Sensor integration for navigation and survey;
the whole is more than the sum of its parts

The introduction of the Global Positioning System (GPS) has revolutionized the way in which defense forces navigate. Initial doubts about its military usefulness were largely dispelled during the 1990-1991 Gulf War, since when GPS has become the sensor of choice for many applications. These are now expanding to include guidance of iron bombs and artillery shells, taking advantage of the shrinking size and plummeting costs of digital circuitry.

GPS is not a panacea, however. Several other more traditional types of sensor provide facilities that neatly complement those of GPS, and defense forces are thus increasingly specifying systems that combine technologies to provide the best of all worlds. Sensors that can benefit from such an approach include inertial navigators, attitude and heading reference systems, Dopplers and digital compasses. This process can also be taken further by slotting other single-board systems - such as digital maps, radar altimeters and air-data computers - into the same box.

The integration of a GPS receiver with an inertial navigation system (INS) combines the inherent low-bandwidth, precision range measurements of the former with the high-bandwidth, low-noise performance of the latter. This leads to advantages both in conventional navigation and in more wide-ranging applications.

Dr. Roger E. Fisher, who leads the Precision Strike thrust in the Office of the US Deputy Under-Secretary of Defense for Advanced Technology, describes INS/GPS as ''becoming the backbone'' of that effort. He sees the technology as providing a reference frame for tactical surveillance and targeting, in addition to weapon delivery and guidance, with synthetic-aperture radar (SAR) incorporating a ground MTI (moving-target indication) mode as becoming ''the workhorse all-weather sensor.''

Norden Systems, which supplies the AN/APG-76 Multi-Mode Radar System to upgrade F-4s of the Israel Defense Force, has conducted flight trials with both the Honeywell H-764G and Litton LN-100G systems (see below) in order to enhance its performance. The availability of accurate positional information allows the SAR to point directly at a ground target when commanded, without the operator having to search throughout the field of view, and assists in stabilizing the radar's sightline when the aircraft is manoeuvring.

In a ''tightly coupled'' INS/GPS installation, the receiver - usually occupying a single electronics board - is physically embedded within the INS. Pseudorange and range-rate data from the GPS receiver are blended with the raw inertial inputs in a Kalman filter to provide a highly accurate navigation solution. The provision of complementary facilities from a single unit results in higher performance than is possible with independent systems, while reducing the total size, weight, power consumption and cost.

The alternative ''closely coupled'' approach involves separate line-replaceable units (LRUs) for the two sensors. These LRUs typically each contain a Kalman filter that produces navigation (position and velocity) data for transmission via databus to the central mission computer, where they are then integrated by a further filter. This filter may also feed aided velocity data back to the GPS receiver in order to assist it in satellite code tracking during vehicle manoeuvres and to re-acquire the signal after jamming.

A tightly coupled installation is more resistant to jamming than an arrangement using separate LRUs. It is also lighter, which can be an important factor in helicopters, missiles and other vehicles where low weight is of paramount importance. A further advantage in heliborne applications is that an embedded INS/GPS installation provides the extremely low output drift necessary for automatic operation in the hover mode. This contributes to mission effectiveness, particularly during target hand-off.

These factors have led the US armed forces generally to favour the tightly coupled approach, although this view is not universal. The French air force has elected to install standalone receivers, communicating with other elements of the navigation system via a databus, in its new combat aircraft. The UK Royal Air Force's initial plans also involve two-box installations using a modified INS and a separate GPS receiver. This is seen as being less expensive than an embedded architecture, where the integration costs can be significant.

European developments

SAGEM is supplying its ULISS-G integrated INS/GPS for aircraft upgrade programs in Poland and Pakistan. The French company is working with PZL Mielec and the Polish aviation institute to develop the M-93 variant of the I-22 Iryda advanced trainer and light attack aircraft, which was due to begin flight trials with ULISS-G in early 1994. SAGEM also received a production order in June 1993 to upgrade the Pakistan air force's Mirage IIIs, including the installation of a pulse-Doppler multi-mode fire-control radar. In both these cases the fit involves a 12-channel C/A-code GPS receiver employing the ''all-in-view'' approach, allowing it to track all available satellites simultaneously. The company can also supply variants of ULISS-G incorporating P(Y)-code receivers, and offers GPS as a complement to the ULISS 30 topographic-survey system for land-based or airborne applications.

