Global Positioning Systems

The Global Positioning System (GPS) satellite network was originally created for military use so that airplanes, ships, and troops could determine their position using signals transmitted from a network of twenty-four satellites that circle the earth every 12 hours. These satellites, which are tracked and maintained by Defense Department base stations around the globe, travel in a predictable orbit that allow a local receiver to determine its own position once it determines the distance to each satellite. The GPS Constellation was completed and became available for civilian use in the early 1990s.

Satellites orbiting 10,900 miles above the earth, operate at two separate frequencies: L1 and L2. An extremely accurate atomic clock onboard each satellite is used to time the transmission of the GPS signals, which are transmitted at regular intervals. These transmissions carry both civilian (C/A - Clear Access) and military (P - Precise) signals, known as pseudocode. That same pseudocode is also generated within the satellite receiver, which attempts to calculate the timing difference between the two signals.

The timing difference is then multiplied by the speed of light (186,000 miles per second) to determine the distance of the receiver to the satellite. A receiver can obtain signals from up to 11 satellites, but it needs to receive signals from at least 4 satellites to accurately locate a position. Because the receiver knows the distance to each satellite as well as the position of the satellite in space, it is able to determine the ground position of the receiver to an accuracy of approximately 100 meters, or 300 feet. This accuracy is adequate enough for most uses, but effective deployment of survey technologies requires more accurate measurements. Therefore, a differential signal must be added to the equation.

Differential correction can be supplied by one of two sources: a Geostationary Satellite Differential which services all of North America (described above), or a Coast Guard Beacon Signal which is a free signal transmitted along the major waterways of the United States.

How Satellite DGPS Works

The differential signal is generated from a network of base stations interspersed across North America. The position of these stations are surveyed down to centimeter level. These fixed base stations calculate the error associated with each satellite signal it receives by comparing its own GPS calculation with its true position. The error is then transmitted to a central uplink point in Houston, Texas, which in turn combines and transmits the corrections to a central satellite above the equator. This satellite transmits the correction signal to the AFS Universal or AFS Satellite Receivers. The receivers combine the GPS and correction signals, pinpointing the actual location within one to two meters. The data string is then transmitted to the yield monitor, which instantaneously records the corrected location information onto the PCMCIA Card.

What are the Advantages of Satellite DGPS?

The biggest benefit of Satellite DGPS is the wide area of satellite coverage. Since the "footprint" is so large, you should have very little concern that you do not fall within the range of a transmitting satellite. This is especially useful in the resale of the unit, for you can sell to someone in an area where there is no tower - based coverage. Also, the satellite system is not susceptible to local electric disturbances such as lightening storms. Satellite systems have more backups in case any one satellite experiences trouble.

How the Coast Guard differential signal works

Coast Guard DGPS works along the same general principles as satellite DPGS, using the actual position of satellite base stations to determine the GPS error. However, the Coast Guard Tower system has three major differences.

• First, the correctional signal is transmitted directly from the local tower. The United States Coast Guard has an established network of tower-based DGPS systems along inland and coastal waterways throughout the United States. Typically, the transmission operates in a frequency range similar to the AM radio band in ranges of up to 200 miles.
• While the DGPS Satellite signal is available almost everywhere, a Coast Guard Beacon signal is not. If you do not live within range of a beacon tower, you will not be able to receive the signal.
• Third, the signal provided by the Coast Guard is FREE. There are no subscription fees associated with the service.

To learn more about GPS, access the online tutorial available at Trimble.com.