What is low probability of intercept radar and how does it differ from conventional radar waveforms?

Low probability of intercept (LPI) radar is designed to detect targets while minimizing the likelihood that enemy ESM receivers can detect the radar emissions. LPI radars achieve this by spreading their transmitted energy in time, frequency, or code, reducing the peak power and power spectral density: (1) Key differences from conventional radar: conventional pulse radar: high peak power (kilowatts to megawatts), short pulse (microseconds), narrow bandwidth (MHz), and high power spectral density (easy for ESM to detect). LPI radar: low peak power (watts to tens of watts), continuous or long-duration waveform, wide bandwidth (hundreds of MHz to GHz), and low power spectral density (difficult for ESM to detect above the noise floor). (2) LPI waveform types: FMCW (Frequency Modulated Continuous Wave): transmits continuously while sweeping the frequency across a wide band. Bandwidth: 100-500 MHz (provides fine range resolution through pulse compression on receive). Peak power: 1-50 W (compared to kW-MW for conventional radar). The received signal is compressed by mixing with the transmitted waveform (dechirp processing), providing the same range resolution as a short pulse but at much lower peak power. Phase-coded CW: the transmitted CW signal is phase-modulated using a pseudorandom noise (PN) code. Bandwidth: set by the code chip rate (100 MHz to 1 GHz). The receiver correlates the received signal with the known code, providing processing gain equal to the code length. Noise radar: transmits a noise-like waveform. The receiver correlates the received signal with a delayed copy of the transmitted noise. The noise waveform has: uniform power spectral density (looks like thermal noise to an ESM receiver), very low probability of detection (no identifiable pulse shape, frequency, or code). (3) LPI metric: the LPI performance is quantified by the intercept factor: alpha = R_radar / R_intercept. Where R_radar = maximum radar detection range and R_intercept = ESM intercept range. For conventional radar: alpha < 0.3 (the ESM detects the radar at 3× the radar detection range). For LPI radar: alpha > 1 (the radar detects the target BEFORE the ESM can detect the radar). Achieving alpha > 1 requires: very low PSD (wide bandwidth, low peak power), high antenna directivity (narrow beam, low sidelobes), and short transmit windows (minimizing the total radiated energy).