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An Introduction to the GPS System
The most widely used SS application is, of course, GPS!


Editor's Note:This article is reprinted from "Spread Spectrum Scene," December 1993


This article represents the first of a new series of articles on the Global Positioning System (GPS). GPS represents what is probably the first truly widespread use of spread spectrum technology in a high volume product.

While GPS is certainly not new (my first involvement with GPS was back in the late 1970's), it is only now getting widespread coverage in the general media. The purpose of this series is to familiarize the reader with the operation, characteristics, and capabilities of the GPS service. In addition, depending on how much equipment I can coerce vendors and manufacturers to loan to me for evaluation, both for my upcoming book and for this column, I will be reviewing the offerings of various manufacturers. Although the primary focus of this column will be strictly on GPS equipment, I am looking for various types of spread spectrum devices to review in my upcoming book, which will focus on GPS, but that will also cover spread spectrum technology in a broad sense.

Personally, I am an engineering consultant with my own consulting firm, Aerospace Consulting. Currently, I am writing a book on GPS and spread spectrum for Artech House. This book should be available in the early summer of 1994. I am also currently looking to develop GPS applications for clients.

History of Navigation

Ever since man first left his home in search of food, wealth, or whatever, he has needed a way to find his way back home. He also needed a way to repeat his trip to a favored trading place, or a prime hunting location. In the earliest days, this was done by simply following landmarks, either along the road, or landmarks that consisted of prominent features on the coastline. This method of navigation was obviously very limited when it comes to traveling across oceans, or even large bodies of water. Navigation by reference to the sun, moon, and the stars was the next logical progression. Techniques related to celestial navigation have improved over the last couple of thousand years to the point where now it is possible to locate a position to an accuracy of better than one hundred feet. However, the instruments required to measure latitude and longitude to an accuracy of one hundred feet or so are definitely not suited to the rapid, dynamic measurements that are necessary for navigation.

Navigation remained little changed from the late 1700's to the early 1900's. The primary instrument was the sextant and a highly accurate chronometer, coupled with a compass and something to measure the distance traveled since the last "fix". The problem with this technique, obviously, is that you can only fix your position with any real degree of accuracy when the sun or stars are visible. During cloudy weather, no reliable means of locating one's position existed. Furthermore, although a compass can tell you in which direction you are headed, it can not tell you in what direction you are actually traveling. These two directions could be somewhat different, depending on conditions of wind, waves, and currents. Thus, dead reckoning is often not a very satisfactory method of navigating for airplanes and ships.

In the early 1900's, electronic means of navigation were developed. These included such systems as radio direction beacons. In these systems, the direction to the transmitter could be determined with an antenna with a cardioid pattern. This type of pattern has a very sharp null in one direction. By pointing this null at the transmitter, the direction to the transmitter could be determined. By determining the direction to two or more transmitters, the user could determine his position from the intersection of the lines pointing to the transmitters. Of course, for this to work the user must know the precise location of the transmitter, and he must be within reception range. Furthermore, since what is being measured is an angle, the precision of the observer's location becomes progressively worse as the distance from the transmitters increases.

In a similar way, the introduction of radar in the 1940's allowed the user to measure both direction and distance to a prominent landmark simultaneously. This simplified the navigation problem somewhat, but radar had the limitations of generally working over a relatively short range -- usually not much longer than over the horizon for the determination of landmarks -- which limited its use to night navigation, or navigation during times when visible landmarks were obscured by fog or heavy precipitation.

Better systems for long range navigation were based on time measurements. Since now the characteristic that is being measured is the time of travel or, more specifically, the difference in travel time, of radio waves, the errors tend to be independent of the distance between the observer and the transmitting locations. This is a big advantage over angular based systems, where the error increases rapidly as distance from the transmitting locations increases. Of course, there are some distortions that get introduced into the time based systems that are based on distance -- such as atmospheric and ionospheric disturbances -- but these are relatively minor compared to the uncertainties introduced by angular measurement over large distances. These time based systems include DECCA, LORAN, and OMEGA.

