Operation and Application of the Global Positioning System

Colleen H. Yinger

GPS was originally designed for defense operations, but civilian receivers now far outnumber military receivers. The number of operational receivers has increased exponentially over the last decade as the technology has moved in diverse and unexpected directions.

The Global Positioning System (GPS) provides timing and navigation for a wide range of applications, from intelligent transportation systems to power control grids. In the short time since its introduction, the technology has established itself as an indispensable component of daily life—even though most of its users know relatively little about it. When asked to describe the uses of GPS, many people mention its highly visible role in navigating airplanes or boats; but based on the number of receivers produced each year, the system's dominant roles are in intelligent transportation systems, telecommunications, and precision delivery of military munitions. Moreover, its use in supporting both critical civil infrastructure and military operations has received new attention since September 2001.

As principal advisor to the Air Force on space acquisitions, The Aerospace Corporation played a significant role in the development of GPS, providing proof-of-concept studies, constellation design and management studies, accuracy improvement initiatives, independent assessments, and operational assistance. With the modernized Block IIR and Block IIF satellites nearing launch—and the GPS III program now in its planning stages—the technology is poised to reach new levels of sophistication unimagined just a few years ago.

Concept Overview

GPS is designed to provide accurate three-dimensional navigation anywhere in the world, at any time, under all weather conditions. Each satellite is essentially an orbiting atomic clock with a radio-frequency transmitter that constantly broadcasts a signal. By comparing the signal received from the satellite with an internally generated signal, a receiver measures the time it takes for the signal to travel from the satellite to the user. Multiplying this time-delay measurement by the speed of light, the receiver calculates the user's pseudorange to the satellite (range plus user clock offset). Using such pseudorange measurements from four or more satellites, the receiver determines the user's three-dimensional position (latitude, longitude, and altitude) and time.

The position of the GPS receiver is where the ranges from a set of satellites intersect at a single measurement time. The range measurements are used together with satellite position estimates based on the precise orbital elements broadcast by each satellite. Four satellites can be used to determine three position dimensions as well as the offset between the receiver's inexpensive clock and a satellite's highly precise atomic clock. Computation of receiver clock offset is critical because a timing error of just 10 nanoseconds would produce 3 meters of ranging error (10 billionths of a second times the speed of light, 3 x 108 meters per second).

The GPS ranging signal is broadcast on two frequencies, 1575.42 megahertz (L1) and 1227.6 megahertz (L2). Each satellite transmits a unique code, enabling all satellites to use the same frequencies (a process known as code division multiple access). A short, unencrypted code (known as the C/A code) with a 1-millisecond period is broadcast on L1 and is generally used for civilian applications. Its short duration allows low-cost equipment to search its code phase quickly, enabling rapid acquisition and tracking. A longer, encrypted code (the P(Y) code) is broadcast on both L1 and L2 for so-called "authorized" users—generally U.S. government agencies and military allies. The P(Y) code provides more accurate ranging with lower risk of spoofing (reception of spurious signals that the receiver accepts as real) and better rejection of multipath (extraneous reflected) signals. Many authorized users initially acquire the C/A code, then transfer to P(Y).

Why are two frequencies needed? Earth's ionosphere delays the arrival of GPS signals, and this discrepancy must be corrected to achieve a precise position fix. Because signals at different frequencies propagate through the ionosphere at different speeds, users that receive both L1 and L2 signals can correct for ionospheric delays. Civil users can make less accurate corrections by using a mathematical model for the ionospheric delays. The parameters for the simple model are transmitted in the data message.

The Elements of GPS

The GPS system is made up of three segments—space, control, and user—all of which contribute to overall accuracy, reliability, and functionality.

Space Segment

The baseline GPS constellation consists of at least 24 satellites in six planes inclined at 55 degrees relative to the equatorial plane. The operational constellation includes additional satellites to ensure that maintenance and anomalies will have minimal impact on service. The satellites are positioned about 20,000 kilometers above Earth in approximately 12-hour orbits. With this configuration, almost every point on Earth can see at least five GPS satellites, and often many more.

Block IIA satellite.

The GPS satellites have solar panels to generate power and use shaped-beam antennas to provide nearly constant signal strength over Earth. Satellite lifetimes typically exceed ten years, thanks to a high degree of system redundancy.

