The Navstar Global Positioning System

What follows is some background information about the Global Positioning System.

1. The Space Segment

The space segment consists of a constellation of 24 active satellites (and one or more in-orbit spares) orbiting the earth every 12 hours. Four satellites are located in each of six orbits. The orbits are distributed evenly around the earth, and are inclined 55 degrees from the equator. The satellites orbit at an altitude of about 11,000 nautical miles.

(Earlier plans for the system called for 18 or 21 active satellites.)

2. How does it work?

Each satellite transmits two signals: L1 (1575.42 MHz) and L2 (1227.60 MHz). The L1 signal is modulated with two pseudo-random noise signals - the protected (P) code, and the clear/acquisition (C/A) code (I have also seen the C/A code called "coarse/acquisition"). The L2 signal only carries the P code. Each satellite transmits a unique code, allowing the receiver to identify the signals.

The receiver measures the time required for the signal to travel from the satellite to the receiver, by knowing the time that the signal left the satellite, and observing the time it receives the signal, based on it's internal clock.

If the receiver had a perfect clock, exactly in sync with those on the satellites, three measurements, from three satellites, would be sufficient to determine position in 3 dimensions. Unfortunately, you can't get a perfect clock that will fit (financially or physically) in a $300 (or even $3000) receiver, so a fourth satellite is needed to resolve the receiver clock error.

Each measurement ("pseudorange") gives a position on the surface of a sphere centred on the corresponding satellite. Due to the receiver clock error, the four spheres will not intersect at a single point, but the receiver will adjust it's clock until they do, providing very accurate time, as well as position.

3. What accuracy can I expect?

The Standard Positioning Service (SPS) available to civilian users should give 20 metre horizontal accuracy, however it is normally degraded to 100 metres (95% of the time) due to Selective Availability (SA). The vertical accuracy is about 1.5 times worse than horizontal, due to satellite geometry. (Satellites are more likely to be near the horizon, than directly overhead.)

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