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AMC1 ACNS.D.ADSB.115 and 120 Horizontal Position and Velocity Total and Uncompensated Latency

Available versions for ERULES-1963177438-15730

ED Decision 2013/031/R found in: GM Remote Aerodrome Air Traffic Services (ED 2019/004/R) Issue 2 (Jul 2020)

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AMC1 ACNS.D.ADSB.115 and 120 Horizontal Position and Velocity Total and Uncompensated Latency

ED Decision 2013/031/R

(a) Time of Applicability

With respect to the latency requirements in [CS ACNS.D.ADSB.115](#_DxCrossRefBm573002793) and [CS ACNS.D.ADSB.120](#_DxCrossRefBm573002795), the initial time of applicability (ITOA) is the time of validity of the position or velocity solution. Hence, the latency between the time of signal in space measurement (TOM) and this time of validity is excluded from the total latency budget.

The transmit time of applicability (TTOA) equals the initial time of applicability plus the amount of compensated latency (CL), as valid at the time at which the ADS-B transmit unit broadcasts the position (or velocity) information (TOT).

(b) Compliance Demonstration

Total latency (TL) is the difference between time of transmission (TOT) and initial time of applicability (ITOA). The analysis of total latency includes the maximum asynchronous delay caused by the time difference of position (or velocity) updates arriving at the ADS-B transmit unit and of transmitting the information. It is noted that for ADS-B transmit units compliant with [AMC1 ACNS.D.ADSB.030](#_DxCrossRefBm573002749), this asynchronous delay can be up to 1.1 second.

Uncompensated latency (UL, or more generically a latency compensation error) is the difference between total latency (TL) and amount of compensated latency (CL) thereof. Therefore, uncompensated latency determines the transmit time of applicability (TTOA). The GNSS time mark if provided to the transmit system, can be used by the ADS-B transmit unit to reduce uncompensated latency. It is possible for compensation algorithms to overcompensate for the effects of latency, also as a result of the desired attempt to account for latency external to the ADS-B transmit unit. This might lead to transmitting a position that is out in front of the actual aircraft position rather than behind the actual aircraft position. This is acceptable as long as the transmitted position is not further ahead than 0.2 s (200 ms).

The various latency related parameters are summarised in Figure 1.

![EASA EAR image](/static/files/file_IMBgPfUsdfy/image030.png)

Figure 1: Latency Parameters

Latency should be addressed through analysis rather than testing. Total and uncompensated latency information should be generated by the respective manufacturers of the position source, ADS-B transmit unit and any interconnecting avionics and should be included as part of the latency analysis.

The latency analysis should determine the latency applicable to each component of the ADS-B Out system. The total of all of the individual component latencies should be established as the sum of their maximum latencies.

ADS-B Out systems whereby the transmit equipment compliant with [AMC1 ACNS.D.ADSB.030](#_DxCrossRefBm573002749) is directly connected to a position source compliant with [AMC1 ACNS.D.ADSB.070](#_DxCrossRefBm573002757), should meet the total latency and uncompensated latency requirements without further analysis.

For other ADS-B Out systems, the applicant should perform a detailed position and velocity latency analysis. This includes systems where ADS-B Out system components are interfaced through a highly integrated architecture.

For detailed guidance on horizontal position and velocity source latency qualification, refer to Appendix H Part 5.

It is expected that this compliance information is supplied by the position and velocity source manufacturer through a Declaration of Design and Performance (DDP) or an equivalent document.

The ADS-B Quality Indicator change latency requirements are driven by the maximum time to alert for the indication of a data integrity failure with respect to exceeding integrity containment bound ([CS ACNS.D.ADSB.070](#_DxCrossRefBm573002772) and related AMC guidance).

For detailed guidance on time to alert qualification, refer to Appendix H Part 5.

(d) Horizontal Position Latency Compensation

The ADS-B transmit unit may compensate for horizontal position latency incurred outside the ADS-B transmit unit (see sub-paragraph 2 above). If such is implemented, a verifiable estimation of the delay between the time of applicability of the position measurement, and the provision of that measurement to the ADS-B transmit unit data interface should be performed.

AMC1 ACNS.D.ADSB.115 and 120 Horizontal Position and Velocity Total and Uncompensated Latency

ED Decision 2013/031/R

(a) Time of Applicability

(b) Compliance Demonstration

The various latency related parameters are summarised in Figure 1.

![EASA EAR image](/static/files/file_39aFuKUGf6f/image030.png)

Figure 1: Latency Parameters

For detailed guidance on horizontal position and velocity source latency qualification, refer to Appendix H Part 5.

It is expected that this compliance information is supplied by the position and velocity source manufacturer through a Declaration of Design and Performance (DDP) or an equivalent document.

For detailed guidance on time to alert qualification, refer to Appendix H Part 5.

(d) Horizontal Position Latency Compensation