Navigation performance is highly dependent on the stability of the cesium and rubidium atomic clocks. These high-quality space-qualified atomic clocks have stabilities of better than 1 part in 10¹³ over a period of one day, which translates to an error buildup of less than 10 nanoseconds (3 meters) per day. To keep accuracy high, Air Force Space Command computes and uploads clock corrections to the satellites, which in turn broadcast this information to the user as part of the data messages. The more stable the atomic clocks, the less frequent the satellite uploads need to be to maintain a desired ranging accuracy.

The first GPS satellite was launched in 1978. Initial operational capability was established in December 1993 when the full constellation of 24 satellites was completed. Final operational capability was announced the following year.

Control Segment

GPS employs a worldwide ground network to monitor the health of the satellites, keep them in their intended orbits, and update their clock and position data.

Five globally distributed monitor stations track the GPS satellites and send ranging data to a master control station in Colorado Springs. The master control station processes the ranging measurements in a Kalman filter every 15 minutes to determine satellite orbit and clock corrections. Periodically, roughly once per day for each satellite, the master control station predicts the orbits and clocks and forms a navigation message. The navigation message is sent to a ground antenna for upload to the satellite on an S-band data link and transmitted to the user on the GPS signal.

The navigation message is transmitted on both the L1 and L2 channels at a rate of 50 bits per second. The message has a 1500-bit frame (30-second duration) consisting of five 300-bit subframes (6 seconds each). Subframe 1 contains clock parameters. Subframes 2 and 3 contain orbit parameters. Subframes 4 and 5 contain almanac data (less accurate orbit data that is used only for signal acquisition), single-frequency ionosphere model parameters, and GPS-UTC (Coordinated Universal Time) offset data.

User Segment

A GPS receiver tracks selected satellites and computes user position. A receiver consists of an antenna (typically omnidirectional), filtering and amplification circuits, and signal-tracking components. Satellite positions are computed from navigation message data. The pseudorange measurements are corrected for satellite clock errors, Earth rotation, ionospheric delay, tropospheric delay, and relativistic effects. The corrected pseudorange data and satellite positions are used to compute user position, velocity, and time. The computation may be done using GPS alone or integrating data from other sensors such as altimeters, compasses, and inertial measurement units. Depending on the application, user position may be superimposed on a map, used to make corrections to a weapon in flight, or transmitted to a central processing facility.

Continuous Development

The Aerospace Corporation has maintained a significant role in all of these system areas. For example, analysts at Aerospace helped define a constellation that would strike the right balance between user coverage and system cost. Aerospace continues to optimize the constellation for competing demands and to assess satellite replenishment strategies.

Aerospace participated in early proof-of-concept studies, algorithm development, and validation efforts to improve ground-station modeling and orbit calculations. Although most space programs need to predict the orbits of their satellites, the needs for GPS exceed those of other programs. Hence, GPS requires more detailed models and more accurate calculations. For example, Aerospace analysts were responsible for the adoption of a technique for modeling solar-radiation pressure, which removed a major impediment to GPS success by enhancing the estimation and prediction of the GPS orbits. Aerospace personnel have also been involved in algorithm enhancements, parameter selection, and similar initiatives that have significantly improved the accuracy of the system. The company remains active in the operations and modernization of the current ground control segment.

Aerospace has also developed new algorithms for jam-resistant receivers. For example, Aerospace is developing and promoting ultratight GPS/inertial coupling techniques that not only increase jamming protection but also improve accuracy, integrity monitoring, and detection and mitigation of multipath signals. Aerospace also played a key role in the development of the Combat Survivor/Evader Locator, a GPS-based rescue system for U.S. military forces.

Aerospace and the Naval Research Laboratory have supported the development of space-qualified atomic clocks for GPS applications since the early 1970s. Aerospace helped analyze and resolve numerous clock anomalies encountered during the early phases of the GPS program. Interestingly, the demand for highly stable clocks is diminishing, thanks to the success of the GPS program, so Aerospace is working with the Air Force to preserve the nation's industrial base for atomic clocks for GPS III and beyond.

