AC 20-199 CHAPTER 3. RADIO ALTIMETER SYSTEM TESTING
3.1 Radio Frequency Interference Testing.
3.1.1 The RFI injection test setup and requirements are specified in appendix B of this AC and may be done:
• At the RA transceiver line replacement unit (LRU), or
• By testing the RF Filter Subassembly (RFFSA) and RA transceiver separately.
3.1.2 For the LRU level, the conducted interference power levels associated with the system ITM (PFD levels) are directly injected into the RA transceiver RF ports. When testing the RFFSA and RA transceiver separately, test the RFFSA at the maximum level (direct conducted) and use the resulting data to reduce the level of interference into the RA transceiver. The equations that define the interference power levels are provided in equations B-1 to B-3 of appendix B in this AC, which assumes specific values for installation parameters, including antenna gain and cable loss. Under these interference power levels, the RA transceiver must meet the measurement validity, accuracy, and noise requirements. As stated in note 1 in appendix B, section B.4, of this AC, the assumed antenna gain must be the gain specified in the RA transceiver IM that is sufficient to maintain the required RA performance, and it may include engineering judgment to bound the antenna gain at all applicable frequencies in real aircraft installations.
3.1.3 Demonstration of compliance may be done using the RA transceiver manufacturer’s data, and an analysis showing that the data is applicable to the installation (i.e., the installed antenna gain and cable loss is within the RA transceiver assumed parameters) across all frequency bands specified in table 2-1 of this AC.
3.2 Installed RA System Testing (as applicable to the change impact).
Installed RA system testing should include testing with the aircraft on the ground using simulated or real inputs or with the aircraft in flight using real inputs. The TC/STC applicant should:
• Demonstrate that the RA system performs appropriately with real terrain returns for all integrated systems. Various types of complex terrain scenarios should be assessed. There is no need to perform this demonstration in the presence of RFI interference.
• Demonstrate that the accuracy and performance are not degraded by installing a new RA system. Negligible degradation may be acceptable if it is shown that all integrated systems continue to meet their intended function, safety, and performance requirements.
• Demonstrate compliance using the manufacturer’s data for the RA transceiver, antenna, or display, as applicable, and an analysis showing the data is applicable to the installation.
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AC 20-199
3.3 Integrated System Testing (as applicable to the change impact).
The TC/STC applicant should perform regression and additional integrated system testing appropriate to the design change and its impact on the integrated systems. When replacing one RA with another, it is acceptable to demonstrate that any degradation in performance (e.g., accuracy) is within the integrated system’s performance requirements. Testing should be conducted as applicable to the systems, functions, and operations integrated into the aircraft. Demonstration of compliance may be accomplished using the RA transceiver, antenna, or by displaying the manufacturer’s data. Refer to appendix A of this AC for additional guidance.
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AC 20-199 Appendix A APPENDIX A. INTEGRATED SYSTEM REFERENCES Table A-1. Standards and Policy for Integrated Systems Operational Application System or Function Reference TSO Reference Advisory Circular Terrain Avoidance1 TAWS FAA or EASA TSO-C151d AC 23-18 AC 25-23 HTAWS FAA or EASA TSO-C194 EASA TSO-2C522 AC 27-1B AC 29-2C Collision Avoidance1 TCAS/ACAS I FAA or EASA TSO-C118a TCAS/ACAS II FAA or EASA TSO-C119e AC 20-131A AC 20-151C ACAS X FAA or EASA TSO-C219a Windshear Protection1 Weather Radar FAA TSO-C63g EASA TSO-C63f AC 25-12 AC 20-182A Predictive Windshear Reactive Windshear FAA or EASA TSO-C117b Flight Guidance Systems Autopilot (AFGCS/FD) FAA or EASA TSO-C198 AC 120-29A/120-28D AC 25.1329-1() AC 20-138() Autothrottle Flight Guidance Alerting Approach and Landing Systems (including Autoflare) 1 These safety systems are required by regulation for certain aircraft operations by multiple state aviation authorities.
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AC 20-199 Appendix A Operational Application System or Function Reference TSO Reference Advisory Circular FMS Guidance/ Approach Flight Control Systems Tail Strike Avoidance Performance and safety requirements tied to aircraft system integration Ground Spoiler/ Speedbrake Deployment Cockpit Information Instrument Approach DH Determination and Alerting (Audio or Visual) Performance and safety requirements tied to aircraft operation Crew Alerting System Triggers or Inhibitions for Various Warnings and Alerts Situational Awareness of Radio Altitude Vision Systems EFIS SAE AS6296, SAE 8034B, FAA or EASA TSO-C209 AC 25-11B EVS/CVS/EFVS AC 20-167() SVS/SVGS AC 20-185() Night Vision Goggle (NVG) Operations FAA or EASA TSO-C164a Surveillance System Transponder FAA or EASA TSO-C166() ADS-B Out AC 20-165 A-2
AC 20-199 Appendix B APPENDIX B. RADIO FREQUENCY INTERFERENCE TOLERANCE TESTING Note: This appendix contains an acceptable means of compliance for RA transceivers or systems to demonstrate compliance with the performance requirements in § 91.220. The TC/STC applicant is only expected to address this testing for RA system installations that have not demonstrated compliance under a TSO authorization.
