Background to and applications of Navstar/GPS.

Background to and Applications of Navstar/GPS

Handover ceremonies of the first examples of new equipment to the US military are always newsworthy, but one held in September 1987 had a unique feature. As the US Air Force Space Division accepted the first production receiver for the Global Positioning System (GPS) it also received a commemorative plaque inscribed "Latitude 42 degrees 01 min 46,093 sec", Longitude 91 degrees 38 min 54.314 sec.>> According to signals received from GPS satellites, these were the exact co-ordinates of the site of the handover ceremony. Transfer this year of responsibility for the GPS spacecraft operational control ground station in Colorado from the USAF's Space Systems Division to Air Force Space Command signals the system's operational status and growing maturity.

The idea of launching satellite as dedicated navaids is not new. The US Navy launched its first Transit navigation satellite in the early 1960s, and had an operational network of five spacecraft in place by 1964. Useful as this system was, it could provide fixes only for slow-moving platforms such a surface ships and the US Navy's fleet of Polaris-armed submarines, while the limited number of satellites meant that fixes were only available at intervals of between 35 and 100 minutes, depending on latitude. The US Navy and US Air Force then each embarked on new navigation satellite programmes known as Timation and 621B respectively; in 1973 these were combined into a single programme known as Navstar GPS.

In 1978, four experimental Navstar spacecraft were orbited, and plans were drawn up for an operation system which would call for a total of 21 spacecraft. This project was revised in 1980 to reduce the number of satellites from 21 to 18 as a cost-saving measure, and to delay deployment of the definitive system until the early 1990s so that radiation-hardening features could be incorporated in the spacecraft design.

Two more spacecraft, Navstar 5 and 6, were launched in 1980. Following the failure of Navstar 1, Navstar 7 should have gone into orbit in December of the following year, but was lost as a result of a booster failure. Navstar 8 restored the orbiting constellation to six in 1983, while Navstars 9,10 and 11 were orbited over the next two years to provide in-orbit spares. These first ten are now referred to as the Block 1 spacecraft. These 1157 lb (525 kg) satellites were designed for a five-year life, and only six are currently usable.

For the definitive spacecraft - the second-generation Block 2 model weighing 1860 lb (844 kg) - the launch vehicle was switched from the General Dynamics Atlas E/F to the new McDonnell Douglas Delta II. First launch was on February 14th 1989, and the satellite was declared operational on March 19th. Subsequent launches have been at intervals of around two months. The seventh Block 2 satellite was launched on March 25th of this year, and three more are to follow by the end of the year. Like the earlier Navstars, all were built by Rockwell International Satellite Systems.

Continental USA now has around 16 hours of two-dimensional coverage per day. Full 24-hour coverage with full accuracy is not expected for several years. The full operational network will have 18 satellites, a figure due to be reached by the end of 1992.

All GPS satellites orbit at an inclination of 55 degrees to the equator (a figure originally chosen to allow use of the Space Shuttle as a launch vehicle). Orbital altitude is 10 900 nm (20 185 km), giving an orbital period of 11 h 58 min. Two complete orbits total 23 h 56 min, a period known to astronomers as a sidereal day, the exact time the Earth takes to complete one rotation on its axis. Each satellite is thus once again over the same spot of the Earth at the end of each sidereal day.

GPS satellites are located in six different orbits at 60 degree intervals around the equator, each of which will eventually contain six spacecraft. Three more will be launched as in-orbit spares, while seven more are available either to replace any which fail to make orbit due to booster problems, or to be kept in storage as future spares. Block 2 satellites have a design lifetime of 7.5 years. Starting in the mid-1990s, the USAF plans gradually to replace the older spacecraft in the constellation with the follow-on Block 2R version.

In 1983 the US decided to make the system available to civil and other non-NATO users. As a result, the spacecraft transmit two types of signal, one low-accuracy transmission freely available to the world community, the other an encrypted high-accuracy transmission for the use of the military forces of the US and its allies.

