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Navigation Satellites & GPS

v2.2.3 / 01 dec 02 / greg goebel / public domain

* One of the first applications of artificial Earth satellites was for navigation. Very early in the Space Age, researchers realized that constellations of satellites could be put in orbit to permit ships, aircraft, or other vehicles to precisely determine their locations.

A number of navigation satellite constellations have been put into orbit, with the most prominent being the US "Global Positioning System (GPS)". The GPS constellation was established by the US military for support of American forces in the field, but it is now in widespread use for public and commercial applications as well.

This document describes the history, principles, and applications of navigation satellite systems, particularly GPS.



* After the Soviets had launched the first artificial Earth satellite, "Sputnik 1", in 1957, some researchers realized that radio transmissions from a satellite in a well-defined orbit could be used to determine the position of a receiving station back on the Earth.

The initial approach towards using a satellite for position location was based on measuring the Doppler shift in the frequency of the satellite's radio transmissions as it passed overheard. The Doppler shift data could determine the location of the satellite relative to the receiver station, and given a precise knowledge of the satellite orbit would then establish the location of the receiver station.

This approach required complicated electronic equipment, as well as measurements from several orbital passes of the satellite to get an accurate position fix. Nonetheless, Doppler positioning was the basis for the first satellite positioning system, known as "Transit", which was introduced by the US military in the 1960s and was mainly used by US Navy ballistic missile submarines.

The Transit system was based on a constellation of six satellites in circular polar orbit at an altitude of about 1,000 kilometers (different sources give somewhat varying values for the altitude), with three ground stations in control of the satellites, and satellite receivers systems carried on submarines and other large naval vessels. Only three of the satellites were actually used for positioning, with the other three set aside as on-orbit spares.

After launch of experimental and then initial operational Transit satellites by Thor-Able and Thor-Able Star boosters, two series of fully operational satellites were launched, the initial 50-kilogram "Oscar" series and the later 160-kilogram "Nova" series. The Oscar and Nova satellites were launched by Scout light boosters, with the boosters also sometimes carrying other payloads.

The Nova series was sometimes described as a separate system from Transit, but the principles of operation of the early Transit, Oscar, and Nova satellites were the same. They transmitted on two frequencies, 149.99 megahertz (MHz) and 399.97 MHz. An Earth receiver measured the Doppler shift of the frequencies and also downloaded the satellite's position coordinates, broadcast by the satellite itself every two minutes.

While receivers could obtain a position fix using only one frequency, much higher accuracy could be obtained by measuring both frequencies, since errors caused by variations in the atmosphere affected the two frequencies differently, allowing the errors to be averaged out. Locations of naval vessels could be determined to an error radius of 80 to 100 meters, adequate for targeting nuclear-armed missiles, but locations of fixed sites could be determined to less than 20 meters using repeated measurements.

* The first Transit launch attempt was on 17 September 1959, less than two years after the launch of Sputnik 1, but the satellite did not make orbit. The first successful launch was on 13 April 1960. Five more experimental Transits were launched, including two failures, leading to the first attempt to launch an operational Transit satellite on 19 December 1962, which was also a failure.

Following yet another failure in April 1963, the first successful operational Transit satellite was put into orbit on 16 June 1963. The system was declared operational in 1964 and was released for civilian use in 1967, though the expense of the receiver system meant that civilian use was limited. The Transit system actually remained in service until 31 December 1996, with the last launch of a Nova satellite in August 1988. Such Nova satellites as are still operational remain in use for ionospheric measurements.

The Transit system was operated by the US Naval Space Operations Center, with headquarters at Point Mugu, California. All Transit technology was designed by the Applied Physics Laboratory of Johns Hopkins University, with RCA performing the actual construction.



* Even before the first Transit satellite was in orbit, researchers were considering a better approach that would eventually result in the modern Global Positioning System.

Suppose an entire constellation of navigation satellites was launched into precisely-known orbits. If a receiver could determine the exact position of several of these satellites at the same time, then the receiver could precisely determine its own position through triangulation.

Determining the position of the satellites was the trick. Ground-based radar tracking was dismissed as impractical, as it required too much equipment and would give away the position of the ground station. A subtler scheme was adopted instead: the satellites would emit signals with distinctive, precisely timed pulse patterns that a receiver could read to determine the distance of each satellite.

The receiver had to be able to distinguish between different satellites from the pulse patterns they emitted. Design of the pulse patterns was based on a technique that had been devised by astronomers for radar studies of the Moon and nearby planets.