B.1 RF Interference Test Setup—Receive Port (RX) Injection.
A simplified block diagram of one possible test setup used for RF interference injection into the receive port of the RA transceiver, either at the LRU level or as a subassembly, is shown in figure B-1 of this AC.
Figure B-1. RF Interference Test Setup—RX Injection The 4.2–4.4 GHz band stop filter is used to suppress broadband noise from the power amplifier when performing out-of-band interference tests. When performing in-band tests where the required interference level does not necessitate the use of the power amplifier, or near-band tests where the use of the band stop filter will unacceptably distort the interference waveform, the band stop filter and power amplifier can be omitted or bypassed.
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AC 20-199 Appendix B B.2 RF Interference Test Setup—Transmit Port (TX) Injection.
A simplified block diagram of one possible test setup used for RF interference injection into the transmit port of the RA transceiver, either at the LRU level or as a subassembly, is shown in figure B-2 of this AC.
Figure B-2. RF Interference Test Setup—TX Injection As in the RX injection test setup, the 4.2–4.4 GHz band stop filter and power amplifier can be omitted or bypassed when performing in-band tests where the required interference level does not necessitate the use of the power amplifier, or near-band tests where the use of the band stop filter will unacceptably distort the interference waveform.
B.3 RF Interference Test Setup—RF Filter Subassembly.
A simplified block diagram of the test setup used for RF interference testing of an RFFSA of the RA transceiver is shown in figure B-3 of this AC. This test setup is used only when the RFI testing is performed through a combination of subassembly tests rather than a full LRU-level test.
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AC 20-199 Appendix B Figure B-3. RF Interference Test Setup Subassembly B.4 RA Transceiver RFI Injection Test.
The RA system must meet the measurement validity, accuracy, and noise when injecting RFI into the RF or TX ports of the RA transceiver with a conducted power level calculated using equation B-1 of this AC:
(Equation B-1) 𝐵𝑊 × 106 𝑐𝑃𝑡𝑒𝑠𝑡(𝑓𝑐) = 𝑃𝐹𝐷 (𝑓𝑐 ± ) + 30 − 10 × log10(4𝜋) + 20 × log10 ( )+ 𝐺(𝑓2 𝑓 𝑐)𝑐 + 10 × log10(𝐵𝑊) − 𝐿𝑐𝑜𝑎𝑥 Where:
𝑃𝑡𝑒𝑠𝑡(𝑓𝑐) = input power at RA Transceiver TX or RX port, in dBm 𝑃𝐹𝐷(𝑓) = power flux-density specified in table 2-1 at the frequency 𝑓, in dBW/m2/MHz 𝑐 = speed of light, in meters per second 𝑓𝑐 = center frequency of waveform specified in table B-1, in Hz 𝐺(𝑓𝑐) = assumed RA antenna gain at the frequency 𝑓𝑐, in dBi 𝐵𝑊 = bandwidth of waveform specified table B-1, in MHz 𝐿𝑐𝑜𝑎𝑥 = coaxial cable loss between the RA antenna and the RA transceiver, in dB Note 1: The assumed antenna gain is the gain specified in the RA transceiver IM that is sufficient to maintain the required RA performance.
Note 2: For RFI injection testing into the TX port, the coaxial cable loss is the minimum required cable loss in the TX path specified in the RA transceiver IM. For RFI injection testing into the RX port, the coaxial cable loss in is the minimum required cable loss in the RX path as specified in the RA transceiver IM.
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AC 20-199 Appendix B B.5 RF Filter Subassembly (RFFSA) and RA Transceiver RFI Injection Test.
If the RFI injection testing is performed using separate tests of an RFFSA and a transceiver subassembly, the measurement validity, accuracy, and noise must meet the requirements with the test levels at the RF port of the transceiver subassembly calculated using either equation B-2 of this AC, if the RFFSA is tested with continuous waveform (CW) only, or equation B-3 of this AC, if the RFFSA is tested with the same waveforms as the transceiver subassembly.
