Civilian and military GPS applications have become so ubiquitous, they are often considered routine appliances of daily life. These applications might not have been possible if early decisions about GPS had been made differently.
GPS IIF satellite
It is hard to imagine life without GPS. You can hop into a taxicab, give the cabbie your destination, and chances are the driver will use a navigation device—costing about $300—to get you there. The device contains local maps and uses signals from satellites orbiting 13,000 miles above to locate your destination. A second GPS device, integrated into the fare box, reports the cab’s position to the dispatcher.
The same GPS is helping U.S. and coalition forces in battle. As forces come under fire, a soldier might use his GPS receiver to identify his position and use a handheld laser range finder to determine the enemy’s position. This information is radioed to nearby airborne forces, who conduct a precision strike on the enemy position. The soldier’s GPS device is heavier and more expensive than the taxi driver’s because it needs to process encrypted military signals and is built to more demanding physical and environmental standards. These signals assure the warfighters that their GPS solutions are highly accurate and resistant to electronic attacks.
The cab driver and the soldier benefit from a team of satellite operators at Schriever Air Force Base in Colorado Springs. Using tracking data from a worldwide network of monitoring stations, these operators calculate the precise position of each GPS satellite, measure the offset and trends of its onboard atomic clock, and predict its future position and clock offset. These predictions are uploaded for each satellite about once a day via a separate network of GPS ground antennas, and the satellite uses them to broadcast its own position in time and space to its worldwide audience. This position, plus the range to the satellite, is the key information needed by a GPS receiver to compute a user’s location.
This history—and future—of GPS is closely tied to visionary work performed by The Aerospace Corporation, which has been supporting those who define, approve, sustain, build, and deploy GPS (and its predecessors) for nearly 50 years. To date, 58 satellites have been launched—one at a time—since the first reached orbit on February 22, 1978, and Aerospace has been involved with every one.
Each GPS satellite is essentially an orbiting atomic clock with a radio-frequency transmitter that constantly broadcasts a signal. The receiver compares the travel time of signals from several satellites to calculate a position.
What GPS Could Have Been
GPS was one of several satellite programs to emerge from the early years of Aerospace. Although it became a formal development program in 1974, it had several progenitors. In fact, it can claim a history back to Sputnik, when scientists at Johns Hopkins University Applied Physics Laboratory used Sputnik’s Doppler signature to determine its orbit. The realization that this process could be reversed led to the Navy Transit system for positioning, which operated for more than two decades (see sidebar, Transit: The GPS Forefather).
Satellite-based navigation might never have progressed beyond Transit, were it not for that system’s various shortcomings (and the shortcomings of the various terrestrial-based navigation systems of the time, such as LORAN and Omega). For example, because Transit relied on Doppler measurements, user motion could degrade the accuracy of the position fix unless it was properly corrected. Radio Determination Satellite Service approaches were considered, where user equipment might actively “ping” a satellite to get a position fix.
A 1966 study examined what was needed in a satellite-based navigation system and proposed that it support high-performance aircraft operations, including blind bombing (bombing through clouds using electronic devices for guidance); provide absolute positioning within 0.1 nautical mile (and relative positioning within 0.01 nautical mile); and be user-passive. It also stipulated that user equipment weigh less than 100 pounds and cost less than $100,000.
Aerospace and other agencies studied architecture options, cost and benefit trades, and performance for a decade before the GPS Joint Program Office was formed in late 1973. Studies for System 621B (the Air Force navigation system) in 1970 considered a constellation of three geosynchronous subconstellations, each containing five space vehicles. These 15 satellites would have the advantage of always appearing in the same place and the disadvantage of having to provide a signal from almost twice the distance as today’s GPS satellites. Thus, either the satellites would need to be much heavier and more complex or (more likely) the devices employed by the taxi and the soldier would need to be larger, heavier, and more expensive. The satellite operator’s job might be easier, since geosynchronous satellites do not really “move” much, while the launch team would need to work with heavier and costlier rockets. An upgrade to the Transit system was also considered. A 1971 study looked at a Navy 30-satellite constellation at 1465-nautical-mile altitude that provided continuous “two-in-view” Transit space vehicles. Either alternative would have led to a much different future for satellite navigation.
The original Transit system required from four to six satellites in low polar orbits. Upgrades to the system were examined during the formative years of GPS, including a 30-satellite constellation that would provide continuous two-satellite coverage.