Each GPS satellite carries a highly stable 10.23 MHz oscillator, whose output is multiplied or divided to create all the required frequencies. The spacecraft transmits two signals. The L1 signal is 1575.42 MHz, and is created by multiplying the main oscillator frequency by x154. The L2 signal on 1227.60 MHz is the result of a x120 frequency multiplication.

These signals are phase-modulated by a pseudo-noise binary code to contain precise time information. L1 carries the C/A (Coarse/Acquisition) code, a 1.023 MHz signal created by dividing the master oscillator frequency by a factor of ten. The L1 signal also carries the P (Precision) code signal, whose 10.23 MHz signal corresponds with that of the master oscillator. L2 is modulated only by the encrypted P-code signal.

The signals from the satellite are modulated with the time of transmission, a parameter which the GPS receiver can compare with the time of reception, thus calculating its distance from the satellite. Simultaneous reception of signals from three spacecraft gives latitude, longitude and time. Signals from a fourth will add altitude, and correct positional errors due to the clocks in the satellites and receiver not being perfectly synchronised. Signals from a fifth will allow the user to monitor a second frequency in order to correct for ionospheric variations (a source of potential error discussed later in this article).

A single-channel receiver tunes to different satellites in quick sequence, while two channels are considered good enough for slow-moving users - ground, ship and slow-speed helicopter. Three and five-channel units are also available, the latter being best suited to fast aircraft.

The basic Coarse/Acquisition (C/A) code signal is available to all users. In theory this can give an accuracy of around 50 feet (15 metres), and early results from Block 1 spacecraft showed accuracies of around 100 feet (30 metres). In 1988 the US Government announced that it would add errors to the signal so as to downgrade accuracy to a level which would be less useful for military purposes. In practice, receivers operating with the coarse signal now give accuracies of typically less than 330 feet (100 metres) in three dimensions. Users cleared to use the encrypted P (Precision) Code signals can obtain accuracies of only a few metres, plus own velocity correct to within 0.3 ft/see (0.1 m/sec). Accuracy should be maintained with up to three satellite failures or two failures of adjacent satellites. In the event of failures, ground control can redistribute the remaining spacecraft as required to improve the coverage.

The main sources of error in the GPS system - apart from those deliberately introduced by the US Government - are errors in the orbit of the individual spacecraft, propagation delays due to the Earth's ionosphere and atmosphere, and multipath effects caused by the signal being reflected by terrestrial features close to the GPS receiver. Some of these can be overcome by receiving both frequencies transmitted by the satellites, a useful feature of five-channel receivers.

Another solution is to use a technique known as Differential GPS. This involves having a GPS ground station whose location has been accurately surveyed. Such a station can measure its position by GPS, note any error between the GPS data and the station's true position, then transmit details of this error to other GPS users in the vicinity. Differential GPS reduces errors to around 6-12 feet (2-4 metres), but the technique does have disadvantages to the military user, since it involves having a ground station which transmits: it may thus be located by hostile direction-finding operations and so is vulnerable to attack.

The largest supplier of GPS user equipment is the Collins Government Avionics division of Rockwell International, which has already delivered around 2 300 systems to the US military. Almost 1 000 of these are five-channel equipments for use on high-performance US Air Force and US Navy aircraft. A $66.4 million contract announced last autumn is just one small part of a larger series of contracts from the US Air Force's Space Division which covers the supply of 632 single-channel systems, 125 two-channel systems and 700 five-channel systems, a programme likely to be worth more than $430 million.

Today's jet fighters all entered production before GPS became available, so this system has in some cases been added to later versions only. For example, GPS was introduced to the F-16 Fighting Falcon with the Block 40/42 model built from late 1988 onwards. A GPS receiver will probably be installed on the Advanced Fighter Technology Integration (AFTI) F-16 trials aircraft during its current update. GPS is likely to revolutionise future close air support, since it can provide both the aircraft and the ground-based forward air controller with accurate information on their current position.

