301(c)(2) Frequency-dependent system response corrections [ETM 3.1.3.2] Corrections for system response may include consideration of some or all of the following frequency-dependent elements to account for variations in sensitivity differences relative to the calibration check frequency (see A36.3.9.1 of part 36).
Note.— To avoid confusion and to promote consistency, it is recommended that all response corrections be determined and presented in such a manner that the correction values are to be added to the measured aircraft noise levels to correctly adjust for the effects of system se nsitivity differences. Example: The correction value for a microphone response at a frequency wher e the microphone is more sensitive to sound than at the calibration check frequency should be presented as a negative number which, when added to the measured aircraft noise levels in that frequency band, lowers them to what would have been measured if the microphone response were flat.
(1) Microphone free-field frequency response Corrections for microphone free-field frequency response may be determined using electrostatic actuator testing in combination with manufacturer-provided corrections for free-field conditions, or alternately from anechoic free-field testing using a method approved by the FAA (see A36.3.9.2 of part 36).
(2) Microphone incidence corrections Microphone incidence corrections may be required when the microphone is not set at nominally grazing incidence. The corrections may be determined using manufacturer’s data, or alternately from anechoic free-field testing using a method approved by the FAA (see A36.3.9.3 of part 36).
(3) Windscreen free-field insertion effects The windscreen free-field insertion effects may be determined using manufacturer’s data, or alternately from anechoic free-field testing using a method approved by the FAA (see A36.3.9.10 of part 36).
Note.- If incidence corrections are required for microphone free-field response, then incidence corrections will likely also be required for the windscreen insertion effects.
(4) System frequency response The frequency response corrections for the measurement sy stem as deployed in the field for aircraft noise measurements (not including the microphone and windscreen, but including any cables, attenuators, gain stages, signal conditioners, etc.) should be determined using a method approved by the certificating authority. Acceptable methods may include the use of discrete- or swept- si ne tones, or random or pseudo-random pink noise. The specific methods and techniques used should be submitted in advance to the FAA for approval. When using pink noise to determine system frequency response, there are additional specifications for noise generator performance and calibration specified in part 36. When using analog (direct or FM) magnetic tape recording, it is required that system frequency response be determined in the field while deployed for aircraft noise measurements (see A36.3.9.5 of part 36).
(5) Effective bandwidth The effective bandwidth (also known as “bandwidth error” or “filter inte grated response”) for one-third octave band analyzer filters may be determined using manufacture r’s data, or from tests using discrete or swept sine signals as described in IEC 61260 (Reference T2). Note that bandwidth corrections might be redundant, dependent on the specific testing methodology used, if the measur ement system response, including the analyzer, is determined using pink noise or other broadband signal testing.
301(c)(3) Single-frequency level calibrations [ETM 3.1.3.3] The following calibrations are typically performed at the calibration check frequency:
(1) Sound calibrator output level The output level of the sound calibrator should be dete rmined using means which are traceable to a national standards laboratory. This may include direct calibratio n by a certified metrology laboratory or comparison calibration methodology. Laboratory calibrations often include a certificate stating the sound pressure level, its tolerance, reference temperature and humidity, and the time period for which the calibration remains valid. This and other documentation may be required by the FAA for its approval. Sound calibrator output level corrections for barometric pressure, coupler volume, and other envi ronmental effects may be determined according to the manufacturer’s specifications.
(2) Calibration of gain or attenuation settings The accuracy of gain or attenuation settings (range change s) for switchable components in signal conditioners, amplifiers, analyzers and sound level meters should be determined from manufactur er’s specifications or by laboratory testing, as approved by the FAA.
(3) Sound pressure level sensitivity of the acoustical measurement system Sensitivity calibrations of the entire system must be performed before and after each day’s aircraft noise measurements using an approved sound calibrator. These calibrations should be performed in such a way that all system components, including cables, are accounted for.
301(c)(4) Field sound pressure level calibration considerations [ETM 3.1.3.4] It is necessary to establish and monitor the overall sound pressure level sensitivity of the measurement system while it is deployed in the field. Considerations for such field sound pressure level calibrations include:
(1) Schedule for sound pressure level calibrations For aircraft noise certification tests defined in Appendices A, G, H and J, an acoustic calibration signal of known amplitude and frequency should be applied to the entire measurement system, including the microphone, while deployed in the field. This calibration should be performed as a minimum at the start and end of each day of aircraft noise measurements. When analog (direct or FM) magnetic recording media are used, the signal from the sound calibrator should also be recorded at the beginning of each physical volume of recording media (e.g. each tape reel, cartridge, or cassette) as well as at the end of the last physical volume of recording media. In addition, sound pressure level calibrations should be considered at regular intervals throughout each day of aircraft noise measurements (see A36.3.9.8, A36.3.9, and J36.109(e)(i) of part 36). As an example, sound pressure level calibrations are sometimes scheduled to coincide with meteorological flights between noise measurement runs, or during test aircraft refueling for longer test programs. Applicants should include the scheduling of such calibrations in the noise measurement test plan.
