[Overview] [What is GPS?] [New Applications] [Limitations] [Major Players] [Future Developments] [Useful Links]

 

 

What is GPS? 

The global positioning system is a satellite-based navigation system, which is funded and controlled by the U.S. Department of Defense (DOD).  GPS consists of a network of 24 orbiting satellites that are eleven thousand nautical miles in space and in six different orbital paths. The satellites are constantly moving, making two complete orbits around the Earth in less than 24 hours.  While there are many thousands of civil users of GPS worldwide, the system was designed for and is operated by the U.S. military.  

The GPS satellites are referred to as NAVSTAR satellites. Of course, no GPS introduction would be complete without learning about the satellites.

  • The first GPS satellite was launched back in February 1978.
  • Each satellite weighs approximately 2,000 pounds and is about 17 feet across with the solar panels extended.
  • Transmitter power is only 50 watts, or less!
  • Each satellite transmits two signals, L1 and L2. Civilian GPS uses the 'L1' frequency of 1575.42 MHz.
  • Each satellite is expected to last approximately 10 years. Replacements are constantly being built and launched into orbit. The GPS program is currently funded with replacements through 2006.

 

One of the biggest benefits over previous land-based navigation systems is GPS works in all weather conditions. No matter what the application is—the GPS receiver will keep right on working.

The GPS signal contains a 'pseudo-random code', ephemeris (pronounced: ee-fem-er-is) and almanac data. The pseudo-random code identifies which satellite is transmitting—in other words, an I.D. code. Satellites are recognized by their PRN (pseudo-random number), from 1 through 32—this is the number displayed on a GPS receiver to indicate which satellite(s) are being received. So why are there more than 24 PRN numbers? This simplifies maintenance of the GPS network. A replacement satellite can be launched, turned on, and used before the satellite it was intended to replace actually fails! They simply use a different number (again from 1 through 32) to identify the new satellite.

Ephemeris data is constantly transmitted by each satellite and contains important information such as status of the satellite (healthy or unhealthy), current date, and time. Without this part of the message, a GPS receiver would have no idea what the current time and date are. This part of the signal is essential to determining a position.

The almanac data tells the GPS receiver where each GPS satellite should be any time throughout the day. Each satellite transmits almanac data showing the orbital information for that satellite and for every other satellite in the system.

 


StreetPilot ColorMap screen examples

Each satellite transmits a message which essentially says, "I'm satellite #X, my position is currently Y, and this message was sent at time Z."  The GPS receiver reads the message and saves the ephemeris and almanac data for continual use.

To determine a position the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received by the GPS receiver. The time difference tells the GPS receiver how far away that particular satellite is. As the distance measurements are added from a few more satellites, a position can be triangulated. With a minimum of three or more satellites, a GPS receiver can determine a latitude/longitude position—what's called a 2D position fix. With four or more satellites, a GPS receiver can determine a 3D position, which includes latitude, longitude, and altitude. By continuously updating a position, a GPS receiver can also accurately provide speed and direction of travel.

GPS provides two levels of service, Standard Positioning Service (SPS) and the Precise Positioning Service (PPS).  SPS is a positioning and timing service that is available to all GPS users on a continuous, worldwide basis with no direct charge.  SPS provides 95% positioning accuracy.  PPS is a highly accurate military positioning, velocity and timing service which is available on a continuous, worldwide basis to users authorized by the U.S.  PPS was designed primarily for U.S. military use and is denied to unauthorized users by the use of cryptography.

To learn more about GPS and its military and civil uses, please visit the Aerospace Corporation's GPS System Principles. For an excellent introduction of GPS policy and description please visit this USNO page.