a) to accomplish the work activity directly associ ated with the purpose for which it is designed;
b) to provide flight crewmember training in the special purpose op eration for which the airplane is designed; or
c) to conduct “nondispensing aerial work operations” related to agriculture, horticulture, or forest preservations.
Chapter 3
TECHNICAL PROCEDURES APPLICABLE FOR NOISE CERTIFICATION OF ALL AIRCRAFT TYPES
CHAPTER TABLE OF CONTENTS
301 NOISE MEASUREMENT PROCEDURES58
301(a) Test Site Selection58
301(a)(1) Terrain58
301(a)(2) Grass59
301(a)(3) Snow59
301(a)(4) Plowed fields59
301(a)(5) Obstructions59
301(a)(6) Anomalous meteorological conditions60
301(b) Precautions Regarding Effects of Precipitation60
301(b)(1) General precautions60
301(b)(2) Effect of moisture on microphones60
301(b)(3) Microphone internal heaters60
301(c) Calibration of Acoustical Measurement System60
301(c)(1) General calibration process60
301(c)(2) Frequency-dependent system response corrections61
301(c)(3) Single-frequency level calibrations62
301(c)(4) Field sound pressure level calibration considerations62
301(c)(5) Application of calibration corrections63
301(c)(6) Calibration traceability63
301(d) Windscreen Insertion Loss64
301(e) Wind Speed Limitations64
301(f) Measurement of Atmospheric Conditions64
301(g) Meteorological Measurement System64
301(h) Aircraft Position Measurement65
301(i) Measurement System Specifications65
301(i)(1) Validity of measurent system configuration65
301(i)(2) Changes in measurement system configuration65
301(j) Noise Compliance Demonstration Plan65
302 FLIGHT PATH MEASUREMENT65
302(a) Radar or Microwave Tracking System65
302(a)(1) Aircraft equipment66
302(a)(2) Ground equipment67
302(b) Kine-theodolite System67
302(c) Radar/theodolite Triangulation67
302(d) Photographic Scaling67
302(e) DGPS-based Time-Space-Position Information Tracking Systems68
302(e)(1) General68
302(e)(2) System design issues69
302(e)(2)(A) Coordinate frames and waypoint navigation70
302(e)(2)(B) Test site survey70
302(e)(2)(C) Receiver output data71
302(e)(2)(C)(i) Data stored during aircraft noise testing when a real-time data link is used72
302(e)(2)(C)(ii) Data stored during aircraft noise testing when a real-time data link is not used72
302(e)(2)(C)(iii) Real-time DGPS messages72
302(e)(2)(C)(iv) Messages for multi-path testing73
302(e)(2)(D) System accuracy and sources of DGPS error73
302(e)(2)(E) Multi-path errors74
302(e)(2)(E)(i) Characteristics74
302(e)(2)(E)(ii) Code-based system ground station74
302(e)(2)(E)(iii) Carrier-based system ground station75
302(e)(2)(E)(iv) Aircraft installation75
302(e)(2)(F) Other sources of DGPS error75
302(e)(2)(F)(i) Correction latency75
302(e)(2)(F)(ii) Tropospheric delay76
302(e)(2)(F)(iii) Mismatched GPS receivers76
302(e)(2)(F)(iv) Mismatched satellite ephemeris/clock data76
302(e)(3) System approval recommendations77
302(e)(3)(A) Design issues77
302(e)(3)(B) Data storage (logging) during noise testing78
302(e)(3)(B)(i) For a system with real-time data link78
302(e)(3)(B)(ii) For a system not using real-time data link78
302(e)(3)(C) Documentation79
302(e)(3)(D) Accuracy verification test79
302(e)(3)(E) Software verification79
302(e)(3)(F) Ground-station multi-path mitigation and verification79
302(e)(3)(F)(i) All systems80
302(e)(3)(F)(ii) Code-based systems80
302(e)(3)(G) Airport survey80
303 ON-BOARD FLIGHT DATA ACQUISITION80
303(a) General80
303(b) Magnetic Tape Recording81
303(c) Automatic Still Photographic Recording81
303(d) Cine Recording81
303(e) Video recording81
304 TIME SYNCHRONIZATION OF MEASURED DATA82
304(a) General82
304(b) TSPI Equipment and Software Approval82
304(c) Continuous Time-Dode Recording82
304(d) Recording of Single Time Marker83
304(e) Measurement of the Interval Between Recorder Start and Overhead83
304(f) Setting of Internal Time-Stamp Clock83
304(g) Additional Time-Synchronization Considerations83
