How do I calculate the maximum detection range of a radar using the radar range equation?
Radar Architecture Comparison
The choice of radar waveform fundamentally determines the system's range capability, resolution, measurement accuracy, and hardware complexity. Each architecture has distinct advantages that make it optimal for specific applications.
Pulsed Radar
Pulsed radar achieves long range by concentrating transmit energy into short, high-power pulses. Peak power of 1 MW is common for air surveillance radars, with pulse widths from 1 to 100 microseconds and PRFs from 200 Hz to 300 kHz. The duty cycle (tau * PRF) is typically 1-30%, limiting average power and transmitter thermal management requirements. Range resolution without pulse compression equals c*tau/2 (150 meters for 1 microsecond pulse), which is inadequate for most target discrimination. Modern pulsed radars use pulse compression (LFM chirp or phase coding) to achieve fine resolution while maintaining long pulse energy. Key challenges: transmit-receive isolation during the pulse (requires a duplexer or separate antennas), blind range zones at short range and at range ambiguity multiples, and eclipse loss when returns arrive during transmit intervals.
FMCW Radar
FMCW eliminates the need for high peak power by transmitting continuously at low power. The transmit signal sweeps linearly in frequency, and the received echo (delayed by round-trip time) is mixed with the current transmit frequency to produce a beat frequency proportional to target range. For a 77 GHz automotive radar with 4 GHz bandwidth: range resolution = c/(2*4e9) = 3.75 cm. Sweep time of 40 microseconds gives a beat frequency of about 250 kHz per meter of range, easily digitized by a low-cost ADC. Velocity is measured from the phase change of the beat signal between consecutive chirps, with unambiguous velocity determined by the chirp repetition rate. The primary challenge is transmitter-to-receiver leakage: because the transmitter operates continuously, even -60 dB of isolation allows the transmit signal to overwhelm nearby target returns. Solutions include separate TX/RX antennas with physical shielding, and digital leakage cancellation.
Interrupted FMCW and Hybrid Approaches
Interrupted FMCW (IFMCW) combines features of both architectures: the transmitter sweeps frequency but is gated off during receive intervals, improving near-range performance and reducing leakage. Stretch processing in pulsed radars borrows the FMCW concept by mixing the received signal with the transmit LFM waveform reference, converting wideband returns into narrowband signals for efficient digitization. This is standard in SAR and high-resolution surveillance radars with bandwidths exceeding 500 MHz, where direct digitization of the full bandwidth would require impractically high ADC sampling rates.
Frequently Asked Questions
Why do automotive radars use FMCW at 77 GHz?
Three factors converge: (1) 77 GHz provides a regulatory allocation of 4-5 GHz bandwidth (76-81 GHz), enabling range resolution under 5 cm for pedestrian and object discrimination. (2) FMCW operates at low power (10-13 dBm EIRP typical), enabling single-chip implementation in SiGe or CMOS at cost under $5 per radar module. (3) The short wavelength (3.9 mm) enables compact antennas with narrow beams from small apertures (2° beamwidth from a 6 cm antenna). Current generation (TI AWR2944, NXP S32R45) integrate 4 TX and 4 RX channels with 4 GHz bandwidth on a single chip.
What is the maximum range of FMCW radar?
FMCW range is limited by transmit power, receiver sensitivity, and beat frequency bandwidth. Automotive: 200-300 meters. Industrial level measurement: 100 meters. Aircraft altimeter: 10,000+ meters. Marine: 50-100 km (25-50W transmit). The beat frequency at maximum range must fall within the ADC bandwidth. For automotive (40 μs sweep, 4 GHz BW): beat frequency at 300m is 80 MHz, requiring an ADC sampling at 160+ MSPS. Increasing range requires either higher power, longer sweep time (slower update rate), or narrower bandwidth (coarser resolution).
Can pulsed radar measure velocity?
Yes, through Doppler processing. A coherent pulsed radar extracts velocity from the phase change of target returns between consecutive pulses. The Doppler shift appears as pulse-to-pulse phase rotation at rate 2*pi*f_d/PRF. Pulse-Doppler processing (FFT across the slow-time dimension) separates targets by velocity and suppresses clutter. The velocity resolution equals lambda/(2*N*T_r), where N is the number of pulses integrated and T_r is the PRI. Pulsed Doppler radar is the primary architecture for airborne and naval fire-control systems requiring simultaneous range and velocity measurement at long range.