28 differing codes from being entered in the transponder and ADS-B system were ineffective and resulted in numerous false and misleading proximity alerts for ATC. Additionally, there are workload and safety concerns of requiring the pilot to enter the Mode 3/A code, IDENT, and emergency codes multiple times. Thus, if you do not provide a single point of entry for the mode 3/A code, IDENT, and emergency code, you must accomplish a human factors evaluation and an additional system safety assessment as follows:
3.7.3.5.1 Human Factors Evaluation.
Installations not providing a single point of entry must accomplish an evaluation of the pilot interface controls to ensure the design minimizes the potential for entry errors by the flightcrew, and enables the flightcrew to detect and correct errors that do occur. Evaluate the system interface design to ensure dual entry of the emergency status, IDENT, and Mode 3/A code does not introduce significant additional workload, particularly when communicating an aircraft emergency. Refer to section 4.1.5.4 of this AC for additional information on the human factors evaluation.
3.7.3.5.2 System Safety Assessment.
Transmission of false or misleading information is considered to be a major failure effect and may not occur at a rate greater than 1x10-5 per flight hour for ADS-B systems. Installations not providing a single point of entry must accomplish a safety assessment that demonstrates that the probability of the transponder and ADS-B system ever transmitting differing Mode 3/A codes is less than 1x10-5 per flight hour. The analysis must consider the potential of all pilot errors.
3.8 ADS-B Antenna Interface.
3.8.1 Antenna Location and Number Required.
The aircraft ADS-B antenna is an important part of the overall ADS-B OUT system because antenna systems are major contributors to the system link performance. The location and number of antennas required for the airborne ADS-B OUT system is a function of the equipment class of the selected broadcast link (UAT or 1090ES). Single bottom-mounted antenna (TSO-C166b and TSO-C154c A1S and B1S classes) installations are allowed. For the UAT link, 16 watts minimum transmit power at the antenna output is required. For the 1090ES link, 125 watts minimum transmit power at the antenna output is required.
3.8.2 Equipment Eligibility.
ADS-B antennas must meet requirements defined in the ADS-B equipment manufacturer’s installation manual.
3.8.3 Installation Guidance.
3.8.3.1 Using an Existing Antenna.
When using an existing antenna system, if the installation does not modify the existing antenna(s), cabling, or output specifications, the antenna installation does not have to be reevaluated.
3.8.3.2 Installing a New Shared Transponder/ADS-B Antenna.
Follow the transponder antenna installation guidance in AC 20-151().
3.8.3.3 Installing a New Stand-Alone UAT ADS-B Antenna.
If the UAT system is installed in an aircraft without a transponder or the installation will not use the existing transponder antenna, use the following guidance:
3.8.3.3.1 Antenna Location.
Mount antennas as near as practical to the centerline of the fuselage and locate them in a position to minimize obstruction in the horizontal plane.
3.8.3.3.2 Antenna Distance From Other Antennas.
The spacing between the UAT antenna and any transponder (Mode S or Air Traffic Control Radar Beacon System (ATCRBS)) antenna must provide a minimum of 20 dB of isolation between the two antennas. If both antennas are conventional omni-directional matched quarter-wave stubs, 20 dB of isolation is obtained by providing a spacing of at least 20 inches between the centers of the two antennas. If either antenna is other than a conventional stub, the minimum spacing must be determined such that 20 dB or more of isolation is achieved.
3.8.3.3.3 Transmit Power.
Transmit power will be verified during ground test.
3.8.3.3.4 Structural Analysis.
You may need to submit a structural analysis of new antenna installation to show compliance with the applicable regulations.
3.8.3.4 Antenna Diplexers.
Diplexers manufactured in accordance with TSO-C154b or TSO-C154c may be installed so UAT ADS-B equipment and a transponder may share the same antenna. The TSO-C154b and TSO-C154c diplexer installation instructions are required to have a limitation that ensures insertion of the diplexer does not exceed the maximum cable attenuation allowance between the transponder and antenna.
3.8.3.5 Single Antenna.
Single antenna systems must use a bottom-mounted antenna.
3.8.4 Configuration of Associated Parameters.
This section provides additional guidance on setting key ADS-B OUT parameters.
Definitions for each of the following associated parameters are included in
4.5.6Appendix A to this AC.
3.8.4.1 GNSS Antenna Offset and Position Offset Applied (POA).
Although not required to comply with § 91.227, it is highly encouraged for ADS-B equipment manufacturers to provide instructions to installers for setting this parameter and for installers to configure the offset during installation. The GNSS antenna offset information will be extremely valuable for surface ATC surveillance and future ADS-B IN surface situational awareness and surface collision alerting applications.
