GB2309348A

GB2309348A - Remote guidance

Info

Publication number: GB2309348A
Application number: GB9525973A
Authority: GB
Inventors: Mikhail Stepanovich Nikitin
Original assignee: HEATVISION TECHNICS CORP
Current assignee: HEATVISION TECHNICS CORP
Priority date: 1995-12-19
Filing date: 1995-12-19
Publication date: 1997-07-23
Also published as: GB9525973D0

Description

Remote Guidance Description The present invention relates to an apparatus and method for determining an attribute of a remote object which can be used in a remote guidance system.

A method of guiding remote objects is known from Zarubezhnaya Radioelektronika, 1988, (6). In the disclosed system, a target is illuminated by electromagnetic radiation with a known wavelength, radiation reflected from the target is registered by a receiver located on the remote object, signals are processed at a frequency equal to the difference between the frequency of the reflected signal and the frequency of a heterodyne located on the remote object and control signals are generated and shaped, and then supplied to the systems controlling the spatial position of the remote object.

However, a number of disadvantages are inherent to this system. These include poor accuracy and the poor noise performance of the receiving systems located on the remote object.

According to the present invention, there is provided an apparatus for determining an attribute of a remote object comprising transmitting means for emitting first and second signals having different frequencies, receiving means arranged for receiving and mixing said first and second signals after refleaion from a remote object to produce an IF signal having a frequency equal to the difference between the frequencies of the first and second signals and processing means responsive to the IF signal to determine the attribute. The emitted signals could be electromagnetic, e.g. rf or optical radiation, or acoustic as in SONAR. Preferably, the transmitting means is arranged such that the first and second signals are emitted as narrow beams. Generally, the beams should be as narrow as possible.

The attribute may be, for example, radial velocity relative to the transmitting means. In which case, the processing means is preferably operative to detect changes in the frequency of the IF signal arising from doppler shifts in the first and second signals caused by refleaion from the remote objea. Other attributes such as azimuth, elevation and range may also be determined. The skilled person will readily find suitable techniques for determining these attributes of remote objects in the arts of RADAR, SONAR and the like.

The present invention also provides a method of determining an attribute of a remote object comprising the steps of: emitting first and second signals, having different frequencies, towards a remote objea; receiving the first and second signals after refleaion from the remote object; mixing the received signals with each other to produce an IF signal having a frequency equal to the difference between the frequencies of the first and second signals; and determining an attribute of the remote object on the basis of the IF signal.

An apparatus for determing an attribute of a remote objea, according to the present invention, may be conveniently incorporated into a remote guidance apparatus. Such a remote guidance apparatus would also comprise a command transmitter for transmitting command signals to the remote objea and a command receiver located at the remote object for receiving the command signals.

Preferably, the command transmitter comprises the transmitting means, the command signals being transmitted by varying the difference in frequency between the first and second signals. Other information, such as instruaions to change position, could be sent concurrently by adjusting the aim of the transmitting means. This is particularly convenient when the transmitting means comprises optical transmitting means.

Advantageously, the command receiver or the receiving means includes a Hgl xCdxTe photosensor.

Preferably, the first and second signals differ significantly in their amplitudes.

An attribute determining method according to the present invention may be used in a remote guidance method. Such a method includes the step of transmitting corrective information to the remote object on the basis of the determined attribute.

The use of lasers as transmitting devices in the guidance system is most preferred. The wavelength of the laser radiation is 10.6 ym. This is due to the fact that X- 10.6 m is within the 8-14 ssm transmittance window of the atmosphere, radiation at this wavelength being absorbed 7-10 times less by the atmosphere and by dust and smoke formations than, for example, radiation at X- 1.06 ym. At the present time, at a wavelength of 10.6 ym is it possible to obtain effeaive heterodyning, thanks to the existence of high-stability lasers of varying power, having a narrow spectral line of radiation, and photoreceivers based on HglxCtxTe (x 0.215), which have sensitivity close to theoretical and a wide pass band.

