text:
Northrop Grumman RQ-4 Global Hawk

The Northrop Grumman RQ-4 Global Hawk is a high-altitude, remotely-piloted surveillance aircraft introduced in 2001. It was initially designed by Ryan Aeronautical (now part of Northrop Grumman), and known as Tier II+ during development. The RQ-4 provides a broad overview and systematic surveillance using high-resolution synthetic aperture radar (SAR) and electro-optical/infrared (EO/IR) sensors with long loiter times over target areas.

The Global Hawk is operated by the United States Air Force (USAF). It is used as a high-altitude long endurance (HALE) platform covering the spectrum of intelligence collection capability to support forces in worldwide military operations. According to the USAF, the superior surveillance capabilities of the aircraft allow more precise weapons targeting and better protection of friendly forces.

Cost overruns led to the original plan to acquire 63 aircraft being cut to 45, and to a 2013 proposal to mothball the 21 Block 30 signals intelligence variants. The initial flyaway cost of each of the first 10 aircraft was US$10 million in 1994. By 2001, this had risen to US$60.9 million (~$ in ), and then to $131.4 million (flyaway cost) in 2013. The U.S. Navy has developed the Global Hawk into the MQ-4C Triton maritime surveillance platform. , the U.S. Air Force plans to retire its Global Hawks in 2027.

Origins
In the 1990s, the Air Force was developing uncrewed aerial intelligence platforms. One was the stealthy Lockheed Martin RQ-3 DarkStar; another was the Global Hawk. Due to budget cuts, only one of the programs could survive. It was decided to proceed with the Global Hawk for its range and payload rather than go with the stealth Dark Star.

The Global Hawk took its first flight on 28 February 1998, at Edwards Air Force Base, California. The first seven aircraft were built under the Advanced Concept Technology Demonstration (ACTD) program, sponsored by DARPA, in order to evaluate the design and demonstrate its capabilities. Demand for the RQ-4's abilities was high in the Middle East; thus, the prototype aircraft were actively operated by the USAF in the War in Afghanistan. In an unusual move, the aircraft entered initial low-rate production while still in engineering and manufacturing development. Nine production Block 10 aircraft, sometimes referred to as RQ-4A, were produced; of these, two were sold to the US Navy and an additional two were deployed to Iraq to support operations there. The final Block 10 aircraft was delivered on 26 June 2006.

To increase the aircraft's capabilities, the airframe was redesigned, with the nose section and wings being stretched. The modified aircraft, designated RQ-4B Block 20, can carry up to 3,000 lb (1,360 kg) of internal payload. These changes were introduced with the first Block 20 aircraft, the 17th Global Hawk produced, which was rolled out in a ceremony on 25 August 2006. First flight of the Block 20 from the USAF Plant 42 in Palmdale, California to Edwards Air Force Base took place on 1 March 2007. Developmental testing of Block 20 took place in 2008.

United States Navy version

The United States Navy took delivery of two of the Block 10 aircraft to evaluate their maritime surveillance capabilities, designated N-1 (BuNo 166509) and N-2 (BuNo 166510). The initial navalised example was tested at Edwards Air Force Base briefly, before moving to Naval Air Station Patuxent River in March 2006 for the Global Hawk Maritime Demonstration (GHMD) program, operated by Navy squadron VX-20.

In July 2006, the GHMD aircraft flew in the Rim of the Pacific (RIMPAC exercise) for the first time. Although it was in the vicinity of Hawaii, the aircraft was operated from NBVC Point Mugu, requiring flights of approximately 2500 mi each way to the area. Four flights were performed, resulting in over 24 hours of persistent maritime surveillance coordinated with the aircraft carrier USS Abraham Lincoln (CVN-72) and amphibious warfare ship USS Bonhomme Richard (LHD-6). For the GHMD program, the Global Hawk was tasked with maintaining maritime situational awareness, contact tracking, and imagery support of exercise operations. Images were transmitted to NAS Patuxent River for processing and then forwarded to the fleet off Hawaii.

Northrop Grumman entered an RQ-4B variant in the US Navy's Broad Area Maritime Surveillance (BAMS) UAV competition. On 22 April 2008, it was announced that Northrop Grumman's RQ-4N had won and that the Navy had awarded a US$1.16 billion (~$ in ) contract. In September 2010, the RQ-4N was officially designated the MQ-4C.

The Navy MQ-4C differs from the Air Force RQ-4 mainly in its wing. While the Global Hawk remains at high altitude to conduct surveillance, the Triton climbs to 50,000 ft to see a wide area and can drop to 10,000 ft to get further identification of a target. The Triton's wings are specially designed to take the stresses of rapidly decreasing altitude. Though similar in appearance to the Global Hawk's wings, the Triton's internal wing structure is much stronger and has additional features including anti-icing capabilities and impact and lightning strike protection.

On 17 June 2022, the Navy brought its last deployed RQ-4A BAMS-D back from the Middle East, ending what started as a six-month experiment but turned into a 13-year deployment. The Navy had acquired five Block 10 RQ-4As and since 2009 at least one had been kept on rotation in the Persian Gulf region. The aircraft accrued over 42,500 flight hours in 2,069 missions; one was lost in an accident and another was shot down by Iran. The BAMS-D was replaced in Navy service with the MQ-4C.

Cost increases and procurement
Development cost overruns placed the Global Hawk at risk of cancellation. In mid-2006, per-unit costs were 25% over baseline estimates, caused by both the need to correct design deficiencies as well as to increase its capabilities. This caused concern over a possible congressional termination of the program if its national security benefits could not be justified. However, in June 2006, the program was restructured. Completion of an operational assessment report by the USAF was delayed from 2005 to 2007 due to manufacturing and development delays. The operational assessment report was released in March 2007 and production of the 54 air vehicles planned was extended by two years to 2015.

In February 2011, the USAF reduced its planned purchase of RQ-4 Block 40 aircraft from 22 to 11 in order to cut costs. In June 2011, the U.S. Defense Department's Director, Operational Test and Evaluation (DOT&E) found the RQ-4B "not operationally effective" due to reliability issues. In June 2011, the Global Hawk was certified by the Secretary of Defense as critical to national security following a breach of the Nunn-McCurdy Amendment; the Secretary stated: "The Global Hawk is essential to national security; there are no alternatives to Global Hawk which provide acceptable capability at less cost; Global Hawk costs $220M less per year than the Lockheed U-2 to operate on a comparable mission; the U-2 cannot simultaneously carry the same sensors as the Global Hawk; and if funding must be reduced, Global Hawk has a higher priority over other programs."

On 26 January 2012, the Pentagon announced plans to end Global Hawk Block 30 procurement as the type was found to be more expensive to operate and with less capable sensors than the existing U-2. Plans to increase procurement of the Block 40 variant were also announced. The Air Force's fiscal year 2013 budget request said it had resolved to divest itself of the Block 30 variant; however, the National Defense Authorization Act for Fiscal Year 2013 mandated operations of the Block 30 fleet through the end of 2014. The USAF plans to procure 45 RQ-4B Global Hawks as of 2013. Before retiring in 2014, ACC commander, General Mike Hostage said of the U-2's replacement by the drone that "The combatant commanders are going to suffer for eight years and the best they're going to get is 90 percent".

During 2010–2013, costs of flying the RQ-4 fell by more than 50%. In 2010, the cost per flight hour was $40,600, with contractor logistic support making up $25,000 per flight hour of this figure. By mid-2013, cost per flight hour dropped to $18,900, contractor logistic support having dropped to $11,000 per flight hour. This was in part due to higher usage, spreading logistics and support costs over a higher number of flight hours. In 2015, the RQ-4 was planned to replace the U-2 by 2019, though Lockheed Martin stated the U-2 can remain viable until 2050. As of January 2018, the U.S. Air Force budget for 2018 had indefinitely postponed the retirement of the U-2. In February 2020, the U.S. Air Force submitted budget documents with confusing language suggesting that it could begin retiring U-2s in 2025 but clarified afterwards that no retirement is planned.

In July 2022, the US Air Force announced plans to retire the Global Hawk in 2027.

EuroHawk

The German Air Force (Luftwaffe) ordered a variant of the RQ-4B, to be equipped with a customized sensor suite, designated "EuroHawk". The aircraft was based on the RQ-4B Block 20/30/40 and was to be equipped with an EADS-built signals intelligence (SIGINT) package; it was intended to fulfill Germany's requirement to replace their aging Dassault-Breguet Atlantique electronic surveillance aircraft of the Marineflieger (Naval Air arm of the German Navy). The EADS sensor package is composed of six wing-mounted pods; reportedly these sensor pods could potentially be used on other platforms, including crewed aircraft.

The EuroHawk was officially rolled out on 8 October 2009 and its first flight took place on 29 June 2010. It underwent several months of flight testing at Edwards Air Force Base. On 21 July 2011, the first EuroHawk arrived in Manching, Germany; after which it was scheduled to receive its SIGINT sensor package and undergo further testing and pilot training until the first quarter of 2012. The Luftwaffe planned to station the type with Taktisches Luftwaffengeschwader 51 ("Reconnaissance wing 51"). In 2011 the German Ministry of Defence was aware of difficulties with the certification for use within the European airspace. During flight trials, problems with the EuroHawk's flight control system were found; the German certification process was also complicated by Northrop Grumman refusing to share technical data on the aircraft with which to perform evaluations.

