Principles of Modern Radar - Volume 2 1891121537

11.2 Colored Space-Time Exploration 501Digital beamforming may provide specific advantages, such as a better visibility ofshort events (e.g. RCS flashes), and a higher Doppler resolution especially useful foridentification purposes, or for detection of slow targets, and effective suppression of noisejammers [4].11.2.1.2 Limitations of Digital Beam FormingHowever, for many applications, the wide illumination on transmit encounters differentlimitations, which may only be alleviated through simultaneous transmission of differentsignals in different directions – colored (space-time) transmissions.For airborne applications, a severe limitation arises from the clutter spreading inDoppler, 1 due to the wider beam on transmit (which is, anyway, difficult to obtain withactive antennas): this leads to a poor minimum detectable velocity, and to a poor clutterrejection, since only half the rejection is obtained (through the receiving antenna diagram),compared to focused beam illumination. This is one clear motivation for turning to coloredtransmission.Another motivation, common to airborne and surface radars, comes from the fact thatthe waveform transmitted through the wide beam is of course the same for every directionin this beam. If, for instance, a surface radar uses a beam widened in elevation, in orderto cover all altitudes simultaneously, the same waveforms will be transmitted for lowelevation angles, where surface clutter is a primary limiting factor, implying the use ofcoherent bursts of pulses, and high elevation angles, where the target ranges are shorterand the surface clutter is largely reduced: since the optimal waveform depends on theelevation angle, it is clear that a widebeam with only one waveform is not optimal.Moreover, when the mission is different for different elevation angles (e.g. detectionof surface targets at 0 ◦ elevation, combined with detection of air targets at higher elevationangles), then again different waveforms are required in the different directions.More generally, considering an active antenna with multiple receiving channels (i.e.multiple sub-arrays on receive), it can be seen that their use results in a difficult dilemma:• If the beam is widened, then the above phenomena (clutter spreading in Doppler, oruniqueness of waveform) actually limit the overall performance;• If the beam is focused, then the reception is not optimal, since each receiving channel hasa wider sector of observation than the sector which is actually illuminated on transmit.Space-time coding on transmit is a way to avoid this dilemma, by transmitting a widebeam while still providing angular directivity on transmit.Last but not least, the directivity on transmit is also necessary when high angularresolution is required, for instance for mitigating multipath effects: the global directivityof the radar being the product of its directivities on transmit and on receive, a radar withdirectivity on transmit will have better performances against multipath than a wide beamsystem. Transmitting simultaneously different signals in different directions is the solutionproviding both wide angular coverage and optimal multipath rejection.1 Main lobe clutter, as seen by a moving platform, has a spectrum whose main lobe is widened by themovement, especially for transverse observations (cf Vol 1, Chapter 5), since different scatterers on theground have different radial velocities with respect to the platform. This widening of clutter spectrum isalso observed when the platform is fixed, but the clutter is moving (atmospheric clutter, or sea clutter).