GB2217137A - Tracking system and method for time-discontinuous signals - Google Patents

Abstract

Broadcast television signals transmitted from a moveable source such as a helicopter may be transmitted using a radio-frequency link. The transmitter may be tracked by a receiving antenna using the principle of higher order mode- synthesis by which the beam of the receiving antenna is squinted to a variety of directions about its boresight direction. The strength of the received signal is measured in each direction and the antenna is steered to adjust the boresight direction accordingly. To alleviate porblems with artefacts caused by the beam squinting, squinting is performed in the field blanking interval. To reduce the effects of multipath propagation caused by helicopter rotor blades squinting in all chosen directions is performed during a single television field blanking interval. The squinting of the receiving antenna may be achieved by providing slots in the walls of the receiving horn antenna, the slots being coupled to waveguide stubs short-circuited by pin diodes, or by a 'back wall'. The auxiliary waveguides may be simple open-ended waveguides, placed above and below the main horn aperture and equally disposed about the H-plane mid-point of the main aperture. Squinting of the beam may thus be carried out in the up, down, left and right directions. <IMAGE>

Description

TRACKING SYSTEM AND METffOD FOR TRANSMIlisD TIME-DISC0NTINUOUS SIGNALS This invention relates to a method and apparatus for tracking time-discontinuous signals transmitted by a moveable radio frequency (R.F) source, for example video signals transmitted from a helicopter.

moving vehicles are often used in Television Outside Broadcast (OB) operations to provide a platform for a camera or for a radio link mid-point. Video signals are communicated from the vehicle to a fixed receiving station using s.h.f. radio links with an omhi-directional antenna on the vehicle and a manually-pointed directional antenna at the receiving station.

The maximum range of communication is limited by the ability of an operator to point the receiving antenna when the vehicle is beyond visual range, or when there are several operating in the same area. Although described in this specification as related to television broadcast use, the invention can be applicable to other systems, particularly those involving a signal which is periodic in nature.

In the BBC such operations are now being improved by the introduction of a tracking receiver system which points the receiving antenna automatically. The tracking system, developed by the BBC is specially tailored to the reairements of a video transmission system and needs only one conventionally transmitted radio-link signal. Also, many components of the system are items of standard radio-link equipment. The two principal applications for this tracking system are operation "in the field", with the receiving antenna mounted on the roof of an Outside Broadcast van, and operation at a semi-permanent receiving station using the roof of a convenient building.

The benefits are an improvement in operational efficiency at the receiving station, a potential increase in the quality and availability of the radio link, and an increase in the maximum usable range.

At the present time the 2.5 GFIz band is conveniently used for such OB links; in the case of mid-point operation this band is used for links both in and out of the vehicle. Although the number of available channels is limited, this band still offers the most favourable combination of available transmitted power and receiving antenna gain.

Firstly, for background information, the existing, manual operation will be described, as it applies to helicopter links although similar schemes are used for other vehicles (for example racing cars).

Special rigs have been constructed for temporary installation in helicopters, containing one or more transmitters, and, for mid-point operations, receivers. It is not exceptional for a helicopter to carry a two channel mid-point transponder and the transmitter for an aerial camera; in this case all three transmitter signals are multiplexed into the same transmitting antenna. The transmitting antenna is either a biconical dipole or a Franklin (collinear) array slung beneath the fuselage of the helicopter on an electric actuator. Both tes of antenna are vertically polarised and have omni-directional radiation patterns in the azirrnath plane, but the Franklin-array has a more restricted pattern in the vertical plane providing some 3 dB gain. (3 dB gain relative to a dipole antenna).

The signal is received with a 1.1 m dish reflector antenna fitted with a dipole-disc feed. This is mounted on a conventional pan and tilt head and is directed manually towards the helicopter. The antenna is connected to a conventional 2.5 GHz radio-link receiver and an a.g.c. meter on the receiver can be used to judge optimum pointing. A 1.1 m dish has a beamidth of only 8 degrees at 2.5 GHz, so a fair degree of precision is required when the helicopter is flying close by. Sometimes two independent receivers are set out each with its oBn operator, offering "human" diversity.

When the helicopter reaches the limit of visual range the a.g.c. meter becomes the only means for tracking. This can be effective because for long range operation the apparent rate of angular motion of the helicopter is very low. However, if the signal is subject to multipath propagation or is lost momentarily because the aircraft flies behind a building, this can mislead the operator into making unnecessary (and potentially disastrous) antenna movements. The problem is that the a. g. c. meter gives no information about the direction in which the antenna should be panned, only that some change might be required.

