NON COHERENT MTI

   

   The composite echo signal from a  moving target and clutter fluctuates in both phase and amplitude.The coherent MTI and the pulse -doppler radar make use of the phase fluctuations in the echo signal to recognize the doppler component produced by a moving target. In these systems,amplitude fluctuations are removed by the phase detector.The operation of this type of radar, which may be called coherent MTI, depends depends upon a reference signal at the radar receiver that is coherent with the transmitter signal                                                                                                           

It is also possible to use the amplitude fluctuations to recognize  the doppler component produced by a moving target. MTI radar  which uses amplitude instead of phase fluctuations is called noncoherent .It has also been  called externally coherent, which is a more descriptive name.The non coherent MTI radar dose not require an internal coherent reference signal or a phase detector as dose the coherent from of MTI. Amplitude limiting cannot be employed in the noncoherent MTI receiver, else the desired fluctuations would be lost.Therefore the IF amplifier must be linear, or if a large dynamic range is required , it can be logarithmic. A logarithmic gain characteristic not only provides protection from saturation, but it also tends to make the clutter fluctuations at its output more uni from  with variations in the clutter input amplitude detector. The detector following the IF amplifier is a conventional amplitude detector. The phase detector is not used since phase information is of no interest to the non coherent radar.The local oscillator of the oscillator of the noncoherent radar dose not have to be as frequency-stable as in the coherent MTI. The transmitter must be sufficiently stable over the pulse duration to prevent beast between overlapping ground clutter, but this is not as severe  a requirement as in the case of coherent radar.                                                                                          

The output of the amplitude contained in the amplitude fluctuations may also be detected by applying component of the amplitude   detector to an A-scope,Amplitude fluctuations due to doppler produce a butterfly modulation similar to that, but in this case, they ride on top of the clutter echoes.Except for the inclusion of means to extract the doppler amplitude component, the noncoherent MTI block diagram is similar to that of a conventional pulse radar.                                                                                    

The advantage of the noncoherent MTI is its simplicity: hence it is attractive for those applications where space and weight are limited. Its chief limitation is that the target must be in the presence of relatively large clutter signal if moving -target detection is desired . The clutter echoes of serves the same function as dose the reference signal in the coherent MTI. If clutter were not present, the desired target would not be detected. It is possible, however, to provide a switch to disconnect the noncoherent. MTI operation  and revert to normal radar whenever sufficient clutter echoes are not present.If the radar is stationary, a map of the clutter might be stored in a digital memory and used to determine when to switch in or the noncoherent                                                                                           

MTI    :   The improvement factor of a noncoherent MTI will not, in general, be as good as can be obtained with a coherent MTI that employs a reference oscillator (coho) The reference signal in the noncoherent case is the clutter itself, which will not be as stable as a reference oscillator because of the finite width of the clutter spectrum caused by its own internal motion. If a nonlinear IF amplifier is used. it will also limit the improvement that can be achieved