(Equation B-2) 𝐵𝑊 𝐵𝑊𝑃𝑇𝑋𝑅(𝑓𝑐, 𝑊𝑛) = 𝑃𝑡𝑒𝑠𝑡(𝑓𝑐, 𝑊𝑛) − min [𝐹𝐷𝑅(𝑓𝑐 − ) , 𝐹𝐷𝑅(𝑓𝑐 + )]2 2 Where:
𝑃𝑇𝑋𝑅(𝑓𝑐, 𝑊𝑛)= input power at transceiver subassembly TX or RX port, in dBm 𝑃𝑡𝑒𝑠𝑡(𝑓𝑐, 𝑊𝑛) = test level at input port of RFFSA calculated from equation B-1, in dBm 𝐹𝐷𝑅(𝑓) = RFFSA frequency-dependent rejection at frequency 𝑓 measured with CW, in dB 𝑓𝑐 = center frequency of waveform 𝑊𝑛, as specified in table B-1, in MHz 𝐵𝑊 = bandwidth of waveform 𝑊𝑛, as specified in table B-1, in MHz (Equation B-3) 𝑃𝑇𝑋𝑅(𝑓𝑐, 𝑊𝑛) = 𝑃𝑡𝑒𝑠𝑡(𝑓𝑐, 𝑊𝑛) − 𝐹𝐷𝑅(𝑓𝑐, 𝑊𝑛) Where:
𝑃𝑇𝑋𝑅(𝑓𝑐, 𝑊𝑛)= input power at transceiver subassembly TX or RX port, in dBm 𝑃𝑡𝑒𝑠𝑡(𝑓𝑐, 𝑊𝑛) = test level at input port of RFFSA calculated from equation B-1, in dBm 𝐹𝐷𝑅(𝑓𝑐, 𝑊𝑛) = RFFSA frequency-dependent rejection at frequency 𝑓𝑐 measured with waveform 𝑊𝑛, in dB 𝑓𝑐 = center frequency of waveform 𝑊𝑛, as specified in table B-1, in MHz Table B-1. RFI Test Waveforms Frequency 3000-4100 MHz 4100-4200 MHz 4200-4400 MHz 4400-4500 MHz 4500-5600 MHz Waveform 2 3 1 3 2 B-4
AC 20-199 Appendix B B.6 RFI Test Waveforms.
1. Band-limited additive white Gaussian noise (AWGN) fully within the 4.2-4.4 GHz frequency band with a minimum bandwidth of 99 percent of the actual bandwidth of the transceiver unit under test (UUT) (a maximum bandwidth of 200 MHz), a minimum peak-to-average power ratio (PAPR) of 10 dB, and a 100 percent duty cycle.
2. Band-limited AWGN fully outside of the 4.2-4.4 GHz frequency band with a minimum bandwidth of 100 MHz, minimum PAPR of 10 dB, and 100 percent duty cycle.
3. Band -limited AWGN fully outside of the 4.2-4.4 GHz frequency band with a minimum bandwidth of 10 MHz, minimum PAPR of 10 dB, and 100 percent duty cycle.
Note 1: The ITM average power represents the constraining scenario from an RFI compatibility perspective. However, to fully characterize the RA performance, peak power testing should be considered to capture data to support any future applications.
Note 2: If testability constraints are encountered for waveform 2, a reduced PAPR may also be acceptable for testing that exceeds the average power to meet an equivalent peak power (i.e., over test of the average power). These methods demonstrate that the RA system can meet or exceed both the ITM average power and any peak power spikes simultaneously.
B.7 RFI Test Altitudes.
Simulated HAGL greater than 10 ft and less than or equal to 75 ft.
Simulated HAGL greater than 75 ft and less than or equal to 200 ft.
Simulated HAGL greater than 200 ft and less than or equal to 500 ft.
Note 1: The test loop losses at each altitude should include the worst-case loop loss in accordance with TSO-C87().
Note 2: RA performance requirements for operations above 500 ft HAGL are not addressed in § 91.220(b). Up to 500 ft represents the constraining scenario for the RA equipment and is critical for compatibility with the RFI environment. However, to fully characterize the RA performance, it is recommended to test using simulated HAGL greater than or equal to 500 ft, less than or equal to 2500 ft, greater than 2500 ft, and up to and just above the maximum HAGL of the RA to capture data to support any future applications.
B.8 RFI Testing with Multiplex Installations.
The ITM specified in table 2-1 of this AC must be met in the presence of all aircraft RA systems (i.e., dual or triple installations).
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AC 20-199 Appendix B B.9 Environmental Degradation.
The ITM specified in table 2-1 of this AC must be met, accounting for any degradation in performance due to the operating environment. The temperature of the equipment has an impact on performance, so the demonstration should be conducted at the maximum equipment operating temperature.