To overcome the shortcomings of manual operation an autoratically-tracking receiver system is highly desirable and to avoid the expense of re-equipping for another frequency band, operation at 2.5 CIt7 is preferred. The few available systems for 2.5 GHz include their own radio-link receiver, antenna support structure, etc. which makes them expensive.

Also, it is desirable that the system should use only the 2.5 GHz radio-link signal radiated from the helicopter to avoid the need for additional radio-frequency spectrum. The system is required to work without leaving visible "artefacts" in the received video signal.

The system is required to track a moving helicopter at a maximum apparent angular rate of about 3 degrees per second.

This is derived from a maximum ground-range of 0.5 mile (800m), a minimum altitude of 500 feet (150m) and a maximum air-speed of 80 knots (40 m/s). The maximum altitude is specified as 2000 feet (600 m), and the maximum range length about 50 miles (80 km), although this should be limited by the f.m. radio-link receiver reaching threshold conditions, not by failure of the tracking system.

Because of the limited beamvidth of the receiving antenna, tracking is required in two planes; typically, azimuth and elevation. A special receiving antenna could be used with a tailored radiation pattern in the vertical plane (cosec2 )to overcome the need for elevation tracking, but to uphold the antenna gain, and therefore the maxImum range, this would be much larger than the standard 1.1 m dish. The range of elevation angles is 0 to 37 degrees, and in the azimuth plane the full 360 degrees coverage is required.

Fundamentally, there are two ways to make an antenna track a moving target: (a) Absolute location: if the 3-dimensional locations of the target and the tracking station are known, trigonometry gives the required antenna pointing angles. This can be Implemented using, for instance, secondary radar with a transponder in the aircraft, or an airborne navigation system relaying to the ground the co-ordinates of the helicopter.

(b) Bearing determination: this provides the pointing angles directly. This can be implemented using a simplified form of secondary radar, an optical tracking system or a r. f.

tracking system responding to a signal transmitted from the helicopter.

Radar requires much additional equipment (inciuding height-finding) and additional r.f. spectrum, and a navigation system would require additional spectrum for relaying the positional information back to the receiving station. An optical tracking system could become confused if a second aircraft was flying in the same area, and its range could be limited by weather conditions.

An r. f. tracking "signal sensing" system could use either the existing radio-link signal or a separate "pilot" signal transmitted from the aircraft. The use of a pilot signal is attractive because in the receiving equipment this could be processed and chopped as necessary without disrupting the radio-link signal, but it would demand additional spectrum and some additional equipment.

Compared to the radar and navigation approaches a signal sensing system which uses only the existing radio-link signal has the disadvantage that tracking can only function when the link transmitter is switched on and a clear, unobstructed path is available between the helicopter and the ground receiver.

However, we have appreciated that these conditions are pre-requisite for communciating television signals, so provided that the system does not take unduly long to re-acquire "track" after a transient outage, the r.f.

signal-sensing method can be equally effective.

There are several ways in which a signal-sensing scheme can be ipplemented. These include: (a) Step track, where the receiving antenna is moved, bodily, in small steps changing the beam direction. The result of each step, judged by whether the received signal level increases or falls, determines the direction of the next step. For this application the process would be carried out, perhaps independently, for two orthogonal planes of motion.

(b) Conical scan, where the antenna beam is moved around a central, boresight direction to gain bearing information. This is usually accomplished by moving only part of the antenna (e.g. the feed). This provides azimuth and elevation information sequentially.

(c) Mbnopulse, where for each plane of interest the antenna is fed in a way which gives "sum" and "difference" radiation patterns. For each plane the solution of simultaneous equations using the "sum" and "difference" signal voltages gives a bipolar signal representing the direction and degree of angular tracking error. These signals can be produced simultaneously and at great speed.

In any case, the antenna can be mounted on a two-axis positioner and the angular error information processed to give the appropriate motor drive signals.

The e speed of a step track system would be limited by the ratio of the inertia of the antenna and positioner to the available motor torque, so rather large motors and very robust mechanical components would be required. Conical scan, with vertical polarisation would require a nutating feed; a complicated arrangement of bearings and flexible or rotary joints.

Monopulse can provide great angular precision and speed of operation, but it is complex and expensive. In this application (using a dish reflector) a multiple feed would be used, with numerous r. f. cables feeding signals either to numerous receivers, or to at least one additional receiver with the multiplexing. The great accuracy of this approach is not really required for an outside broadcast link.