On 13 May 2013, German media reported that the EuroHawk would not be certifiable under ICAO rules without an anti-collision system; thus preventing any operations within European airspace or the airspace of any ICAO member. The additional cost of certification was reported to be more than €600 million (US$780 million). On 15 May 2013, the German government announced the immediate termination of the program, attributing the cancellation to the certification issue. Reportedly, the additional cost to develop the EuroHawk to the standards needed for certification may not have guaranteed final approval for certification.

German defense minister Thomas de Maizière stated EuroHawk was "very important" for Germany in 2012, then referred to the project as being "a horror without end" in his 2013 statement to the Bundestag. The total cost of the project before it was canceled was €562 million. Northrop Grumman and EADS have described reports of flight control problems and high costs for certification as "inaccurate"; they have stated their intention to provide an affordable plan to complete the first EuroHawk's flight testing and produce the remaining four aircraft.

On 8 August 2013, the EuroHawk set an endurance record by flying continuously in European airspace for 25.3 hours, reaching an altitude of 58600 ft. It was the longest flight by an unrefueled UAS weighing more than 30000 lb in European skies. On 5 October 2014, German Minister of Defence Ursula von der Leyen was reportedly considering reactivating the EuroHawk program to test its reconnaissance abilities over a long period at altitudes of up to 20000 m. Attempting to test the recon system on Airbus aircraft and an Israeli drone as alternate platforms had proven unsuccessful.

The Bundeswehr would use it to detect, decrypt, and potentially interfere with enemy communications signals. If tests prove successful, a carrier would be purchased, likely "similar" to the U.S. Global Hawk. Germany is considering installing the EuroHawk's SIGINT payloads onto the U.S. Navy MQ-4C Triton Global Hawk derivative, as the electronic and communications intelligence sensors would be more difficult to place on other substitute aircraft. It already has icing and lightning-strike protection, and was built with certification over civilian airspace in mind, meeting the STANAG 4671 requirements that had ended the EuroHawk program.

As of March 2021, Germany plans to put the single RQ-4E aircraft on display in the Bundeswehr Military History Museum by 2022.

Universal Payload Adapter and new payloads
In January 2014, President Obama signed a budget that included a $10 million (~$ in ) study on adapting the U-2's superior sensors for the RQ-4. In April 2015, Northrop Grumman reportedly installed the U-2's Optical Bar Camera (OBC) and Senior Year Electro-Optical Reconnaissance System (SYERS-2B/C) sensors onto the RQ-4 using a Universal Payload Adapter (UPA). Successful testing indicated that all RQ-4s could be similarly retrofitted.

On 14 July 2015, Northrop Grumman and the USAF signed an agreement to demonstrate an RQ-4B fitted with the U-2's OBC and SYERS-2C sensors. Two Global Hawks are to be fitted with the UPA, involving the installation of 17 payload adapter fixtures and a new payload bay cover, as well as software and mission system changes for each sensor. The UPA can support 1200 lb of sensors and will create a canoe-shaped sensor bay on the fuselage's underside.

Northrop Grumman also expects to receive a contract to integrate the UTC Aerospace Systems MS-177 multispectral sensor used on the Northrop Grumman E-8C JSTARS onto the RQ-4. The MS-177 will replace the SYERS-2 and includes modernized optronics and a gimbaled rotation device to increase field of view by 20 percent. The RQ-4B flew with the SYERS-2 on 18 February 2016.

Raytheon developed the AN/ALR-89 self-protection suite consisting of the AN/AVR-3 laser warning receiver, AN/APR-49 radar warning receiver, and jamming system, along with the ALE-50 towed decoy for the Global Hawk.

Range Hawk
Although the Global Hawk is being retired from combat use, the Department of Defense's Test Resource Management Center (TRMC) is acquiring them to support the SkyRange program to test hypersonic missiles by 2024. Tests are currently monitored by ships, but it can take a ship 21 days to be positioned and outfitted for use, limiting flights to about a dozen airborne demonstrations a year. By using unmanned aircraft to track hypersonic systems, faster availability and deployment could support a test rate of up to one per week. To perform this new mission, the Global Hawk is retrofitted into the Range Hawk, which involves configuring it to look up rather than down by repositioning onboard avionics and installing new sensors and instrumentation suites to track an overhead hypersonic vehicle. The program will use four Block 20 and 20 Block 30 airframes retired from USAF service. Converted Block 10 Range Hawks supported 10 hypersonic flight tests in 2023, including two tests in spring 2023 over the Atlantic and Pacific Oceans taking place only 10 days apart.

Overview
The Global Hawk UAV system comprises the RQ-4 air vehicle, which is outfitted with various equipment such as sensor packages and communication systems; and a ground element consisting of a Launch and Recovery Element (LRE), and a Mission Control Element (MCE) with ground communications equipment. Each RQ-4 air vehicle is powered by an Allison Rolls-Royce AE3007H turbofan engine with 7050 lbf thrust, and carries a payload of 2000 lb. The fuselage uses aluminum, semi-monocoque construction with a V-tail; the wings are made of composite materials.

There have been several iterations of the Global Hawk with different features and capabilities. The first version to be used operationally was the RQ-4A Block 10, which performed imagery intelligence (IMINT) with a 2000 lb payload of a synthetic aperture radar (SAR) with electro-optical (EO) and infrared (IR) sensors. Seven A-model Block 10s were delivered and all were retired by 2011. The RQ-4B Block 20 was the first of the B-model Global Hawks, which has a greater 3000 lb payload and employs upgraded SAR and EO/IR sensors. Four Block 20s were converted into communications relays with the Battlefield Airborne Communications Node (BACN) payload.

The RQ-4B Block 30 is capable of multi-intelligence (multi-INT) collecting with SAR and EO/IR sensors along with the Airborne Signals Intelligence Payload (ASIP), a wide-spectrum SIGINT sensor. The RQ-4B Block 40 is equipped with the multi-platform radar technology insertion program (MP-RTIP) active electronically scanned array (AESA) radar, which provides SAR and moving target indication (MTI) data for wide-area surveillance of stationary and moving targets.

The RQ-4 is capable of conducting sorties lasting up to 30 hours long and scheduled maintenance must be performed sooner than on other aircraft with less endurance. However, since it flies at higher altitudes than normal aircraft, it experiences less wear during flight. It can survey as much as 40000 sqmi of terrain per day, an area the size of South Korea or Iceland.

System and ground facilities
Raytheon's Integrated Sensor Suite (ISS) consists of the following sensors:

 * a synthetic aperture radar (SAR)
 * electro-optical (EO)
 * thermographic camera (IR)

Either the EO or the IR sensors can operate simultaneously with the SAR. Each sensor provides wide area search imagery and a high-resolution spot mode. The SAR has a ground moving target indicator (GMTI) mode, which can provide a text message providing the moving target's position and velocity. Both SAR and EO/IR imagery are transmitted from the aircraft to the MCE as individual frames, and reassembled during ground processing. An onboard inertial navigation system, supplemented by Global Positioning System updates, comprises the navigational suite.

The Global Hawk's camera is capable of identifying objects on the ground as small as 30 cm in diameter from 20 km in the air.

The Global Hawk is capable of operating autonomously and "untethered". A military satellite system (X Band Satellite Communication) is used for sending data from the aircraft to the MCE. The common data link can also be used for direct down link of imagery when the UAV is within line-of-sight of compatible ground stations. For dense flight areas the autonomous navigation is switched off and the RQ-4 is remote controlled via the satellite link by pilots on the ground who are supplied with the same instrument data and who carry the same responsibilities as pilots in crewed planes.

The ground segment consists of a Mission Control Element (MCE) and Launch and Recovery Element (LRE), provided by Raytheon. The MCE is used for mission planning, command and control, and image processing and dissemination; an LRE for controlling launch and recovery; and associated ground support equipment. The LRE provides precision Differential GPS corrections for navigational accuracy during takeoff and landings, while precision coded GPS supplemented with an inertial navigation system is used during mission execution. By having separable elements in the ground segment, the MCE and the LRE can operate in geographically separate locations, and the MCE can be deployed with the supported command's primary exploitation site. Both ground segments are contained in military shelters with external antennas for line-of-sight and satellite communications with the air vehicles.

Radar
The Global Hawk carries the Hughes Integrated Surveillance & Reconnaissance (HISAR) sensor system. HISAR is a lower-cost derivative of the ASARS-2 package that Hughes developed for the U-2. It is also fitted to the US Army's de Havilland Canada RC-7B Airborne Reconnaissance Low Multifunction (ARLM) crewed aircraft, and is being sold on the international market. HISAR integrates a SAR-MTI system, along with an optical and a thermography imager.

All three sensors are controlled and their outputs filtered by a common processor and transmitted in real time at up to 50 Mbit/s to a ground station. The SAR-MTI system operates in the X band in various operational modes; such as the wide-area MTI mode with a radius of 62 mi, combined SAR-MTI strip mode provides 20 ft resolution over 23 mi wide sections, and a SAR spot mode providing 6 ft resolution over 3.8 mi2.

In July 2006, the USAF began testing the Global Hawk Block 30 upgrades in the Benefield Anechoic Facility at Edwards AFB. Upgrades include the Advanced Signals Intelligence Payload, an extremely sensitive SIGINT processor. and a specialist AESA radar system, the Multi-Platform Radar Technology Insertion Program, or MP-RTIP. In 2010, Northrop disclosed the sensor capabilities of the new Block 40 aircraft, including the MP-RTIP radar, emphasising surveillance over reconnaissance.

