Understanding the Global Positioning System (GPS)

Diana Cooksey
MSU GPS Laboratory
Department of Land Resources and Environmental Sciences
Montana State University-Bozeman

This document provides a mostly verbal explanation of the Global Positoning System (with a few graphics). Please see the GPS slide shows for an explanation which includes text and color graphics.

Contents

What is GPS?

Space_Segment:_The_GPS_Constellation
Satellite_Orbits
Satellite_Signals
Control_Segment:_U.S._DoD_Monitoring
User Segment: Military and Civilian GPS Users
GLONASS
GALILEO

How Can GPS be Used?

GPS_Applications_in_Agriculture
GPS_Navigation:_Land,_Sea_and_Air
GPS_Applications:_Mapping_and_Surveying
Other GPS Applications: The Possibilities are Endless

How Does GPS Work?

Calculating_a_Position
Measuring_Distance_to_Satellites
Using the Distance Measurements to Calculate a Position
GPS_Error
Geometric_Dilution_of_Precision_(GDOP)
Selective_Availability_(SA)
How_to_Reduce_GPS_Error
Differential_Correction
Differential_Correction_Sources


What is GPS?

The Global Positioning System (GPS) is a satellite-based system that can be used to locate positions anywhere on the earth. Operated by the U.S. Department of Defense (DoD), NAVSTAR (NAVigation Satellite Timing and Ranging) GPS provides continuous (24 hours/day), real-time, 3-dimensional positioning, navigation and timing worldwide. Any person with a GPS receiver can access the system, and it can be used for any application that requires location coordinates.

The GPS system consists of three segments: 1) The space segment: the GPS satellites themselves, 2) The control system, operated by the U.S. military, and 3) The user segment, which includes both military and civilian users and their GPS equipment.

Space Segment: The GPS Constellation

The first GPS satellite was launched by the U.S. Air Force in early 1978. There are now at least 24 satellites orbiting the earth at an altitude of about 11,000 nautical miles. The high altitude insures that the satellite orbits are stable, precise and predictable, and that the satellites' motion through space is not affected by atmospheric drag. These 24 satellites make up a full GPS constellation.

The GPS satellites are powered primarily by sun-seeking solar panels, with nicad batteries providing secondary power. On board each GPS satellite are four atomic clocks, only one of which is in use at a time. These highly accurate atomic clocks enable GPS to provide the most accurate timing system that exists.

Satellite Orbits
There are four satellites in each of 6 orbital planes. Each plane is inclined 55 degrees relative to the equator, which means that satellites cross the equator tilted at a 55 degree angle. The system is designed to maintain full operational capability even if two of the 24 satellites fail.

GPS constellation

GPS satellites complete an orbit in approximately 12 hours, which means that they pass over any point on the earth about twice a day. The satellites rise (and set) about four minutes earlier each day.

Satellite Signals
GPS satellites continuously broadcast satellite position and timing data via radio signals on two frequencies (L1 and L2). The radio signals travel at the speed of light (186,000 miles per second) and take approximately 6/100ths of a second to reach the earth.

The satellite signals require a direct line to GPS receivers and cannot penetrate water, soil, walls or other obstacles. For example, heavy forest canopy causes interference, making it difficult, if not impossible, to compute positions. In canyons (and "urban canyons" in cities) GPS signals are blocked by mountain ranges or buildings. If you place your hand over a GPS receiver antenna, it will stop computing positions.

Two kinds of code are broadcast on the L1 frequency (C/A code and P code). C/A (Coarse Acquisition) code is available to civilian GPS users and provides Standard Positioning Service (SPS). Using the Standard Positioning Service one can achieve 15 meter horizontal accuracy 95% of the time. This means that 95% of the time, the coordinates you read from your GPS receiver display will be within 15 meters of your true position on the earth. P (Precise) code is broadcast on both the L1 and L2 frequencies. P code, used for the Precise Positioning Service (PPS) is available only to the military. Using P code on both frequencies, a military receiver can achieve better accuracy than civilian receivers. Additional techniques can increase the accuracy of both C/A code and P code GPS receivers.