Radar operates by transmitting a radio pulse, and then measuring the time it takes the pulse to travel to a target, be reflected, and travel back to the original transmitter. Since sending a radar pulse to the Moon or a nearby planet involved a long time delay, the radar pulse had to have a distinctive pattern to ensure that the exact timing of pulse transmission and reflection could be determined.

To accomplish this, astronomers used a set of pulses emitted in a "pseudo-random noise (PRN)" sequence. The pulse pattern appeared to be random noise that did not repeat itself, at least over the length of time of interest, but was generated by a predictable algorithm. The reflected pulse could be compared against the original pulse pattern using a procedure known as "correlation" to determine the exact timing.

The engineers working on the navigation satellites like the idea of using PRN sequences, though rather than transmitting an on-off pulse pattern, the satellites would send the pulse pattern using a continuous signal that varied between two frequencies, an approach known as "frequency shift keying (FSK)".

Furthermore, the PRN sequences could be designed so that each navigation satellite had its own unique, non-overlapping sequence. In formal terms, the sequences were said to be "orthogonal". This allowed all the satellites to transmit on the same frequency and not be confused by a receiver. A receiver then only had to pick up a single frequency, reducing cost and complexity.

* For this scheme to give precise locations, the clocks on the satellite and on the receiver had to be precisely synchronized. A timing error of no more than a microsecond would result in an error of about 300 meters. While the satellites could carry precision clocks, such clocks would be far too clumsy and expensive for the Earth-based receivers. However, there turned out to be a way to build a receiver that could provide accurate position data using a clock no more accurate than a typical cheap digital wristwatch.

Suppose the receiver's clock is used to help determine the distances to four satellites. Due to clock inaccuracy, these distances will be inexact, and so are known as "pseudo-ranges". The receiver's position could be thought of as being at the precise intersection of four invisible spheres, with the radius of each defined by the receiver at one end and a satellite at the other. The pseudo-ranges could give an estimate for the length of all four of these radii, but due to the clock error the invisible spheres would not intersect. It is, however, straightforward algebra to then use the intersection error to compute the clock error, subtract it, and make the spheres link up.

* The last major issue was the altitude of the GPS satellite constellation. Satellites are generally either placed in low Earth orbit, a few hundred kilometers high, or in geostationary orbit over the equator, 36,000 kilometers high, where they take 24 hours to orbit the Earth and remain in a fixed position relative to the Earth as it turns under them.

Putting the satellites into low Earth orbit would reduce the size and cost of the boosters required to launch them, and would also reduce the power required for the transmitters on the satellites. However, obtaining adequate coverage would demand a large number of satellites. Putting them into geosynchronous orbit would reduce the number of satellites, but it would require more powerful launchers and transmitters, and it would not provide good coverage of the polar regions.

The altitude finally chosen was a compromise: a circular orbit with an altitude of 20,200 kilometers and a period of 12 hours. At that altitude, 17 satellites would be enough to make sure that four of them, the minimum number needed to establish a position, would always be visible from any location on the Earth's surface.

The GPS constellation finally implemented actually has 24 satellites, consisting of 21 in active operation, plus three spares. The 24 satellites operate in six different orbital "planes" (an orbital path shared by multiple satellites), with four satellites in each plane. The planes are inclined 55 degrees with respect to the equator. The GPS satellites are also fitted with nuclear blast detectors as a secondary mission, replacing the early "Vela" nuclear blast surveillance satellites in this role.



* The technologies on which GPS is based were initially tested on three "Timation" satellites, with the first launched on 31 May 1967, and the other two launched in 1969 and 1974.

The first actual GPS satellite, named "Navstar", was launched by a US Air Force (USAF) Atlas-Centaur booster in February 1978. That satellite, plus the ten after it, were designated "Block I", and were built by Rockwell International. They were intended as technology demonstrators, and differed from the later operational GPS satellites in that they were placed into orbits with an inclination of 63 degrees, not 55 degrees.

The last Block I satellite was launched in 1985. The seventh Block I satellite was destroyed in a launch failure in 1981, and so far has been the only GPS satellite to be fail to reach operational status.

The first "Block II" operational satellite was launched by an Air Force Delta booster on 14 February 1989. Another eight Block II satellites were launched, to be followed by 15 slightly improved "Block IIA" satellites. Both the Block II and Block IIA satellites were also built by Rockwell.

