SPG-55B Fire Control Radar

How Many 55B Radars Did A Leahy Class DLG Initially Have?

LT Dean F. Dunlop, who served on the mighty USS England from Dec 1983 to April of 1988 wrote with a solution as to why the CG model kits came with only one 55B radar each fore and aft. He worked in Forward Missile radar otherwise know as Forward Control. He says that the first six Leahy class cruisers where all built with just two 55B, one on each end. LT Dunlop goes on to say, "This story was told to me from a guy that commissioned the England in the early 60's. I met this person from a port visit at the Portland Rose festival. Sperry the original makers of the radar had a strike during the building of the Leahy and the Yarnell DLG/CG-16 & 17. Because the production of the Radar's were halted and only 16 radar's existed the Navy had to decide to either put one radar per end or two radar's on just one end. Since the Navy wasn't sure how long this strike would be or how long it would take to populate the rest of the Radar's they put one Radar per end. I don't know if that was the reason for your model only have one per end but it seems to fit with the story that the guy gave me one Oregon afternoon." Further proof of there being a time when some of these ship had only one 55B on each end is given in this picture of the USS Worden CG-18 below.

USS Worden CG-18 showing only two 55B Radars - one forward and one aft!

Info supplied by Jerry Gay (Source: http://www.ussengland.net/cg22/newsletters/june_2001.htm)

And How Did The SPG-55B Radars Function?



   Click on image at left

Flight sequence of Terrier beamriding missile (antiaircraft mode). System reset is where computer automatically adjusts system if manual reset has not been performed.

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Flight sequence of Terrier/Standard semiactive homing missile (antiaircraft mode). After target acquisition, missile follows `up-and-over' trajectory while maintaining constant collision angle. Computer automatically resets system if manual reset has not been performed.


Terrier C-band monopulse guidance radar, the successor to SPQ-5. It is distinguishable from the latter by its far shorter length, the pointed radome of SPG-55 revealing its Cassegrain feed system, in which power fed from the rear of the antenna is reflected off a secondary antenna in front of the main reflector. Such a system blocks less of the antenna than does a more conventional feed. SPG-55 must control not only beamriding Terriers, in particular at present the nuclear Terrier BT(N), but also the newer semiactive types.

A typical SPG-55 consists of a main antenna with a capture antenna for CCM operation and a Custer horn antenna above the doppler antenna. In beamriding operation, the capture antenna conically scans by means of a nutating feed. Tracking is either pulse, via the main antenna, or CW doppler; there is also a special low-elevation track mode. In a heavy jamming environment the system can operate passively, using the jammer as a beacon (Passive Angle Track mode); it can also accept external range data, for example from NTDS. A Local Range Track mode is used when the operator can distinguish the target through the jamming but cannot achieve range location; he manually tracks the target by operating a range rate control, and the computer of the fire control system maintains the range rate at the value last entered before Local Range Track was initiated. Finally, there is a Coast mode in which tracking is given up, but values of range and range rate are projected from the last pre-jamming figures.

Doppler tracking employs the same CW illuminator signals which are used to control semiactive Terriers in flight; the reflected signal is received by the doppler antenna, and a shield between it and the main antenna minimizes spillover from the transmitter. Maximum range in this mode is reportedly 200,OOOyds. In beam-rider operation, the broad capture beam nutates about an axis close to that of the main radar, to gain control of the missile within about 4 seconds; it steers the missile into the 1.6° main beam, which nutates about 30 cycles per second in sychronism with the capture beam, and the nutation axis of the guidance beam coincides with the track beam locked on the target. In semiactive operation, two X-band beams are radiated: a narrow beam, emitted at the point of the radome and focussed by the main reflector, illuminates the target for the missile seeker. The other side of the X-band feed emits into space, where it illuminates the rear of the missile to establish a rear reference signal for the guidance computer in the missile. The missile compares CW energy reflected by the target to this reference signal. Both of these modes are effective mainly at considerable ranges. However, the recent development of antiship missiles requires an auxiliary short-range system. In the case of current SPG-55s, this is provided by the Custer horn, a pillbox antenna (12° x 3° beam) intended to illuminate shortrange, low-elevation targets while at the same time providing the necessary rear reference signal. The Custer horn is used in Sector Scan Engagement and Continuous Boat Track modes of operation.

Peak power in C-band in reportedly 1MW, with alternative pulsewidths of 0.1 microsecond compressed from 12 microseconds; of 1 microsecond compressed from 13; and of 0.1 microsecond uncompressed, with a PRF of 427 and an instrumented range of 300,OOOyds. The CW illuminator reportedly has an average power of 5kW and a 0.8° x 0.8° beam. Prime contractors - Sperry and RCA.

Info supplied by Ken Deshaies
(Source: Naval Radar by Norman Friedman - Naval Institute Press 1981 - ISBN: 0870219677)