305 CALCULATION OF CONFIDENCE INTERVALS84
305(a) Introduction84
305(b) Confidence Interval for the Mean of Flight Test Data84
305(b)(1) Confidence Interval for the Sample Estimate of the Mean of Clustered Measurements84
305(b)(2) Confidence interval for mean line obtained by regression85
305(c) Confidence Interval for Static-Test-Derived NPD Curves87
305(d) Confidence Interval for Analytically Derived NPD Curves88
305(e) Adequacy of the Model88
305(e)(1) Choice of engine-related parameter88
305(e)(2) Choice of regression model88
305(f) Worked Example of the Determination of 90 percent Confidence Intervals89
305(f)(1) Introduction89
305(f)(2) Confidence interval for a clustered data set90
305(f)(3) Confidence interval for a first order regression curve91
305(f)(4) Confidence interval for a second order regression curve95
305(f)(5) Confidence interval for the pooled data set96
305(g) Student’s t-distribution (for 90 percent Confidence) for Various Degrees of Freedom96
306 ADJUSTMENT OF AIRCRAFT NOISE LEVELS FOR THE EFFECTS97
306(a) Introduction97
306(b) Definitions98
306(c) Background Noise Adjustment Procedure99
306(c)(1) Assumptions99
306(c)(2) Step-by step description100
306(c)(2)(A) Determination of pre-detection noise100
306(c)(2)(B) Determination of post-detection noise100
306(c)(2)(C) Testing of pre-detection noise versus post-detection noise100
306(c)(2)(D) Determination of masking criteria100
306(c)(2)(E) Identification of masked levels101
306(c)(2)(F) Determination of Last Good Band101
306(c)(2)(G) Adjustment of valid levels for background noise101
306(c)(2)(H) Adjustment of valid levels for measurement conditions101
306(c)(2)(I) Reconstruction of low frequency masked bands101
306(c)(2)(J) Reconstruction of levels for masked high frequency bands102
306(c)(2)(J)(i) Frequency extrapolation method102
306(c)(2)(J)(ii) Time extrapolation method103
306(c)(2)(K) Handling of spectra after reconstruction of masked bands103
306(d) General Considerations104
306(d)(1) Limitations and requirements for any background noise adjustment procedure104
306(d)(2) Rejection of spectra due to masking104
305(d)(3) Special tone correction considerations due to masking104
306(d)(4) Handling of masked data in reference conditions data sets105
307 NOISE REDUCTION SYSTEMS105
307(a) Variable Noise Reduction Systems105
307(a)(1) Reference procedures106
307(a)(2) Test conditions and procedures106
307(a)(3) Adjustments to measured noise data106
307(a)(4) Guidance for specific VNRS107
307(b) Selectable Noise Reduction Systems107
308 CALCULATION OF THE SPEED OF SOUND107
309 REFERENCE TABLES USED IN THE MANUAL CALCULATION OF107
301 NOISE MEASUREMENT PROCEDURES [ETM 3.1] The applicant should submit to the FAA for its review an d approval technical procedur es for the measurement of aircraft noise certification levels that the applicant will use. The technical procedures described in the following sections of this chapter should be considered as genera lly appropriate for all aircraft types. See Chapter 4 for procedures specific to Appendices A and H. See Chapter 5 for procedures specific to Appendix G. See Chapter 6 for procedures specific to Appendix J.
301(a) Test Site Selection [ETM 3.1.1] For airplanes, when the flight path intercept equivalent test procedure is used, and for helicopters, it may not be necessary for the test site to be located at an airport. De tails of the proposed noise certification test-site locations should be submitted to the FAA for review and approval. Some test-site criteria that could support selection of a nonairport test site include level terrain, reduced air traf fic, reduced ambient noise, improved weather conditions (temperature, humidity and wind), improved microphone placement, availability of field surveys, improved locations for aircraft position monitoring and improved pilot sight and handling.
