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[aprssig] Ambiguity

Robert Bruninga bruninga at usna.edu
Tue Aug 21 20:28:25 UTC 2007


> Yes.  That's why I've never liked the scheme.  
> I'd much prefer a separate ambiguity magnitude 
> indicator, so the client would know a 
> center position and how 'fuzzy' to make it.

Its there.  The magnitude of the ambiguity is given by the
number of blank fields omitted by the ALT or LONG.  If the
position is transmitted as:

39__.__ then it has 60 mile ambiguity (1 deg)
385_.__ then it has 10 mile ambiguity (10 min)
3849.__ then it is 1 mile ambiguity   (1 min)
3848.7_ then it is 0.1 mile ambiguity
3848.73 then it is to nearest 60'

> On a related subject, I'd really like to see 
> some sort of flag that'd indicate a questionable 
> ? position. 

Its in there.

There are 6 methods:
- the five shown above if you have an idea of the degree of
quesionableness
- the "\." symbol which is called the "ambiguous plot" symbol.

This symbol was displayed as a big questionmark.

> The T2 reports 'NO FIX' in the comment 
> if there's no GPS fix ... but it'd be 
> nice to have a standard so that client 
> software could still show a position but 
> also indicate clearly that the station 
> might NOT be there.

That was the intent of the above system.

> I think this is most critical for safety-
> related functions, like if you've got a 
> bunch of storm chasers that you want to be 
> sure are accounted for and not where they 
> shouldn't be.  

Yes, from day one, APRS was designed to transmit no more
precision than the sender intended.  Too many clients ignored
this fundamental principal.

> If it's not valid, you still want the station 
> to be able to report its last known position, 
> since that's still useful information.

Dead reckoning and the moving dead-reckoning display was
intended to also convey this age of last report info.  The
longer the DR line connecting the the last received position to
the currently estimated position, the more out-of-date the
plotted symbol was becoming.  Again, too many clients ignored
the DR plot completely as well as the display of latency
(different color for older objects) that most people do not see
all these fundamentls of APRS any more.

This web page shows how the original APRS symbols were intended:
http://www.ew.usna.edu/~bruninga/aprs/symbols.html

Bob, WB4aPR





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