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 time of each echo (Distance from a target to the radar). This section covers the parameters that characterise a pulse radar — for an EW specialist these are exactly the parameters by which you identify a radar.
Pulse Repetition Frequency
The PRF is how many pulses a radar transmits per second. Few pulses leave time to wait for echoes from far away; many pulses give many measurements per second, good for velocity, but distant echoes then get mixed up.
In practiceAn air defence search radar uses a low PRF to measure range unambiguously out to hundreds of kilometres. A fighter's radar switches between low, medium and high PRF depending on whether it is searching, tracking a target or guiding a missile — and an RWR recognises those modes precisely by that rhythm (Radar Warning Receiver (RWR)).
The Pulse Repetition Frequency (PRF) is the number of pulses the radar transmits per second, expressed in Hz or pulses per second (pps). Typical values range from a few hundred Hz (long-range surveillance) to hundreds of thousands of Hz (pulse-Doppler tracking).
The PRF is perhaps the most important design choice of a pulse radar, because it sets a fundamental trade-off:
- Low PRF → plenty of listening time between pulses → large unambiguous range (Maximum Unambiguous Range), but the Doppler measurement becomes ambiguous: velocities are not unique.
- High PRF → unambiguous Doppler measurement (good for velocity determination and clutter suppression), but the unambiguous range becomes small: range measurements become ambiguous.
- Medium PRF → both somewhat ambiguous; modern radars resolve this by alternating between several PRFs (PRI modulation).
The PRF is a main component of a radar's electronic fingerprint. ESM systems classify intercepted emissions first of all by frequency, PRF and pulse width — from these the radar type, and often the weapon system behind it, can be deduced.
Pulse Repetition Interval and Pulse Width
The Pulse Repetition Interval (PRI, also called pulse repetition time) is the time from the start of one pulse to the start of the next. PRI and PRF are each other's inverse:
The Pulse Width (PW, pulse duration, τ) is how long the transmitter is actually on per pulse, typically 0.1 – 100 µs. The ratio between transmit time and total time is called the duty cycle:
The pulse width is a compromise in itself:
- Short pulse → fine range resolution (Range resolution) and short minimum range (Minimum range), but little energy per pulse → less range.
- Long pulse → a lot of energy per pulse → more range, but coarse range resolution and a large minimum range. Pulse compression (Pulse compression and pulse integration) breaks this dilemma.
So within one PRI this happens: transmit (PW), then listen until the next pulse. The listening time (PRI − PW) determines from how far away an echo can still return before the next pulse leaves.
Peak power and average power of the pulse radar
With a pulse radar you have to keep two powers apart:
- Peak power (Pp): the power during the pulse itself — often hundreds of kilowatts to megawatts.
- Average power (Pavg): the power averaged over the whole time, including the long silences. This determines the energy budget (power supply, cooling) and — averaged over time — the detection capability.
A radar has Pp = 500 kW, PW = 2 µs and PRF = 500 Hz (PRI = 2,000 µs). Duty cycle = 2/2000 = 0.001. Pavg = 500,000 × 0.001 = 500 W. Half a megawatt at peak, but on average no more than an electric kettle.
This explains why pulse radars can deliver such enormous peak powers: the transmitter tube only has to work a fraction of the time and can "rest" in between. It also explains an EW fact: a receiver listening to the radar picks up the full peak-power pulse — that is why an RWR can hear a radar at a much greater distance than the radar can see the aircraft.
Minimum range
As long as the transmitter is on, the receiver cannot listen — the duplexer has connected the antenna to the transmitter and the receiver is shielded. An echo that returns while the pulse is being transmitted is therefore lost. As a result every pulse radar has a minimum range: targets closer in are invisible.
PW = 2 µs → Rmin = (3 × 108 × 2 × 10-6) / 2 = 300 m. Everything within 300 metres is a blind zone for this radar. With PW = 100 µs (a long-range pulse) that grows to 15 km!
That is why radars that also need to see close in (navigation radars, short-range defence) use very short pulses, and why many surveillance radars alternate long pulses (to see far) with short pulses (to see close in).
Maximum Unambiguous Range
After each pulse the radar listens until the next pulse leaves. The radar implicitly assumes that every received echo belongs to the most recently transmitted pulse. If an echo from a very distant target only arrives after the next pulse, it is attributed to the wrong pulse and placed at far too short a range: a second-time-around echo. The range within which no confusion can arise is called the Maximum Unambiguous Range (Rmax,unamb):
PRF = 1,000 Hz → PRI = 1 ms → Rmax,unamb = (3 × 108 × 0.001)/2 = 150 km. A target at 180 km returns its echo only 1.2 ms after the pulse — i.e. 0.2 ms after the next pulse — and appears falsely at (0.2 ms × c)/2 = 30 km.
Here you see the PRF trade-off of Pulse Repetition Frequency in its purest form: the higher the PRF, the smaller the unambiguous range. An early warning radar that has to see 400 km may use a PRF of at most ≈ 375 Hz; a pulse-Doppler radar with a PRF of 100 kHz has only 1.5 km of unambiguous range and has to reconstruct its range measurements with clever tricks (PRI modulation). What "ambiguous" means exactly here — and why the same problem also exists in the velocity domain — is explored in depth in Ambiguity.
