The Ministry of Defence has bought a third solar-powered Zephyr-S surveillance drone for £4.3m, and says it “performs more like a satellite than a conventional UAV”.

With its 25 metre wingspan covered in solar panel cells, this latest Zephyr-S is the third unmanned aerial vehicle (UAV) to be delivered as part of the MoD's £13m UAV contract with Airbus.

According to Airbus the Zephyr is capable of flying at altitudes of up to 70,740ft, while an early prototype managed to stay aloft for 14 days over the Arizona desert.

The MoD, however, reckons the Farnborough-built Zephyr-S is capable of staying airborne for 45 days. With modern advances in aerodynamics technology, this is not out of the question – though the figure is likely to be an extreme endurance one rather than its routine operational endurance.

Both manufacturer and customer refer to the Zephyr-S as a High Altitude Psuedo-Satellite. The idea is that the drone uses its 5kg payload to carry a surveillance camera, which Airbus says can provide up to 15cm resolution of objects on the ground from an altitude “in excess of” 65,000ft.

A product brochure from Airbus tells us that the Zephyr 8 can be supplied with cameras capable of producing NIIRS level 6 photos, which is good enough to “identify the shape of antennas on EW/GCI/ACQ radars as parabolic, parabolic with clipped corners or rectangular” according to this handy NIIRS guide.

Some educated guesswork tells us, then, that the Zephyr's normal operating altitude is somewhere between 65,000ft and a service ceiling of 70,000ft – double the usual cruising height of transatlantic airliners and well above most usual cloud formations, enabling it to make the most of its solar charging panels. It is powered by a 300W electric motor driving two two-bladed propellers and energy storage is through lithium-ion batteries.

As well as surveillance, the Zephyr-S is capable of being used as a communications relay satellite. Interestingly, Airbus says it can be fitted with Tetra-compatible equipment, Tetra being the emergency services communications system that Britain's blue light services are desperately trying to get rid of.

The MoD's three Zephyr-Ses are operational concept demonstrators and so won't be deployed to trouble spots. Instead MoD boffins will fly them around the UK and determine what they can be used for and how. One of their ambitions is to use two to demonstrate how a persistent surveillance capability can be “sustained indefinitely”, with the drones swapping over every so often.

The Zephyr-S started life with British defence technology firm Qinetiq before the programme was sold to Airbus in March 2013. The aircraft are designed and built in Surrey. ®