A Casestudy in Managing Obsolescence
Managing obsolescence over many decades is rare in civilian climes, but an often-encountered reality in the defence and aerospace sector. Whilst many aircraft have in-service lives of several decades, ships can see service lengths of 50 years or more, including some of the systems they are equipped with. Though multiple upgrades and retrofits are often carried out, integrating the very latest that technology has to offer is not necessarily top of the military and defence contractor’s main agenda. Rather, the key requirements of maximum reliability and ruggedness ensure that proven solutions are the order of the day. And in certain cases, older technology has unforeseen benefits to offer, as the example of the Royal Navy’s Sea Dart illustrates.
Sea Dart is one of the Navy’s most reliable and effective missile delivery systems. Designed by Hawker Siddeley Dynamics in the early 1960s and first entering service in 1973, this British surface to air missile has now protected the Royal Navy’s fleet for five decades. When Sea Dart was designed, it wasn’t envisaged that it would be in service for so long. However, thanks to its successful performance during the Falklands Campaign in 1982 and subsequent use in the Gulf War of 1990/1, Sea Dart is due to remain in service until around 2015.
Within the defence community, Sea Dart is perhaps best-known for its ease of manoeuvrability, which is in part down to the way it navigates, and also down to the way in which it is propelled. It is what’s known as a two-stage missile, using a solid fuel booster for the first phase of flight and then a Ramjet for cruising.
Sea Dart’s simple navigation system is based on low frequency analogue modulation. In Sea Dart’s nose cone is an analogue interferometer that can be steered to point the missile body towards a target. Four microwave antennas that extend out of the nose cone are used to determine the target’s location. Sea Dart calculates the difference in distance of the target’s signal as applied to each of these antennas in order to work out the precise angle at which to point the interferometer dish. The dish then sends a command signal to turn the fins on the end of the missile body. A likely reason for Sea Dart’s many life-extensions is that being an older, analogue-based system, it has high immunity to electro-magnetic interference.
Sea Dart’s target identification and navigation system used a long range semi-active radar called the Type 965, later replaced by the Type 1022, which also provides information to the weapon control system. The system also uses a Type 996 or 992 3-D surveillance radar, whilst a Type 909 radar (which is specific to Sea Dart) is used for the illuminator and rear reference radar. The missile itself operates in a small window within J-Band (12.4 to 18 GHz).
Custom to COTS-based test
As with any critical system that needs to be maintained over the long term, managing the equipment used to test it is an important task in itself. When the original test system was configured, it was essentially a custom-designed solution, using very little commercial off-the-shelf (COTS) equipment. This was due to its ‘cutting edge ‘ technical requirements and the concern that COTS instrumentation would not necessarily be supported for as long as defence contractors might require.
Subsequently, GEC-Marconi – Sea Dart’s first test contractor – turned to Rohde & Schwarz to assist with providing an RF source for the missile’s test system. Indeed, Rohde & Schwarz has continued to support the Sea Dart test system throughout the missile’s lifetime, which has so far spanned three major upgrades. Its instrumentation has provided not just the intermediate frequency generation part of the system, but more recently, the final microwave (J-Band) signal output within the system as well.
When it comes to testing the missile, the test engineer needs to be able to generate signals that simulate those that the radar generates to control the missile and potentially, inject other signals that represent interferers. For Sea Dart, the RF Source test cabinet needs to provide two J-Band FM/AM modulated RF signals: one to simulate the rear signal, which is generated by the illuminating radar on a Type 42 Destroyer and one to simulate a front signal, which simulates the reflected signal returned from the target. Notably, as the previously mentioned change from one long-range radar system to another only really affected the weapon control system and the 909 hasn’t changed, there were no implications on the missile design itself, nor its test philosophy throughout the years.
