Patent GB-684089-A (GB)
Title: System of radio-navigation
Filed: 1948-09-22
Abstract: 684,089. Radio navigation. OFFICE NATIONAL D'ETUDES ET DE RECHERCHES AERONAUTIQUES. Sept. 22, 1948 [Sept. 25, 1947; April 17, 1948], Nos. 24842/48 and 24843/48. Class 40 (vii). In a radio navigation system of the type in which patterns of equi-signal contours are created in space, the contours are created by synchronized radiations from spaced transmission points and spaced receivers, usually mounted on the craft to be navigated, are used to provide outputs from which a position determination may be obtained. In a preferred form of the invention the spaced transmitters F1 and F2, which may be placed so that the perpendicular bisector of the line joining them lies along the runway 2 of an airport, radiate synchronously frequency modulated carrier waves so that hyperbolic contours 3 and 5 of equal received frequency difference (beat frequency between the radiations from F1 and F2) are created. An aircraft 1 obtaining navigational information is provided with two spaced receiving antennse A and B situated at the wing tips feeding receivers which indicate the beat frequencies of the signals received on the two antennae. The outputs of the two receivers are also combined in two further indicators showing the sum and difference of the beat frequencies, Fig. 7 (not shown). The beat frequencies are equivalent to path difference values and where 2a=/AF2 - AF1/ and 2b=/BF2-BF1/ 2a and 2b are directly indicated by the separate beat frequency meters. Thus the readings of the sum and difference indicators are given by: 2/a+b/=/AF2-AF1/+/BF2 - BF1/ and 2/a-b/=/AF2-AF1/-/BF2-BF1/ Computation from these equations is dependent on the sign of the respective terms and two conditions arise: The first in which A and B are on the same side of y the perpendicular bisector of F1, F2 in which case 2/a+b/1=/AF2+BF2/ - AF1+BF1/ and 2/a-b/1=/AF2-BF2/-/AF1-BF1/ and the second where A and B are on opposite sides of y when 2/a+b/2=/AF2 - BF2/z-z/AF1 - BF1/ and 2/a-b/2=/AF2+BF2/-/AF1+BF1/ The suffixes are used to indicate which condition is which. Now denoting the midpoint of AB by M and regarding the distance AB as small compared with the distance between the craft and either transmitter: Thus inside the central region where A and B are on opposite sides of y the /a - b/ indicator, i.e. the difference of the beat frequencies indicator, will give an indication which when maintained constant will enable the craft to fly on a hyperbolic course line such as 24, passing within a distance d/2 of the centre of the line F1, F2. Outside the central region the sum indicator will give an indication which corresponds to one of a series of hyperbolµ 21 having foci F1 and F2 and by maintaining the indication constant the craft may be flown to a point on the line F1, F2 along such a hyperbola. It should be noted that consideration of which is the smaller of the readings on the separate path difference indicators enables the craft to be steered to the central region for landing down this region on to the runway. To fly the craft directly along the linefy it should be flown so that (after the central region has been reached) the difference indicator (a - b) reads as near zero as possible. If the craft is being flown along one of the hyperbolic position lines 21 or 24 AB is a tangent to an ellipse with foci F1 and F2 and AMF1 = BMF2 so that: Thus outside the central region the /a - b/ indicator values correspond to arcs of circles 23 standing on F1, F2 as a chord and the readings of the difference indicator and sum indicator respectively determine an arc of a circle and a hyperbola whose intersection defines the craft's position. Within the central region the arcs of a circle 22 are defined by the /a+b/ or sum indicator and the hyperbola (24) by the difference indicator to give a similar position determination. The sum and difference indicators may be connected alternatively to two further meters, changeover being effected when proceeding into or out of the central region so that one meter alway refers to arcs of circles and the other to hyperbolic position lines. In the central region the sum indicator will show to a reasonable degree of accuracy the craft's distance from the mid-point of the line F1, F2. The transmitters. A circularly sealed grounded grid triode in a coaxial transmission line may be used to feed dipoles at the transmission points F1 and F2 through coaxial feeders. The wavelength used may be 50 centimetres and the frequency modulation variation 40 cycles with a period of a hundredth of a second. The frequency of the radiations transmitted by one of the transmitting aerials may be made smaller than that of the radiations from the other by a fixed amount so that there is a finite frequency difference along the perpendicular bisector direction y. This may be done either by using aerial feeders of different lengths or by feeding one aerial directly and the other through a system combining the fre. quency modulated wave with a fixed frequency and separating out by filtering the required wave.