if real-time system) Assigned frequency, data rate, error detection/ correction, flexible interface 302(e)(3)(B) Data storage (logging) during noise testing [ETM 3.2.5.3.2] 302(e)(3)(B)(i) For a system with real-time data link [ETM 3.2.5.3.2.1] For applicants employing a real-time data link, the ground-station GPS receiver should output RTCM/SC-104 Type- 1 messages at a rate of 0.5 Hz or great er, which should be transmitted to and us ed by the aircraft GPS receiver. The applicant’s aircraft computer should collect data from the aircraft GPS receiver and generate permanent data files containing:
a) the three-dimensional aircraft pos ition copied directly from the receiver’s data port (i.e., in raw/native form) and not processed;
b) if waypoint navigation is used, the waypoints (i.e., latitude, longitude and altitude) used to define the local coordinate frame;
c) the time (e.g., UTC or GPS time), with or without a local offset, associated with each sample of position data copied directly from the receiver’s data port; and
d) the data quality/validity indication associated with each sample of position data.
If waypoints are used they should be included in the header of each data file. New waypoints should not be able to overwrite existing waypoints. If new waypoints are defined then a new data file should be created.
For consistency with the noise data collected during a certif ication test, it is recommended that the data associated with a), c) and d) above be saved in th e GPS receiver’s raw/native format at a rate greater or equal to 2 Hz, the rate associated with the noise data. However, if hardware limitations preclude following this recommendation, a sampling rate of 0.5 Hz or greater is acceptable.
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