Patent GB-997273-A (GB)
Title: Doppler system test set
IPC: G01S7/40
Filed: 1963-09-05
Abstract: 997,273. Aerials; radar. BENDIX CORPORATION. Sept. 5, 1963 [Sept. 20, 1962], No 35179/63. Headings H4A and H4D. A testing apparatus for Doppler radar navigation system comprises a plurality of horn aerials directed to receive respectively energy transmitted on individual aerial beams by the Doppler navigation system, the horn aerials being associated with a transmission path for that energy including attenuator means, delay means and modulator means and there being provided means for retransmitting from the horn aerials the attenuated, delayed and modulated energy which has passed through the transmission path. The embodiment relates to the testing of the Doppler system whilst the aircraft on which it is fitted is on the ground, the testing apparatus being on a trolley 71, Fig. 3, provided with arrangements whereby the distance of horn platform 11 from radome 73 on aircraft 72 is maintained constant as slight motion of the aircraft occurs, e.g. due to persons boarding it. The aircraft radar tested is of the kind in which energy is transmitted sequentially on four differently-directed beams. The radiation axes of horns 12, 13, 14, 15, Fig. 1, are directed toward the respective beam axes of the aircraft aerial system. In order to measure the magnitude of energy received by each horn calibrated attenuators 17 are provided. Horn pairs 12, 14 and 13, 15 are connected to 3dB couplers 18, 19 respectively and output waveguides 22, 24 feed into a further 3dB coupler 29; the couplers are reciprocal devices. The sequential outputs in waveguide 31 pass through an attenuator 34, calibrated so as to introduce attenuation corresponding to that experienced when the navigation system is at maximum altitude, and a delay line 35, affording a " round-trip " delay corresponding to that altitude, to a phase modulator 36 providing a cyclic phase shift greater than a multiple of 180 degrees to mask any effects of a possible carrier phase nul. The output of phase modulator 36 passes to a ferrite rotator device 38 which has two conditions in one of which there is complete transmission of energy from modulator 36 and in the other of which there is complete reflexion thereof; by application of a sinusoidal control current on line 39 the energy passing from generator 38 to the modulator 36 is thereby amplitude modulated sinusoidally and proceeds back to the respective horn for transmission to the system under test. The control frequencies on line 39 correspond to a predetermined ground speed and drift angle; since in a fixed aerial system drift angle is indicated by different Doppler shifts for the diagonal pairs of beams, Doppler simulator 42 applies a correspondingly different control frequency. Doppler simulator, Fig. 2.-Detectors 25, 27, Fig. 1, supply switching square waves of opposite polarity to respective terminals 51, 52. If the energy received by each horn is equal, a symmetrical bi-polar wave 53 is obtained and can be observed on a cathode-ray oscilloscope 50; if this is not so, comparators 54, 55 prevent the normal operation of switch 56 and this effect is sensed by the system under test. In normal operation switch 56 connects line 57 alternately to inputs from oscillators 61, 62 which represent predetermined ground speed and drift angle. The arrangement for controlling phase modulator 36, Fig. 1, comprises a generator 68 synchronized by the output of comparator 54. Mechanical arrangements. Fig. 3.-Trolley 71 carries aerial platform 11 which is coated on its upper surface with microwave absorbent material 16; the platform is raised by cam 78 and is supported by a ball joint 82. A spring 83 urges the platform against radome 73 on the aircraft carrying the system under test. Rods 84 make contact with indexing flats on radome 73. Microwave components are suspended under platform 11 in an enclosure 75 and delay line 35 is coiled around the latter to provide a flexible connexion between the attenuator 34, Fig. 1, mounted under the platform, and phase modulator 36, Fig. 1, located within the trolley. The arrangement is such that the microwave assembly is free to move into position below the aircraft without any changes arising in the calibration of the waveguide paths. Sockets are provided on platform 11 for rods 84, Fig. 4 (not shown), different sets of rods being made available to cater for different aircraft types.