The full 24-satellite operational constellation was finally completed with the launch of a Block IIA satellite on 9 March 1994. The Block II/IIA satellites have a design lifetime of over 7 years.

Replacement satellites, designated "Block IIR" and built by Lockheed Martin, are being launched as needed to maintain the constellation. The first Block IIR satellite was launched on a Delta II booster on 27 March 1996, and six Block IIR satellites have been launched as of early 2001.

A total of 20 Block IIR spacecraft were produced and stockpiled, with the last scheduled to be launched in 2009. After manufacture of the Block IIR spacecraft, the Air Force then decided to initiate a degree of improvement in GPS services, and so the last twelve of the batch are being modified to the "Block IIR-M" ("M" for "Modernized") standard, with additional GPS signals. The first Block IIR-M satellite is currently scheduled for launch in 2003.

The Block IIR / IIR-M satellites will be be followed by further improved "Block IIF" satellites, with the first Block IIF spacecraft scheduled to be launched in 2005. A total of 12 Block IIF spacecraft will be built by Boeing, which bought Rockwell's space assets in 1996.

Ground system improvements will be implemented as part of the Block IIF program. The ground system includes a number of elements. Overall GPS direction resides at the "GPS Master Control Station (GPS MCS)" located at Falcon Air Force Base, outside Colorado Springs, Colorado. The MCS is linked to four remote active-tracking ground antenna stations and five passive-tracking monitor stations. The ground stations, which are at precisely-known locations, forwards GPS satellite broadcasts to the GPS MCS. The GPS MCS measures the timing of the signals, and then uploads any necessary corrections.

* The USAF is now funding initial studies of a follow-on "GPS III" or "Block III" system, with the first Block III satellites to be launched in 2009. However, while plans are going forward on this future generation of GPS satellites, the Air Force is becoming very concerned about keeping the existing GPS constellation operational until these improved spacecraft can be launched.

By late 2002, about half the GPS satellites in orbit were no longer fully functional, and a number of accidents had delayed launch of new spacecraft. The USAF has enough satellites available or on order to keep the constellation healthy, but the Air Force doesn't have enough launch vehicles in the pipeline to put the satellites in orbit.

What complicates matters is that if the Air Force were in a position to launch new GPS satellites more quickly, they would be Block IIR / IIR-M spacecraft, which would rob funds from the Block IIF effort. That would delay full implementation of the Block IIF constellation, with the delays in turn affecting schedule for deployment of Block III. The situation puts the USAF in the position of praying that the current constellation keeps on working until the long-term fixes are ready, but nobody has any real idea of what might fail next.

The Air Force is also complaining about having to bear the full cost of the GPS constellation, since the military is now a minority user of the system. Broadening the funding mechanism would require a major change in the system's charter and setting up a US national program office.



* The US military, having designed GPS to support their operations, wanted to make sure that they were the only ones entitled to the full accuracy of the system, given as roughly 10 meters (different sources give different spins on the exact value). They introduced "noise" or "dithering" into the signals transmitted by the satellites, coarsening the accuracy for civilian users to about 100 meters. The "noise" is apparently generated as a classified code pattern that military GPS receivers can screen out.

The highly accurate military service is known as the "precise positioning service", while the degraded civilian service is known as the "standard positioning service". The military refers to this scheme as "selective availability (SA)".

Selective availability proved controversial. Civilian users felt that the value of precise positioning was great enough for civilian applications that it was wrong-headed to deny it. What made the argument even more troublesome was that there were ways to get around selective availability.

In 1980, MIT researchers demonstrated a method of greatly reducing the uncertainty in non-military GPS positioning. Since they knew the satellite orbits with precision, then if they had a ground receiver whose exact position was also known by other means, they could then measure the distances to the satellites using the coded signals, and calculate the difference between the true distance and the distance given by the coded signals. The corrections could then be broadcast locally to GPS receivers in the area to allow them to correct their own positions accordingly.

This scheme became known as "differential GPS", and allowed cheap GPS receivers to obtain locations to within about ten meters, with the aid of a error correction signal. The availability of differential GPS, in the view of many civilian GPS enthusiasts, made selective availability a joke. What made the joke even more ironic was that some US government organizations implemented GPS error-correction broadcast networks, also known as "GPS augmentation services", that were accessible by civilians.

In particular, the US Coast Guard established a "National Differential GPS (NDGPS)" network that originally provided differential GPS error correction signals in coastal areas, but has been extended, partly with help of the US Department of Transportation, to nearly all of the United States.

