The Type 901 guidance radar.
The Type 901 radar project started during the second world war as part of the projected 'Long Range System 1' (LRS1) gunnery control system. LRS1 was never completed but the radar was utilised for the Seaslug missile project. Operating between 9·0GHz and 9·8GHz, it first tracked an aircraft as early as November 1946. At that stage the system looked very different to the tapered drum of the operational system; instead it consisted of two offset feed dishes on a wartime 2pdr pom-pom mounting. One dish was for the tracking beam and the other dish was for the gathering/guiding beams. By mid 1951 experimental missiles were riding a stationary beam, and by the end of that year they were riding a jitter-free moving beam. By the spring of 1952 missiles had been gathered at 'a relatively high elevation' and had successfully ridden a beam in the presence of jitter.
Later trials confirmed the tracking and guidance beams had a 0·8° semi cone angle (at 6db) with a 13° control cone although it was felt that a 5° cone would be better. Minimum elevation was 2° with a smooth sea surface and 1·5° with waves of 6" RMS or more, and sea reflections would be reduced with a 5° gathering deflection. This upward gathering deflection was, in fact, subsequently adopted; further improvements to the aerial system reduced the minimum elevations to 1·25° for a smooth sea and 0·75° for 6" waves.
In 1960-61 exercises were carried out to establish the tracking discrimination of the type 901 radar. These proved that the radar required a separation of 20 minutes of arc (0·33°) between two targets to be able to track either of them without jitter; this was half the predicted value, and was later reduced to 18 minutes of arc. The radar could similarly fully discriminate between targets with a range separation of only 75 yards.
Initially the Type 901's acquisition process was manual as the target tracking cone was much narrower than either the Type 992 or
278 radars fitted to the County Class destroyers. The Missile Officer (MD) points to a target on the Target Indication Unit (TIU)
display. The TIU operator switches from 'standby' to 'slew' and uses his joystick to put a marker strobe onto the echo. When the echo
is strobed he switches to 'track' and reports 'tracking' and switches to auto-track when ready. Once tracking has started the Missile
Control Officer pushes his start search button and the search pattern starts. The TS operators (Aimer and Rangetaker) observe their
displays; when an echo is seen the Aimer switches to 'Hand' and then takes control of the Type 901 and repositions it so it is looking
at the target. When the target re-appears the Rangetaker moves his range strobe to align with the echo; both operators select 'auto range'
and 'auto aim', target acquisition is complete.
This procedure, which is clearly based on earlier anti-aircraft gunnery techniques, took about 20 to 30 seconds.
Since writing this I have been contacted by a former Aimer who stated that with practice the teams could achieve acquisition within
The Type 901 radar has three separate radar transmitters which require to be triggered in a definite sequence and at intervals related to the
instantaneous position of the conical scan. The sequence of pulses is referred to as the "502 pulse pattern".
The gathering antenna's spinner (which is referred to as the "master spinner" drives a disc, the circumference of which carries 25 equally
spaced narrow strips of mu-metal. A fixed magnet and pick-up coil are mounted near the periphery of the disc, and this arrangement is
referred to as the "magnetic trigger initiator" and produces 25 initiating pulses per revolution of the conical scan -ie at a fixed repetition
rate of (25x4000)/60 =1667 pulses per second. One of these pulses is arranged to occur exactly at "top dead centre" of the conical scan, and
is made a double pulse so that it can be identified.
The pulse generator uses this to generate two pulses from the gathering antenna with a separation of 2, 4 or 6 microseconds followed by an
equally spaced pair from the guidance antenna, the complete group of four pulses being sinusoidally time modulated at a frequency of 66 2/3
Hertz with a maximum time excursion of ·75 microseconds. This is "pulse position modulation" analogous to phase modulation in ordinary
communication practice. It results in the "instantaneous repetition rate" of the gathering-guiding pulse group varying in accordance with
the position of the conical scan; in this way information is transmitted to the missile which, in effect, enables the position of the conical
scan to be reproduced in the missile resolving system.
The Seaslug system was required to be operable in a wide range of climatic environments; this included Arctic conditions with
the risk of ice forming on the surface of the Type 901 radar, which would severely affect its performance. Direct electric
heating of the of the face of the radame was considered and rejected, as was indirect hot-air heating; the latter would have
a peak (ten minute) load of 200kW! In the end a far more ingenious solution was selected, using the latent heat of fusion.
Many chemical reactions are exothermic, and one is the dissolving of ammonia gas into water. The gas is piped to the periphery
of the radome and then through micro channels between the main surface and a porous top layer through which the ammonia could
readily percolate. On contact with the ice it would start to dissolve and liberate heat. Tests on a model in an icing chamber
showed that the ice on the radome started to melt after 25 seconds, and within five seconds of this the ice slid off in a single
There was a fear that when the de-icing operations ceased, water would travel back into the channels as the residual ammonia
dissolved and might later freeze. To prevent this, warm dry air was blown into the system for a few minutes after de-icing operations
to ensure no ammonia was left in the channels, and there was also a thermostatically controlled electric heating element around
the periphery of the radome.
Each guidance group consists of 5 pulses and the gathering group consists of 3 pulses; they both have a
38 µS duration.
These pulses are time modulated according to the formula: T+m(sin wt) where T is the mean time interval
between pulses, for a spinner speed of 4000 rpm and 24 pulses per scan cycle this is equal to 625 µS.
m is the modulation depth and is 3% of the mean time interval, ie 18·75 µS.
wt is the angle turned through by the spinner flare...and may be shown to equal 72 µS.
The tracking pulse is similar but a form of random pulse modulation is called for to prevent the enemy
using a deception pulse type countermeasure system. The depth of this is limited to +/- 12·5 µS about
the mean position of the tracking pulse.
Sources: ADM 220/196, /342, /355, /549, /957, /960, /1925, /1996, /2062, /2345, /2353 & AVIA 6/18743. The National Archives, Kew, TW9 4DU.
This page copyright SR Jenkins January 2014; reproduction without prior approval is prohibited.
Page last updated: 23rd January 2019.
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