BaE Systems ALARM
The BaE Systems ALARM (Air Launched Anti Radiation Missile) is an anti-radiation missile (ARM), with a different design approach than the U.S. AGM-88 HARM and the French Matra Armat. All attack enemy radar, often as part of an integrated air defense system. The three have complementary design approaches, but all are precision-guided munitions (PGM) that will fly into the radar antenna.
While the original response to a U.K. Ministry of Defence initiative, started as a study in the 1970s and awarded in 1983 was from BaE Systems, corporate realignments now make the ALARM a joint BaE-Matra product. The manufacturer described the ALARM as "more advanced" than the HARM, but the two reflect different design philosophies; the HARM missile proper is faster, while the ALARM guidance system is more flexible. . In its Armat missile, MATRA went for a third set of design choices, emphasizing range and destructive power. In comparison with Armat, both ALARM and HARM are more for the defense of a general air attack mission than as an independent mission to destroy long-range radar. Britain also constrained the requirement with a policy of not wanting dedicated ARM carrier aircraft.
The missile can be launched by aircraft with a minimum of missile specific hardware modifications. IT offers a modern seeker, a unique loitering mode, a highly lethal terminal attack profile, a standard MIL-STD-1553B interface, a lower launch weight than competitive missiles, and a compact form factor. Both HARM and ALARM are more flexible when the launching aircraft can give them range information. The ALARM's complexity results in higher cost than competitive missiles, which is its principal limitation.
In the MoD study, the U.S. (and German) model of dedicated "Wild Weasel" suppression of enemy air defense (SEAD) aircraft, such as the F-4G of the time or today's F-16CJ was rejected. Instead, the basic requirements were:  The first was that the RAF did not wish to incur the cost and in-service support overheads of deploying sophisticated and complex Emitter Locating Systems (ELS) such as the AN/APR-38 or AN/APR-47 fitted to the F-4G, or the AN/ASQ-213 HARM Targeting Set pod of the F-16CJ and EF-18 Growler. The UK position was that this would cut into force sizes, reducing the deployable number of aircraft which could be dedicated to SEAD operations.  It should be noted that Germany and Italy, but not the U.K., elected to use the dedicated electronic combat/reconnaissance version of the Panavia Tornado, the Tornado ECR. British Tornadoes were either the air superiority (Tornado ADV) or ground attack (Tornado GR.4) versions
"It was further considered that the adoption of the Weasel operational model would expose SEAD aircraft to attack more frequently, this in turn incurring high loss rates. In early 1978 the RAF stated its requirements for a new ARM. The weapon was to have the following attributes:
- no aircraft were to be permanently dedicated to the SEAD mission, and no specialised sensors such as ESM or ELS were to be fitted to aircraft tasked with the SEAD mission.
- the ARM should be capable of launch from aircraft flying at high speed and very low altitude from outside the range of a hostile air defence weapon.
- the weapon should be small enough that:
- a Tornado IDS tasked with SEAD could carry a substantial number
- Smaller aircraft, such as the Harrier and Hawk, could use it
- Tornado IDS on non-SEAD missions still could carry some defensive ARMs
ALARM has five basic operating modes, which divide into two categories, direct and indirect. In the direct modes the pilot launches the missile against a designated target. In the indirect modes the missile can be programmed to seek for a range of radar emissions. Once fired it climbs to altitude (around 40,000ft) and there it either deploys a parachute and loiters, dives directly onto an emitter, or seeks a target on a corridor glide path. 
The ALARM can operate in any of five distinct operating modes, selected prior to launch. Depending on the way ALARM is integrated with the aircraft electronics, they may be selected in flight rather than only on the ground. All of these modes employ the vertical attack trajectory, which imposes specialized terminal homing and fuzing. Two modes are used for situations where the position of threat emitters is not known prior to launch.
- Direct Mode is the most common ARM mode, where the missile is fired straight at an active radar. If, in a given mission, both ALARM and HARM are available, HARM may be superior in its direct mode due to its greater speed.
- Loiter Mode is unique to ALARM, but, before firing, needs to know the range and bearing to the target. The missile will climb above the target and then slowly descend on a parachute. If the radar shuts down, the ALARM will still descend, and, whenever the radar turns on, the missile will dive into it.
- Dual mode allows the missile, if given range and bearing, to start in Direct Mode but switch to Loiter Mode if the target shuts down.
The remaining two modes are optimized for attacking mobile SAMs.
- Corridor/Area Suppression Mode involves a low altitude launch, steep climb, and then a low coast (without parachute) looking for targets meeting preprogrammed criteria.
- Universal Mode is similar, but takes advantage of a medium to high altitude launch, which allows more search time.
ALARM is programmed with target radar parameters, operating modes, fusing altitudes and target priorities prior to launch. Aircraft with a MIL-STD-1553B bus and proper onboard software can reprogram the missile at any time prior to firing. Otherwise, the missile must be programmed on the ground.
Three missile interfaces are available, of which the second and third require a MIL-STD-1553B bus and electrical power to the missile on its launching rail. For all, the aircraft must have ALARM specific rail launchers. The first, two signal interface, simply requires the aircraft to point at the target and send a two-pulse firing command to the missile. This mode does not allow a position or programming interface to be sent to the ALARM.
