Data Hacking

Examining the Data
When you connect a GPS to another device (such as a PC or an
iPAQ) or to a software application, it is quite probable that the
communication protocol used by this connection will be based
on a standard known as NMEA. NMEA is crucial to this communication,
and by understanding the format that NMEA takes and how the data is
structured, you are in a position to directly examine the data stream. If you
can read NMEA, you can troubleshoot GPS problems, directly examine the
data retrieved, and also check for corrupted data.
In this chapter, we are going to take a close look at the NMEA protocol and
the format in which the data is output by a GPS receiver. By the end of this
chapter, you will be familiar with the following topics:
Communicating with other devices
Software applications that enable you to capture and store NMEA
data
Troubleshooting
The more you know about and understand the data, the more you can do
with it.
NMEA
NMEA is the protocol most used by GPS receivers to communicate with
other devices — either for data transfer to and from the unit, or for communication
with other devices. The fact that this standard is used for GPS
betrays its marine roots.
NMEA stands for National Marine Electronics Association, which is the
body that sets and defines the standard. The full name of the standard most
commonly used by GPS receivers is NMEA 0183. This standard covers not
only GPS receivers but a variety of other electronic devices.
Most computer programs that provide real-time position information are
capable of understanding the NMEA format. Some can only understand
the NMEA plaintext format, while others use a variety of different data
formats.
NMEA data sent from a GPS contains full PVT (Position, Velocity, and
Time) data calculated by the GPS unit.
chapter
in this chapter
˛ Introduction to
NMEA
˛ NMEA sentence
structure
˛ A tour through
common NMEA
sentences
˛ A different
standard: SiRF
˛ NMEA tools
˛ Recording real
and simulated
NMEA data
˛ Troubleshooting
with NMEA