Control Segment: U.S. DoD Monitoring

The U.S. Department of Defense maintains a master control station at Falcon Air Force Base in Colorado Springs, CO. There are four other monitor stations located in Hawaii, Ascension Island, Diego Garcia and Kwajalein. The DoD stations measure the satellite orbits precisely. Any discrepancies between predicted orbits and actual orbits are transmitted back to the satellites. The satellites can then broadcast these corrections, along with the other position and timing data, so that a GPS receiver on the earth can precisely establish the location of each satellite it is tracking.

User Segment: Military and Civilian GPS Users

The U.S. military uses GPS for navigation, reconnaissance, and missile guidance systems. Civilian use of GPS developed at the same time as military uses were being established, and has expanded far beyond original expectations. There are civilian applications for GPS in almost every field, from surveying to transportation to natural resource management to agriculture. Most civilian uses of GPS, however, fall into one of four categories: navigation, surveying, mapping and timing.

GLONASS

The Russian government has developed a system, similar to GPS, called GLONASS. The first GLONASS satellite launch was in October 1982. The full constellation consists of 24 satellites in 3 orbit planes, which have a 64.8 degree inclination to the earth's equator. The GLONASS system now consists of 12 healthy satellites. GLONASS uses the same code for each satellite and many frequencies, whereas GPS which uses two frequencies and a different code for each satellite. Some GPS receiver manufacturers have incorporated the capability to receive both GPS and GLONASS signals. This increases the availability of satellites and the integrity of combined system.

GALILEO

Galileo is Europe's contribution to the next generation Global Navigation Satellite System (GNSS). Unlike GPS, which is funded by the public sector and operated by the U.S. Air Force, Galileo will be a civil‑controlled system that draws on both public and private sectors for funding. The service will be free at the point of use, but a range of chargeable services with additional features will also be offered. These additional features would include improved reception, accuracy and availability. Design of the Galileo system is being finalized and the delivery of initial services is targeted for 2008.

How can GPS be used?

GPS Applications in Agriculture

More and more producers today are using precision farming techniques that can help increase profits and protect the environment. Precision, or site-specific farming involves applying fertilizer, pesticides and other inputs only where they are needed. GPS-guided equipment is often used for variable rate application of fertilizer (based on soil tests) or pesticides (based on pest survey). GPS can also be used to develop the initial reference maps upon which variable rate applications are based. A GPS system on a combine with a yield monitor can be used to develop an on-the-go yield map or can be used to map weed locations from the combine when harvesting. Mounted in an airplane, GPS can be used to guide aerial spraying operations.

GPS can be used to locate weed, insect or diseases infestations and monitor their spread. It can also be used to navigate back to previously mapped infestations to apply controls. A field map can be created using GPS to record the coordinates of field borders, fence lines, canals, pipelines, and point locations such as wells, buildings, and landscape features. The resulting field map might be the first layer a producer would develop for an on-farm GIS (Geographic Information System). Additional layers showing crop damage from hail or drought, and riparian areas or wetlands could be mapped using GPS. Ranchers could use GPS to develop rangeland utilization maps and to navigate back to previously mapped areas or monitoring sites.

GPS Navigation: Land, Sea and Air

GPS is being used for emergency response (fire, ambulance, police), search and rescue, fleet management (trucking, delivery vehicles, and public transportation) and for automobile guidance systems. Recreational uses of GPS include navigation while hiking, hunting, or skiing. GPS is even used on golf courses to track golf carts, and to let players know how far it is to the center of the greens.

On our nation's waterways, GPS is being used for recreational sailing and fishing and for commercial shipping fleet management. Assisted steering, risk assessment and hazard warning systems for marine navigation are being developed using GPS.

