Page 317 - Fundamentals of Radar Signal Processing

FIGURE 4.57 Autocorrelation of two L = 130 mismatched filters for the same N
= 64 MPS code. Solid line: Optimal PSR filter response. Dotted line: Optimal
ISR filter response. (After Keel and Baden, 2012.)

The dotted line is the result of an L = 130 filter designed to minimize the
ISL and also normalized to an energy of 64. The PSL is now about –23.1 dB, 1
dB worse than the matched filter and 7.3 dB worse than the minimum-PSL filter.

However, the ISL is now –12 dB, 5.3 dB better than the matched filter and 2.2
dB better than the minimum-PSL filter. The LPG is 0.85 dB relative to the
matched filter, a 0.26 dB improvement compared to the minimum-PSL filter.

4.11 Costas Frequency Codes

Costas waveforms are a class of pulse compression waveforms having aspects
of both phase-coded and stepped frequency pulse burst waveforms (Costas,
1984). A Costas waveform is similar to a polyphase waveform in that it is a

single pulse waveform divided into N subpulses. It is similar to the linearly
stepped frequency waveform in that, rather than maintaining a constant
frequency and altering the phase of each subpulse, it alters the subpulse
frequencies, stepping through a set of N frequencies that differ by ΔF Hz. Unlike
the stepped frequency pulse burst, however, the Costas waveform does not step
through the frequencies in linear order. The Costas pulse can be expressed as

(4.140)

where the sequence c [n] denotes the ordering of the stepped frequencies.