APA NOTAM

The notion of using a high power electronically steered radar to 'fry' the onboard electronics of a guided missile is neither new nor particularly original. The idea emerged during the 1990s in the aftermath of the debate surrounding E-bombs and other electromagnetic weapons. The essence of the concept is that modern missile seekers and guidance systems are complex digital electronic devices which can suffer electronic upsets or even electrical damage if exposed to microwave radiation of sufficiently high intensity.

The reasoning behind this regime of electronic attack is that rather than to deceive the inbound missile as to the location of the target, i.e. defending aircraft, the target actively defends itself by using the very high power rating of the radar and its exceptional beamsteering agility - virtually identical for all electronically steered US AESAs and Russian hybrid ESAs or AESAs - to illuminate the incoming missile and cause either an upset to its guidance electronics or lethal electrical damage to its analogue and/or digital electronic hardware.

The idea was so popular during this period that it spawned a specialised product, the ground based Raytheon Vigilant Eagle, essentially a very large AESA radar intended to protect airliners from shoulder launched missile attacks by illuminating them with an microwave beam of very high power to cause electronics failures. There are however some important differences between fighter borne AESA/ESA radars and the multiple square metre array of the Vigilant Eagle system, primarily in the intensity of microwave illumination they can produce.

To what extent are claims of using a fighter AESA radar as a microwave beam weapon to electrically kill inbound missiles reasonable?

A senior US Air Force officer was quoted in the September 5, 2005, issue of Aviation Week & Space Technology, thus: "AESA radars on fighter aircraft aren't particularly suited to create weapons effects on missiles because of limited antenna size, power and field of view...".

This observation is entirely correct, for a number of good technical reasons. These all have to do with how much microwave power is needed to disrupt or damage electronic components versus how much power can be delivered by a fighter carried radar into the target missile.

A number of studies have been performed in recent years to determine the electrical field strengths needed to achieve disruptive and lethal effects against electronic equipment, mostly in the context of High Power Microwave weapons such as E-bombs. The results are essentially that electrically lethal effects are produced at field strengths of kiloVolts/metre, and disruptive effects at hundreds of Volts/metre. These studies generally involved commercial electronic equipment, rather than hardened military equipment, and usually involved direct exposure of the equipment to microwave radiation.

If we plot the achievable field strength against distance, for a number of Russian phased array radars, we get interesting results:

Potentially lethal effects are produced only inside 100 metres range, and disruptive effects at distances of the order of one kilometre. Radars with lesser power-aperture performance, such as the APG-79 (Super Hornet) and APG-81 (JSF) would produce lesser effect at a given distance. This is contingent on the assumption that the internal electronics of the missile are exposed to the full intensity of the impinging microwave beam.

The latter is very optimistic for a variety of reasons. Microwave radiation can couple into a missile via two paths.

Direct coupling occurs when an antenna is illuminated and becomes a path into the internals of the missile. Typical air to air missiles have a nose mounted seeker antenna pointing at the target, and if equipped with a radio or radar proximity fuse, side mounted fuse antennas, and if the missile is built for beyond visual range combat, an aft mounted datalink antenna.
Indirect coupling occurs when radiation enters the target via a path other than an antenna, such as through a gap between panels or some other exposed area, such as the bulkhead openings behind the missile's radome or infrared window.

Fighter radars largely operate in the X-band (~7 to 12 GHz). The most frequency agile AESAs might be capable of covering most or all of this band, but no more. At the upper end of the X-band, the physical spacing of antenna elements restricts how far the antenna can be steered, and at the lower end of the band, the cutoff frequency of the individual elements comes into play.

If the aim is to couple into the missile via its seeker antenna, this will only be feasible for older semi-active homing missiles like the AIM-7 and R-27R series, which rely on illumination by the launch aircraft and thus must operate in the same band as the radar guiding the missile. Most active radar guided missile seekers operate in the Ku-Band or above it, as a result of which most of the impinging X-band radiation will couple in very poorly as the missile antenna is designed for half the wavelength, or less, compared to an X-band radar. Another consideration is that many missile seekers in this class will include active protection devices designed to protect the sensitive receiver circuits from leakage from the missile's transmitter circuits. So what X-band radiation can get in via the antenna is apt to be soaked up by the protection devices.

Radar fuses and datalink antennas are potentially more susceptible to penetration as they are low gain designs which are inherently wideband, and likely to lack protection devices. However, the fuse antennas point sideways relative to the target until the missile is within milliseconds of impact, and the datalink antenna is always pointing away from the target. Therefore the combination of antenna location and low gain makes them poor candidates for delivering a lethal dose of X-band radiation. The electronic warfare literature is very specific about the challenges in jamming these channels, as exceptionally high power is required for effect. Microwave lethal effect requires even more power.

Indirect coupling via cables and through hole apertures behind an antenna or infrared seeker head, or via the missile umbilical connector on its back should also be considered, as these are the only other apertures usually available on an air to air missile.

The former might be feasible if the missile designers did not take care to put protection devices and proper shielding in. If the drive transistor on the antenna gimbal servo melts, the missile will be killed. Unfortunately for the attacker in this game, such components are very robust, and shielding and protection devices easy to add in.

The bottom line in this game is that other than some very specific missile types with X-band antennas, and specific vulnerabilities in particular active radar guided or infrared homing missiles, the opportunities to deliver lethal electrical damage with forseeable fighter radar technology will not be many. The defensive countermeasures an opposing missile designer can apply are neither expensive nor technically difficult to implement. Most would not require replacement of the missile seeker, but rather depot level fixes which could be applied during scheduled missile servicings.

So Colonel Medved's arguments stand up to scrutiny here, and only a very courageous air force would rely on using a fighter radar to burn out an incoming missile guidance system in a real combat environment.