[aprssig] Can objects/items dead-reckon?
bruninga at usna.edu
Mon Feb 27 10:57:14 CST 2012
> 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?
All APRS clients are supposed to DR all objects and stations on receipt on a
1 minute basis. The display of such a DR should be clearly obvious to even
the most casual observer. The way this was done in the original APRSdos is
to mark the reported position with a DOT or small circle, then draw a DR
line to the new position and put the Symbol at that location including the
CSE/SPD header. The circle (dot) and DR line and symbol and header were all
shown. But the SYMBOL was light-blue colored to clearly show that it was a
DR symbol and not the exact reported position.
If the DR was beyond the edge of the current viewed map, then the DR was not
shown, but the symbol was shown in a darker gray to show that it was OLD.
HOWEVER, for EVENTS, I also wrote a special version of APRSdos called
APRSdr. It's job was to do DR on TRANSMIT for EVENTS which follow a given
track. You could place just about any observed runner on the APRSdr map and
it would DR each such object along the event track. Anytime a fresh visual
sighting came in by voice radio, then the OBJECT was selected, moved to its
new reported position, and the DR was recaulclated to give him a new
velocity based on the time.
This way, everyone else was seeing all of these 1 minute moving objects as
if they had their own GPS on them. Give the LEAD a 9 knot velocity and the
pack and 8 Kt velocity and tail-end-charlie a 3 kt and it would do a pretty
good job of letting everyone with an APRS display see the progress of the
> If so, how long should such dead-reckoning be carried on
> before the original position/velocity report is considered
> too stale to continue extrapolating?
For APRSdos, anything beyond a DAY was assumed irrelevant. But for a 3 hour
event, then at the end of the 3 hours.
Some people do not like seeing the long lines of old objects. But to me, it
is better to see the long DR line on the map and KNOW that the object is
stale and by how much, than to see a nice clean object on the map that is an
hour old but looks the same as if it was 10 secondds old....
We found the APRSdr to be so reliable at showing the movement of the lead
runner, we never outfitted the lead again with a GPS. For a marathon, even
if he falls over dead, then someone else becomes the lead and he still also
is running at about 9 knots. So the moving object LEAD on the map still is
the best information available at how the event is progressing.
Andrew Pavlin, KA2DDO
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