On 14 April 2014, a Block 40 Global Hawk completed the first Maritime Modes program risk-reduction flight to enhance the Air Force's maritime surveillance capabilities. Maritime Modes is made up of a Maritime Moving Target Indicator and a Maritime Inverse synthetic aperture radar (MISAR) that function together to provide ISR information on vessels traveling on the water's surface. During the 11.5-hour flight off of the California coast, the MISAR collected data on over 100 items of interest. Maritime Modes is planned to be integrated with the RQ-4B's existing MP-RTIP radar to detect and produce synthetic aperture radar imagery of ground vehicles.

In November 2015, Northrop Grumman selected the Garmin International GSX 70 weather radar to be installed on Air Force Global Hawks. The GSX 70 is designed to provide operators with real-time weather information, offering horizontal scan angles of up to 120 degrees for better visibility into the strength and intensity of convective activity and a vertical scanning mode to analyze storm tops, gradients, and cell buildup activity. It also has a Turbulence Detection feature to identify turbulence in air containing precipitation and other airborne particulates and Ground Clutter Suppression that removes ground returns from the display so operators can focus on weather. Installation is expected to begin in early 2016. Installation of weather radars on the Global Hawk fleet completed in late 2019.

Visible light/infrared
The visible and infrared imagers share the same gimballed sensor package, and use common optics, providing a telescopic close-up capability. It can be optionally fitted with an auxiliary SIGINT package.

U.S. Air Force
Following the September 11th attacks, the normal acquisition process was bypassed almost immediately and early developmental Global Hawk models were employed in overseas contingency operations beginning in November 2001. Global Hawk ACTD prototypes were used in the War in Afghanistan and in the Iraq War. Since April 2010, they fly the Northern Route, from Beale Air Force Base over Canada to South-East Asia and back, reducing flight time and improving maintenance. While their data-collection capabilities have been praised, the program lost four prototype aircraft out of six or seven to accidents.

The crashes were reported to be due to "technical failures or poor maintenance", with a failure rate per hour flown over 100 times higher than the F-16 fighter. Northrop Grumman stated that it was unfair to compare the failure rates of a mature design to that of a prototype aircraft. In June 2012, a media report described the Global Hawk, the General Atomics MQ-1 Predator and the MQ-9 Reapers "... the most accident-prone aircraft in the Air Force fleet." On 11 February 2010, the Global Hawks deployed in the Central Command AOR accrued 30,000 combat hours and 1,500 plus sorties.

Initial operational capability was declared for the RQ-4 Block 30 in August 2011. The USAF did not plan to keep the RQ-4B Block 30 in service past 2014 due to the U-2 and other platforms being less expensive in the role. Congress sought to keep it in service until December 2016. The USAF had 18 RQ-4 Block 30s by the time of the passage of the National Defense Authorization Act for Fiscal Year 2013, which directed a further three RQ-4s to be procured as part of Lot 11. The USAF felt that additional aircraft were "excess to need" and likely become backup or attrition reserve models.

Despite the potential retirement of the Block 30 fleet due to low reliability, low mission readiness, and high costs, the USAF released a pre-solicitation notice in September 2013 for Lot 12 aircraft. In planning the USAF's FY 2015 budget, the Pentagon reversed its previous decision, shifting $3 billion from the U-2 to the RQ-4 Block 30, which had become more competitive with the U-2 due to increased flying hours. Factors such as cost per flight hour (CPFH), information gathering rates, mission readiness, adverse weather operational capability, distance to targets, and onboard power still favored the U-2.

After the 2011 Tōhoku earthquake and tsunami, RQ-4s flew 300 hours over the affected areas in Japan. There were also plans to survey the No. 4 reactor at the Fukushima Daiichi Nuclear Power Plant.

By November 2012, Northrop Grumman had delivered 37 Global Hawks to the USAF. By March 2014, 42 Global Hawks were in use around the world, with 32 in use by the USAF.

The USAF stated that U-2 pilot and altitude advantages allow better functionality in the stormy weather and airspace restrictions of the East Asia region and its altitude and sensor advantages allow it to see further into hostile territory. In October 2013, the U.S. secured basing rights to deploy RQ-4s from Japan, the first time that basing rights for the type had been secured in Northeast Asia. RQ-4s are stationed at Andersen Air Force Base in Guam, but bad weather often curtailed flights. Basing in Japan as opposed to Guam enhances spying capabilities against North Korea by eliminating range as a factor.

Two RQ-4s moved from Anderson AFB to Misawa Air Base in mid-2014 in the type's first deployment to Japan. They were speculated to have focused on maritime patrol missions. The two RQ-4s successfully performed their missions from Misawa AB during a six-month deployment, with none cancelled due to poor weather. It was the first time that they had operated out of a civil-military airport, sharing airspace and runways with commercial aircraft safely without additional restrictions, usually taking off and landing during quieter periods of air traffic. Officials only stated that they had operated at "various places around the Pacific."

On 19 September 2013, the RQ-4 Block 40 Global Hawk conducted its first wartime flight from Grand Forks Air Force Base.

In November 2013, an USAF RQ-4 deployed to the Philippines after Typhoon Haiyan to assist in relief efforts. It flew from Andersen Air Force Base in Guam to relay imagery of afflicted areas to response personnel and ground commanders.

In planning for the FY 2015 budget, the U-2 was to be retired in favor of the RQ-4, made possible by reductions of RQ-4 operating costs and would be the first time an uncrewed aircraft would completely replace a crewed aircraft. The U-2 will continue to fly through 2018 without replacement.

In May 2014, a U.S. Global Hawk conducted a surveillance mission over Nigeria as part of the search for the kidnapped Nigerian schoolgirls. The Global Hawk joined MC-12 crewed aircraft in the search.

The Global Hawk was used in Operation Inherent Resolve (OIR) against the Islamic State of Iraq and the Levant (ISIL). The aircraft provided real-time imagery and signals intelligence to identify friendly and enemy forces, do long-term target development, and track enemy equipment movement, enabling combatant commanders to act on better information and make key decisions. The BACN version allowed ground troops to contact aircraft when they were in need of assistance, such as close air support.

On 11 November 2015, an EQ-4 became the first Global Hawk aircraft to reach 500 sorties. All three EQ-4s in operation supported OIR. Upon landing, maintainers could complete ground maintenance and make the aircraft mission ready again within five hours. Missions could last up to 30 hours, with each aircraft getting a "day off" in between combat flights. On 1 April 2017, the EQ-4 program completed 1,000 continuous sorties, without incurring a single maintenance cancellation, while supporting OIR.

On 4 April 2016, it was reported that a USAF Global Hawk had completed its third flight over Germany under an initiative (the European Reassurance Initiative) to reassure NATO members concerned over Russian involvement in the conflict in Ukraine. Germany opened its airspace for up to five Global Hawk flights a month until the middle of October 2016. The Naval Air Station Sigonella, Sicily-based Global Hawk flies over Italian and French airspace and an air corridor through Germany with its sensors switched off on its way to its area of operations over the Baltic Sea.

In 2017, the USAF decided to begin the process of training enlisted airmen to fly the RQ-4 due to a shortage of pilots and an increased demand for the Global Hawk's capabilities. The RQ-4 is currently the only aircraft enlisted pilots are flying.

On 16 August 2018, a Global Hawk, assigned to 12th Reconnaissance Squadron, took off from Beale AFB, California, and landed at Eielson Air Force Base, Alaska for Red Flag – Alaska. This was the first time an RQ-4 had landed in Alaska during a simulated combat training exercise.

On 21 April 2021, a Global Hawk was reported to have made a reconnaissance flight in an airspace off the coast of southern Crimea which Russia had temporarily closed up to 19000 m from Sevastopol to Feodosiya, issuing a relevant NOTAM. The Global Hawk reportedly departed from Naval Air Station Sigonella on Sicily.

On 22 February 2022, a Global Hawk was reported to have made a reconnaissance flight over Southeastern Ukraine coinciding with a NOTAM order by Ukrainian government and increased Russian military activity. The Global Hawk departed from Naval Air Station Sigonella on Sicily.

Records
On 24 April 2001, a Global Hawk flew non-stop from Edwards AFB to RAAF Base Edinburgh in Australia, making history by being the first pilotless aircraft to cross the Pacific Ocean. The flight took 22 hours, and set a world record for absolute distance flown by a UAV, 13219.86 km.

On 22 March 2008, a Global Hawk set the endurance record for full-scale, operational uncrewed aircraft UAVs by flying for 33.1 hours at altitudes up to 60,000 feet over Edwards AFB.

From its first flight in 1998 to 9 September 2013, the combined Global Hawk fleet flew 100,000 hours. 88 percent of flights were conducted by USAF RQ-4s, while the remaining hours were flown by NASA Global Hawks, the EuroHawk, the Navy BAMS demonstrator, and the MQ-4C Triton. Approximately 75 percent of flights were in combat zones; RQ-4s flew in operations over Afghanistan, Iraq, and Libya; and supported disaster response efforts in Haiti, Japan, and California.

From 10 to 16 September 2014, the RQ-4 fleet flew a total of 781 hours, the most hours flown by the type during a single week. 87 percent of flights were made by USAF RQ-4s, with the rest flown by the Navy BAMS-D and NASA hurricane research aircraft.

The longest Global Hawk combat sortie lasted 32.5 hours.

