Which of the following is an example of a phase-coded waveform used in radar systems?

• A. Linear frequency modulation
• B. Barker codes
• C. Random noise
• D. Constant phase

Answer: (B)

Phase-coded waveforms encode information in the phase of the carrier rather than changing its frequency. Barker codes are a classic set of binary phase sequences used to modulate the phase of successive pulse segments. This means the transmitted signal keeps a steady carrier frequency while its phase hops between two values according to the code, producing a waveform with well-controlled autocorrelation properties. When such a phase-coded pulse is received and processed with a matched filter, the result is a sharp, well-localized peak with low sidelobes, making it easier to detect targets and resolve range.

In contrast, a linear frequency modulation waveform changes the frequency over time (a chirp), which is a frequency-encoded technique rather than phase-coded. Random noise lacks a deterministic phase structure suitable for precise autocorrelation, and a constant phase waveform has no phase variation to encode a code. Thus Barker codes stand out as the phase-coded example used in radar.