Silent spies are listening.

Although widely associated with photo-reconnaissance and surveillance, UAVs can also be used for sigint (signals intelligence) tasks. Early in the Vietnam War, a pilotless aircraft was given the task of flying deep into North Vietnamese airspace. When attacked by one of the then recently-installed SA-2 Guideline surface-to-air missiles, it successfully detected the radio transmissions from the Soviet missile's proximity fuze--information urgently needed by the designers of American electronic warfare systems. The mission demonstrated that the UAV could gather vitally needed sigint data, but only today is the concept approaching technological maturity.

In December 2000, an RQ-5A Hunter drone fitted with an experimental sigint payload flew a mission over Southern Arizona, successfully detecting and locating different types of radio-frequency emitter scattered over a 40 square kilometre area. As the payload located each emitter, it transmitted the location to a ground station via a datalink. Once received, the information was immediately displayed on a digital map, demonstrating the feasibility of a US Army division-level requirement for real-time electronic mapping of the battlefield. The transmitted data also included information on the emitter's characteristics so that operators could identify the type of radio being used.

Although elint systems can be ground-based or shipborne, fitting them to an airborne platform has many advantages. Since the strength of a radio frequency signal decreases with range, elint systems can have difficulty detecting low power signals because of the distances between the collection platforms and the emitters. An airborne sensor can be deployed closer to the target and, in wartime, can operate forward of the battle line.

The traditional platform for airborne elint is either a dedicated reconnaissance aircraft such as the Lockheed U-2, or a converted bomber, maritime-patrol aircraft or airliner. As the existing elint fleets in service with the world's air forces become older, some form of long-term replacement is clearly required. However, there is little chance that existing airframes can be replaced on a one-for-one basis. As a result, some air forces are considering replacing at least part of the existing fleet with unmanned aircraft.

As the landing in China by a US Navy EP-3 Aries II elint aircraft on 21 March 2001 following a mid-air collision with a Chinese F-8 fighter demonstrated, an elint mission which goes wrong can present a potentially unfriendly nation with a massive intelligence bounty in the form of captured high-technology electronic systems. Worse, it can expose aircrew to the risk of death, or of being retained as hostages, prisoners of war or as the subjects for propaganda.

Using a UAV as an elint platform may not prevent the capture of hardware (unless the UAV has enough payload to carry a reliable sensor-destruct package), but will keep highly trained personnel out of harm's way.

Unmanned aircraft with the capacity to carry a high performance electro-optical payload such as a stabilised television camera have the payload volume, payload-carrying capacity and electrical power needed for an elint payload. As a result, many patterns of UAV currently in production or under development are being offered as potential elint platforms. This article will concentrate on UAVs which are known to have carried elint systems, either in operational service or on trials.

In 1998, Cecom's Intelligence and Information Warfare Directorate (I2WD) and L-3 Communications began the Multifunction Sigint Payload (MFSP) programme. As the US Army's interest in sigint drones grew, the Program Executive Office Intelligence, Electronic Warfare and Sensors (IEW&S) asked I2WD to perform a feasibility demonstration.

There are two forms of sigint:

* comint (communications intelligence) targeted against communications emitters and

* elint (electronics intelligence) primarily targeted against radars.

For the demonstration, the US Army decided to focus on comint. This was due to the service's greater interest in intercepting radio traffic and also because of the difficulty in fitting a relatively-small tactical UAV with the large antennas required for the elint mission.

Since the Army's new Shadow 200 TUAV was not yet available, the established Hunter was chosen as the airborne platform for the demonstration. An aerial version of the Prophet communications intelligence system, a tactical common data link and an inertial navigation system were installed in the Hunter. The successful demonstration in late 2000 gave a boost to the US Army's Division TUAV Signals Intelligence Program (more below).

The TRW/IAI RQ-5A Hunter used for the US Army's December 2000 trial is unique in that it saw operational service in 1999 after being cancelled in 1995. Twelve Hunters were withdrawn from storage and deployed to Skopje, Macedonia, in late March 1999 to monitor the Serbia/Kosovo conflict. Half of the force has been lost by the time that the Nato air campaign ended two months later.

Its basic payload was an IAI Tamam Mosp combined television and flir sensor, but the vehicle was demonstrated with alternative payloads which included a lightweight comint system. In 1998, a Hunter fitted with a Litton LR-100 radar warning receiver was used in suppression of enemy air defence trials by the US Air Force's UAV Battle Lab. The aircraft was able to detect emitters and use a datalink to transmit targeting information to two Block 50 F-16 fighters.

The Hunter currently serves with Belgium (where it replaced the Epervier), France and Israel, but it is not known if any have been adapted for elint. The reason for the US Army decision to cancel its Hunter programme was that it threatened to starve the follow-on Alliant RQ-6A Outrider programme of funding. However, this joint Army/Navy programme was abandoned in 1998 when it became obvious that the design could not meet the joint requirement.