3.8.4.2 If the ADS-B equipment is interfaced to multiple GNSS position sources that use GNSS antennas in different locations on the aircraft, the installation must have provisions to ensure the appropriate GNSS antenna offset is being transmitted when the ADS-B equipment switches from one position source to another.
3.8.4.3 The POA setting of the GNSS antenna indicates if the broadcast position of the vehicle is referenced to either a) the aircraft’s ADS-B position reference point, or b) the lateral distance from centerline and longitudinal distance from the most forward part of the aircraft, (reference 4.5.6B.4.1).
Note: Either the transmitted position should be adjusted to the reference point described in paragraph 3.8.4.4 OR the GNSS antenna offsets should be provided. It is not required to do both.
3.8.4.4 The ADS-B position reference point is the center of the rectangle used to describe the length and width of the aircraft in the length and width code.
Refer to section 3.2.3.2 and figure 2 of this AC. For a more detailed description of POA, refer to RTCA/DO-338, Minimum Aviation System Performance Standards (MASPS) for ADS-B Traffic Surveillance Systems and Applications (ATSSA), section 3.2.4.1.
31 Figure 3. Position Offset Longitudinal Axis Reported Length (using next higher value) Reported Width (using next higher value) Position of GNSS antenna ADS-B reference point Center of defining rectangle (center of solid box) ● Center of leading edge of Length Width code “box” Antenna longitudinal offset Antenna lateral offset Lateral Axis
3.8.4.5 Single Antenna Bit.
For aircraft using a single antenna, this parameter should be set to one, “True”.
3.8.5 Mutual Suppression.
Follow the ADS-B equipment manufacturer’s guidance on interfacing the ADS-B OUT equipment to the mutual suppression bus.
3.9 Vertical Rate Source.
We recommend that the ADS-B system output the vertical rate field when available.
The vertical rate may come from a barometric air data computer, a GNSS source, or a system that filters barometric and geometric vertical rates. Vertical rate will typically come from a position source or an air-data computer. This section addresses this unique parameter, and augments section 3.3 and 3.4 of this AC, as applicable.
3.9.1 Equipment Eligibility.
Unlike position accuracy, vertical velocity accuracy is not transmitted in ADS-B messages. Thus it is important that vertical velocity sources integrated into the ADS-B
32 system meet minimum performance requirements at installation. Use the following guidance:
3.9.1.1 Hybrid Vertical Rate Source.
Vertical rate may be taken from a hybrid system that filters barometric vertical rate with an Inertial Reference Unit (IRU) vertical rate and GNSS vertical rate, provided the hybrid system was tested and approved to provide a vertical rate output with an accuracy that is at least as good as barometric vertical rate sources (such as TSO-C106). Hybrid vertical rate could come from a Flight Management System (FMS), Air Data and Inertial Reference System (ADIRS), or IRU. ADS-B equipment should transmit hybrid vertical rate solutions as barometric vertical rates.
3.9.1.2 Blended Vertical Rate Source.
Vertical rate may be taken from a blended system that filters IRU vertical rate and barometric vertical rate, provided the blended system was tested and approved to provide a vertical rate output with an accuracy that is at least as good as barometric vertical rate sources (such as TSO-C106).
Blended vertical rate could come from an FMS, ADIRS, or IRU. ADS-B equipment should transmit blended vertical rate solutions as barometric vertical rates.
3.9.1.3 Barometric Vertical Rate Source.
Barometric vertical rate may be taken from an air data computer meeting the minimum performance requirements of any revision of TSO-C106 or a vertical velocity instrument meeting the minimum performance requirements of applicable revisions of TSO-C8, Vertical Velocity Instruments (Rate-of-Climb). We recommend you use any revision of a TSO-C106 compliant air data computer if you interface barometric vertical rate to the ADS-B OUT equipment.
3.9.1.4 GNSS Vertical Rate Source.
Geometric vertical rate may be taken from any revision of TSO-C129, TSO-C145, TSO-C146, or TSO-C196 GNSS equipment if the position source has been qualified to provide vertical rate in accordance with
4.5.6Appendix B of this AC. Do not interface GNSS vertical velocity if the equipment has not been qualified in accordance with
4.5.6Appendix B.
3.9.1.5 Inertial Vertical Rate Source.
Vertical velocity from an inertial sensor that is not blended with barometric altitude should not be transmitted from the ADS-B system.
3.9.1.6 Barometric Altitude Source.
ADS-B systems should not derive a barometric altitude rate by sampling barometric altitude measurements. This could lead to misleading vertical
33 velocity information. If barometric vertical rate is not available, use geometric vertical rate.