* The military was still reluctant to give up selective availability, but it was finally turned off by executive order on 2 May 2000. Measurements of GPS accuracy performed by the US National Oceanic & Atmospheric Administration showed that before selective availability was turned off, 95% of the position readings sampled fell within a 45 meter radius, and then zoomed to a 6.3 meter radius.

Interestingly, even with selective availability disabled, differential GPS provides such high accuracy that the military is investigating differential GPS guidance systems for their systems. Military differential GPS systems are being developed under a US Air Force GPS accuracy improvement initiative, which also involves distribution of more accurate data for GPS satellite orbits and other GPS parameters. The goal is to improve accuracies to less than five meters.

The US military is working on "local denial" techniques to prevent adversaries from making use of GPS in a combat theater. Details are classified, but observers suspect local denial will involve some type of selective jamming from an aircraft or ground station, with US military GPS receivers able to operate in the presence of such jamming.

* GPS augmentation services are now being planned for air-traffic control (ATC) systems. Civil air traffic is now becoming increasingly dependent on GPS. Up to the mid-1950s, air traffic control in the US was based on controllers using radio communications and handwritten notes to direct traffic. After a disastrous mid-air collision in 1956, the US Federal Aviation Administration (FAA) set up a system of radars and computers to keep closer track of airline traffic.

Up to the introduction of GPS, long-range airliner navigation was handled by radio beacons on land and aircraft-based inertial navigation systems over oceans. On approach to the runway, an airliner was directed by radio-based "VHF omnidirectional range (VOR) localizers" and "distance measuring equipment (DME)" to ensure that the aircraft was on the proper approach path. Large airports used automated "instrument landing systems (ILS)" to bring aircraft down safely in day or night, in any weather.

GPS has made long-distance navigation much simpler, eliminating the reliance on ground-based navigational beacons. However, unaugmented GPS does not have the resolution needed for approach and landing systems, and so the the US Federal Aviation Administration (FAA) is now working on a GPS "Wide Area Augmentation System (WAAS)", which would reduce airliner position errors from a hundred meters with unaugmented GPS to less than 3 meters.

WAAS will be based on a network of 25 ground stations at precisely-known positions around the US. These stations will not directly transmit error signals to airliners. Instead, they will pick up GPS signals, determine errors, and transmit the error data to one of two master stations. The error correction data will then be transmitted to a communications satellite in geostationary orbit, and relayed in turn to airliners with WAAS gear.

Interestingly, the error correction data will be transmitted by the communications satellites in the same frequency band as the GPS signals themselves. The airliners will also have "Automatic Dependent Surveillance B (ADS-B)" gear to transmit their locations to ATC centers.

Once WAAS is implemented in the US, it is expected to be implemented in the rest of North America. As WAAS doesn't have the accuracy for blind landings in bad weather, the FAA is also considering a "Local Area Augmentation Systems (LAAS)", based on GPS augmentation transmitters located at airports. LAAS would provide 1-meter navigation accuracy within a radius of 30 to 45 kilometers around airports, and would be used both for landing guidance and runway taxi navigation.

Yet another GPS tool for airliners now under development is the "Traffic Collision Avoidance System IV (TCAS-IV)", which will use ADS-B to obtain precise locations of airliners and determine if they are on a collision course. The current "TCAS-III" system uses radar.

Such widespread use of GPS for air-traffic control would mean that the abrupt failure of the GPS network might have disastrous consequences. As a result, WAAS also includes "integrity services" that provide notification to airliners through the communications satellites if the GPS network goes down.

* Both Japan and the European Space Agency (ESA) are now working on GPS augmentation systems, with the main function of both networks being air traffic control.

The Japanese system is known as "MTSAT (Multifunction Transport Satellite) Space-based Augmentation System" or "MSAS", and is being implemented by the Japanese Meteorological Agency and the Japanese Ministry of Transport, hence the name of the satellite. The MTSAT spacecraft will be a combination meteorological and communications satellite, and will be placed in geostationary orbit over the eastern Pacific. The satellite will relay GPS augmentation and integrity data to aircraft, along with other communications services.

The first MTSAT launch was in November 1999, but the spacecraft failed to reach orbit. A replacement spacecraft, designated "MTSAT-1R", will be launched in the late summer of 2003, with a second, similar satellite designated "MTSAT-2" scheduled for launch in the summer of 2004. The satellites are built by Space Systems / Loral, and are based on standard Loral satellite buses.