New competitions were launched by the Army and the US Navy/US Marine Corps to find suitable tactical UAVs and, in December 1999, the Army selected the AAI Corporation Shadow 200 for service as the RQ-7A. Initial operational test and evaluation started in April 2001, although was suspended for about six months following a number of accidents it was successfully completed. Full production was authorised in late 2002.

Just one year later, following the success of the Hunter-based demonstration of late 2000, the US Army started its Division TUAV Sigint Program (DTSP), which is intended to create a sigint and EW platform capable of detecting, identifying and locating hostile communications networks and tactical radars such as counter-mortar, counter-battery and ground surveillance sets, even if these use Low Probability of Intercept technology.

Development contracts have been awarded to Applied Signal Technology, BAE Systems and Raytheon. The system is designed to fit the RQ-7A, but hardware could begin flight tests using a Hunter testbed.

Several types of UAV developed by General Atomics have carried sigint payloads. In 1994, the Gnat 750 was deployed by the CIA from a base in Albania and used to conduct image-gathering missions over Bosnia. By the end of that year these UAVs had been fitted with a high-resolution Mitsubishi thermal imager, plus a sigint sensor.

In 2001, the US Navy and US Marine Corps were conducting sigint and other surveillance missions from Camp Pendleton in California, using an Altus I high-altitude drone, a design which General Atomics had developed for Nasa's Environmental Research Aircraft and Sensor Technology upper-atmosphere research programme.

The best-known General Atomics aircraft is probably the MQ-1 and RQ-1 Predator, a medium-altitude, long-endurance drone which has seen action over Kosovo, Afghanistan, and Iraq. The Predator normally carries an electro-optical or synthetic aperture radar payload, and company officials become strangely tight-lipped when asked about possible sigint payloads. However, in October 1998 the company began flight tests of aircraft P011, which carried what was simply identified as an RF payload. This used a total of seven antennas, three on each wing leading-edge, and one on the nose, and may have been the prototype of a reported sigint variant.

Northrop Grumman's rotary wing RQ-8A Fire Scout is designed to carry a multimission sensor suite combining infrared and television sensors plus a laser designator/range-finder, but potential alternative payloads are known to include a sigint package, although a version with a synthetic aperture radar was recently announced as a possible entry in the British Watchkeeper programme competition. The Fire Scout can carry a payload of more than 90 kg, and can loiter in the target area for up to four hours, but the more recent four-bladed rotor RQ-8B considerably augments its lift or range capability and would enable the craft to carry both synthetic aperture radar/moving target indicator and conventional flir sensors.

The wars against Afghanistan and Iraq saw the operational use of the Northrop Grumman RQ-4A Global Hawk. Originally developed by Teledyne Ryan (now part of Northrop Grumman), the Global Hawk first flew in February 1998. A total of five had been delivered to Edwards AFB by the end of 1999, by which time the second aircraft--the first to carry a full sensor suite--had been lost in a crash, while another had been written off after a taxiing accident. EMD started in March 2001 and included orders for aircraft six and seven, the final two prototypes.

Late in 2001, the three remaining Global Hawks were deployed to the Middle East to support combat operations in Afghanistan. Operations began on 27 November and the aircraft proved able to remain on station for around 18 hours at a time, but two of the three were lost in accidents.

The loss of Air Vehicle 5 on 30 December 2001 was probably due to the failure of a rudder actuator. Controllers commanded the aircraft to return to its base at Al Dhafra in Abu Dhabi, but while it was still en route, flapping motions of the rudder caused a catastrophic failure.

The programme's newest Global Hawk, Air Vehicle 5 carried the only available example of Raytheon's Integrated Sensor Suite, which teams a synthetic aperture radar with electro-optical and infrared sensors and a Northrop Grumman LR-100 ESM (electronic support measures) system. The others had only a synthetic aperture radar payload.

Although Northrop Grumman completed aircraft 6 in early 2002, its delivery was delayed due to the need to 'cannibalise' it for parts needed to support the Middle Eastern deployment.

Aircraft seven was delivered on 14 February 2003, and a week later Northrop Grumman was given a $ 74.5 million contract to prepare two RQ-4s, a single set of ground facilities and associated support for deployment to the Middle East.

A low-rate initial production (Lrip) contract was placed in February 2002. This covered the first two Lrip aircraft (P1 and P2), which are due to carry an I/J-band synthetic aperture radar in addition to electro-optical and infrared sensors.

In 2001, a Global Hawk fitted with an Inmarsat communications link and carrying an operational payload which included a Litton LR-100 sigint made the first fully autonomous Pacific crossing by a UAV, then flew eleven demonstration missions over Australia and the surrounding seas. These tests involved a total of 238 flight hours, the purpose of which included an evaluation of the Global Hawk's ability to meet a Royal Australian Air Force strategic surveillance requirement. Australia is reported to be considering the procurement of up to six Global Hawks.

