Patent GB-1069382-A (GB)
Title: A radar system for ships
IPC: G01S7/40
Filed: 1964-06-24
Abstract: 1,069,382. Ship's radar. BOFORS A.B. June 24, 1964 [June 28, 1963], No. 26190/64. Heading H4D. In a ship-borne radar system using a vertical fan beam (A), Fig. 1 (not shown), and comprising an expanded scale display 7, Fig. 2, of part of the area scanned by the antenna, bearing fluctuations in the display caused by lurching movements of the ship, are corrected for by obtaining and feeding to the radar display a signal proportional to the difference between the angle (x). Fig. 1, in the horizontal plane (H) between the bearing (A 1 ) to a target (M) in the beam (A) and visible on the display and the cant axis (O) about which the rotational plane (D) of the antenna is momentarily inclined relative to the horizontal plane, and the angle (#) in said rotational plane, between the antenna bearing (A 2 ) when receiving echoes from the target and said cant axis. Fig. illustrates the involved space geometry wherein (A) represents the antenna lobe which is perpendicular to the antenna plane (D) and intercepts the horizontal plane (H) in line (A 1 ), including the target (M), and the antenna plane (D) in line (A 2 ). Assuming that the antenna plane is fixed and parallel to the ship's deck, the fore and aft antenna direction is represented by line (L) having a pitch angle (#s) and the athwart antenna direction is represented by line (T) having a roll angle (#r). Line (L) when perpendicularly projected on plane (H) gives line (L 1 ), and line (T) similarly gives line (T,). The angle between the planes (H) and (D) is (#). The - bearing error caused by the lurching of the ship is # = α- # which can be shown to be equal to α - arctan (Cos # Tan α). This is in a form unsuitable for computation and the following approximate formula is used: # = [2# s .# r . Cos 2# + (# s <SP>2</SP> - # r <SP>2</SP>) Sim 2#], where (#) is the angle in the horizontal plane between line (A 1 ) . and line (T 1 ). Approximations to angles (# s ), (Or) and (#) will be used in practice. In the system of Fig. 2, echo signals from a transmitter/ receiver 3 are fed to a PP1 display 5, displaying the whole of the area scanned by antenna 1, and to a display 7 showing, on an enlarged scale; a section of the display 5 including a selected target M. The X and Y deflection-controls 12 are respectively fed with signals from resolver R 1  representing B Cos y and B Sin y where B is the scanning waveform shown, and y is the angle of the antenna with respect to north, the bearing of the ship being added to the angle of the antenna relative to the ship's fore and aft axis, by means of gyro compass 11 and differential gear D 1 . The target M appears on display 5 at a range l and a bearing 8. A potentiometer P 1  is controlled by handle 20 to produce a signal representing a range l, which is fed to a resolver R 3  having a rotor controlled by handle 15. Handle 15 is turned until the resolver produces two signals representing l Sin # and l Cos # which are fed to X and Y deflector terminals 24 to cause a circular marker, produced by generator 23 to surround the target M. Handle 15 also controls resolver R 2  which is fed with the output of resolver R 1'  such that it produces signals representing B Sin (γ - 8) and B Cos (y - 8). The former signal is fed to the X deflector terminal 19 of display 7 whilst the latter signal is fed to the Y deflector terminal together with the output of potentiometer P 1 . The deflection voltages and amplification of the deflector circuits, is such that only that part of the display 5 immediately surrounding target M is reproduced on display 7. If it were not for the correcting means of the invention lurching of the ship would cause the response of target M on display 7 to move in the X direction and make a reliable and constant bearing position of shaft 16 impossible to obtain. A gyro 29 is provided having an inner gimbal 30 rotating on an axis parallel to the athwart antenna direction when the antenna plane is horizontal, and an outer gimbal 31 rotating on an axis parallel to the fore and aft antenna direction. Generators G 2  and G 3  measure the rotation of the outer and inner gimbals respectively and produce signals respectively representing angles #r<SP>1</SP> and #s<SP>1</SP> which are good approximations of the roll and pitch angles #r and #s respectively. The approximate valve #<SP>1 </SP>used in lieu of angle # is the angle # represented by the rotation of shaft 16, converted from being with respect to north to being with respect to the athwart direction of the antenna. This is achieved by means of a differential gear D2 having one input shaft coupled to shaft 16 and the other coupled to servo motor S2 fed with the output of gyro compass 11. The signals representing #r<SP>1</SP>, #s<SP>1</SP> and #<SP>1</SP> are fed to a computer 28 together with a signal representing range l. The computer produces an output signal representing l.#<SP>1</SP>, which is fed to the X deflector terminal of display 7. The response of target M now remains stationary and the setting of shafts 16 and 21, representing the true bearing and range of the target may be fed to a weapons control system. In a modification the computer 28 produces a signal representing #<SP>1</SP> which is fed to potentiometer P 1  to produce the signal representing l.#<SP>1</SP>. If the antenna platform is semistabilized such that the antenna does not participate in the rolling motion of the ship, generator G2 may be omitted and the computer designed to compute the formula