B.10 UUT and Test Equipment Tolerances.
The testing should account for unit-to-unit variations in the UUT and test equipment uncertainty.
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AC 20-199 Appendix C APPENDIX C. RELATED MATERIAL Note: The regulations and guidance should be applicable to the aircraft or design change certification basis or to a later revision.
C.1 Title 14, Code of Federal Regulations (14 CFR).
The following 14 CFR regulations are related to this AC. You can download the full text of these regulations from the Federal Register website at http://www.ecfr.gov/.
• § 23.2335, Lightning protection.
• § 23.2500, Airplane level systems requirements.
• § 23.2505, Function and installation.
• § 23.2510, Equipment, systems, and installations.
• § 23.2515, Electrical and electronic system lightning protection.
• § 23.2520, High-intensity Radiated Fields (HIRF) protection.
• § 23.2600, Flightcrew interface.
• § 23.2605, Installation and operation.
• § 23.2610, Instrument markings, control markings, and placards.
• § 23.2615, Flight, navigation, and powerplant instruments.
• § 23.2620, Airplane flight manual.
• § 23.1529, Instructions for Continued Airworthiness.
• § 25.777, Cockpit controls.
• § 25.581, Lightning protection.
• § 25.1301, Function and installation.
• § 25.1302, Installed systems and equipment for use by the flightcrew.
• § 25.1309, Equipment, systems, and installations.
• § 25.1310, Power source capacity and distribution.
• § 25.1316, Electrical and electronic system lightning protection.
• § 25.1317, High-intensity Radiated Fields (HIRF) Protection.
• § 25.1321, Arrangement and visibility.
• § 25.1322, Flightcrew alerting.
• § 25.1329, Flight guidance system.
• § 25.1331, Instruments using a power supply.
• § 25.1333, Instrument systems.
• § 25.1351, General.
• § 25.1431, Electronic equipment.
• § 25.1529, Instructions for Continued Airworthiness.
• § 27.610, Lightning and static electricity protection.
• § 27.777, Cockpit controls.
• § 27.1301, Function and installation.
• § 27.1303, Flight and navigation instruments.
• § 27.1309, Equipment, systems, and installations.
• § 27.1316, Electrical and electronic system lightning protection.
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AC 20-199 Appendix C
• § 27.1317, High-intensity Radiated Fields (HIRF) Protection.
• § 27.1321, Arrangement and visibility.
• § 27.1322, Warning, caution, and advisory lights.
• § 27.1329, Automatic pilot and flight guidance system.
• Part 27, Appendix B / 29.1431.
• § 27.1529, Instructions for Continued Airworthiness.
• § 29.610, Lightning and static electricity protection.
• § 29.777, Cockpit controls.
• § 29.1301, Function and installation.
• § 29.1303, Flight and navigation instruments.
• § 29.1309, Equipment, systems, and installations.
• § 29.1316, Electrical and electronic system lightning protection.
• § 29.1317, High-intensity Radiated Fields (HIRF) Protection.
• § 29.1321, Arrangement and visibility.
• § 29.1322, Warning, caution, and advisory lights.
• § 29.1329, Automatic pilot and flight guidance system.
• § 29.1331, Instruments using a power supply.
• § 29.1333, Instrument systems.
• § 29.1351, General.
• § 29.1431, Electronic equipment.
• § 29.1529, Instructions for Continued Airworthiness.
• § 91.220, Radio Altimeter Systems.
• § 135.160, Radio altimeters for rotorcraft operations.
C.2 FAA Advisory Circulars.
The following ACs are related to the guidance in this AC. The latest version of each AC referenced in this document is available on the FAA website at FAA Advisory Circulars and on the Dynamic Regulatory System (DRS).
• AC 20-73A, Aircraft Ice Protection.
• AC 20-115D, Airborne Software Development Assurance Using EUROCAE ED- 12() and RTCA DO-178().
• AC 20-131A, Airworthiness Approval of Traffic Alert and Collision Avoidance Systems (TCAS II) and Mode S Transponders.
• AC 20-136B, Aircraft Electrical and Electronic System Lightning Protection.
• AC 20-138D, Airworthiness Approval of Positioning and Navigation Systems.
• AC 20-151C, Airworthiness Approval of Traffic Alert and Collision Avoidance Systems (TCAS II), Versions 7.0 & 7.1 and Associated Mode S Transponders.
• AC 20-152A, Development Assurance for Airborne Electronic Hardware.
• AC 20-155A, Industry Documents to Support Aircraft Lightning Protection Certification.
• AC 20-158B, The Certification of Aircraft Electrical and Electronic Systems for Operation in the High-Intensity Radiated Fields (HIRF) Environment.
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