Introduction
Radar stands for RAdio Detection And Ranging: detecting and measuring distance with radio waves. The basic principle is strikingly simple — and has not changed since the 1930s:
- The radar transmits a short burst of electromagnetic energy through a directional antenna.
- The energy travels at the speed of light, strikes an object (the target) and is partly reflected.
- The radar receives that weak echo and measures how long the signal was under way and from which direction it came.
Everything follows from those two pieces of data: the travel time gives the range, the antenna direction gives azimuth (and, for 3D radars, elevation), and the frequency shift of the echo gives the radial velocity (Doppler effect, Doppler effect).
The strength of radar lies in everything it does not need: no light, no thermal contrast, no cooperation from the target. The price: radar transmits actively and is therefore detectable itself, and the echo is extremely weak — the received power is often a factor of 1018 smaller than the transmitted power. Almost all radar technology in this chapter revolves around one question: how do you reliably extract that minuscule echo from the noise?
Distance from a target to the radar
A radar measures how long it takes for the echo to come back. Radio waves travel at the speed of light, so we are talking about millionths of a second. Because the wave travels out and back, you divide the distance by two.
In practiceRadar operators work with the "radar mile": an echo that arrives 12.36 µs after the pulse is at 1 nautical mile (1852 m). That is how the radar draws the range rings on the display. A target at 200 nautical miles only returns an echo after just over 2.5 ms — and that determines how soon the radar may send the next pulse (Maximum Unambiguous Range).
Radio waves travel at the speed of light c. If the radar measures a travel time Δt between transmitting and receiving, the signal has covered the distance to the target twice in that time (out and back). The distance (range R) is therefore:
A radar receives an echo 200 µs after transmitting the pulse. R = (3 × 108 × 200 × 10-6) / 2 = 60,000 / 2 = 30 km.
Radio waves cover ~300 m per microsecond. Each microsecond of travel time therefore corresponds to 150 m of range (out and back). Handy: 1 km of range ≈ 6.7 µs of travel time; 150 km ≈ 1 ms.
This simple formula is the heart of every pulse radar. Anything that disturbs the time measurement — a next pulse that comes too early (Maximum Unambiguous Range), a target echo arriving while the transmitter is still on (Minimum range) — directly disturbs the range measurement.
Clutter and noise
The target echo is not the only thing entering the receiver. Two categories of unwanted signals determine whether a target is visible:
Noise
Noise is random electrical energy, largely generated in the receiver itself (thermal noise) and partly coming from the environment and from space. Noise is always present, at all frequencies. A target echo is only detectable if it rises sufficiently above the noise floor; the ratio between signal power and noise power is called the signal-to-noise ratio (SNR). Detection is ultimately a statistical game: the threshold (Threshold, Constant False Alarm Rate and Automatic Gain Control) must be high enough not to keep mistaking noise for targets, and low enough not to miss real targets.
Clutter
Clutter consists of real echoes from objects you are not interested in: the earth's surface, sea waves, rain, hail, flocks of birds, wind turbines, buildings. Clutter can be many times stronger than the echo of an aircraft — a mountainside reflects millions of times more energy than a fighter.
| Type | Source | Characteristic | Countermeasure |
|---|---|---|---|
| Ground clutter | terrain, buildings | very strong, stationary | MTI / Doppler filtering (Pulse-Doppler radar and Moving Target Indicator) |
| Sea clutter | sea waves | moves slowly, rough | CFAR, polarisation, Doppler |
| Weather clutter | rain, hail, snow | voluminous, moves with the wind | circular polarisation (Polarisation of the electromagnetic wave), Doppler |
| Angels | birds, insects | small, slow | velocity filters |
The essential difference: noise is random and everywhere; clutter is a real reflection with a fixed location or a velocity of its own. That is why they are countered differently: you counter noise with more power, better receivers and integration (Pulse compression and pulse integration); you counter clutter with filters that discriminate on velocity (Doppler/MTI) and with adaptive thresholds (CFAR, Threshold, Constant False Alarm Rate and Automatic Gain Control).
Jamming adds artificial "noise" or false echoes and so enters precisely this fight: the jammer tries to ruin the radar's SNR, while the radar tries to see the target anyway through filtering, frequency changes and signal processing.