3.9.2 Installation Guidance.
The vertical rate field can be populated with either barometric vertical rate or geometric vertical rate. There is no requirement to interface multiple vertical velocity sources.
We recommend that you use the following priority scheme when selecting or interfacing multiple vertical rate sources:
1. Hybrid vertical rate or blended vertical rate.
2. Barometric vertical rate.
3. GNSS vertical rate.
3.9.3 Configuration of Associated Parameters.
This section provides additional guidance on setting key ADS-B OUT parameters:
3.9.3.1 Vertical Rate.
Interface vertical rate from one or more of the sources listed in section
3.9.1 above. Ensure the source provides vertical rate in feet per minute, or ensure the ADS-B equipment can recognize the vertical rate basis and convert the vertical rate to feet per minute.
3.9.3.2 Vertical Rate Source.
The source bit for vertical rate should be coded as barometric when using barometric rate from an air data computer, or when using a blended or hybrid vertical rate. The source bit for vertical rate should only be coded as geometric when using vertical rate from a GNSS source.
3.10 Air-Ground Considerations.
3.10.1 Length and Width Code.
The length and width code is required by § 91.227, and is only transmitted in the surface position message. Thus, to comply with the rule, the aircraft must automatically determine its air-ground status and transmit the surface position message, which includes the length and width code, when on the ground.
3.10.2 Air-Ground Status.
For aircraft with retractable landing gear, the air-ground status determination is typically provided through a landing gear weight-on-wheels (WOW) switch. For aircraft that have fixed gear, the ADS-B system must still be able to determine the air-ground status of the aircraft. Installations that provide a means to automatically determine air-ground status based on inputs from other aircraft sensors may be acceptable if they can be demonstrated to accurately detect the status. For example, air-ground status may be derived from WOW switch and GPS velocity; or GPS
34 velocity, an airport database, and geometric altitude; or GPS velocity and airspeed.
These algorithms should be tested and validated during the installation approval.
Note 1: We recommend that any automatic air-ground determination be more robust than just a simple comparison of ground speed to a single threshold value. Field experience has shown that this method can lead to false air-ground status.
Note 2: Manual selection of the air-ground status is not acceptable.
Note 3: Rotorcraft may require unique logic for providing an accurate air-ground state.
A reliable method to determine the air-ground state should consider training requirements. Rotorcraft may consider hover taxi as in the air.
3.10.3 Mode S Transponder Inhibit.
TSO-C112d and TSO-C112e, Air Traffic Control Radar Beacon System/Mode Select (ATCRBS/Mode S) Airborne Equipment, requires Mode S transponders to inhibit the reply to Mode A/C/S all-call and Mode S-only all-call interrogations on the surface.
Mode S transponders with ADS-B functionality will now remain “ON” during surface operations; thus it is imperative that you ensure the transponder interface to the airground status is installed correctly and that the transponder does not reply to Mode A/C/S all-call or Mode S-only all-call interrogations on the surface.
Note: In deploying Airport Surface Detection Equipment version X (ASDE-X) at various airports, we have found transponder installations that have been improperly wired and therefore inappropriately respond to ATC and TCAS interrogations while on the airport surface.
3.11 Foreign Airspace Requirements.
3.11.1 Optional Parameters.
If operations are planned in a country that requires parameters not mandated in the United States, such as selected heading and selected altitude, follow the ADS-B equipment manufacturer’s installation guidance to interface those parameters.
35 CHAPTER 4. TEST AND EVALUATION
4.1 Ground Test.
4.1.1 Systems Interface Testing.
Verify the installed ADS-B equipment meets its intended function and transmits the appropriate information from each of the interfaced systems (including the position source, barometric altitude source, heading source, TCAS II, pilot interface, etc).
Coordinate with local ATC before broadcasting over the air to prevent being a source of interference to ATC or ADS-B IN equipped aircraft in the area. For example, transmitting airborne position reports with simulated airborne altitudes while on the surface will produce false targets for the ATC surveillance systems or airborne ADS-B IN equipped aircraft.
4.1.2 System Latency.
Latency is addressed through analysis rather than testing. Refer to section 3.1.3 and
4.5.6Appendix C of this AC.
4.1.3 Rule Compliance.
Ensure the ADS-B system meets the requirements of § 91.227.
4.1.3.1 Accuracy and Integrity Performance.
Ensure the installed system meets its stated accuracy and integrity performance under expected operating conditions. We recommend that you accomplish a GNSS performance prediction for the applicable time of your test to ensure the ADS-B system meets the predicted performance.
In absence of predicted GNSS performance, demonstrate that you meet all § 91.227(c)(1) requirements as listed in Table 2.
