GB1382150A - Navigation systems - Google Patents

Abstract

1382150 Aerials; radio navigation; transponders SPERRY RAND CORP 28 June 1972 [6 July 1971] 30239/72 Headings H4A H4D and H4L A navigation system comprises transmitter/ receiver means for sending out signals which are wept in frequency over a fine-determined range, and for receiving said signals after redirection by a passive beacon. Said beacon comprises aerial means whose directivity is frequency-dependent, and which is adapted selectively to redirect towards the transmitter/ receiver means a signal therefrom having a frequency within the fine-determined range. As shown in Fig. 4, a ship 15 is moving in a direction 17, and has aboard an azimuth scanning radar system, including an aerial 16 which is rotating as indicated by the arrow 18. The carrier frequency of the radar pulses from the aerial 16 is cyclically swept between the values f c1 to f cn at a rate which is rapid compared with the rate of rotation of the aerial. A shore radio beacon comprises a receiver-radiator device 1 which, when the radar aerial is at the location 16, directs an echo signal of frequency fc 1 thereto. As the ship moves along the coast to locations 16a, 16b, ... echo signals of respective frequencies f c2 , f c3 , ... are directed to the radar aerial. In this way enhanced indications of such beacons are given on the radar display, as compared with ordinary objects. A modulator 12 may be associated with the receiver-radiator device 1, for coding the re-directed signals. Said device comprises an elongated hollow transmission line adapted to propagate a travelling electromagnetic wave with a velocity that is proportional to frequency, energy coupling means extending along the length of the line, and reflecting means at one end of the line so that an incoming signal may be returned along said line for re-radiation. As described, the transmission line is a length of rectangular waveguide which is terminated at one end 3, Figs. 1, 2, with a shorting wall 13, and at the other end with a wave-absorbing portion 4. An array of radiating slots 6, 6a, 6b, ... is provided along one broad face 5 of the waveguide, the interior of which may be fully or partially filled with low-loss dielectric material. The inner conductor 10 of a coaxial cable 2 extends into the waveguide near the end wall 13, and is connected in series with a controlling diode 9. A modulator 12 varies the bias on the diode 9, and produces a variation in the apparent radar scattering crosssection of the beacon. The image of the beacon on the radar display will nicker, in consequence, and coding information may be extracted by such means as tuned filters. The radar system is of conventional type, except that a frequency sweep generator is coupled to the transmitter and is controlled by a pulse counter using the synchronizing trigger pulses which are supplied to the transmitter (Fig. 5, not shown). The system may be adapted to operate in elevation rather than in azimuth, e.g. for use with aircraft.