[aprssig] GPS NMEA and APRS Trackers

Steve Dimse steve at dimse.com
Thu Dec 29 22:04:49 EST 2011

On Dec 29, 2011, at 9:55 PM, Andrew Rich wrote:

> Hello
> Looking at making use of as much data as I can send over 19k2 EasyRadio modules.
> I would like to hear from the hardware guys how they handle the fact that not all NMEA strings seem the same.
> Some have fixed character lengths, and some have zero padding.
> How do you get around the fact you can't control what sort of GPS someone might hook up to a tracker ?
> How do you get by without having to specify a NMEA format that a devices complies with ?

You have to be willing to parse different strings (GLL and VTG, or RMC for Lat Lon, Course, and Speed for example) depending on the GPS. Within a given string though the number of comma-delimited fields and the meaning of the data in each is fixed and easy to separate.

There were a couple occasions of old trackers breaking when the number of digits changed early in the days of APRS, but people learned and essentially any legal number needs to get parsed. Most development environments these days include a library to convert floating point numbers, if not you need to roll your own with care towards tolerance.

Steve K4HG

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