The first widespread use ofsatellites for navigation (and surveying) was the Transit system. This is a dual frequency system that operates at 150 and 400 MHz. It determines position by measuring the Doppler shift of radio signals from a satellite as it passes within range of the observer. Since the location and orbital path of the satellite is known with a high degree of precision, it is possible to determine the location of the observer by observing the time-varying Doppler shift of the two transmitted frequencies. It is currently in widespread use to determine the location of ships at sea and to determine the position of an observer where other precision navigation systems are not available. This system can provide position fixes as good as about 30 to 100 meters. This offers good precision in non-dynamic situations, but position fixes are only available when the satellites are in view, which, at best, is perhaps once every hour.

GPS, unlike any of these other systems, offers a real-time position fix with an accuracy of 3 to 100 meters on a 24 hour per day basis. It also offers a way to determine time to a precision of better than a few hundred nanoseconds almost anywhere on / around the surface of the earth. These are capabilities that will completely revolutionize many areas of life in the near future.

Basics of GPS

The GPS system consists of a constellation of 24 satellites. While not officially declared fully operational, for all practical purposes the system is now fully operational. These satellites orbit the earth at an altitude of about 10,900 miles and at an inclination of 55 degrees. As I will demonstrate in my next column, this orbit translates to an orbital period of 12 hours. The orbits are distributed around the earth in such a way that at least 4 satellites are always visible from virtually any point on the surface of the earth. This provides a means of precisely determining the position of the user in longitude, latitude, and altitude. The satellites operate at two frequencies, known as L1 and L2. These two frequencies are 1575.42 MHz and 1227.6 MHz, respectively.

The whole system operates at a system clock frequency of 10.23 MHz, which is an exact submultiple of the L1 and L2 frequencies. The two transmission frequencies are modulated with a pseudo-random signal to produce spread spectrum signals. The L1 channel is modulated with both a 1.023 Mbps pseudo-random code known as the C/A (course/acquisition) code and a 10.23 Mbps PN code known as the P (precision) code. The L2 channel is only modulated with the P code. The two codes are considerably different in characteristics. The L1 code repeats every 1023 bits, or every 1 millisecond. The P code, on the other hand, only repeats itself every 267 days. Furthermore, the P code can be encrypted by the Department of Defense, so as to make it unavailable to civilian (or unauthorized) users. This limits the best accuracy obtainable by unauthorized users to about 30 meters, while allowing authorized users to achieve accuracies of up to 3 meters. Additionally, the DOD, at its discretion, can disseminate slightly inaccurate information pertaining to the location of the satellites, so as to further degrade the accuracy obtainable by unauthorized users to about 100 meters. These accuracy degradation capabilities are important, since hostile nations could use the information against us in times of war, as the Iraqi government did for positioning SCUD missile launchers during the Gulf War, according to a February 7, 1991 article in the Wall Street Journal.

As time has gone by, however, more potential applications have been developed for GPS and many techniques have been developed to augment the accuracy available to unauthorized users. Techniques like carrier phase tracking and differential GPS can allow users to obtain centimeter level accuracy, especially in cases where measurements are being made at a fixed location. However, it is well established that even aircraft positions can be determined to an accuracy of better than several meters, even in real time. This technique is also being widely used to navigate ships in narrow channels and along the coast of the U.S., by taking advantage of differential correction signals that are being transmitted over various navigation beacons by the U.S. Coast Guard.

Other applications include moving map displays in cars and trucks. Attitudes of aircraft and spacecraft can also be determined with GPS. GPS equipment is currently set-up in the San Francisco area to allow researchers to measure the amount of shifting in the earth's surface during the next earthquake. GPS was also recently used to measure the height of Mount Everest and K2. Forest fire fighters use GPS to define the extent of fires and townships are using GPS equipped vans to map roads in a small fraction of the time that would be required for conventional surveying tech- niques.