Active interface gives the missile a position and speed reference to the target, which the missile remembers even if the aircraft makes radical turns before firing. Full intelligent interface allows reprogramming of operating mode and all other relevant parameters, as well as displaying the status of the ALARM.
The ALARM is launched directly off the rail in the same manner as a Sidewinder.After release, its aerodynamic control surfaces unlock, it stablilize itself, and then begins to climb to its search altitude. Remember that low-altitude launch is a requirement for ALARM.
The intelligent interface gives the most capability, but also requires the aircraft electronic intelligence/electronic support and stores management to be ALARM-aware. The ALARM will work best, however, when the launching aircraft can tell the missile the range to the target, which would be known to the aircraft's electronic warfare suite -- if it has one. While the RAF required that the ALARM could be launched from simple aircraft such as the Harrier, ALARM can make good use of more information from a more capable aircraft. Even if the Full Intelligent interface is used, the lack of real-time range information will require some ALARM programming on the ground.
While this may not be an issue for a set piece sortie into a known fixed land based air defence system, it would limit flexibility in more dynamic situations. Indeed this the reason why the USAF and USN are fitting rangefinding receivers to the F-16C HARM shooters and possibly also the F/A-18C (see the upcoming Update on the Loral/LMC TAS). The RAF model is analogous to the traditional USN approach, which is to bypass defences en route, and use the ARM in a pre-programmed or reactive mode to engage terminal defences. The composition and location of target defences is determined a priori by electronic or other reconnaissance, and the ARMs programmed before the sortie.
"In a range known mode, launched at high altitude, the missile is credited with a range in excess of 50 NM. Tossing the missile at 4G during launch adds an additional 10% to the achievable range performance."
The flight regime has launch, homing, and terminal modes. ALARM exceptionally wide range of homing modes. and its terminal guidance is complementary to its radar detection and homing.
The Mission Control Unit contains the flight control/navigation computer and the mission managemnt software, connected to a MIL-STD-1553 bus. The missile, therefore, has a standard means to communicate with its launching aircraft.
Like the HARM, the ALARM can be programmed to recognize a wide range of potentially threatening receivers, principally based on their signal wavelength and pulse repetition frequency( designed to recognise the characteristic Pulse Repetition Frequencies (PRF) of programmed threat emitters. The radar intercept receiver itself appears to be a "conventional quartet of cavity backed spiral antennas, forming a fixed two axis interferometer with lower mid-band to hi-band coverage. The UK have neither disclosed nor confirmed any performance figures for frequency coverage.
It has electronic protection (i.e., electronic counter-countermeasures) features designed to defeat the enemy's attempt to confuse it. If the target shuts down, it will select the next highest priority target. As oppposed to the AGM-88 HARM, assuming it is in one of its operating modes where it loiters while hanging from a parachute, it can wait if all radars are shut down and it has no target; if it can keep the radars shut down while the strike aircraft attack, it has achieved its purpose with a "soft kill".
Clearly the ALARM is a more complex missile in comparison with conventional ARMs. The vertical attack terminal trajectory requires a unique fuse arrangement and the loitering mode requires the parachute system. Both are however used for good technical reasons.
The parachute loitering capability and the technique of terminal guidance and detonation add complexity, but complexit seen as worthwhile. Most ARMs home on the main radar beam or its sidelobes, the latter being incidental emissions. The more the sidelobes are suppressed, which, for many reasons, a very good idea in radar design, and the more directional the main beam, the harder it will be for the ARM to use the radar signals for continuous guidance. Narrow main beams and suppressed sidelobes will give the ARM a solid reference signal only when the beam is pointed at it, and the beam, in normal operation, sweeps the sky.
With respect to final guidance, if, for example, the radar has a rotating antenna, the missile may be lured more by the last solid fix than the actual position of the radar, and may detonate near it but not on it. Such an offset thus requires a bigger warhead.
Sidelobes exist in the horizontal and vertical planes, and there is usually less suppression of the vertical sidelobes. If an ARM can attack straight down, it can home on the vertical sidelobes, which stay constant while the antenna sweeps the main beam. While the strength of these sidelobes vary, they never fully disappear as might the main beam or horizontal sidelobes.
One reason ALARM needs an extensive library of radar characteristics is that its fuze needs to know the height of the target antenna, so it can detonate one meter above it. Mast mounts are increasingly common, not to confuse ARMs, but to improve low altitude coverage of the radar. ALARM exploits this trend, so it can be as effective as a missile with a larger warhead, if it detonates its warhead very close to the target.
It is worth noting that mast mounted antenna variants are employed by systems such as the Russian S-300/SA-10 Grumble, the SA-11 GADFLY and the SA-12 Gladiator SAMs. A wide variety of widely exported European and Indian radars also use mast mounts.
After ALARM entered service in 1990 it faced an early opportunity to demonstrate its worth during the 1991 Gulf War. During this conflict the RAF fired 123 rounds, essentially the entire stockpile of the new weapon. It is unclear, but likely, that NATO has used it in the Balkans. It was used in Iraq in 2003. 
The ALARM is 4.3 m long, has a wing span of 0.72 m and a diameter of 0.224 m, weighing 584 lb (265 kg) at launch.