We have appreciated that a sampled scan approach would be advantageous, in which a number of discrete beam directions about the boresight direction (e.g. up, down, right and left) are used, giving information for the two planes sequentially.

The sampled scan approach has been used in some commercial helicopter tracking systems using a multiple feed. An example is the use of a central horn at the relector' s focus, surrounded by four peripheral horns. The radio-link signal is received conventionally via the central horn, and a separate tracking receiver, connected in commutation to each of the peripheral horns, derives the tracking information. The peripheral horns, offset from the focus, give the antenna radiation patterns which are squinted away from the central, boresight direction. This method provides sate simplification over the monopulse approach but it still needs a dedicated s.h.f. tracking receiver.

We have also appreciated that this disadvantage can be overcome by the use of a type of antenna feed which allows high-speed electronic control of the beam direction, and tracking without a second, dedicated receiver. Such a feed uses "higher order mode synthesis".

In this a central horn feed is used, surrounded by four peripheral, open-ended waveguides. The waveguides are coupled parasitically to the horn and only the horn is connnected to the receiver. Under electronic control, and independently, each waveguide is selectively terminated either by a p.i.n.

(P-type, intrisic, N-type) diode biased into conduction, or by a short-circuit "back wall". The change of electrical length introduced by the p.i.n. switch alters the relative phase of the signal re-radiated by the each waveguide, and this can be arranged to introduce a phase slope into the illumination of the reflector. Consequently the antenna beam can be squinted away from boresight in one of the four directions; up, down, right and left, each with the same degree of angular offset.

The action of introducing phase-shifted elements into the focal field of the reflector is analogous to the introduction of suitably phased higher-order modes into the waveguide horn; in this case the effect is synthesised, but the end result is the same.

When a symmetrical reflector antenna is pointed accurately at a distant transmitter, by design the electro-magnetic fields in the region of feed (about the focal point) are syrtrnetrical.

If the antenna is de-pointed by a small angle, asymmetry is introduced and this usually causes a reduction in the received signal strength. However, by applying "mode converters" to the antenna feed it is possible to transfer same of the power fram the asymmetric component (or mode) into the required symmetric mode which couples to the feed. If the "mode converters" are switched on and off on demand then tracking information can be gained from variations in the strength of the received signal. This process effectively squints the beam of the receiving antenna about the boresight direction, and by using mode conversion in two orthogonal planes a form of conical scan or pyramidal scan is achieved with no moving parts.

MDde conversion is more easily applied to a waveguide horn feed than to the type of dipole-and-disc feed used conventionally with the existing 2.5 GIz radio link equipment, but we have appreciated that a waveguide horn feed can be used in this application with e.g. standard 1.1 m reflector with only a small reduction in the aperture efficiency of the antenna. The mode conversion can be switched on and off, or more appropriately, phased using p. i. n. diode switches which can operate at very high speeds.

In order that higher-order mode synthesis is understood the technique will now be described with reference to figures 1 to 4 of the accompanying drawings. For ease of understanding the description will be given with reference to an example rectangular waveguide horn feed, mounted with its aperture at the focus of a symmetrical parabolic reflector. The antenna will be considered as transmitting, although in practice it will be receiving, but reciprocity may be applied for the receiving case.

The E and H fields in the mouth of the horn are similar in distribution to those in a smooth-walled rectangular waveguide, and these cause an approximately spherical TEM wave to be radiated towards the ref lector. By applying geometric-optics principles it can be shown that the ref lector converts this to a plane wave which forms the secondary beam.

If the centre of the horn's aperture is on the axis of the reflector the secondary beam is directed along that same axis (ie. in the boresight direction). If the horn is offset to one side then optical reflection dictates that the beam will be offset to the other side.

In practice, with a ref lector which may be small in terms of wavelengths the accuracy of such optical principles becomes questionable and a more thorough analysis is possible using "physical optics"; the generation of surface currents in the reflector by the incident radiation from the feed, and derivation of the secondary beam from the radiation caused by these currents. Nevertheless, the qualititive effects on the main lobe of the secondary beam, predicted by either method, are fundamentally the same.

The amplitude distribution of the E field across a waveguide supporting the lowest order, dominant mode, TE10, is a half-sinusoid with a peak in the middle and zero at the side walls where it is effectively cancelled by equal and opposite components caused by currents flowing in the side walls. This is illustrated in Fig 1. Such a symmetrical distribution in the mouth of the feed horn maps to a symmetrical distribution in the secondary (reflected) wave, giving a symmetrical beam.

