Patent GB-747146-A (GB)
Title: Tracking system for radio position locator
IPC: G01S13/70
Filed: 1954-04-12
Abstract: 747,146. Radar. HAZELTINE CORPORATION. April 12, 1954 [May 13, 1953], No. 10667/54. Class 40 (7). A pulse radar system includes a single stroke generator and means for causing the stroke to search for and lock on to an echo signal in such a way that the echo signal is maintained substantially centrally within the stroke pulse. This is achieved by gating the echo signal with the stroke and using the pulse passed to generate a first signal pulse of predetermined length commencing at the front edge of the pulse from the gating circuit. The degree of overlap between the first signal and the stroke is determined in a coincidence detector to provide a second signal which is in turn compared with the first signal to give an error potential for use in controlling the stroke to follow the echo signal. The embodiment described includes means for interrogating a radio beacon by the radiation of a pair of pulses of predetermined separation and means for selecting and following the first pulse of a pair of reply pulses having a known spacing. To radiate the interrogating pulses the transmitter 11 is triggered by the timing circuit 10 directly and, after an interval determined by control 86, through delay network 12 to produce pulses for radiation by antenna 16. Reply pulses are derived from aerial 16 and fed by receiver 15 to coincidence detectors 47 and 64. At the commencement of the search operation ganged switches 40 and 75 are manually closed for a short period; switch 40 applies a negative potential to the control grid of valve 36 rendering its anode potential a maximum and switch 75 applies a positive potential to the grid of valve 66 and charging capacitors 72 and 74. Valve 66 is therefore rendered and maintained conductive and the potential at Y increases sufficiently to cause the gas-filled tube 76 to conduct and raise the potential at point X. The potential at point Z is maintained at + B<SP>1</SP> under these conditions by means of diode 85 and this potential is fed through resistor 57 and diode 54 to cause the control grid potential of valve 36 to gradually increase over the search sweep period of about 20 seconds. This increase causes a corresponding decrease in the anode potential of valve 36 which is coupled to the anode of diode 35. The cathode of this diode is directly coupled to the anode of valve 20 and the potential on its anode is the limiting potential below which the potential of valve 20 anode cannot fall. Valve 20 forms a part of circuit 19 which when triggered at 13 by a negative input pulse coincident with the second transmitted pulse A generates a negative-going sawtooth waveform C of constant slope which terminates when the anode of valve 20 reaches the potential at the anode of valve 36 and, at the second grid of valve 20, a positive square waveform E having the same duration as the sawtooth. As the anode of valve 36 gradually decreases in potential the length of the sawtooth waveforms produced by circuit 19 progressively increases and a train of square pulses of the transmitter recurrence frequency and having progressively increasing durations is fed to differentiating circuit 32. The rear edge of each square pulse E produces on differentiation a short negative-going pulse F which triggers the stroke pulse generator 17 to produce a stroke pulse of standard length G. This stroke pulse will appear at progressively increasing time intervals after the transmitted pulse pair and will eventually overlap (coincide in time with) the first of the reply pulses received. When this happens an output pulse corresponding to the overlap period is fed to wide pulse generator 48 which generates the first signal pulse of predetermined length (J) referred to above. The length of this pulse is preset by control 83 and is' adjusted to correspond with the separation between the reply pulses of the beacon interrogated. The first signal pulse J and the stroke pulse G are fed to coincidence detector 49 to produce the second signal K which is a square negative-going pulse of a duration equal to the degree of overlap; this is equivalent to the distance from the leading edge of the first reply pulse to the rear edge of the stroke pulse. The amplitude of the pulse is adjusted as shown at L to give a pulse whose energy content, when the reply pulse is central in the stroke pulse, is equal to that of the negative-going part of waveform M (shown separately at O) obtained by differentiating the first signal pulse J by circuit elements 56 and 57. These two pulses L and O are used for tracking, as will be described later, when the search operation of the system has been stopped. Stopping of the search occurs when a first signal pulse is produced by generator 48, indicating overlap between a first reply and the stroke, and a gate pulse N derived from the rear edge of this first signal pulse coincides with the second reply pulse, causing coincidence detector 64 to provide a negative pulse which renders diode 70 conductive and causes valve 66 to become temporarily non-conductive. This in turn causes tube 76 to cease to conduct and applies a potential of - C to diode 54, the grid of valve 36 then ceases to increase in potential and the search is stopped. The circuit of valve 66 is such that this condition persists as long as reply pulse-pairs are being received and for a memory period thereafter dependant on the length of time that tracking has been taking place; the operation is more fully described in connection with Specification 747,147. When searching ceases tracking automatically commences. The pulses L being fed through diode 53 to decrease the potential at the grid of valve 36 and the pulses O being fed through diode 54 to increase the potential of the grid. When the first reply pulse is central in the stroke no change in grid potential occurs and the stroke remains stationary, if however the reply pulse shifts towards the front edge of the stroke the pulse L lengthens and there is a resultant negative shift at the grid of valve 36 causing the stroke to move inwards. A corresponding reply pulse shift towards the back edge of the stroke reduces the length of pulse L and the stroke is caused to shift outwards. The position of the stroke and, therefore, the range to the beacon being tracked is indicated on meter 45 which shows the anode potential of the valve 36. Automatic tracking thus takes place. A modified circuit arrangement in which coincidence detectors 47 and 49 are combined is described with reference to Fig. 4 (not shown).