For new aircraft, GPS is being incorporated in the original design. In many cases, GPS is being combined with more traditional navaids such as the Inertial Navigation System (INS) to create newer and more effective systems which overcome the limitation of the individual systems. One advantage of teaming GPS with other navaids is that the latter can take over at times when gaps in the constellation or geographic area limit the availability of GPS signals. This is also of interest to civil operators. Honeywell Sperry offers the GPIRS (Global Positioning/Inertial Reference System), while companies such as Sextant Avionique offer an integrated Omega VLF/GPS system.

For the military user, a more significant feature is that INS may be aligned in flight by means of GPS data, so a fighter will be able to conduct faster scramble take-offs. Data from an INS or other system can also be used to help the GPS receiver re-acquire a signal lost due to hostile jamming. This was demonstrated several years ago in the United Kingdom during tests of Plessey's five-channel PA9051 receiver.

The Rafale ACT (Avion de Combat Tactique) will have a SAGEM Sigma RL90 laser-gyro INS coupled with a Crouzet GPS receiver, plus other navaids. The latter will probably include a terrain profile-matching system. The European Fighter Aircraft (EFA)is also expected to carry a combined INS/GPS, as will Japan's new FS-X F-16 derivative.

GPS also forms part of the Integrated Communication Navigation Identification Avionics (ICNIA) proposed for new US combat aircraft such as the Advanced Tactical Fighter (ATF) and late-model A-12 Advanced Tactical Aircraft. As its name suggests, ICNIA will combine the functions of current communication systems, navaids and IFF systems.

GPS receivers form part of the retrofits planned for several types of military aircraft. The first production receiver whose handover was described at the start of this article was a five-channel unit destined for installation in a B-52 bomber. For the moment, GPS retrofits seem confined to NATO US Navy P-3C Orion ASW aircraft undergoing the planned Update IV rebuild are to be fitted with a Collins five-channel Receiver 3A, a system able to interface with the standard P-3 dual LTN-72 INS. Update IV avionics are also due to be installed on the US Navy's new P-7 ASW aircraft.

As part of their current Pacer Strike avionics rebuild, 79 F-111D and 84 F-111E are being fitted with a Rockwell-built GPS receiver, plus the US Air Force's new ring-laser gyro INS. A GPS receiver and new standard attitude and heading reference system similar to those on the F-14D Tomcat and the F/A-18 Hornet are also being added to the Grumman EA-6B Prowler Electronic Warfare aircraft as part of the planned ADVCAP (ADVanced CAPability) upgrade.

Western Europe's biggest GPS order so far is for 65 receivers plus the associated antennae and cabling needed to update the navigation system of French Air Force C.160 Transall transports. This attracted bids from Sextant Avionique and TRT-Defense, but both were turned down in favour of the STC Navigation Systems STRNAV 2500 series receiver. This is a five-channel P-code system. The French order is worth around $2 million, and deliveries are to begin at the end of this year.

Companies are now bidding for the task of updating the navigation systems of the Royal Air Force's C-130K Hercules fleet. The programme, expected to be worth up to $ 30 million, will include an INS, air-date computer and GPS receiver. STC will naturally have its eye on this programme, but faces competition from other companies such as Plessey. The deal might be even bigger, including the upgrading of the navaids on the BAe Nimrod Mk. 2, but recent reports suggest that this part of the programme has been cancelled due to the lower-than-expected life remaining on the airframes.

GPS is also of interest to commercial airlines, since it allows aircraft to operate safely in areas where conventional ground-based navaids are rare or even non-existent. An indication of what is possible came last year when a USAF KC-135 transport aircraft flew round the world communicating and navigating solely by satellite. Civil systems also serve with paramilitary users. For example, more than a year ago the US Drug Enforcement Administration was an early customer for Global-Wulfsberg's new five-channel GPS receivers, fitting these to its two Rockwell Commander 1000 aircraft.

GPS is equally useful for ground and naval forces. Even before a sufficient number of satellites was available to provide good coverage, Magnavox had created a receiver able to operate with signals from either the GPS satellites or the older Transit spacecraft. This was the Vehicle Integrated Navigation System (VINS), a system intended for use on land vehicles.

