[aprssig] APRS Speed Spec? (Above Mach 1)

Robert Bruninga bruninga at usna.edu
Thu Oct 12 09:16:26 EDT 2017

In a move to keep APRS relevant, there was some useful feedback on the idea
to extend the speed range of APRS reporting.  I like the transition at Mach
1 (670 kts).

So here is the way to handle speeds above Mach 1 and it works for both MicE
and CSE/SPD values of X speed field.

If X Less than or equal to 670 then SPEED = X in Kts

If X is greater than 670 then SPEED is 135X -90000  in Kts

You do not even need to do the math.  You can just use some pre-calculated
values shown here:

779 = 15,200 knots space station

682 =     2100 knots Mach 3

678 =     1600 knots military

677 =     1340 knots Mach 2

675 =     1200 knots Concorde

670 =       670 knots Mach 1

These remain in Knots because that is the APRS standard for speed, but of
course you can also display them in MPH if you want to make that conversion
which I think all PILOTS use.

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