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Global Positioning Systems: An Introduction

Rohini Prinja (Graduate Student)

(Department of Computer Science, University of Minnesota)

  • GPS


The Global Positioning System (GPS) is a navigational system involving satellites and computer systems that can determine location (latitude, longitude and altitude) of any point on Earth. The satellite transmits signals which are received on Earth by a relatively in-expensive receiving device. By calculating the distance between four or more satellites and a receiver, a three-dimensional position on Earth is determined.

As described in [2], this system allows a person to figure out his/her location with the help of a simple radio receiver. The accuracy of the data (i.e. co-ordinates of a location) depends upon the quality of the receiver and procedures applied to collect the data. It can range from as good as a few millimeters to around a hundred meters. In addition to giving the current location, some GPS receivers can also store past data by transferring the data collected at a regular interval to a computer memory. This data can then be utilized for other applications such as Geographic Information Systems (GIS).

The main global positioning system in US is NAVSTAR (NAVigation System with Time And Ranging). The term GPS usually refers to NAVSTAR, which is a constellation of 27 Earth-orbiting satellites (24 in operation and three extras in case one fails), developed by the US Department of Defense (DOD). GLONASS (GLObal NAvigation Satellite System) and the Galileo Positioning System are the Russian and European Union counterparts of NAVSTAR respectively.

Historical Background

The modern history of navigation systems began with radio-based systems developed in the early twentieth century and used extensively for military purposes in World War II. The drawback of such systems was the tradeoff between the area that can be covered and accuracy as high frequency radio-waves are accurate but can cover only a small area.

Satellite navigation systems were built to overcome the drawback of radio-based navigation systems. The idea of satellite based navigation using radio waves started when Sputnik was launched into space by Russia. After the launch, researchers of the Massachusetts Institute of Technology determined the orbit of the Russian satellite by noting that Sputnik's radio signal increased as it approached and decreased as it left. So the fact that a satellite's position could be tracked from the ground was the first step in recognizing that a subject's whereabouts on the ground could be determined using radio signals from the satellite [5].
The U.S. Navy experimented with satellite navigation. In the mid-sixties, the Transit System was developed to cater to the navigational needs of submarines carrying Polaris nuclear missiles. The earlier method used in submarines called gyroscope-based navigation was not accurate enough. The Transit system comprised a half-dozen satellites that would circle the earth continuously in polar orbits. By measuring the Doppler shift of the radio signals, the submarines could locate its position within fifteen minutes. But this method was not foolproof.
To develop a foolproof (and/or error-proof) system, various ideas based on the earlier satellite navigation systems developed by or for the US DOD were brain-stormed in 1973 and the result was the NAVSTAR Global Positioning System, now commonly known GPS. In the beginning, three satellites, which were called ‘Block I’, were launched to test the proposed system, and in 1989 another set of ‘Block II’ satellites was launched. As mentioned in [4], the current GPS installation consists of 24 Block II/II A satellites and 1 Block I experimental satellite. The next generation of satellites, named Block IIR is also in orbit.
Scientific Fundamentals

Architecture of GPS:

The technical and operational characteristics of GPS are organized into three distinct segments: the space segment, the operational control segment, and the user equipment segment. The GPS signals, which are broadcast by each satellite and carry data to both user equipment and the ground control facilities, link the segments together into one system. Figure 1 below briefly characterizes the signals and segments of the Global Positioning System:

Figure1: Architectural Diagram of GPS explaining various segments

Space Segment

The space segment consists of 24 earth-orbiting satellites. Each satellite orbits the earth in a way so that every point on the planet is in radio contact with at least four satellites. A GPS satellite broadcasts three different kinds of signals as explained below:

  1. Almanac: This includes coarse time information (upto seconds precision) and status information about the satellite.

  2. Ephemeris: This sends the information with which a receiver can determine the exact position of the satellite. It is also called the Navigation Message or NM.

