When Saab first developed the JAS-39 Gripen fighter for Sweden’s defense, the aircraft included an integrated navigation package. However, to become part of the NATO rapid deployment force, yet maintain vigilance over the homeland, the Gripen nav system had to be upgraded. After six years of development, the solution, says developer Saab Aerospace, is at hand.
Saab boldly claims that its New Integrated Navigation System (NINS) is the only integrated nav system in the world capable of meeting today’s stringent military tactical and safety-critical requirements for accuracy, integrity and continuity. This new approach to integrated navigation may be applicable not only to fighters and other military aircraft, but also to helicopters, cruise missiles and unmanned air vehicles (UAVs).
Today the Swedish Defense Procurement Agency (FMV) is close to validating NINS, after which the system will be installed in all new Swedish Air Force Gripen fighters and retrofitted into the more than 100 Gripens operational with the service. So far, 36 aircraft have been equipped with NINS and installations on a second batch of 40 aircraft have begun.
What is NINS?
According to Saab, NINS is based on an inertial navigation system (INS) tightly integrated with a terrain-referenced navigation algorithm called TERNAV. The new system also benefits from navigational information supplied from the air data computer (ADC), radar altimeter (RALT) and such infrastructure systems as GPS and DME. An integrated navigational system certainly is not a new concept; however, this technology has matured only recently to result in both tactical and safety applications.
TERNAV has been operational in the Swedish Air Force since 1995, when it was integrated into the older Saab Viggen fighter aircraft. The system uses terrain variations to estimate an aircraft’s position in three dimensions. To accomplish this, TERNAV applies elevation and ground cover databases and altitude measurements from the radar altimeter, along with information on the aircraft’s changes of position.
Saab believes TERNAV has reached a performance level comparable to military GPS. But unlike GPS, TERNAV is not susceptible to intentional and unintentional jamming or spoofing, which makes it an attractive software component. Also offering vertical accuracy, TERNAV complements GPS, which has very high accuracy over water and at high altitudes.
Integration of the various NINS elements–including TERNAV, GPS, ADC and DME–is accomplished through an extended Kalman sensor fusion filter that can estimate the errors or faults in the sensors to provide the aircraft the best possible navigation performance. A key feature in NINS, the Kalman filter ensures that the system will always be aware of an aircraft’s position accuracy. Processing NINS data has required approximately twice as much computing power in the avionics processor.
A major software component, the geographical information system (GIS), contains databases for terrain elevation, obstacles, ground cover and runway data used in TERNAV. The GIS predicts the need for data from the databases, decompresses the data in real time, and enables quick data retrieval. The GIS can handle simultaneous readings in all databases. The new terrain database, which FMV provides to Saab Aerospace, comes from the Swedish government’s Administration of Land Surveying. The challenge with the large database is to transfer data quickly from bulk storage to the avionics computer for processing.
While INS, ADC and RALT constitute the base of NINS, support sensors, such as imaging systems and electronic compasses, can improve system safety and availability. These sensors are not required for accuracy but can enhance NINS performance. They rely, however, on infrastructures, or ground-based systems, which may be unavailable in certain situations.
Crucial to the Future
According to Mats Thorbiarnsson, chief Gripen test pilot, the fighter’s original INS is based on gyros and accelerometers. It has its strengths; its estimates are delivered at a high rate with good accuracy during dynamic maneuvering.
But INS has drawbacks, as well. The main one, says Thorbiarnsson, is error growth. Over time, the error increases. "In the old INS, after one hour we could be a few hundred meters off course," he says. "With NINS, the positive accuracy is very much improved." Test flights show the new system to be in error by only 10 to 30 meters in the same amount of time–a vital improvement during critical missions.
And now those missions may become part of NATO rapid deployment operations. "It is understood that we must be ready to participate in those international operations within two years," says Thorbiarnsson. The NINS terrain database must be compatible with the NATO software to permit rapid deployment to foreign terrain.