Changing to the optical wavelength range (with the sole serious disadvantage of such systems being the dependence of range on the state of the atmosphere) imparts a multiplicity of positive operational qualities to guidance systems compared with the radio waveband. These are primarily the possibility of forming narrow radiation diagrams (fields of view) of systems, which makes it possible substantially to improve the accuracy of determination of the spatial coordinates of a remote objea. In faa, if one assumes that a transmitting antenna with diameter dt is uniformly irradiated by the nominal power of the transmitter, then the calculated size of the irradiated fragment of space at a distance R is determined by: D = 2.44 A-R (1) D = 2.44 dr .R It is obvious that, other conditions being equal, the value of D in the millimetre band ^-8-10-3 m is roughly 800 times greater than at #-10-5 m (10 ym).

The possibility of implementing coherent processing of location information in guidance systems makes it possible to provide in the systems: - high sensitivity; - spatial and frequency seleaivity; - noise immunity, since when two spatially superimposed coherent electromagnetic waves with frequencies fo (reference) and f, (signal) are incident on the sensitive area of a photo receiver, in the post-detector processing channel, formed at an intermediate frequency fif~sfos the signal at the output will be proportional to the resultant of the intensities of the reference and signal radiation sources ~E,-Eo. Processing the signal at an intermediate frequency ensures high frequency selectivity in the system. The heterodyne method of reception in laser finding ensures spatial filtration of the optical fields being received. This is due to the fact that the effective field of view of a heterodyne receiver is limited by diffraction in the entry aperture (diameters to , where the width of the main lobe is given by

The high spatial and frequency selectivity determine the high noise immunity of guidance systems using the heterodyne method of receiving radiation.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a block diagram of the ground sub-system of a laser instrument landing system (LILS); Figure 2 is a block diagram of the on-board sub-system of a laser instrument landing system; and Figure 3 illustrates a photosensor used by the on-board sub-system of Figure 2.

A laser instrument landing system (LILS) provides a means of controlling the decent of an aircraft onto a runway, particularly where the runway is small or visibility is poor. The laser instrument landing system (LILS) is intended to set the heading and glide path for aircraft and also the speed at various points in the glide path and to provide automatic landing conditions at various times of the day. LILS makes it possible to determine and then set for an aircraft the following parameters of its motion: - the direction and amount of deviation of an aircraft from the optimal glide path; - the approach speed of an aircraft at various distances from the runway; - control of aircraft movement after landing.

The LILS comprises a coherent laser information system, providing: 1) reception of preliminary target acquisition data; 2) seeking and locking-on to an aircraft, tracking it in respect of angular coordinates and speed; 3) transmission of data to the aircraft on optimal (required) aircraft motion parameters in the descending glide path; 4) identification of error signals, characterising deviation of parameters from the optimal; 5) transmission of these to the aircraft automatic systems controlling the state of its control surfaces and engine thrust.

Referring to Figure 1, the ground sub-system of the LILS includes a transmitting section and a receiving section. The transmitting seaion comprises a main laser 1 for generating a laser beam having frequency / a subsidiary laser 3 for generating a beam having frequency f2 , a control circuit 5 for controlling the frequencies of the lasers 1, 3, a feedback photosensor 7 for detecting the beams from the lasers 1,3 and feeding back a control signal to the control circuit 5 and beam directing means 9 for direaing the beams at an aircraft 11 to be guided. fi and f2 are in the region of 28.3THz.

The receiving seaion also employs the beam directing means 9 where it is used to capture light reflected from the aircraft 11. The other components of the receiving section are a receiver photosensor 13, formed from HgX xcdxTe (x = 0.215), onto which are directed captured reflected light and a proportion of the beam from the main laser 1 and a signal processing circuit 15 for processing the signal output by the receiver photosensor 13.

A control computer 17 receives the output of the signal processing circuit 15 and provides control signals to the control circuit 5 and the beam directing means 9.