The ESA network is known as the "European Geostationary Navigation Overlay System (EGNOS)". Like MSAS, EGNOS will transmit augmentation and integrity data to aircraft through geostationary communications satellites. It is currently scheduled to go into operation in 2003, and will use the INMARSAT AOR-E and IOR commercial communications satellites and the European Space Agency Artemis experimental communications satellite. The satellites will provide coverage of subpolar areas ranging from the east coast of the United States to Japan.



* The Soviets introduced a network of navigation satellites apparently similar to the US Transit system. The first launch of an experimental satellite, "Cosmos 192", was in 1967, leading to first launch of an operational satellite, "Cosmos 700", in 1974. Nine more satellites were placed in orbit to establish the military "Parus" navigation constellation.

The Parus system is secret, but a similar commercial constellation designated "Tsikada" was also put into orbit, with first launch of a Tsikada satellite, "Cosmos 883", in 1976. The Parus system is sometimes referred to as "military Tsikada" or "Tsikada-M". Tsikada itself was heavily used by the Soviet merchant marine.

Both the Parus and Tsikada systems seem to still be in operation. The Soviets followed them with their own answer to GPS, with the English name of "Global Navigation Satellite System (GLONASS)".

Like GPS, the full GLONASS network is to include 24 satellites, consisting of 21 operational satellites and three spares. All the satellites are to transmit identical codes but at different frequencies, exactly the reverse of the scheme used for GPS.

The orbits are at an altitude of 19,100 kilometers, slightly lower than that of the GPS satellites, with the satellites placed in three orbital planes, each containing eight satellites. Each satellite completes an orbit in 11 hours 15 minutes. The planes have orbital inclinations of 64.8 degrees. GLONASS is supposed to have location accuracy capabilities roughly similar to those of GPS, but it does not impose selective availability on civilian users.

GLONASS launches began in 1982, but due to the troubled circumstances of the Soviet and successor Russian states, the full constellation has never been implemented. As of mid-2002, only eight GLONASS satellites are operational. The Russians are planning to launch more when they can get the money.

They are also working on a next-generation "GLONASS-M" satellite, with improved signal characteristics and a design lifetime of up to eight years, rather than the current 3 year design lifetime. They ultimately hope to go to "GLONASS-K", which will be smaller and will have a design lifetime of ten years.

* There has been some effort towards building receivers that can obtain signals from both GPS and GLONASS, providing substantially greater accuracy than would be possible from either by itself. Use of two satellite systems also gives users a "backup" operational capability if one of the systems is disabled. The European Community is now implementing the "Global Navigation Satellite System 1 (GNSS-1)", which will integrate services from GPS, GLONASS, and various augmentation networks.

One of the problems in combining use of GPS and GLONASS is that they use different global coordinate systems. GPS uses a coordinate system named "WGS-84", in which the precise location of the North Pole (which drifts a bit) is fixed at its location in 1984. GLONASS uses a coordinate system named "PZ-90", in which the precise location of the North Pole is given as an average of its position from 1900 to 1905. Trying to link the two coordinate systems has proven difficult, particularly because there are far fewer GLONASS receivers than GPS receivers.

* GNSS-1 is regarded as a stepping stone to a completely independent European "GNSS-2". GNSS-2, or "Galileo" as it has been named, will be based on an entirely new satellite constellation, consisting of 21 or 36 satellites that will be integrated with ground augmentation networks. Galileo will be compatible with GPS, but unlike GPS will be under complete civilian control. European military forces have expressed interest in making use of Galileo, but so far have not offered to help with funding.

Basic Galileo positioning services will be offered free, but the system may include paid-access services, such as navigation-related telecommunications channels, to help defray costs. A tax on receivers is also being considered. The Galileo system is expected to begin operation no earlier than 2006.

The Russians and the Japanese may join the effort. While GNSS-2 is still in the definition phase, enthusiasm for the concept is high among potential European participants in the program. Although determining the precise status of such multinational programs at any one time is an exercise in frustration, GNSS-2 does seem to be gradually moving from the discussion phase to the implementation phase.

* China has now introduced their own first-generation satellite navigation system. The "Beidou (Big Dipper) Navigation Test Satellite 1" was launched by a Chinese Long March 3M booster on 31 October 2000, into a geostationary orbit slot at 140 degrees East Longitude, to the east of China.

