588,763. Radiolocation. BOHM, O. M., and WRIGHT, C. S. June 12, 1944, No. 11250. [Class 40 (v)] A system for determining the elevation of a distant object at a low elevation angle which makes use of rays reflected from the surface of the earth, the beam being wider in the vertical plane than the angle of elevation, comprises means in which a pulsed beam of electromagnetic or acoustic radiation is transmitted, and echo signals reflected by the object are received, by aerial systems, or equivalent acoustic means, at the same height having two different polar diagrams. The strength of the echo signal received with an aerial having a first polar diagram, produced by transmission with the same diagram can be compared (a) with the echo signal strength received with an aerial having a second diagram, produced by the first transmission, or (b) with the echo signal strength received with an aerial having a second diagram, produced by transmission with the second diagram, or (c) with the echo signal strength received with an aerial having the first diagram, produced by transmission with the second diagram. As the aerial heights are the same, the phase difference between the direct and ground reflected rays will be the same for the two diagrams so the interference patterns will be exactly similar. For a symmetrical vertical-polar diagram such as I, Fig. 2, which is broad compared with the angle between the direct and ground reflected ray, the received signal strength E1 produced by transmission over the same diagram is (M/D)<2> sin<2> #, where M is the maximum amplitude of the diagram, D the distance of the object and 2# the phase difference between waves traversing the direct and indirect paths. If reception of the same transmission is by asymmetrical diagram II then the received Mk# signal strength is E 12 Mk# /D<2> sin # cos #, # being the angle of elevation and k the gradient of the diagram. For transmission and reception using diagram II, the echo signal strength is E2 =-(k#/D)<2> cos<2> #. By taking readings of the values of two of these signal strengths at times when they are a maximum (sin #=Œ1, sin # cos # = Œ ¢, cos # = Œ< >1) the elevation angle # can be found as M and k are constants which can be determined. Polar diagrams I and II may be obtained by feeding a parabolic mirror from two dipoles or waveguides arranged one above the other and connected in phase or in antiphase. If the amplitude of diagram I and the gradient of diagram II are not constant over the angle between the direct and indirect rays as is assumed above correction curves can be applied. Owing to the time interval between moments when the distant object passes through positions providing suitable maxima, fading effects may cause considerable inaccuracy, so, in a second and preferred system the target is observed simultaneously, or nearly so, using two symmetrical diagrams of different shape. It is stated that asymmetrical diagrams could be used, but are not so satisfactory owing to the weaker echo signal strengths involved. Using symmetrical diagrams such as shown at " a " and " b," Fig. 3, and taking measurements simultaneously, the echo signal strengths corresponding to the three methods of operation will all be proportional to different functions of the elevation angle, the nature of the functions being dependent on the shape of the polar diagrams. It follows that if two signal strength readings are taken simultaneously, the elevation angle can be determined, if they are taken using identical receivers and with a variable control device in the system feeding the broader diagram, the elevation angle may be directly read off from the control device when the two outputs are adjusted to be equal. With suitable switching arrangements indicated in Figs. 1 to 7 of the Complete Specification .(not shown), a single transmitter and receiver may be used, the second signal strength reading being taken as soon after the first as possible. Two similarly fed mirrors of different shape or different methods of feeding the same or two similar mirrors are suitable means of producing the different diagrams. The height of the object can be found from the elevation angle if the distance to it is determined by normal radar technique.