In the air, GPS is being used for en-route navigation (helicopter, airplane, hot-air balloon), aircraft landing, and air-collision avoidance systems.

GPS Applications: Mapping and Surveying

GPS applications in natural resource management include inventory and mapping of soils, vegetation types, threatened and endangered species, lake and stream boundaries and wildlife habitat. GPS has been used to aid in damage assessment after natural disasters such as fires, floods and earthquakes. GPS has also been used to map archaeological sites and for infrastructure (streets, highways and utilities) mapping, management, and planning for future growth. Engineers use GPS for surveying when building roads, bridges and other structures.

Other GPS Applications: The Possibilities are Endless

Other uses of GPS include real estate valuation and taxation assessment, air quality studies, environmental protection, demographic analysis including marketing studies, atmospheric studies, oil and gas exploration, and scientific exploration. There are many additional current and possible uses for GPS. Any application where location information is needed is a possible candidate for GPS.

How does GPS work?

Calculating a Position

A GPS receiver calculates its position by a technique called satellite ranging, which involves measuring the distance between the GPS receiver and the GPS satellites it is tracking. The range (the range a receiver calculates is actually a pseudorange, or an estimate of range rather than a true range) or distance, is measured as elapsed transit time. The position of each satellite is known, and the satellites transmit their positions as part of the "messages" they send via radio waves. The GPS receiver on the ground is the unknown point, and must compute its position based on the information it receives from the satellites.

Measuring Distance to Satellites
The first step in measuring the distance between the GPS receiver and a satellite requires measuring the time it takes for the signal to travel from the satellite to the receiver. Once the receiver knows how much time has elapsed, it multiplies the travel time of the signal times the speed of light (because the satellite signals travel at the speed of light, approximately 186,000 miles per second) to compute the distance. Distance measurements to four satellites are required to compute a 3-dimensional (latitude, longitude and altitude) position.

In order to measure the travel time of the satellite signal, the receiver has to know when the signal left the satellite and when the signal reached the receiver. Knowing when the signal reaches the receiver is easy, the GPS receiver just "checks" its internal clock when the signal arrives to see what time it is. But how does it "know" when the signal left the satellite? All GPS receivers are synchronized with the satellites so they generate the same digital code at the same time. When the GPS receiver receives a code from a satellite, it can look back in its memory bank and "remember" when it emitted the same code. This little "trick" allows the GPS receiver to determine when the signal left the satellite.

Travel time of satellite signals


Using the Distance Measurements to Calculate a Position
Once the receiver has the distance measurements, it's basically a problem of geometry. If it "knows" where the four satellites are, and how far it is from each satellite, it can compute its location through trilateration. Here's an illustration of how it works.

1) The GPS receiver "locks on" to one satellite and calculates the range to be 12,000 miles. This fact helps narrow the receiver location down, but it only tells us that we are somewhere on a sphere which is centered on the satellite and has a 12,000 mile radius. Many of the locations on that sphere are not on earth, but out in space.

1 measurement

2) Now, consider that the receiver picks up a signal from a second satellite and calculates the range between the receiver and the satellite to be 11,000 miles. That means we are also somewhere on a sphere with an 11,000 mile radius with the second satellite at the center. We must, therefore, be somewhere where these two spheres intersect. When the two spheres intersect, a circle is formed, so we must be somewhere on that circle.

2 measurements

3) If the receiver picks up another satellite, say at 11,5000 miles away, another sphere is formed, and there are only two points where the three spheres intersect.

3 measurements

Usually the receiver can discard one of the last two points because it is nowhere near the earth. So, we're left with one point which is the location of the GPS receiver.

4) In practice, a fourth measurement is needed to correct for clock error.

