A study of uncompensated latency in ADS-B reports.

The total latency (TL) in ADS-B is the difference between the true TOA of the original position measured by the onboard GPS and the TOA in the ADS-B report at the GS. We processed the ADS-B reports extracted from live-data recorded at the Chicago to produce the results in this report. We observed that 99% of aircraft in our data set were broadcasting in non-UTC coupled mode. For these aircraft, STARS exclusively used ADS-B reports to update tracks when available. The calculated ULs for the non-UTC coupled ADS-B reports were well within the budget (-200 ms to +400 ms) allowed by the DO-260B MOPS. Moreover, the mean latencies from both methods were less than +/-50 ms. The latencies obtained from the MTPES method were observed to be in close agreement with the means from the ATPE method. About 1% of aircraft were broadcasting in UTC coupled mode. The reports from these aircraft were identified to have a non-compliant link version, causing STARS to reject these reports for tracking. Moreover, the reported position and TOA were found to be unsynchronized, which conflicts with the MOPS requirement for UTC coupled operation.