[aprssig] Can objects/items dead-reckon?

Lynn W. Deffenbaugh (Mr) ldeffenb at homeside.to
Mon Feb 27 11:02:23 CST 2012

On 2/27/2012 11:12 AM, Andrew P. wrote:
> I was looking at the spec regarding Objects and Items, and it's not clear whether a station sending such APRS messages can add a course and speed for Objects.
> Can course and speed be specified for an Object? I would presume so, given the stuff about tracking storms/hurricanes/etc., but can it be done for generic Objects?

Yes, in a generic sense, an object packet can include a course and 
speed.  NWS objects, in particular carry this information to good use, 
but see below for more on these objects.

> If Objects can have a course and speed, is the sending station and/or the receiving stations supposed to dead-reckon the future position of the Object based on that course/speed?

If you do dead-reckoning, I would do it on anything that specifies a 
course and speed.  But there's more to it that is non-obvious.

> If so, how long should such dead-reckoning be carried on before the original position/velocity report is considered too stale to continue extrapolating?

Personally, I chose 5 minutes as the limit in APRSISCE/32.  This is 
applied to all station or objects that have included course and speed 
information.  If I haven't heard anything within 5 minutes, then the 
dead-reckoned extrapolation of the last heard posit is removed.

Now, consider this little conundrum with course and speed on objects.  
If an object is coded in most object configurations with a course and 
speed, and that object posit is not changed by the object generator, 
then the dead reckoning would restart from the same position every time 
the object is received.

However, NWS objects include an additional bit of information, the alert 
issue time.  In APRSISCE/32, I use that timestamp as the origin of the 
dead reckon and continue to dead reckon said object through the 
NWS-specified expiration time (no 5 minute limit here).  Non-NWS objects 
don't have a start time, so dead reckoning can only start at the 
received object location.

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

> Andrew Pavlin, KA2DDO
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