RAF Sentinel Quick Look

ASTOR System Links

With the announcement of a single RAF Sentinel R Mk 1 (Number 5 (Army Cooperation) Squad­ron) being deployed to support the French operation in Mali I thought a quick look at what it is would useful.


The Sentinel R Mk1 has a fairly long history.

The Airborne Stand odd Radar (ASTOR) is very simply a well connected ground surveillance system that uses an advanced radar sensor to gather data that is analysed and sent to friendly forces for action.

Jane’s has a good description;

ASTOR is a ground surveillance system designed to provide information regarding the deployment and movement of enemy forces. It uses MTI and SAR technology to obtain high-resolution imagery of static features and to identify and track moving vehicles. It is based on a modified Global Express airframe carrying the radar, datalinks and DAS, which will transmit near-realtime imagery to a network of distributed ground stations. The ground stations will be deployed with the front line forces and will display, analyse and interpret the imagery

In the 1980’s the MoD formulated a requirement designed to track moving Warsaw Pact Panzers on their way to the English Channel, this was called the Corps Airborne Stand-Off Radar (CASTOR) programme. CASTOR was a medium altitude aircraft equipped with a Moving Target Indicator (MTI)/Synthetic Aperture Radar (SAR) system, tracking Russian panzers streaming through Germany on their way to the Channel.

This programme got as far as a technology demonstration of the CASTOR I on a Britten-Norman Islander aircraft with a Ferranti radar before being abandoned because basically, it wasn’t good enough with a the usual dash of inter service politics and budgetary concerns, although the basic concept was well proven.


CAPTOR was designed to provide advanced warning of Warsaw Pact vehicles prior to them crossing into Western Europe, think of it as an AWACS for tanks.

From the ashes of CAPTOR came ASTOR

The Airborne Stand-Off Radar (ASTOR) programme picked up where CASTOR left off continuing development using a modified Thorn EMI Searchwater radar. It also blended this with technology from the Raytheon’s HISAR radar, the very same as used on the Global Hawk UAV and U2 ASARS-2 improvement programme.

Through a technology demonstration the programme the system matured, the demonstration phase followed and despite several false starts Raytheon System won the contract in 1999.

You can read the full history at the Jane’s link above, pretty painful!

In Service was declared in late 2008.

To say it was a troubled project with a protracted development history would be an understatement of Nimrod proportion but any less than other major systems, arguably not.

Spyflight has a great read up on ASTOR, click here to read.

Sentinel is operated by 5 (Army Cooperation Squadron), comprising roughly 160 RAF and 140 Army personnel, from RAF Waddington, the ISTAR hub. There are also a small number of Royal Navy personnel but the majority of imagery analysis work is carried out equally by RAF and Army personnel.

The radar produces a near photo quality image in SAR mode and can track many moving targets in GMTI mode.

ASTOR Image Sample
ASTOR Image Sample
ASTOR Image Sample
ASTOR Image Sample

In addition to the aircraft, Sentinel also has an advanced training systema number of ground stations (6 ‘tactical’ and 2 ‘operational level’) and a support segment.

Both the TGS and OLGS are C130 transportable and modular, mounted in 20ft containers and carried on 6×6 Pinzgaur 718k Improved Medium Mobility vehicles with HGI trailer mounted generators

ASTOR Tactical Ground Station
ASTOR Tactical Ground Station

It can operate in two modes, on and off tether (real time or store and analyse later)

ASTOR System Links
ASTOR System Links

The cost of the complete programme was just under £1.1billion with the 5 air vehicles production cost at £76m each and the 8 ground systems having a production cost of 15m each (MoD figures)

What did we get for £1.1billion plus the cost of CASTOR and all the time spent on development, especially interesting as we are about to chuck it into the round filing cabinet?

By all accounts it is a very effective system, able to track targets, create high resolution imagery and carry out pattern/scene analysis.

The Bombardier Global Express XRS, the donor aircraft for Sentinel, has prodigious range and endurance, relatively high speed and altitude. These qualities were deemed essential for the synthetic aperture radar but I have read there is a performance penalty due to the weight of the systems.

Additional weight is possible but will require recertification and a mid life upgrade would have possibly seen an in flight refuelling probe fitted and a replacement of some of the communications, computing and display equipment which would have freed up weight. One of the potential upgrade options was fitting the Goodrich DB-110 optical sensor as fitted into the RAF’s highly regarded RAPTOR pods into the read of the canoe fairing in place of a data link antenna. This was actually part of the original project, the mission software even takes imagery direct from the sensor and the ground stations are designed to handle it.

There is room for growth but with the 2010 SDSR decision to withdraw it, none is now planned.

Without knowing the performance, which of course only a few people will, it’s hard to make any judgements but most commentators and publicly available information seems to point to it being ‘a bit special’

Further details of the system are here and here.