Now consider the case when the waveguide supports the second order mode TE20. In this case the E field distribution is a complete sinusoid; zero at the side walls, as before, and zero in the middle but with two antisymmetrical peaks between the zeros. This is illustrated in Fig. 2. In the oommunications sense a feed supporting only this mode would not be very useful because the antiphased radiation from the two halves of the aperture would give a bifurcated secondary beam; an antiphased pair of beams, each slightly offset fram the boresight direction, with a null in the boresight direction.

If the horn supports both modes then some more useful results follow, especially if nost of the transmitted power arrives at the horn aperture in the symmetrical TE10 mode and a low level of TE20 is present, for instance -lOdB. The result is then dependent on the relative phases of the fields carried by the two modes. If one (spatial)half of the TE20 mode is co-phased with the TE10 then an amplitude slope is produced across the horn aperture and an asymmetrical secondary beam results.

However, if the two halves of the TE20 mode are in quadrature with the Toe10 mode (one half leading and the other lagging) then very little amplitude disturbance occurs, but a phase slope is produced across most of the horn aperture. This is illustrated in Fig. 3, and gives a similar result to offsetting the feed to one side of the reflector's focus; the beam is offset, or squinted, at an angle to the boresight direction.

The magnitude of the offset angle is proportional to the degree of phase slope, which in turn is proportional to the ratio of the powers in the two modes, so the greater the proportion of TE20 mode added the greater the off set. The limit is set by the degree of amplitude disturbance that can be tolerated, and -lOdB of second order mode is about the maximum for the case in hand. By reversing the polarity of either mode the beam is squinted to the other side of boresight.

Whether or not a uniform waveguide will support the TE20 mode is dependent on its H-plane width. If power is injected into the waveguide in the TE10 mode, for instance using a simple co-axially fed probe, it is possible to convert a proportion into higher-order modes by introducing a discontinuity into the guide. An example is a step change of guide (H-plane) width; just beyond the step a cclmplicated mixture of modes will exist, but only those which are not "cut off" (evanescent) will propagate.To determine the degree of conversion to a particular higher-order mode is not a simple task, and in the past this has been handled enpirically. btre recently a numerical "mode matching" technique has been reported, which uses the principles of conservation of energy and uniformity of tangential E-field either side of the non-uniformity. Also a mode converter can be designed to favour a particular higher-order mode by observing the required spatial distribution of E- (or H-) field. For instance a TE10 to TE20 mode converter may use a shunt susceptance such as an E-plane conductive post or fin, offset in the H-plane to one side of the centre of the waveguide.

The local E-field is concentrated in this "capacitor" and an asymmetric component of E-field is derived. This corresponds to a proportion of power in the second order mode. The actual position of the post or fin may then be chosen to suppress the third-order mode, TE30, if this can propagate, by placing it at one of the TE30 E-field nulls (each one third of the way across the waveguide).

The requirement of a tracking feed, to be able to switch the beam squint direction fran one side to the other, means that the phase of the converted mode must be under electronic control. This can be achieved by using p.i.n. diodes to control the electrical position of the discontinuity with respect to the horn aperture, and schemes have been devised using slots in the walls of the horn to provide the discontinuity. The slots couple to short-circuited waveguide stubs and the position of each short-circuit is determined either by a p.i.n. diode, when it is biased into conduction, or by a "back wall" when the diode is reverse biased. The diode and back wall are separated by one quarter guide-wavelength so switching the diode on and off changes the phase of the reflected wave by 180 degrees.An antisymmetrical pair of such stubs can give the required TE2O conversion in either sense with respect to the exciting TE10 mode. The distance between the slots and the horn aperture is chosen to give the desired phase difference between the two modes since they have different phase velocities in the flare of the horn.

Another implementation of this process is to simulate the higher-order mode in the focal region of the reflector by using two auxilary waveguide horns, located either side (in the H-plane) of the main horn aperture. These are either driven (using discrete co-ax or waveguide couplers), or excited parasitically from the main aperture; the latter method is easily capable of providing the -lOdB c(rOonent.

Each auxillary waveguide is terminated in a short-circuit provided by either a forward biased p.i.n. diode or a "back wall", and by suitable positioning it is possible to arrange for the re-radiated field components to be mutually antisymmetric and in quadrature with the radiation fram the main aperture. The oombination of the TEM fields radiated by these three apertures can be made an adequate simulation of that radiated by a single aperture with a component of the true higher-order mode. In practice the auxiliary waveguide horns can be simple open-ended waveguides and the assembly can be fabricated more easily than a horn with true mode converters built into its flare.

