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[aprssig] Dead Reckoning in APRS

Robert Bruninga bruninga at usna.edu
Sat Jun 17 22:22:27 UTC 2006


>>> ve7gdh at rac.ca 06/17/06 5:23 PM >>>
>> 12)  Dead reckoning so that moving objects keep moving and
>>       conveyed unambiguously to the viewer
>
>Just curious... what happens if the mobile station just parks 
>and turns  his "tracker" off? Does dead reckoning make it 
>keep going in APRSdos...

Yes, but you have to understand how APRSdos presents this 
information to the viewer.
1) There is always a small circle at the exact reported position.
2) Then a bright line connects to the estimated "present position"
3) A BLUE SYMBOL is then shown at that location
4) The symbol also still has its CSE/SPD vector.

Since APRS clearly displays all mobile symbols in WHITE (if message 
capable) or Gray (if not message capable), then a viewer instantly 
recognizes the following when he sees a BLUE symbol:

a) THe fact that it is now BLUE means it is an estimated position
b) the circle at the white dot shows where he was when his last
    report was transmtted
c) the length of the line between the last reported position and the
    estimated position shows the viewer the length of time that
    has elapsed since that last position was valid
d) This presentation clearly gives the viewer everything he needs
   to know in a glance.

Not only where he was at his last report, but how old that
position is, and how far he could have traveled during that
time and where he could be now if he continued on that
course and speed.

No APRSdos user would ever missinterpret that display as being
a valid position.  But just what it is, a display in space and
time of what is happening and the best info available.

Our web pages are down right now, but See the examples on

http://www.ew.usna.edu/~bruninga/aprs/symbols.html

Because of the Dead Reckoning color attribute for all
mobiles on an APRSdos map, the BLUE and WHITE symbols
instantly tell who has been updated within the last minute
and who's positions are getting stale.  The length of the
DR line to the blue ones tell you *how* stale.  ALl of this
at a glance without having to click on anything.  This
is very important for public displays where each viewer
cannot click on each mobile he might need to check on.

Hope that helps.
Bob, WB4APR








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