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[aprssig] old formats

Jason Winningham jdw at eng.uah.edu
Thu Jan 5 16:57:55 UTC 2006


On Jan 5, 2006, at 10:27 AM, Bill Vodall wrote:

> 2. The raw nmea strings are the only source I know of that contains 
> true
> time-stamp information.  While not needed for basic APRS it is 
> essential
> to a project I'm working on.

Not sure what resolution you need, but APRS has timestamps with 1s 
granularity (assuming this hasn't been repealed in a later version of 
the spec than 1.0.1, which I have on hand).

>  So not all of those microseconds of air time is wasted.

Well it may not be many microseconds, but in % it's between 200% and 
500% the size of APRS formated packets. Grabbing 3 examples from an old 
log file:

$GPGGA sentence: 69 byte payload
standard APRS position report, with time, speed, and heading: 35 bytes
standard APRS position report, position only: 20 bytes
Mic-E packet from a D700: 14 bytes

$GPGGA uses about 5 times as much payload bandwidth than Mic-E, and 
includes less useful information (except for the timestamp).  Even 
including the headers, it's double (or worse).

This also does not address altitude; you need two different NMEA 
strings to get complete position and heading information from a 
balloon, but a "dumb" tracker is _so_ dumb it can do it in a single 
packet using about 10 or 15% of the air time (and battery power!) of 
the KPC-3+.

-Jason
kg4wsv





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