(2) Aircraft noise data validity Aircraft noise data that are not preceded and succeede d by valid sound pressure level calibrations are not acceptable for aircraft noise certification purposes. Part 36 specifies that the difference between pre- and postcalibrations must not be greater than 0.5 dB. Because of this limitation, frequent checks of system sensitivity and functionality are advised (see A36.3.9.8 and J36.109(e)(2)(ii) Page 62
(3) Insert “checks” Depending upon the specific measurement system config uration, and with the approval of the FAA, level sensitivity checks (e.g. charge or voltage “insert” calibrations, which involve electrical signals injected into the microphone preamplifier) at frequent intervals (e.g. every two hours of aircraft noise measurement deployment) may be used to supplement the requirement for sound pressure level calibrations at the start and end of each day of aircraft noise measurement. Employing such a procedure can facilitate continuous testing activity, and provide supporting data to isolate and identify potential system failures. Insert checks should not be considered as substitutes for required sound pressure level calibrations.
(4) System configuration during sound pressure level calibrations in the field During field sound pressure level calibration, all components of the measurement system including cables, attenuators, gain and signal-conditioning amplifiers, filters (including pre-emphasis, if used) and power supplies, but excluding the windscreen, should be in place. Where used, attenuators and gain stages should be set to prevent overload and to maintain the calibration signal level within the reference level range specified in A3.6.6 of part
36. If any switchable filters that could affect the calibration signal are utilized during measurements, then sound pressure level calibrations should be performed both with and without these filters enabled. Components of the electrical system should not be added, removed or replaced without re-calibrating any part of the system affected by that component immediately before and after making each change.
(5) Adjustments for differences between pre- and post- calibrations Section A3.9.8 of part 36 requires that the arithmetic m ean of the sound pressure level calibrations before and after a group of aircraft noise measurements must be us ed to represent the acoustical sensitivity level of the measurement system for those measurements. No such requirement is provided for in Appendices G or J of part
36. Nevertheless the FAA may approve such an adjustment to the system’s acoustical sensitivity level. In all cases, and as an alternative to the ar ithmetic mean of the before and after sound pressure level calibrations, the FAA may accept that for a particular aircraft noise ev ent the acoustical sensitivity level of the system be represented by a linear interpolation over time of the sound pressure level calibrations before and after the aircraft noise measurement to the time of the data acquisition for that aircraft event.
301(c)(5) Application of calibration corrections [ETM 3.1.3.5] It is recommended that all calibration corrections be determ ined and presented in a manner such that they are to be added to the measured aircraft levels to properly correct for system instrumentation effects (see Note to 301(c)(2)).
All calibration corrections should be determined individually, reported with full documentation of the method of determination, and applied to the measured aircraft noise levels during data processing of analyzed one-third octave band sound pressure levels. This includes sound calibrator output adjustments for ambient conditions such as temperature and atmospheric pressure, coupler volume, etc. (see A36.3.9.7 of part 36), as well as any sensitivity “drift” corrections, or any system response corrections.
Note 1.— Such adjustments should not be applied by adjusting the calibration value in the analyzer or sound level meter.
Note 2.— In the event that the sound pressure level sensitiv ity of the analyzer or sound level meter needs to be reset after the initial calibration at the start of the test day, and after aircraft noise measurements have already been obtained, then prior to such reset, the sound pressure level calibration signal should be measured and recorded at the existing level sensitivity. In this way, a traceable record of the system sensitivity can be maintained.
301(c)(6) Calibration Traceability [ETM 3.1.3.6] Page 63
All sound calibrator output levels and performance testin g of calibration instrumentation should be traceable to a national standards laboratory. The method of traceability may in some cases include documentation of any comparison calibration methods performed by the applicant, instrumentation manufacturer, or another third party. All methods of traceability must be approved by the FAA.
301(d) Windscreen Insertion Loss [ETM 3.1.4] The physical condition of a windscreen can significantly affect its performance due to insertion loss. Only new or clean, dry windscreens should be used.
Insertion loss data adjustments for wi ndscreens used during aircraft noise evaluations under the provisions of Appendices A and H of part 36 may be obtained from manufacturer’s data or by free-field calibration in an anechoic chamber (see 401(c)(14) of this AC).
For aircraft noise evaluations made under the provisions of Appendices G and J of part 36, windscreen insertion loss data adjustments may be required.
301(e) Wind Speed Limitations [ETM 3.1.5] The wind speed should be monitored against the specified wind speed limits. In cases where these limits are exceeded during an aircraft test run, that test run is invalid and might have to be repeated. No method has been approved for making data adjustments for wind speed or direction.
301(f) Measurement of Atmospheric Conditions [ETM 3.1.6] The temperature and relative humidity near the earth’s surface can be affect ed by numerous factors including solar heating, surface winds, local heating or cooling, increased or decreased local humidity, etc. To avoid localized anomalous conditions that often occur n ear the ground, meteorologi cal measurements are to be made 33 ft (10 m) above the surface for aircraft noise measurements made unde r the provisions of Appendices A and H of part 36 and between 4 ft (1.2 m) and 33 ft (10 m) for aircraft noise measurements made under the provisions of Appendices G and J of part 36. The meteorological conditions measured above the ground are assumed to be constant from the ground surface to the height at which they are measured.
