GB2156622A - Target location method and apparatus - Google Patents

Abstract

In a method and apparatus for locating and determining the direction of a source of radiation, especially a target which radiates heat, radiation from the source is passed through a modulator 2 and detected by a photoelectric cell 1, the modulator being arranged such that the phase of the modulation of the output from the cell is representative of the physical deviation of the source from a reference axis. The signal from the cell 1 is amplified and passed to a digital frequency and phase analyser to generate digital values for feeding to a microprocessor 5 together with a reference frequency signal from the modulator 2. These signals are processed by the microprocessor and, as a result, the deviation of the source from the reference axis is established. This digital method has advantages over analogue systems in that it reduces the volume of electronic circuitry and residual ripple on the deviation voltages. <IMAGE>

Description

SPECIFICATION Target location method and apparatus This invention relates to a method of locating a radiation source such as a heat radiating target, and for determing its direction relative to a reference axis.

It is known from German Auslegeschrift No.

1 3 03 486 and German Offenlegungsschrift No. 14 23 696 to determine the location of a heat radiating target in terms of polar coordinates representing the magnitude and angle of the target deviation with respect to a reference axis and reference plane. Radiation from the source is received by an objective and passed through a mechanical modulator before being converted into an electrical signal by a radiation detector located in the image plane of the objective. The signal produced is frequency modulated and the modulator is arranged such that the magnitude and phase of frequency deviation are representative of the magnitude and angle respectively of the target deviation.

The amplified output voltages are evaluated on an analogue basis which necessitates the use of comparatively expensive electronic circuitry.

An apparatus for locating a target object or for determining its location in space is also known from German Offenlegungsschrift No.

32 1 9 826, this apparatus converting analogue signals to digital signals which are further processed by means of a microprocessor.

It is an object of the present invention to provide an improved method and apparatus in order to achieve a reduction in the signal evaluation electronics and in residual ripple on the deviation voltages. According to the invention, a method of determining the location of a source of radiation relative to a reference axis comprises receiving radiation from the source or radiation representative of the source in an objective, passing the received radiation through modulating and nutating means to produce modulated radiation having a modulation frequency of which the frequency deviation and phase is representative of the location of the source relative to the axis, and converting the modulated radiation into an electrical signal using a radiation detector, wherein the electrical signal is fed to a frequency analyser having a clock frequency which is at a fixed ratio with respect to the nutation frequency, frequency analysis is performed with regard to amplitude and phase, digital values resulting from the analysis are fed to a microprocessor to obtain values of the deviation of the source from the axis, and the resulting deviation values are displayed or processed further. Use of this digital method enables the signal to be smoothed out and can be relatively economical in terms of electronic components.The invention also includes apparatus for determining the direction of a radiation source relative to a reference axis, wherein the apparatus comprises an objective for receiving radiation from the source or radiation representing the source, modulating means for modulating the received radiation, a radiation detector for converting the modulated radiation into an electrical signal, an electrical circuit means for evaluating the signal to produce values representing the direction of the source relative to the axis, wherein the modulating means comprises a mechanical modulator arranged to produce a nutational movement with a predetermined nutation frequency whereby the electrical signal from the detector is frequency modulated and has a frequency deviation representative of the source direction, and wherein the electrical circuit means includes (a) a frequency analysis circuit which receives a clock signal of a frequency having a fixed ratio relative to the nutation frequency and which produces output signals representative of the amplitude and phase of the frequency deviation. and (b) a microprocessor programmed to convert the said output signals into source deviation values.

The invention will now be described by way of example with reference to the drawing which is a block diagram.

The illustrated embodiment of the invention has a radiation detector in the form of a photoelectric cell arranged to receive heat radiation from a target, the radiation having passed through an objective to be modulated at a constant frequency in the optical axis by a modulation disc 2, the said target generally being a missile, e.g. a rocket. The illustrated apparatus is part of a semi-automatically operating guidance sight with which the missile is guided to its destination by an operator constantly aiming to bring the missile and the destination into coincidence in the guidance sight. Visible radiation is used for observing the missile and the point of destination, and the heat radiated by the target serves for calculation of deviation voltages with respect to the intended flight path, by means of a socalled goniometer.As soon as the missile deviates from the sight line (which acts as a reference point), the modulated signal exhibits a deviation from the mean frequency. The frequency deviation magnitude is a measure of the deviation of the missile from the sight line, while the time displacement of the zero axis crossing of the frequency deviation, i.e.

its phase, is a measure of the angular location of the missile in a polar co-ordinates system centred on the sight line. These values are fed via an amplifier 3 to an electronic evaluating unit, corresponding correcting pulses being then transmitted by wire or radio to the missile.