  3. Coarse Acquisition Code (C/A) and Precise code (P Code): These are the forms of two types of accurate clock information; the former is used for civilian purposes and the latter for governmental purposes. Each satellite sends a distinct C/A code, which allows it to be identified. C/A is broadcast at 1.023 MHz and P-code at 10.23MHz. Also, in normal mode, the P code is first encrypted into the Y-code, or P(Y), which can only be decrypted by units with a valid decryption key.

All three signals, NM, C/A and P(Y), are mixed together and sent on the primary radio channel, L1, at 1575.42 MHz. The P(Y) signal is also broadcast alone on the L2 channel, 1227.60 MHz ([5]).
Operation Control Segment

As noted in [4], the Operational control segment (OCS) consists of the master control station (MCS), located at Falcon Air Force Base in Colorado Springs, Colorado; remote monitor stations, located in Hawaii, Diego Garcia, Ascension Island, and Kwajalein; and uplink antennas located at three of the four remote monitor stations and at the MCS. The four remote monitor stations track and send information about the satellite to the MCS. The frequency with which this is done varies from 20 to 21 hours per day. Land-based and space-based communications are used to connect the remote monitoring stations with the MCS.

The MCS is responsible for overall satellite command and control, which includes maintaining the exact orbits of each satellite and determining any timing errors that may be present in the highly accurate atomic clocks aboard each satellite. Errors in a satellite's orbital position or in a satellite's timing are determined by analyzing the same signal and navigation message from each satellite that is used by the GPS receiver equipment.
User Segment

GPS user equipment varies widely in cost and complexity, depending on the receiver design and application. Receiver sets, which currently vary in price from approximately $400 or less to $30,000, can range from simple one-channel devices that only track one satellite at a time and provide only basic positioning information, to complex multi-channel units that track all satellites in view and perform a variety of functions. Most GPS receivers, however, consist of the same three basic components: (1) the antenna, which receives the GPS radio signal and in some cases provides anti-jamming capabilities; (2) the receiver-processor unit, which converts the radio signal to a useable navigation solution; and (3) a control/display unit, which displays the positioning information and provides an interface for receiver control.

GPS positioning capability is provided at no additional cost to civilian and commercial users worldwide but the accuracy level is restrained. The accuracy levels are of two types:

  1. Standard Positioning Service (SPS): Provided for civilian and commercial users and involves accuracy level of 100 meters.

  2. Precise Positioning Service (PPS): The U.S. military and its allies, and a select number of other authorized users, receive a specified accuracy level of 16 meters.

The full accuracy capability of GPS is denied to users of the SPS through a process known as Selective Availability, or SA. SA is the purposeful degradation in GPS navigation accuracy that is 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 100-meter positioning accuracy to users of the SPS. PPS receivers with the appropriate encryption keys can eliminate the effects of SA.

How GPS works:

This section describes how the above individual segments gel together and function:

Locating the User Device: The basis of GPS is ‘triangulation’ from satellites. It is a method of determining the relative positions of objects using the geometry of triangles. This can be thought of as a venn diagram as follows:

Figure2: Venn diagram showing how 3 satellites cover a particular area on earth’s surface.

S1, S2 and S3 are three satellites and User represents a GPS user on Earth’s surface. In Figure 2, the intersection of 3 circles is a collection of points but when this diagram is extrapolated to a 3-dimensional surface, it can be found that the 3 spheres intersect at 2 points. By measuring the distance from one other satellite (S4), the accurate position of the User on Earth can be found, since geometrically, distances from 4 spheres will intersect at only one point.
Measuring Distance from Satellite: Distance to a satellite is determined by measuring how long a radio signal takes to reach the receiver from that satellite. Multipling that travel time by the speed of light gives the distance.

    1. Each satellite in the message it sends, includes the time (T1) at which it sends message based on the atomic clock inside the satellite. When the signal hits the receiver, that time T2 is noted. T2-T1 gives the total time (T) taken by the signal in space.