It was followed by "Beidou 1B" on 21 December 2000, which was placed in a geostationary slot at 80 degrees East longitude. This allows the two satellites to provide navigational coverage over the entire country. Specific details of the system have not been released, but the Chinese state that they are working on a second-generation navigation satellite system.



* All GPS satellites up to and including the Block IIR satellites broadcast two microwave carrier channels, with timing based on two rubidium and two cesium atomic clocks. The first carrier is at the "L1" frequency, 1,575.42 MHz, and the second is at the "L2" frequency, 1,227.60 MHz. The two carriers provide somewhat different sets of signals:

The civilian L1 signal is known as the "coarse acquisition (C/A)" signal. This signal carries a 1,023-bit PRN code, which as mentioned earlier uniquely identifies a particular GPS satellite.

The L1 and L2 military signals are both based on "precision (P)" PRN codes, about 6 x 10^12 bits long, with a cycle time of a week. As delays in the propagation of radio waves through the atmosphere change more or less predictably with frequency, the use of a P signal on each carrier allow military receivers to provide some compensation for such delays.

During military operations, the P codes can be encrypted by another level of coding, known as a "Y code", to prevent an adversary from trying to "spoof" GPS receivers with phony GPS signals. The two P codes are combined with the C/A signal to provide high-resolution position data.

* The Block IIR-M satellites will add a new military or "M" code to both carrier frequencies, and also provide a new L2 code for civilians designated "L2C". The dual M codes will provide increased resistance to jamming by using "spread spectrum" techniques, and also are believed to provide a capability to deny an enemy use of the GPS signal, though details are classified. The second civilian signal will give civilian users increased ability to compensate for atmospheric delays. Full operational capability of the Block IIR-M constellation is expected in 2007.

The Block IIF satellites will add a third carrier signal designated "L5" at 1,176.45 MHz. The new L5 signal is intended for civilian applications in air traffic control. Full operational capability of the Block IIF constellation is not expected before 2011.

By that time, the Air Force expects to be putting GPS III spacecraft into orbit. GPS III remains largely undefined at present, but the Air Force is pushing for award of a development contract in 2003. Current thinking about GPS III envisions that it will provide 100 times greater signal power, mostly through the use of spot beams, and allow the targeting of GPS-guided munitions to less than a meter. GPS III is expected to reduce the number of orbital planes from six to three, using nonrecurring orbits. The Air Force also wants to improve the reliability and security of GPS.

* The guts of a GPS receiver consists of three functional blocks:

The internal electronics are enclosed in a protective case. The case also contains batteries and access to external power, along with a keyboard and display, and may possibly include digital interfaces to allow the receiver to be hooked up to a computer.

Early single-channel military receivers were big and heavy, weighing about 9 kilograms, but modern GPS receivers are light and compact. GPS chipsets are available from a number of manufacturers, and are also sold in some cases as complete modules that can be interfaced into an electronic system. The cost of a GPS receiver system is steadily falling, and as the price drops the number of applications increases.

* As GPS usage and accuracy increases, concerns over sources of error has increased as well. There are three primary sources of error, including ionospheric interference, multipath reflections, and interference:

Incidentally, the accuracy of a GPS position fix is also partly dependent on the positions of the visible satellites in the sky. Position fixes are about two or three times more accurate if the satellites are scattered all over the sky than they are if they are clustered close together.

* Since the GPS satellites carry precision time references, they can be used to provide timing information accurate to within 100 nanoseconds of the Universal Time Coordinated (UTC) atomic clock. The equipment required to obtain this accurate timing is much more expensive than standard GPS receivers, costing thousands of dollars. Such hardware is nonetheless in demand for applications such as telecommunications and scientific research.

* One of the peculiarities of the GPS satellites is that they operate on a clock that "rolls over" to zero every 1,024 weeks, with the calendar beginning on at 00:00 hours (midnight) on 5 January 1980. This "week number roll-over (WNRO)" was part of the spec for satellite operation, and so should not have been a surprise to anyone building a GPS receiver.

However, the concerns over potential "Year 2000 (Y2K)" bugs that might afflict a wide range of computing equipment were aggravated in the case of GPS receivers by fears that they might also have problems with the very first WNRO, which happened just before midnight on Saturday, 21 August 1999. The uncertainly that older GPS receivers could properly handle the discontinuity sent manufacturers into a frenzy of testing and updates. In any case, the GPS network passed over both the WRNO and Y2K hurdles with no major problems.



