Method for HPRF-radar measurement

#This application claims the priority of 011 27 990.8, filed Nov. 24, 2001, the disclosure of which is expressly incorporated by reference herein.

#The invention relates to a method for High Pulse Repitition Frequency (HPRF) radar measurement of target range and Doppler frequency, such as may be used, for example, in an airborne radar system.

#The use of a well known radar waveform with two slightly different transmit frequencies is often referred to in the literature as a frequency shift keying (FSK) technique. Using such a technique, target range and velocity can be measured unambiguously even in multi target situations. FIG. 1 shows the two carrier frequencies f1 and f2 as a function of time. Each frequency represented by a horizontal line corresponds to a coherent processing interval (CPI), and includes a pulse sequence of N-2048 pulses each.

#Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.

#FIG. 1 is a graphic representation of the transmit frequency for FSK-radar as a function of time;

#FIGS. 2.a and 2.b are constellation diagrams which show the phase steps from pulse to pulse for the transmit signal according to the invention;

#The digital PSK waveform can be described easily by the constellation diagram of the phase states of the different pulses in the pulse sequence. In a preferred embodiment a distinction is made between a fixed phase modulation of the first sequence, in which all pulses have the same phase state (for example zero phase) for each pulse, as shown in FIG. 2.a. In this case, the second pulse sequence will have a linearly increasing phase modulation with a fixed phase difference from pulse to pulse, as shown in FIG. 2.b.

#For this digital PSK waveform the transmit signal consists of two interleaved pulse sequences with a fixed transmit frequency, but one having a constant phase (first pulse sequence), and the other being modulated by a linear phase modulation technique (second pulse sequence), as shown in FIG. 3.

#The phase modulation from pulse to pulse can be realized for example by using the quadrature modulation technique which is well known in digital radio transmission. The amplitude envelope of the waveform is constant, so that no distortion due to nonlinearity of the individual amplifier need be considered. The linear phase modulation of the second pulse sequence is carried so that the phase shift from pulse to pulse satisfies the condition that in the complete pulse sequence the phase rotates exactly (for example) 20 times in the constellation diagram. This condition leads to a shift in the Doppler frequency domain of exactly 20 FFT bins.