After detection and identification the weapon system has to take over the target. That takeover is called target acquisition: the tracking sensor is pointed in the right direction on cue from the surveillance radar, searches a small volume, and "locks on". From that moment tracking begins: continuously measuring and predicting the target position, precisely enough to aim a weapon at it.
Tracking can be done with radar (Target tracking by radar: the Target Tracking Radar to Techniques of the target tracking radar) or passively with IR/EO (Target acquisition and tracking by IR) — and modern systems combine both.
Target tracking by radar: the Target Tracking Radar
The Target Tracking Radar (TTR) is in every respect the opposite of the early warning radar — because its task is the reverse: not many targets coarsely, but one target (or a few) tracked extremely precisely.
| Characteristic | EW radar | TTR |
|---|---|---|
| Frequency | VHF – L | X – Ka (short wavelength → narrow beam, Angular resolution) |
| Beam | wide (fan) | very narrow (pencil, ~1°) |
| PRF | low | high (fast updates, unambiguous Doppler) |
| Scan | 360° all-round search | fixed on the target |
| Range | very long | shorter (sufficient for the weapon's range) |
| Output | plots/tracks of the air picture | continuous fire control solution: Az, El, R, v at a high rate |
The TTR tracks the target in three dimensions at once using three "gates": a range gate (a time window around the expected echo), an angle track (the antenna stays pointed at the target, Techniques of the target tracking radar) and, in Doppler radars, a velocity gate (a filter around the expected Doppler shift). The tracker continuously predicts where the target "should" be and corrects for the difference.
A TTR that has your aircraft "in track" is the direct precursor of a shot — which is why radar warning receivers distinguish the modes of a radar (search / acquisition / track / missile guidance) by PRF, scan pattern and waveform, and why a "lock" triggers an audible alarm. The three gates are also the points of attack for deception jamming: range gate pull-off, velocity gate pull-off and angle deception each pull their own gate away from the target.
Techniques of the target tracking radar
To keep the beam exactly on the target, the radar has to know whether the target is slightly to the left, right, above or below. In the past the beam went round in circles and the radar compared the echoes (conical scan); modern radars compare four sub-beams at once within a single pulse (monopulse).
In practiceBecause monopulse obtains the direction from one single pulse, simple jamming tricks with modulated pulses do not work against it. That is why practically all modern fire-control radars and missile seekers use monopulse — and a jammer needs expensive, complicated techniques to deceive the angle (Angle deceptive jamming).
How does a TTR know that the target is not exactly in the centre of its beam — and which way it has to correct? Successive generations of angle-tracking techniques were developed for this:
Sequential lobing / lobe switching
The beam is alternately placed just left/right (and above/below) of the expected target direction. If the echo is stronger in one position than in the other, the target is off-centre and the radar corrects. Simple, but slow and sensitive to targets whose echo strength fluctuates.
Conical scan
The beam rotates continuously in a small cone around the line of sight. If the target is exactly on the axis, the echo is constant; if it is beside it, the echo strength rises and falls with the rotation — and the phase and depth of that modulation give the direction and size of the error.
Conical scan was standard for decades, but it has a notorious weakness: the target can measure (or guess) the scan frequency and modulate its own "echo" in the same rhythm with a jammer — inverse gain jamming — so that the radar corrects the wrong way and loses its track.
Monopulse
The modern standard. The antenna receives every echo simultaneously in several feed horns (typically 4 quadrants). By forming sum and difference signals (Σ and Δ), the radar measures from a single pulse exactly how far and to which side the target is off-centre:
- No scan rhythm → nothing for inverse gain to play on: much more resistant to angle deception.
- Angle error per pulse → fast, precise tracks, even on fluctuating targets.
Track While Scan (TWS) and phased array tracking
TWS lets a searching radar track after all: the antenna keeps scanning, and software builds tracks of many targets at once in between the "hits". Less precise per target than a dedicated tracker, but the target does not notice it is being tracked — the radar simply keeps searching. Phased arrays (Antenna shapes) take this to its final stage: the beam jumps electronically between the search volume and dozens of tracked targets, hundreds of times per second.
Target acquisition and tracking by IR
Tracking is also possible without ever transmitting. An IR tracker keeps the target's thermal contrast in the centre of its field of view, with the same control loop as a radar tracker — except that the "echo" here is the target's own radiation.
Forms:
- IRST (InfraRed Search and Track): a passive "surveillance + tracking" sensor on fighters and ships. It scans the horizon, detects hot targets at tens of kilometres and builds angle tracks of them.
- EO/IR directors in air and coastal defence: a stabilised camera head tracks the target and steers the fire control, often as a silent back-up to the TTR.
- Seeker tracking in the missile itself: from the rotating reticle seekers of early heat-seekers to modern imaging-IR seekers that recognise the target as an image and even aim for a specific part of it (Passive homing).
The trade-off against radar tracking is exactly the passive/active trade-off from Military applications within the EMS and Infrared:
| Radar track (TTR) | IR track | |
|---|---|---|
| Emission | active — the target is warned (RWR) | none — the target notices nothing |
| Range / velocity | measured directly | not available (direction only); LRF needed (Laser rangefinder) |
| Weather | almost all weather | degraded by fog/rain/cloud |
| Vulnerable to | chaff, RF jamming, anti-radiation missiles | flares, DIRCM, smoke, thermal crossover |
That is why modern practice is sensor fusion. The TTR provides range and velocity, the IR/EO head provides silent, jam-resistant angle tracks; if one drops out (jamming, weather), the other carries the track. For the attacker this means that a single countermeasure is rarely enough any more.