Data Hacking

Chapter 9 — Examining the Data
Well, partly, yes! However, there is a serious side to examining NMEA. Let’s look at two
aspects now.
Ensuring That Your GPS Works
A GPS, like all other electronic gadgets, is prone to failures. If, one day, you are unable to
switch on your GPS even after you change the batteries for fresh ones, then you know it’s dead.
But electronic devices are just as prone to erratic problems as a result of failures.
A good way to determine whether your GPS (or antenna) is picking up satellites properly is to
hook the device up to a PC or portable device and look at the NMEA being produced. Does
what you see look like valid NMEA? Do all the sentences look okay or do you find that there
are blank lines being displayed onscreen?
If your GPS is outputting valid NMEA that contains accurate positional information, then the
GPS is working (even if other components of the GPS, such as the screen or input buttons, aren’t).
I’ve seen GPS receivers with broken screens or nonfucntioning buttons put to excellent use
when connected to another device.
If your NMEA sentences seem wrong or cut off or include blank lines, before you condemn
your GPS to the trash can (or recycling depot), check all the cables and connections. Follow
the instructions in the cable chapters to test your cables (remember that heat and vibration can
affect cables, so test them under these conditions if possible). If possible, try a different cable to
see if the problem persists.
Avoiding Data Corruption
When you have a GPS connected to a PC or portable device for navigating with (in-car especially),
you’ll likely notice that most of the track information for your journey is smooth and
accurate and follows the road closesly. However, you might also find that occasionally you get
what are known as spikes in the data. Spikes are when you appear to have moved a great distance
very fast indeed. Sometimes the jumps can be very small (a few hundred meters), while at other
times the spikes can be huge, momentarily propelling you hundreds, if not thousands, of miles
off course before bringing you back to where you are suppoed to be.
These can be irritating and make you lose faith in your system, and it’s a good idea to try to
pinpoint the cause of the problem.
This kind of problem usually indicates one of two root causes:
A problem with the GPS
A problem with the device to which the GPS is connected
If the problem is with the actual GPS itself, then the way to test it is simple: First make a log of
the NMEA sentences. Then, when you see a problem with the tracklog produced, look through
the NMEA sentences for data that corresponds to the glitch. You can save yourself a lot of time
by only looking at the $GPGGA sentences that contain the position, velocity, and time data.
With practice, just a cursory look through the listing will highlight problems. Generally, the
numbers represeting coordinates and velocity change gradually over time, and sudden changes are
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