Patent GB-1366726-A (GB)
IPC: G01S3/14
Filed: 1972-04-27
Abstract: 1366726 Radio direction-finding INTERNATIONAL STANDARD ELECTRIC CORP 27 April 1972 [7 May 1971] 19630/72 Heading H4D An automatic direction finding system comprises two angularly disposed loop aerials, a sense aerial, two amplifiers, bearing evaluation means, and timing means defining a recurring series of operational periods. Each amplifier is normally connected to a respective loop aerial, and provides output to the bearing evaluation means. However, during a sense determination part of each operational period, the sense aerial is connected to one of the amplifiers in place of its associated loop aerial, when quadrant information for determining a bearing angle unambiguously is obtained on the basis of vectorial combination of the signals from the sense aerial and the other loop aerial. The value of the bearing angle is measured during a bearing part of each operational period, and is obtained either with respect to a main reference direction, or with respect to an auxiliary reference direction. As described, a crossed loop aerial system is provided in which one loop Y, Fig. 3, is in a fore-to-aft plane of a ship, and the other loop X is in a port-to-starboard plane. The bearing of a signal is measured with respect to the fore direction when its angular displacement α therefrom, in a clockwise sense, lies between 45 and 135 degrees and between 225 and 315 degrees. Otherwise, said bearing is measured with respect to the port direction, i.e., # = α+90 degrees (Fig. 7, not shown). A vertical whip type sense aerial V, Fig. 4, is provided, which is connected through a phasing circuit 20 to the 0 contacts of relays 21, 22. The circuit 20 brings the signal s v  from the aerial V substantially into phase with the signals s y , s x  from the aerials Y, X when the bearing of the source is in the first quadrant (α = 0 to 90 degrees). The aerials Y, X are normally connected through the 1 contacts of respective relays 21, 22 to the inputs of respective amplifiers 6, 7. The amplifier 6 has two phase-opposed outputs S y , -S y  (with input s y ), and the amplifier 7 has two pairs of phase-opposed outputs S x , -S x  and jS x , -jS x  (with input s x ) which are mutually in quadrature. Vectorial adders 23, 24, 25 with respective limiters 26, 27 28 produce from the above outputs the combinations S y +jS x , S y -jS x  and -S y -jS x . The phase angle between the first two is representative of the bearing angle α, and the phase angle between the last two is representative of the bearing angle #. Flip-flops 31, 32 have respective inputs from the said pairs of combinations, and change their states according to the zero crossings thereof. One or other of the flip-flops is selected by a switch 30, and controls a gate 34, which opens to admit an appropriate number of pulses from a source 35 to an adder 36. Pulses from a sense storing circuit 10, in number according to the quadrant in which the signal source is located, are directed to the adder 36 through a gate 37, and the output of said adder, which is representative of an unambiguous bearing, is displayed on a digital device 12. The bearing is also shown on a cathode-ray tube (CRT) 8, which receives outputs S y , S x  from the amplifiers 6, 7. Sequence of operations and sense determination.-A timing unit sets up the following sequence during each successive operating period: Adjusting (U): the gain and phase of each amplifier 6, 7 are adjusted Comparison (C): the amplitudes of the signals S y , S x  from the loops are compared in a circuit 5 Sense determination (S): the selector switches 29, 30 are operated in accordance with the result of the above comparison. If S y  is greater than S x  the normally neutral switch 29 moves to the 0 position, and the relay 22 is operated, so that the signal s y  is substituted for the signal s x . The signals from the limiters 26, 27 may now be represented as S y +jS v , S y -jS v , and they operate the sense control switch 33 appropriately. The selector switch 30 is also set up, in this instance to the 0-position. At the end of the (S) period the switch 29 returns to the neutral position. Had S x  been greater than S y , the switch 29 would have been operated to the 1 position, so that the relay 21 was operated, and the signal s v  substituted for the signal s y  Bearing (B): bearing information is obtained from the signals s y , s x  of the loops X, Y and an ambiguous result is presented to the adder 36 either relative to the fore direction (switch 30 in the 1 position) or relative to the port direction (switch 30 in the 0 position) Adding (A): sense (quadrant) information from the storing circuit 10 is added to the ambiguous result in dependence upon the settings of the switches 30, 33 Display (D): correct bearing information is transferred to the digital display device 12, and remains until the next display period. The CRT 8 does not receive signals during the periods (U), (C), (S), but afterglow ensures a continued display. A signal under control of the switches 30, 33 and a pulse shaping circuit 38 blanks out the CRT 8 except when the correct radius vector is to be dispalyed. Reference is made to Specification 1,365,703.