[aprssig] old formats
jdw at eng.uah.edu
Thu Jan 5 10:57:55 CST 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
> time-stamp information. While not needed for basic APRS it is
> 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granularity (assuming this hasn't been repealed in a later version of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
$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
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