lesson 4 - TF/TA Systems ( stealth technology )

( based on research-data )

This was an air-to-air flight test in which the target was an F-111. The target was co-altitude with the stealth-radar-carrying-aircraft. The total maximum power in this test was 4 W. The three panels shows the display during search mode, acquisition, and track mode. The display is completely clean of false alarms. The target was acquired and place in track at 25 km and continued to track all the way back to gimbal lock ( very close range ). The stealth radar was never detected, even when the two aircraft were visible to one another. The annotation on the display shows the running time, flight number, horizon, threshold multipliers, target range, azimuth, elevation, heading, and velocity.    

Even a very low-cross-section vehicle can be detected if it is totally in the clear. Operating at low altitude in clutter is best. Additional stealth for aircraft is obtained by flying low and fast with terrain following ( TF ) or terrain avoidance ( TA ) systems. The objective of a TF system is to fly over higher terrain, whereas TA systems fly around higher terrain. Such systems have been used for more than 30 years ( correct me if  I’m wrong ) in cruise missiles and manned aircraft. The concept for stealth terrain following or avoidance uses three sets of measurements, real time altitude and terrain as well as a prestored hypsographic map. The concept is shown below :

TF/TA requires a three –dimensional map of the region along the aircraft flight path with accuracy of about 10 percent of the desired flight altitude. the object for low-flying aircraft is to maintain a relatively fixed altitude above the terrain without hitting anything. Unfortunately, the geometry of TF/TA doesn’t allow high duty ratios, so such as a prestored map database. TF/TA systems must measure some small targets, such as suspended were noncoherent, but this leads ti unnecessary processing losses for a stealth sensor.

The three sets of measurements used in the concept in picture ( above ) must be comparable scale and accuracy, The first set of measurements is a prestored digital terrain elevation database made from survey data, maps, satellite measurements and so on. The prestored terrain database provides control points for long range TF or TA maneuvers. Because the terrain elevation database is not updated very quickly when new features arise, and it contains some known schematic errors in the distance between measured points as well as height inaccuracies, real-time measurement are also required. The second set of  measurements is terrain profile data from a TF/TA radar. The radar must also detected and measure obstacles too small to be in the database but still lethal to aircraft. The third set of measurements is the actual height of the aircraft above the terrain which, when coupled with inertial data to remove flight perturbations, allows matching and registration of the prestored profile with the actual measurements. This is, in essence, a calibration of all three sets of measurements.