From the beginning, military applications—such as bombing, missile guidance, navigation of ships, and coordination of ground forces—were part of the planning. Within the government, civil use of GPS for similar functions, such as commercial aircraft guidance, were recognized as potential outcomes. The 1980 Federal Radionavigation Plan acknowledged the potential for both civil and military users, but envisioned a system in which the civil function is limited and constrained by national security. Although the use of GPS by the civilian, commercial, and scientific communities was anticipated at the outset, no one anticipated the widespread integration of low-cost chip-sized GPS receivers and precision navigation into everyday life.
The GPS that might have been could have accelerated some of today’s uses and stifled others. For example, some of the concepts from early studies could not have supported the delivery of precision-guided munitions, where the host aircraft initializes the munition, based on targeting information received from the ground observers, all within the local GPS coordinate system. Civil analogs—such as mapping the perimeters of forest fires, tracking the locations of felons or packages, or turn-by-turn navigation in a strange city—likewise might never have emerged.
On the other hand, some decisions, such as removing selective availability (in which the signal available for civilian use is intentionally degraded) and expanding the size of the constellation, might have accelerated the introduction of vital military and civil applications if they had been made earlier. But in the 1980s, the only competitor to GPS on the horizon was the Soviet Global Navigation Satellite System (GLONASS). DOD had not embraced the radical changes in battlefield operations that were to come when GPS, battlefield communications, geographic databases, networked operations, and information operations merged.
What GPS Has Become
The most reported military application of GPS today is the use of GPS for targeting and delivery of precision-guided munitions. Live television from downtown Baghdad demonstrated the surgical bombing of a city with little “collateral damage.” Less dramatic, but vital, is the use of GPS and geographic information systems to coordinate battlefield movements, to ensure ships avoid hidden hazards, and to allow the aircraft carrying those precision-guided missiles to stay out of harm’s way. In his autobiography, Chuck Yeager, retired Air Force general and test pilot, poignantly describes the stress and risk of doing a midocean refueling operation before GPS. Today, it is a routine operation, thanks to better communication and to GPS.
GPS is one of the success stories in Aerospace’s history, but it is also something more: Like the Internet, GPS has become a global utility without which the world as we know it would cease to function.
The original military P(Y) and civil C/A signals had considerable spectral overlap, which made it difficult to boost the power of the P(Y) code signal.
On the GPS civil side are many visionary and unanticipated applications. While at Stanford University, Bradford Parkinson, former GPS program manager and Aerospace trustee, pioneered the use of GPS for precision operations, such as the unaided landing of an aircraft, precision farming, and the development of Gravity Probe B to explore whether gravity waves affect satellites as Einstein predicted. GPS anklets to track paroled convicts, among other uses, were not envisioned in the 1960s.
The M-code signal uses a binary offset carrier to spread the signal energy away from the center frequencies, which is the location of C/A and Y-code signals.
Precision farming may seem unnecessary, but consider that when a farmer combines it with soil analyses of a field, fertilizers can be applied exactly where needed in varying amounts, thereby avoiding excessive use of fertilizers that may then drain into waterways. Throughout California, a network of GPS receivers monitor the subtle motions of Earth’s tectonic plates and faults. Emergency location features in cellphones make use of GPS, either via a receiver in the phone itself, or via GPS-based timing to accurately synchronize the cell-tower network. Likewise, the banking industry indirectly uses GPS to synchronize the timing for electronic fund transfers. When a customer swipes a credit card at a retailer, GPS may be part of the service.
A second signal for the civil aviation community, L5, will be introduced on an upcoming IIF satellite. A third civilian signal, L1C, will be included on GPS III.
A two-frequency, precise-positioning service was developed to provide global, all-weather navigation and timing service for the DOD, its allies, and other authorized entities. Even before GPS reached its full operational capability in 1995, the revolutionary impact of precise-positioning-service GPS on military operations had been proven in battle. In Operation Desert Storm, even the partially complete GPS constellation allowed U.S. and coalition armored forces to precisely execute a massive flanking maneuver over featureless desert terrain that would have been unthinkable only a few years before. These large-scale maneuvers were a key reason the ground campaign of Desert Storm was completed in 100 hours, with minimal U.S. and coalition casualties. Desert Storm provided the first real demonstration of the value of GPS to military operations and evidence of its power as a “force multiplier,” allowing military objectives to be achieved with smaller forces, fewer casualties, and less collateral damage.
