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A Case for Replacing the RAF’s Sentinel R.1 Fleet with Additional P-8A Poseidon Aircraft

Justin Bronk
RUSI Defence Systems, 13 May 2020
Air Power and Technology, Equipment and Acquisitions, Martial Power Programme
The Sentinel R.1 Airborne Standoff Radar aircraft are superb and relatively efficient aircraft which provide critical airborne intelligence gathering services for joint commanders around the world. However, as a small and bespoke fleet they are expensive and vulnerable to cuts. A highly classified US Navy radar pod for the P-8 Poseidon could give the RAF a more efficient solution.

The RAF’s five Sentinel R.1s were designed to fulfil a similar role to the US Air Force’s much larger and older E-8 Joint STARS – ground moving target indicator (GMTI) and high-resolution synthetic aperture radar (SAR) mapping of areas of interest from standoff ranges. As such, the Sentinel carries three mission system operators and mounts a large active electronically scanned array (AESA) radar in a pod under the fuselage, as well as a wide array of communications equipment including a prominent SATCOM array in a dorsal blister. Thanks to their ability to provide all-weather wide-area surveillance, target detection, classification and tracking services the Sentinel fleet has been one of the most important RAF contributions to coalition operations around the world since their entry to service in 2008. Despite being marked for retirement as part of the 2010 Strategic Defence and Security Review, heavy demand for Sentinel taskings over Iraq, Libya, Mali and Syria, and excellent performance during all those campaigns have led to the fleet being repeatedly saved from retirement, often for a year or two at a time. Most recently in 2017 the fleet was cut from five aircraft to four but had its retirement date extended from 2018 to 2021.

Unfortunately, there have been several negative consequences of the multiple retirement date announcements and postponements for the Sentinel fleet. First and foremost, long-term investment requirements such as training simulator upgrades and base infrastructure components have been difficult to resource due to constantly looming official out-of-service dates. The aircraft themselves have been upgraded and overhauled as mission tempo and changing real-world missions required, but without a standardised and sustainable structure in place for the Sentinel ecosystem as a whole. Personnel issues have also resulted since training pipeline sizing requires stability and predictable demand levels in order to optimise. With the official out-of-service date for Sentinel looming through most of the 2010s, the need to maintain a high mission tempo has been coupled with a deficit of newly trained crews arriving from training, which has put additional strain on currently serving squadron personnel. These issues have not prevented the Sentinel force from delivering outstanding service on operations, but they have raised costs on what should be a relatively efficient aircraft type, reduced availability at times and made the fleet an attractive target for future cuts despite its utility.

The P-8A Poseidon is, on the face of it, a rather different type of aircraft. Heavily optimised for the maritime environment as befits its primary role as a Maritime Patrol Aircraft (MPA), it carries a range of sensors and weapons systems specifically for the detection, tracking and (if necessary) destruction of hostile submarines and surface vessels. However, it is also capable of mounting under-fuselage radar pods of a similar sort to that carried by the Sentinel. The US Navy previously developed the highly classified and reportedly extremely capable AN/APS-149 Littoral Surveillance Radar System (LSRS) for carriage by specialised units within its P-3C Orion MPA fleet. That radar was highly classified in part due to its ability to work well over maritime, littoral and overland environments. Traditional wide-area surveillance, GMTI and SAR radars have had to be specially adjusted for a primary environment. Since 2015, an evolved variant called the AN/APS-154 Advanced Airborne Sensor (AAS) has been flying on a small number of US Navy P-8As. Like the LSRS, the AAS is highly classified in terms of its capabilities but it is known to be a highly advanced wide-area surveillance AESA type array which can scan on both sides of the aircraft at once and extends down once in flight to provide a clear field of view past the underwing engine nacelles.

The P-8A is an expensive aircraft – the UK’s acquisition of nine aircraft, associated spares, support packages and infrastructure cost an estimated $3.2 billion (or about £2.6 billion). Whilst a significant proportion of that acquisition cost will be for fixed non-recurring costs, the taxpayer still ends up contributing around £288 million per aircraft which the RAF will eventually operate. However, the P-8A is undoubtedly the finest MPA in the world at present, is the backbone of US Navy anti-submarine warfare (ASW) capabilities which will ensure its sensors and mission system continue to be upgraded at the cutting edge of technology throughout its service life, and it is used by other close allies including Norway and Australia. The airframe is also efficient in terms of operating costs and performance, being derived from the modern and commercially highly successful 737-NG which helps keep spares and consumables costs down.