SAGEM can additionally provide a GPS complement to its Sigma ring laser gyro (RLG) inertial navigator, which in its basic form will equip Rafale fighters and has recently undergone trials aboard a Puma helicopter at the French flight-test centre. The company's TERCOR (Terrain Correlation) system, which the French air force has selected for its Mirage 2000Ns and 2000Ds and which will also equip Rafales, may be implemented in either the ULISS or Sigma navigators.

The Royal Norwegian Navy has adopted the Sigma 40 variant to upgrade its Oslo-class frigates. This integrates an RLG-based attitude and heading reference system (AHRS) with a GPS receiver and logs. The Royal Danish Navy is buying SAGEM's GPS 40 6/12 military 12-channel P(Y)-code receiver for mast-mounting aboard all its surface vessels, providing outputs for integration with log, attitude and heading information.

Sextant Avionique's TOPSTAR 100P miniature 10-channel P(Y)-code receiver will equip the French air force's Mirage 2000Ds and Rafale Ds, together with the navy's Rafale Ms. Following the supply of pre-production receivers in 1991-1992, Sextant delivered the first production batch for the Mirage 2000D. last year. The unit has also undergone trials in a Mirage III, and in Puma and Gazelle helicopters. The French air-force selected TopStar in 1993 for its Congar upgrade program, and Eurocopter has adopted the equipment for exporting applications. Sextant is additionally promoting a C/A-code variant.

TOPSTAR provides position, velocity and time outputs at a 10Hz update rate from a box that occupies a volume of some two litres and draws less than 20W of power. The receiver is intended for use aboard highly manoeuvrable platforms under severe environmental conditions. Even without inertial aiding, it is designed to maintain lock on the satellite signal at accelerations of up to 50G and speeds of 10,000m/s, and to re-acquire the signal in less than 1s following shadowing.

LITEF has developed its two-box LSR-85G, which interfaces the Germany company's LSR-85 gyrocompass and vertical-reference system with a Magnavox GPS receiver, to equip new warships and upgrade those in service. The system has undergone trials aboard a fast patrol boat, and is seen as being particularly suitable for platforms - such as mine-countermeasures vessels, surface-effect ships and hydrofoils - where precise velocity information may be difficult to obtain. The LSR-85G weighs less than 20kg, including the associated antenna and control-and-display unit, and can operate in a differential GPS (DGPS) mode to provide highly accurate positional information. LITEF is additionally developing the airborne LCR-88G, which couples its LCR-88 AHRS with a Magnavox DGPS receiver.

The UK Royal Air Force is installing the FIN 1075G two-box INS/GPS system - which combines the GEC-Marconi Avionics FIN 1075 inertial navigator with the PA9056 five-channel P(Y)-code GPS receiver provided by the Electronic Systems Division of GEC-Marconi Defence Systems - in some of its Harrier GR.7s. As a result of its experience during the 1990-1991 Gulf War, the RAF has also elected to upgrade its Tornados in a similar way. GEC-Marconi is offering the FIN 1010G, which integrates the FIN 1010 inertial navigator with a GPS receiver, for this application. Other potential customers for the FIN 1010G include Germany, Italy and Saudi Arabia for their Tornados.

In the mean time, the RAF is rationalizing its existing fits. Northern Telecom Europe (formerly STC Navigation Systems) supplied 21 examples of its STR 2510 receiver - six to equip Nimrod MR.2 maritime-patrol aircraft, and 15 for Tornados - during the 1990-1991 Gulf War. In parallel, GEC-Marconi Defence Systems (formerly GEC-Plessey Avionics) provided 46 GPS receivers: five for the RAF's Nimrod R.1 electronic reconnaissance aircraft, in which they interface with a Delco Carousel IV inertial system, and 41 for Tornados.