In accordance with the invention, amplified output voltages from the photoelectric cell 1 are fed to a digital circuit 4 in which the signal period is measured. In other words, the circuit 4 carries out a frequency analysis of the amplified voltage signal with reference to amplitude and phase and feeds corresponding values to the microprocessor 5. For this, a period or frequency analyser is used, having a clock frequency input which is at a fixed ratio to the nutation frequency of the modulation disc 2. This ratio is fixed by connecting the digital circuit 4 with the modulation disc 2 by a link 7 which takes the form of a phase lock loop (PLL) circuit 8.

Frequency analysis is carried out by detecting the zero axis crossings of the amplified voltage signal, for instance by means of a comparator and, in the time between any two zero axis crossings, counting the pulses of the clock frequency or pulses of a frequency which is a multiple of the nutation frequency.

A further reference frequency signal originating from the modulation disc 2 is fed via line 9 directly to the microprocessor 5. The line 9 is also connected to the PLL circuit 8 by a connection 10. The microprocessor 5 evaluates the pulses and in a further processing stage allocates deviation voltages to the amplified voltage signal according to frequency and phase.

As a result, then, the deviation of the target from the reference point in the field of view of the sight is provided in the form of coordinates.

Claims (11)

1. A method of determining the location of a source of radiation relative to a reference axis, comprising receiving radiation from the source or radiation representative of the source in an objective, passing the received radiation through modulating and nutating means to produce modulated radiation having a modulation frequency of which the frequency deviation and phase is representative of the location of the source relative to the axis, and converting the modulated radiation into an electrical signal using a radiation detector, wherein the electrical signal is fed to a frequency analyser having a clock frequency which is at a fixed ratio with respect to the nutation frequency, frequency analysis is performed with regard to amplitude and phase, digital values resulting from the analysis are fed to a microprocessor to obtain values of the deviation of the source from the axis, and the resulting deviation values are displayed or processed further.

2. A method according to claim 1, wherein the source is a heat radiating target lying within a predetermined solid angle centred on an optical axis constituting the reference axis, the received radiation is modulated by a mechanical modulator which itself executes a nutating movement relative to the received radiation, the radiation detector is disposed in the image plane of the objective, the electrical signal from the detector has zero frequency deviation when the target is coincident with the optical axis, and the frequency deviation of the electrical signal has a amplitude representative of the magnitude of the deviation of the target from the optical axis and a zero crossing time shift representative of the angle of the target deviation in a polar co-ordinate system.

3. A method according to claim 1 or claim 2, wherein the frequency analyser detects the zero-axis crossings of the electrical signal, and, in the time interval between any two zero axis crossings, counts the pulses of a pulse signal having a frequency which is a multiple of the nutation frequency.

4. A method according to claim 3, wherein the zero axis crossings are detected by means of a comparator.

5. Apparatus for determining the direction of a radiation source relative to a reference axis, comprising an objective for receiving radiation from the source or radiation representing the source, modulating means for modulating the received radiation, a radiation detector for converting the modulated radiation, into an electrical signal, an electrical circuit means for evaluating the signal to produce values representing the direction of the source relative to the axis, wherein the modulating means comprises a mechanical modulator arranged to produce a nutational movement with a predetermined nutation frequency whereby the electrical signal from the detector is frequency modulated and has a frequency deviation representative of the source direction, and wherein the electrical circuit means includes (a) a frequency analysis circuit which receives a clock signal of a frequency having a fixed ratio relative to the nutation frequency and which produces output signals representative of the amplitude and phase of the frequency deviation, and (b) a microprocessor programmed to convert the said output signals into source deviation values.

6. Apparatus according to claim 5, wherein the source deviation values comprise the amplitude and angle of the source deviation from the reference axis.

7. Apparatus according to claim 5 or claim 6, wherein the relationship between the clock signal and the nutation frequency is fixed by a phase-locked loop.

8. Apparatus according to any of claims 5 to 7, wherein the modulation means includes a rotatable modulation disc.

9. Apparatus according to any of claims 5 to 8, wherein the microprocessor is operable such that, in a further processing stage, deviation voltages are generated, which voltages are associated with the signal from the radiation detector according to frequency and phase.

10. A method of determining the location of a source of radiation, the method being substantially as herein described with reference to the drawing.

11. Apparatus for determining the direction of a radiation source, the apparatus being substantially as herein described and shown in the drawing.