Using Complementary Sequences with Doppler Tolerance for Radar Sidelobe Suppression in Meteorological Radar

The use of complementary binary phase codes in a pulse compression radar system provides a way to reduce received clutter power from range sidelobes. Complementary codes are such that perfect cancellation of range sidelobes occurs under zero or known doppler frequency shift. However, like other forms of sidelobe suppression, complementary codes are extremely sensitive to doppler phase shifts across the return pulse. The usefulness of these codes is therefore limited by this sensitivity to doppler. We offer a processing technique for alleviating this sensitivity. The method consists of transmitting a set of pulses; half of them modulated with one of the pair of complemenatary phase codes and the other half of the sequence modulated with the other of the pair. The processing consists of pulse to pulse doppler filtering and each doppler filter output having the doppler phase shift along the pulse removed by heterodying with a wave having the opposite doppler frequency. INTRODUCTION Dispersed pulse transmission and pulse compression upon reception is frequently used in radar to achieve high energy per pulse with low peak power while maintaining large bandwidth for fine range resolution. The fine range resolution is obtained in the receiver by pulse compression or matched filtering. Matched filtering results in range sidelobes that can be troublesome in an environment of extended scatterers (such as preceipitation and other meteorological phenomena). The reason is echo “flooding” into the measurement of their properties. While we have investigated other means for sidelobe suppression, we offer here an alternative that can suppress these sidelobes to a very low level. This alternative uses paris of complementary sequences. Dopper Tolerant Sidelobe Elimination Basic complementary sequences are pairs of biphase sequences with the property that the sum of the time autocorrelation functions (obtained with pulse compression) has no sidelobes outside of the main lobe. This is illustrated in Figure 1. The absence of range sidelobes is a very desirable trait, but complete suppression of these sidelobes depends on the absence of a doppler frequency shift or the knowledge of the doppler shift so that it can be compensated. The technique for range sidelobe elimination involves the separation of the echo sequences into their respective doppler bins before matched filtering so that doppler tolerance is achieved. A typical transmitted set of pulses is illustrated in Figure 2. Of a total of 2L pulses the first L are modulated with phase code #1 of a complementary pair and the last L are modulated with the second phase code of the complementary pair. This sequence is processed as illustrated in Figure 3. In a digital or discrete time embodiment, the doppler filter bank is achieved by means of a discrete Fourier transform (DFT) (usually in the form of a Fast Fourier Transform (FIT) algorithm). The DFT operates on a sequence of L input echoes. L of the echoes come from one of the two sequences of the complementary sequence pair and the other L echoes come from the other sequence of the complementary sequence pair. The fact that all of mixer outputs have had all doppler removed results in all filters being designed for zero doppler frequency. All filters pairs are therefore identical. In this paper, the deleterious effects of range sidelobes are measured by the “integrated sidelobe Ibl COMPRESSED SEQUENCE A SEQUENCE A SEQUENCE B IC) COMPRESSED SEOUENCE B (dl SUM OF COMPRESSED SEOUENCES Figure 1. Example of a p,air of basic biphase complementary sequences, each of length 4. cp(t) is the pattern of phase changes of the transmitted waveform. ’The sum of the compressed sequences has no sidelobes. kLSEQUENCE # 1 PULSES + LSEOUENCE # Z PULSES -1 Figure 2. Transmit pulse train for doppler tolerant complementary code pulsa compression radar. DOPPLER Am FILTER DELAY LT12 -% MATCHED FILTER # 2 I DETECT, , TRACK. WEATHElR PROCESS. ETC.