Patent GB-684088-A (GB)
Title: Improvements in distance measuring systems
IPC: G01S11/04
Filed: 1948-09-10
Abstract: 684,088. Radio navigation. OFFICE NATIONAL D'ETUDES ET DE RECHERCHES AERONAUTIQUES. Sept. 10, 1948 [Sept. 23, 1947; Sept. 27, 1947; Feb. 19, 1948], Nos. 23863/48, 23864/48 and 23865/48. Class 40 (vii). Relates to systems for determining at a mobile receiver the distance of the receiver from a radio beacon in which the receiver comprises two horizontally spaced non-directional aerials means being provided for determining the angle subtended at the beacon by the base line of the receiver aerials, the magnitude of this angle being inversely proportional to the required distance. According to the invention the beacon transmits a bearing dependent signal means being provided at the receiver for determining the difference between the bearings of the two receiver aerials from the beacon. Rotating-beam beacon systems. In one embodiment, Fig. 7 and Figs. 1-6, 8 and 9 (not shown), the beacon radiates a narrow beam rotating at constant speed and the required angle is proportional to the time separation between the signals received by the two aerials which is measured by deriving a short pulse from each sig, Fig. 3 (not shown), applying the pulses to a trigger circuit to give a pulse of duration equal to the separation of the applied pulses and applying the output pulses from the trigger circuit through a low-pass filter to give a D.C. voltage of average amplitude proportional to the duration which is applied to a meter calibrated directly to indicate the required distance, Fig. 4 (not shown). Alterna. tively the separation of the two pulses may be determined by a cathode-ray tube, Figs. 4 and 5 (not shown), whose horizontal time base is triggered by the first pulse, the second pulse being applied to produce a vertical deflection or both pulses may be applied to cause signal deflections of a trace. A separate receiver and pulse shaper may be connected to each aerial or a angle receiver and shaper may be switched from one aerial to the other under the control of the first received signal. In a modification, Fig. 6 (not shown), the first received pulse is delayed by a time t0 corresponding to the radial distance from the beacon of a predetermined point, e.g. a landing point for aircraft, so that the final indicator displays the radial distance of the craft from the predetermined point. In the above forms of the first embodiment, the receiver base line must be perpendicular to a radius vector from the beacon, thus necessitating a preliminary orientation of the craft and in order to avoid the necessity for such action two orthogonal base lines AB and DC, Fig. 7, may be employed having any arbitary orientation. As shown, the base lines subtend at the beacon O angles #e<SP>1</SP> and #<SP>2</SP> equal to # sin α and # cos α respectively where e is the angle subtended by a single base line perpendicular to OM and D.C. voltages proportional to #1 and #2 are applied to the crossed stator coils of a goniometer, Fig. 9 (not shown), having a constantly rotating rotor so that the amplitude of the A.C. voltage output from the rotor is proportional to e. If the base lines are of unequal length, Fig. 8 (not shown), the input to one of the goniometer stators must be multiplied by a correction factor. Path-difference beacon system, Fig. 13 and Figs. 10-12, 14 (not shown). In this embodiment the beacon comprises two spaced transmitter aerials F1 and F2 energized cophasally by an R.F. carrier whose frequency is cyclically varied in a saw-tooth manner so that the loci of constant beat frequency between the radiations from the two aerials are hyperbolµ 61 which for distances D greater than the aerial separation approximate to radii from O the mid-point of the aerial base line, the axis OY being the zero beat frequency locus. An aircraft 1 having spaced receiving aerials A and B follows the axis OY by maintaining the beat frequencies produced in separate detectors connected to aerials A and B equal and when this condition obtains a quantity proportional to the sum of the beat frequencies provides a measure of the angle subtended by the aerials A and B at O and thence of the distance D from O. Specification 22690/48 (as open to inspection under Sect. 91), is referred to.