PRI modulation
Many radars do not keep their PRI constant but vary it deliberately: PRI modulation. The main forms and their purpose:
| Form | What happens | Why |
|---|---|---|
| Constant (fixed) | fixed PRI | simplicity; but predictable and ambiguous |
| Staggered PRI | fixed sequence of 2–8 different PRIs, repeating | eliminating MTI blind speeds (Pulse-Doppler radar and Moving Target Indicator), unmasking second-time-around echoes |
| Jittered PRI | PRI varies (pseudo-)randomly from pulse to pulse | hard for deception jammers to predict; protects against false-echo techniques |
| PRF switching / dwell switch | blocks of pulses, each with a different PRF | resolving range and Doppler ambiguities (medium-PRF radars) |
| Sliding PRI | PRI gradually increases or decreases | among other things preventing eclipsing, shifting ambiguities |
How does PRI variation resolve ambiguities? A genuine echo from within Rmax,unamb stays at the same range whatever PRI you choose. A second-time-around echo "jumps" in apparent range as soon as the PRI changes — and so gives itself away. The same principle applies to velocities in the Doppler domain.
PRI modulation is a double-edged sword. For the radar it is protection; for the ESM operator the pattern of the PRI modulation (stagger sequence, jitter depth) is precisely an extra identification feature. Modern deception jammers must predict the PRI in order to place false echoes ahead of the real one — jitter makes that fundamentally harder.
Threshold, Constant False Alarm Rate and Automatic Gain Control
A radar has to decide what is a target and what is just noise. To do so it sets a bar: anything above it counts as a target. If things get noisier, the bar automatically moves up — otherwise the radar would see false targets everywhere.
In practiceThat is exactly where noise jamming takes hold: the jammer raises the noise so much that the bar goes up and weak echoes disappear below it. On the display a sector "snows up" and the targets in it vanish (Introduction).
When does a radar "see" a target? When the received signal rises above a threshold. Choosing that threshold is a fundamental dilemma:
- Threshold too low → noise peaks exceed it → false alarms, the screen "snows up" and the processor is overloaded.
- Threshold too high → weak target echoes stay below it → missed detections.
A fixed threshold works poorly because the background (noise + clutter + jamming) differs enormously by direction and range. That is why radars use a Constant False Alarm Rate (CFAR) processor: for each cell under test, the processor measures the background level in the surrounding cells and sets the threshold a fixed margin above it. The result: the number of false alarms per second stays constant, regardless of whether the background is quiet or full of clutter.
In addition, Automatic Gain Control (AGC) protects the receiver: AGC automatically turns the gain down for strong signals (nearby targets, strong clutter, jamming) so that the receiver does not saturate, and up for weak signals. Without AGC a single strong signal would swamp the entire receive chain.
Threshold logic is a point of attack for EW. Noise jamming pushes the CFAR threshold up so that real targets disappear below it (the radar "sees" nothing, but knows it is being jammed). And AGC can be exploited: deception techniques such as AGC deception/inverse gain play with strong pulses to upset the gain control of tracking radars.
Pulse compression and pulse integration
A long pulse carries a lot of energy but measures range fuzzily; a short pulse measures sharply but carries little energy. The trick: transmit a long pulse with a rising tone, and let the receiver compress it into a short one.
In practiceModern solid-state radars, which use transistors instead of transmitter tubes, cannot produce huge peak powers. They therefore transmit long, modulated pulses and still achieve long range and a sharp range measurement. A bonus: a jammer without an exact copy of that modulation loses much of its effect (Digital Radio Frequency Memory).
Two signal processing techniques lift the performance of pulse radars far beyond their "bare" specifications.
Pulse compression
The dilemma of Pulse Repetition Interval and Pulse Width: a long pulse gives range, a short pulse gives resolution. Pulse compression delivers both at once. The radar transmits a long pulse that is internally modulated — for example with a rising frequency (chirp / linear FM) or a phase code (Barker code). The receiver "knows" that modulation and compresses the long echo with a matched filter into a single short, high peak.
A 100 µs chirp pulse with 10 MHz of bandwidth behaves like a 100 µs pulse in terms of energy (long range) but like a 0.1 µs pulse in terms of resolution: ΔR = 15 m instead of 15 km.
Pulse integration
A single echo can disappear below the noise, but a target is hit by dozens of pulses during one antenna sweep. Pulse integration adds those echoes together. Noise is random and partly averages out; the target echo is always in the same place and grows. This raises the effective SNR.
- Coherent integration (before detection, using phase information): n pulses → SNR gain up to a factor of n.
- Non-coherent integration (after detection, amplitude only): less gain (~√n), but simpler.
Pulse compression also makes radars resistant to jamming: the matched filter only amplifies signals with exactly the right internal modulation. A jammer that does not reproduce the chirp exactly loses the compression gain — often a factor of 1000 — relative to the real echo. That is why modern DRFM jammers (Digital RF Memory) invest precisely in recording and replaying the pulse shape exactly.