The expansion of this idea to provide beam squints in the other, orthogonal plane is something like a duplication of this process, with two more auxiliary waveguides above and below the main aperture. This is a simplification because with linear polarisation the parasitic excitation of two of the auxiliary waveguides will be stronger than of the other two, so their positions around the main aperture need careful adjustment to give equal offset angles in the two planes. In the chosen implementation the auxiliary waveguides are actually placed in pairs above and below the main horn aperture, each equally disposed about the H-plane mid-way point of the main aperture, as shown in Fig 4. In this way the parasitic excitations are equal, and squinting in each of the four directions requires one pair of adjacent p.i.n.

diodes to be forward biased, while the other pair is reverse biased. Further reference is made to the two papers, namely; 1. WATSON, B., DANG, N. and GHOSH, S.; 1981; "A mode extraction network for r.f. sensing in satellite ref lector antenna"; Proc. of 2nd International Conference on Antennas and Propagation.

2. DANG, R., WATSON, B. and DAVITS, I.; 1985; "Electronic tracking systems for satellite ground stations"; Proc. of 5th European Microwave Conference, A mode-convertor of the type outlined above is disclosed in EP-A-171149. The specification describes how higherorder mode synthesis may be used to track distant objects such as artificial satellites in orbit around the earth. Use is made of the satellite beacon signal as a measure of signal intensity whilst the antenna beam is squinted or offset to obtain tracking information. By commutating the currents biasing the p.i.n. switches, the beam direction can be comnatated for the required sampled scan.If the transmitting helicopter is away from the boresight direction then for each step of the scan a different signal level is received. Then by resolving the envelope of the received signal with reference to the p.i.n. switch driving waveforms, differential information is gained about the degrees of tracking error for the azimuth and elevation planes, independently. Using sample-and-hold circuits a pair of error signals can be derived, and these are amplified and fed in the appropriate senses to the positioner motors.

Because the error signals are derived differentially (e.g.

from the difference between the "up" and "down" signal levels), their magnitudes are essentially independent of the mean signal level, so they are not influenced by changes of the range, transmitter power or receiver gain at rates much less than the sampled scan rate.

Thus an independent pair of control loops is formed, and these act to reduce the angular tracking errors. When the errors are zero, and the helicopter appears in the boresight direction, the same signal level is received for each step of the scan.

The degree of beam squint is only a small fraction of the beanm7idth.

For small tracking errors the sampled scan imposes a very small degree of amplitude modulation on the received signal.

In steady-state conditions, within the bounds of stability and signal-to-noise ratio (SNR), the tracking errors can be made as small as desired by increasing the control loop gains.

Obviously, the SNR degrades as the range length inc increases and the mean signal level decreases, but the radio-link f.m.

de.7xxiulator will fail when received carrier-to-noise ratio falls to about 11 dB in the 20 NHz channel bandtwidth. The control loop bandwidths are very much smaller, about 5 Hz is sufficient to follow all expected helicopter manoevres, so at maximum range the SRN of the a.m. envelope signal passed to the tracking system is still very large.

Although we have realised that higher-order mode sythesis is, in theory suitable for use in outside broadcast links, there are problems which arise from the existing implementations such as that described in EP-A-171149 for example. The system of EP-A-171149 relies on a beacon signal entirely separate from the conmnunications signal, to obtain tracking information. If this system were to be used with the radio-linked video signals, there is a danger that the amplitude modulation, produced by the beam squinting could disrupt and impair the pictures received.

Furthermore, there is an additional problem which arises from the proximity of the helicopter rotor blades to the transmitter. The level of the radio-link signal received from the helicopter is subject to a degree of amplitude modulation due to multipath propagation caused by the presence of the blades near the transmitting antenna. This modulation can be at a rate close to an appropriate sampled scan rate (eg. 50H3) and could interfere with the operation of the tracking system.

The invention aims to overcome the above described disadvantages and, thus, to provide a system which can utilize high order mode synthesis to track sources transmitting time-discontinuous signals such as television signals.

The invention is defined in the claims to which reference should now be made.