The Global Hawk will be subjected to a process of spiral development, with improvements gradually being added. P-9 is expected to be the first RQ-4B, a version the stretched fuselage and extended wingspan of which will provide the payload capacity for the planned Spiral 2 upgrades.

The Spiral 3 standard currently under development will be the first to be able to carry a sigint payload. In its initial form, this could be based on the Northrop Grumman LR-100 receiver used in the Australian trials. The definitive system is supposedly intended to meet a demanding requirement which asks for sigint capabilities similar of those of the U-2 from a sensor payload significantly smaller and lighter than that carried in the manned aircraft.

The US Air Force has asked Northrop Grumman to study the advantages which could be gained by operating Global Hawks in pairs. A pair of sigint aircraft flying a distance apart could team their sensors to create a high-resolution synthetic-aperture antenna which would be able to determine the location of detected emitters to a much higher degree of accuracy than is possible with a single airborne sigint sensor.

As part of its Broad Area Maritime Surveillance project, the US Navy is considering the Global Hawk as a maritime surveillance system to supplement its ageing P-3/EP-3 Orion fleet. Two air vehicles are to be modified to handle new payloads; one is expected to receive an elint system to supplement its normal sensor fit. The receiving system to be used has not been disclosed but there is speculation that it could be the Northrop Grumman LR-100.

Germany is seen as another potential Global Hawk customer. A German elint package is reported to have been installed on a Global Hawk for trials at Nordholz airbase in northern Germany. Known as the Euro Hawk, the modified aircraft is envisaged as a possible replacement for Germany's ageing fleet of Dassault-Breguet Atlantic 2 maritime patrol aircraft. These are due to be retired in 2008. Work is underway to demonstrate it with an Eads-developed sigint suite. The tests were expected to take place earlier, but until recently the owner of the airframe--the US Air Force--wanted to keep it as a back-up for operations in Afghanistan and Iraq.

Russia has been in the drone field for a number of years, but rather more in the tactical and observation segment with the Pechora. At the Maks air show (near Moscow) in August 2003, however, fighter aircraft designer Sukhoi clearly showed its intention to enter the heavyweight market. At this stage Sukhoi told Armada "we are currently awaiting detailed responses from our potential customers [...] and studying the possibilities of working with French companies, including Sagem". The aircraft is known as the Zond-1, which is a two to three tonne aircraft featuring an anhedral tail and a pusher propeller, and the Zond-2 and Zond-3, which are jet-powered and rather more heavy at approximately twelve tonnes. Under consideration for the latter two is twin-engine propulsion to enable them to fly over towns. Depending on the response to the preliminary studies, the first prototypes could have their maiden flights in 2006.

Several years ago, the Italian companies Meteor and Elettronica teamed to develop and market a reconnaissance version of the Mirach 100/5 target drone. The 350 km-range design would carry an ALR-733 C-J band elint receiver. Originally developed for use on helicopters and light to medium-weight patrol aircraft, this can automatically analyse and identify RF emitters.

France's sigint requirements are believed to include a high-altitude long-endurance (Hale) unmanned aircraft able to fly 48-hour missions at heights of up to 65,600 ft. Under its Fregate programme, Eads has proposed designs heavier than the Global Hawk. Powered by twin engines, this would have the payload capacity needed to house a complex sensor suite combining optical, synthetic aperture radar and sigint sensors. Rival designs of sigint payloads are reported to be under development by Thales and CS Communications/Onera. In the summer of 2000 Eads and Northrop Grumman signed an agreement to co-operate on Hale technology.

Eads has teamed with IAI to develop the Eagle, which is a Male (medium-altitude long endurance). Intended for French Air Force use, this is based on the Israeli company's 16.6-metre span Heron. It has a large payload compartment which could house radar, electro-optical or sigint sensors, but the French sensor configuration seems to be based on the IAI Elta EL/M-2055 radar and the IAI Tamam Mosp stabilised payload.

Silver Arrow, a division of Elbit, offers the Hermes 450 (a single-engined aircraft similar in general concept to the Predator) and the larger, twin-engine Hermes 1500. One goal of the latter design was to offer an internal volume and electrical-power capacity able to allow the use of multiple payloads. Potential payloads include sigint sensors and EW systems, says the company.

At least one Israeli type may already be in service as a sigint platform. At the Paris Air Show in mid-2003, IAI Elta Systems showed the EL/K-7071 comint payload for such applications. This forms part of a complete comint system. A datalink is used to transmit information on the intercepted signals to a ground comint operator's station.

For the moment, the subject of sigint UAVs remains very much 'under wraps'. While relatively little hard information is available, the concept is obviously attracting interest. At least half a dozen electronics companies already offer sigint payloads and that number is certain to grow. As ageing sigint aircraft are retired, something will have to take over their duties. In many cases, that 'something' is likely to be an unmanned aircraft.