Abstract:
A radio navigational device for determining the angle of a mobile craft by analyzing the signals arriving at each element of a linear array of n equally spaced elements. A series of received R.F. pulses emanating from the mobile craft by transmission or reflection provide contemporaneous signals at the antenna elements with a phase difference ψ between successive elements in accordance with the angle of arrival of the R.F. pulses. Using &#34;zero&#34; IF techniques, each signal of the series is processed to provide signal outputs in I and Q form. Sequential or parallel sampling of these I and Q signals provides a synthesized scanning beam signal from which ψ is determined.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to radio direction finding equipment. 
     2. Description of the Prior Art 
     Radio direction finder systems exist in the prior art in a number of forms. The type of direction finder to which the present invention relates is the type which determines the angle of arrival of the phase front of a signal transmitted by, or reflected from, an object to be located in angle. 
     A system of the particular type to which the present invention applies is described in U.S. Pat. No. 3,924,236. That prior art, sometimes referred to as a synthetic aperture system, requires a delay line constructed to exacting requirements and, therefore, relatively expensive. 
     The manner in which the present invention improves upon the prior art, in view of the aforementioned prior art disadvantage, and also in other ways will be evident as this description proceeds. 
     SUMMARY OF THE INVENTION 
     According to the invention there is provided radio direction finding equipment for deriving angular information in respect of a mobile craft from radio frequency pulses emanating from (received from or reflected from) the craft, including means responsive to each of the pulses to produce a series of n simultaneous pulse signals of equal frequency with a phase difference ψ between successive signals of the series as determined by the angle of arrival of the pulses from the craft. For each of the n signals, a first mixing network is provided for mixing the signal with a first phase of a common local oscillator at the same frequency as that of the received signal. In parallel with this first network, a second mixing network is provided for mixing the signal with a second phase of the local oscillator in quadrature with the aforementioned first local oscillator phase for each of said first and second mixing networks. Means responsive to the respective zero frequency outputs of the two mixing networks provide outputs given respectively by I j  and Q j  where I j  is proportional to cos j ψ  and Q j  is proportional to sin J ψ, j representing the position of the zero frequency responsive means. Means are also included for sampling said n pairs of zero frequency responsive means to derive from each pair outputs in the form I j  Q j  -I j  -Q j , and for processing the n derived outputs to determine therefrom the phase difference ψ and hence the angle of arrival of the pulses from the craft. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a first embodiment of a radio direction finding equipment in accordance with the invention. 
     FIGS. 2, 3, and 4 show waveforms relevant to the operation of portions of the embodiment of FIG. 1. 
     FIG. 5 shows the circuit diagram of a practical implementation of a portion of the system of the first embodiment. 
     FIG. 6 shows the overall relationships between the various waveforms occurring throughout the first embodiment. 
     FIG. 7 shows portions of a second embodiment. 
     FIG. 8 shows waveforms relevant to the operation of the second embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1 successive radio frequency pulses of frequency fc, typically at 1 GHz, each of duration T, typically 1 μsec, and spaced at typically 1 m/sec., are received from an aircraft, either by reflection therefrom in a radar application or by generation thereat, on array of n equally spaced antennas, A1 to An (typically numbering 25) with 2λ or 4λ spacing between centers so that each antenna may have directional receiving properties. FIG. 1 indicates simply the effective centers of each antenna. 
     Assuming the equipment is required to give elevation angle information, a vertical array with boresight normal to the array axis is employed. Each successive r.f. pulse arrives at angle θ, and each such received pulse induces a series of simultaneous pulse signals of frequency fc and duration T, which signals are fed to individual mixers M1- Mn and down-converted to a frequency fo. This frequency fo is typically 5 MHz, and the conversion is effected by a common local oscillator L01 operation at the appropriate off-set frequency of fc-fo. Between the signals of the array elements there is a phase difference ψ given by 2π d sin θ, where d is the spacing between adjacent elements of the array in units of wavelength. 
     Two stages of frequency conversion (and amplification) are preferably employed so that the normal advantages of a double IF system can be achieved. This does not however affect the subsequent signal processing which is described with reference to the n simultaneous signals of frequency fo, duration T and with mutual phase differences ψ determined by the angle of arrival of the pulses. 
     Each of the fo signals is applied to two mixing networks, MW1A and MW1B providing one paralleled I and Q pair thereof. The first of each of the n pairs of mixing networks is connected to a common local oscillator L02 having a frequency of fo (i.e. equal to that of the signal). The mixers of the network, i.e. one mixer in each pair is connected to oscillator L02 via a 90° phase shifter PS. 
     FIG. 2 shows how the outputs from each of the pair of mixers associated with an antenna element (I and Q outputs) contain a packet of &#34;zero frequency&#34; with amplitude proportional to the amplitude at the zero IF and the cosine and sine respectively of the phase difference between the signal and the local oscillator L02. 
     With an input from the j th  antenna element equal to cos (ωot+ jψ), where ψ = 2πd sin θ radians, the outputs of the j th  I and Q channels are of the form cos jψ and sin jψ. 
     Each of the pairs of mixers (for example MW1A and MW1B) have their outputs coupled via suitable enabling switches, such as ES1A and ES1B, to a pair of integrating networks such as IN1A and IN1B, each containing a reset switch typically RS1A. 
     The function of the integrators, which each include an amplifier and, feedback capacitor, is to provide optimum performance in conditions of low signal to noise. They are reset prior to receiving the pulsed signal through these reset switches activated by a suitable common first timing circuit TC1. These integrators are only enabled for the pulse signal duration by the switches ES (ES1A and ES1B being one typical pair of ES switches) by a suitable common second timing circuit TC2, to provide a &#34;range-gate&#34; function. The integrators provide filtering properties equivalent to a filter matched to the signal pulse. 
     The outputs of the j th  pair of integrators 
     