As is the case with any RF system, sensitivity is limited by the phase noise characteristics of the system. Hence, one requirement from the outset for Sea Dart’s transmitter was low phase noise. Bryan Cooper, Engineering Group Lead within SELEX Galileo’s Support & Test Services division – the organisation that now supports Sea Dart’s test systems – explains why this was the case: “High levels of phase noise would effectively raise the noise floor for other signals close to carrier. Although the 909 only transmits a single frequency carrier-wave, the target is effectively a close to carrier sideband (separated only by Doppler difference). In order to achieve Sea Dart’s high Sum channel sensitivity, it is important that the illuminator does not degrade the noise level and hence reduce the front threshold. There are many other problems associated with phase noise, but my opinion is that the effect on front threshold was the uppermost concern for the designers back in the 60s, such that the level of the target return would not be below the noise level and hence masked by the test equipment.”
From the test equipment perspective, the signal generator for the Sea Dart ATE needs to have very low phase noise characteristics too. This is an important requirement when multiple low frequency external modulations are applied to the generator, as well as when there is a phase difference between generators, as is the case here.
The original test system used special-to-type (STT) generators designed by a third party. These were specific to Sea Dart but, when a second series of RF Source was introduced in the mid 80s, it was appreciated that Rohde & Schwarz’s SMK, a fully programmable AM-FM synthesized generator with a 10Hz-140MHz frequency range and very low phase noise performance, would be an effective replacement. Eleven systems, with two SMK’s required per system, were built and managed as part of a strategic spares capability programme.
Maintaining test continuity
During the 80s and 90s, with several company names changes, the team continued to support the test equipment, and in the mid 90s, with the SMK coming to the end of its life and various other components becoming more difficult to service, it was time for an upgrade. This time, Rohde & Schwarz’s SME02 was used, with two units needed per system again. Being a more broad band generator, the SME02 was slightly less capable than the SMK within the specific frequency range, so Rohde & Schwarz provided models that were selected on test in the factory to ensure maximum performance for this application. Notably, within a signal generator there are some 10,000 components, hence select on test was adopted to minimise the risk of variation between units..
Subsequently in 2008, SELEX Galileo’s Support and Test Services Group (known at the time as Test Solutions), which specialises in the development and delivery of test systems for military and civil applications, was commissioned to develop Series Three of the Sea Dart ATE RF Source, as part of a Through Life Support Contract. Two new Sea Dart RF Cabinets were required, enabling two of the existing cabinets to be released for spares. For this, Rohde & Schwarz supplied two SMF100A’s per system. Due to its superior phase noise performance and modulation characteristics, SELEX Galileo was able to use the SMF100A to effectively replace the entire IF/RF chain within the test system. The company also procured Rohde & Schwarz FSU26 spectrum analysers to calibrate the Series 3 RF Source and to help diagnose faults as part of its support facility. The FSU26 can emulate the command set of many existing and obsolete instruments and hence its selection minimised development time of the support solution.
Notably, SELEX Galileo’s Support & Test Services division, which supports test systems covering applications as far ranging as avionics, communications, surveillance equipment and underwater systems, also supports the Sea Skua UK short-range air-to-surface missile. To maintain a consistent approach to the design architecture, and adopt a common spares strategy, the company chose the same SMF100A signal generators that are used for Sea Dart for Sea Skua.
For a long-life program such as this, it is typical for the engineers responsible for the test system to review its composition annually, checking for any design and development changes that might be prompted by new regulations or standards. Other factors that might prompt a test upgrade include how the test system (or system under test) is performing in terms of safety, supportability, maintenance and repair.
When selecting an instrument supplier to support your test capability for a long-life program, there are several requirements to bear in mind. Firstly, you will need to extract a written support guarantee for the instrument/s in question – Rohde & Schwarz tends to guarantee to continue servicing parts until ten years after the last date of manufacture. Other considerations to bear in mind include the need for detailed technical specifications and a fast, accurate response to any technical queries you may have. A designated point of contact at your test system supplier is advisable, as is guaranteed advance notice of any decisions to discontinue support for products, to allow you to obtain spares or an alternative supplier of calibration. It is also wise to evaluate a suppliers’ longevity, as continuity of support is paramount in such situations.
Author profile:
Keith Randall is Sales Manager - Aerospace & Defence Group, Test & Measurement Division -
Rohde & Schwarz UK (www.rohde-schwarz.co.uk).
More information about SELEX Galileo can be found at www.selexgalileo.com.
