*47 the checksum data, always begins with *
As an example, some “real” records taken during a flight looked like this:
$GPGGA,171634,3406.6883,N,11757.4427,W,1,9,0.9,181.1,M,-33.2,M,,0000*6A
This record was taken at 17:16:34 UTC (actually 9:16:34 PST), at 34°6.6883’’ N,
117°57.4427’W. There was a valid GPS fix (the “1” after the “W”), from 9 satellites. You
can pretty much ignore the rest of the data, including the altitude; GPS altitudes are only
accurate when the ellipsoid is taken into account, and they tend to lag behind the
latitude/longitude for several seconds at least; that’s why they’re not generally used to measure
real-time altitude in rockets except at extreme altitudes. Also, consumer-grade GPS units have
a velocity and altitude lockout feature that suppresses data when you get moving quickly
and/or at high altitudes; barometric altitude and accelerometer data have no such limitations, so
they tend to be better real-time indicators of altitude.
Now that you know a little about the GPS records, all you need to do is to look at the $GPGGA
records, and if you either a) stop getting records, or b) see that nothing is changing, then use
the last $GPGGA record to figure out where your rocket landed.
Of course, you’ll still need to figure out how to get to your rocket… that’s a little further on.
Using MapSphere to Track Your Rocket
MapSphere is a really good Windows program for taking the GPS input from the Eggfinder
RX’s serial cable and displaying a real-time track of where your rocket is going. Best of all,
it’s free. You can download maps ahead of time, so that when you get out to the range where
there is most likely no cellular Internet coverage, you still have a nice satellite or street map to
help guide you. You can save your track, and it’s a simple text file (although it’s not the raw
NMEA data), so you can go back and look at it with a text editor if you wish, and you can load
and “play” them back to retrace your flight.
To use MapSphere, turn on your Eggfinder TX board, plug in your Eggfinder RX board to the
USB port, and launch MapSphere. (You must load the serial drivers for the USB-Serial cable
first, of course). MapSphere has a “community” function that you can use with their web site
to share tracks; if you want to use that, read about it on their web site, otherwise we
recommend when the “Login” box displays you should simply cancel out of it. Go to the
Toolbar, click on the GPS tab, then Connect. It should automatically find the serial port that’s
being used to receive the Eggfinder data, and as soon as valid location data is available you’ll
start seeing a track on the map.
