Seaslug firing modes.


Line Of Sight Beam Riding

The standard method of firing against aircraft. The beam tracks the target and the missile rides up the beam.

The following modes were only available to the GWS2 system:


Constant Angle of Sight with Terminal Dive.

The firing method used against low-level targets. The missile is fired at a fixed angle of elevation -usually half a degree. At the appropriate time the 'dive' command is transmitted and the missile enters a 45° dive This technique may seem to involve a bit of wishful thinking but I have known a Stilletto target travelling at 80 feet to be physically hit by a Seaslug fired in this mode. The range staff were most upset to lose the target drone, cameras and all.


Up and Over is a surface-to-surface mode, the Type 901 radar follows the Type 903 (of the MRS3) in bearing and the 'Up and Over' computer uses the range data to control the elevation of the beam. The Type 903s fitted to the County Class had an increased range scale of 40,000 yards from the standard 30,000. The missile is fired at a high elevation and then depressed at a rate calculated to intercept the target, when the beam is depressed to 0.5° the 'Glide' command is sent and the final approach is made with the missile autopilot demanding zero acceleration in the pitch plane. The missile strikes the target in a steep dive; as the fuze is not activated it cannot detonate the warhead but the impact may cause a full or partial detonation.
The advantage of this firing mode is the higher missile speed and longer range as part of the missile's flight is at a high altitude and thus thinner air.


MIdcourse Constant Angle of sight With Beam Riding

If a closing target is flying at between 500 and 800 feet altitude at the moment of firing, it will be above the sea reflection zone (0·5°) at normal interception range. In this instance, although the engagement is started in the constant angle of sight mode, the system will automatically change to the line of sight mode beam riding when the angle of sight rises above 0·5°. If the target dives below 500 feet after the mode has changed, line of sight beam riding is continued.

Sea Skimmer

This mode was suggested by Hawker Siddeley Dynamics as an alternative to CASWTD mode for use against surface targets. The maximum range was the same as CASWTD at 30,500 yards but the minimum range was reduced from 12,000 yards to 6,000 yards. It required a radio altimeter to be fitted, either the Honeywell AN/APN-198 or the HSD FM/CW. Like CASWTD the missile was fired with the Type 901 at its minimum elevation (0.5°); once it had been gathered and was riding the beam satisfactorily the 'Glide' command would have been transmitted and the missile autopilot would follow the beam in azimuth only, and use the on-board radio altimeter to control its altitude turning Seaslug into a supersonic sea-skimmer. Again the fuze would not be activated, but a physical hit would destroy a fast patrol boat and seriously damage anything bigger.
Actual maximum range would have been limited by the TV height-finding system, which needed a clear view of both the target and the horizon. Unlike CASWTD and UAO modes it would have permitted salvo firing, and the missiles could be preset to one of a number of different altitudes.
I have not yet found any evidence of this mode being used and I don't remember any reference to altimeters in the Seaslug components lists, there are just tantalising hints to this mode in some of the surviving documentation. The HSD FM/CW radio altimeter was built, it was fitted to the RN Phantoms as part of the modifications to enable them to be used on the British carriers.

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Last updated: 26th March 2019.