Historically, with the advances in technology for rockets and guided missiles, there are three primary means of navigation that missiles use. These are celestial systems, Doppler navigation and inertial navigation. Lately, however, inertial systems have been combined with GPS (Global Positioning System) to navigate missiles more accurately. This combination is used in ICBMs (Inter-continental Ballistic Missiles). Not stressing much on the celestial system and the Doppler navigation, let’s look at what the latest weapons of war use.
Inertial navigation is a self-contained system that does not depend on any visual or electronic information from outside the missile. The heart of the system is an accelerometer (a device that measures acceleration of a moving craft along N-S and E-W axes). The acceleration data is with reference to the launch point of the missile. This data is taken by onboard computers and converted onto precise position of the missile.
However, even with the best inertial systems available, missiles suffer from a phenomenon called drift. This is measured in distance (meters) per hour. For example, during the making of the Tomahawk Cruise missiles it was determined that even with the inertial navigation system, it would have a drift of 900 meters per hour. This essentially means that if the missile flew for one hour, it could miss the target by as much as 900 meters! Further, while ICBMs travel at sonic and supersonic speeds, smaller Cruise’s speed was subsonic. So, the chances of missing the target are higher.
TERCOM guidance system
As a solution to the drift problem, a guidance system called TERCOM (Terrain Contour Matching) is used. Though very old as a technology (first tested in 1961 onboard an aircraft), this could not become popular because of the payload size of the computational resources needed for TERCOM was huge at that time. With faster yet smaller computer systems, this technology was re-looked at for the purpose of missile navigation. Here is how TERCOM works.
The system uses radar to scan the ground/terrain that the missile is passing over. The terrain data is compared to the digital maps stored in the computer on the missile. These stored digital maps are of the area along the intended flight path of the missiles and are gathered by satellites and reconnaissance planes. If a drift is noticed, the inertial navigation system is corrected and a course correction is made to put the missile back on path. This process is used a number of times to ensure that the missile does not drift too much before a flight correction is made. Owing to this, the projectiles fall within 50 meters of their target (most fall within 10 meters!).
An added advantage of TERCOM is that the missiles fly very low, almost skimming the ground. Coupled with their small size and radar cross-section, low infrared signature, they become virtually undetectable by enemy radar. Also, a flight path can be input into the onboard system to make the missile ‘fly around’ known enemy defenses!
Another alternative is to use GPS to complement the navigational data computed by inertial system. GPS is based on an array of low-earth satellites. Computers onboard the missile communicates with these satellites to accurately determine their instantan- eous location. GPS is a system that is widely used nowadays for navigation, both in defense and civil applications.
For long India has had to import sophisticated weapons, but the Brahmos supersonic cruise missile, which marries the propulsion of the Russian Yakhont missile with onboard computer guidance systems, developed by Indian scientists will soon change that. This low-radar signature, anti-ship missile has a range of 290 km and can fly up to 2.8 mach. It is also a ‘one of its kind’ in the world.
Germany and France have recently revived efforts to build a similar supersonic missile after shelving it sometime back. Even France’s famous Exocet is a subsonic missile with a far lesser precision and range. Brahmos, which is an Indo-Russian joint venture, will greatly increase Indian military reach in the Indian Ocean vis-à-vis the Chinese Navy, after induction in 2003. The Brahmos is also expected to provide a $ 4 billion market, once planned exports to friendly nations start.
Benoy George Thomas