Data Hacking

Appendix A — GPS Primer
Receivers generally use four or more satellites, however, to improve accuracy and provide precise
altitude information.
In order to perform this simple calculation and determine where you are, the GPS receiver has
to know two things:
The location of at least three satellites above you
The distance between you and each of those satellites
The more accurate these values are, the more accurate your position will be.
The GPS receiver determines both the location of the satellites and your distance from them by
analyzing the high-frequency, low-power radio signals from the GPS satellites. Better units have
multiple (or parallel) receivers, so they can pick up signals from several satellites simultaneously.
If you buy a GPS receiver nowadays, it will be a multi-channel receiver (12 or even 14 channels
simultaneously).
The radio waves from the GPS SVs, like all other radio waves, are electromagnetic energy, which
means that they propagate at the speed of light (roughly 186,000 miles per second, or 300,000 km
per second, in a vacuum). A GPS receiver can figure out how far a signal has traveled by timing
how long it took the signal to arrive. The mechanism by which it does this is quite clever.
At a particular time (midnight, for example), the SVs begins transmitting a long digital pattern
called a pseudo-random code. This code consists of a carrier wave that transmits the digital “chips”
that make up the code. The receiver knows when this code starts (this information is transmitted
to the GPS along with the signal) and begins running the same digital pattern at exactly the same
time. When the satellite’s signal reaches the receiver, its transmission of the digital pattern will
lag slightly behind the digital pattern that the receiver would expect. This lag between the two
corresponds to the time delay from the satellite sending the signal and the receiver receiving it.
Speed × Time = Distance
Therefore, if it takes 0.08 of a second for a satellite’s signal to reach the GPS receiver, the
distance between the two must be 14,880 miles (186,000 miles per second × 0.08 seconds =
14,880 miles). The GPS receiver must be located somewhere on an imaginary sphere that has
a radius of 14,880 miles.
One thing that GPS relies on is accurate timing. To provide this, every GPS satellite carries
four atomic clocks on board that provide an extremely accurate timing signal. The GPS receiver,
however, doesn’t contain an atomic clock (this would make it preposterously expensive!). Instead,
it contains an inexpensive quartz clock, and the internal timekeeping is updated when the
receiver is capable of locking on and receiving a signal from four or more SVs.
GPS Signal Errors
The GPS system isn’t absolutely perfect, however, because the radio signal has to propagate
through the atmosphere to the receiver. The receiver isn’t perfect either. The following table
describes the sources of the possible errors present.
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