As a countermeasure to drone swarm attack, an effective way is to use electromagnetic (EM) drone jamming to disrupt the GPS and communication system of the drones. Which one among the following is the correct frequency range used for EM drone jamming ?

Correct — B, (b) Microwave frequency. A jammer has to work in the SAME band as the link it is trying to break, so the answer is fixed by what frequencies drones actually use.

A small drone depends on two radio links. Its navigation comes from satellite signals — the GPS civil signals are broadcast at about 1.58 GHz and 1.23 GHz — and its command, control and video links run in the licence-free bands at about 2.4 GHz and 5.8 GHz, with some longer-range control links near 900 MHz. Every one of those frequencies lies in the MICROWAVE region of the electromagnetic spectrum, conventionally taken as roughly 300 MHz to 300 GHz, wavelengths from about a metre down to a millimetre.

Jamming is deliberate interference: the jammer radiates noise or a false signal in that same band at a power high enough to swamp what the drone's receiver is trying to hear. Cut the control link and the drone loses its operator; cut the satellite signal and it loses its position fix, after which most machines either hover, drift, land or attempt a return-to-home that no longer works. Against a swarm this is the attraction of the method — one emitter covers a volume of sky and affects every drone in it at once, without a projectile and without a kill for each target.

The other three options name optical bands, and the reason they cannot be the answer is the same for all three: drones neither navigate nor take commands over infrared or visible light, so radiating energy at those frequencies interferes with nothing. Light-based counter-drone systems do exist, but they belong to a different category — a high-energy laser DESTROYS or blinds a target rather than jamming a radio link, and it must be pointed at one drone at a time.

Why the others are wrong

• (a)Far-infrared frequency — Far-infrared is thermal radiation, the band in which warm bodies glow and in which thermal cameras see. It is a good band for DETECTING a drone's motors against a cold sky, and that is where a candidate's association with counter-drone work may come from, but it carries no part of the drone's navigation or control signal, so flooding it interferes with nothing the drone is listening to.
• (c)Visible frequency — Visible light is used against drones only in the sense that a bright beam can dazzle a camera. That degrades an operator's video feed at best; it leaves the satellite fix and the control link untouched, and it needs line of sight to each individual machine. The question asks for the band used for jamming the GPS and communication systems, and the drone receives neither over visible light.
• (d)Infrared frequency — Infrared appears twice in this option set, plain and far, which is a sign that the examiner expected candidates to hesitate between them. Neither is right, and for the same reason: infrared is used for imaging and for short-range line-of-sight links such as a remote control across a room, not for the satellite navigation or the 2.4 and 5.8 GHz control channels that a drone actually depends on.

Electronic jamming exploits the fact that a receiver locked to a weak, distant signal can be deafened by a stronger local one in the same band. Satellite navigation signals are extremely weak by the time they reach the ground — they are transmitted from an orbit some twenty thousand kilometres away — so a modest jammer nearby can override them, which is why GPS denial is easier than it sounds. Counter-drone systems combine detection, by radar, radio-frequency scanning, acoustic sensors or thermal cameras, with a means of defeat: radio-frequency jamming, spoofing a false satellite fix, net capture, or in some cases a directed-energy weapon. Jamming is the method that scales against swarms, because a single emitter covers everything within its beam.

Key facts

• The microwave region runs roughly from 300 MHz to 300 GHz, that is wavelengths of about a metre down to a millimetre.
• GPS civil signals are transmitted at about 1575.42 MHz (L1) and 1227.60 MHz (L2), inside that region.
• Common drone control and video links use the licence-free bands near 2.4 GHz and 5.8 GHz, and some longer-range links near 900 MHz.
• Jamming means radiating interfering power in the same band as the target link so that the receiver can no longer read the wanted signal.
• Satellite navigation signals arrive very weak, which makes them relatively easy to jam close to the receiver.
• Spoofing differs from jamming: it feeds a receiver a false but plausible signal instead of drowning the real one.
• Infrared and visible bands are used against drones for detection and for dazzling optics, and by directed-energy weapons for destruction, not for jamming radio links.

Common traps

• Choosing an optical band because counter-drone systems are often pictured with cameras and lasers; sensing and jamming are different jobs.
• Treating 'Far-infrared' and 'Infrared' as meaningfully different in an item where neither can be right.
• Confusing jamming with spoofing when a question describes a drone being made to land somewhere else.
• Forgetting that a jammer must occupy the same band as the link it attacks — that single rule answers this question.

• practice — not a real PYQ

  Which one among the following describes 'spoofing' of a satellite navigation receiver ?

  • (a)Radiating high-power noise so that the receiver hears nothing
  • (b)Feeding the receiver a false but plausible signal so that it computes a wrong position
  • (c)Physically destroying the receiver's antenna with a laser
  • (d)Blocking the receiver's view of the sky with smoke

  Answer(b) Feeding the receiver a false but plausible signal so that it computes a wrong position