the spectrum shown in fig1 permits the basic principles of the invention to be understood . the main peak of the spectrum is due to the main vehicle ( body ) echo , its frequency being shifted with respect to the repetition frequency fo of the transmitted signal by a quantity fd . this frequency shift is caused by the radial component of the vehicle &# 39 ; s own speed ( fd representing the doppler frequency ). at frequency fo , we find the ground ( clutter ) echo due to slow moving objects and those whose average speed of motion is zero . another peak shown in fig1 is shifted in frequency from the frequency fo by an amount equal to twice the doppler frequency fd . this peak represents the echo from the vehicle &# 39 ; s caterpillar tracks . in fact , if the vehicle is moving at a speed v with respect to the ground , it will be realized that the upper part of the tracks is then moving at a speed of 2v . this invention is designed to make use of this characteristic to automatically recognize the presence of tracked vehicles in a zone scanned by radar . the principle employed for the recognition of track vehicles is based on the simultaneous observation of the two characteristic frequencies fd and 2fd . however , as shown in fig1 the doppler component corresponding to the movement of the ( caterpillar ) tracks is at a much lower level ( due to the much smaller area ) than the echo from the vehicle body and furthermore non - linearities of the radar receivers can cause the 2nd harmonic of the doppler signal received from a wheeled vehicle to appear . the result can therefore be false identification if the criterion for recognizing tracked vehicles is based solely on the simultaneous presence of the two characteristic frequencies fd and 2fd . a closer analysis of the signal received from a tracked vehicle shows that the doppler frequency of the signal reflected by a track is slightly shifted and thus no longer exactly twice the doppler frequency fd of the body echo . this peculiarity is due to the fact that there is always a slight slippage between track and ground , especially when the tracked vehicle is travelling over rough terrain . the diagram in fig2 shows how this latter characteristic is used to advantage by the invention to produce an automatic track vehicle reconnaissance device operating even in the presence of high harmonic distortion of the signal received from the body of the vehicle ( main vehicle echo ). the characteristic body echo line at the doppler frequency fd is shown in fig2 along with the 2nd harmonic of the body echo at the frequency 2fd . on either side of the spectral line at 2fd , the spectrum of the signal reflected by the tracks is seen to cause a slight shift to appear with respect to the frequency 2fd . it is this phenomenon which is used in the reconnaissance device described below in connection with fig3 . referring now to fig3 the radar signal , for example , supplied by a typical range selector of a doppler mti radar system receiver . in practice , reconnaissance of track vehicles should be carried out for each resolution cell , and a radar receiver adapted for this purpose will contain as many duplicate devices , such as the one in fig3 as there are range selectors ( i . e ., range &# 34 ; bins &# 34 ;) in the system . the device in fig3 includes a frequency doubler 1 supplying , from the radar signal at 10 , a signal doubled in frequency . this doubled - frequency signal is then applied to an amplitude limiter 2 in which it is standardized at a value not exceeding a predetermined maximum . the output signal from circuit 2 is then applied to a multiplier 3 which also receives the incident radar signal on line 9 . a narrow - band ( a few hz ) filter circuit 4 selects , from the output signal of circuit 3 , only the signal component due to the echo of the vehicle tracks . in other words , the filter 4 allows only signals having a frequency of 2 to 5 hz ( for example ) to pass . this frequency range represents the transposed doppler frequency ranges on both sides of the spectral line at 2fd of fig2 . the presence of an output signal from the filter circuit 4 therefore indicates the presence of track echoes . the amplitude detectors 5 and 6 supply a signal proportional to the amplitude of the incident radar signal ( circuit 6 ) and a signal proportional to the amplitude of the filter 4 output signal ( circuit 5 ). circuit 7 calculates the quotient of the output signal of circuit 5 over the output signal of circuit 6 . the quotient computer 7 normalizes the signal due to the track echo . this operation is necessary when the criterion of recognition of tracked vehicles is based on whether or not a signal at the frequency 2fd ± ε exists which is a given ratio with the received signal of frequency fd . to do this , the output signal from the quotient computer 7 is compared to a fixed threshold in the comparator circuit 8 . the output of comparator 8 therefore indicates the presence or absence of a track - type vehicle in the resolution cell examined . the value of the threshold ( which may be empirically determined and set into 8 ) applied to the comparator circuit 8 corresponds to the average value of the ratio of the amplitudes of the signals reflected by the body of the vehicle to that from the ( caterpillar ) tracks , the average value of this ratio being approximately 30db . although this invention has been described in connection with a particular application , it is clearly not limited to the said application and is capable of other variants or modifications . accordingly , it is not intended that the drawings or this description should be regarded as limiting the scope of the invention , these being intended to be typical and illustrative only .