[aprssig] APRS Speed Spec? (plan B)
bruninga at usna.edu
Fri Oct 13 10:45:09 CDT 2017
> This is not really needed...
The current speed field is limited. To stay relevant, APRS needs a speed
field that can allow for all speeds on or around earth.
> and it's yet another special case kludge which doesn't work too well....
Putting in a single knee point is a common way to preserve scales that have
a range of very common and frequently used values while allowing expansion
over another magnitude of range that are rarely if ever used. And the speed
of sound, Mach 1 is a perefect such inflection point. We should not leave
it as is where there is no way to present these speeds.
And it is trivial to implement as a simple breakpoint at Mach 1. Takes two
lines of code to decode:
If speed S is Less than or equal to 670 then SPEED = S in Kts
If speed S is greater than 670 then SPEED is 135*S -90000 in Kts
It preserves the better than 0.2% accuracy up to Mach 1 and gives about 1%
resolution at orbital velocity. We could have used a power-law formula to
preserve accuracy, but the simple linear equation with one multiplication
seemed so much easier.
Here are the examples:
779 = 15,200 knots space station
682 = 2100 knots Mach 3
678 = 1600 knots military jets
677 = 1340 knots Mach 2
675 = 1200 knots Concorde commercial type jets
670 = 670 knots Mach 1
No one is required to implement this, and no one has used any speeds of that
ilk to date anyway. But this gives us a way forward that does not break
anything, yet allows those that might have the need to at least have a way
to transmit it.
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