APRS duplicate removal – trial #3

The undetected long-delayed duplicate posits that are a feature of APRS VHF are a significant corruption of mapping.

In an attempt to limit the propagation of posits and hence the probability of corruption / delay etc, I have experimented with a path of WIDE1-1 on a recent trip to Canberra (about 400km for the round trip).

Whilst this should prevent packets getting to the Wagga, Newcastle and Tamworth regions which have been the main cause of corrupted posits and mapping defects, it does so at the risk of some loss of posits as some digi infrastructure was never updated to the “New N paradigm” of more than a decade ago and they ignore WIDE1.

Screenshot - 06_03_16 , 14_15_51

Above is a zoomed in view of the Canberra end of the trip, and I am pleased to say that the zig zag double backs that have been evident in recent trips did not occur. The principal reason is that with a path of WIDE1-1, the packets did not pass through VK2RWG-1/VK2KAW.

The cyan trace is from a GPS logger, the red trace from APRS.

The map can be browsed interactively at http://owenduffy.net/map/20160305.htm.

Close examination of the detail shows some interesting anomalies where the direct NMEA log shows some significant errors at times, and it appears to occur when parked near some tall buildings, so it is possible that limited sky view manifests itself as poor mapping.

The fundamental flaw in the design and implementation of APRS is its dependence on source routing. Smarter digis would make routing decisions optimised for their own environment, and packets would only be carried far enough to be reliably heard by at least one iGate. Smart digis could prevent propagation into areas with known packet corrupters, and so enhance mapping accuracy.

Conclusions

  • The architecture of the wider APRS network makes it vulnerable to corrupted posits including delayed duplicated posits and without a robust implementation of eradicating them in the core infrastructure and preventing them in the future.
  • A path of WIDE1-1 can be used to limit packet propagation and the risk of corruption, albeit at a risk of packet loss due to nodes that were not updated to the “New N paradigm”.