GPS Error

There are many sources of possible errors that will degrade the accuracy of positions computed by a GPS receiver. The travel time of GPS satellite signals can be altered by atmospheric effects; when a GPS signal passes through the ionosphere and troposphere it is refracted, causing the speed of the signal to be different from the speed of a GPS signal in space. Sunspot activity also causes interference with GPS signals. Another source of error is measurement noise, or distortion of the signal caused by electrical interference or errors inherent in the GPS receiver itself. Errors in the ephemeris data (the information about satellite orbits) will also cause errors in computed positions, because the satellites weren't really where the GPS receiver "thought" they were (based on the information it received) when it computed the positions. Small variations in the atomic clocks (clock drift) on board the satellites can translate to large position errors; a clock error of 1 nanosecond translates to 1 foot or .3 meters user error on the ground. Multipath effects arise when signals transmitted from the satellites bounce off a reflective surface before getting to the receiver antenna. When this happens, the receiver gets the signal in straight line path as well as delayed path (multiple paths). The effect is similar to a ghost or double image on a TV set.

Geometric Dilution of Precision (GDOP)
Satellite geometry can also affect the accuracy of GPS positioning. This effect is called Geometric Dilution of Precision (GDOP). GDOP refers to where the satellites are in relation to one another, and is a measure of the quality of the satellite configuration. It can magnify or lessen other GPS errors. In general, the wider the angle between satellites, the better the measurement (see GPS Basics slide show for an illustration). Most GPS receivers select the satellite constellation that will give the least uncertainty, the best satellite geometry.

GPS receivers usually report the quality of satellite geometry in terms of Position Dilution of Precision, or PDOP. PDOP refers to horizontal (HDOP) and vertical (VDOP) measurements (latitude, longitude and altitude). You can check the quality of the satellite configuration your receiver is currently using by looking at the PDOP value. A low DOP indicates a higher probability of accuracy, and a high DOP indicates a lower probability of accuracy. A PDOP of 4 or less is excellent, a PDOP between 5 AND 8 is acceptable, and a PDOP of 9 or greater is poor. Another term you may encounter is TDOP, or Time Dilution of Precision. TDOP refers to satellite clock offset. On a GPS receiver you can set a parameter known as the PDOP mask. This will cause the receiver to ignore satellite configurations that have a PDOP higher than the limit you specify.

Selective Availability (SA)
Selective Availability, or SA, occurred when the DoD intentionally degraded the accuracy of GPS signals by introducing artificial clock and ephemeris errors. When SA was implemented, it was the largest component of GPS error, causing error of up to 100 meters. SA is a component of the Standard Positioning Service (SPS), which was formally implemented on March 25, 1990, and was intended to protect national defense. SA was turned off on May 1, 2000.

Table 1. lists the possible sources of GPS error and their general impact on positioning accuracy.

Table 1. GPS Error Budget

Error source Potential error Typical error
Ionosphere 5.0 meters 0.4 meters
Troposphere 0.5 meters 0.2 meters
Ephemeris data 2.5 meters 0 meters
Satellite clock drift 1.5 meters 0 meters
Multipath 0.6 meters 0.6 meters
Measurement noise 0.3 meters 0.3 meters
Total ~ 15 meters ~ 10 meters



How to Reduce GPS Error
You've probably heard people talk about getting 1 to 5 meter accuracy with a GPS receiver, or even centimeter or millimeter accuracy. Is there a way to cancel out the errors and get better than 15 meter accuracy? The answer is yes, but the level of accuracy depends on the type of equipment you are using. The following discussion describes a technique used to achieve 1 to 5 meter accuracy using mapping (resource) grade receivers. Some mapping grade receivers are even capable of sub-meter accuracy, but the increased accuracy comes at a price. Survey grade receivers are the most accurate, capable of centimeter or even millimeter accuracy, depending on the equipment, but they use more advanced techniques to achieve this level of accuracy and, naturally, are more expensive. Recreational grade receivers usually can receive real-time differential corrections, but they cannot store a file that can be differentially corrected using post-processing methods.

