Data Hacking

Appendix A — GPS Primer
Standard Positioning Service (SPS)
The SPS is a positioning and timing service that is available to all GPS users on a continuous,
worldwide basis with no direct charge. SPS is provided on one of the frequencies that the
GPS satellites use, called L1. It contains a coarse acquisition (C/A) code and a navigation
data message.
Precise Positioning Service (PPS)
The Precise Positioning Service (PPS) is a highly accurate military positioning, velocity, and
timing service that is available on a continuous, worldwide basis to users authorized by the U.S.
The P(Y) code–capable military user equipment provides robust and predictable positioning
accuracy of at least 22 meters (95 percent) horizontally and 27.7 meters vertically, and time
accuracy to within 200 nanoseconds (95 percent).
PPS is the data transmitted on both GPS frequencies: L1 and L2. PPS was designed primarily
for U.S. military use and access to it is controlled by encrypting the signal.
Anti-spoofing (A-S) measures guard against fake transmissions of satellite data by encrypting
the P-code to form the Y-code. This is only activated periodically when deemed necessary.
How GPS Works
The basic principle behind GPS is straightforward: The GPS receiver picks up a signal from
three or more of the satellites and then uses this information to calculate the distance to the
satellites. This information is, in turn, used to determine a location on the globe where the
receiver is at that time. This whole process is based on a system called trilateration.
Trilateration is easy to visualize. Looking at the map in Figure A-1, assume that you are positioned
somewhere on it.
A
B
C
FIGURE A-1: Assume you’re somewhere on this map.
Now assume that you know that you are within a certain distance of Point A (see Figure A-2).
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