Referring to Figure 2, the on-board sub-system comprises redirectable optics 21 which include a corner reflector for returning a portion of the beams from the ground sub-system, an objective lens 23, a photosensor 25 formed from HglxcdxTe (x=0.215), a signal processing circuit 27, a navigational computer 29, avionics instruments 31 and control signal shapers 33.

The operation of the system shown in Figures 1 and 2 will now be described.

Initially the ground sub-system operates in a preliminary target acquisition mode. In this mode, the control computer 17 instruas the control circuit 5 to operate the main and subsidiary lasers 1,3 at fi and f2 respectively. At the same time, the control computer 17 causes the beam directing means 9 to scan a region of space beyond the end of the runway. When the beams strike the corner reflector of an on-board sub-system, mounted to an aircraft 11, the beams are reflected back to the beam directing means. The reflected beams are directed onto the photosensor 13. The concurrent illumination of the photosensor 13 by both beams means that the electrical output of the photosensor 13 includes a signal at a frequency fd equal to the difference between, and,. The signal processing circuit 15 has a channel tuned to fd and processes signals at this frequency to determine whether reflected beams are being received. If reflected beams are being received, the signal processing circuit 15 notifies the control computer 17. When the control computer 17 is so notified, the initial target acquisition mode is terminated and a control mode begins.

In the control mode, the range of the aircraft 11 is determined using conventional means and the control computer 17 controls the beam direaing means to keep the beams trained on the corner reflector of the on-board subsystem. Techniques for this are also known in the art. The azimuth and elevation of the aircraft and its range provide the three co-ordinates required to fix its position in space relative to the ground sub-system..

The radial velocity of the aircraft relative to the runway is derived from the Doppler shift of the reflected beams. The Doppler shift is given by the formula: = 2fgV (3) c where A is the transmitted beam frequency, V is the radial speed of the aircraft and c is the velocity of light.

However, in the present system the reflected beams are not detected independently. Therefore, the radial velocity of the aircraft is determined by the control computer 17 from the IF signal's frequency using the following formula: V (t - (4) 2fd where fd is the difference between g, and and is the difference in the frequencies of the received beams.

Once the control computer 17 determined the current position and radial or approach speed, it compares the speed, azimuth and elevation of the aircraft with the corresponding values of the optimal glide path. Any deviation from the optimal glide path needs to be corrected, so the ground sub-system must transmit correcting information to the aircraft 11.

The speed correction information is transmitted to the aircraft by changing the difference between, and by Afd where: Afd = 2f1AV (5) c In other words, the signal processing circuit 15 determines the frequecy of the IF signal and outputs this value to the control computer 17. The control computer 17 calculates the approach speed of the aircraft from this value and the current value of -f2 , and then compares the actual approach speed with the optimal glide path value for the type of aircraft at the current range and under the prevailing conditions. The control computer 17 then sends suitable tuning control signals to the control circuit 5, instructing it to adjust the frequencies of the lasers 1,3 by the desired amount.

In order to communicate position correaion information, the control computer 17 determines whether the aircraft 11 is on the optimal glide path.

If it is not, the control computer 17 determines the direaion in which it should move to get onto the optimal glide path. This correction is then communicated to the aircraft by slightly redirecting the beams towards the desired position for the aircraft 11.

At the on-board sub-system, the portions of the beams, which are not reflected by the corner reflector, are directed by the redireaable optics 21, through the objective lens 23 onto the photosensor 25. Referring to Figure 3, the photosensor 25 comprises four square photosensitive elements 25a arranged into a square. The outputs of the photosensitive elements 25a are fed to the signal processing circuit 27. A navigation computer 29 receives the output of the signal processing circuit 27 and the outputs of the aircraft's avionics systems 31 and outputs control signals to a the control signal shaper 33, the redireaable optics 21 and the signal processing circuit 27. The control signal shaper 33 conditions control signals for controlling the control surfaces and engine(s) of the aircraft 11.

