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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 
C 
B 
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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A |
You are here – somewhere! |
|
C 
B 
FIGURE A-2: Now assume you’re somewhere within this circle.
The area you are in falls within the circle. It’s still quite a big area, but it narrows it down quite a bit. Now suppose that you also know your distance from Point B (see Figure A-3).
A 
You are here
C 
B 
FIGURE A-3: Knowing your distance from Point A and
Point B narrows the field.
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Knowing your distance from Point C further refines the positional information (see Figure A-4).
A 
You are here
C 
B 
FIGURE A-4: Knowing your distance from three points provides a lot of information.
This is an example of two-dimensional trilateration (2-D trilateration). What GPS does is take this into three dimensions (3-D trilateration or triangulation).
In principle, three-dimensional trilateration doesn’t differ much from two-dimensional trilateration, but it is trickier to grasp. What you need to do is imagine the radii of the circles from the preceding examples going off in all directions, so instead of a getting a series of circles, you get a series of spheres.
If you know you are fifteen miles from Point A (or satellite A in the sky), you could be anywhere on the surface of a huge, imaginary sphere with a fifteen-mile radius. If you also know you are eighteen miles from satellite B, you can overlap the first sphere with second, larger sphere. These spheres all intersect in a perfect circle. Finally, if you know the distance to satellite C, you get a third sphere, which will intersect with the other circles at two points, as shown in Figure A-5.
The Earth itself acts as another sphere. It is assumed that you are on the Earth, so you can eliminate the other point in outer space, as shown in Figure A-6.
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Intersection 2
A 
B 
C 
Intersection 1
EARTH
FIGURE A-5: Using three-dimensional trilateration to find your position on the Earth
Eliminate this intersection as it
is in outer space
A 
B 
C 
You are here 
EARTH
FIGURE A-6: By eliminating the point in space, you find your exact location.