An RQ-4 Global Hawk seen in a hangar (Picture: US Air Force).
By David Hambling, technology expert
On the morning of 20 June, Iran's Revolutionary Guard said it had shot down an American RQ-4 Global Hawk drone which had violated their airspace in the Straits of Hormuz.
Officials from the United States confirmed that an unmanned aircraft had been shot down, with initial reports suggesting it could have been a slightly different aircraft, an MQ-4C Triton.
The US said the drone had been flying in international airspace at the time.
What would an American drone be doing in that area and what are its capabilities?
The exact identity was initially puzzling. The RQ-4 is a large surveillance drone operated by the US Air Force and other nations, while the MQ-4C Triton is the naval variant flown only by the US Navy.
In theory, the MQ-4C has not even been deployed yet; its first posting will be Guam, far from the Persian Gulf, later this year.
Online aviation enthusiasts looking back over flight plans noted that a US Navy MQ-4C flew from Pax River Naval Air Station in Maryland in the US to Al Dhafra Airbase, Qatar on 15 June.
The ID number reveals that this particular aircraft is not a 'true' MQ-4C, but was originally an RQ-4 modified into a Broad Area Maritime Surveillance Demonstrator (BAMS-D).
This an experimental model which preceded the MQ-4C Triton proper, since confirmed by US officials as an RQ-4A Global Hawk.
The first thing to note about this type of aircraft is that they are big.
While the MQ-1 Predator may be not much larger than a light aircraft, the Global Hawk is more like an airliner - its wingspan of 40 metres being bigger than a Boeing 737.
It cruises at a modest 380 mph, driven by a single Rolls Royce turbofan engine, being designed not for speed but for long endurance at high altitude.
The Triton flies at 60,000 feet, almost twice as high as a typical airliner, on missions lasting up to 30 hours.
The Triton is also a very pricey piece of kit.
The US Navy is thought to be buying two MQ-4Cs this year at a cost of over $260m each, making them twice as expensive as an F-35 fighter.
The Triton’s role is ocean surveillance. Its main sensor is a huge radar set, the AN/ZPY-3 which provides a 360-degree view of the surface below.
In its maritime surface search (MSS) search mode, the radar can locate and track ships, while ISAR mode allows it to zoom in to see more detail and identify individual vessels.
Software allows it to switch rapidly between modes, while an artificial intelligence system can make identifications without human assistance.
Additional radar modes allow the Triton to look at targets on land.
The Triton also carries an Electronic Support Measures suite, a set of sensors which can pick up, locate and identify radar emitters.
This is used both to track ships which are using radar and to identify surface-to-air missile radars based on land.
It may also locate and tap into radio communications, collecting what is known as signals intelligence.
At lower altitudes, the Triton uses a Multi-Spectral Targeting System (MTS), a long-range camera which operates in the visual and infra-red bands for day and night sensing.
The MTS also includes a laser designator to highlight targets for smart laser-guided bombs.
Recent events in the area have included attacks on oil tankers, apparently using limpet mines placed on high-speed boats.
Nobody seems to have seen who placed the mines, but a US helicopter took pictures of Iranian Revolutionary Guard removing an unexploded mine.
Iran, however, denies involvement.
The role of the Triton would be to keep track of ship movements in the area.
It would be able to follow oil tankers and note any Iranian naval vessels engaged in suspicious activity, cueing ships, manned aircraft or helicopters to move in close if they posed a threat to commercial shipping.
Drones are expendable in a way that manned aircraft are not, and the loss of a pilot would be far more serious than any piece of hardware. But losing a $260m plane is a blow.
The incident demonstrates that while flying at 60,000 feet may keep it well above the height of shoulder-launched missiles, the Triton is still vulnerable to larger systems, such as the one alleged to have been used against a US Reaper drone last week.
The Iranians claim the drone was shot down by their Ra’ad (Thunder) system which can reach up to 70,000 feet.
The Triton is not stealthy and does not have the same array of defences as a fighter plane; high-speed manoeuvring to avoid a missile, even if the operators were aware it was approaching, would be impossible.
In the short term, the incident will further raise tensions in the region and also will also fuel the debate about drones.
One side will argue that by using an unmanned aircraft, the US avoided the political reaction when a pilot was killed or captured, proving the value of drones for risky surveillance missions.
Others might say that the Iranians would not dare shoot down a manned plane and sending a drone was a provocation that invited a missile.
Exactly what happens next may well shape how drones are used in future.