740,666. Potentiometric voltage measurements. BENDIX AVIATION CORPORATION. Dec. 2, 1953 [Dec. 10, 1952], No. 33528/53. Class 37. [Also in Group XL (c)] In a radar system having a first antenna scanning in one co-ordinate and a second antenna movable in the same co-ordinate and means associated with the antennµ generating first voltages of magnitudes representative of the positions thereof, together with a range mark signal generator modulating the beam of a co-ordinate and range display C.R. oscillograph to exhibit ranging marks, there is provided an indicator system to indicate on the display the position of the second antenna, wherein means responsive to the respective first antennµ positional voltages produce two second voltages having a substantially constant sum and varying between predetermined limits differentially in accordance with variations of the first voltages within predetermined limits of relative value thereof, and means responsive to the difference of the second voltages when less than a predetermined magnitude for generating a control signal which is combined in summation means with the range mark generator to modify the range mark display over a region representing the effective radiation of the second antenna in the plane of the co-ordinate scan. The directive elevation antenna 11 (Fig. 1) of a ground controlled approach radar system for guiding aircraft to a landing runway is scanned through an angle of elevation by elevation scanning means 12 and is adjusted in azimuth over a limited range by elevation servomechanism 13, while a directive azimuth antenna 14 is scanned in azimuth by azimuth scanning means 15 and adjusted in elevation over a limited range by azimuth servosystem 16. A slider 17 of a potentiometer 18 across a D.C. supply circuit of Œ 150 volts is mechanically ganged to the elevation scanner 12 while a slider 19 of a potentiometer 21 across the same supply is ganged to the azimuth servosystem 16. The voltages on the sliders representing the scanning position of the elevation antenna and the elevational position of the azimuth antenna are applied to the grids of similar amplifying triodes 22, 23 having equal plate resistors and a common cathode circuit comprising a variablybiased constant current tube 24, all supplied from the Œ 150 v. source; the anodes of triodes 22, 23 being respectively directly connected to a - 250 v. D.C. source over potentiometers 26, 27 whose sliders are adjustable to supply balanced signals to the grids of a pair of triode amplifiers 28, 29 having a common anode resistance connected to the + 150 v. supply and earthed cathodes. The signal at the common anodes is directly connected to the grid of a cathode follower triode 32 whose anode is supplied from a + 300 v. D.C. supply and whose cathode resistor is returned to an adjustable positive bias source, and the cathode voltage is directly coupled to the suppressor grid of a range-marking amplifier 36 whose control grid receives range marking impulses 37 and whose output is applied through a video mixing and gating circuit 38 for display on a C.R. oscillograph indicator 39. As the voltage of slider 17 moves positively with the elevation scan from a value at which triode 22 is cut off, the anode potential of the latter falls from value 41 to value 43 (Fig. 2), and due to the constant current in the common cathode circuit of triodes 22, 23, the anode potential of the latter rises from value 42 to value 44 (which are determined by the setting of slider 19 responsive to the azimuth servosystem). During these potential changes, the grid voltages of triodes 28, 29 respectively pass through saturation points 45, 48 and cutoff points 46, 47 so that over a region of the elevation scan both triodes 28, 29 are cut off and the common anode potential rises from normal. level 49 to a more positive level 51 in the form of a square pulse whose width is determined_ by the constant current through tube 24 controlled by its variable bias, and whose position in the elevation scan range is determined by the potentialat slider 19 controlled by the azimuth servosystem. The pulse is applied over cathode follower 32, which superimposes thereon a reference voltage determined by the variable cathode positive bias, to increase the gain of amplifier 36 whereby the intensity of the range marks applied to the C.R. oscillograph 39 is increased over the duration of the pulse to brighten the trace over a sector corresponding to values of the voltages representing the instantaneous scan angle of the elevation antenna and the elevation angle to which te azimuth antenna is set, falling within a predetermined range of proximity, irrespective'of the actual voltage values; the duration of the gating pulse depending on the angular location in elevation of the azimuth antenna. The range marks 57 (Fig. 4) appearing on the elevation display 55 of the cathode-ray oscillograph are intensified over sectors 58 as described and the range marks of the azimuth display 56 may be similarly intensified by a duplicate system responsive to the azimuth antenna scan and the elevation antenna position in azimuth; the intensified portions corresponding in position to the positions of the respective antennµ in space and in width to the effective width of the radiation beams at different range values.