“Satellites are brilliant at global coverage, but they have a problem with persistence. Aircraft are good for local stuff, but they’re not very persistent and they’re very local,” says Paul Brooks, head of HAPS business development at Airbus Defence and Space.
The solution is Zephyr, which Mr Brooks calls “a satellite with wings.”
Zephyr is a lightweight unmanned aircraft with an airframe made of carbon fibre, the upper surface of its wings crammed with solar cells. It was originally developed by QinetiQ in 2003 and the program has since been taken over by Airbus.
There are two versions of Zephyr, the current Zephyr 8 or S and the future Zephyr T. The S version has a 25-metre wingspan but weighs less than 75 kilos – so light it is launched by being thrown into the air by the ground crew after a take-off run.
It cruises at around 70,000 feet, twice the altitude of most airliners and far above clouds and bad weather. This ensures continuous sunlight during daytime to power the eight propellers and recharge the batteries.
At night, the Zephyr runs on a bank of special high-capacity lithium-ion batteries which make up almost a third of the aircraft’s weight. These store about 30% more energy than conventional batteries, and the silicon nanowire technology behind them is being developed for electric cars and other applications.
The Zephyr S carries a five-kilo payload, enough for a camera able to distinguish objects about 15 centimetres across on the ground, making it as good as the best current satellites.
The difference is that while a satellite can only take a few snaps as it whizzes past on each orbit, the Zephyr hangs around taking as many pictures as needed and can home in on objects of interest before they disappear.
Another advantage of Zephyr is that, unlike a satellite, it can easily return to base for repairs or upgrades to sensors or other equipment.
The Zephyr T model will be slightly larger, with a twin tail and a 33 metres wingspan. It will weigh almost twice as much as the S at 140 kilos, including a significantly bigger payload.
The T version is likely to be optimised for maritime surveillance and communications relay.
The Caihong T-4 first flew in 2017. It is bigger than Zephyr, with a 40-metre wingspan, making it the size of a 737 airliner. It has double-bodied fuselage to help distribute its weight of some 400 kilos across the airframe.
It flies at 65,000 feet and the designers say it will ultimately be able to stay in the air for months at a time, though so far the T-4 has been restricted to relatively short flights.
High-altitude pseudo-satellites may be a game changer on land and at sea.
They have the potential to provide a stream of high-resolution images in real time and leaving the opposition with nowhere to hide.