    2. The Speed of light is taken to be 3 X 108 m/sec

    3. Distance (in meters) = Speed X T

As in the formula presented for calculating distance, it is obvious that finding the distance accurately is dependent upon measuring time accurately Satellites are accurate because they have atomic clocks on board. But receivers generally have a Quartz clock which is not as accurate as an atomic clock. However, to rectify this problem of inaccuracy at the receiver’s end, signals are exchanged between satellite and receiver to synchronize the clocks. The receiver calculates the necessary adjustment in its clock that will cause the four spheres (as described in #1) to intersect at one point. Based on this, it resets its clock to be in sync with the satellite's atomic clock. The receiver does this constantly whenever it's on, which means it is nearly as accurate as the expensive atomic clocks in the satellites.
Measuring Satellite Positions: To use the satellites as references for range measurements requires that their exact position be known. GPS satellites are so high up that their orbits almost always follow the same designated path. Minor variations in their orbits are measured by the Department of Defense and the error information is sent to the satellites, to be transmitted along with the timing signals.
Error Corrections: Finally, the delay that the signal experiences has to be corrected. There can be a number of reasons for the delay which are explained in the next sections. The delay in the signal in turn affects the distance calculation and hence the calculation of the triplet (longitude, latitude, altitude).
Sources of Error:

There are three main potential sources of errors in the position as calculated by any GPS receiver: the current time, position measurement of the satellite and delay in the signal. Errors in the current time can generally be reduced as described in the previous section, so the accuracy of GPS device is generally affected by the latter two factors. The following paragraphs mention briefly the different kind of errors that can affect GPS accuracy:

Selective Availability: Selective availability is the intentional error introduced to prevent the system from being exploited by hostile agents; this is explained in the Users Segment section.
Clock and Ephemeris errors: This deals with the errors that crop up due to erroneous clock time and position of the satellite.
Atmospheric Delays: The signal sent from the satellite gets delayed as it passes through various layers of atmosphere (mainly troposphere and ionosphere layers). This form of error can be eliminated using something called dual-frequency observation but it is not accessible to civilian users.
Multi-path errors: These errors occur when GPS signals get reflected from obstacles (such as buildings etc.) on the way to the receiver.

Receiver’s noise: Thermal noise produced at the receiver’s end can cause faulty observations, resulting in inaccurate output by the GPS receiver.
Poor satellite geometry: Satellite geometry in GPS allows any user to have access to signals from four satellites; however, based on the position on earth, this might not be possible for the simple reason that due to obstacles, a receiver cannot access the four satellites whose distances converge at a single point.
Measures of Capability of GPS:

GPS capability can be measured not only in terms of accuracy as described above but also in terms of integrity, availability, continuity of service, and resistance to radio frequency (RF) interference.

Accuracy is the ability to provide accurate coordinates of a point on earth and integrity is the ability of GPS to broadcast timely warnings to the receiver if the system is not to be used.
Another parameter is availability which is the percentage of time the system can be used (i.e. is available). Continuity of Service is the ability of the system to provide continuous service without interruption for a specified interval of time, also referred to as reliability.
If there are unwanted radio signals, the accuracy of a GPS receiver can be degraded. Hence resistance to radio frequency refers to the ability of the receiver to cancel out the effects of noise signals while calculating the position of co-ordinates.
Enhancements to GPS:

Many techniques have been designed to improve the accuracy of GPS. Some of them are:

    1. Differential GPS

    2. Carrier Phase (Interferometric) GPS

    3. Pseudolites

    4. Receiver Autonomous Integrity Monitoring (RAIM)

    5. Combined Use of GPS and GLONASS

    6. GPS/Inertial Navigation System (INS) Integration

    7. GPS and Loran-C

For more discussion on this topic, refer to [4].

Key Applications

GPS has become a vital global utility that is used in many arenas, some of which are described below:

  1. Military: GPS allows accurate targeting of various military weapons including cruise missiles and precision-guided munitions, as well as improved command and control of forces through improved locational awareness.

  2. Navigation: GPS is used by people around the world as a navigation aid in cars, airplanes, and ships. Hand-held GPS devices in the form of Personal Navigation Devices (PND) are used by mountain climbers and hikers.

  3. Location-based services: GPS functionality can be used by emergency services and location-based services to locate mobile phones. Assisted GPS is a GPS technology often used by the mobile phone because it reduces the power requirements of the mobile phone and increases the accuracy of the location obtained.

