A sensor chain has to answer four questions: is something there? (detection), where is it? (position: azimuth/elevation), how far away? (range) and how fast is it moving? (velocity). Different parts of the spectrum answer these questions in different ways and with different accuracy. We cover them from low to high frequency.
3 Hz – 100 GHz: Radar
The radio-frequency part of the spectrum is the domain of radar (RAdio Detection And Ranging) and radio communication. Radar actively transmits energy and listens for the reflection; this makes it the only sensor family that can detect at long range, day and night and in almost any weather, and measure range and radial velocity directly.
Within this enormous region there are clear patterns:
| Sub-region | Typical frequencies | Application |
|---|---|---|
| HF | 3 – 30 MHz | over-the-horizon radar (OTH), long-range communication via the ionosphere |
| VHF / UHF | 30 MHz – 1 GHz | early warning radars with very long range; relatively good against stealth |
| L- / S-band | 1 – 4 GHz | surveillance and air traffic radars, long range |
| C- / X-band | 4 – 12 GHz | multifunction and fire-control radars: precise aiming and tracking |
| Ku / Ka / mm-wave | 12 – 100 GHz | missile seekers, short-range sensors, very high resolution |
The choice of frequency is always a trade-off. An early warning radar chooses a low frequency: long range, large antenna, coarse positioning — enough to give warning. A fire-control radar chooses a high frequency: smaller antenna, narrow beam, accuracy sufficient to aim a weapon — but less range. Radar technology is worked out in full in Radar; Executing the fire control sequence using the EMS links the radar types to the fire control sequence.
Because radar transmits actively, it gives itself away: an opponent with receiving equipment (ESM/RWR) detects the radar at a greater distance than the radar can see him. This fundamental drawback of active sensors drives the race between radar, stealth and jamming.
300 GHz – 394 THz: Infrared
Above the radio waves lies the infrared (IR): thermal radiation. Every object warmer than absolute zero emits IR (Temperature) — engines, exhausts, people, tyres, heated vehicles. IR sensors are passive: they emit nothing themselves, but pick up the radiation of the target.
Strengths of IR for detection and positioning:
- Undetectable: a passive sensor does not give itself away. The target does not know it is being observed or tracked.
- Very precise angle measurement: thanks to the short wavelength, small optics can image the target sharply — good enough to guide a missile.
- Day and night: thermal contrast is always there; darkness and most camouflage do not help.
- Immune to radar jamming and stealth: radar-absorbent shaping does not lower the IR signature.
But there are fundamental limitations:
- No direct range or velocity measurement: IR measures direction only (azimuth/elevation). Distance requires a second means, for example a laser rangefinder.
- Weather-sensitive: rain, fog, cloud and dust attenuate IR strongly; the range is much shorter than that of radar.
Applications: thermal imaging cameras (FLIR), IR seekers of air- and ground-launched missiles (including MANPADS), IRST systems (InfraRed Search and Track) on fighter aircraft and ships, and missile warning sensors. Infrared goes deeper into infrared.
394 THz – 789 THz: visible light
The narrow band that the human eye can see — from red (~394 THz / 760 nm) to violet (~789 THz / 380 nm) — remains militarily indispensable. Sensors in this region are called electro-optical (EO) sensors: the eye itself, daylight cameras, periscopes, sights and TV guidance heads of weapons.
Properties:
- Passive (uses ambient light) and therefore undetectable.
- Highest image quality and recognisability: identifying a target ("is this a lorry or a launcher?") is ultimately often done visually. That is why many engagement procedures end with an EO verification step.
- Only with sufficient light and visibility: night, smoke, fog and camouflage are effective countermeasures. Image intensifiers (night vision) stretch the usable envelope by amplifying scarce light (stars, moon) thousands of times.
- No range or velocity: like IR, visible light measures direction only; distance requires stereoscopy, a map or a laser rangefinder.
In modern sensor turrets (targeting pods, ship directors, vehicle sights) an EO camera, IR camera and laser rangefinder sit together in one stabilised housing: each compensates for the weaknesses of the others.
Laser
Laser is not a separate part of the spectrum but a special way of generating radiation, usually in the visible or infrared region. Laser light is monochromatic (one wavelength), coherent (all waves in phase) and extremely collimated. This allows a lot of energy to be concentrated on a small spot at long range.
Military applications for detection and positioning:
- Laser rangefinder: measures the distance to a target to within centimetres from the travel time of a short laser pulse — the missing range measurement for EO/IR sensors.
- Laser designator: "paints" a target with coded laser pulses on which laser-guided bombs and missiles home.
- LIDAR: the laser counterpart of radar, for very precise 3D imaging and obstacle detection.
- In addition: laser communication (hard to intercept), dazzlers against optical sensors and — emerging — high-energy lasers as weapons against drones and missiles.
The flip side of the narrow beam: a laser must be aimed exactly at the target and needs a clear line of sight; smoke, dust and fog weaken it strongly. Moreover, targets with laser warning receivers can detect that they are being illuminated. Laser is covered in detail in Laser and military applications.