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Please note: we are not currently updating this site with new content, but please continue to explore our resources. The system was originally developed by the US government for military navigation but now anyone with a GPS device, be it a SatNav, mobile phone or handheld GPS unit, can receive the radio signals that the satellites broadcast.

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Each one transmits information about its position and the current time at regular intervals. These signals, travelling at the speed of light, are intercepted by your GPS receiver, which calculates how far away each satellite is based on how long it took for the messages to arrive. Once it has information on how far away at least three satellites are, your GPS receiver can pinpoint your location using a process called trilateration. Imagine you are standing somewhere on Earth with three satellites in the sky above you.

If you know how far away you are from satellite A, then you know you must be located somewhere on the red circle. If you do the same for satellites B and C, you can work out your location by seeing where the three circles intersect. This is just what your GPS receiver does, although it uses overlapping spheres rather than circles. The more satellites there are above the horizon the more accurately your GPS unit can determine where you are.

GPS satellites have atomic clocks on board to keep accurate time. General and Special Relativity however predict that differences will appear between these clocks and an identical clock on Earth. General Relativity predicts that time will appear to run slower under stronger gravitational pull — the clocks on board the satellites will therefore seem to run faster than a clock on Earth.

Each satellite completes one orbit in one-half of a sidereal day and, therefore, passes over the same location on earth once every sidereal day, approximately 23 hours and 56 minutes. With this orbital configuration and number of satellites, a user at any location on Earth will have at least four satellites in view 24 hours per day.

Washington, D. The four stations can track and monitor the whereabouts of each GPS satellite 20 to 21 hours per day. Land-based and space-based communications connect the remote monitoring stations with the MCS. GPS user equipment varies widely in cost and complexity, depending on the receiver design and application. Each satellite in orbit transmits a continuous radio signal with a unique code that includes data about the satellite's position and the exact time the coded transmission was initiated, as kept by the on-board atomic clocks. A pseudorange measurement is created by measuring the distance between a user's receiver and a satellite by subtracting the time the signal was sent by the satellite from the time it was received by the user.

GPS segments

In general, a user's three-dimensional position can be determined by simultaneously measuring the ranges from a user's receiver to three satellites.. However, because the GPS satellites and receiver clocks are not perfectly synchronized, observations from a fourth satellite are needed to eliminate the receiver clock bias that is common to all the pseudorange measurements.

Figure illustrates the GPS pseudoranging concept. The P-code is normally encrypted using National Security Agency cryptographic techniques, and decryption capability is available only to the military and other authorized users as determined by the U. Department of Defense. The encryption process, known as anti-spoofing A-S , denies unauthorized access to the P-code and also significantly improves a receiver's ability to resist locking onto mimicked GPS signals, which could provide incorrect positioning information to a GPS user. P-code availability on both the L1 and L2 carrier signals through decryption capability provides authorized users with more accurate positioning and is known as the Precise Positioning Service PPS.

Introduction to GPS

SA is a deliberate degradation in GPS accuracy accomplished by intentionally varying the precise time of the clocks on board the satellites, which introduces errors into the GPS signal, and by providing incorrect orbital positioning data in the GPS navigation message. SA is normally set to a level that will provide meter 2 drms positioning accuracy to users of the SPS. SPS accuracy is normally represented using a horizontal 2 drms measurement, or twice the root mean square radial distance error.

Normally, 2 drms can be graphically represented as a circle about the true position containing approximately 95 percent of the position determinations. GPS Selective Availability SA within a decade in a manner that allows adequate time and resources for our military forces to prepare fully for operations without SA. In practice, there are several sources of error other than SA that can affect the accuracy of a GPS-derived position. These include unintentional clock and orbital errors, errors caused by atmospheric delays, multipath errors, errors caused by receiver noise, and errors due to poor satellite geometry.

The differential GPS DGPS method is based on knowledge of the highly accurate, geodetically surveyed location of a GPS reference station, which observes GPS signals in real time and compares their ranging information to the ranges expected to be observed at its fixed point.

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Differential corrections can be broadcast to GPS users, who can apply the corrections to their received GPS signals or computed positions. Corrections can also be stored for later analysis and dissemination. Differential techniques are used in many civilian applications to eliminate the effects of SA. This technique, known as carrier phase tracking, works by determining which portion of the 19 centimeter L1 and 24 centimeter L2 carrier waves are striking the antenna at a given instant in time, thus revealing the phase of the received signals.


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With subsequent signal processing that includes squaring or cross-correlating the carrier waves, carrier phase tracking can produce very precise measurements, sometimes as good as 1 to 5 millimeters. Thus, the technique is valuable for high-performance applications. The difficulty with using carrier phase tracking is determining the exact number of carrier wave cycles along the path from the GPS satellites to the receiver's antenna, also known as ambiguity resolution.

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For static positioning, this can be accomplished by various techniques that include knowing the approximate location of the antenna and simultaneously tracking signals from all satellites in view of the antenna. The process is more difficult for real-time dynamic positioning but can still be used. A variety of federal, state, county, and public sector organizations and their counterparts in other countries are establishing or planning to use networks of GPS reference stations, utilizing both differential and carrier phase tracking techniques, for either real-time navigation or post-processed positioning.

The use of GPS networks for research in the Earth and oceanic sciences has been well established for a number of years.


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