[aprssig] APRS Speed Spec? (plan B)

Jason KG4WSV kg4wsv at gmail.com
Fri Oct 13 10:09:25 CDT 2017




-Jason
> On Oct 12, 2017, at 9:47 PM, WB2OSZ <wb2osz at comcast.net> wrote:
> 
> Must have exactly 6 digits.  No need for
> following delimiter.

Fixed width fields are a bad idea - a design decision opting for a fixed width is why we're having this conversation.

Parsing a well defined variable length field is not that difficult. If we take the example of the /A= altitude extension, we can use the existing /[identifier]=[value] format, but use a /, a space, or the end of packet to terminate the field. That way we don't waste space (think about wasted bandwidth of all the extra zeroes in altitude reports, since almost all of us live and report below 10k' and only use 3 or 4 digits) and we don't limit further expansion (e.g. we can't report the ISS altitude with the existing /A= because it's over a million ft). It is trivially parseable by even 8 bit microcontrollers. And it's still human readable. 

As for the format of the identifier after the /, I'll admit I'm ambivalent. I have a strong preference for readable identifiers (e.g. /SPD or /VEL OR /speed), but we are dealing with a relatively small packet size so there's a good argument for single character identifiers, too. 

-j



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