However, by the 1990s the shortcomings of the military precise-positioning service, based on encrypted “Y-code” signals, became apparent. Paul Kaminski, undersecretary of defense, asked the Air Force to study options for addressing potential future shortfalls, particularly countermeasures to jamming. One outcome of these “navigation warfare” studies was a proposed new architecture for a military “M-code” signal to replace the Y-code. To design the signals, a team developed metrics to gauge how well alternatives addressed needs for future accuracy, antijam, signal security, signal exclusivity, operational flexibility, and integrity. In addition, the new signals had to coexist amicably with the existing civil and military signals and receivers. The team consisted of the GPS Joint Program Office, Aerospace, MITRE Corporation, the National Security Agency, and satellite and receiver manufacturers.
A candidate design was briefed in 1999; in 2000, the Joint Program Office was directed to begin retrofitting the GPS IIR satellites, which were then in manufacturing, and the GPS IIF satellites, then in design, with the new M-code signals. Aerospace provided technical guidance to the Joint Program Office and the satellite contractors on generating and testing the new signals. The first “modernized” IIR-M satellite was launched in September 2005. In 2009, the final two IIR-M satellites were launched, and attention shifted to the first modernized IIF launch scheduled for 2010.
Until the late 1990s, selective availability artificially degraded civilian accuracy worldwide.
Aerospace engineers thought that the signal features should be capable of supporting warfighter operations and what is now called effects-based operations. Aerospace enabled warfighter operations within a region by using high-antijam regional M-code signals while continuing to support the larger worldwide DOD/allied communities with different signals and security on an Earth-coverage M-code. A feature introduced by the M-code design was the use of a binary offset carrier to spread the signal energy away from the L1 and L2 center frequencies, where the older coarse/acquisition (C/A) and Y-code signals are found. However, to those on the study, spectral separation allowed for boosting the power in the regional M-code signals by 20 decibels or more without degrading C/A or Y-code use in the same region.
Improvements in onboard clocks and ground systems continue to improve accuracy for military users.
Spectral separation is the key to the principle behind the modernized military GPS. The original military signal, P(Y) code, and the original civil signal, C/A code, were basically two versions of the same modulation scheme, known as binary phase shift keying, with the power of P(Y) code spread over 10 times as much bandwidth as C/A code. Since these two signals also shared a common center frequency, there was considerable “spectral overlap” between them. This presented undesirable constraints on military operations using P(Y) code, including the ability to boost the power of the P(Y) code signal. This problem was addressed with the modernized military signal, M code, which was designed to have spectral separation from C/A code and enables much greater flexibility for military operations.
The C/A code deserves credit for the acceptance of GPS as the first global utility and for the resulting billion-dollar worldwide industry in GPS goods and services. However, this legacy signal was designed for 1960s-era technology; the field of digital signal processing has seen more than 30 years of advances since the first GPS satellite was launched.
In the mid-1990s, the Departments of Defense and Transportation assembled interdepartmental teams that sought to define a new civil frequency. In the end, they got two. Aerospace proposed a draft design for a civil signal on L2 (L2C) that was reworked by a design team into a useful low-cost second frequency. It was designed for better tracking than the short C/A code and featured a variable navigation message structure that corrected the limitation of the fixed C/A message structure. After additional work, a multiagency team produced a second signal for the civil aviation community—L5. It, too, featured a more robust signal with a modern message structure. L2C was introduced on the modernized IIR-M satellites, and L5 will be introduced on the upcoming IIF satellite. A third modernized civil signal, L1C, will be included on the next generation of GPS satellites, GPS III.
Each modernized GPS signal includes some form of forward error correction, which allows receivers to detect and correct bit errors when demodulating the navigation data message, which in turn allows receivers to achieve faster “first fixes” in challenging radio-frequency interference situations. Each modernized signal also includes some form of pilot or “dataless” channel, which allows receivers to track the signal at lower signal-to-noise ratios that may be encountered in urban canyons, in dense foliage, or inside some buildings where C/A code receivers lose lock today.
The “spreading code sequences” of GPS civil signals are the bit sequences that uniquely identify each satellite and form the basis of the “pseudorange” measurement, the distance from the receiver to the GPS satellite upon which GPS navigation is based. The spreading codes of modernized civil GPS signals also have improved autocorrelation and cross-correlation properties over the comparatively short (1023-bit) C/A code-spreading code sequences.
When all three modernized civil GPS signals reach full operational capability, civil users will receive a variety of benefits, depending on their GPS application. Surveyors and high-precision users will benefit from dual-frequency ionospheric refraction corrections, primarily using L1C and L5. Mass-market GPS applications in cellphones and automobiles will benefit from many improvements in the L1C and L2C signals to improve performance in urban canyons, and even deeper into some buildings. Differential users will achieve greater accuracy over longer distances. And some applications may choose to track all three signals—L1C, L2C, and L5—to achieve unprecedented accuracy for carrier phase-based applications.
