[aprssig] APRS Speed Spec? (plan B)

WB2OSZ wb2osz at comcast.net
Thu Oct 12 21:47:04 CDT 2017


Kenneth proposed:

> Why not just leave the field in knots and just 
> have the ISS beacon their location with "120/15200"?

That's a good suggestion in many ways.  It is clean, simple, no compromise,
and human readable.  Unfortunately it wouldn't be compatible with the
official protocol spec.

APRS has several "data extensions" that can be at the beginning of the
comment field:

999/999	course and speed
PHG9999	power, height, gain
RNG9999	radio range
DFS9999	df signal strength
T99/C99	area object descriptor

All of these are exactly 7 characters.   The comment text starts immediately
after the fixed length field.  There is no need to insert a space or other
delimiter. 

So if you had "123/45678" the speed would be 456 and the 78 would be part of
the comment.  If you continued gathering up digits, you might end up
consuming part of something else such as a frequency specification.  Here is
an example from the "APRS Freq Spec."

!DDMM.mmN/DDDMM.MMW$CSE/SPDFFF.FFFMHz ...  w/o delimiter

Some might want to put a space in there for readability but it is clearly
stated as being optional.  

Out of all the suggestions, there is only one that is:

- Clean, simple. 
- Not some special case kludge.
- No compromise.
- Human readable.
- Completely compatible with original official protocol spec.
- Follows earlier precedent.

We already have the form:

/A=999999	for altitude.  Must have exactly 6 digits.  No need for
following delimiter.

Similar to what Jason suggested, we could follow earlier precedent and have:

/S=999999	for speed.  Again, must have exactly 6 digits for
consistency.

If this appears in the comment, it would override any earlier speed value.  
If the receiving software does not recognize this pattern as being something
special, it's just part of the comment text and human readable without any
math or obscure information required to decipher it.







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