Downing by Iran
On 19 June 2019, a U.S. Navy BAMS-D RQ-4A from NAS Patuxent River flying over the Persian Gulf near the Strait of Hormuz was shot down by a 3rd Khordad surface-to-air missile fired from near Garuk, Iran. Iranian Foreign Minister Javad Zarif said that the drone had been in Iranian airspace, while the United States maintained that the drone was in international airspace 18 nautical miles (34 km) away from Iran.

NASA

In December 2007, two Global Hawks were transferred from the USAF to NASA's Dryden Flight Research Center at Edwards AFB. Initial research activities beginning in the second quarter of 2009 supported NASA's high-altitude, long-duration Earth science missions. The two Global Hawks were the first and sixth aircraft built under the original DARPA Advanced Concept Technology Demonstration program, and were made available to NASA when the Air Force had no further need for them. Northrop Grumman is an operational partner with NASA and will use the aircraft to demonstrate new technologies and to develop new markets for the aircraft, including possible civilian uses.

It was reported in the March 2010 issue of Scientific American that NASA's Global Hawks were to begin scientific missions that month, and had been undergoing tests in late 2009. Initial science applications included measurements of the ozone layer and cross-Pacific transport of air pollutants and aerosols. The author of the Scientific American article speculates that it could be used for Antarctic exploration while being based in Chile. In August–September 2010, one of the two Global Hawks was loaned for NASA's GRIP (Genesis and Rapid Intensification Program) mission.

Its long-term on station capabilities and long range made it a suitable aircraft for monitoring the development of Atlantic basin hurricanes. It was modified to equip weather sensors including Ku-band radar, lightning sensors and dropsondes. It successfully flew into Hurricane Earl off the United States East Coast on 2 September 2010.

NATO
In 2009, NATO announced that it expected to have a fleet of up to eight Global Hawks by 2012 to be equipped with MP-RTIP radar systems. NATO had budgeted US$1.4 billion (€1 billion) for the project, and a letter of intent was signed. NATO signed a contract for five Block 40 Global Hawks in May 2012. 12 NATO members are participating in the purchase. On 10 January 2014, Estonia revealed it wanted to participate in NATO Global Hawk usage. In July 2017, the USAF assigned the Mission Designation Series (MDS) of RQ-4D to the NATO AGS air vehicle.

The first RQ-4D aircraft arrived at Sigonella Air Base on 21 November 2019. At that time, all five aircraft were undergoing developmental test flights. Initial operational capability was expected in the first half of 2020.

In October 2018, Italy certified five of the drones for use in Sigonella, Sicily in 2020. However, by 23 December 2019, there were regulatory issues for the Global Hawks concerning shared space between Germany and Italy. German government officials criticized the new drones for their lack of technology to avoid collisions with other aircraft.

South Korea
In 2011, South Korea's Defense Acquisition Program Administration (DAPA) expressed interest in acquiring at least four RQ-4Bs to increase intelligence capabilities following the exchange of the Wartime Operational Control from the U.S. to the Republic of Korea. Officials debated on the topic of the Global Hawks and domestic UAV programs. In September 2011, the US and South Korea discussed aircraft deployments near its land border to view North Korea and the North Korea–China border.

In January 2012, DAPA announced that it would not proceed with a purchase due to a price rise from US$442M to US$899M, and that other platforms such as the AeroVironment Global Observer or the Boeing Phantom Eye were being investigated. However, in December 2012, South Korea notified Congress of a possible Foreign Military Sale of 4 RQ-4 Block 30 (I) Global Hawks with the Enhanced Integrated Sensor Suite (EISS) at an estimated cost of $1.2 billion (~$ in ). On 5 July 2013, the Korean National Assembly advised the government to re-evaluate the RQ-4 purchase, again citing high costs.

On 17 December 2014, Northrop Grumman was awarded a $657 million contract by South Korea for four RQ-4B Block 30 Global Hawks. The first RQ-4 arrived on 23 December 2019 at a base near Sacheon. The second arrived on 19 April 2020, and the third by June. The fourth and final Global Hawk was delivered in September 2020.

Japan
On 24 August 2013, Japan announced that the Japan Air Self-Defense Force planned to operate one Global Hawk jointly with the U.S. by 2015. On 21 November 2014, the Japanese Ministry of Defense officially decided to procure the Global Hawk instead of the General Atomics Guardian ER; Japan has also been interested in the purchase of three aircraft. The first Japanese Global Hawk landed at Misawa Air Base on 12 March 2022.

Potential operators
Australia considered the purchase of a number of Global Hawks for maritime and land surveillance. The Global Hawk was to be assessed against the General Atomics MQ-9 Mariner in trials in 2007. The Global Hawk aircraft would have operated in conjunction with crewed Boeing P-8 Poseidon aircraft, as a replacement of aging Lockheed AP-3C Orion aircraft. In the end, the Australian government decided not to proceed and canceled the order. In 2012, a procurement effort for seven UAVs by 2019 was initiated. In May 2013 the Australian government confirmed its interest in acquiring the MQ-4C Triton maritime surveillance variant.

Canada has also been a potential customer, looking at the Global Hawk for maritime and land surveillance as either a replacement for its fleet of Lockheed CP-140 Aurora patrol aircraft or to supplement crewed patrols of remote Arctic and maritime environments, before withdrawing from the joint effort in August 2011. Spain has a similar requirement, and has existing contacts with Northrop Grumman.

The New Zealand Defence Force is studying the Global Hawk, which has the range to conduct surveillance in the Southern Ocean around Antarctica, and in the Pacific Islands. The acquisition process has not moved beyond an expression of interest.

The Indian Navy has expressed interest in acquiring six to eight MQ-4C Maritime Surveillance Unmanned Aircraft Systems.

In September 2018, Transport Canada was looking into buying a former German Air Force EuroHawk for surveillance missions in the Arctic. The EuroHawk cannot currently fly and has no equipment inside such as GPS and navigation tools.

Variants

 * RQ-4A: Initial production version for the USAF, 16 built.
 * RQ-4B: Improved version with increased payload, wingspan increased to 130.9 ft and length increased to 47.7 ft. Due to the increased size and payload the range is reduced to 8,700 nmi.
 * RQ-4D Phoenix
 * NATO Alliance Ground Surveillance (AGS).

 * RQ-4E Euro Hawk
 * Version for the Bundeswehr based on RQ-4B and equipped with an EADS reconnaissance payload for SIGINT. Germany canceled its order in May 2013; it received one of five Euro Hawks originally ordered.

 * MQ-4C Triton
 * For USN Broad Area Maritime Surveillance (BAMS) role; previously known as the RQ-4N; 4 ordered, 68 total planned.

 * EQ-4B
 * Equipped with the Battlefield Airborne Communications Node (BACN) system.

 * KQ-X
 * Proposed autonomous tanker variant.

 * Model 396
 * Scaled Composites and Northrop Grumman also offered an armed, 50% smaller version of the RQ-4A, known as the Scaled Composites Model 396, as part of the USAF Hunter-Killer program. The aircraft was rejected in favor of the MQ-9 Reaper.

Operators

 * Republic of Korea Air Force – Ordered 4 in 2014. First aircraft delivered on 23 December 2019.
 * Japan Air Self-Defense Force – Ordered 3 in November 2018, to be delivered by 1 September 2022. The purchase was made under a contract worth $USD1.2 billion.
 * NATO
 * Alliance Ground Surveillance – Ordered 5 aircraft, first delivered 21 November 2019.
 * United States Air Force
 * Air Combat Command
 * 319th Reconnaissance Wing – Grand Forks Air Force Base, North Dakota
 * 319th Operations Group
 * 4th Reconnaissance Squadron – Andersen Air Force Base, Guam
 * 7th Reconnaissance Squadron – Naval Air Station Sigonella, Italy
 * 12th Reconnaissance Squadron – Beale Air Force Base, California
 * 348th Reconnaissance Squadron – Grand Forks Air Force Base, North Dakota
 * 53d Wing
 * 53d Test and Evaluation Group
 * 31st Test and Evaluation Squadron – Edwards Air Force Base, California
 * Air Force Reserve Command
 * 940th Wing – Beale Air Force Base, California
 * 940th Operations Group
 * 13th Reconnaissance Squadron – Beale Air Force Base, California
 * 380th Expeditionary Operations Group – Al Dhafra AB, United Arab Emirates since early 2002
 * RQ-4B (Block 30), RQ-4B (Block 40), EQ-4B (BACN), RQ-4A (BAMS-D)
 * NASA
 * Dryden Flight Research Center
 * RQ-4B (Block 30), RQ-4B (Block 40), EQ-4B (BACN), RQ-4A (BAMS-D)
 * NASA
 * Dryden Flight Research Center

Accidents

 * 29 March 1999: USAF RQ-4A 95-2002 crashed at China Lake Naval Weapons Center.
 * 30 Dec 2001: USAF RQ-4A 98-2005 crashed while returning to al-Dhafra Air Base, UAE.
 * 10 Jul 2002: USAF RQ-4A 98-2004 crashed near Shamsi AB, Pakistan due to engine failure.
 * 21 August 2011: USAF EQ-4B crashed southeast of Jalalabad, Afghanistan.
 * 11 June 2012: USN RQ-4A assigned to the Navy's BAMS program crashed near Naval Air Station Patuxent River, Maryland, US.
 * 21 June 2017: USAF RQ-4B crashed near Lone Pine, California, US.
 * 26 June 2018: USAF RQ-4B crashed into the sea off Naval Station Rota, Spain.
 * 20 June 2019: An RQ-4A aircraft was shot down by the IRGC, near Mogh-e Qanbareh-ye Kuh Mobarak in Hormozgan province, using the Sevom Khordad air defense missile system.
 * August 2021: USAF RQ-4B crashed near Grand Forks Air Force Base, North Dakota, US.

title: Northrop Grumman RQ-4 Global Hawk


text:
AAI RQ-2 Pioneer

The AAI RQ-2 Pioneer was an unmanned aerial vehicle (UAV) that was used by the United States Navy, Marine Corps, and Army, and deployed at sea and on land from 1986 until 2007. Initially tested aboard USS Iowa, the RQ-2 Pioneer was placed aboard Iowa-class battleships to provide gunnery spotting, its mission evolving into reconnaissance and surveillance, primarily for amphibious forces.

