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

[Matthew Chambers]

I agree with Kenneth’s proposal. Someday, we’ll need to be able to track objects up to 582,749,918 knots and if the speed field isn’t digit limited we’ll be able to. I’m a bit rusty on parsing APRS packets myself so I don’t recall what comes after the speed field. But if it has any kind of delimiter on it, it would be trivial to tweak an APRS client to look for that instead of counting characters.


Matthew Chambers
M Chambers Communications Engineering LLC
Broadcast engineering services
Icom 2-way radios sales and service
mchambers at showmeham.info



> On Oct 12, 2017, at 11:30 AM, Kenneth Finnegan <kennethfinnegan2007 at gmail.com> wrote:
> 
> 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/ <http://blog.thelifeofkenneth.com/>
> On Thu, Oct 12, 2017 at 6:16 AM, Robert Bruninga <bruninga at usna.edu <mailto: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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