Which GPS Unit Should I pick?
Michalis "BIG Mike" Kotzakolios
Kotzakoliou, SSA

Introduction

The GPS unit has comprehensively demonstrated its worth in military applications, especially in Operation Desert Storm, in 1991 when U.S. and allied forces faced a vast, endless desert. Without a dependable navigation system, complicated troop maneuvers were not possible and could not have been executed.

While the GPS receiver was originally manufactured to meet the requirements of the U.S. military, of late new commercial applications are continually being discovered. Nowadays markets have changed considerably, and more than five hundred types of GPS receivers are available. Presently, a GPS unit price varies from less than hundred dollars for a simple handheld receiver, to fifteen thousand dollars for the advanced geodetic quantity receiver.

How does it work?

To attain a location lock, or gather an almanac from a navigation satellite, the GPS unit must be held or placed in direct view of the satellite overhead. If the signal appears to be blocked or weak, moving a few feet in any direction will strengthen the signal. This is particularly true in an area of dense vegetation, like forests or if surrounded by tall structures, like buildings. A GPS unit will not receive signals if objects or people block its view of the satellites, or if one attempts to use the GPS unit indoors without any external antenna attached to it.

Obtaining position fix with Global Positioning System and an ingenious receiver is effortless for the user. The GPS unit uses the information gathered from at least four satellites to solve an elementary geometric equation and then presents it in simple navigation displays.

The GPS unit accesses the transmission or broadcasting time and signal quality from each satellite. Each satellite is approximately 11,900 miles away, so there is a calculable delay from the time when the code is broadcasted, until it is received. The receiver multiplies the difference in the transmission time by the speed of light (186,000 miles per second) to arrive at an estimation of the satellite's distance from the unit. The receiver then computes and displays a position fix.

Conclusion

The simplest of the GPS unit have just one channel, and search for multiple satellites using a sequencing arrangement that rapidly switches between satellites. More sophisticated models have parallel tracking capability and assign a separate channel to each satellite in view. These receivers typically track from four to twelve satellites at once (although it is rare that more than eight will be available). By averaging data from multiple satellites, the GPS unit eliminates some of the effects of selective availability or SA mode, and reduces the frequency uncertainty.