Over the past year, the RAF has removed the STR 2510s from Tornados and transferred them to maritime-patrol Nimrods. Cossor Electronics, which bought Northern Telecom's GPS business in 1991, is supplying further receivers to complete re-equipment of the Nimrod MR.2 fleet. Similarly, GEC-Marconi Defence Systems provided 15 additional GPS receivers to replace the STR 2510s in Tornados.

Cossor's standalone GPS receivers, which employ software written in the XD-Ada high-level language, incorporate barometric height-aiding and clock-hold facilities. This allows them to navigate when only two satellites are visible to the antenna. The production units that Cossor is supplying to equip the UK Royal Navy's EH101 Merlin helicopters supply primary navigation data over a MIL-STD-1553B interface and via an ARINC 429 interface for aiding of the INS/AHRS. The company is also collaborating with the UK Defence Research Agency (DRA) on a GPS integration Kalman filter, located within the GPS security boundary, that maximizes the synergy with the INS. Work now in progress with extend the application of this filter to navigators using fibre-optic gyros (FOGs).

The Royal Navy's Sea Harrier FRS.2s are a candidate for a similar upgrade to that for the RAF's Harrier GR.7s, with a GPS receiver being linked to the FIN 1031B NAVHARS (Navigation Heading and Attitude Reference System) via the aircraft's MIL-STD-1553B databus. Alternatively, an INS with an embedded GPS receiver may prove more attractive. A request for proposals to equip Sea Harriers is not expected to be issued before 1995. The MoD has, however, released such a request for GPS receivers to equip RN Sea King and Lynx helicopters.

Flight trials of GEC-Marconi Avionics' new RLG-based FIN 3110 inertial navigation system are due to begin at the Aircraft and Armament Evaluation Establishment (A&AEE), Boscombe Down, in May 1994. The FIN 3110, which measures some 17.8x17.8x28cm and weighs less than 12kg, is substantially smaller and lighter than earlier designs such as the FIN 1010. Derivatives are planned for land-based, maritime and commercial applications.

GEC-Marconi Avionics is marketing the FIN 3110 family both with and without GPS facilities. The DRA at Bedford has already ordered two examples of the FIN 3110G variant, which incorporates an embedded receiver, to equip its Harrier T.4 and Wessex helicopter. An operational variant of the FIN 3110G could use either the GEM III receiver developed by Rockwell's Collins Avionics & Communications Division (see below) or GEC-Marconi Defence Systems' PA9300 series of single-card receivers.

The latter, which is already under development for the Eurofighter 2000 program, has a radio-frequency section comprising a single component rather than an entire board, with one device performing signal processing for all channels. The receiver will provide similar facilities to the existing PA9000 series, including a 10Hz update rate and dual-frequency (L1/L2) operation. The five-channel PA9351 variant uses 60 correlators to provide faster acquisition, re-acquisition and enhanced anti-jam performance. The module can also be extended in software to form the six-channel PA9361.

Phased upgrading

British Aerospace (Systems and Equipment) has adopted a phased approach to upgrading its RLG-based LINS 300 inertial systems by adding GPS and the company's TERPROM terrain-referenced navigation system. The baseline variants comprise the LINS 300-10 series, using a MIL-STD-1553B interface, for retrofit to the Northrop F-5E and to equip the British Aerospace Hawk 100/200; and the LINS 300-20 family, employing ARINC 429 interfaces, for the EH Industries EH101 and NH Industries NH90. The LINS 300-21 for the EH101 is already integrated with a Doppler navigator, via a Kalman filter, to provide in-air alignment.

BASE will initially upgrade the LINS 300-10 to the LINS 300-30 standard, and the LINS 300-20 to the LINS 300-40, by interfacing them with separate Cossor Electronics GPS receivers and incorporating an enhanced air-alignment filter in the helicopter variant. BASE demonstrated the LINS 300-30 aboard the A&AEE Comet during the Aries 1993 exercise last year, which included extended high-latitude flights. The system's free-inertial performance was within the US standard SNU 84-1 requirements, consistently achieving errors of less than 0.3nmi/h and maintaining accuracy during extended GPS outages. In-air alignment times were less than 8min, and heading errors within 0.05 . Production units are available for delivery from mid-1994.

