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Hi All,<br>
<br>
Corruption is commonplace in APRS.<br>
<br>
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.<br>
<br>
But, one posit in those 2880 is interesting:<br>
<br>
<span class="raw_line_err">2014-09-24 03:09:34 UTC: <b><a
href="http://aprs.fi/?c=raw&limit=&call=VK2TGB-7">VK2TGB-7</a></b>>S3T2W3,WIDE1-1,WIDE2-1,qAR,<a
href="http://aprs.fi/?c=raw&limit=&call=VK2NR-5">VK2NR-5</a>:'N?9l <span
class="raw_bin"><0x1c></span>>/ <b>[Duplicate position
packet]</b></span><br>
<span class="raw_line">2014-09-24 03:11:34 UTC: <b><a
href="http://aprs.fi/?c=raw&limit=&call=VK2TGB-7">VK2TGB-7</a></b>>S3T0W3,WIDE1-1,WIDE2-1,qAR,<a
href="http://aprs.fi/?c=raw&limit=&call=VK2MB-1">VK2MB-1</a>:'N?9l <span
class="raw_bin"><0x1c></span>>/</span><br>
<span class="raw_line_err">2014-09-24 03:13:34 UTC: <b><a
href="http://aprs.fi/?c=raw&limit=&call=VK2TGB-7">VK2TGB-7</a></b>>S3T2W3,WIDE1-1,WIDE2-1,qAR,<a
href="http://aprs.fi/?c=raw&limit=&call=VK2MB-1">VK2MB-1</a>:'N?9l <span
class="raw_bin"><0x1c></span>>/ <b>[Duplicate position
packet]</b></span><br>
<span class="raw_line_err"><br>
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).<br>
<br>
I have noticed certain stations (including VK2MB) commonly figure
in corruptions whereas others don't so it points to those stations
as probable corrupters.<br>
<br>
Perhaps APRSdroid is the future, it bypasses all the problems
inherent in radio links and flawed digis and iGates!<br>
<br>
Owen</span> <span class="raw_line_err"></span>
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