The UK is an island nation dependent on sea trade, with a submarine-based nuclear deterrent and aircraft carriers which must be defended to be credible, which is once again having to guard against regular incursions by modernised Russian submarines. One of the UK’s foremost NATO responsibilities during the Cold War was ASW operations alongside the US Navy to keep the Russian fleet from penetrating the Greenland–Iceland–UK Gap. With relations between Russia and NATO remaining adversarial, and the US under the Trump Administration loudly demanding that other NATO members do more to ensure European defence, ASW must be a priority for UK defence as a whole. The importance of uninterrupted sea lanes to the global economy, especially after the damage wrought by the coronavirus pandemic, also points to a high level of demand for fixed wing MPAs to perform surface-vessel related ISTAR taskings around the world for decades to come. Finally, the P-8A is also a potent signals intelligence gathering, communications and network relay, translation and coordination node which is central to the US Navy’s future vision for network-centric warfighting alongside allies such as Japan and Australia in the Indo-Pacific. For the RAF, the P-8A will offer significant network enabler capabilities for the joint force in many situations outside the already broad MPA mission set, and as such may be called upon to perform overland missions regularly. 

It is thus a fair assumption that the RAF’s P-8A Poseidon fleet will find itself heavily utilised, and with only nine aircraft there will be an inherent and inescapable trade-off between the capacity and crew proficiency for the core (and critical) ASW mission set, and other global surveillance and enabler missions. This brings us to the central point. If the UK can buy its way into the highly capable AAS radar programme, which is not a certainty but there are multiple historical precedents for, then the RAF could replace the remaining Sentinel R.1s one for one with additional P-8As equipped with the AAS pod. This would ensure that the Sentinel’s valuable wide-area surveillance, SAR mapping and GMTI capabilities are not lost during the cuts which are likely as the global economy contracts post coronavirus, when small bespoke fleets of enabler aircraft will be the juiciest targets. It would give the P-8A fleet a world-class standoff radar capability which would allow it to perform at least most of the Sentinel R.1’s ISTAR taskings overland as well as improve the capability of the MPA fleet to perform surface search and track taskings in the maritime and littoral domain. Dedicated mission system crews would likely be required for the AAS-dependent roles due to the complexities inherent in those missions and the core MPA tasks respectively. Nonetheless, a common airframe, training pipeline elements, support structures and so on would still deliver significant efficiencies and availability benefits for both the core MPA and AAS-equipped sub-fleets.

It would also give the RAF commonality with the US Navy on a system which sits at the heart of the latter’s naval integrated fire control – counter-air (NIFC-CA) construct for advanced networked warfighting, and provide an additional tool for enabling UK joint force integration at the tactical level. AESA radars such as the AAS are potentially extremely effective electronic warfare and high-bandwidth communications tools in addition to their role as sensors, and the AAS will be progressively upgraded by the US Navy for decades with such capabilities. Whilst the additional aircraft would be expensive to procure, the fixed costs for P-8A operations have already been paid by the UK, and additional aircraft will be significantly cheaper than those bought already. With the RAF’s E-3D Sentry AWACS fleet being replaced by the E-7A Wedgetail based on the same 737-NG airframe as the P-8, replacing Sentinel with additional P-8s with AAS would also allow the RAF to fully exploit the efficiencies of standardising most of its fixed-wing ISTAR fleets on one airframe. Finally, additional P-8As with AAS to replace the Sentinel fleet would add sorely needed resilience to the ASW capabilities of the UK as a whole, as aircraft equipped with the radar pod could also perform other MPA tasks if demand for overland missions is lower – and at the very least will reduce the otherwise almost inevitable tendency for the existing P-8 fleet to be drawn off its core mission by other commitments.

Justin Bronk

Research Fellow for Airpower, RUSI.

Banner Image: US Navy P-8 of VP-8 "The Fighting Tigers" equipped with the APS-154 Advanced Airborne Sensor.

Used with permission of photographer ‘Fred’, Twitter handle: @cvvhrn


Justin Bronk
Research Fellow, Airpower and Technology

Justin Bronk is a Research Fellow specialising in combat airpower and technology in the Military Sciences team at RUSI. He is also... read more

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