Getting lost while driving, or stopping at a gas station to ask for directions, has become a thing of the past. With GPS in your car’s navigation system, in the portable navigation device on your dashboard, or in your smartphone, it is easy to pull up a map and see where you are, or get directions to where you are going. GPS makes you safer, routes you around traffic delays and helps you find nearby services.
GPS (Global Positioning System), a technology we now take for granted, started life as a military-first radio-navigation system meant to increase the accuracy of aircraft, submarines and their ordnance. The first satellite went up in 1978 and the system became fully operational in 1995. Civilian use, however, was not allowed until the 1980s. Until 2000, the signal available to civilians was degraded to provide less accuracy. The unencrypted time signal sent to non-military users was randomly offset to provide no better than 100-meter accuracy. It meant that a car’s location might be off by one or two city blocks. Today, the GPS receiver in a car is accurate to 10-15 meters. However, the latest enhancement uses an L5 band. This has a much higher accuracy and can pinpoint a location to within an incredible 30 centimeters, or 11.8 inches.
Since 1994, two dozen GPS satellites have been orbiting the earth 13,000 miles up in six groupings, or planes.. They weigh about 2,000 pounds (on the ground) and are 17 feet in diameter including the solar panels that power the satellite. They are good for about 10 years of operation and replacements are constantly built and launched. Other countries have GPS satellites, as well.
They are not stationary overhead, but move west-to-east at about 8,000 mph and orbit the earth (pass over the same spot) twice a day. On board is an atomic clock. Each satellite continuously reports the following information:
- Pseudorandom code, the ID for each satellite.
- Ephemeris data, the current date and time, and whether the satellite is healthy or unhealthy (“unhealthy” might mean the satellite is being re-positioned or re-calibrated; it’s not always a goner).
- Almanac data, where the satellite should be at any time of the data (also almanac data for every GPS satellite).