Where the transmitted signal is a video signal the beam squinting during which tracking is acquired by sampled scanning is performed in the field blanking periods of the video signal. It is to be understood that for the case of a video signal, periods when no active information is transmitted refers to that part of the video signal that does not convey picture information. Squinting, also known as de-pointing, in the field blanking periods has the advantage that any artifacts caused by the beam steering which might degrade picture quality can be removed by simple video processing such as clamping.

The method and system of the invention may be used to track a variety of different sources, for example radio cameras or transmitters mounted in helicopters or other vehicles. The receiver river may be stationary or moveable, for example located in a land vehicle.

Perferably the whole squinting operation through the four successive squint positions, up, down, left, right, is performed during a single field blanking interval. Such an operation alleviates the problems of rotor blade multipath propagation outlined above. Preferably each squint takes place over two television lines which means that the complete cycle is accomplished in the period of four pairs of television lines or 512 microseconds. Were one squint to be performed per blanking interval, the complete cycle would take four fields or 80 milliseconds. Over 512uS the influence of the rotor blades is insignificant.

Preferably the differential pairs of squints are arranged to appear consecutively so that e.g. lines 318 and 319 could be the UP squint, lines 320 and 321 the down squint etc. Each squint may occupy a single television line length, although this would require faster processing.

The invention will now be described in more detail in terms of one specific example with reference to the drawings, in which: Figures 1 to 4 are used in the preceding discussion to illustrate the basic principles of higher-order mode synthesis; Figure 5 is a block circuit diagram of a receiver embodying the invention; Figure 6 illustrates the squinted beams; and Figure 7 illustrates the cyclic sampling of the received signal.

A block diagram of the complete system is illustrated in Figure 5. The standard 1.1 m dish reflector 10 is mounted on an elevation-over-azi77.7uth motorised positioner 12 and the higher-order mode synthesis feed 14 is mounted at its focus.

The positioner is supported by a standard radio-link tripod.

The feed 14 is of the type described in European Patent Application 171,149 to which reference should be made for further details of its construction and operation. Further reference may be made to European Patent Application No.

014,692.

A single r.f. cable 16 connects the feed to the r.f. head 18 of the conventional link receiver, which to economize on positioner torque, is located nearby on the floor (or roof).

A low-power, low-frequency control cable 20 connects the p.i.n. switches in the feed to a remote electronics package, the "processor" 22. Only these two cables are subject to continual flexing. Stationary cables 24 connect the positioner motors to a remote servo-amplifier uZt 26. The processor and servo amplifier can be located, with the remote-control unit 28 for the link receiver, some 50 m away.

The remote control unit provides the demodulated video output from the receiver.

A 70 MEz intermediate frequency signal which is not limited nor subject to automatic gain control is passed from the link receiver to a logarithmic amplifier 30 which detects the a.m.

envelope of the received signal. The signal output by the log amplifier 30 is an analogue signal of magnitude proportional to the received signal strength measured in dBs. In the processor the signal output by the log amplifier is sampled, as illustrated in Figure 6 separating the azimuth and elevation information to derive the tan error signals. The processor 22 is also fed a field sync waveform derived from the video signal by a "flywheel" sync separator 34. Then, under normal conditions the error signals are sarpled-and-held at television field rate, filtered and passed to the servo amplifier unit. This contains independent gain and power output stages for the two axes, and provides the drive currents for the azimuth and elevation positioner motors.

With reference to Figure 6, depending on the states of the p. i. n. switches, the antenna beam is squinted to one of four directions, UP, DOWN, LEFT and RIGHT, each equally offset from the boresight direction shown at +. By comtrLitatg the switches the "pyramidal" scan is produced and when the transmitting source (the helicopter) appears at any bearing other than the boresight direction this will give rise to a cyclic variation in the received signal level.

The signal level is monitored by the log. amplifier and in the processor its output signal is samplea at points corresponding to the UP/DOWN and RIGHT/LEFT beam offsets, as illustrated in Figure 6. Two square waveforms are produced with peak to peak magnitudes proportional to the azimuth and elevation offset angles, and fram these are derived the error signals for driving the antenna positioner. Thus the pair of control loops is completed; as the antenna is moved bringing the boresight direction closer to the actual bearing of the transmitting source, so the magnitudes of the error signals decrease. When the antenna is aligned precisely the same signal level is received for each of the beam directions and the square waves disappear.One advantage in the use of a logarithmic level detector is that it will output a constant voltage difference (square wave magnitude) for a given ratio of input levels, irrespective of the mean level of the input signal. Thus the gains of the control loops will remain essentially constant as the mean signal level varies.

