Types of NAVSTAR Satellite data

GPS units receive two types of data from the NAVSTAR satellites.

• Almanac: Almanac data contains the approximate positions of the satellites. The data is constantly transmitted and is stored in the GPS receiver’s memory.
• Ephemeris: Ephemeris data has the precise positions of the satellites. To get an accurate location fix, the receiver has to know how far away a satellite is. The GPS receiver calculates the distance to the satellite by using signals from the satellite.

Using the formula Distance = Velocity X Time, a GPS receiver calculates the satellite’s distance. A radio signal travels at the speed of light, which is 186,000 miles per second. The GPS receiver needs to know the amount of time that the radio signal takes to travel from the satellite to the receiver in order to figure out the distance. Both the satellite and the GPS receiver generate an identical pseudo-random code sequence. When the GPS receiver receives this transmitted code, it determines how much the code needs to be shifted (by using the Doppler-shift principle) for the two code sequences to match. The shift is multiplied by the speed of light to determine the distance from the satellite to the receiver. Both the satellite and the GPS receiver clocks must be synchronized for accurate measurements to take place. Because putting a $50,000 atomic clock in each GPS receiver wouldn’t make much sense, receivers use a much cheaper quartz clock that is kept up-to-date and synchronized by the satellite signals.