Table 2. Accuracy and Integrity Requirements NIC ≥ 7 Rc < 370.4 m (0.2 nm) NACP ≥ 8 EPU < 92.6 m (0.05 nm) NACV ≥ 1 < 10 m/s SIL ≥ 3 ≤ 1x10-7 per-hour or per sample SDA ≥ 2 ≤ 1x10-5 per-hour
4.1.3.2 Parameters.
Per § 91.227(d), ensure the following parameters are properly populated and transmitted.
4.1.3.2.1 The length and width of the aircraft;
4.1.3.2.2 An indication of the aircraft's latitude and longitude;
4.1.3.2.3 An indication of the aircraft's barometric pressure altitude;
4.1.3.2.4 An indication of the aircraft's velocity;
4.1.3.2.5 An indication if TCAS II or ACAS is installed and operating in a mode that can generate resolution advisory alerts;
4.1.3.2.6 If an operable TCAS II or ACAS is installed, an indication if a resolution advisory is in effect;
4.1.3.2.7 An indication of the Mode 3/A transponder code specified by ATC;
4.1.3.2.8 An indication of the aircraft's call sign that is submitted on the flight plan, or the aircraft's registration number, except when the pilot has not filed a flight plan, has not requested ATC services, and is using a TSO-C154c self-assigned temporary 24-bit address;
4.1.3.2.9 An indication if the flightcrew has identified an emergency, radio communication failure, or unlawful interference;
4.1.3.2.10 An indication of the aircraft's “IDENT” to ATC;
4.1.3.2.11 An indication of the aircraft assigned ICAO 24-bit address, except when the pilot has not filed a flight plan, has not requested ATC services, and is using a TSO-C154c self-assigned temporary 24-bit address;
4.1.3.2.12 An indication of the aircraft's emitter category;
4.1.3.2.13 An indication of whether an ADS-B In capability is installed;
4.1.3.2.14 An indication of the aircraft's geometric altitude;
4.1.3.2.15 An indication of the Navigation Accuracy Category for Position (NACP);
4.1.3.2.16 An indication of the Navigation Accuracy Category for Velocity (NACV);
4.1.3.2.17 An indication of the Navigation Integrity Category (NIC);
4.1.3.2.18 An indication of the System Design Assurance (SDA); and
4.1.3.2.19 An indication of the Source Integrity Level (SIL).
4.1.3.3 Position Accuracy.
Position the aircraft on a surveyed location and validate the position transmitted from the ADS-B system. Ensure the position transmitted is within the allotted NACP accuracy limit. For example, if the aircraft reports a NACP = 8, the ADS-B position should be within 92.6 meters,
0.05 nm. If the aircraft reports a NACP = 10, the ADS-B position should be within 10 meters. Refer to 4.5.6Appendix A of this AC for a complete list of NACP values. If the transmitted position accuracy is smaller or equal to the resolution of the test equipment, it is acceptable to use plus or minus one Least Significant Bit as the pass/fail criteria.
4.1.3.4 Barometric Altitude Accuracy.
Validate that the barometric altitude transmitted from the ADS-B system is accurate to within 125 feet. If the aircraft has a transponder installed, you must also validate that the ADS-B barometric altitude matches the transponder barometric altitude.
4.1.4 Electromagnetic Interface (EMI)/Electro Magnetic Compatibility (EMC) Testing.
Provide an EMI/EMC test plan that demonstrates compliance with 14 CFR 23.1431(a) and (b), 25.1353(a) and (b), 25.1431(a) and (c), 27.1301, 27.1309, 29.1353(a) and (b), and 29.1431(a) and (b) as appropriate. Accomplish EMI/EMC testing to ensure the ADS-B equipment does not provide an interference source on other installed systems on the aircraft. Additionally, ensure equipment already installed in the aircraft does not interfere with the ADS-B system. If the STC or TC only involves a software change to an existing approved Mode S transponder installation, and the software update will not affect the systems response to EMI, you do not need to accomplish EMI testing again.
4.1.5 Human Machine Interface.
Evaluate the flightcrew interface for the ADS-B OUT system, including the human-system interface and system behavior. The ADS-B OUT system must be compatible with the overall flightdeck design characteristics (such as access to controls, sunlight readability, night lighting, etc.) as well as the aircraft environment (such as vibrations).
4.1.5.1 Information Display.
Evaluate the ADS-B OUT system to ensure displayed information is easily and clearly discernible, and has enough luminance, size, and visual contrast for the pilots to see and interpret it. Ensure the pilots have a clear, unobstructed, and undistorted view of the displayed information elements.