It was developed jointly by AAI Corporation and Israel Aircraft Industries. The program grew out of successful testing and field operation of the Tadiran Mastiff UAV by the American and Israeli militaries.

Essentially, the Pioneer is an upgraded IAI Scout which was re-engined to accommodate a greater payload by request of the US Navy. To accomplish this, the original "Limbach" two-cylinder two-stroke engine was replaced with a Fichtel & Sachs two-cylinder two-stroke. The Limbach motor used a 71 cm propeller from Propeller Engineering and Duplicating, Inc. of San Clemente, California. The newer, more powerful Fichtel & Sachs motor was outfitted with a 74 cm propeller (which spins in the opposite direction) from the Sensenich Propeller Manufacturing Company of Lancaster, Pennsylvania.

Operation

Launched by rocket assist (shipboard), by catapult, or from a runway, the Pioneer recovers into a net (shipboard) or with arresting gear after flying up to five hours with a 75 lb payload. It flies day or night missions with a gimbaled EO/IR sensor, relaying analog video in real time via a C-band line-of-sight (LOS) data link. Since 1991, Pioneer has flown reconnaissance missions during the Persian Gulf, Somalia (UNOSOM II), Bosnia, Kosovo and Iraq conflicts. In 2005, the Navy operated two Pioneer systems (one for training) and the Marines operated two, each with five or more aircraft. It is also operated by Israel and the Republic of Singapore Air Force. In 2007 Pioneer was retired by the US Navy and was replaced by the Shadow UAV.

Internationally, Pioneer drones are perhaps most remembered for their role in the 1991 Gulf War, when a Pioneer launched by the Iowa-class battleship USS Wisconsin (BB-64) observed Iraqi troops on Failaka Island surrendering shortly after USS Missouri (BB-63)'s attack on their trenchlines. When navy officials offered to transfer a Pioneer to the Smithsonian Institution, curators at the National Air and Space Museum specifically asked for the UAV that Iraqi troops surrendered to during the Gulf War.

In the 1991 Gulf War, the US Army operated a UAV Platoon from Ft. Huachuca, Arizona. The UAV Platoon conducted flight surveillance and target acquisition missions from KKMC and later, the unit pushed north (Operation Sand Hawk) where US Army combat engineers built a metal runway for the aircraft to launch and recover.

The "R" is the Department of Defense designation for reconnaissance; "Q" means unmanned aircraft system. The "2" refers to its being the second of a series of purpose-built unmanned reconnaissance aircraft systems.

Specifications (RQ-2)

 * Primary Function: Artillery Targeting and Acquisition, Control of Close Air Support, Reconnaissance and Surveillance, Battle Damage Assessment, Search and Rescue, Psychological Operations

Operators

 * United States Marine Corps
 * VMU-1 'Watchdogs': Marine Corps Air Ground Combat Center Twentynine Palms, California
 * VMU-2 'Night Owls': MCAS Cherry Point, North Carolina
 * 🇱🇰 Sri Lanka
 * Sri Lanka Air Force
 * Sri Lanka Navy
 * Sri Lanka Navy

Former operators

 * United States Army
 * UAV platoon deployed in Fort Huachuca, January-May 1991.
 * United States Navy
 * VC-6 "Firebees": Naval Station Norfolk (decommissioned)
 * Training Air Wing 6 UAV Detachment: Naval Air Station Whiting Field (decommissioned)
 * Training Air Wing 6 UAV Detachment: Naval Air Station Whiting Field (decommissioned)

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: AAI RQ-2 Pioneer


text:
IAI RQ-5 Hunter

The IAI RQ-5 Hunter unmanned aerial vehicle (UAV) was originally intended to serve as the United States Army's Short Range UAV system for division and corps commanders. It took off and landed (using arresting gear) on runways. It used a gimbaled EO/IR sensor to relay its video in real time via a second airborne Hunter over a C-band line-of-sight data link. The RQ-5 is based on the Hunter UAV that was developed by Israel Aerospace Industries.

Design and development
System acquisition and training started in 1994 but production was cancelled in 1996 due to concerns over program mismanagement. Seven low rate initial production (LRIP) systems of eight aircraft each were acquired, four of which remained in service: one for training and three for doctrine development, exercise, and contingency support. Hunter was to be replaced by the RQ-7 Shadow, but instead of being replaced, the Army kept both systems in operation because the Hunter had significantly larger payload, range, and time-on-station capabilities than the Shadow.

Operational history

In 1995, A Company, 15th Military Intelligence Battalion (Aerial Exploitation) out of Fort Hood, TX was the first Army field unit equipped with the Hunter. A Company conducted multiple successful training rotations to the National Training Center. Then in March 1999, they were deployed to the Republic of Macedonia in support of NATO operations in Kosovo where one was shot down by a Yugoslav Mil Mi-8 Side 7.62mm Machine Gun. During the 78 days of the NATO led campaign, at least 7 Hunter UAV were lost. During the 7 month operation, the Hunter was flown for over 4,000 hours. Significant operational success in Kosovo led to resumption of production and technical improvements. Hunter was used in Iraq and other military operations since then. The system was also armed with the Viper Strike munitions.

The Army's Unmanned Aircraft Systems Training Battalion at Fort Huachuca, AZ trained soldiers and civilians in the operation and maintenance of the Hunter UAV.

In 2004, the United States Department of Homeland Security, Bureau of Customs and Border Protection, Office of Air and Marine utilized the Hunter under a trial program for border patrol duties. During this program, the Hunter flew 329 flight hours, resulting in 556 detections.

A version armed with the Northrop Grumman GBU-44/B Viper Strike weapon system is known as the MQ-5A/B.

As of October 2012, the U.S. Army had 20 MQ-5B Hunters in service. Retirement of the Hunter was expected to be completed in 2013, but Northrop was awarded a support contract for the Hunter in January 2013, extending its missions into 2014.

On 7 October 2013, the U.S. Army opened a UAS facility at Vilseck Army Airfield in Germany. A letter of agreement between the U.S. and Germany allows the 7th Army Joint Multinational Training Command to use two ‘air bridges’ in the east of the country to train operators, marking the first time a U.S. UAV will fly beyond the limits of military training areas. Two unarmed MQ-5B Hunters were used solely for training drone operators.

From 1996 to January 2014, the MQ-5B Hunter unmanned aerial system flew over 100,000 hours with the U.S. Army.

On 14 March 2014, an RQ-5 was reported downed by a Crimean self-defense unit over Russian occupied Ukrainian territory, although Russia did not substantiate the claim and the Pentagon denies it operated such a vehicle over Crimea.

On 16 December 2015, the Hunter flew its final flight in Army service at Fort Hood. Since entering service in 1995, the aircraft had been deployed to the Balkans, Iraq, and Afghanistan. It was deployed to the Balkans four times between 1999 and 2002, accumulating 6,400 flight hours, and was the first Army UAS to cross into Iraq in 2003, proving itself for the first time in contingency operations as an intelligence asset to commanders at all levels and flying more hours than any other NATO reconnaissance platform. One capability unique to the Hunter was its relay mode that allowed one aircraft to control another at extended ranges or over terrain obstacles. By the end of Operation New Dawn in 2011, Hunters had flown more than 110,000 hours, its battlefield success clearly showing the value of UASs in combat operations as a direct result. While Army operators transitioned to the larger and more capable General Atomics MQ-1C Gray Eagle, the Hunter is being transferred to government-owned, contractor-operated units supporting operations overseas.

International use
In 1998, the Belgian Air Component purchased three B-Hunter UAV-systems, each consisting of six aircraft and two ground control stations. Operational from 2004 in the 80 UAV Squadron, 13 aircraft were in service in 2020. The last Hunter was withdrawn from Belgian service on 28 August 2020, to be replaced by the MQ-9B SkyGuardian.

title: IAI RQ-5 Hunter


text:
Relative density

Relative density, also called specific gravity, is a dimensionless quantity defined as the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water at its densest (at 4 °C); for gases, the reference is air at room temperature (20 °C). The term "relative density" (abbreviated r.d. or RD) is preferred in SI, whereas the term "specific gravity" is gradually being abandoned.

If a substance's relative density is less than 1 then it is less dense than the reference; if greater than 1 then it is denser than the reference. If the relative density is exactly 1 then the densities are equal; that is, equal volumes of the two substances have the same mass. If the reference material is water, then a substance with a relative density (or specific gravity) less than 1 will float in water. For example, an ice cube, with a relative density of about 0.91, will float. A substance with a relative density greater than 1 will sink.