The second stage of upgrading adds an interface to TERPROM, which enhances the navigator's performance in the areas of terrain following, target acquisition, weapon aiming and ground proximity warning. BASE also plans to provide a one-box system by integrating a single-card GPS module into LINS 300 during 1994.

The British Army, RAF and RN upgraded all their helicopters used in the 1990-1991 Gulf War - CH-47 Chinooks, Pumas, Sea King Mk4s and Lynx AH.7s - by adding GPS receivers to the existing fit of Racal Avionics Doppler navigators, and by replacing their TANS (Tactical Navigation System) computers with the newer RNS 252 SUPERTANS. The latter combines any of Racal's family of Dopplers with a six-channel C/A-code GPS receiver, such as Trimble's TNL 8000.

The MoD has subsequently selected Racal Avionics as prime contractor for the more comprehensive three-year Puma Navigation Upgrade (PNU) program, worth 13 million. The company has issued tenders for a fully encrypted P(Y)-code GPS receiver, TACAN VOR/ILS/marker system and IFF Mode C height encoder, to be integrated with its own Doppler and computer. Racal Avionics is additionally developing an integrated system combining an RLG-based inertial navigator, GPS receiver and Doppler for Eurocopter France, on behalf of an unspecified European customer.

US programs

The US Air Force was about to announce the result of its competition to provide the tri-service EGI (Embedded GPS/INU) as this feature went to press. The EGI program will involve the supply of up to 3,600 systems to equip the six initial ''Table 1'' aircraft for which it has already been adopted and funded: US Air Force F-15s; US Marine Corps AH-1Ws; and US Army Apaches (both the AH-64A+ and Longbow variants), OH-58Ds, and special-forces helicopters (MH-47Es and MH-60Ks). Further procurements will follow to outfit the 20 to 25 other ''Table 2'' platforms of all three services as their upgrading is approved.

All EGI bidders had to demonstrate that they could meet the entire initial requirement, although the USAF was considering three potential approaches to procurement. Under the first of these, a single contractor would provide all the units. The second possibility was for one company to supply those for the US Army aircraft, with another delivering those for the F-15 and AH-1W. The third approach was to split the purchase into the F-15 and the Apache variants on one hand, and the remainder of the aircraft types on the other.

The leading contenders for selection as the EGI were regarded as being variants of the Litton Guidance & Control Systems LN-100G and the Honeywell Military Avionics Division H-764G, each incorporating a Collins GEM III (GPS Embedded Module). The US Naval Air Warfare Center has already selected the LN-100G with an embedded GEM III as the AN/ASN-166 GINA (GPS Inertial Navigation Assembly) to equip its T-45A Goshawk training aircraft, replacing the present Kearfott Standard AHRS, as part of the Cockpit 21 upgrade.

Litton has an initial contract worth US$4.5 million to supply seven units, including five for flight trials that were due to begin last month (March). Options for production of up to 390 systems, to equip Goshawks and potentially other US Navy carrier-based aircraft, could raise the program value to more than US$35 million. The Japanese Air Self-Defense Force has also adopted the LN-100G to equip its E-767 AWACS (Airborne Warning And Control Systems), each of which will have two such installations.

Litton has additionally developed the LN-93G, a variant of its LN-93 inertial navigator with an embedded GPS receiver, on behalf of an overseas air force. Although neither Litton nor Rockwell will identify the customer, IDR understands that the LN-93G will initially equip the Israel Defense Force's 60 Block 40 F-16C/Ds under the Peace Marble III program. Litton will deliver engineering development models in the early summer of 1994 for in-country flight trials that are planned for July and August, followed by production units from the fourth quarter of the year.

A Collins GEM II has replaced the repackaged version of the same company's MAGR (Miniature Airborne GPS Receiver) that was originally planned for the LN-93G, thereby providing commonality of software with GINA and the LN-100G. The GEM II also has a faster reaction time, provides GPS aiding even in the absence of four satellites, and supplies a line-of-sight solution.

