EW Academy · Contents · Chapter 5 · Nederlandse versie
Chapter 5: Radar
The complete radar chapter: from basic operation and the radar equation to pulse radar, Doppler, CW/FMCW, passive radar, antennas and resolution.
5.1Basic operation of radar
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:
Read §5.1 →5.2Components of the radar system
Every pulse radar consists of the same functional blocks:
Read §5.2 →5.3Polarisation of the electromagnetic wave
Polarisation is the direction in which the electric field of an EM wave oscillates. The antenna determines the polarisation of the transmitted signal — and an antenna receives waves with the "wrong" polarisation poorly or not at all.
Read §5.3 →5.4Doppler effect
If something is coming towards you, the waves it reflects are squeezed together slightly: the frequency goes up. If it is moving away, the frequency goes down. That small shift gives the speed towards or away from the radar.
Read §5.4 →5.5Radar Cross Section
RCS says how "big" something looks to a radar. Shape and material count for far more than actual size: an inside corner, like the corner of a box, reflects the energy straight back, while an angled surface deflects it away.
Read §5.5 →5.6Inverse square law
If you stand twice as far from a lamp, four times less light falls on you. A radar echo has to travel that path twice — out to the target and back — and so becomes weaker very rapidly with distance.
Read §5.6 →5.20Radar horizon
The Earth is round and radar waves travel (almost) in straight lines. A radar cannot see anything flying low beyond the horizon. The higher the radar or the target, the further it can see.
Read §5.20 →5.23Terrain masking
A radar cannot see what is behind a hill — any more than you can see round a corner. Whoever uses that terrain cleverly stays out of sight.
Read §5.23 →5.21Decibels and logarithms
Decibels (dB) work with ratios instead of plain numbers. +3 dB is roughly a doubling, +10 dB ten times as strong and −3 dB a halving. That turns multiplying into simple adding.
Read §5.21 →5.7Gain
An antenna cannot create energy, only concentrate it — just as a torch with a reflector shines much further than a bare bulb of the same power. That gain in one direction is called gain.
Read §5.7 →5.27The full radar equation: noise, losses and detection
The simplified radar equation from Inverse square law tells you how much power comes back. But a radar does not detect on power — it detects on the difference between echo and noise . Every receiver generates noise of its own, and an echo…
Read §5.27 →5.8Frequency bands
Radar frequencies are designated by letter bands. The (IEEE) classification dates from the Second World War, when the letters were deliberately chosen to be cryptic, and it is still the standard:
Read §5.8 →5.9Types of radar
Radars can be classified along several axes. The most important classification is by function :
Read §5.9 →5.10Pulse radar
The pulse radar is the classic and still most common form of radar: the transmitter is on for a very short time (the pulse ) and then off for a relatively long time while the receiver listens for echoes. The range follows from the travel…
Read §5.10 →5.28The transmitter: from magnetron to solid-state
The transmitter ( Components of the radar system ) generates the radio-frequency pulses. There are two fundamentally different designs for this, and the difference determines what the radar can do:
Read §5.28 →5.29The receiver: from echo to plot
An echo at 3 or 10 GHz is hard to amplify and filter directly. Almost every radar (and every ESM receiver) therefore uses the superheterodyne principle: a mixer combines the echo with a local oscillator (LO) and converts it to a fixed,…
Read §5.29 →5.30Radar displays: A-scope, B-scope and PPI
Modern radars show synthetic symbols on a map, but the classic display types still shape the jargon — and they make visible what jamming does to a radar.
Read §5.30 →5.11Pulse-Doppler radar and Moving Target Indicator
Over land the echo from the ground is often much stronger than that of an aircraft. The radar therefore looks at what is moving (the Doppler shift) and throws away echoes that stand still.
Read §5.11 →5.22Ground clutter
Clutter is everything the radar sees but does not want to see: waves, trees, buildings, rain. Against a busy background a small target is hard to find.
Read §5.22 →5.12Continuous wave radar
A Continuous Wave (CW) radar does not transmit in pulses but transmits a single constant frequency without interruption , and receives at the same time. Because transmitting and receiving happen simultaneously, separate transmit and…
Read §5.12 →5.13Variation on the CW radar
An FMCW radar transmits continuously, with a tone that keeps rising. By the time the echo comes back, the radar is already transmitting a higher tone; the difference between the two reveals how long the echo was under way, and therefore…
Read §5.13 →5.24LPI — Low Probability of Intercept
An LPI radar whispers across a wide stretch of the spectrum, so that only someone who knows exactly where to listen picks up the signal. The radar sees the target, but the target hears the radar late or not at all.
Read §5.24 →5.14Passive radar
A passive radar (passive coherent location, PCL) transmits nothing at all itself . Instead it uses existing third-party transmitters — FM broadcast, DAB/DVB-T transmitters, mobile phone masts — as "illumination". The system receives two…
Read §5.14 →5.31Bistatic and multistatic radar
In a bistatic radar the transmitter and receiver are in different places. A passive radar goes further still: it transmits nothing itself and uses the signals of other people's transmitters, such as radio and TV.
Read §5.31 →5.25Triangulation
A single receiver picking up a signal only knows from which direction it comes — a bearing, in other words a line on the map, not a point. Triangulation (cross-bearing) solves this: if you measure the same source from two or more…
Read §5.25 →5.15Antenna characteristics
An antenna never radiates its energy perfectly in one direction. The actual radiation pattern consists of:
Read §5.15 →5.162D and 3D radar
The difference lies in the number of coordinates measured (see Azimuth, elevation and range ):
Read §5.16 →5.26Examples: techniques in present-day systems
By way of illustration — which techniques from this chapter are found in well-known radar systems described in open sources? (Broad outline only; actual performance is often classified.)
Read §5.26 →5.17Radar resolution
Resolution is whether a radar sees two aircraft flying close together as two. A wide beam or a long pulse makes them merge into a single blip.
Read §5.17 →5.18Ambiguity
If the radar transmits a new pulse before the echo from a distant target has returned, it does not know which pulse the echo belongs to. The target then appears at a wrong, too short range: a ghost target.
Read §5.18 →5.32Imaging radar: SLAR, SAR and ISAR
A radar on a moving aircraft or satellite sweeps past the area and combines all the echoes as if it were one enormous antenna. The result is a sharp image — at night and through cloud as well.
Read §5.32 →5.33Secondary radar and IFF
An ordinary radar only sees a reflection. A secondary radar asks a question ("who are you?") and a transponder in the aircraft answers with a code.
Read §5.33 →5.19Conclusion
This chapter forms the technical core of the course. The main points:
Read §5.19 →