Ashtech has recently enhanced the performance of its GPS positioning systems by combining GPS receivers, with GLONASS receivers. GLONASS is the Russian equivalent of GPS--by combining GPS and GLONASS, you have a combined satellite constellation with many more than the standard 24-satellite constellation of GPS alone, which offers much better system availability and integrity.

GPS has wide applicability. Linked to a vehicle, it becomes a tool of navigation. Within the context of a coordinate system, it is an instrument of surveying. With a cellular phone or transceiver, it becomes a method of tracking vehicles or people. With a digitized map base, it provides an all-electronic chart. For weapons guidance, it is unsurpassed.

GPS works on the principle of triangulation. By knowing its distance from three or more satellites, the receiver can calculate its position by solving a set of equations. Information from three satellites is needed to calculate longitude and latitude at a known elevation; four satellites are needed to include altitude as well.

Satellites orbit the earth twice a day at an altitude of 10,900 miles, repeatedly broadcasting their position and the time. The atomic clocks aboard each satellite keeps time by the vibration of atoms and are accurate to one second in 30 years!

In theory, the distance from satellite to receiver can be calculated by multiplying the time it takes for the signal to arrive by the speed at which it travels -- the speed of light. In practice, more sophisticated calculations are required to account for the fact that receiver clocks are not as accurate as satellite clocks. Upper atmospheric conditions and solar disturbances can also interfere with signal reception. All GPS receivers need direct and unobstructed line of sight access to each satellite.

While a single receiver can provide accurate positioning to about 100 feet, accuracy to a fraction of an inch is possible by using two receivers. One is fixed to a spot whose coordinates are already known. The other, whose location is sought, logs the same satellite data and the errors are resolved, in real-time for pin-point navigation or by post-processing for precision geodetic surveying.

Because GPS was designed for military use, it contains a number of features to limit its use to national defense. Each satellite broadcasts two signals, one for commercial use (C/A-Code) and a more accurate one for military use (P-Code). The Pentagon can encrypt the P-Code signal ("anti-spoof" mode or AS) so that unauthorized receivers cannot understand the information, however more advanced commercial receivers such as the Ashtech Z-12 can compensate by correlating the components of the P-Code for continued use in high resolution positioning.

Another restriction is "selective availability", or SA, in which the data transmitted by the satellites contain deliberate errors for all but military receivers. If SA is turned on, the accuracy of commercial receivers droops from 100 feet to 300 feet. (However, using two receivers together in a differential mode can correct for this misinformation).

Commercial use of GPS has proven invaluable in many fields. It has revolutionized surveying. It can be used to track everything from migrating animal herds to the creep of the earth's crust. Using GPS provides an entirely new way of navigating and piloting, on land, sea or in the air. It is one of our best space adventures yet