CS ACNS.D.ADSB.070 Horizontal Position and Velocity Data Sources
ED Decision 2013/031/R
(See AMC1 ACNS.D.ADSB.070)
(a) The horizontal position is derived from GNSS data.
(b) The GNSS receiver based horizontal position and velocity data source is approved and performs, as a minimum, horizontal position receiver autonomous integrity monitoring (RAIM) and fault detection and exclusion (FDE).
(c) Horizontal velocity data stems from the same source as horizontal position data.
AMC1 ACNS.D.ADSB.070 Horizontal Position and Velocity Data Sources
ED Decision 2013/031/R
(a) GNSS Standards
(1) Basic GNSS System Approval
To be approved, the horizontal position and velocity data source should hold an EASA equipment authorisation in accordance with either ETSO-C129a, or ETSO-C196, or ETSO-C145/ETSO-C146, including the additional qualification requirements as specified in paragraph (2) below.
(2) Additional GNSS Receiver Qualification Requirements
In order to fully address the standard associated with ADS-B Out, an ETSO authorisation alone may not be sufficient to ensure ADS-B compatibility. The position and velocity source should also comply with the following requirements (i) to (vi).
It is expected that compliance with these requirements is demonstrated by the equipment manufacturer and documented in the Declaration of Design and Performance (DDP), or an equivalent document. Detailed guidance material on the qualification requirements is provided in Appendix H Part 5.
(i) GNSS system must provide a latitude and longitude output.
Note: ETSO-C129a does not cater for full compliance with this requirement.
(ii) The horizontal position integrity containment should have been qualified as per Appendix H Part 5 paragraph 1;
Note: Horizontal Uncertainty Level (HUL) information does not fulfil CS ACNS.D.ADSB.070.
(iii) The maximum time to alert for the indication of a signal-in-space data integrity failure should be 10 seconds as per Appendix H in Part 5 paragraph 1;
(iv) Navigation modes that would force the NIC value temporarily to ‘zero’ whilst the actual horizontal position integrity containment bound would meet the NIC requirements in Appendix H Part 3 Table 20, should not be installed.
(v) The horizontal position source accuracy output should have been qualified as per Appendix H Part 5 paragraph 2;
(vi) The horizontal position source latency and timing characteristics should have been documented (see Appendix H Part 5 paragraph 3);
(vii) The horizontal velocity accuracy output should have been qualified. If a dynamic horizontal velocity accuracy output is not provided, the transmitted horizontal velocity accuracy should be based on a worst case accuracy. If a dynamic horizontal velocity accuracy output is provided, the source should have been qualified for this quality indication accordingly as per Appendix H Part 5 paragraph 4.
In addition, a means should be provided to establish the condition when the horizontal velocity track angle accuracy exceeds plus/minus ‘eight’ degrees as per Appendix H Part 5 paragraph 4.
(3) Interface Interoperability Aspects
It should be verified that the position and velocity information (including their respective quality indicators) received from the source are correctly interpreted by the ADS-B equipment.
(i) Horizontal Position Integrity Containment Bound
Some approved horizontal position sources may incorrectly output horizontal position integrity containment bounds of less than 75 meters. In such cases, it is accepted that the transmit unit limits the NIC value to ’eight’.
It is expected that the ADS-B transmit unit manufacturer supplies compliance information through a Declaration of Design and Performance (DDP), or an equivalent document.
(ii) Horizontal Velocity Format
The position and velocity source manufacturer should provide information describing how the horizontal velocity information is output (i.e. in a ground speed/track angle format versus north/east velocity format) and the protocols used.
(4) Data Quality Indicator Testing
By design and under nominal GNSS satellite constellation conditions, an ADS-B Out system that is compliant with CS ACNS.D.ADSB.070 should meet the required values of the horizontal position NIC, NACp, SIL and horizontal velocity NACv quality indicators (refer to Appendix H Part 3 Table 20).
(b) Installation Guidance
The GNSS based position sources should be installed in accordance with FAA AC 20-138B (or later).
Note: EASA is developing GNSS installation guidance, once published, should be used instead of the FAA material.
(c) Multiple Position and Velocity Data Sources
(1) Multiple Source Approval
Any position and velocity source that is interfaced to the ADS-B transmit unit, should meet the requirements of CS ACNS.D.ADSB.070.
(2) Source Priority
If multiple horizontal position data sources are interfaced with the ADS-B transmit unit, priority should be given to the source that provides the best ADS-B performance with respect to the horizontal position integrity containment bound (NIC)..
A change of the selection between sources should only take place when the not selected source has exceeded the NIC performance of the selected source for several seconds.
(d) Interconnecting Avionics
Interconnecting avionics between a horizontal position and velocity data source and the ADS-B transmit unit are not recommended.
If installed, interconnecting avionics should:
(1) not output horizontal position and velocity data that has been blended with data from other sources;
(2) use GNSS horizontal velocity data to extrapolate the horizontal position data if extrapolation is deployed; and
(3) maintain full source resolution of the horizontal position and velocity data.
Interconnecting avionics that do not comply with the above may dilute the horizontal position accuracy achieved with GNSS-based sources, with detrimental effects on the usability of the ADS-B Out system.
Note: closely coupled GPS/IRS systems are not considered as interconnecting avionics.