In Closing

These and other applications, as well as more specifics on the operation of the system, will be included in future columns. If you have a spread spectrum product that you would like to see mentioned in my upcoming book on GPS and spread spectrum, please contact me.


1. GPS World, March 1991, Newsfront, p. 18.

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More on GPS

The Global Positioning System

The Global Positioning System (GPS) is a satellite-based navigation system developed and operated by the US Department of Defense. GPS permits land, sea and airborne users to determine their three-dimensional position, velocity and time. This service is available to military and civillian users around the clock, in all weather, anywhere in the world.

GPS uses the NAVSTAR (NAVigation Satellite Timing and Ranging) satellites. The current constellation consists of 21 operational satellites and 3 active spares. This provides a GPS receiver with 4 to 12 useable satellites 'in view' at any time. A minimum of 4 satellites allows the GPS card to compute latitude, longitude, altitude (with reference to mean sea level) and GPS system time.

The Space Segment

The space segment is composed of the 24 NAVSTAR GPS satellites. They orbit the earth at an altitude of 10,898 nautical miles in six 55 degree orbital planes, with 4 satellites in each plane. The orbital period of each satellite is approximately 12 hours.

The GPS satellite signal contains information used to identify the satellite, as well as provide position, timing, ranging data, satellite status and the updated ephemeris (orbital paramaters). The satellites are identified by either the Space Vehicle Number (SVN) or the Pseudorandom Code Number (PRN).

GPS satellites transmit on two L-Band frequencies: 1.57542 GHz (L1) and 1.22760 GHz (L2). The L1 signal has a sequence encoded on the carrier frequency by a 'spread spectrum' modulation technique which contains two codes, a precision (P) code and a Coarse/Acquisition (C/A) code. (The C/A code is also sometimes incorrectly referred to as 'Civillian/Acquisition' code.) The L2 carrier contains only P code which is encrypted for military and authorized civillian users. Most commercially available GPS receivers utilize the L1 signal and the C/A code. There are a few civillian receivers capable of utilizing the L1 P code without actually decoding it.

The Control Segment

The control segment consists of a worldwide system of tracking and monitoring stations. The 'Master Control Facility' is located at Falcon AFB in Colorado Springs, CO. The monitor stations measure signals from the GPS satellites and relay the information they collect to the Master Control Station. The Master Control Station uses this data to compute precise orbital models for the entire GPS constellation. This information is then formatted into updated navigation messages for each satellite.

The User Segment

The user segment consists of the GPS receivers, processors and antennas utilized for positioning and timing by the community and military. The GPS concept of operation is based on satellite ranging. Users figure their position on the earth by measuring their distance to a group of satellites in space. Each GPS satellite transmits an accurate position and time signal. The user's receiver measures the time delay for the signal to reach the receiver. By knowing the distance to four points in space, the GPS receiver is able to triangulate a three-dimensional position.


  • Precise Positioning System (PPS) - P Code
    • 17.8m horizontal accuracy
    • 27.7m vertical accuracy
    • 100ns time accuracy
  • Standard Positioning System (SPS) - C/A Code
    • 100m horizontal accuracy
    • 156m vertical accuracy
    • 167ns time accuracy

Sources of error

  • Bias error from Selective Availability (SA)
  • Other bias errors
  • Blunders or receiver errors (from hardware or software failures)
  • Noise errors
    • PRN code noise
    • Internal receiver noise
In order to compensate for these errors, a correction signal is needed. This signal, called 'Differential Correction', allows civillian users of GPS to achieve far greater accuracy than the GPS signal alone is able to provide.

Differential GPS

Differential GPS (DGPS) is the regular GPS with an additional correction signal added. DGPS uses a reference station at a known point (also called a 'base station') to calculate and correct bias errors. The reference station computes corrections for each satellite signal and broadcasts these corrections to the remote, or field, GPS receiver. The remote receiver then applies the corrections to each satellite used to compute its fix. Real time corrections are typically transmitted over a radio link.

Editor's Note:For more information and links to other GPS-related web sites, please see our GPS Stuff Page.

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