GPS error sources include satellite clock and position errors, propagation errors, and user receiver errors such as noise and multipath signals. Military users can generally eliminate the ionospheric effect by using dual-frequency measurements. Civilians apply a single-frequency model that reduces ionospheric error. Multipath error is caused by reflection of GPS signals off nearby surfaces and depends on antenna-to-satellite geometry. Proper antenna design and placement can minimize multipath errors by eliminating reflected signals. Troposphere is modeled in the receiver.

Error Sources

GPS navigation performance is determined by the accuracy of the ranging signal and the quality of the user-satellite geometry. Sources of ranging-signal errors include signal-in-space errors (uncertainties in satellite position and clock data), signal propagation delays through the ionosphere and troposphere, and receiver errors. Satellite geometry determines how the ranging errors affect user navigation error (see sidebar, Signal-in-Space Errors). An ideal four-satellite geometry would have one satellite directly overhead and three satellites equally spaced around the user's horizon. In general, more satellites are better. Newer receivers generally implement "all-in-view" satellite selection, as opposed to "best-of-four" criteria, and 12-satellite civil receivers are common (see sidebar, Signals Through the Ionosphere).

Navigation error is roughly the expected ranging error multiplied by the "position dilution of precision," an instantaneous measure of the geometric quality of the satellite configuration selected by the GPS receiver. Actual values typically range between about 2 and 3 for the operational constellation because most sites will see more than enough satellites, though their geometry will probably not be ideal. Locations and times with high position dilution of precision (often defined as greater than 6) produce less accurate navigation or a navigation "outage." The position dilution of precision concept provides a convenient way to predict user navigation performance, analyze alternate constellations, and study the impact of satellite failures.

How Good Is It?

The original GPS specification called for a military three-dimensional position accuracy of 16 meters and a civilian horizontal accuracy of 100 meters (civilian accuracy was intentionally degraded—a protocol known as "selective availability"). Actual GPS navigation accuracy depends on the user's receiver, location, and dynamics. Military performance is now on the order of a few meters, constrained by signal-in-space errors and receiver performance. With selective availability set to zero in May 2000, stand-alone civilian systems can typically achieve performance in the 10–20 meter range, limited primarily by single-frequency ionospheric modeling constraints.

In addition to exceeding original accuracy expectations, GPS has also exhibited an impressive history of reliability, integrity, and availability. GPS satellites are outliving their specified mean mission life (six years) by a factor of nearly two. Aerospace studies of satellite reliability enabled the Air Force to revise its procurement schedules, thereby saving hundreds of millions of dollars without interrupting user coverage. In the eight years since full operational capability was declared, only one service failure occurred in which a satellite generated an unusually large error without either being declared unhealthy or being corrected immediately. The Air Force continues to look into ways to improve its responsiveness to the rare occurrence when a satellite inadvertently broadcasts incorrect information.

Augmentation

Applications requiring greater navigation accuracy can take advantage of a technique known as differential GPS. In this case, GPS satellites are tracked from one or more reference sites whose positions are precisely known, thereby determining the ranging errors to each satellite. The reference site transmits the ranging corrections to users in the vicinity in real time. Since the dominant error sources are common to the user and a nearby reference site, most errors can be eliminated. The accuracy of a differential system degrades with separation distance between the reference site and the user.

Numerous differential systems are currently operational or planned. The maritime differential GPS developed by the U.S. Coast Guard operates more than 50 differential sites around U.S. coastal areas, harbors, and rivers. The system provides better than 10-meter accuracy and was originally designed for harbor approach, vessel tracking, and buoy positioning.

Nationwide Differential GPS is a planned improvement and expansion of the maritime system to more than 120 sites to provide free differential corrections throughout the United States. Applications include train control, intelligent transportation systems, crop dusting, precision mining and farming, and snowplow management. For example, Nationwide Differential GPS—in conjunction with gyros, axle generator interfaces, track databases, and communication links—can help prevent train collisions and improve railroad track utilization. Several commercial differential systems are also available in the United States and internationally, some providing corrections via communication satellites.

Block IIR satellite. (Lockheed Martin Missiles and Space)

Surveyors and geologists studying plate tectonics achieve centimeter-level accuracy or better using a combination of differential techniques and carrier tracking. Carrier tracking uses the GPS radio-wave phase rather than standard code tracking to obtain ranging resolution that is a small fraction of the 19-centimeter wavelength (as small as 2 millimeters, or 1/100th of the wavelength). Carrier tracking is not appropriate for all users because it requires greater signal strength and resolution of the cycle ambiguity (i.e., which carrier cycle is being tracked). Also, it may be challenging for dynamic applications.

