Global Positioning System – GPS

The Global Positioning System (GPS) is a radio-navigation system that employs RF transmitters in 24 satellites. The satellite configuration when completed will guarantee that a GPS receiver located anywhere on earth can receive RF signals from at least four satellites 24 hours a day (with unobstructed visibility).

For commercial use, each satellite transmits unique bi-phase pseudo-random- noise codes on the L-band carrier frequency of 1.57542 GHz. A GPS receiver decodes the spread-spectrum modulations and uses triangulation techniques on the signals to calculate precise latitude, longitude, altitude and timing information from a position on earth.

GPS, officially known as the NAVSTAR GPS (Navigation System with Timing and Ranging Global Positioning System) is operated by the Department of Defense (DoD). It consists of 21 operational satellites and 3 spares circling the earth once every 12 hours.

The GPS receiver when used as a synchronization source for a CDMA cell-site, offers several significant advantages over the other alternatives.

The system provides world-wide coverage, absolute system time information, excellent accuracy, and all at a relatively low cost. There are however some limitations associated with the use of GPS which prevent it from being the total system solution for base station synchronization. The requirement for an unobstructed view of the satellite orbits force several restrictions on antenna placement and cabling at a cell-site.

Several environmental factors (i.e. snow, sleet, debris, RF interference) can severely degrade receiver performance. Field tests have shown that some receivers can be jammed by intentional or non-intentional low power interference sources over a fairly wide range (1 Watt up to 14 miles).

Since the entire network is under DoD control and has important military significance, the uninterrupted availability of the system can also be cause for concern. Selective Availability (SA) can degrade the accuracy of the GPS system for civilian use, by intentionally introducing random jitter into the timing signals being transmitted.

For normal levels of SA, the GPS system with proper filtering can still provide acceptable accuracies to satisfy CDMA base station timing requirements. Accuracies of +/- 150 nS without and +/- 400 nS with SA are typical.

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