The US Air Force is focusing research efforts around the concept of a sensor-laden UAV


At the US Air Force Research Laboratory (AFRL) they are called "integrating concepts", ideas around which technologies can be gathered to deliver new warfighter capabilities. Unmanned vehicles are key among these integrating concepts. Short-term efforts are focused on the ground-breaking Boeing X-45 unmanned combat air vehicle (UCAV) technology demonstration, while long-term work includes concepts such as swarming unmanned air vehicles (UAVs).

In the medium term lies the Sensor Craft, an intelligence, surveillance and reconnaissance (ISR) UAV concept that is bringing together the work of AFRL's Air Vehicles, Sensors, Propulsion and Materials, and Manufacturing directorates. The vision for Sensor Craft is global ISR, and to "provide the warfighter with the eyes and ears to respond to dynamic combat operations with the right force on target", says Sensor directorate senior electronics engineer Ron Crumbacher.

The US Air Force's shift to an Expeditionary Aerospace Force, and the need to reduce the "footprint" of deployed forces, means as much equipment as possible must be retained in the USA. This drives the requirement to "reach back", which demands high-capacity communications links and leads to increased use of unmanned ISR assets, be they UAVs or space-based, to reduce the logistics "tail".

Today, USAF ISR assets include surveillance platforms such as the Boeing E-3 Airborne Warning and Control System (AWACS), the Northrop Grumman E-8 Joint Surveillance Target Attack Radar System (JSTARS), the Lockheed MartinU-2 reconnaissance aircraft and the Boeing RC-135 Rivet Joint intelligence platform, plus UAVs like the General Atomics RQ-1 Predator. All are available in only limited numbers, meaning that ISR coverage available to expeditionary forces cannot be continuous.

The Northrop Grumman RQ-4 Global Hawk UAV is to be developed as an "adjunct" to the USAF's limited ISR assets. In the longer term, unmanned systems will play an even greater role, with the Sensor Craft potentially joining advanced versions of the Global Hawk, possible UCAV derivatives, swarming micro unmanned air vehicles and unattended ground sensors - all linked together in an "information-centric" infrastructure: "Sensor Craft is a fully integrated ISR enterprise linked to space and ground assets as well as other air vehicles, "Crumbacher says.

The concept is for two long-endurance Sensor Craft to provide a sustained presence over a large area of terrain, supplying ground commanders with updates every 15min. This would eliminate problems encountered in Kosovo in 1999 when it was known Serb forces had deployed, but not where, because ISR was not continuous.

AFRL's Sensor Craft concept requires a number of breakthroughs, notably in airframe and propulsion technology, to enable the vehicle to fly missions of up to 80h at high altitude in all weathers, while doubling its electrical power generation capacity to drive the multi-sensor suite. Technologies to make this possible are emerging, says Ron Kaehr, Sensors directorate technical director, strategic planning.

The approach being taken is to design the air vehicle around its sensors. Typically, aircraft have been built and then sensors added, with inevitable compromises in performance. With the requirement to carry sensors which need long antennas, it becomes more practical to build the air vehicle around the sensor components. This leads to the diamond-shaped joined-wing planform of AFRL's Sensor Craft.

Industry is getting to grips with this concept. Under AFRL's new Air Vehicle Technology Integration Programme (AVTIP), contracts are being awarded for configuration studies. The relatively modest contracts require manufacturers to formulate air vehicle concepts, identify potential applications and define candidate configurations for the long-endurance, high-altitude Sensor Craft. The work includes modelling and simulation to verify the aerodynamic and performance characteristics of the resulting designs. Boeing was the first to receive a Sensor Craft study contract under AVTIP late last year.

Design requirements

While AFRL's clean-sheet configuration is a "conceptual image", designed "to get the audience's reaction" like a motor show concept car, Sensor Craft concept images released by Lockheed Martin and Northrop Grumman show diamond-shaped fuselages mated to high-aspect-ratio wings to provide high-altitude, long-endurance (HALE) capability without obstructing the sensors' field of view.

One requirement is for long leading and trailing edges to house the apertures for a foliage-penetration radar, which operates at low frequency and therefore requires long antennas. Foliage penetration to detect concealed targets was a key ISR need identified in the lessons learned from 1999's Operation Allied Force.

The potential sensor suite also includes air and ground moving-target indication (AMTI and GMTI) radar; synthetic-aperture radar; electronic support measures; infrared search and track; hyperspectral imaging; and three-dimensional laser imaging. The system will be "multi-static", illuminating targets for and tracking targets illuminated by other UAVs, satellites and unattended ground sensors.

Ideally, a single aperture will be used for all radar functionality, says Kaehr, with software-controlled electronically scanned active arrays providing flexibility and programmability. Also, the same aperture could be used for communications and act as a receive-only electronic support measures sensor.

This will require the development of antennas able to operate in multiple frequency bands, and technologies enabling electronic beam-forming when the aperture is in a double-curvature surface such as a wing.

Other challenges facing the designers include reducing sensor weight and cost while increasing capability, particularly in AMTI and GMTI modes. Conformal apertures will help, but trade-offs will be required between the need for a thick aerofoil for sensor performance and a thin aerofoil for aerodynamic performance. Powering and cooling the sensor suite on long-duration missions will also be a something of a challenge.

Crumbacher says goals include reducing the cost of the radio-frequency (RF) suite by a factor of 10 to $5-10 million from $50-100 million today. Other targets include a 1,000-times reduction in the size and weight of RF receivers; and a weight reduction of 100 to 1,000 times for the whole system.

Sophisticated self-protection techniques will also be required to harden the system against electronic warfare. The Sensor Craft is not a formal USAF programme, but it is hoped that funding will be secured to allow development work to get under way in 2007-13, leading to an initial operational capability after 2018.

Source: Flight International