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Prevent rejected CPR positions from validating repeats - #204
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Using PMS3 we found an issue with incorrectly decoded positions (e.g., above the oceans out of reach of receivers). The issue was found to be with PipeDecoder. PipeDecoder adds CPR positions to its position history even when the motion check rejects them. A repeated or duplicated receiver (from another receiver) message can then be accepted because it matches the previously rejected position. This reproduced the incorrect positions observed in the FMI and ADSB-SUP streams.
Proposed change:
Add a position to the trusted history only when the motion check accepts it. Update the existing test to verify that replaying the same rejected CPR pair is rejected again.
Consideration:
This changes the current behavior, which allows a sequence of rejected positions to contribute to the history. If recovery from such sequences is still needed, handle it through a separate candidate history with explicit confirmation. Rejected positions should not validate themselves.