  4. Road and Highways: GPS can be used in conjunction with GIS to route traffic effectively to avoid congestion on busy highways.

  5. Space: GPS has provided many low-cost alternatives to activities in the space domain, for e.g.: The need of having costly atomic clocks on board spacecraft is eliminated by substituting them with relatively low-cost GPS receivers.

  6. Aviation: Aviators use the Global Positioning System to increase the safety and efficiency of flight. Space-based position and navigation enables three-dimensional position determination for all phases of flight from departure, en route, and arrival, to airport surface navigation.

  7. Marine: GPS provides the fastest and most accurate method for mariners to navigate, measure speed, and determine location. This enables increased levels of safety and efficiency for mariners worldwide

  8. Environment: GPS coupled with GIS can provide better data, which can be very helpful in predicting and reacting to environmental conditions.

  9. Surveying: More costly and precise receivers are used by land surveyors to locate boundaries, structures, and survey markers, and for road construction

  10. Agriculture: GPS Machine Guidance is used for tractors and other large agricultural machines via auto steer or a visual aid displayed on a screen, which is extremely useful for controlled traffic and row crop operations and when spraying.

  11. Precise time reference: Many systems that must be accurately synchronized use GPS as a source of accurate time. For instance, the GPS can be used as a reference clock for time code generators or NTP clocks. Also, when deploying sensors (for seismology or other monitoring application), GPS may be used to provide each recording apparatus with a precise time source, so that the time of events may be recorded accurately.

  12. Public Safety and Disaster Relief: Time plays an important role in effective rescue operations after a disaster. Knowing the precise location of landmarks, streets, buildings, emergency service resources, and disaster relief sites reduces that time -- and saves lives. GPS has made accurate data available easily.

Future Directions

GPS is a relatively new technology and many improvements are being suggested and implemented in the current system.

As described by the US government, future work is to do an extensive modernization program, including the implementation of a second and a third civil signal on GPS satellites. The second civil signal will improve the accuracy of the civilian service and supports some safety-of-life applications. The third signal will further enhance civilian capability and is primarily designed for safety-of-life applications, such as aviation

Improving the accuracy of systems is another promising area for future work in GPS. As described in [4], although GPS provides information fairly accurately, improvement is still required in certain operations like: all-weather aircraft carrier landings, surveying and mapping etc.

    1. Use of a 24-Satellite Ensemble Clock: Currently, clock offset corrections are determined on the ground and then sent to the individual satellites once a day as they pass over a GPS monitoring station. The Block IIR satellites will have the capability to determine their clock offsets autonomously relative to a space-based ensemble clock and exchange clock information with other satellites via cross-links every 15 minutes.

    2. Satellite-Based Integrity Monitoring: Perhaps the most innovative and promising method of signal integrity monitoring is through space-based monitoring, rather than ground-based monitoring. This capability, known as Satellite Autonomous Integrity Monitoring, or SAIM, would require the instrumentation of GPS satellites to monitor transmitted L-band signals from each other for accuracy and usability.

In addition, new applications need to be developed that make use of the data provided by GPS and build upon the key applications explained in the last section
How satellites are structured in space can have a considerable impact on how efficiently a user can locate their positions and calculate the mutual distance. Many satellite navigations systems have been built so far, each having their pros and cons. This information can be utilized in developing a fool-proof system which uses a mix of principles used in the past. This is a rich area for future research.
Cross References

  • Satellite Navigation Systems, NAVSTAR, GIS, GLONASS, Galileo

Recommended Reading

[1] El-Rabbany Ahmed. Introduction to GPS: The Global Positioning System. Artech House Publishers, 2002.

[2] Kennedy Michael. The Global Positioning System and GIS: An Introduction. Ann Arbor Press, Inc., 1996.

[3] GPS for the Military and Civilians. Crosslink (The Aerospace Corporation magazine for advances in aerospace technology), Volume 1, Number 2. Summer 2000

[4] Aeronautics and Space Engineering Board, National Research Council. The Global Positioning System: A Shared National Asset. The National Academic Press, 1995

[5] GPS on wikipedia.

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