222 Part III — Data Hacking

usually a result of either the systyem being switched off and back on again, the GPS reaquiring satellite lock after losing it, or an intermittant problem with the receiver. Here is an example (the last part of the sentences is omitted for clarity):

$GPGGA,182129.04,5040.642,N,00102.255,W, $GPGGA,182130.03,5040.643,N,00102.256,W, $GPGGA,182131.02,5040.644,N,00102.258,W, $GPGGA,182132.01,5040.645,N,00102.259,W, $GPGGA,182133.05,5040.646,N,00102.261,W, $GPGGA,182134.04,5040.647,N,00104.339,W, $GPGGA,182135.03,5040.648,N,00102.264,W, $GPGGA,182136.02,5040.649,N,00102.266,W, $GPGGA,182137.00,5040.650,N,00102.267,W,

Take a look at line 5. Notice how the longitude reading jumps from 102.261 degrees west to 102.261 degrees west before returning back to normal? That’s the kind or error you are looking for, and it stands out clearly when you learn how to read NMEA.

However, you might find nothing wrong with the NMEA sentences and that the spike isn’t present in the actual data received from the GPS. If this is the case, then switch your focus to look for applications that might be interfering with the mapping software. Shut down any unwanted applications. In addition, check for electrical interference from other devices, especially cell phones. If none of these tactics work, consider either reinstalling the software that you use or trying it on a different device.

While most applications have the capability to log NMEA received, some don’t; and this can present a problem when trying to diagnose ptoblems. In this case, you should use an application such as GpsGate, described in the preceding section.

Summary

In this chapter, you looked at the two communication protocols that GPS uses with external devices: NMEA and SiRF.

You examined in detail many types of NMEA sentences that a GPS can output and looked at the structure of the sentences and what the individual parts mean.

You also looked at software that enables you to access the raw data that is output from the GPS. This can be useful for GPS diagnostics and giving you experience in decoding NMEA sentences.

Finally, you looked at software that enables you to create simulated NMEA sentences in order to practice decoding them and gain experience in troubleshooting them for a variety of situations.