Principles of Modern Radar - Volume 2 1891121537

524 CHAPTER 11 Space-Time Coding for Active Antenna Systems⎪⎨T (v, t) = Abs⎧The literal expression of “Migration Target Indication”, which subtracts two imagesobtained by overlapping bursts: pulses 0 to M −M 0 −1, and pulses M 0 to M −1, is writtten:⎧⎫⎪⎩∑f =0,...,N−1r=0,...,M−M 0 −1[ ] y f,r exp(2π j ft ) (exp −2π jr 2v ) (T r exp −2π jrf δF )2v⎪⎬T rNλ 0 F 0 λ 0⎪⎭⎫⎪⎨−Abs⎪⎩∑f =0,...,N−1r=M 0 ,...,M−1[ ] y f,r exp(2π j ft ) (exp −2π jr 2v ) (T r exp −2π jrf δF )2v⎪⎬T rNλ 0 F 0 λ 0⎪⎭(11.19)In the example of Figure 11-27, the clutter is made of 2 slowly moving echoes (1 m/sand 0,5 m/s), with a delay of 17 ns (arbitrary origin), and two targets are inserted, 23 dBbelow clutter level, one at the same range as clutter (delay of 17 ns) et with velocity 16 m/s(slightly above the first ambiguous velocity of 15 m/s), the other exo-clutter at delay 10ns and velocity 18 m/s.On the left, only clutter echoes are visible, with residual ambiguous sidelobes around16 m/s and 31 m/s, due to the repetition frequency 1 kHz (ambiguous velocity 15 m/s).On the right, these clutter residues are cancelled, leaving the two doublets correspondingto the two targets.This simple example (which has then been validated with experimental trials on realclutter described in [22]) clearly illustrates the improvements that can still be obtained inclutter rejection with wideband radars – which naturally can be combined with space-timewaveforms, to combine longer time on targets with wider bandwidths, and thus improveclutter and disturbances rejection.11.6 CONCLUSIONThe different techniques briefly presented may be combined for simultaneous optimizationof space exploration and target analysis and detection. For example, multifrequencytransmission through different sub-arrays, or circulating chirp coding, with a total instantaneousbandwidth between 200 MHz and 500 MHz, or interleaved multifrequency pulsetrains, will allow beamforming on transmit and receive and high resolution in range andDoppler, thus providing better detection, location, and classification of multiple surfaceand low-flying targets.Basically, the main advantages to be gained are a better extraction of targets –especially slow targets – from clutter, multipath, and noises (including repeater jammers),and a better identification of targets, obtained through longer observation times and widerbandwidths.More generally, it may be interesting to note that, while the second half of 20thcentury has seen major developments in radar waveform design and “Time/Doppler” signalprocessing, this early decade of 21st century is now focused on antenna developmentsaround phased array design and “Space/Time” processing, on transmit and receive.