I was using the Adafruit GPS receivers to calculate the bearing and distance
between two boxes that had radio tranceivers in them. The control box polls
the remote box position and does simple trig on the difference between the GPS
coordinates. I noticed that the Adafruit GPS seems to go into some kind of
funky "position hold" mode if thinks that it is not moving (or is actually
moving very slowly)... the GPS coordinates stop updating until the box moves
some distance.
I got much better results using a CN06 GPS module that is based upon the Ublox
Neo-6M. You need to solder a wire to the Neo module to access its PPS output.
HobbyKing sells them for cheap... also lots of sellers on Ebay. I could put
one box on the ground, and move the other one around it only a couple of feet
away. It gave remarkably good results considering the boxes were much closer
than the average GPS error... it is basically a differential GPS application.
Also, the processor is an AVR running single precision floats.
I laid out a breakout board that can take the Adafruit, CN06, or Trimble
Resolution T GPS boards and convert the TTL/CMOS levels to true RS-232 levels
via a MAX3232 chip to a DE-9 connector. PPS polarity is jumperable.
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