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The format of the M17 GPS Data
#3
Hi Jon, 
  Thanks for the precise description of  GPS encoding of data into the M17 digital side channel.  It looks very good. 

I can accept feet for altitude as it is the basis for altitude in the aviation industry and is a wide spread global industry standard. 

Although I have to disagree on the use of MPH as the basis for speed..  SI  units are meter per second.

Directly encoding 1 byte as m/s really does not give the range for normal speed of vehicles. 

Encoding the speed and a fixed float in meters per second  (6 bits for matissa, 2 bits for exponent) 

speed  = mantissa * 10^(exponent + 1 )  m/s 

(Note the plus one in the decoding of the exponent, however that is not really necessary and without it will allow you to track the speed of garden snails as will as F35 fighter jets) 


This give a range of:

0.35mph to 22370 mph
0.563kph to 36000 kph

I do not agree that what APRS-IS uses should be a basis for how we encode the values. There will always be software that takes the encoded speed form m17 and converts it for presentation to APRS-IS or even to the user interface. 

In fact encoding the altitude in meters in a similar manner would also significantly increase the range over which the altitudes can be reported.

Yes this is more complicated than just taking the byte as a integer number, but I do not think an 8 bit integer number can reliably convey the precision and range these quantities needed. 


With regards to device type information, I would consider making source 0 "default", then the other numbers more specific types, because I can guarantee that there will be a lot of devices that have the default set, regardless of whether they are "fixed" "mobile" or "portable".  This situations detracts from the quality the information of those that have taken the time to set their "fixed" type correctly..

Under the current scheme there will be a lot of "fixed" stations traveling 60mph down the freeway, assuming 0 is the default. 



Cheers, 
David
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Messages In This Thread
The format of the M17 GPS Data - by G4KLX - 10-04-2021, 10:44 PM
RE: The format of the M17 GPS Data - by G4KLX - 10-05-2021, 11:36 PM
RE: The format of the M17 GPS Data - by VK3DCU - 10-08-2021, 12:57 AM

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