The NINS test program is nearly complete. It has undergone ground rig testing and flight testing in the Swedish Air Force SK-60 training aircraft before being installed for tests in the Gripen. Two Gripens are used in the Saab flight tests.
There are seven Gripen test pilots, including Thorbiarnsson. He has been a Saab test pilot for two and one-half years and, before that, was a test pilot with FMV for six years. He is considered a temporary civilian while at Saab, but he also is a Gripen pilot in the Swedish Air Force.
Several of the latest production Gripens have been delivered to FMV to validate NINS, as well as other avionics software improvements. FMV will fly about 80 sorties to validate the NINS and other updated systems. The new software upgrades require five Mil-Std-1553 buses, up from four in the original configuration. NINS communications requirements, alone, represent only a small increase in bus communications needs.
FMV validation could be completed this autumn. The Swedish Air Force then will begin to integrate the new avionics updates, including NINS, this fall. Meanwhile, serial production of the fighter aircraft continues, which means operational Gripens will have to be retrofitted with NINS.
From a pilot’s point of view, NINS changes the Gripen cockpit’s look very little. The big difference is the software.
One objective is to use NINS in developing a new integrated landing system (NILS) for the Gripen, which would not depend on any ground navigational aid. "We are now beginning that approach, but it is very early in the program," says Thorbiarnsson. "One big problem is how to qualify the NILS and integrate it into current aircraft."
NILS takes advantage of the NINS concept by creating a glidepath based on its navigation information and coordinates for the runway. The independent infrastructure would reduce cost and increase flexibility, as the aircraft could operate in all weather conditions from austere and dispersed air bases that have no ground-based landing equipment. NILS will be certified for a landing minimum corresponding to Category I requirements.
Another potential NINS upgrade could integrate a position reporting system, supplying air traffic control (ATC) with vector, identity and information for surrounding aircraft. These improvements could include increased situational awareness, midair and ground collision avoidance, fleet management and rescue services. Reporting systems could be a tactical data link like the Joint Tactical Information Distribution System (JTIDS) or VHF data link (VDL) Mode 4.
In short, NINS is certifiable as the primary navigation system for both tactical and safety-critical applications. Other typical applications include random route navigation, weapons utilization and inputs to head-up or head-down displays with synthetic terrain presentations.
The Saab Gripen manager for flight navigation, Predrag Pucar, says a potential working relationship with Boeing and Lockheed Martin on a NINS-based ground collision avoidance system could have large implications for the program’s future.
"NINS could be the basis for many more systems in future navigation systems," Pucar claims. "Each program involving NINS is a separately funded package." However, he also says that, from a military point of view, there is no intention to export the system beyond the Swedish Air Force Gripen program.
Saab JAS-39 Gripen: Up Close
The Gripen multirole fighter is the first, and so far the only fourth-generation European fighter to enter service. (Saab defines "fourth generation" to mean an aircraft that can switch in flight between air superiority, close air support and reconnaissance roles.) The single-seat, single-engine aircraft replaces all Drakens and Viggens in the Swedish Air Force. Unique Gripen design features include:
A triplex fly-by-wire system from Lockheed Martin to allow an unstable aerodynamic configuration.
Modern systems with considerable self-test for ease of operation and maintenance. Actual production costs are lower than with the Viggen. The Gripen can operate from ground highways.
Man-machine interface systems. Features include ministick in the center, HOTAS (hands on throttle and stick) philosophy, wide-angle head-up display and three large, full-color liquid crystal displays.
High-speed data link.
Ericsson PS-05/A radar.
Extensive use of carbon/fiber and other composite materials to decrease weight.
There is a two-seat variant, designated the JAS-39B, with a lengthened fuselage under a modifed canopy.
Under the current contract, the Swedish Air Force has ordered more than 200 Gripens, and more than 100 are in service. International marketing of the Gripen is a cooperative effort with British Aerospace. Major contractors for the Gripen are Saab Aerospace, Ericsson Microwave, Saab Ericsson Avionics, Volvo Aero Corp, and FFV Aerotech.