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[aprssig] APRS return to basics (DR) (new images)

Bob Bruninga bruninga at usna.edu
Thu May 24 13:32:03 UTC 2012

> I've never used a system that used dead reckoning, 
> but I don't think I would like the feature. 

That's what they said about the telephone, the car, the telegraph and
facebook.  It also shows why I had asked all authors to have DR enabled by
default.  SO that people would get a taste of what real-time tactical
awareness really looks like when they came into APRS.  If it got too messy,
they could turn it off.  But if it is messy, then it is really telling you
something important about the latency of your data!  Hiding it is hiding
that important element.

Again,  DR is fundamental to all military and aviation and other tracking
systems.  If something is moving then it has FOUR dimensions.  LAT, LONG,
COURSE and SPEED.  TO ignore CSE and SPD in the display is like looking at a
Black and white TV and it hides the age of the position.

> but my feeling is that I would rather know what is fact... 

The last known position is always clearly shown with the circle and the
line.  The length of the line from that last known position to the estimated
DR'ed position shows the passage of time and the age of the "fact".

> if they kept moving, completely missing the next bend in the road...

A symbol DR'd at 70 MPH off the interstate because the road turned still
conveys exactly the same information.  He is that far along his route.
Looking at the map you can easily see where the road went with him still on
it and can visualize where he is.  If the DR is very long, such as 5 or 10
miles, then clearly he can be 5 or 10 miles from that last known position
and so its "factual" value is vanishing to practically nothing.

In fact at 60 MPH, the error in that "fact" is more than a mile off after
even just a minute.  Hence, the DR'ed position is always more factual than
the old stamp on the road just getting older and older.


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