Differential Correction
Differential correction is a method used to reduce the effects of atmospheric error and other sources of GPS positioning error (differential correction cannot correct for multipath or receiver error; it counteracts only the errors that are common to both reference and roving receivers). It requires, in addition to your "roving" GPS receiver, a GPS receiver on the ground in a known location to act as a static reference point. This type of setup is often called a GPS base station. Since the base station "knows" where it is, it can compute the errors in its position calculations (in reality, it computes timing errors) and apply them to any number of roving receivers in the same general area. This requires that the base and rover receivers "see" the same set of satellites at the same time.

The base station, depending upon how it is configured, can correct roving GPS receiver data in one (or both) of two ways. 1) In the first method, called real-time differential correction or real-time differential GPS (DGPS), the base station transmits (usually via radio link) error correction messages to other GPS receivers in the local area. In this case, the positions you read on your GPS receiver while you are out collecting data, are the corrected positions. 2) The second method, called post-processed differential correction, is performed on a computer after the roving receiver data are collected. While you are out in the field collecting data, the positions you read on your roving GPS receiver are uncorrected. It is not until you take your rover files back to the office and process them using differential correction software and data from the base station file, that you get corrected positions. The base station file contains information about the timing errors. This information allows the differential correction software to apply error corrections to the roving receiver file during processing. Since the base and rover receivers have to "see" the same set of satellites at the same time, the base file has to start before the rover file starts, and end after the rover file ends (a base station is normally set up to track all satellites in view, insuring that it will "see" at least the four satellites that the roving receiver is using to compute positions). Post-processed differential correction, then, requires both base and rover receivers that are capable of collecting and storing files. Most recreational grade receivers cannot collect and store files that can be differentially corrected.

Differential Correction Sources
Several options are available for obtaining differential corrections: 1) use a local base station, 2) use one of the wide-area differential GPS (WADGPS) services.

You can set up your own local base station or share a base station with other GPS users in your area. If you are using post-processed differential correction, the base station can usually serve users in an area with about a 2 to 300 mile radius (the further the roving receiver is from the base station, the less accurate the corrections). If you are using real-time differential correction, you must establish radio links, and your coverage area is limited by the strength of the radio transmissions. If you plan to set up your own base station, make sure the manufacturer can supply all the necessary components including base and rover receivers, radios (if using real-time correction) and differential correction software (if using post-processed correction).

Many government agencies operate GPS base stations and may provide correction files for post-processed differential correction. If you plan to use files from an operating base station, determine the manufacturer of the base station receiver. If you purchase a roving receiver from the same manufacturer, the base and rover files will be compatible and your rover files can be differentially corrected using software provided by the manufacturer. If your rover is made by a different manufacturer, you will probably have to convert the base files to Receiver Independent Exchange (RINEX) format before they can be used to differentially correct your rover data. Make sure your differential correction software can use a RINEX base file. If not, the rover file has to be converted to RINEX format and then differentially corrected using software provided by the base station manufacturer. In this situation, any attribute data stored in your roving receiver file will be lost because the RINEX format supports conversion of position data only. If you need to use a RINEX conversion, make sure you test it thoroughly before purchasing a receiver.

Some companies, such as Omnistar and RACAL provide differential corrections in real-time via their own communication satellite systems. To receive their signals you must purchase a special satellite receiver as well as the subscription service. The signals from satellites are generally available over a widespread area, hence the term wide-area differential GPS (WADGPS). Your GPS receiver must be able to receive the correction data from the satellite receiver and apply those corrections to the data it collects. Some companies offer an integrated GPS/satellite correction receiver so you don't have to purchase a separate GPS receiver. Just be sure the system will allow attribute data collection and can provide any other features you need.

The Nationwide Differential GPS (NDGPS) system is another source for differential correction data. NDGPS stations exist around the country, and the system is currently being expanded, with the hope of providing full coverage throughout the continental United States . An additional source of real-time differential correction data is the Wide Area Augmentation System (WAAS) operated by the Federal Aviation Administration (FAA). Many receivers are now WAAS compatible.