Temperature and pressure must be specified for both the sample and the reference. Pressure is nearly always 1 atm (101.325 kPa). Where it is not, it is more usual to specify the density directly. Temperatures for both sample and reference vary from industry to industry. In British brewing practice, the specific gravity, as specified above, is multiplied by 1000. Specific gravity is commonly used in industry as a simple means of obtaining information about the concentration of solutions of various materials such as brines, must weight (syrups, juices, honeys, brewers wort, must, etc.) and acids.

Basic calculation
Relative density ($$RD$$) or specific gravity ($$SG$$) is a dimensionless quantity, as it is the ratio of either densities or weights $$\mathit{RD} = \frac{\rho_\mathrm{substance}}{\rho_\mathrm{reference}},$$ where $$RD$$ is relative density, $$\rho_\mathrm{substance}$$ is the density of the substance being measured, and $$\rho_\mathrm{reference}$$ is the density of the reference. (By convention $$\rho$$, the Greek letter rho, denotes density.)

The reference material can be indicated using subscripts: $$RD_\mathrm{substance/reference}$$ which means "the relative density of substance with respect to reference". If the reference is not explicitly stated then it is normally assumed to be water at 4 °C (or, more precisely, 3.98 °C, which is the temperature at which water reaches its maximum density). In SI units, the density of water is (approximately) 1000 kg/m3 or 1 g/cm3, which makes relative density calculations particularly convenient: the density of the object only needs to be divided by 1000 or 1, depending on the units.

The relative density of gases is often measured with respect to dry air at a temperature of 20 °C and a pressure of 101.325 kPa absolute, which has a density of 1.205 kg/m3. Relative density with respect to air can be obtained by $$\mathit{RD} = \frac{\rho_\mathrm{gas}}{\rho_\mathrm{air}} \approx \frac{M_\mathrm{gas}}{M_\mathrm{air}},$$ where $$M$$ is the molar mass and the approximately equal sign is used because equality pertains only if 1 mol of the gas and 1 mol of air occupy the same volume at a given temperature and pressure, i.e., they are both ideal gases. Ideal behaviour is usually only seen at very low pressure. For example, one mol of an ideal gas occupies 22.414 L at 0 °C and 1 atmosphere whereas carbon dioxide has a molar volume of 22.259 L under those same conditions.

Those with SG greater than 1 are denser than water and will, disregarding surface tension effects, sink in it. Those with an SG less than 1 are less dense than water and will float on it. In scientific work, the relationship of mass to volume is usually expressed directly in terms of the density (mass per unit volume) of the substance under study. It is in industry where specific gravity finds wide application, often for historical reasons.

True specific gravity of a liquid can be expressed mathematically as: $$ SG_\mathrm{true} = \frac {\rho_\mathrm{sample}}{\rho_\mathrm{H_2O}},$$ where $$\rho_\mathrm{sample}$$ is the density of the sample and $$\rho_\mathrm{H_2O}$$ is the density of water.

The apparent specific gravity is simply the ratio of the weights of equal volumes of sample and water in air: $$ SG_\mathrm{apparent} = \frac {W_{\mathrm{A},\text{sample}}}{W_{\mathrm{A},\mathrm{H_2O}}},$$ where $$W_{A,\text{sample}}$$ represents the weight of the sample measured in air and $${W_{\mathrm{A},\mathrm{H_2O}}}$$ the weight of an equal volume of water measured in air.

It can be shown that true specific gravity can be computed from different properties: $$ SG_\mathrm{true} = \frac {\rho_\mathrm{sample}}{\rho_\mathrm{H_2O}} = \frac {\frac{m_\mathrm{sample}}{V}}{\frac{m_\mathrm{H_2O}}{V}} = \frac {m_\mathrm{sample}}{m_\mathrm{H_2O}} \frac{g}{g} = \frac {W_{\mathrm{V},\text{sample}}}{W_{\mathrm{V},\mathrm{H_2O}}},$$

where g is the local acceleration due to gravity, V is the volume of the sample and of water (the same for both), ρsample is the density of the sample, ρH 2O is the density of water, WV represents a weight obtained in vacuum, $$\mathit{m}_\mathrm{sample}$$ is the mass of the sample and $$\mathit{m}_\mathrm{H_2 O}$$is the mass of an equal volume of water.

The density of water and of the sample varies with temperature and pressure, so it is necessary to specify the temperatures and pressures at which the densities or weights were determined. Measurements are nearly always made at 1 nominal atmosphere (101.325 kPa ± variations from changing weather patterns), but as specific gravity usually refers to highly incompressible aqueous solutions or other incompressible substances (such as petroleum products), variations in density caused by pressure are usually neglected at least where apparent specific gravity is being measured. For true (in vacuo) specific gravity calculations, air pressure must be considered (see below). Temperatures are specified by the notation (Ts/Tr), with Ts representing the temperature at which the sample's density was determined and Tr the temperature at which the reference (water) density is specified. For example, SG (20 °C/4 °C) would be understood to mean that the density of the sample was determined at 20 °C and of the water at 4 °C. Taking into account different sample and reference temperatures, we note that, while SGH 2O = $1$ (20 °C/20 °C), it is also the case that SGH 2O = $0.998/1$ = $0.998$ (20 °C/4 °C). Here, temperature is being specified using the current ITS-90 scale and the densities used here and in the rest of this article are based on that scale. On the previous IPTS-68 scale, the densities at 20 °C and 4 °C are $0.998$ and $1$ respectively, resulting in an SG (20 °C/4 °C) value for water of $0.998$.

As the principal use of specific gravity measurements in industry is determination of the concentrations of substances in aqueous solutions and as these are found in tables of SG versus concentration, it is extremely important that the analyst enter the table with the correct form of specific gravity. For example, in the brewing industry, the Plato table lists sucrose concentration by weight against true SG, and was originally (20 °C/4 °C) i.e. based on measurements of the density of sucrose solutions made at laboratory temperature (20 °C) but referenced to the density of water at 4 °C which is very close to the temperature at which water has its maximum density, ρH 2O equal to 999.972 kg/m3 in SI units ($1 g/cm^{3}$ in cgs units or 62.43 lb/cu ft in United States customary units). The ASBC table in use today in North America for apparent specific gravity measurements at (20 °C/20 °C) is derived from the original Plato table using Plato et al.‘s value for SG(20 °C/4 °C) = $0.998$. In the sugar, soft drink, honey, fruit juice and related industries, sucrose concentration by weight is taken from a table prepared by A. Brix, which uses SG (17.5 °C/17.5 °C). As a final example, the British SG units are based on reference and sample temperatures of 60 °F and are thus (15.56 °C/15.56 °C).

Given the specific gravity of a substance, its actual density can be calculated by rearranging the above formula: $$\rho_\mathrm{substance} = SG \times \rho_\mathrm{H_2O}.$$

Occasionally a reference substance other than water is specified (for example, air), in which case specific gravity means density relative to that reference.

Temperature dependence

 * See Density for a table of the measured densities of water at various temperatures.

The density of substances varies with temperature and pressure so that it is necessary to specify the temperatures and pressures at which the densities or masses were determined. It is nearly always the case that measurements are made at nominally 1 atmosphere (101.325 kPa ignoring the variations caused by changing weather patterns) but as relative density usually refers to highly incompressible aqueous solutions or other incompressible substances (such as petroleum products) variations in density caused by pressure are usually neglected at least where apparent relative density is being measured. For true (in vacuo) relative density calculations air pressure must be considered (see below). Temperatures are specified by the notation (Ts/Tr) with Ts representing the temperature at which the sample's density was determined and Tr the temperature at which the reference (water) density is specified. For example, SG (20 °C/4 °C) would be understood to mean that the density of the sample was determined at 20 °C and of the water at 4 °C. Taking into account different sample and reference temperatures we note that while SGH 2O = 1.000000 (20 °C/20 °C) it is also the case that RDH 2O = $0.9982008⁄0.9999720$ = 0.9982288 (20 °C/4 °C). Here temperature is being specified using the current ITS-90 scale and the densities used here and in the rest of this article are based on that scale. On the previous IPTS-68 scale the densities at 20 °C and 4 °C are, respectively, 0.9982041 and 0.9999720 resulting in an RD (20 °C/4 °C) value for water of 0.99823205.

The temperatures of the two materials may be explicitly stated in the density symbols; for example:
 * relative density: 8.15$20 °C 4 °C$; or specific gravity: 2.432$15 0$

where the superscript indicates the temperature at which the density of the material is measured, and the subscript indicates the temperature of the reference substance to which it is compared.

Uses
Relative density can also help to quantify the buoyancy of a substance in a fluid or gas, or determine the density of an unknown substance from the known density of another. Relative density is often used by geologists and mineralogists to help determine the mineral content of a rock or other sample. Gemologists use it as an aid in the identification of gemstones. Water is preferred as the reference because measurements are then easy to carry out in the field (see below for examples of measurement methods).

As the principal use of relative density measurements in industry is determination of the concentrations of substances in aqueous solutions and these are found in tables of RD vs concentration it is extremely important that the analyst enter the table with the correct form of relative density. For example, in the brewing industry, the Plato table, which lists sucrose concentration by mass against true RD, were originally (20 °C/4 °C) that is based on measurements of the density of sucrose solutions made at laboratory temperature (20 °C) but referenced to the density of water at 4 °C which is very close to the temperature at which water has its maximum density of ρ equal to 0.999972 g/cm3 (or 62.43 lb·ft−3). The ASBC table in use today in North America, while it is derived from the original Plato table is for apparent relative density measurements at (20 °C/20 °C) on the IPTS-68 scale where the density of water is 0.9982071 g/cm3. In the sugar, soft drink, honey, fruit juice and related industries sucrose concentration by mass is taken from this work which uses SG (17.5 °C/17.5 °C). As a final example, the British RD units are based on reference and sample temperatures of 60 °F and are thus (15.56 °C/15.56 °C).