When any degree of error exists, as will be the case when the helicopter is in motion, the coinutating action will cause amplitude modulation of the received signal and ideally this will have no effect on the operation of the f.m. link receiver. However, if the entire dynamic range of the receiver is to be used, to achieve the longest possible range, it may happen that the limiter in the receiver will allow some degree of a.m. to reach the f.m. demodulator. Thus, the commutation is synchronised to the television field rate its effect on the resulting video signal can largely be removed by clamping.To this end the generation of the p.i.n. switch driving waveforms is synchronised to the field sync. waveform derived from the outgoing video signal by the sync, separator.

A "flywheel" action is desirable to prevent abrupt changes occurring during short fades (such as when the helicopter flies behind a building), and the sync. separator should be arranged to free-run at 50 Hz when no video signal is present. The remaining "glitches" will occur in the field blanking intervals and provided that their magnitude does not Upset the first field sync. regenerator in the programme transmission chain (probably in a television synchroniser) they will be lost without trace.

We have appreciated that an adverse effect produced by rotating helicopter blades can be overcome. The level of the radio-link signal received from a helicopter is subject to a degree of amplitude modulation due to multipath propagation caused by the presence of the blades near the transmitting antenna. Tunis a.m. can be at a rate close to television field rate, and this could interfere with the operation of the tracking system if the cointrL2tation cycle was simply stepped through at the television field rate.

Thus, we prefer to perform the whole up-down-left-right conmutation cycle during each field-blanking interval. An exfflmple is to squint the receiving antenna beam UP during television lines 6 and 7, DXXAN during lines 8 and 9, LEFT during lines 10 and 11, and RIGHT during lines 12 and 13 of each television picture, and then again for the other field, with the same sequence on lines numbered 312 higher (e.g. UP during lines 318 and 319, etc.). This method has two advantages. Firstly, the complete cycle is accomplished in the period of four pairs of television lines, 512 microseconds, as opposed to four fields, 80 milliseconds, and over 512 microseconds the rotor blade effect is insignificant.

Secondly, the differential pairs, UP/DOWN and LEFT/RIGHT are arranged to appear consecutively and this further reduces the influence of spurious a.m. relative to an up-left-down-right sequence. The use of pairs of television lines, rather than single lines, is optional and is chosen simply to reduce the required speed of the sampling circuits in the "processor".

With this modification, downstream processing to remake "artefacts" may have to insert eight blank lines during each field-blanking interval.

During the periods when the television signal is conveying picture information the antenna beam can either be left squinted to one of the offset directions (UP, DOWN, LEFT or RIGHT), or returned to the boresight direction. As noted before the difference in received signal strength in these two directions signals, is small and does not affect the picture quality. However, as a consequence of squinting the beam, a practical higher order mode synthesis feed can offer an improved radiation pattern for the complete antenna (with respect to side lobe levels) in the non-suinted boresight case. The latter case has been chosen for the reception of active periods of the television signal.

By suitable choice of loop gains and loop filtering, the simple sampled linear control systems thus described can provide tracking for most expected conditions of constant position, constant velocity and constant acceleration, but for initial acquisition of track and to cater for transient outages some non-linear assistance is required. Preferably the processor includes a micro-computer for this purpose, such as a BBC microcomputer.

The micro-camputer can provide a graphic display of the system's status and add a degree of. "intelligence" to its operation. In normal conditions it simply monitors the levels of the received signal and the tracking error signals, but for initial acquisition and during outages it can be programed either to offer manual control, with a joystick, or to iirplement a step track algorithm.

When the system is first switched on the micro-computer directs the operator to run through a "preamble" check-list.

This includes setting the azimuth and elevation to the direction from which the helicopter is expected to appear. If automatic acquisition is selected the micro-computer then steers the antenna through a limited scan, logging signal levels, and if the signal is detected the step track process begins and the antenna direction is optimised. When the error signals are sufficiently small the system changes over to sampled scan for "linear" operation. However, if the signal is not found, the width (and height) of scan can be increased in steps. The strategy exists as software so a great deal of sophistication is possible with very little extra hardware.

When an outage occurs, if automatic operation is selected, the system can implement the same step track process to try to regain track. The preamble includes options for delaying the orset of re-acquisition and maintaining the previous antenna direction and angular velocities.

The system illustrated has the advantages that: (a) It uses readily-available radio-link equipment, including the receiver, and does not require a separate tracking receiver.

(b) It uses a tracking feed which requires only one r.f.

connection and can be operated at very high scanning rates.

