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By Cobham Sensor Systems

As the Global Positioning Satellite system (GPS) becomes more of a "household" word, and as the GPS industry matures from a few specialist systems into a ubiquitous utility, new applications are being tested and accepted by the consumer market as well as by commercial and industrial areas. The automotive industry in particular is examining a variety of ways that GPS can effectively be used to help the driver in terms of safety, emergency assistance, and efficient navigation in unknown areas. This is a case where an industry is struggling to understand concepts of a new system, while doing all possible to implement that system quickly. The goal of this paper is to help the user better understand the performance requirements that drive the choice of antenna and cable in an automotive GPS system.

In order to accomplish this goal, a brief description of the GPS system will be given. Second, issues affecting GPS performance in the automotive environment will be discussed. One specific issue, system sensitivity, will be described in detail. The effect of the antenna and cable on the GPS receiver sensitivity will be shown, in theory, and example. Methods to determine the best antenna will be given. Last, data taken in automobile driving tests will be shown to demonstrate the effect of antenna choice.

The GPS System
The GPS system consists of 3 segments. The space segment has 24 satellites orbiting in 6 orbital planes. The ground control segment, which monitors satellite health, consists of several monitor stations, a Master Control Station, and upload stations. The upload stations communicate with each satellite to update information on orbits and satellite health, which will be transmitted to users. The user segment is made up of receivers used in various applications.

The operational principles of GPS depend on a combination of high school trigonometry and sophisticated signal processing. Satellite signals are received by the GPS unit, and from the knowledge of the satellite position and the travel time of the signal, the distance from each satellite is calculated. With distance from four satellites, the intercept point of the four distances can be calculated, giving a position in latitude, longitude, and elevation. Four satellites are required for three-dimensional positioning, as precise time is actually a variable to be solved as well as (x, y, z).

The actual operation of GPS is impressive in it's ability to give position to a precision of better than 100 meters anywhere on the globe, with the use of fairly inexpensive receivers that can be as small as a credit card. Much of the sophistication is in the satellites themselves. The satellites transmit current information to their precise location, as well as orbit information that will help predict future position. The satellites also transmit extremely precise timing information, derived from on board atomic clocks. This feature allows the ground-based receivers to use inexpensive oscillators as their internal clock source. For automotive applications, this means that GPS receivers are able to be purchased in a cost range acceptable to consumers. As will be discussed in the next section, purchasing the receiver is not the only consideration.

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