What Is Spread Spectrum Radar?

Spread spectrum radar is a class of radar system that distributes the transmitted signal's energy over a bandwidth much wider than the minimum needed to convey the ranging information. By spreading energy across a large frequency band, these systems achieve high range resolution through pulse compression, reduce the peak power required for a given detection range, and present a signal that is difficult for adversaries to detect or jam. The technique draws on the same wideband signaling principles used in spread spectrum communications, but applies them to the problem of target detection and ranging rather than data transmission.

The fundamental tradeoff is that wider bandwidth demands more of the radio spectrum and more complex signal processing at the receiver, while providing better range resolution and improved resistance to interference. Spread spectrum radar is employed extensively in military sensing, synthetic aperture radar imaging, and increasingly in commercial automotive and ground-penetrating radar applications.

Chirp Modulation and LFM Waveforms

The most widely used spread spectrum radar waveform is the linear frequency-modulated (LFM) chirp, in which the instantaneous frequency sweeps linearly across the transmission bandwidth during each pulse. A pulse of duration T swept over bandwidth B achieves a time-bandwidth product of BT that equals the pulse compression ratio: the ratio of the transmitted pulse width to the compressed output pulse width. This compression delivers range resolution proportional to 1/B, independent of the pulse duration. Research on LFM signal detection and chirp rate estimation has demonstrated that wideband LFM pulses are the dominant waveform in low-probability-of-intercept (LPI) radar designs because they spread energy too thinly across frequency for simple narrowband receivers to detect reliably. Non-linear FM variants modify the frequency sweep profile to shape range sidelobes without an additional weighting filter.

Low Probability of Intercept and Electronic Countermeasure Resistance

A radar system is classified as low-probability-of-intercept (LPI) when its signal is sufficiently broadband and low in average power that a passive intercept receiver cannot detect or localize it before it detects the target. Spread spectrum techniques reduce the signal's spectral density far below the noise floor of a narrowband receiver, requiring the interceptor to integrate over the full wideband signal to accumulate enough energy. This property directly degrades the effectiveness of electronic countermeasures that rely on detecting and characterizing the radar signal before responding. OSTI research on coded LFM waveforms for spectrum-shared radar illustrates how waveform coding adds another layer of ambiguity for potential interceptors by varying chirp rates across pulses.

Waveform Diversity and Spectrum Sharing

Modern spread spectrum radar systems often employ waveform diversity, changing the spreading code, chirp rate, or center frequency from pulse to pulse to reduce predictability and limit cross-interference when multiple radars operate in proximity. Orthogonal coded LFM sets allow several radars to share the same frequency band with controlled mutual interference, a technique studied for spectrum coexistence between radar and communications systems. PMC research on optimized coded LFM waveforms for spectrum-shared radar quantified the interference reduction achievable through spread spectrum coding in a shared-spectrum scenario.

Applications

Spread spectrum radar has applications in a range of fields, including:

• Military airborne and ground surveillance, where LPI properties reduce the radar's detectability by adversary intercept systems
• Synthetic aperture radar (SAR) imaging, where wide bandwidth is required to achieve sub-meter ground range resolution
• Automotive radar for collision avoidance, where FMCW spread spectrum waveforms provide simultaneous range and velocity measurements
• Ground-penetrating radar for subsurface sensing in construction, geology, and unexploded ordnance detection