Owen Duffy owen at owenduffy.net
Thu Sep 25 07:28:01 EST 2014

Hi All,

Corruption is commonplace in APRS.

This morning, looking at VK2TGB-7 which has been stationary for the
entire 48 hour history in aprs.fi and has beaconed every 2 minutes (yes,
that is 2880 beacons for a station which did not move, repeated by
several digis into the Sydney region and until recently, gated from
APRS-IS back to RF by VK2ASY in Orange so that it travelled back via
Orange, Ginini and Illawarra/Southern Highlands digs to the Sydney
region again). Until Monday, I would hear each those posits 4 or 5
times, so on average a VK2TGB-7 packet very 25-30s... if VK2ASY ceases
his gating of this stuff to RF, it is an average rate of one posit every
60s (via Vk2RHR-1 and VK2AMW-1)... and yes, for a stationary tracker.

But, one posit in those 2880 is interesting:

2014-09-24 03:09:34 UTC: *VK2TGB-7
<http://aprs.fi/?c=raw&limit=&call=VK2NR-5>:'N?9l <0x1c>>/ *[Duplicate
position packet]*
2014-09-24 03:11:34 UTC: *VK2TGB-7
<http://aprs.fi/?c=raw&limit=&call=VK2MB-1>:'N?9l <0x1c>>/
2014-09-24 03:13:34 UTC: *VK2TGB-7
<http://aprs.fi/?c=raw&limit=&call=VK2MB-1>:'N?9l <0x1c>>/ *[Duplicate
position packet]*

The middle one is a most unlikey position change... exactly one
character in the MICE encoded position changed. If this occurred in the
AX.25 link, it should have been detected by bad CRC and discarded... so
it has happened in the tracker, or in the iGate / APRS-IS infrastructure
(and it is a single bit error in that case).

I have noticed certain stations (including VK2MB) commonly figure in
corruptions whereas others don't so it points to those stations as
probable corrupters.

Perhaps APRSdroid is the future, it bypasses all the problems inherent
in radio links and flawed digis and iGates!

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