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

[Kenneth Finnegan]

Ok, let's take a step back here, because this is getting a little crazy.
Let's examine the original problem statement:

"The APRS course/speed data extension doesn't support speeds above 999
knots"

This means that no current APRS implementations support any changes we make
here, and any changes we make will not be parsed correctly by any existing
software.

For example, given this latest proposal, if the ISS were to beacon a speed
of 779, every existing APRS implementation would rightfully so parse this
as 779 knots, which is really really incorrect.

Given that none of the existing parsers will correctly handle these
packets, why are we limiting ourselves to three digits? Why not just leave
the field in knots and just have the ISS beacon their location with
"120/15200"?

 1. Existing parsers were going to get it wrong anyways, so they're going
to misinterpret this course/speed as 152 knots instead of 779, an
additional error of only 4%. They're already off by 95%, so the difference
is immaterial.
2. Humans reading the packets will parse the packet correctly, which is
certainly not the case with the proposed 135X-90000 equation.

So my proposal would be:
"For speeds above 999 knots, use more than three digits."

1. It's simple and human readable
2. It doesn't break the current expectation that the speed is in units of
knots
3. It's limitlessly extensible, for when someone wants to encode any speed
between or above any of the magic 99X values proposed earlier.
4. It doesn't suffer from reduced resolution for speeds above mach 1.


--
Kenneth Finnegan
http://blog.thelifeofkenneth.com/

On Thu, Oct 12, 2017 at 6:16 AM, Robert Bruninga <bruninga at usna.edu> wrote:

> 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.
>
>
>
> Bob, WB4APR
>
>
>
>
>
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>
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