Patent GB-915399-A (GB)
Title: Fm doppler navigation system utilizing first fm sideband
IPC: G01S13/60
Filed: 1959-12-10
Abstract: 915,399. Doppler radar. COLLINS RADIO CO. Dec. 10, 1959 [April 2, 1959], No. 42060/59. Class 40 (7). Relates to an airborne F.M. C.W. Doppler radar navigation system comprising at least two fixed pencil beams directed downwardly for and aft. In such a system the echo signal # R  received by each aerial comprises frequency components # c Œn# m Œ# d  when f c  is the carrier, f m  is the modulation signal, f d  is the Doppler shift frequency and n is an integer, the amplitude of the component of order n being given by the Bessel function J n . When each echo signal is transposed in frequency to an intermediate frequency # s2 , the corresponding components # s2 Œn# m + # d  and # s2 Œn# m  -# d  derived from the forward and rear signals are shown in Figs. 4 and 5 respectively, the corresponding derived components for a stationary aircraft, i.e.# d  = 0, being shown in Fig. 3. According to the present invention the first order component # s2 +# m +# d  of the forward I.F. signal is selected by a filter having a bandpass characteristic 102, Figs. 3 and 4, between the frequencies (# s2 +# m ) and (# s2  + 2# m ) and the first order component # s2 -# m -# d  of the rear I.F. signal is selected by a filter having a bandpass characteristic 106, Figs. 3 and 5, between the frequencies (# s2  # m ) and (# s2  - 2# m ), the selected components being applied to corresponding discriminators to give the corresponding Doppler shift frequencies. Alternatively the sidebands of the rear signal may be inverted to give a resultant I.F. signal # s2  + n# m +# d  and in this case, Fig. 6, the front and rear signals are applied sequentially to a single filter having a band-pass characteristic 102, Figs. 3 and 4. The value of the modulation frequency f m  is sufficiently low, e.g. 8 kc/s., so that the spectrum of unwanted signals due to noise, local reflections (e.g. from rain below the aircraft) and transmitter leakage is substantially confined between the first upper and lower sidebands as indicated by the curve 100 in Figs. 3-5. It is shown that by using the first order sideband with a low modulation frequency (1) " altitude holes " are eliminated in the entire altitude range of present aircraft, (2) the amplitude of the first order sidebands is substantially constant up to an altitude corresponding to a delay time of 0.9/# m , and (3) fixed pencil beams may be employed. The invention is compared with a prior system using a higher modulation frequency #m, e.g. 500 kc/s., in which the received signal is heterodyned with the transmitted signal, the Doppler frequency being derived from the third or fifth order sideband components. The invention is described as applied to a system having a single rear beam and two forward beams symmetrically inclined to the longitudinal axis of the aircraft, Fig. 1 (not shown), the Doppler frequency outputs from the three corresponding discriminators 57, 58, 59, Fig. 2, each of the type disclosed in Specification 889,413, being applied to a known form of computer 61 to give the ground speed and drift angle; the frequency transposition of the received signals may be effected in two stages (as described in Figs. 2 and 6) or in a single stage. Simultaneous beams, Fig. 2. The carrier f c  is frequency modulated in an oscillator 13 by a frequency # m  from an oscillator 14 and the resultant signal # T  is applied through circulators 16, 17, 18 to the three aerials 10, 11, 12. The resultant echo signals are applied to respective receivers 20, 25, 30 and heterodyned in corresponding mixers 21, 22, 23 with a signal # H1  = # c  - # s1  obtained by heterodyning the signals # c  and # s1  from oscillators 13 and 28 in a mixer 26 and selecting the difference frequency in a filter 27. The resultant first I.F. signals are applied to corresponding I.F. amplifiers 43, 44, 45 which include notch filters to remove the carrier and noise components and the outputs therefrom are heterodyned in corresponding mixers 47, 48, 49 with a signal # H2  = # s1  -# s  2 obtained by heterodyning the signals # s1  and f s2  from oscillators 28 and 31 in a mixer 32 and selecting the difference frequency in a filter 33. The two resultant forward second I.F. signals are then applied to filters 51 and 52 having a band-pass characteristic 102, Fig. 4, to select the upper first order sideband # s2 +# m +# d  and the resultant rear second I.F. signal is applied to a filter 53 having a band-pass characteristic 106, Fig. 5, to select the lower first order sideband # s2  - fm -# d  Sequential beams. Fig. 6. In Fig. 6, components corresponding to those in Fig. 2 are indicated by corresponding reference members with a prefix 1. In this case the output # T  from the oscillator 113 is applied sequentially to the aerials 110, 111, 112 by a switching means 170 (see below) controlled by a timing source 180 and echo signals are transposed to the second I.F. # s2  by the mixers 121 and 147. During reception of the forward signals a heterodyne signal # H2  =# s1 -f s2  is applied to the mixer 147 as in Fig. 2, but during reception of the rear signal a heterodyne signal #<SP>1</SP> H2  = # s1 +f s2  is applied to the mixer 147 so that the sideband components are inverted as described above. The signals # H2  and #<SP>1</SP> H2  are produced by applying the output from the mixer 132 to filters 133, 134 and selecting the outputs therefrom sequentially by a switch 136 controlled by the timing source 180. Aerial beam switching, Fig. 6. The transmitter oscillator 113 is coupled to a switched circulator 171 which is controlled by the timing source 180 so that during the 1 sec. periods C, Fig. 7, one output is coupled to the rear aerial 110 and during the intervening 2 sec. periods D the other output is coupled through an unswitched circulator 172 to another switched circulator 173 whose two outputs are coupled sequentially to the two front aerials 111, 112 during periods A and B respectively.