301(a)(1) Terrain [ETM 3.1.1.1] Page 58
Uneven terrain having features such as mounds or furrows can result in reflections that could influence the measured sound levels. Vegetation can reduce the amount of sound that is reflected from the ground surface. In most cases this effect results in a reduced sound level, but under some circumstances the level may be higher. Testing over a smooth hard surface, such as a paved area, will generally result in a higher sound level.
301(a)(2) Grass [ETM 3.1.1.2] For noise measurement points under the flight path 25 ft (7.5 m) radius circles of mowed grass (not exceeding 3 in (8 cm) height) are acceptable. For noise meas urement points located to the side of the flight path, the grass may be mowed in a semicircle of 25 ft (7.5 m) radius facing the line of flight.
301(a)(3) Snow [ETM 3.1.1.3] Snow in the area surrounding the noise measurement points may provide excessive absorption of aircraft sound reflected from the ground. Noise measurement points have been approved when snow within a 50 ft (15 m) radius of the noise measurement points has been removed. However, snow should not be piled at the borders facing the line of flight.
301(a)(4) Plowed fields [ETM 3.1.1.4] Earthen or sandy surfaces within a 25 ft (7.5 m) radius of the noise measurement points must be reasonably tamped down. Plowed furrows, silt or soft, powdered surfaces are unacceptable.
301(a)(5) Obstructions [ETM 3.1.1.5] Obstructions in the vicinity of the noise measurement points such as buildings, walls, trees, vehicles and test personnel, if close enough, may be unacceptable because of reflections that influence measured noise levels. There should be no obstructions that significantly influence the sound field from the aircraft with in a conical space above a point on the ground vertically below the microphone at each noise measur ement point. The cone is defined by an axis normal to the ground and by a half angle of 80º (75º for light propeller-driven airplanes) from the axis as illustrated in Table 2.
Figure 2. Obstruction-free cone defined from the base of the measurements microphone Page 59
301(a)(6) Anomalous meteorological conditions [ETM 3.1.1.6] Certain geographical areas are more susceptible to anomalous meteorological conditions than others (i.e., large variations, or inversions, of temperature or humidity, excessive turbulence, or thermally induced vertical winds). The applicant may conduct certification testing only as approved by the FAA.
301(b) Precautions Regarding Effects of Precipitation [ETM 3.1.2] 301(b)(1) General precautions [ETM 3.1.2.1] Fog, rain, drizzle and snow can have a number of adverse effects. Changes in sound generation and propagation under these conditions are not well documented. Most of the equipment used for measuring noise is not intended for use during conditions of precipitation, and the effects can range from changes in microphone and windscreen sensitivity or frequency response, to arcing of conventional condenser microphones, to possible failure of equipment because of electrical short circuits.
301(b)(2) Effect of moisture on microphones [ETM 3.1.2.2] Most microphones that are used during noise certification testing are susceptible to moisture. Precipitation, including snow, drizzle and fog, or excessive humidity may induce electrical arcing of the microphone sensors, making measured noise data unacceptable. Howeve r, some pre-polarized microphones are le ss susceptible to electrical arcing during high-moisture conditions (consult the equipment manufacturer’s specifications). Special care should be taken to ensure that any windscreens exposed to precipitation be thoroughly dry, inside and out, before use. Foam windscreens can trap water and wet foam windscreens should be avoided.
301(b)(3) Microphone internal heaters [ETM 3.1.2.3] When internal heaters are prov ided, microphones are less likely to be affected by moisture in wet, humid, cold or freezing atmospheric conditions.
301(c) Calibration of Acoustical Measurement System [ETM 3.1.3] 301(c)(1) General calibration process [ETM 3.1.3.1] The process for the calibration of an acoustical measurem ent system (see A36.3.1.1, A36.3.3.1, G36.103, and J36.109(c) of part 36 for definition of the “measurement system”) for purposes of noise certification testing consists of two parts:
a) determination of system response corrections; and
b) sound pressure level calibrations and testing in the field.
Some of these calibrations must be conducted at the aircraft noise test site, with the measurement system configured and deployed for aircraft noise measurement. Note that additional calibrations and checks may be allowed, or required, by the FAA.
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.