Measurement
Relative density can be calculated directly by measuring the density of a sample and dividing it by the (known) density of the reference substance. The density of the sample is simply its mass divided by its volume. Although mass is easy to measure, the volume of an irregularly shaped sample can be more difficult to ascertain. One method is to put the sample in a water-filled graduated cylinder and read off how much water it displaces. Alternatively the container can be filled to the brim, the sample immersed, and the volume of overflow measured. The surface tension of the water may keep a significant amount of water from overflowing, which is especially problematic for small samples. For this reason it is desirable to use a water container with as small a mouth as possible.

For each substance, the density, ρ, is given by $$\rho = \frac{\text{Mass}}{\text{Volume}} = \frac{\text{Deflection} \times \frac{\text{Spring Constant}}{\text{Gravity}}}{\text{Displacement}_\mathrm{Water Line} \times \text{Area}_\mathrm{Cylinder}}.$$

When these densities are divided, references to the spring constant, gravity and cross-sectional area simply cancel, leaving $$ RD = \frac{\rho_\mathrm{object}}{\rho_\mathrm{ref}} = \frac{\frac{\text{Deflection}_\mathrm{Obj.}}{\text{Displacement}_\mathrm{Obj.}}}{\frac{\text{Deflection}_\mathrm{Ref.}}{\text{Displacement}_\mathrm{Ref.}}} = \frac{\frac{3\ \mathrm{in}}{20\ \mathrm{mm}}}{\frac{5\ \mathrm{in}}{34\ \mathrm{mm}}}=\frac{3\ \mathrm{in} \times 34\ \mathrm{mm}}{5\ \mathrm{in} \times 20\ \mathrm{mm}} = 1.02. $$

Hydrostatic weighing
Relative density is more easily and perhaps more accurately measured without measuring volume. Using a spring scale, the sample is weighed first in air and then in water. Relative density (with respect to water) can then be calculated using the following formula: $$RD = \frac{W_\mathrm{air}}{W_\mathrm{air} - W_\mathrm{water}},$$ where
 * Wair is the weight of the sample in air (measured in newtons, pounds-force or some other unit of force)
 * Wwater is the weight of the sample in water (measured in the same units).

This technique cannot easily be used to measure relative densities less than one, because the sample will then float. Wwater becomes a negative quantity, representing the force needed to keep the sample underwater.

Another practical method uses three measurements. The sample is weighed dry. Then a container filled to the brim with water is weighed, and weighed again with the sample immersed, after the displaced water has overflowed and been removed. Subtracting the last reading from the sum of the first two readings gives the weight of the displaced water. The relative density result is the dry sample weight divided by that of the displaced water. This method allows the use of scales which cannot handle a suspended sample. A sample less dense than water can also be handled, but it has to be held down, and the error introduced by the fixing material must be considered.

Hydrometer
The relative density of a liquid can be measured using a hydrometer. This consists of a bulb attached to a stalk of constant cross-sectional area, as shown in the adjacent diagram.

First the hydrometer is floated in the reference liquid (shown in light blue), and the displacement (the level of the liquid on the stalk) is marked (blue line). The reference could be any liquid, but in practice it is usually water.

The hydrometer is then floated in a liquid of unknown density (shown in green). The change in displacement, Δx, is noted. In the example depicted, the hydrometer has dropped slightly in the green liquid; hence its density is lower than that of the reference liquid. It is necessary that the hydrometer floats in both liquids.

The application of simple physical principles allows the relative density of the unknown liquid to be calculated from the change in displacement. (In practice the stalk of the hydrometer is pre-marked with graduations to facilitate this measurement.)

In the explanation that follows,
 * ρref is the known density (mass per unit volume) of the reference liquid (typically water).
 * ρnew is the unknown density of the new (green) liquid.
 * RDnew/ref is the relative density of the new liquid with respect to the reference.
 * V is the volume of reference liquid displaced, i.e. the red volume in the diagram.
 * m is the mass of the entire hydrometer.
 * g is the local gravitational constant.
 * Δx is the change in displacement. In accordance with the way in which hydrometers are usually graduated, Δx is here taken to be negative if the displacement line rises on the stalk of the hydrometer, and positive if it falls. In the example depicted, Δx is negative.
 * A is the cross sectional area of the shaft.

Since the floating hydrometer is in static equilibrium, the downward gravitational force acting upon it must exactly balance the upward buoyancy force. The gravitational force acting on the hydrometer is simply its weight, mg. From the Archimedes buoyancy principle, the buoyancy force acting on the hydrometer is equal to the weight of liquid displaced. This weight is equal to the mass of liquid displaced multiplied by g, which in the case of the reference liquid is ρrefVg. Setting these equal, we have $$mg = \rho_\mathrm{ref}Vg$$

or just

Exactly the same equation applies when the hydrometer is floating in the liquid being measured, except that the new volume is $V − AΔx$ (see note above about the sign of Δx). Thus,

Combining ($$) and ($$) yields

But from ($$) we have $V = m/ρ_{ref}$. Substituting into ($$) gives

This equation allows the relative density to be calculated from the change in displacement, the known density of the reference liquid, and the known properties of the hydrometer. If Δx is small then, as a first-order approximation of the geometric series equation ($$) can be written as: $$RD_\mathrm{new/ref} \approx 1 + \frac{A \Delta x}{m} \rho_\mathrm{ref}.$$

This shows that, for small Δx, changes in displacement are approximately proportional to changes in relative density.

Pycnometer

A pycnometer (from πυκνός), also called pyknometer or specific gravity bottle, is a device used to determine the density of a liquid. A pycnometer is usually made of glass, with a close-fitting ground glass stopper with a capillary tube through it, so that air bubbles may escape from the apparatus. This device enables a liquid's density to be measured accurately by reference to an appropriate working fluid, such as water or mercury, using an analytical balance.

If the flask is weighed empty, full of water, and full of a liquid whose relative density is desired, the relative density of the liquid can easily be calculated. The particle density of a powder, to which the usual method of weighing cannot be applied, can also be determined with a pycnometer. The powder is added to the pycnometer, which is then weighed, giving the weight of the powder sample. The pycnometer is then filled with a liquid of known density, in which the powder is completely insoluble. The weight of the displaced liquid can then be determined, and hence the relative density of the powder.

A gas pycnometer, the gas-based manifestation of a pycnometer, compares the change in pressure caused by a measured change in a closed volume containing a reference (usually a steel sphere of known volume) with the change in pressure caused by the sample under the same conditions. The difference in change of pressure represents the volume of the sample as compared to the reference sphere, and is usually used for solid particulates that may dissolve in the liquid medium of the pycnometer design described above, or for porous materials into which the liquid would not fully penetrate.

When a pycnometer is filled to a specific, but not necessarily accurately known volume, V and is placed upon a balance, it will exert a force $$ F_\mathrm{b} = g\left(m_\mathrm{b} - \rho_\mathrm{a}\frac{m_\mathrm{b}}{\rho_\mathrm{b}}\right),$$ where mb is the mass of the bottle and g the gravitational acceleration at the location at which the measurements are being made. ρa is the density of the air at the ambient pressure and ρb is the density of the material of which the bottle is made (usually glass) so that the second term is the mass of air displaced by the glass of the bottle whose weight, by Archimedes Principle must be subtracted. The bottle is filled with air but as that air displaces an equal amount of air the weight of that air is canceled by the weight of the air displaced. Now we fill the bottle with the reference fluid e.g. pure water. The force exerted on the pan of the balance becomes: $$ F_\mathrm{w} = g\left(m_\mathrm{b} - \rho_\mathrm{a} \frac{m_\mathrm{b}}{\rho_\mathrm{b}} + V\rho_\mathrm{w} - V\rho_\mathrm{a}\right). $$

If we subtract the force measured on the empty bottle from this (or tare the balance before making the water measurement) we obtain. $$F_\mathrm{w,n} = gV( \rho_\mathrm{w} - \rho_\mathrm{a}),$$ where the subscript n indicated that this force is net of the force of the empty bottle. The bottle is now emptied, thoroughly dried and refilled with the sample. The force, net of the empty bottle, is now: $$F_\mathrm{s,n} = gV(\rho_\mathrm{s} - \rho_\mathrm{a}),$$ where ρs is the density of the sample. The ratio of the sample and water forces is: $$SG_\mathrm{A} = \frac{gV(\rho_\mathrm{s} - \rho_\mathrm{a})}{gV( \rho_\mathrm{w} - \rho_\mathrm{a})} = \frac{\rho_\mathrm{s} - \rho_\mathrm{a}}{\rho_\mathrm{w} - \rho_\mathrm{a}}. $$

This is called the apparent relative density, denoted by subscript A, because it is what we would obtain if we took the ratio of net weighings in air from an analytical balance or used a hydrometer (the stem displaces air). Note that the result does not depend on the calibration of the balance. The only requirement on it is that it read linearly with force. Nor does RDA depend on the actual volume of the pycnometer.

