The Basics of the GPS System
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November 13 2009
GPS System Basics
The Golbal Positioning System (known as GPS) is the backbone of the vehicle tracking industry. The GPS System was developed and deployed by the United States (specifically the United States Department of Defense) as a military system only. In 1980, by executive order, part of the GPS System was made available for civilian use. The GPS System is satellite-based and available in all weather conditions worldwide 24 hours a day. There are three distinct parts to the GPS System - the Space Segment (the satellites), the Control Segment (ground stations operated by the US Air Force), and the User Segment (the GPS Receiver - for vehicle tracking a device located in the vehicle which also contains a radio modem and logic circuits to upload the position information to the Tracking System Controllers and Servers). The Space Segment of the GPS System consists of a constellation of satellites in 6 orbital planes (4 satellites per plane) at 10,900 Nautical Miles altitude and 55 degrees inclination, with a speed of approximately 7,000 miles per hour. Each satellite completely orbits the earth every 12 hours. The satellites are solar powered but also have backup batteries so that the satellite can continue to function during an eclipse. They are also equipped with small rocket motors to allow repositioning the satellite as needed to correct the orbit. The satellites transmit on two L-band (UHF) frequencies: L1 = 1575.42 MHz and L2 = 1227.6 MHz. At these frequencies the signal is line-of-sight and will penetrate glass, foam, plastic and glass but not metal, trees or most buildings. This is important for the selection of a proper location for the GPS Receiver Antenna. At the present time, our civilian receivers only use the L1 frequency. The L2 frequency is used to improve the accuracy of the positioning signals by measuring the delay caused by the ionosphere. Further, there is a Course Acquisition code which is available to civilians and additional Precise Code which is restriced from use by most non-government agencies by encripting the signals. All of the GPS satellites transmit on the same 2 frequencies at the same time but the signals from each satellite contain a unique identifier so that the signals can be used reliably. Tha main purpose of these coded signals is to allow the user's receiver to calculate the time it takes for the signals to reach the receiver plus information on the location of each satellite by a navigation message containing the satellite's orbit, clock corrections and system parameters. Visit the U.S. Naval Observatory Home Page for up-to-date information on the current GPS constellation.
The Control Segment consists of five Monitor Stations (Hawaii, Kwajalein, Ascension Island, Diego Garcia, Colorado Springs), three Ground Antennas, (Ascension Island, Diego Garcia, Kwajalein), and a Master Control Station (MCS) located at Schriever AFB in Colorado. The monitor stations passively track all satellites in view, accumulating ranging data. This information is processed at the MCS to determine satellite orbits and to update each satellite's navigation message. Updated information is transmitted to each satellite via the Ground Antennas. The MCS can then upload correction information to the satellites via the Ground Antennas. This keeps the information in the satellites updated with accurate clock and orbital data.
The User Segment includes the Receiver and the User Interface. The Receiver converts the signals received from the satellites into a positional fix, velocity and time estimates. Since the receiver "learns" where each satellite is in the sky, and since radio signals travel at a constant speed (the speed of light), and Velocity x Travel Time = Distance. As long as the time clock in the satellite is syncronized with the time clock in the receiver (this is a primary component of the signals sent from the satellite). The satellite has 4 highly accurate atomic clocks on board which compare the time of each clock for accuracy. The time delay between the satellite clocks and the receiver clock (determined by the calculation of a pseudo-random code) is the time it took the signal to travel from each satellite to the receiver. This comparison of time delays, combined with a list of where each satellite should be at a given moment in the sky (called an Almanac), is what allows the receiver to achieve a "fix" at a place on or above the surface of the earth. Good signals from 3 satellites are the minimum for a "2-Dimensional fix" on the surface of the earth, with 4 satellites a "3-Dimensional fix" including altitude can be determined. In practice, GPS receivers look for up to 12 satellite signals at a time because the more spread out in the sky the satellites being used to determine position are, the more accurate the location calculation - this is referred to as geometric dilution of precision (GDOP). The farther apart the satellites are, the smaller the GDOP (meaning there is little dilution of precision). Better quality receivers can selectively rely less on calculations based on satellites that are close together in the view of the sky and use those that provide a better angle - even if the signal is not quite as strong.
One point to note is that the time clocks on the satellites are combined with the Master Control Clock to acieve GPS time which does not have "leap" seconds, so the GPS time is not exactly the same as UTC time. GPS Time is a little bit ahead of UTC time - a matter of less than a minute, but using GPS time as a clock on earth has this limitation. In practice for GPS Tracking purposes, this time advance partially offsets the time it takes for signals transmitted from the vehicle to the data center servers (network delay). GPS-UTC is at this writing 15 (GPS WILL BE AHEAD OF UTC BY 15 SECONDS) - for updates see NOTICE ADVISORY TO NAVSTAR USERS.
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