Continuous-Wave (CW) Radar — Applications

Learning Objectives

By the end of this topic, the student should be able to:

1. Explain the operating principle of simple CW, CW-Doppler and FM-CW radars.
2. Derive and apply the Doppler frequency relation fd = 2vr/λ.
3. Analyse FM-CW range measurement using the beat-frequency technique R = c·fb/(2·(ΔF/Tm)).
4. Describe CW radar applications: police/sports speed radar, Doppler navigation, satellite tracking via Doppler, radar altimeters, weather radar, proximity fuzes and biomedical sensing.
5. Discuss practical CW radar problems: transmitter–receiver leakage/isolation, noise, and the need for modulation to obtain range.
6. Solve numerical problems on Doppler shift and FM-CW beat frequency.

1. Theory of CW Radar

1.1 What is CW radar?

A continuous-wave (CW) radar transmits an unbroken (or frequency-modulated) sinusoidal signal rather than pulses. Because there is no time reference between transmission and echo, a simple CW radar cannot measure range directly — but it measures velocity with very high accuracy through the Doppler effect. Adding linear frequency modulation (FM-CW) gives both range and velocity, at low peak power and small hardware size.

1.2 The Doppler effect in radar

The received frequency is fc ± fd: approaching targets → higher (positive Doppler), receding → lower. Only the radial component matters — a target crossing the beam at constant range produces zero Doppler.

1.3 Simple CW Doppler radar — bistatic block diagram

In a bistatic CW Doppler radar the transmitter and receiver are at separate sites, separated by the baseline d. The transmitter chain (oscillator → power amplifier → TX antenna) radiates continuously. The echo is collected by a physically separate RX antenna, amplified by an LNA / RF preamplifier, and mixed with a local-oscillator pick-off taken from the same oscillator (sent over a cable or derived coherently) so the phase coherence needed for Doppler detection is preserved. The mixer output at f_d passes through a Doppler (audio) amplifier and filter to a detector/threshold and processor/display.

Bistatic operation removes the TX→RX leakage problem of monostatic CW radar by antenna spatial separation (the residual direct-path leakage between sites is handled by nulling and Doppler filtering). The bistatic Doppler shift depends on the two look angles θ_t (at transmitter) and θ_r (at receiver):

The bistatic arrangement is widely used for fencing (continuous surveillance lines — e.g., the historic SPASUR satellite fence), forward-scatter intrusion detection, and measurement scenarios where placing the receiver away from the transmitter avoids noise, interference or physical crowding at the target site.

1.4 FM-CW radar — adding range capability

If the transmitted frequency is swept linearly (e.g., triangular modulation, bandwidth ΔF, period T_m), the echo — delayed by the two-way time t_r = 2R/c — differs in frequency from the currently-transmitted signal by the beat frequency:

Application 1 — Doppler Speed Measurement (Police & Sports Radar)

The simplest and most common CW application. A Gunn-diode or dielectric-resonance-oscillator (DRO) transmitter at 10.525 GHz (X-band, λ ≈ 2.85 cm) or 24.125 GHz (K-band) illuminates a vehicle; the Doppler echo is down-converted and the audio-frequency beat is counted. Typical values: 1 mph ≈ 31 Hz at 10.5 GHz — the "tone" rises with speed. Modern units add pseudo-random FM to defeat detectors and to reject moving clutter (e.g., patrol-car motion).

Why CW here?

• No minimum range — can measure a car 30 m away with the same unit that reaches 1 km.
• Low peak power → compact, battery-operated hand-held units.
• Only radial speed is needed — the geometry (roadside beam) makes the cosθ correction small.

Other speed applications: sports radar guns (cricket/baseball), conveyor and industrial speed monitoring, ground-speed measurement of aircraft (Doppler velocity sensors), and water-current measurement (river/sea surface scatter).

Application 2 — Distance Measuring Equipment (DME) & FM-CW Ranging

Distance measurement requires a time or frequency reference. CW systems implement it in two ways:

• FM-CW two-way ranging: sweep the carrier (e.g., 4.2–4.4 GHz radar-altimeter band or 5.2–5.7 GHz); measure beat frequency → slant range directly. Used in precision distance measuring equipment and in industrial level/position radars.
• Phase-comparison CW: transmit a CW tone and compare the phase of the returned tone; range follows from phase ambiguity resolution (used in surveying EDM — electro-optical distance meters are CW phase-comparison systems).

