Hi all
Gunnar, sa6bqz, noticed yesterday that his position beacons from his AIS
and pskmail systems differed somewhat. There was abt 18 meters
difference between them. I investigated this issue somewhat today and
figured it best to post a short notice here should anyone else start to
notice something similar. The GPS delivers position data through the
nmea GGA message (and others). The position message format for NMEA GGA
is like llll.ll,a,yyyyy.yy,a. A real world example of that is what I get
here: 5901.3658,N,01756.0467,E. When we transmit this we have to format
it in accordance with the APRS protocol spec. That spec says that the
latitude format should be:
"Latitude is expressed as a fixed 8-character field, in degrees and
decimal minutes (to two decimal places), followed by the letter N for
north or S for south."
To create a concrete example for APRS the position is formatted like:
5901.37N. And there we have it in a nutshell, NMEA data 5901.3658 gets
reported as 5901.37 and a slight difference in position is noticeable
(not the rounding, thats expected).
Also, when looking at this I would prefer to never use the decimal
position format. Incoming NMEA is in degrees and minutes, outgoing APRS
is in the same format (less precise but still degrees and minutes).
Decimal degrees as presented in jpskmail is never used, only presented.
My sailor eyes hurt when looking at the decimal format but I guess some
want it anyway?
73 de Per, sm0rwo
P.s. Spending the afternoon painting the boat.