The on-board system derives speed correction information in the following manner. fd and f are known for the system. The signal processing circuit 27 determines the frequency fd of the IF signal output by the photosensor 25.

The navigation computer 29 then calculates the correa approach speed for the aircraft using formulas (4) and (5) and outputs the appropriate correaive control signals.

If formula (5) is used to set fd, Doppler effects become vanishingly small. For example, for an aircraft having an approach speed of 300ms l and td - 1GHz, the Doppler induced change in the IF signal frequency would be approximately 200Hz, representing a determined speed error of approximately 1.2 x 10-3 ms-'.

Figure 3 is a front view of the photosensor 25. In the figure, the lefthand side of the photosensor 25 is illuminated by the beams 35. Consequently, the outputs of the different photosensitive elements 25a differ. The relative magnitudes of these signals are detected by the signal processing circuit 27 and used by the navigation computer 29 to generate control signals for manouevering the aircraft 11. It will be remembered, that the ground subsystem communicates position correction information by slightly redirecting the beams. Thus, Figure 3 represents the situation where the ground subsystem has determined that the aircraft should move to its right.

It will be appreciated that the foregoing guidance process is iterative and is repeatedly performed until the aircraft has landed.

Although the present invention has been described with reference to an instrument landing system for aircraft, it will be appreciated that it has many applications. These include piloting of ships into docks and narrow fairways amd docking of space craft.

Claims

1. An apparatus for determining an attribute of a remote object comprising transmitting means for emitting first and second signals having different frequencies, receiving means arranged for receiving and mixing said first and second signals after reflection from a remote object to produce an IF signal having a frequency equal to the difference between the frequencies of the first and second signals and processing means responsive to the IF signal to determine said attribute.

2. An apparatus according to claim 1, wherein the attribute is radial velocity relative to the transmitting means and the processing means is operative to detect changes in the frequency of the IF signal arising from doppler shifts in the first and second signals caused by reflection from the remote object.

3. A remote guidance apparatus comprising an apparatus according to claim 1 or 2 and a command transmitter for transmitting command signals to the remote object and a command receiver located at the remote object for receiving said command signals.

4. An apparatus according to claim 3, wherein the command transmitter comprises the transmitting means, the command signals being transmitted by varying the difference in frequency between the first and second signals.

5. An apparatus according to claim 3 or 4, wherein the transmitting means comprises optical transmitting means.

6. An apparatus according to claim 5, wherein the command receiver comprises a multi-element optical receiver and the transmitting means is arranged such that the first and second signals concurrently illuminate the same portion of the optical receiver, the command receiver being responsive to the portion of the optical receiver so illuminated to generate a position control signal.

7. An apparatus according to claim 5 or 6, wherein the command receiver or the receiving means includes a HgxCdxTe photosensor.

8. An apparatus according to any preceding claim, wherein the first and second signals differ significantly in their amplitudes.

9. A method of determining an attribute of a remote object comprising the steps of: emitting first and second signals, having different frequencies, towards a remote objea; receiving the first and second signals after reflection from the remote object; mixing the received signals with each other to produce an IF signal having a frequency equal to the difference between the frequencies of the first and second signals; and determining an attribute of the remote object on the basis of the IF signal.

10. A method according to claim 9, wherein the radial velocity of the remote object is determined from a change in the IF frequency arising from doppler shifts induced in the first and second signals by reflection from the remote object.

11. A method of guiding a remote object according to claim 9 or 10, including the step of transmitting corrective information to the remote object on the basis of the determined attribute.

12. A method according to claim 11, wherein the corrective information is transmitted by varying the difference in frequency between the first and second signals.

13. A method according to claim 11, wherein the corrective information is transmitted by varying the direction in which the first and second signals are emitted.

14. A method according to any one of claims 9 to 13, wherein the first and second signals are optical signals.