Ensure information elements are distinct and permit the pilots to determine the source of the information elements if necessary, when there are multiple sources of the same kind of information.
4.1.5.2 Controls and Labeling.
Evaluate the controls for the pilot interface to ensure they are plainly marked as to their intended function, provide convenient operation, and prevent confusion and inadvertent operation of both the ADS-B system, and the other systems with which they interact. Evaluate the acronyms, labels, and annunciations to ensure they are used consistently in the flightdeck, and do not cause confusion or errors. If a control performs more than one function, evaluate the labels to ensure the labels include all intended functions, unless the function of the control is obvious. During evaluation, consider line select keys, touch screens or cursor controlled devices (such as trackballs) as these can be susceptible to unintended mode selection resulting from their location in the flightdeck (for example, proximity to a footrest or temporary stowage area).
4.1.5.3 Annunciations and Alerts.
Evaluate all ADS-B annunciations and alerts to ensure they are clear and unambiguous, and provide attention-getting and saliency appropriate to the type of alert. Compliance with AC 25.1322-1, Flightcrew Alerting;
AC 27.1322 (in AC 27-1B, Certification of Normal Category Rotorcraft);
and AC 29.1322 (in AC 29-2C, Certification of Transport Category Rotorcraft) should be considered when evaluating ADS-B annunciations and alerts. The colors yellow/amber and red should be restricted to cautions and warnings, respectively. Evaluate the annunciations and indications to ensure they are operationally relevant and limited to minimize the adverse effects on flightcrew workload. When an annunciation is provided for the status or mode of a system, it is recommended that the annunciation indicate the actual state of the system, and not just the position of a switch.
4.1.5.4 Pilot Interface Errors.
Installations not providing a single point of entry for the ADS-B and transponder for the Mode 3/A code, IDENT, and emergency status must accomplish an evaluation of the pilot interface controls to determine that they are designed to minimize entry errors by the flightcrew, and enable the flightcrew to detect and correct errors that do occur. System interface design must also be evaluated to ensure dual entry of the Mode 3/A code, IDENT, and emergency status does not introduce significant additional workload, and that the controls are acceptable for data entry, accuracy, and error rates, particularly when communicating an aircraft emergency.
Evaluations should consider pilot-detected and undetected error rates, pilot workload, and training times. Refer to section 3.7.3.5 of this AC for additional information on transponder and ADS-B system single point of entry.
4.1.5.5 Lighting.
Evaluate all foreseeable conditions relative to lighting, including failure modes such as lighting and power system failure, and day and night operations.
4.1.6 Transponder Regression Testing.
At a minimum, use the procedures outlined in AC 43-6(), Altitude Reporting Equipment and Transponder System Maintenance and Inspection Practices, to validate that the transponder is operating normally following the ADS-B installation. Use the procedures outlined in AC 20-151() for ADS-B systems that include installation of a new or modified Mode S transponder. If you are installing a new air-ground status capability for the ADS-B system and this functionality is also interfaced to the transponder, you must ensure replies to the Mode A/C and ATCRBS/Mode S all-call interrogations are inhibited on the ground.
4.1.7 ICAO 24-Bit Address.
For U.S. civil aircraft, demonstrate that the 24-bit address transmitted by the system correlates to the aircraft registration number. If the system has a separate Mode S transponder and UAT ADS-B system installed, ensure both the transponder and ADS-B system transmit the same correct ICAO 24-bit address. For non-U.S. registered aircraft, verify that the ICAO 24-bit address is the address assigned to the aircraft by the responsible State authority.
4.1.8 Self Test.
Evaluate the ADS-B self-test features (if provided) and failure mode annunciations to ensure the pilot is able to determine whether the system is functioning properly.
4.1.9 Position Source Failure.
Demonstrate that a failure or loss of the position source results in an indication to the operator of an ADS-B function failure. If a secondary position source is interfaced to the ADS-B equipment, ensure it meets all guidance in this AC. If the change from the primary position source to the secondary position source requires a change in SIL or SDA, ensure these changes are accomplished within 10 seconds.
4.1.10 Air-Ground Status.
Verify that the air-ground inputs (or algorithms) are functioning properly and that the ADS-B system transmits the appropriate airborne messages or surface messages based on the air-ground status. This can be accomplished with simulated inputs to the appropriate sensors or accomplished in conjunction with the flight test. Rotorcraft may consider hover taxi as in the air.
4.1.11 Transmit Power.
Transmit power testing must be accomplished if a new antenna has been installed, an existing antenna has been relocated, a diplexer has been installed into an existing antenna system, or the output specifications on the transponder have changed. Perform the following testing to validate transmit power: