Replacing Watchkeeper: Staying Behind the Curve

The British Army is reviewing proposals for a reconnaissance drone to provide ‘Land Tactical Deep Find’ capability with 24-hour endurance under project Corvus. As is found with newer platforms such as Elbit’s 650 Spark or DZYNE Technologies ULTRA, 24 hour endurance, with related sensor payloads, can only realistically be achieved by a UAV in the NATO Class 3 Medium-Altitude Long Endurance (MALE) category.

The decision to acquire such a drone is inconsistent with both the MOD’s assessment of the threat environment and data from contemporary conflicts. Corvus will replace the troubled Watchkeeper drone program which took 14 years to reach full operational capability and was declared obsolete last year by Minister of State Lord Coaker, who argued that lessons from Ukraine showed that MALE UAVs were not survivable. This is in line with a 2020 assessment by the US Air Force which concluded that this class of drones cannot survive in contested airspace.

The vulnerability of such UAVs is currently being demonstrated in Yemen. The Market Engagement Notice for Project Corvus comes after at least 15 US MQ-9 Reaper drones have been shot down over Yemen since late 2023, a toll that spiked after the Trump administration’s renewed air campaign on 15 March 2025. Losses, according to the Houthis, could be as high as 22 airframes, although the group often makes false claims. This is a strong indicator that Corvus is not a sound investment. But that doesn’t mean the Army can do without a means of finding targets.

The deep battle

Replacing Watchkeeper’s capability for the Deep Find is an urgent requirement for the British Army. Ukraine has once again shown the importance of striking enemy logistics over 50 km behind the front line. Although Russia has reduced its losses to HIMARS and ATACMS by dispersing its enablers, this has permanently reduced the efficiency of Russian forces.

Russia’s adaptations also reinforce the need for persistent surveillance to find concealed and separated targets. In Ukraine this has largely been provided through US satellite reconnaissance and private geospatial imagery providers such as Maxar. But satellite reconnaissance has significant latency, whereas live observation offers the opportunity to engage more dynamic targets.

The UK and European allies lack comparable persistent space-based ISTAR to the US. Until this is rectified, the British Army may struggle to achieve the tight deep-battle targeting cycles needed for peer conflict, even as the Army invests heavily in longer range weapons systems like the M270 that need low-latency target identification to be effective.

MALE drones can provide persistent surveillance, including through clouds with Synthetic Aperture Radar (SAR), but only if they can survive. And as that survivability is now highly questionable, it seems that the UK must look for alternative approaches.

The Lack of Permissive Airspace

The Houthis have a long history of rapid tactical innovation alongside their Iranian advisors, and clearly have capable air defences. This was illustrated in 2018 when a Saudi Arabian F-15S, tracked by the Houthis with a FLIR system, deployed flares and accelerated before being hit by a surface to air missile (SAM).

Several US Reapers were lost to the group before the current conflict, including one shot down in 2019 which CENTCOM assessed was engaged with Soviet-era SA-6 and SA-7 SAMs.

More recently, the loitering 358 Saqr Iranian SAM used by the Houthis, Iran-backed militias in Iraq and Hezbollah, have had far more success. Israeli losses of Hermes MALE drones over Lebanon including the 450 model that the Watchkeeper was based on, and the larger 900 model, are more evidence that MALE drones are at acute risk. And yet, the Houthis cannot be considered a peer adversary.

Evidence of MALE drone performance in peer conflict is demonstrated by the near total destruction of Ukraine’s Bayraktar TB2 fleet after a few days of successful strikes at the beginning of Russia’s full-scale invasion of Ukraine, with one Ukrainian pilot calling the drones ‘almost useless’ against Russia’s layered air defences.

The point demonstrated here is that even in the airspace above non-state actors MALE drones lack survivability. But the British Army wants to use such a platform as a Corps reconnaissance asset, and therefore suggests it is to be used in peer-warfighting.

Cost Versus Reward

There is a strong argument to be made that so long as the cost of a UAV can be brought down to the point where it is attritable then it could be used even if the enemy can theoretically knock it down. The key metric is whether it is cheaper than what is destroying it.

Reapers are often said to cost $30 million, varying significantly depending on the configuration. If Watchkeeper is a guide, the sustainment of these systems can significantly raise initial cost estimates. That said, drone technology is getting cheaper. Wide Area Motion Imagery (WAMI) sensors are one example. They process imagery from hundreds of CMOS sensors - equivalent to several hundred smartphone cameras and enable ‘super resolution’ video of towns. In the early 2010s, WAMI sensors on Reapers produced 2 gigapixel imagery and the latest Gorgon Stare systems using AI image processing can generate 8.1 gigapixel imagery. There are now compact WAMI sensors and depending on the required resolution, these can weigh just a few pounds.

Falling cost of computer vision is creating opportunities for navigation in GNSS (satnav)-denied environments, fusing imagery with radar, on relatively cheap UAVs. This is a Corvus requirement.

The US Air Force’s ULTRA drone, for example, has a claimed endurance of 80 hours and a claimed price of $8.75 million, based on a civilian sports glider. However, at equivalent losses to Reapers in Yemen, the loss of 15 ULTRAs would cost $131 million, close to the MOD initial offering to fund Corvus (£130 million/$173 million). Assessments in Ukraine would suggest that the point at which a UAV becomes attritable is a unit price below $200,000 for ISR.

While a lower cost UAV could be a solution, therefore, it seems that this would require the British Army to pursue a price point at the expense of capability on the platform. The requirements for Corvus suggest that there is a desire for it to be cheaper than Watchkeeper, but still far higher than the price point where it could be considerable attritable.

Alternative Approaches

The trajectory among peers suggests that the UK will need to be able to find targets against capabilities that exceed those currently fielded by Russia. PLA Active Electronically Steered Array (AESA) systems such as the PLA’s JYL-1 could readily proliferate, while armies around the world are actively investing in systems for countering UAVs.

The good news is we are already building alternative capabilities to MALE drones that can replicate some of their capabilities. There is however, little margin for error when choosing capabilities that may seem attritable to the US (the ULTRA drone) but may not be affordable in numbers for the UK.

Alternative approaches for the UK could include investing in low-earth orbit satellites, and in the near term, sourcing affordable options such as high-altitude balloons in the stratosphere (a current US project) or tethered aerostats (available through UK providers), which support an array of sensors providing surveillance over hundreds of kilometres. There is also the prospect of massively capable stand-off sensing using technologies like quantum sensors, where the UK has significant sovereign capability. What is clear, however, is that the British Army cannot simply replace Watchkeeper on what is close to a like-for-like basis and expect to remain competitive.

© RUSI, 2025.

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