         I.sub.j = .sub.o ∫.sup.T i.sub.j dt  and  Q.sub.j = .sub.o ∫.sup.T q.sub.j dt 
    
     are given by: 
     
         I.sub.j = T cos jψ 
    
     
         Q.sub.j = T sin jψ 
    
     The remaining portion of FIG. 1, comprising an analog demultiplexer AD with its timing control TC3, and a sector filter SF, are only intended to outline the subsequent signal processing. FIG. 4 shows this in more detail and will be described later, however it is convenient now to outline the basis of this subsequent processing, which involves sampling the outputs from each of the integrator pairs in a 1 to n sequence at the same sampling rate. The outputs from each pair of integrators are formed into the sequence (for the j th  pair) I J  Q j  -I j  -Q j . 
     As shown in FIGS. 3 and 4, if the duration of each sample in the sequence is τ (typically 1 μ sec) then the waveform obtained is identical to sampling and holding a sine wave of frequency π/4 and phase jψ. 
     Thus the complete sequence I 1  Q 1  -I 1  -Q 1  , I 2  Q 2  -I 2  -Q 2   . . . I n  Q n  -I n  -Q n  is identical in form to samples of a Doppler beat envelope from n antenna elements with one cycle of beat per antenna element period and a phase jump of ψ radians between adjacent elements. 
     The peak spectral component is at ##EQU1## 
     The use of a sector filter SF with a bandpass of ##EQU2## for azimuth (with a horizontal array) and ##EQU3## for elevation will result in a signal waveform indistinguishable from that of a conventional Doppler scan, and so may be further processed by suitable known techniques using for example a frequency tracking filter to derive the required angle θ in accordance with the Doppler signal frequency. 
     As shown in FIG. 5, the balanced mixer function of the mixing networks is conveniently performed by analog switches such as MOS transistors MOS1A, MOS1B, etc., controlled by &#34;inphase&#34; and &#34;quadrature&#34; square waves of frequency fo derived from the local oscillator L02 (now of frequency 4fo) via a shift register SR whose successive outputs form one input to a pair of AND gates G1, G2 having a common other input enabling the mixer switches. 
     Derivation of the I j  Q j  -I j  -Q j  sequence is effected by appropriate timing of the demultiplexer (AD of FIG. 1), shown in FIG. 5 as a commutator COM. The signal inversions required by the demultiplexing sequence are provided by a single inverting amplifier IA at the output of the multiplexer and a &#34;toggling&#34; switch TS. 
     Control timing is shown in FIG. 6. The sampling sequence may be repeated in the inter-pulse period (of 1 m sec) thereby producing multiple scan signals. 
     In the second embodiment illustrated, the basic difference in the processing of the series of n simultaneous signals from that described so far, is in the sampling of the integrating networks. Accordingly, FIG. 7 only shows this portion of the circuit, the other aspects of the second embodiment remaining the same. 
     It will be recalled that the sequence I j  Q j  -I j  -Q j  represents four samples of a single cycle of a sinusoid. 
     With each sample in the sequence of duration τ then the repeated cyclic commutation of these values represents a sine wave of frequency 1/4τ. 
     The rate of this cyclic commutation is a free choice and does not destroy the basic information. In principle, in the second embodiment, the sequences associated with the respective pairs of integrating networks are cyclically commutated in parallel at different rates: 
     Commutation rate of the first sequence= F 
     Commutation rate of the second sequence= F-f 
     Commutation rate of the third sequence= F-2f&#39; 
     Commutation rate of the n th sequence= F- (n- 1 )f&#39;. 
     As shown in FIG. 7 this is implemented by individual sampling switches S1, S2, etc., each including the two necessary inverting amplifiers such as IA1A, IA1B, with successive switches in a 1 to n sequence being driven at the respective frequencies of F, F-f 1 , F-2f 1 , etc. by suitably controlled commutation oscillators OSC 1 , OSC 2  - - - OSC n , those frequencies being readily derived by logic divider circuits. 
     With the aforementioned commutation oscillators mutually in phase at time t 1 , the sequences correspond to sinusoids with different frequencies and phases are described by: 
     
         SIN 1= A cos 2πFt 
    
     
         SIN 2= A cos 2π (F-f&#39;)t+ ψ 
    
     
         SIN 3= A cos 2π (F-2f&#39;)t+ 2ψ 
    
     
         SIN n= A cos 2π [ F -(n-1)f]t+ (n-1)ψ 
    
     where ψ = 2πd sin θ. 
     As shown in FIG. 8 this parallel sampling sequence adds the signals together, the shape of the envelope in time of the resultant signal being the radiation pattern of the array which, implicit in this description, is of sin x/x form. The timing t 2  of the peak of the pulse in relation to the time t 1  (i.e. Δt), when all the commutation oscillators are in phase (at the commencement of the sampling sequence) is a measure of the sine of the angle of arrival θ of the pulse with respect to the boresight of the array, and is effected by suitable circuitry CIR. This circuit CIR will be recognized as a Δ t computer, i.e. by solving the equation Δt= ψ/(2πf), by known analog techniques. The multiple oscillators for the sampling sequence can be at conveniently low frequencies allowing implementation by digital methods. 
     The above described sampling technique can operate satisfactorily in multiple target situation. 
     It is to be understood that the foregoing description of specific examples of this invention is made by way of example only and is not to be considered as a limitation on its scope.