Adoption of the GEM II has additionally freed up a spare card slot, which is available to provide GPS steering as an eventual replacement for TACAN. The LN-93G can use either GEM II, which operates at IF (intermediate frequency) because of the long distance from the antenna in its initial application, or the RF (radio-frequency) GEM III.

Litton says that several other overseas potential customers are evaluating the LN-93G, and that the unit is a candidate for retrofitting in the US services' aircraft that will not receive the EGI. Redesign and consolidation of the LN-93G's internal modules has allowed the box to remain the same size and weight as the LN-93 inertial navigator, and to draw the same power, so that it is fully interchangeable. The LN-93G continues to operate in the same way as the standard INS if the GPS module is removed, and it can emulate standalone INS and GPS functions if a customer wants these inputs to be fed individually to a separate mission computer.

Litton has built two examples of the LN-200G, which embeds a GPS receiver in its FOG-based LN-200 AHRS, and is beginning flight trials in a company Cessna Citation 2. Potential applications include the US Army's RAH-66 Comanche scout/light attack helicopter, which will carry two LN-210s as part of the fire-control system (FCS) and a single LN-100G that acts as the primary navigator and provides the third channel of the FCS. Embedding a GEM III in the LN-210s would enhance their contribution to the fire-control solution. Litton is also supplying a variant of the LN-200G with an embedded Magnavox MX 7212 DGPS receiver to SAIC for the US Air Force's LATR (Large Area Tracking Range) program.

Honeywell has developed the H-764G INS/GPS system, which combines three of the company's GG1320 RLGs and the same number of Sundstrand QA2000 accelerometers with a GPS receiver such as the GEM III.

MAPS potential

In February 1993, the US Army selected the company as sole source for integration of the Rockwell PLGR (Precise Lightweight GPS Receiver) into the H-726 MAPS (Modular Azimuth Position System). The receiver will be incorporated in new production units and will also be available as a field-retrofittable kit. The integration program includes the incorporation of a MIL-STD-1553B databus, replacing the present RS-422.

Potential applications of the GPS-equipped MAPS include the M109A6 Paladin howitzer, the follow-on AFAS (Advanced Field Artillery System), the TPQ-36(V)7 variant of the Firefinder counter-battery radar, the LAMPS (Large-Area Mobile Projected Smoke) system, and the replacement for the US Army's FIST-V vehicles. Programs in Europe and the Pacific Rim are other possible markets.

The US Army's M1 Abrams main battle tanks and Bradley Fighting Vehicles at present carry no compasses, since they were designed primarily to fight Warsaw Pact forces on well surveyed terrain in Western Europe, and the cost of equipping them with gyrocompasses was regarded as prohibitive. This proved a significant disadvantage during the 1990-1991 Gulf War, when commanders were forced to leave their vehicles and walk several tens of metres away - beyond the immediate magnetic field - in order to take readings with handheld lensatic compasses.

KVH Industries, which has developed a comparatively inexpensive digital electronic compass that operates effectively within an armoured vehicle, provided units to equip US Marine Corps LAV-25s during the Gulf War and has since supplied others for the Saudi Arabian National Guard's LAV-25s. In the latter case they interface with Trimble Navigation AN/PSN-10 Trimpack GPS receivers.

In October 1993, KVH Industries received a contract worth up to US$18 million to provide a variant that will interface with a PLGR and tactical computers aboard US Army Bradleys. Production is scheduled to begin in early 1995 and continue throughout the decade. The company is also working on an electronic compass for the M1.

The US Army Communications-Electronics Command has assumed responsibility for the Gun Laying and Positioning System (GLPS), which previously formed part of the Army Space Exploitation Demonstration Program. The man-portable GLPS permits accurate and rapid alignment of weapons and electronic equipment without having to wait for non-organic survey assets to arrive. Potential applications include howitzers other than Paladin, mortars, Patriot air-defense batteries and direction-finding systems.

