The First GPS Receivers

Soon after 1978, the NAVSTAR GPS (Navigation Satellite Timing and Ranging Global Positioning System) became operational in the United States. This was the first Global Positioning System (GPS) and was designed for the use of the United States military and other friendly forces. At that time, GPS receivers were considerably larger than they are today; however, they were already relatively small as the users of most of the receivers at this time were anticipated to be on the battlefield or in the nosecone of a missile seeking a course correction.

Currently, there are 30 active satellites in the constellation of satellites that defines the current GPS, where three or four of these are denoted as backup satellites. Although the operation of the GPS is managed by the National Space-Based Positioning, Navigation, and Timing (PNT) Executive Committee, and supported by the PNT Executive Secretariat, operational and logistics support is in the hands of the US Space Command. The objective of this GPS continues to be to provide land, sea, and airborne travelers the information to know their three dimensional position, velocity, and time, 24 hours a day, in all weather, and anywhere in the world.

The Role of the GPS Receiver

A GPS receiver’s job is to locate four or more of these satellites in order to use a process known as trilateration to determine the GPS receivers’ position. The more reference points (satellites) the GPS receiver can tune into, the more accurate the positioning calculation will be. The GPS receiver actually performs the calculations itself. It starts by analyzing the high-frequency, low-power radio signals from the GPS satellites which are traveling with the velocity of the speed of light. The signal from the satellite is itself encoded with a message that says when the signal was sent. The GPS receiver knows when the signal was received. The velocity of the wave is known, because the receiver has now calculated how long it took for the signal to get to it. This tells the receiver the distance, in miles, from that satellite to the receiver. The GPS receiver does this for three, four or more satellites, and from this information, it can “triangulate” its own position. The more satellites a receiver can tune into, the more accurate the position will be.

The Components of a GPS Receiver

Most standard GPS receivers must have an antenna that is tuned to the frequencies transmitted by the satellites, custom receiver-processors, and a very highly-stable clock which is often a crystal oscillator. Particular models may also have a continuously updating display showing location, usually in latitude and longitude, and will sometimes include the velocity as well. Although we now have an almost universal capability across the world with phones, PDA’s and cars, all having their own GPS receivers, it is important to note that this system is still operated by the United States military and is provided primarily for the public good.

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