Data Hacking

312 Appendix A — GPS Primer
Error Amount of Error (feet/meters)
Ionosphere 13.1/4.0
Clock 6.9/2.1
Ephemeris 6.9/2.1
Troposphere 1.3/0.7
Receiver 1.6/0.5
Multipath 3.3/1.0
TOTAL 33.1/10.4
The errors resulting from the ionosphere and the troposphere are due to the slowing down of
the signal (remember that the speed of light quoted is the speed of light in a vacuum). Clock
errors are due to inaccuracies in timing. Even using atomic clocks, the speed of light is so fast
and the distance that it has to travel is so short that small errors in timing add up. Ephemeris
errors are due to the fact that it is impossible to know absolutely the orbits of the satellites.
Slight variations cause small errors.
Receiver error is also significant; this is the error in the antenna and the delays in processing
caused by the internal circuitry (also due to temperature changes affecting the internal clock).
Finally, there are multipath errors. This error is caused by the signal being reflected or bounced
off things (such as a building or the ground).
There are other issues with the signal. Because it is a microwave signal, it is absorbed by water,
so a GPS receiver won’t work underwater. Fortunately, not many people need to use it underwater,
so this isn’t a huge problem, but water can affect the system in other ways:
Summary
Leaves contain water and can absorb the GPS signal, dramatically reducing signal strength
and quality. The denser the leaves, the more the signal is degraded. Worse than leaves are
pine needles. These not only contain water, but are the right size and shape to act as miniantennas,
catching the signal. This usually means that if you are traveling though dense
forests (especially conifers), you will need to find a clearing to gain a good signal lock.
Humidity in the air, such as rain, snow, or fog, can also weaken the signal. In addition,
areas of high mist (such as near waterfalls) can be problematic because the high water
content of the air exacerbates the problem.
A layer of water on the antenna can dramatically reduce the signal’s quality.
This appendix provided a brief look at the GPS network and what makes it work. GPS is a
vastly complex multi-billion-dollar system, and certain key parts of it are classified. Nonetheless,
this appendix should give you a good working knowledge of the system and what can affect its
accuracy.