The 2010 SDSR announced the withdrawal of Sentinel


Bob Ainsworth (Coventry North East, Labour)

To ask the Secretary of State for Defence what plans his Department has made for the replacement of the capability currently provided by Sentinel R1 aircraft after 2015.


Peter Luff (Parliamentary Under Secretary of State (Defence Equipment, Support and Technology), Defence; Mid Worcestershire, Conservative)

The strategic defence and security review announced the decision to delete the Sentinel capability in 2015. The Ministry of Defence is developing plans to address the capability gap and expect to reach conclusions in the autumn. The plans are likely to involve the use of Watchkeeper, an unmanned air vehicle, and future systems such as the Crowsnest programme from 2016, and through the development of Scavenger, an unmanned air system.



Madeleine Moon (Bridgend, Labour)

To ask the Secretary of State for Defence which aircraft have been identified as potential replacements for the Sentinel R1; and if he will make a statement.


Peter Luff (Parliamentary Under Secretary of State (Defence Equipment, Support and Technology), Defence; Mid Worcestershire, Conservative)

There is currently no plan to replace the Sentinel R1. The withdrawal of this capability will be mitigated by utilising a number of other platforms and assets including unmanned air systems such as Watchkeeper.

The official withdrawal point was defined as ‘when it is no longer required to support operations in Afghanistan.

But since the SDSR there have been numerous reports of efforts ongoing to reverse the decision.

Sentinel was deployed on Operation ELLAMY during which it flew about 50 sorties, contributing hugely to the NATO targeting and surveillance effort.

Reportedly, the Sentinels output was the only one used for daily briefings to the NATO commander.

In the Defence Select Committee report on Operations in Libya it quoted Air Marshal Sir Christopher Harper, UK Military Representative to NATO.

It [Sentinel] played  a key and pivotal role in the operation. There is no question about that. This is a highly capable ISR [intelligence, surveillance and reconnaissance] platform that is able to detect movement on the ground with extraordinary high fidelity and provide that information in real time. Discussion with the air commander would indicate that he relied extremely heavily on its capability and on similar capabilities provided by other platforms. So, without that capability I do not think that we would have seen the rapid success that has been achieved.

Air Chief Marshal Sir Stephen Dalton was also quoted;

It [Sentinel] was fundamental. We were able to link up and securely pass information from the Sentinel aircraft providing the ground-mapping capability through the AWACS in E3 aeroplanes, through secure satellite comms, through data links to the Typhoon and from Typhoon to Tornado and onwards. All that was done. Without that combat ISTAR […] the ability to do something  about what you find on the ground at the same time—this would  undoubtedly have been a more complex operation. The technical capability is there, and it has proven itself to be combat ready and combat capable.

If the cost of the system as a whole could be reduced then it is entirely feasible that the MoD will reconsider the withdrawal decision but it is also a basic requirement that no future planned equipment could duplicate the ASTOR capability.

In order to see how this could be done it is important to realise that it is not an aircraft but a system and in that system are people.

Reduce the number of people to deliver the capability and the system becomes cheaper.

If it could be operated with a reduced ground element, as recent operational use seems to point to, a large cost element could be removed. This is a reaction to a change in doctrine and the ability to transmit back to the UK via satellite and then if necessary, onward, or back to the operational theatre over DII.

Its a different way of using the system that better reflects current thinking.

This ‘different way’ was revealed post ELLAMY when it became clear that a newly introduced portable integrated mission planner had been used for mission planning and post flight imagery analysis. Raytheon had self funded the addition of satellite and networking functionality to the new ISO container mounted mission planners including a range of COTS computing, network and display equipment. One of our commenters also stated that one of the data links had been replaced in favour of better satellite communications equipment.

This would seem to point to the TGS being somewhat surplus to requirements as the likelihood of them being forward deployed into the field in tactical conditions overtaken by technology.

Eliminate the TGS and you can perhaps eliminate a lot of cost of the system as a whole.

The ongoing support costs, bandwidth requirements and support cost of the aircraft themselves would also have to be considered as part of a re think of the SDSR decision, nothing is certain and the people costs I alluded to might not be able to be reduced that much anyway. Reducing or eliminating the TGS would not be without impacts, it would be a compromise of course.

So the deployment to Africa in support of Operation Serval looks very much like it will be another  argument in favour of retention, albeit in a slightly different deployment model.

If SDSR 2015 does confirm retention it might allow the RAPTOR sensor to be fitted, resulting in a very long range, high endurance ISTAR collection and analysis system with a range of ‘cross cueable’ sensors. The ability to link back to the UK and other in theatre receiving stations is crucial for ongoing development.

The ground station infrastructure, systems, training and equipment could also possible be integrated with Watchkeeper and other collection platforms to create an air deployable multi technology hub.

A bit of forward and joined up thinking could result in an improved capability and reduced cost.



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