[aprssig] APRSIS32 sending conflicting position reports

Lynn W Deffenbaugh (Mr) KJ4ERJ at arrl.net
Sat Nov 7 20:28:29 EST 2020

On 11/7/2020 7:35 PM, Eric H. Christensen via aprssig wrote:
> I think I just found a flaw in APRSIS32 (but it may not be limited to that software).

It's not a flaw, it's a feature.

> I've been seeing stations slide across my screen over the past few weeks.  I couldn't quite put my finger on it until tonight.  Here's what I'm seeing:

It's helpful if you supply specific examples so that we can adequately 
research the details.  Especially what software you are using that shows 
"stations slide across my screen".

> A non-mobile station will suddenly relocate itself a distance away only to move back to where it came from a short time later.  After working with one station, and looking at the packets being sent directly to the IS, I noticed two things: 1) the position reports are not changing, and 2) the status reports (of which I've never found to be useful) contain a properly formatted status report *with Maidenhead Grid Locator*.

If a station is reporting two different locations with different 
precision and/or different center locations, then yes, some software 
will display them ping-ponging between the two locations.   APRSISCE/32 
remembers the most precise coordinate received and will ignore the less 
precise coordinate (typically the grid square) to avoid this issue.

> So, what I think is happening is my client is seeing the position report and plotting the station as expected. It is then seeing the grid locator in the status text and moving the station to the middle of the grid, as it should per the APRS spec[0].

Again, I'd be curious as to what client you are running.  The author of 
that client might be able to implement the same approach that APRSIS32 uses.

> Having never run APRSIS32, I'm wondering if this is a setting somewhere in the software to include the grid in the status text.  I've seen so many of these stations sliding across my screen it would seem to be odd that it is an individual effort.

It is a mis-configuration by the operator who doesn't understand the 
effects s/he is causing in other clients.  Yes, there is a checkbox in 
the status configuration of APRSIS32 that includes the GridSquare in the 
status report.  But status reports are not enabled by default, nor is 
this box checked by default.  The option is there for someone that 
doesn't want to beacon positions but only do Status Reports with 
GridSquare included.   Anyone that enables both, again, doesn't 
understand what they are doing.

So, if you want the station to stop doing that, contact the operator and 
educate them.  Tell them that they are beaconing two different positions 
and to disable one or the other.  They can keep the Status Report, but 
just don't include the GridSquare within it.   And point them to the 
wiki page


that even says:

> *GridSquare*- If checked, the current GridSquare will be included in 
> the Status Report. Use this with caution if you are also transmitting 
> position beacons as the position resolution between the GridSquare and 
> Lat/Lon coordinates is significantly different, possibly resulting in 
> a station that ping-pongs between two locations depending on which 
> packet was heard most recently.

Lynn (D) - KJ4ERJ - Author of APRSISCE for Windows Mobile and Win32

> 73,
> Eric WG3K
> [0] http://www.aprs.org/doc/APRS101.PDF (pages 8, 27, and 81-82)
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