301(g) Meteorological Measurement System [ETM 3.1.7] Atmospheric conditions should be measured within 6562 ft (2000 m) of the microphone locations and must be representative of the conditions existing over the geographical area in which noise measurements are conducted.
If an applicant can show that measurements from a fixed meteorological station, such as might be found at a nearby airport comply with these requirements, then subject to the approval of the FAA, this facility may be used. Approval will normally require the applicant to show that the measurement systems have been calibrated within 90 days prior to the tests.
In general, an approved portable system is preferred. This will be especially important when any of the meteorological conditions, in particular wind speed, are near their limits. Applicants should note that in order to determine the crosswind component an accurate measure of not only wind speed but also wind direction is required. Some wind direction indicators installed at airfields and airports have a slow response to rapid changes in wind direction and are not sufficiently accurate.
302 301(h) Aircraft Position Measurement [ETM 3.1.8] Appendices A, G, H and J specify that the aircraft position must be determined by a method independent of normal flight instrumentation. For this purpose the FAA has approved differential global positioning systems (DGPS), radar tracking, theodolite triangulation or photographic scaling techniques (see 302).
301(i) Measurement System Specifications [ETM 3.1.9] 301(i)(1) Validity of measu rent system configuration [ETM 3.1.9.1] Each applicant should submit information to the FAA about the measurement system used. The FAA will determine whether any listed components require approval.
301(i)(2) Changes in measu rement system configuration [ETM 3.1.9.2] If an applicant makes changes to the approved measurement system configuration, the FAA should be notified before aircraft noise certification testing to determine whether additional evaluation and approval are required.
301(j) Noise Compliance Demonstration Plan [ETM 3.1.10] An applicant should prepare a noise compliance demonstration plan (see 20 1(d)(1) of this AC), that specifies the proposed certification process, including the use of any equivalencies. This plan is to be submitted to the FAA allowing sufficient time to permit adequate review and possible revisions prior to the start of any noise certification testing.
FLIGHT PATH MEASUREMENT [ETM 3.2] The criteria for the measurement of aircraft height and late ral position relative to the intended track are described in A36.2.3.1, F36.109(f)(6), G36.109(f), H36.101(d)(2), and J36.101(d)(2) of part 36. Examples of methods used include:
a) radar tracking system;
b) theodolite triangulation;
c) photographic scaling; and
d) differential global positioning system (DGPS)-based time-space-position information tracking systems.
Practical examples of aircraft tracking systems employing one or more of these techniques are described in subsequent sections. Other tracking systems such as inertial navigation systems (INS) and microwave systems, which have a high degree of accuracy, have been installed in aircraft and consequently have been accepted by several FAA for use during noise certification. These techniques may be used singly or in combination. This is not intended to be an exhaustive list and additional information will be included as more experience is acquired.
302(a) Radar or Microwave Tracking System [ETM 3.2.1] One example of a radar position tracking system is shown in Figure 3. It operates on the principle of pulse radar with a Page 65
radar interrogator (receiver/transmitter) located on the aircraft and a radar transponder (receiver/transmitter) positioned at each reference station. The elapsed time between the receiver/transmitter pulse and reception of the pulse returned from the reference station transponder is used as the basis for determining the range of each reference station. This range information, together with the known location of the reference stations, can be used to obtain a fix on the position of the aircraft in three dimensions.
4 32 1 Flight path Transponder site Microphone sites Acoustics test range position transponder layout K/T D BA C APATS positioning system with four reference stations Figure 3. Example radar/microwave position tracking system A pulse coding system is employed to minimize false returns caused by radar interference on reflected signals. The system performs the following basic functions during noise certification:
a) continuously measures the distance between the aircraft and four fixed ground sites;
b) correlates these ranges with In ter-Range Instrumentation Group Standard Serial Time Code Format B (IRIG B) time code and height information and out puts these data to a pulse code modulation (PCM) recorder;
c) converts the aircraft range and height information into X, Y and Z position coordinates in real time; and
d) uses the X, Y, Z data to drive a cockpit display providing the pilot with steering and position cueing.
The accuracy of the coordinate calculation depends on the flight path and transponder geometry. Errors are minimized when ranges intersect, and the recommended practice is to keep the intersection angle near to 90. The four transponder arrangements shown in Figure 3 produce position uncertainties from ±3 ft to ±7 ft (±1.0 m to ±2.0 m).
At low aircraft heights some inaccuracies can be introduced with the use of microwave systems. The use of a radioaltimeter can reduce these errors. The height data are recorded and synchronized with the microwave system.
302(a)(1) Aircraft equipment [ETM 3.2.1.1] The distance measuring unit computer and transponder beacon are connected to a hemispherical antenna which is mounted under the fuselage, on the aircraft centerline, preferably as close to the aircraft center of gravity as possible.
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.