Just as the navigation warfare studies addressed the future needs of DOD users, a series of civil studies addressed what a future GPS civil service—the “standard positioning system” (SPS)—should do. Initially, SPS was largely the C/A signal on the L1 frequency. In the 1980s and 1990s, Aerospace was asked to propose new features to support trade studies of future satellite buys. A second civil frequency was a proposed addition to both the IIR and IIF satellites, but was not a part of the final packages.
Selective availability was a major factor in delaying additional civil signals. Until the late 1990s, selective availability artificially degraded C/A accuracy worldwide. Since the second military signal served mainly to correct ranging errors introduced during signal passage through the ionosphere, and since selective availability dominated the resulting SPS accuracy, there was little civil interest in adding a second civil frequency. However, political changes in the 1990s led to a reevaluation. The end of the Cold War and emergence of the Galileo system in the European Union tipped the political scales toward phasing out selective availability.
This single decision by the Air Force triggered an explosion of high-accuracy consumer GPS applications that are found almost everywhere today. In addition, each generation of GPS satellites flies better and better atomic clocks, and the operational control segment has made continuous improvements in its tracking and orbit prediction accuracy.
Meanwhile, other navigation satellite systems began adopting variants of GPS civil signals. For example, the Federal Aviation Administration Wide Area Augmentation System uses a member of the C/A family, as does its European counterpart. Japanese, Indian, European, and Chinese systems, as well as the Russian GLONASS, developed new designs for civil signals. Aerospace and MITRE conducted studies of many of these signals to verify their compatibility with GPS civil and military operations (see sidebar, A Global Standard).
GPS coordinates have become the world’s de facto standard. As a result, England’s Greenwich Meridian is no longer the point of zero longitude; according to GPS coordinates, the world’s prime meridian is now some 30 meters to the east of the Greenwich meridian.
What GPS Might Be
Since the mid-1990s, Aerospace has been involved in studies of future capabilities and the system architecture that is needed to support them. As in the early 1970s, the program is trying to accommodate a list of military, and now civil, needs—and as in the 1970s, those needs could be addressed in several ways. Study members are ever mindful of capabilities the solutions may enable or discourage.
Aerospace has been helping the government envision the next leap in capability, GPS III. Beginning with the systems architecture and requirements development studies in 2000 and proceeding through the technology development phase, Aerospace and the government considered a wide range of future architectures.
With two and sometimes three contractor teams working on proprietary architectures, the government needed its own satellite definition capability—an independent technical baseline. Such a baseline would achieve two aims. First, nonproprietary government designs could be shown to the government and contractors to help explain how decision-makers reached their conclusions on program design, risk, cost, and complexity. Second, it could be used in independent government trade studies to explore the performance and design trades of the proposed architectures. In this way, government decision-makers could understand both the merit of the proposed program and the design decisions that made it stand out among a myriad of others.
To create this definition, the government turned to the Aerospace Concept Design Center (CDC). Over the course of the system definition phase, the CDC created a series of reference designs. It explored various concepts for incremental “blocks” of satellites and for the preliminary impact of alternative design elements. For example, after other trade studies determined the preferred frequency band for the new GPS III crosslink, CDC studies examined the options for antenna placement on the satellite and the numbers of crosslink transmitters. Additional studies defined concepts for dual launch of GPS IIIA satellites, based upon the CDC baseline for satellite size and weight. Other studies incorporated proprietary features from contractors to allow independent government evaluation.
Syntheses of opinions, facts, capabilities, and requirements into a development program also continue, as well as upgrading and adding capability via GPS III, OCX (the future operational control segment), and MGUE (military GPS user equipment). And the GPS that might be could take several forms as needs and visions evolve.
Ubiquitous Radio Signals
The last 20 years have demonstrated the power of combining precise real-time positioning, geographical information (e.g., maps), and communications. Civil and military applications are increasingly highly integrated, rather than a stand-alone GPS receiver that displays one person’s latitude and longitude. This “wired world” can lead to highly flexible civil and military applications that integrate positioning information from many sources, not just GPS satellites. Cellphone networks, television stations, foreign satellite navigation systems, or roadside transponders all could supplement GPS satellites to provide future positioning in populated areas. In remote areas, foreign systems like Galileo, GLONASS, Gagan (India), or QZSS (Japan) could potentially augment GPS. In either case, supplemental communications provide information that allows the user’s receiver to integrate all ranging sources.
