[aprssig] Proximity detection

[Robert Bruninga]

> Bit hard to do in a PIC chip Wes
>> ... you could use cosine corrected Pythagorean theorem.... 
>
>>> If you get a micro, and you know from a GPS 
>>> where you are, and then you pick up another 
>>> position, one would assume you can then apply 
>>> a basic "is there anyone in my box" calculation ? 

Simplify it to a frist order solution and just take the
difference

>> dist=sqrt((COS(avg lat)x[delta long]^2)+[delta lat]^2).  
>
Becomes

Dist= DeltaLONG + DeltaLAT

Which is simple addition and subtraction.  The distance will not
be scaled and will result in a box that is not square, but even
that can be corrected for.  Trivial to do, just by comparing
lat/longs.

Bob