Radar Waveform
Understanding Radar Waveforms
The transmitted waveform fundamentally determines what a radar can and cannot measure. Different waveforms provide different trade-offs between range resolution, Doppler resolution, ambiguity, and signal processing complexity.
Common Radar Waveforms
- Simple pulse: Unmodulated RF burst. Range resolution = c*tau/2. Simple but limited.
- Linear FM chirp (LFM): Frequency sweeps linearly across the pulse. Pulse compression = TB. Most common advanced waveform.
- Phase-coded (Barker): Pulse divided into sub-pulses with binary phase pattern. Good sidelobe properties for short codes. Codes up to length 13.
- FMCW: Continuous chirp sweep. Used for automotive radar, altimeters. Simultaneous range and velocity.
- Stepped frequency: Discrete frequency steps within a pulse. Ultra-wide effective bandwidth from narrowband steps.
Frequently Asked Questions
What is a radar waveform?
The radar waveform is the modulation applied to the transmitted signal. It determines range resolution, Doppler resolution, and ambiguity properties. Common types: simple pulse, LFM chirp, phase-coded, FMCW, and stepped frequency.
Why use LFM chirp instead of a simple pulse?
LFM chirp provides pulse compression: the range resolution of a short pulse with the energy of a long pulse. A 100 us LFM with 10 MHz bandwidth provides 15 m resolution (same as a 0.1 us pulse) with 1000x more energy for detection range.
What is the ambiguity function?
The ambiguity function is a 2D plot of the radar's joint range-Doppler response. It shows the trade-off between range and Doppler resolution inherent in any waveform. The ideal waveform has a thumbtack ambiguity function (narrow in both dimensions).