The result is the same slant-range information a pulsed DME provides, with the low-peak-power and short-minimum-range benefits of CW — valuable at small airfields and in helicopter operations.

Application 3 — Satellite Ground Control & Tracking (CW Doppler)

The historic birth of space tracking was pure CW Doppler: the 1957 Sputnik 1 beacon (20.005/40.002 MHz) produced the famous descending-tone recordings from which amateur and professional stations derived orbit parameters. Today every satellite TT&C ground station still relies on coherent CW techniques:

• Two-way coherent Doppler: the ground uplinks a stable CW carrier; the satellite transponder coherently turns it around (translates by a fixed ratio, e.g., 221/240 for S-band). The ground measures the received frequency with mHz-class precision → range-rate accuracy of mm/s at GEO distances.
• Doppler curve of a pass: as the satellite rises, approaches (max Doppler), passes overhead (zero Doppler — closest point), and sets, the recorded curve f(t) lets a single station determine orbit elements; this is how early networks (Minitrack, SPASUR fence) worked.
• Ranging tones / PN codes on the CW carrier (phase-comparison, like EDM) give absolute range to ~1 m, closing the same loop as the DME principle at 36 000 km.
• Deep space: DSN combines CW Doppler, ranging tones and delta-DOR (differential VLBI) for interplanetary navigation.

Application 4 — FM-CW Radar Altimeters

Aviation radio altimeters (4.2–4.4 GHz) must measure height from 0 ft (wheels on runway) to 2 500 ft with feet-class accuracy — impossible for pulsed radar (huge dynamic range, short-range eclipsing). The FM-CW solution: sweep ~150 MHz in ~1 ms, mix TX and RX, count the beat. Because the beat at 40 m is only ~40 kHz, cheap digital counters suffice; because the signal is continuous, there is no minimum range. Modern combined altimeter/weather radar modules use the same FM-CW front-end.

• Low-altitude mode: fast sweep (small ΔF) for wide unambiguous range window near ground.
• High-altitude mode: slower sweep, larger ΔF for resolution; tracking filters follow the ground return through overpass (e.g., over water the return point migrates with roughness — sea-state bias).

Application 5 — Doppler Weather Radar

Weather radars must measure both reflectivity (rain intensity) and radial velocity of hydrometeors (wind inside storms). The Doppler measurement uses the pulsed-Doppler variant (magnetron/klystron with phase-coherent processing), but the underlying measurement is pure CW-Doppler physics applied per range cell: successive pulses sample the phase of the echo from each cell; the phase progression rate = Doppler frequency.

• Velocity aliasing (folding): maximum unambiguous velocity v_max = ±λ/(4·PRI) — at 3 GHz (10 cm) with PRI 1 ms, v_max ≈ ±25 m/s. Beyond this, velocities "fold" — correction requires dealiasing algorithms.
• Products: base velocity, storm-relative velocity, spectrum width (turbulence), and mesocyclone/tornado vortex signatures (TVS) that trigger warnings.
• Applications: nowcasting, aviation hazard (wind shear, microbursts), rainfall estimation for flood warnings — e.g., Doppler radars such as those operated by national meteorological services and the NEXRAD network.

Application 6 — Other CW Radar Applications

Interactive Simulations

Summary

• CW radar trades direct range measurement for simplicity, low peak power and excellent velocity resolution via the Doppler effect (f_d = 2v_r/λ).
• Applications span police/sports speed radar, DME-equivalent FM-CW and phase-comparison ranging, satellite ground control via coherent two-way Doppler (the Sputnik principle), radar altimeters, Doppler weather radar, proximity fuzes, Doppler navigators and vital-sign monitors.
• FM-CW recovers range from the beat frequency between the swept TX and the delayed echo: R = c·T_mf_b/(2ΔF).
• Key engineering issue: TX→RX isolation; key physics issues: cosθ geometry, velocity aliasing in pulsed-Doppler weather radar, and modulation linearity.

Review Questions