* Although GPS had been used by US forces in a limited fashion during the "tanker war" in the Persian Gulf in the late 1980s and during the US invasion of Panama in 1989, the military effectiveness of GPS was dramatically proven during the Persian Gulf war in 1990:1991. GPS was used in its planned roles to guide bombers to targets, allow infantry and armored units to locate their locations in the featureless desert, and position artillery for precise fire. It was also used to guide experimental missiles to their targets.

GPS was a great success, even though it was not fully operational. When the crisis started in August 1990, only 14 GPS satellites were in orbit. Two more were launched and put into service in record time, providing a constellation of 16 satellites when the ground war began. This was enough to give military forces continuous two-dimensional positions, but only intermittent three-dimensional positions.

There was also a lack of military-qualified GPS receivers. Only 4,000 were available when the crisis began, and so the military simply ordered thousands of commercial handheld GPS receivers. Antennas had to be improvised to allow the use of the receivers inside sealed-off armored vehicles. Ironically, to use the civilian GPS receivers the military had to turn off selective-availability coding, and the critics made much of this inconsistency. Saddam Hussein's forces did not exploit this opportunity to use GPS in their own operations.

* The US military has expanded their use of GPS since the Gulf War, acquiring bombs, missiles, and even artillery shells with GPS or differential GPS guidance. GPS guidance allows such weapons to accurately strike targets in any weather, day or night. GPS-guided weapons were used extensively during the Balkans bombing campaign in spring 1999 and the Afghanistan campaign in the winter of 2001:2002.

Ideally, targets can be acquired and located by the launch aircraft or other platform using imaging radar or other sensors, with target coordinates downloaded immediately into the weapon using a hardwired connection or infrared link. The weapon is then launched and guides itself to the target coordinates without further operator intervention. A GPS guidance system is much cheaper than most other types of weapons guidance system, though for absolute accuracy a weapon may also be fitted a "terminal seeker", such as an optical or infrared camera, for pinpoint targeting.

The US military is applying GPS in other imaginative ways. For example, work has been done to develop cargo parasails that can be dropped at high altitude by a transport aircraft, and then sail to a remote location, automatically guided by a GPS receiver, allowing the transport to remain out of range of air defenses near the landing zone.

The US Air Force and Navy are also developing an augmented GPS approach and landing control system for military aircraft, known as the "Joint Precision Approach & Landing System (JPALS)", which will be compatible with the US civilian WAAS and LAAS systems. "Hands-off" aircraft carrier landings have been performed by pilots in fighter jets using prototype JPALS technologies, and JPALS promises to be very useful for controlling the new generation of "unmanned aerial vehicles (UAVs)" now in development.

The US military is trying to understand the full strategic implications of GPS, or what they call "navigation warfare (NAVWAR)". NAVWAR involves the coordinated use of GPS weapons, jammers, and electronic countermeasures to make the best use of GPS for their own purposes while denying it to an adversary.

* Originally, the military simply regarded GPS as a navigational aid. The idea of, say, fitting a GPS receiver to an artillery shell was not seriously considered, and so the problem of jamming wasn't seriously considered. The GPS signals are highly vulnerable to jamming as they are extremely weak, providing about the equivalent energy as a household light bulb thousands of kilometers away, a billion times weaker than the signals picked up by a broadcast television set.

GPS can be effectively jammed with a brute-force "broadband" jammer that throws out radio noise all over the spectrum. A specialized GPS jammer that selectively operates on GPS frequencies would be even more effective, and both Russian companies and American academic institutions have developed such jammers. GPS signals have actually been jammed by accident on a number of occasions, interfering with the navigation systems of aircraft.

There is no shortage of antijamming ideas. Of course, GPS-guided smart munitions always have a backup inertial navigation system to take over when the GPS signal has been demonstrably compromised, though the accuracy of the INS is poorer than that of GPS. Weapons can also be designed to have a "home-on-jam" capability to attack the GPS jammer. However, trading a GPS-guided munition that costs tens of thousands of dollars at minimum for a $500 USD jammer would not be a bargain.

GPS receivers can improve their resistance to jamming by improving the selectivity of their reception. One approach is to use multiple separated antennas so that the angle of the signal being received can be determined, assisting in the rejection of signals coming from the ground and not the sky. Increasing GPS satellite power output would help, and in fact the GPS III satellites now under consideration may use focused "spot beams" to ensure much higher signal power in specific combat theaters.