Collins' manpack GPS receiver for the US forces weighs just under 17 lb (37 kg), and contains enough battery power to operate continuously for up to 12 hours, or intermittently for up to 48 hours. This unit is similar in size and weight to a conventional backpack radio, so not surprisingly, paratroops and other highly-mobile users such as special forces have looked for lighter systems. The result has been a rapid decrease in weight.

In recent years, US companies have vied with one another to create eversmaller GPS receivers, with considerable success in the civil and military fields. Magellan's GPS NAV 1000 might at first sight be confused with a scientific calculator. As its civilian appearance suggest, it is being aimed at non-military maritime users. Powered by six AA alkaline batteries, it measures only 8.75 in X3.5 in X2.25 in (22.2 cm X8.9 cm X5.7 cm), and weighs 1.76 lb (0.8 kg). Given a pricetag of only $ 3000, it should find a ready market aboard commercial and even recreational vessels. A militarised NAV 1000M model is also offered at $ 3 500. An evaluation batch has been ordered by NATO, and an US Marine Corps order is expected this year.

Inevitably, it is the military who spearhead the drive toward miniaturisation. Early last year the Pentagon issued a requirement for a Small Lightweight GPS Receiver, a unit whose acronym SLGR tends to be pronounced "Slugger". Just how small a stand-alone receiver might be has been demonstrated by two US programmes. The Defence Advanced Research Projects Agency (DARPA) SUNS is the acronym for Small Unit Navigation System, but the agency also sponsored the developed by Collins of a GPS receiver which probably takes miniaturisation as far as it can profitably go in the immediate future. As its name "Virginia Slims" suggests, the prototype unit is about the size of a pack of cigarettes, and weighs only 0,5 lb (0.23 kg), yet inside this tiny case is a receiver operating from the P (Precision) code. Airborne systems are also shrinking in size. Collins now offers a 9.9 lb (4.5 kg) unit able to match the performance of its standard 3/8 ATR-sized receiver.

The Soviet Union has developed a rival system. Known as Glonass, this will have a constellation of 21 operational spacecraft plus three in-orbit spares. These will be in circular orbits at an altitude of 19 100 km, and with an orbital period of 11 h 15 min. First launch was in October 1982. Studies of the signals from the Soviet spacecraft have led researchers to conclude that the onboard clocks on current Glonass satellites are similar in accuracy to the cesium clocks on the Block 1 GPS. Coverage of the system lags behind that of GPS, and the full system is not expected to be in orbit until 1995.

Like GPS, Glonass satellites transmit two different signals. This is done at frequencies only slightly higher than those used by the US, but with a lower bandwidth. The latter feature implies that the accuracy of the Soviet system will be less than that of GPS.

The two systems are sufficiently alike that receivers able to operate from either type of spacecraft will probably be developed and fielded. Earlier this year, INMARSAT awarded a contract to Trimble Navigation, already an established source of GPS receivers, for the development of a receiver which could be deployed to monitor the integrity of both spacecraft types. The US company is working in conjunction with Leeds University and Worcester Polytechnic Institute on the new design, which is intended to detect satellite malfunctions within seconds. Warnings of the failure would then be transmitted via INMARSAT's third-generation satellites.

Even with Glonass available as an alternative, non-NATO users face the problem that the availability of GPS will probably result in older navaids being phased out. A document published by the US makes it clear that the Department of Defense hopes to abandon the military use of area navigation systems such as Omega and Loran, route-navigation systems such as Tacan, and VOR/DME, and even the ILS landing aid. Just how rapidly this plan will be carried out, and the degree to which civil users will follow the military lead are open to speculation, but there is the long-term prospect that the USA and USSR will eventually enjoy a global monopoly of long-range navigation systems. By degrading the accuracy of the C/A-code signal, the US Government has already demonstrated its ability to dictate the level of performance which the civil GPS user will enjoy.