This integrated approach addresses the main drawback of GPS today: availability. Every time a building or tree gets between a user and a GPS satellite, the signal is blocked and the user has one less satellite with which to operate. When the number of satellites in view drops below four, the GPS receiver cannot compute a precise location. The United States can address this issue by launching more satellites, but it takes a great number of satellites to ensure that every user can navigate even when surrounded by tall buildings, mountainous terrain, or trees. A radio device that can receive and process these various types of signals, together with the ancillary information necessary for navigation, might be heavier and consume more power than today’s devices, but significant benefits might make this cost acceptable.
Vehicle navigation, such as the taxi driver enjoys, would become far more precise in urban environments, such as downtown Chicago, Manhattan, Tokyo, or Delhi. Soldiers would be able to operate more effectively, calling for support without having to leave the protection of cover. These benefits to the end users would come at a cost to the GPS satellite operators, who would have to coordinate carefully with other satellite providers to ensure that the signals do not interfere with each other and that satellites are placed in optimal orbits so that the various systems complement each other while still being able to operate as stand-alone systems. The launch community might feel a little less time pressure, since the delay of a single additional satellite to a combined constellation of 60 or even 120 satellites becomes less important.
No Satellite Signals
GPS could also evolve into a surveyor’s utility, with little or no use by actual end users. One way this might come about is through the proliferation of millions of local beacons, each surveyed by GPS. Imagine if every highway reflector, every road stripe, every light switch, and every electrical outlet contained a small beacon—perhaps something like the RFID (radio-frequency identification) chips common in employee ID cards, antishoplifting devices, and new passports. An inexpensive, low-powered device—perhaps embedded in a watch or a cell phone—would identify its proximity to these ubiquitous little “landmarks” and derive position from this information. For this to work, all the little beacons would have to be surveyed—probably using GPS—and the user would need access to a network to get necessary information about the beacons themselves.
This would require more work to set up, but would provide exceptionally light and inexpensive equipment. Not only could a taxi navigate in a dense city—or even in tunnels and under bridges—but also pedestrians could navigate to specific offices within multistory buildings. Soldiers fighting in surveyed cities would be able to use the same technology, but there would be a greater burden on the military to provide ubiquitous small beacons in remote jungles or deserts. The satellite operators and launch communities would continue to operate as they do now, because the existing GPS architecture would be essential for surveying the locations of all the little beacons.
The Aerospace Concept Design Center created a series of reference designs for GPS III, examining features such as the number and placement of crosslink antennas.
An “Internet” in the Sky
Aerospace is working with the government to investigate how to employ new high-frequency, satellite-to-satellite crosslinks to enhance the next generation of GPS III satellites. A concept under consideration would have future GPS III satellites, starting with GPS Block IIIB, sharing data with (and perhaps ranging to) three or four of its fellow satellites that are equipped with new crosslinks. Because at least one of these satellites would always be in view of the U.S.-based master control station, the satellite operators would have real-time status, command, and control of every GPS satellite equipped with the new high-frequency, satellite-to-satellite crosslinks. This would usher in an unprecedented level of service. Any satellite with the new crosslinks that is performing even slightly below its optimal capability would instantly be detected and adjusted, enabling previously unsustainable levels of accuracy, reliability, and availability.
The taxi might still wind up taking a wrong turn, but this would most likely be caused by errors in the mapping software employed by the GPS receiver rather than something wrong with the space portion of the system itself. And because the increased service would enable more airplanes to travel at the same time and to arrive on time, the taxi driver might see more fares to and from the airport. The U.S. and allied warfighters who trust their lives to GPS would be able to do so with a higher degree of assurance. The operators of the system would be able to focus on deploying new services and pushing the system to ever higher levels of performance rather than solving the ever more complex scheduling problem of which individual satellite needs to be uploaded from which ground station.
Although they surely had ideas for how the system would be used by both military and civil users, the original architects of GPS could not have envisioned the myriad applications and worldwide acceptance of GPS as the first truly global utility. Parts of their original vision for GPS have become reality, but others have not. What GPS will become during its modernized era cannot be accurately predicted today; however, it can be assumed that GPS will remain an engine of innovation in position, navigation, and timing applications. Many of the best applications of GPS are still to come.
As of 2010, the NAVSTAR Global Positioning System is fully operational, with 31 healthy satellites providing service to civil and military users worldwide. This represents a remarkable success rate, not only in the development and deployment of these spaceborne devices, but for the launch community as well. Aerospace participated in the launch of the first experimental Block I satellite in February 1978, and in every one of the launch attempts thereafter—60 in all. Of these attempts, 58 have been successful—a remarkable record by any standard.
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