Further manipulation and finally substitution of RDV, the true relative density (the subscript V is used because this is often referred to as the relative density in vacuo), for ρs/ρw gives the relationship between apparent and true relative density:

$$RD_\mathrm{A}= {{\rho_\mathrm{s} \over \rho_\mathrm{w}}-{\rho_\mathrm{a} \over \rho_\mathrm{w}} \over 1 - {\rho_\mathrm{a} \over \rho_\mathrm{w}}} ={RD_\mathrm{V}-{\rho_\mathrm{a} \over \rho_\mathrm{w}} \over 1 - {\rho_\mathrm{a} \over \rho_\mathrm{w}}}.$$

In the usual case we will have measured weights and want the true relative density. This is found from $$RD_\mathrm{V} = RD_\mathrm{A} - {\rho_\mathrm{a} \over \rho_\mathrm{w} }(RD_\mathrm{A}-1).$$

Since the density of dry air at 101.325 kPa at 20 °C is 0.001205 g/cm3 and that of water is 0.998203 g/cm3 we see that the difference between true and apparent relative densities for a substance with relative density (20 °C/20 °C) of about 1.100 would be 0.000120. Where the relative density of the sample is close to that of water (for example dilute ethanol solutions) the correction is even smaller.

The pycnometer is used in ISO standard: ISO 1183-1:2004, ISO 1014–1985 and ASTM standard: ASTM D854.

Types

 * Gay-Lussac, pear shaped, with perforated stopper, adjusted, capacity 1, 2, 5, 10, 25, 50 and 100 mL
 * as above, with ground-in thermometer, adjusted, side tube with cap
 * Hubbard, for bitumen and heavy crude oils, cylindrical type, ASTM D 70, 24 mL
 * as above, conical type, ASTM D 115 and D 234, 25 mL
 * Boot, with vacuum jacket and thermometer, capacity 5, 10, 25 and 50 mL

Digital density meters
Hydrostatic Pressure-based Instruments: This technology relies upon Pascal's Principle which states that the pressure difference between two points within a vertical column of fluid is dependent upon the vertical distance between the two points, the density of the fluid and the gravitational force. This technology is often used for tank gauging applications as a convenient means of liquid level and density measure.

Vibrating Element Transducers: This type of instrument requires a vibrating element to be placed in contact with the fluid of interest. The resonant frequency of the element is measured and is related to the density of the fluid by a characterization that is dependent upon the design of the element. In modern laboratories precise measurements of relative density are made using oscillating U-tube meters. These are capable of measurement to 5 to 6 places beyond the decimal point and are used in the brewing, distilling, pharmaceutical, petroleum and other industries. The instruments measure the actual mass of fluid contained in a fixed volume at temperatures between 0 and 80 °C but as they are microprocessor based can calculate apparent or true relative density and contain tables relating these to the strengths of common acids, sugar solutions, etc.

Ultrasonic Transducer: Ultrasonic waves are passed from a source, through the fluid of interest, and into a detector which measures the acoustic spectroscopy of the waves. Fluid properties such as density and viscosity can be inferred from the spectrum.

Radiation-based Gauge: Radiation is passed from a source, through the fluid of interest, and into a scintillation detector, or counter. As the fluid density increases, the detected radiation "counts" will decrease. The source is typically the radioactive isotope caesium-137, with a half-life of about 30 years. A key advantage for this technology is that the instrument is not required to be in contact with the fluid—typically the source and detector are mounted on the outside of tanks or piping.

Buoyant Force Transducer: the buoyancy force produced by a float in a homogeneous liquid is equal to the weight of the liquid that is displaced by the float. Since buoyancy force is linear with respect to the density of the liquid within which the float is submerged, the measure of the buoyancy force yields a measure of the density of the liquid. One commercially available unit claims the instrument is capable of measuring relative density with an accuracy of ± 0.005 RD units. The submersible probe head contains a mathematically characterized spring-float system. When the head is immersed vertically in the liquid, the float moves vertically and the position of the float controls the position of a permanent magnet whose displacement is sensed by a concentric array of Hall-effect linear displacement sensors. The output signals of the sensors are mixed in a dedicated electronics module that provides a single output voltage whose magnitude is a direct linear measure of the quantity to be measured.

The relative density in soil mechanics
The relative density $$D_\mathrm{R}$$ a measure of the current void ratio in relation to the maximum and minimum void rations, and applied effective stress control the mechanical behavior of cohesionless soil. Relative density is defined by $$D_\mathrm{R}=\frac{e_\mathrm{max}-e}{e_\mathrm{max}-e_\mathrm{min}} \times 100\%$$ in which $$e_\mathrm{max}, e_\mathrm{min}$$, and $$e$$ are the maximum, minimum and actual void rations.

Examples
(Samples may vary, and these figures are approximate.)

Substances with a relative density of 1 are neutrally buoyant, those with RD greater than one are denser than water, and so (ignoring surface tension effects) will sink in it, and those with an RD of less than one are less dense than water, and so will float.

Example: $$RD_\mathrm{H_2O} = \frac{\rho_\mathrm{Material}}{\rho_\mathrm{H_2O}} = RD,$$

Helium gas has a density of 0.164 g/L; it is 0.139 times as dense as air, which has a density of 1.18 g/L.

 * Urine normally has a specific gravity between 1.003 and 1.030. The Urine Specific Gravity diagnostic test is used to evaluate renal concentration ability for assessment of the urinary system. Low concentration may indicate diabetes insipidus, while high concentration may indicate albuminuria or glycosuria.
 * Blood normally has a specific gravity of approximately 1.060.
 * Vodka 80° proof (40% v/v) has a specific gravity of 0.9498.

title: Relative density


text:
User:Aidan Elliott-McCrea

Aidan Elliott-McCrea, occasional editor since 2001.

title: User:Aidan Elliott-McCrea


text:
D'Urville Island

D'Urville Island, Māori name Rangitoto ki te Tonga, is the largest island in the Marlborough Sounds, on the northern coast of the South Island of New Zealand. It was named after the French explorer Jules Dumont d'Urville. With an area of approximately 58 sqmi, it is the eighth-largest island of New Zealand, and has around 52 permanent residents. The local authority is the Marlborough District Council.

History
The original Māori name of the island is Rangitoto, meaning "blood red sky" (rangi means sky; toto means blood). It is an ancient name brought from Hawaiki, the ancestral homeland of the Māori in the Pacific. The name was later lengthened to Rangitoto ki te Tonga, the suffix meaning "of the south", to distinguish it from places in the North Island called Rangitoto. The official name of the island is Rangitoto ki te Tonga / D'Urville Island.

The island was a traditional source of argillite (pakohe) for Māori, used in the production of stone tools such as adzes during the Archaic period (1300–1500). From the 1600s until the early 1800s, the island was a part of the rohe of Ngāti Tūmatakōkiri. In the present day, the island is within the rohe of Ngāti Koata and Ngāti Kuia.

Geography
The island has a convoluted coastline, as is frequently found with islands formed from peaks between sea-drowned valleys. It extends for some 35 km northeast/southwest, and is a little over 10 km wide at its widest point. The eastern coast of the island is relatively smooth, marked mainly by the small D'Urville Peninsula, some halfway along its length. In contrast, the west coast is marked by three large inlets: Port Hardy in the north, Greville Harbour in the centre, and Manuhakapakapa in the south. Numerous smaller islands lie off the coast, notably Stephens Island, which lies 3 km off D'Urville's northernmost point, Cape Stephens. The island's highest point, Takapōtaka / Attempt Hill (729 m) lies close to the centre of the island, due east of Greville Harbour. Most of the island's residents live close to the more sheltered east coast, with the localities of Patuki and Mukahanga being close to the northern tip of the island.

The island is separated from the mainland by the dangerous French Pass, known to Māori as Te Aumiti, through which water passes at up to 8 kn at each tide. Several vortices occur near this passage. d'Urville investigated the passage for several days in 1827, and damaged his ship passing through it.

Flora and fauna
There is roughly 6,000ha (60km2) of public conservation land on D'urville Island, mainly through the centre of the island. The conservation land consists of regenerating farmland, coastal broadleaf, and beech forest.

D'Urville Island is free of possums, feral goats, ship rats, Norway rats and, weasels making it important ecologically. Red deer and feral pigs are present on the Island and a permit can be obtained from the New Zealand Department of Conservation to hunt on the public conservation land.

Stoats on the island have caused the local extinction of the little spotted kiwi, South Island kākā and yellow-crowned kākāriki. D'Urville Island Stoat Eradication Charitable Trust started in 2003 to attempt remove stoats from the island. A large scale stoat eradication programme was funded for the Island but after issues with land access the funding was withdrawn.

There is a population of the rare South Island long-tailed bat on the island.

Transport
There is a small airstrip at Moawhitu, Greville Harbour, that is maintained by the Department of Conservation. Pelorus Air has flights to D'Urville Island from Picton, Wellington and Paraparaumu.

A barge service is operated by D'Urville Island Crossings between French Pass village and the settlement of Kapowai. There is also a water taxi operating between the D’Urville Island Wilderness Resort at Catherine Cove and French Pass.

Driftwood Eco Tours  take small group tours to the island, focusing on its heritage and ecology and meeting with residents to learn about its unique features. Driftwood Eco Tours donate each year to 'DISECT' (D'Urville Island Stoat Eradication Trust). In 2016, Motueka-based Abel Tasman Sea Shuttles hosted a number of charity cruises around D'Urville Island in conjunction with the Rotary Club of Motueka.

title: D'Urville Island