Data Hacking

206 Part III — Data Hacking
SiRF Another protocol that you might hear of is the SiRF protocol. This is a protocol developed by
SiRF Incorporated to work with their latest hi-tech GPS devices.
The SiRF protocol is different from NMEA sentences in several fundamental ways:
The SiRF protocol is binary. This offers a lot of advantages over using ASCII as the
medium for transmitting information.
SiRF is faster. The NMEA protocol specifies a speed of 4800 bits per second. This is
okay, but it can create a bottleneck if you want to transmit large amounts of data. SiRF
supports data speeds of 38,400 bps and greater.
SiRF can carry more data. NMEA sentences are generally limited to 80 characters.
A single SiRF instruction string (called a payload) can be as long as 2 × 10 15 bytes
(actually 2 × 10 15 – 1, or 2 billion gigabytes!).
SiRF has greater information integrity. Not only does SiRF use a checksum to check for
data integrity, but it also contains other message validation code.
SiRF uses message encapsulation. The message is encapsulated by specific start and stop
characters that also form part of the message validation code, meaning that sentence
confusion cannot happen.
However, the SiRF protocol is more complex and harder to navigate than NMEA. Because
of this, I’m not going to cover it in greater detail here, but if you want more information
(free, as opposed to paying for the NMEA-0183 standard), visit www.sirf.com/reps/
Technical.html. Here you will find tools, information, and utilities to help you make the
most out of SiRF.
Using NMEA Sentences
Let’s take a look at how you can make use of NMEA sentences directly from your GPS. All
you need is a GPS connection between your GPS and your PC or PDA. In order to be able to
use these applications, you need to make sure that your GPS is set to output NMEA data.
Examining the NMEA output is a great way to learn more about GPS and how it interfaces
with software and other devices. It is also a great way to troubleshoot problems — you can test
for incorrect output, which can lead you to solutions to your problems.
In addition, if you are a programmer, you may see something that you can do with the NMEA
data that cannot currently be done — with a bit of programming and the ability to decode
NMEA, you’ll be able to leverage your existing skills in a completely different arena.
A lot of available software will enable you to read and log NMEA sentences from your GPS
receiver. The following sections describe a few of the better applications.