the signal processing portion of a pulse doppler radar system constructed in accordance with the principles of the present invention is shown in fig1 . the system has a transmitter sen connected to a transmit / receive circuit s / e having an antena ant . a demodulator dem is connected to the transmit / receive s / e , and a coherent oscillator coho is connected to each of the transmitter sen and the demodulator dem . the two outputs i and q of the demodulator dem are connected to analog - to - digital converters 1 and 2 for converting the video signals into digital form . the further stages of the processing branch of the receiver include a coherent integrator 3 for vectorial integration , a threshold circuit 4 , and an indicator means 5 . a modified coherent integration of the reflected pulses is undertaken by the integration circuit 3 in order to considerably reduce the filter outlay which would otherwise be required for conducting coherent integration in a pulse doppler radar having variable pulse repetition frequency . the preconditions for executing a modified coherent integration are a specific staggering of the pulse repetition frequency , and a phase backing or reference for the signal voltages which has a predetermined constant value . optimum target detection is guaranteed as a result of the modified coherent integration . the reflected pulses of a range window are vectorially added over a plurality of reception periods . over - the - horizon clutter can be largely supressed by this measure when the mean power of the over - the - horizon echos is distributed to a plurality of range windows . the desired effect can be achieved by means of a specific staggering of the pulse repetition frequency . when the different pulse repetition periods are thereby statistically connected to each other , a considerable number of coherent filters is required in conventional means for undertaking coherent integration in order to cover the entire doppler frequency range of interest . the system disclosed and claimed herein considerably reduces the outlay for undertaking modified coherent integration , and over - the - horizon echos can be spectrally distinguished from the moving targets within the range of unambiguity with a small number of filters . the underlying effect of such an arrangement is that of obtaining a comb - like filter structure even given a quasi - statistical pulse - repetition frequency . the transfer function of the filter becomes more regular as the staggering of the pulse repetition frequencies becomes more ordered . a schematic diagram for a circuit for undertaking a modified coherent integration of the reflected pulses is shown in fig2 which is utilized in the coherent integrator 3 shown in fig1 . digital signals are supplied at the input side of the circuit to a series connection of n shift registers sr 1 through sr n . the number of shift registers corresponds to the number of signal values to be integrated . the signals are tapped before the input of each shift register and are supplied to respective multipliers m 1 m n + 1 . the output of each multiplier is connected to a common adder ad . each shift register sr 1 through sr n effects a delay corresponding to the pulse repetition period prt of the radar means . in order to undertake vectorial addition of the samples for coherent integration , a phase reference for the individual samples is required . this must normally be derived from the momentary pulse repetition time and the center filter frequency . in the modified coherent integration for the system disclosed and claimed herein , however , all signals tapped before the inputs of the shift registers sr 1 through sr n are multiplied by a weighting factor of e - j · ω . sbsp . 0 . sup .· n · prt in the multipliers m 1 through m n + 1 . this weighting effects a constant phase reference for the samples so that the required phase condition for vectorial addition of the sample is restored after two phase reference steps . in the weighting factor , ω 0 = 2πf 0 , where f 0 is the center filter frequency . as described above , n in the expression is the number of samples and prt is the mean period duration for the pulse repetition frequency . in order to obtain a comb - like filter transfer function in the integration circuit , even given a quasi - statistical pulse repetition frequency , the staggering of the pulse repetition times is selected such that the respective sums of two successive pulse repetition times produces a value which remains constant for all pairs of successive pulses . examples of staggerings of the pulse repetition periods are shown in fig3 through 5 based on transfer functions calculated with modified coherent integration . fig3 shows and example of staggering wherein two pulse repetition periods are employed which differ from the mean pulse repetition period prt by a value δt , and which can be determined according to the following equation : a further example of prt staggering is shown in fig4 utilizing eight different pulse repetition periods with a linearly rising deviation from the mean pulse repetition period prt . these pulse repetition frequencies are calculated according to the equation : whereas the examples shown in fig3 and 4 have an ordered staggering , the examples shown in fig5 is based on a completely statistical staggering , wherein there is no longer any pair - wise allocation of periods . the individual pulse repetition periods are calculated in the example shown in fig5 according to the following equation : ## equ1 ## two comb - like filter transfer functions are shown in fig6 and 7 respectively based on the examples employing an ordered staggering shown in fig3 and 4 . the transfer functions are derived from a modified coherent integration of sixteen pulses . both transfer functions are based on a mean pulse repetition frequency prf of 4 khz with a corresponding pulse repetition period prt = 250 microseconds . the values for the staggering of the pulse repetition periods are shown in fig6 below the transfer function . as can be seen in fig6 the periods of successive sampling pulses alternate between two values 2 / 3 · prt and 4 / 3 · prt . the mean value of any two successive pulse repetition periods assumes a constant value , namely 250 microseconds , and δt assumes the value 1 / 3 · prt . the phase reference for the signal values thus is achieved with a constant value δφ = 2π · f 0 · prt , wherein f 0 = 2 khz is the center filter frequency for this example . vectorial addition of the samples is enabled on the basis of the constant phase reference and clearly exhibits the comb - like structure transfer function . a further important feature of this transfer function is that the frequency range around the doppler frequency 0 hz is in the stop band , thus indication of a moving target is guaranteed within that range as a result thereof . the transfer function shown in fig7 based on a prt staggering according to fig4 is not restricted to the use of two different pulse repetition frequency , but can theoretically employ an arbitrarily large number . the essential feature of the staggering in the embodiment shown in fig7 is , as in the embodiment shown in fig6 that any two successive pulse repetition period pairs assume a constant mean value . calculation of a transfer function according to fig7 is based on a change of the pulse repetition period δt with the value 1 / 3 · 1 / 16 · prt . the constant phase backing of the samples which is essential for the modified coherent integration again is δφ = 2π · f 0 · prt . the comb - like structure of the transfer function and the null in the region of the doppler frequency 0 hz can also be clearly seen in fig7 . although modifications and changes may be suggested by those skilled in the art it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art .