Author: MJT
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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