Radar MASINT - Non-Cooperative Target Recognition

Non-Cooperative Target Recognition

Driving research into Non-Cooperative Target Recognition (NCTR) is the fratricide problem, which, according to Army Maj. Bill McKean, is that "... our weapons can kill at a greater range than we can identify a target as friend or foe. Yet if you wait until you're close enough to be sure you are firing at an enemy, you've lost your advantage." The procedural approach of more restrictive rules of engagement (ROE), according to McKean, "What they found was, if you tighten the rules of engagement to the point that you reduce fratricide, the enemy begins inflicting greater casualties on you. "Waiting until you're sure in combat could mean becoming a casualty yourself.". Technical approaches to fratricide prevention include:

1. Systems that align with the weapon or weapon sight and are pointed at the intended target, and send an identification friend or foe (IFF) signal at it. If it responds correctly, it is treated as friendly, but otherwise unknown. Challenges here include the interrogation becoming a source of electronic targeting for the enemy, and trusting a response.
2. "Don't shoot me" systems use a mesh of IFF interrogators that send challenges at a given position. Friendly forces identify in response, and the interrogators share the data. This may not work in terrain that may mask the challenge, response, or response sharing.
3. Situational awareness systems rely on periodic updates of positional data to help users locate friendly forces, as long as the responses are timely and not masked by terrain
4. Noncooperative target recognition systems measure signature using acoustic and thermal radiation, radio emissions, radar techniques, etc. Comparing the measurements to classic MASINT signatures characterize the target.

Radar offers the potential of non-cooperative target recognition (NCTR). These techniques, which could work if IFF systems fail, have been especially secret. No one has yet proposed, however, NCTR that will be effective if a coalition partner is flying the same aircraft type as the enemy, as in Desert Storm. IFF, presumably with encryption, probably is the only answer to that problem.

One open-literature study combined several pieces of radar information: cross-section, range, and Doppler measurements. A 1997 Defense Department report mentions "Air Force and Navy combat identification efforts focus on noncooperative target recognition technologies, including inverse synthetic aperture radar imaging, jet engine modulation (JEM), and unintentional modulation on pulse-based specific emitters".

NCTR on JEM specifically depends on the periodic rotation of the blades of a turbine, with variations caused by the geometry of the elements of the engine (e.g., multiple rotors, the cowling, exhaust, and stators). More generally, the idea of "micro-Doppler" mechanisms, from any mechanical movements in the target structure ("micro-motion dynamics"), extends the problem to cover more than rotating aircraft structures, but also automatic gait recognition of human beings. The micro-Doppler idea is more general than those used in JEM alone to consider objects that have vibrational or other kinds of mechanical movement. The basisc of JEM is described in . One non-rotational effect would be the surface vibrations of a ground vehicle, caused by the engine, which would be different for gas turbines of tanks and diesel engines of trucks. ISAR is especially useful for NCTR, since it can provide a two-dimensional map of the micromovements.

Moving surfaces cause amplitude, Doppler frequency, and pulse modulation of the return. The amplitude modulation comes from moving surfaces of different reflectivity and angle of reflection. Doppler shifting of the returned signals is a function of the radar carrier frequency, as well as the speed of the radar source and target, with positive Doppler shift from surfaces moving toward the illuminator and negative shift of surfaces moving away from it. Moving surfaces impose a pulse width modulation.

Detecting modulation depends on the angle of the source versus the target; if the source is too far off-center with a turbine or other moving surface, the modulation may not be evident because the moving part of the engine is shielded by the engine mounting. Modulation increases, however, when the source is at right angles to the axis of rotation of the moving element of the target. For fully exposed moving elements, (e.g., propeller blades or helicopter rotors), modulation is a function of the radar beam being off-center to the center of the moving element.