The RQ-4 Global Hawk is a high altitude, long endurance (HALE) unmanned aerial system (UAS) currently operated by the U.S. Air Force. The aircraft has a range of 12,300NM, service ceiling of 60,000ft, and cruises at 310 knots (U.S. Air Force, 2014). The Global Hawk’s greatest strength is arguably its 34 hour endurance which makes it ideal for providing persistent intelligence, surveillance, and reconnaissance (ISR). While the RQ-4 has had limited success with operational users when trying to replace the Lockheed U-2’s capabilities, it has significantly contributed to disaster relief efforts in Japan, Haiti, and the Philippines.
RQ-4 Sensor Capabilities
The Block 20 RQ-4 can carry up to 3,000lbs of mission payloads. The standard configuration is a Raytheon Integrated Sensor Suite (ISS) comprised of a Hughes Integrated Synthetic Aperture Radar (HISAR), 0.4-0.8um Electro-Optical (EO) sensor, and 3.6-5.0um Infrared (IR) sensor (Kable Intelligence Ltd, 2015). HISAR is capable of operating in the following modes (Bayma, 1996):
- Wide Area Search: 24m resolution over a 60o sector.
- Strip Map: 6m resolution over a continuous 37km path.
- Spot Image: 1.8m resolution over a 4.8x2.8km area.
- Ground Moving Target Indicator (GMTI): Position/velocity within 45o of broadside.
The HISAR transmitter operates in X-band with a 600MHz bandwidth. Block 40 RQ-4s have recently been delivered with a Multi-Platform Radar Technology Insertion Program payload which enables new capabilities to be quickly fielded across different vehicles by using common hardware/software interfaces (Pultrich, 2010). The ISS EO/IR sensors share an optical path through a 10in reflecting telescope that can generate 2km2 images that are geo-rectified to within 20m. The ISS requires up to 3.5kW of power (Kable Intelligence Ltd, 2015), likely supplied by an engine-driven 11.2kW generator (Cessna, 2013).
Data Format, Protocols, and Storage
The primary method of transferring sensor data to a ground-based processing, exploitation, and dissemination (PED) function is Ku or UHF satellite data link (Kable Intelligence Ltd, 2015). The HISAR contains a dual-channel receiver which passes raw signals to an Analog-to-Digital converter. The digitized raw data is held in a high capacity buffer until the onboard processor can finish generating an image, which is then downlinked for PED (Bayma, 1996). One of the strengths of this system is that the only the finished product is transmitted from the aircraft, which requires much less bandwidth than the raw data. One of the key features of HISAR is its ability to simultaneously operate in GMTI mode, while performing wide area or strip mapping. In this case, the processed images will be overlaid with GMTI position/velocity information. In the RQ-4, only high-resolution still frames from the ISS EO/IR sensors are downlinked, however the MQ-4 Triton variant flown by the U.S. Navy has the capability to downlink low-resolution video.
Recommendations
The HALE ISR mission is perfect for a UAS. When comparing the performance of the RQ-4 with the U-2, the unmanned platform clearly has an advantage in terms of persistence. Additionally, a pilot’s life is not risked by enemy fire or having to ditch over inhospitable environments. The best recommendation I would make is to continue developing the RQ-4 reliability and sensor capability, while employing open architectures to facilitate future enhancements, so that it can seamlessly fill the role of the U-2. Work is currently underway by Northrop Grumman to build a Universal Payload Adaptor, that would enable the RQ-4 to “carry the Senior Year Electro-Optical Reconnaissance System-2B/C and the Optical Bar Camera” (Malenic, 2015), which are currently the only internationally-recognized systems for aerial treaty enforcement.
References
Cessna Aircraft Company. (2013). Citation X: Specification & Description. Retrieved from http://cessna.txtav.com/~/media/Files/citation/x/xsd.ashx
Note: The Citation X has the same basic power plant model as the RQ-4.
Kable Intelligence Ltd. (2015). RQ-4A/B Global Hawk HALE Reconnaissance UAV, United States of America. Air Force Technology. Retrieved from http://www.airforce-technology.com/projects/rq4-global-hawk-uav/
Malenic, M. (2015, April 29). Northrop Grumman to test U-2 sensors on Global Hawk. Jane’s 360. Retrieved from http://www.janes.com/article/51076/northrop-grumman-to-test-u-2-sensors-on-global-hawk
Pultrich, G. (2010, August 16). Next generation of Global Hawks ready to roll. Flight Global. Retrieved from http://www.flightglobal.com/news/articles/next-generation-of-global-hawks-ready-to-roll-346116/
U.S. Air Force. (2014, October 27). RQ-4 Global Hawk Fact Sheet. Retrieved from http://www.af.mil/AboutUs/FactSheets/Display/tabid/224/Article/104516/rq-4-global-hawk.aspx
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