Aerospace was instrumental in the implementation and testing of a worldwide accuracy enhancement system for military users. By providing more frequent clock corrections in the GPS navigation message, this system reduces signal-in-space errors by 20–30 percent for suitably equipped military users. Aerospace demonstrated the potential of this method and other differential techniques to improve GPS navigation performance for several munitions, including the conventional air-launched cruise missile and the Joint Direct Attack Munition.

For the civilian aviation sector, the biggest navigation challenge is service integrity—that is, how does one guarantee that GPS is not broadcasting misleading information that could result in injuries or death? The Federal Aviation Administration Wide Area Augmentation System is being tested to meet the stringent requirements of the civilian aviation industry for accuracy, integrity, and system availability. The Wide Area Augmentation System processes tracking data from 25 reference stations throughout the United States to compute and disseminate GPS corrections and integrity information to aviation users via geostationary satellites. Since accuracy of civil users is generally constrained by the accuracy of the GPS single-frequency ionospheric model, the Wide Area Augmentation System will transmit a more complex and accurate grid-based ionospheric model to its users. One of the most challenging technical areas is the accurate determination of this time-varying, geographically dependent ionospheric grid. Aerospace used its long-established ionosphere modeling expertise to test the ability of this algorithm to satisfy the Wide Area Augmentation System ionosphere correction requirements.

Applications above Earth

Although GPS was originally designed for terrestrial and airborne use, its applications now extend far above Earth. Aerospace has been active in many of these applications. A number of engineering studies by the company have supported the role of GPS on missiles and launch vehicles. As early as 1987, the Aerospace Range Systems Architecture Study recommended the "transition from radar to the Global Positioning System (GPS) as the primary source of tracking" at the Western and Eastern Space and Missile Centers. A 1999 Space-Based Range Feasibility Study reaffirmed GPS capabilities to meet most tracking requirements. Recent analysis has demonstrated GPS capabilities to satisfy launch vehicle tracking requirements for real-time range safety. All of these studies support increased use of GPS for range standardization to reduce operational expenses.

Flight experience has demonstrated GPS applicability on many satellites in orbits ranging from low Earth to geostationary. For example, Radcal—a radar calibration satellite launched by the Air Force in 1993—demonstrated a precision orbit determination capability using an inexpensive GPS receiver. Flight-data processing at Aerospace produced a postflight orbit accurate to 5 meters, satisfying requirements for the worldwide Department of Defense radar-calibration system.

Aerospace provided key technical guidance in the development of the Combat Survivor/Evader Locator (CSEL). This rescue radio uses GPS to communicate survivor position to rescue forces, enabling rapid rescue with minimal exposure to hostile conditions. (The Boeing Company)

Aerospace has also provided performance assessments for the more challenging mission of high-altitude spaceborne users. These users, well above the GPS constellation, receive some GPS signal spillover from the far side of Earth. Aerospace has shown that by using sophisticated orbit modeling and measurement processing, GPS can meet the orbit determination needs of many high-altitude space systems.

The Future of GPS

GPS is playing an increasingly important role in all aspects of military operations—from ground troop maneuvers to precision weapon delivery. But the role of GPS in civilian applications is expanding even faster. As navigation technology matures, the trend will continue toward embedded GPS applications integrated with communication systems and large databases. For example, integrated systems could provide immediate traffic information and route alternatives to rush-hour drivers or advertise a particular restaurant to potential customers in its vicinity as the dinner hour approached. In fact, given the emphasis on complete system integration, future users may not even be aware that satellite navigation technology will be at work in their daily lives (see sidebar, GPS in Action).

Promising applications are abundant in the transportation arena: real-time traffic information, route guidance, fleet control, collision avoidance, automated accident reporting, and automated toll charges, to name just a few. Other uses—such as auto insurance pricing based on when, where, and how fast a car is driven—might not be so popular with the general public.

GPS has become an essential element in the global infrastructure and has exceeded the expectations of even its early developers. Aerospace played a prominent role in the development of this dual-use space system, and will continue to guide and support its future evolution.

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