GLPS integrates a PLGR with an advanced wire-suspended north-seeking gyro and an eyesafe laser rangefinder, providing self-location to GPS accuracies and azimuth alignment/pointing to 0.2mils. Positioning accuracy is about 12m CEP, compared with the US Army's 17.5m requirement for howitzers. GLPS permits laying of a battery in 6-8min rather than up to two hours using PADS (Position & Azimuth Determining System) and an aiming circle.

Miniature systems

The development of miniature and inexpensive integrated INS/GPS systems is expanding the range of applications to tactical missiles, guided munitions and unmanned aerial vehicles. Programs that are planned to adopt such an approach include the US armed forces' Joint Direct Attack Munition (JDAM) and Joint Stand-Off Weapon (JSOW).

Honeywell Military Avionics Division is promoting a family of systems based on either its GG1308 RLGs or FOGs in order to meet different customer preferences. Several Rockwell companies are collaborating on the MIGITS (Miniature Integrated GPS/INS Tactical System) family, which combines a variety of GPS engines and INS assemblies with a common core navigation filter and input/output processor module.

The Collins Avionics & Communications Division has delivered about 10 examples of C-MIGITS (C = C/A-code), which incorporates a Rockwell NavCore V receiver, for laboratory testing and has recently begun flight trials of the unit operating in DGPS mode.

Collins has also provided about the same number of the P-MIGITS (P = P-code) version to other Rockwell divisions and will deliver a further 40 to 50 this year, mainly to the Tactical Systems Division for integration into the US Air Force's AGM-130 stand-off missile. P-MIGITS, which has already undergone trials in the GBU-15 guided bomb, assists in pointing the seeker so that the target remains within its field of view.

Other applications include a US Army mine-clearance program, in which it aids in levelling the vehicle's blade, and to replace the earlier C-MIGITS in the General Atomics Tactical Endurance UAV.

Digital watch

M-MIGITS (M = Micro), which will be available in February 1995, incorporates a Digital Quartz IMU (inertial measurement unit) based on the ISA (instrument sensor assembly) that Rockwell's Autonetics Electronic Systems Division has co-developed with the Systron Donner Inertial Division of BEI Electronics. The ISA, which consists of three quartz angle-rate sensors and the same number of quartz accelerometers, exploits technology originally employed in digital watches and is claimed to be significantly cheaper than alternative approaches. Rockwell says that the DQI, which will be available in production quantities from July, provides substantially better performance over the full military temperature range than was available from any earlier quartz solid-state system. M-MIGITS is small enough to be packaged into a pod 12.7cm (5in) in diameter, making it suitable for range applications.

ARPA (the US Advanced Research Projects Agency) is sponsoring the GPS Guidance Package (GGP) program, which is intended to provide a family of solid-state packages that could replace all present designs. Litton Guidance & Control Systems leads the GGP Phase 1B team, which includes Rockwell's Collins Avionics & Communications Division and Boeing Military Airplane Company. The team will deliver two brassboard units in September 1994 for government testing aboard land vehicles and high-performance aircraft.

The brassboard model combines a Litton LN-250 MIMU (Miniature IMU), based on three fibre-optic gyros and a silicon accelerometer, with a Collins 10-channel MGR (Miniature GPS Receiver) using gallium arsenide technology and providing all-in-view tracking. The MIMU has a gyro bias stability of better than 0.01 /h, while the MGR provides rapid acquisition (15s) and can search the sky in 86s. The MIMU and MGR are tightly coupled to provide an accuracy of less than 10m SEP (standard error probable) when processing GPS signals, and 20m SEP for the first 5min after signals are lost. The complete unit has a volume of just under 4.9 litres (300in3), weighs approximately 6.8kg and draws 31.5W of power.

The Phase 2 contract, which will be opened to competition, is expected to be awarded in the autumn of 1994. This effort is aimed at providing a miniature unit that weighs 3.2kg, has a volume of about 1.6 litres (100in3) and would cost US$15,000 in production. The US Army Missile Command will assume responsibility for this phase in succession to the US Naval Command, Control and Ocean Surveillance Center, which managed Phase 1. Initial testing of Phase 2 hardware will take place aboard land vehicles rather than long-range missiles, as had earlier been foreseen.