The US Defense Advanced Research Projects Agency (DARPA), which performs research on advanced military technologies, is also working on a concept in which a network of ground stations and high-flying, long-endurance UAVs could produce high-power location signals that could be used by standard GPS receivers. DARPA refers to the ground stations and UAVs as "GPX pseudolites". The current program will last through 2003, terminating with a wide-area demonstration of the concept.



* The first civilian application of GPS was on large ships, where the relatively high expense of the early GPS receivers was not such a problem. As prices have fallen, GPS receivers have become common on smaller vessels as well.

GPS receiver systems are now being incorporated into cars as well. While they remain mostly curiosities in the US so far, they have proven popular in Japan, where consumers are more gadget-happy. Such systems may interact with the car's CD-ROM player to obtain map information and present it on a dashboard video display.

Inexpensive handheld GPS systems are in increasing use in the US by outdoor enthusiasts, and GPS chipsets are becoming cheap enough to consider their use in common consumer items like cellphones. Further cost reductions in GPS electronics are needed to attain this level of universality, but the US government is pushing an "Enhanced 911 (E911)" that requires location of a cellphone being used to make an emergency call. Many cellphone manufacturers see GPS as the best option for implementing this service.

Interesting civilian applications of GPS under consideration or being implemented include a flight-data recorder, or aircraft "black box", that tracks the position of an aircraft over time; robotic earth excavation; disposal of toxic substances; monitoring of suspension bridges to warn of impending bridge failure; and space capsules for scientific and commercial experiments that would, after reentry, deploy a parasail and glide to a predetermined landing site for recovery.

* Geophysicists have been exploiting GPS since the mid-1980s, using it to measure continental drift and the movement of the Earth's surface in geologically active regions. They have been able to obtain accurate surface measurements to within a few millimeters through a procedure known as "carrier tracking", which is even more accurate than differential GPS. Carrier tracking actually senses the phase of the carrier signals on which the location code sequences are broadcast. It is, not surprisingly, a tricky and subtle procedure, and not applicable for general use.

A particularly interesting potential scientific application of GPS is in observations of changes in the ionosphere, the ionized layer of the upper atmosphere from 80 to 600 kilometers, through a procedure known as "radio occultation".

Radio occultation has long been used by interplanetary probes. All it consists of is tracking changes in the probe's radio signal as it passes behind another planet, in order to obtain information about the planet's atmospheric density and other parameters.

Radio occultation experiments in Earth orbit would involve the launch of satellites carrying GPS receivers. Once in space, ground controllers would observe the timing shifts in the precise GPS signals as the sensing satellites fell under the horizon from the line of sight to a GPS satellite.

A radio occultation experiment built by the US National Aeronautics & Space Administration's Jet Propulsion Laboratory (NASA JPL) was put into orbit in July 2000 on board the German "Challenging Minisatellte Payload (CHAMP)" spacecraft for Earth studies.

JPL's "Blackjack" package carried on CHAMP features a rearward-facing GPS antenna to perform the radio occultation experiments, and also features a downward-facing antenna to pick up GPS reflections from the ocean surface. The downward-facing antenna is highly experimental, with researchers using it to see if GPS signals can be used to determine ocean surface heights and wave conditions. In principle, the heights could be determined from the time of signal travel, and wave conditions from the spreading of frequencies and travel times by choppy seas, a procedure known as "scatterometry".

The Blackjack package also includes a top-mounted GPS antenna for fixing its own position. An improved follow-on to the Blackjack package named "TurboRogue", built by JPL with help from Italy, was flown on the Argentine "Satelite de Aplicationes Cientificas-C (SAC-C)" satellite, launched in November 2001.



* Sources include:

* Revision history:

   v1.0   / 05 dec 96 / gvg / Introduced as "The Global Positioning System".
   v1.1   / 07 dec 98 / gvg / Minor cosmetic update.
   v2.0   / 01 jun 99 / gvg / GNSS, Y2K problems, general rewrite.
   v2.1   / 01 jun 01 / gvg / More details on WAAS & so on, plus GPS jamming.
   v2.2.0 / 01 nov 01 / gvg / Added updates, data on other systems, new title.
   v2.2.1 / 01 mar 02 / gvg / Minor fixes.
   v2.2.2 / 01 jul 02 / gvg / GPS III comments.
   v2.2.3 / 01 dec 02 / gvg / Comments on GPS constellation degradation.
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