Patent GB-1040585-A (GB)
Title: Radio navigation apparatus
IPC: G01S13/60
Filed: 1963-05-31
Abstract: 1,040,585. Doppler radar navigation. NORTH AMERICAN AVIATION Inc. May 31, 1963 [June 8, 1962], No. 21995/63. Heading H4D. A Doppler radar navigation system for determining on a vehicle, the speed of that vehicle, comprises a pulse transmitter, a directional receiving antenna receiving reflections of the transmitted energy, said antenna being caused to scan both sides of a reference azimuth plane, a pair of filters arranged to select two components of the Doppler beat signal at different frequencies, means for generating a signal representative of the angle of scan of the antenna relative to the reference plane, and a sampling device for sampling the said angle representative signal whenever the amplitudes of the components in the filters reach a predetermined value relative to each other. The invention is based on the discovery that when the antenna is scanning either side of a reference plane the ratio of the amplitudes of two frequency components of the Doppler signal spectrum reaches a predetermined value at an antenna scanning angle which varies in dependence on the vehicle's speed. Fig. 4 (not shown) shows the variation of six different frequency components with the antenna scanning angle γ, at a speed parallel to the reference plane of 890 knots. The lower frequency curves drop in amplitude with increased speed, whilst the higher frequency curves increase in amplitude with increased speed. In the embodiments described the components at frequencies of 100 and 500 c.p.s. are chosen, and the amplitude of the 500 c.p.s. signal is amplified five times. Fig. 5 (not shown) shows the resulting variation of the two components with scanning angle, for five speeds. It is seen that the amplitudes of the two components (after one is amplified as described) are equal at any one speed, at an antenna scanning angle which is a non-linear function of the speed. This function is shown in Fig. 6 (not shown). The function may be made linear by varying the amplification of the 500 c.p.s. signal with the scanning angle. It is seen that the function of Fig. 6 is symmetrical with respect to the reference plane of the antenna. This, however, is only the case when the velocity vector of the vehicle is parallel to the said plane. When the vehicle is subject to a drift angle 8, then if V=f(γn) is the equation of Fig. 6 and γ1 nR  and γ<SP>1</SP> nL  are the right-hand side and the left-hand side equi-signal scan angles, angle 8 is given by and γ nL  = γ nR  = Y<SP>1</SP> nL  + # = γ<SP>1</SP> nR  - 8, where γ nL  and γ nR  are the angles to be used in the equation V=f(γ n ). An arrangement for determining the speed V and the drift angle 8 is shown in Fig. 9. An antenna 30 scans either side of the reference plane under the control of antenna drive means 32 which also supplies signals representing the direction of scan of the antenna. The reflection signals received by the antenna (the transmitting means not being shown) are passed to a receiver 31 to produce the Doppler signal. The low-frequency component is separated out from the Doppler signal by a band-pass filter 33 and the high-frequency component is similarly separated out by filter 34. The high-frequency component is amplified in gain changer 36 and is then fed to an equality detector 35 together with the low-frequency component from filter 33. Detector 35 gives a signal to AND gates 41 and 42 when the amplitude of the low-frequency component is equal to the amplitude of the amplified high-frequency component. Gate 41 is also fed with a signal from the drive unit 32 when the antenna is scanning to the left of the reference plane, whilst gate 42 is fed with a signal from unit 32 when the antenna is scanning to the right of the reference plane. Gates 41 and 42 feed signals to open gates 39 and 40, respectively, to pass a signal from unit 32, representative of the antenna scanning angles γ<SP>1</SP> nL  and γ<SP>1</SP> nR  to storage means 37 and 38, respectively. The outputs from the storage means are passed to a difference deriving means 44 to produce a signal representing the drift angle 8, and to a sum deriving means 45 to produce a signal representing the drift angle 8, and to a sum-driving means 45 to produce a signal representing the antenna angle γ nL  or γ nR . This signal is passed to a voltmeter 46 with a suitable non-linear read-out scale to provide an indication of the velocity V. In an alternative embodiment, Figs. 10 and 11 (not shown), more than two frequency components may be selected and an average taken of the values of 8 and V obtained from the various frequency pairs available. In this alternative embodiment the amplification of the gain changes is altered in dependence on the scanning angle.