Given the modest size of the receivers, GPS is obviously a prime candidate for the guidance of medium and long-range missiles and remotely-piloted vehicles (RPVs). Litton Italia and STC have jointly developed the LISA-6000 INS/GPS for use on the now cancelled Modular Stand-Off Weapon (MSOW). This 7 kg unit is based on the inertial components of the LISA-4000 attitude and heading reference system used in the Agusta A.129 Mangusta and the EH Industries EH.101, and incorporates a five-channel P-code receiver.

Collins is currently developing a two-channel receiver for use on Block 3 versions of the General Dynamics Tomahawk land-attack cruise missile. This will be linked to the missile's on-board digital scene-matching area correlator, and will probably be used to update the INS.

The McDonnell Douglas AGM-84 Stand-Off Land Attack Missile, a derivative of the Harpoon anti-ship missile, has a longer range than the original weapon, and was an ideal missile application for GPS. For mid-course guidance it relies on a modified version of the anti-ship missile's normal midcourse guidance unit, teamed with a Collins GPS receiver and a datalink. Using GPS data, the weapon is able to determine its position to within 50 feet (15 metres), and so can be flown close enough to the target to allow final homing by means of a nose-mounted imaging infrared (IIR) seeker, a unit "borrowed" from the AGM-65D version of the Maverick.

Long-duration RPVs such as Boeing's twin-piston-engined Condor pose unique navigation problems, since INS systems drift with time. The 200 ft (61-metre) span Boeing aircraft has already demonstrated its ability to remain airborne for two-and-a-half days. Currently, the prototype relies on a strap-down INS, but production aircraft will probably be fitted with GPS.

The potential usefulness of GPS as a missile navaid may have been a factor in the US Government's decision deliberately to downgrade the accuracy of the C/A Code signal. During the "War of the Cities" conducted in the final stages of the recent Iran/Iraq conflict, the accuracy of the extended range Scud missiles fired by Iraq was poor, with rounds falling a mile or more from their nominal aiming point. In military terms, the weapon was no more effective than the A-4 (V-2) rockets fired at London in 1944. Any future ability to exploit the C/A-code for aim correction in the final stage of flight would make Third World missiles much more effective, even given the code's degraded level of accuracy.

Much attention has been paid to the threat posed by Third World ballistic missiles programmes. The prospect of these being joined by relatively inexpensive GPS-guided cruise missiles is disturbing, given recent reports of the chemical warfare capability already being developed by nations such as Iraq.

Another question mark over the future of GPS concerns the long-term security of the P-mode signal. Although this is encrypted to ensure its availability to the US and close allies only, GPS exists in a world where intelligence services spend vast sums of money on code-breaking. How long can the Precision mode remain secure? How long will it be before some nations develop the ability to read and exploit P-code signals? To take a simple analogy, for close to a decade manufacturers of computer games have tried to make their products uncopiable, but the "hackers" - computer enthusiasts skilled at stripping away such protection - have always managed to cope. Here is a case where a small number of amateurs pit their skills against teams of professional programmers, yet the former always win.

In the case of the P-code, and indeed the general problem of crypt-analysis, teams of highly qualified professionals compete with one another. The struggle is less one-sided, and the history of codebreaking is littered with accounts of the breaking of "uncrackable" codes. The secrets of the P-code may not stay inviolate for long.

PHOTO : Magellan Systems Corp. has developed the NAV 1000M, a pocket computer-size C/A code GPS receiver which provides a position accuracy of 25 metres.

PHOTO : Collins is the main supplier of GPS units to the US forces. The units shown here (rear, l. to r.) are naval, manpack, vehicle and aircraft (foreground).

PHOTO : Advances in integrated circuit technology have reduced the size of receivers. In less than ten years the number of components has dropped from 77 to 24.

PHOTO : In late 1988 this US Coast Guard HH-65A Dolphin rescue helicopter was the first to have a fully integrated GPS/aircraft navigation system.

PHOTO : This small 5.67 kg unit forms part of the guidance system of the US Navy's SLAM version of the Harpoon.

PHOTO : This Rockwell system, which squeezes better accuracy out of the GPS C/A code, could arouse great interest among those not granted access to the P code.

PHOTO : Datum Timing has recently introduced this two-channel airborne and portable GPS receiver.