The system is synchronised to television field rate to make the tracking function invisible to the viewer.

(c) The system has immunity to rapid acting multipath effects such as those caused by helicopter rotor blades.

(d) It incorporates a micro-compllter which can apply sophisticated strategies in exceptional circumstances, such as automatic re-acquisition of track using a step track algorithm.

The system has been described for use in the 2.5 GF band but could be used at other frequencies, in particular at frequencies above 1 GHz. The system could be used with other time-discontinuous signals, such as digital packet-data from a transmitter to a receiver, one or both of which are moving.

Claims (23)

1. A radio-frequency tracking system for tracking time-discontinuous signals transmitted from a moveable source, comprising a receiver including a steerable receiving antenna, means for squinting electronically the main beam of the antenna successively into a plurality of positions about the antenna boresight axis, means for measuring the strength of the received signal in each of the squinted beam directions and the boresight direction, means for steering the receiving antenna in response to the signal strength measurements to optirise received signal strength in the boresight direction, and synchronising means for synchronising the Squinting means with the timing of the received signal to squint the antenna beam during periods when no active information is transmitted.

2. A system according to claim 1, wherein the squinting means squints the beam into each of the plurality of squint directions during a single period when no active information is transmitted.

3. A system according to claim 1 or 2, wherein the transmitted time-discontinuous signals are video signals and the synchronising means synchronises the squinting means to squint the antenna beam during the field blanking intervals of the video signal.

4. A system according to claim 3 wherein the squinting means squints the beam into each of the squint directions during successive video lines or groups of lines, of the field blanking interval.

5. A system according to claim 3 or 4 wherein the squinting means squints the beam into each of the squint positions for two video line periods.

6. A system according to any preceding claims wherein the squinting means squints the beam into four positions substantially equidistant from the boresight axis.

7. A system according to any of claims 2-6 wherein the synchronising means synchronises the squinting means to the field synchronising pulses of the video signal.

8. A system according to claim 7 wherein the synchronising means comprises a flywheel separator for separating field synchronising pulses from the received video signal.

9. A system according to any preceding claim comprising signal acquisition means for picking up the signal upon switch-on or after temporary signal loss, the aoquisition means comprising means for steering the antenna through a limited scan, means for monitoring received signal levels during the limited scan, means for initiating a step-track steering of the antenna on detection of a signal and means for initiating the squinting and measuring means when the error in the received signal level falls to a predetermined level.

10. A radio-frequency communications system comprising a moveable transmitter and a tracking system according to any preceding claim.

11. A system according to any preceding claim wherein the receiver is moveable.

12. A system according to any preceding claim wherein the transmitter is located in a helicopter.

13. A system according to any preceding claim wherein the transmitter is a radio-carera.

14. A method of tracking a movable source transmitting timemdiscontinuous r. f. signals using a steerable receiving antenna having means for electronically squinting the antenna beam about its boresight axis, the method comprising the steps of coarsely aligning the antenna to receive the transmitted signals, electronically squinting the main beam of the antenna successively into a plurality of positions about the boresight axis, measuring variations in intensity of the received signal in each of the squint positions and the boresight direction, adjusting the boresight direction to an optimum direction in response to the measured intensity in the squint positions, wherein the beam squinting is synchronised to be performed during a period which no active information is transmitted from the source.

15. A method according to claim 14, wherein the means is squinted into each of the plurality of positions during a single period when no active information is transmitted.

16. A method according to claim 14 or 15 wherein the transmitted signal is a video signal and the beam squinting is synchronised to be performed during the field blanking intervals.

17. A method according to claim 16 wherein the beam is squinted into each of the squint directions during successive video lines or groups of video lines of the field blanking interval.

18. A method according to claim 17, wherein the beam is squinted in each of the squint directions over a period of two video lines.

19. A method according to any preceding claim wherein the beam is squinted to four positions substantially equidistant from the boresight axis.

20. A method according to claims 17 or 19 wherein the beam is squinted in up, down, left and right directions from the boresight and the up and down squints and the left and right squints appear on successive video lines or groups of video lines.

21. A method according to any of claims 15 to 20 wherein the squinting means is synchronised with the field synchronising pulses of the video signal.

22. A method according to claim 21, wherein the field synchronising pulses are separated using a flndheel separator.

23. A method according to any of claims 14 to 22 wherein the step of coarsely aligning the antenna comprises steering the anteanna through a limited scan to detect a transmitted signal, on detection of a transmitted signal performing a step-track operation to increase the received signal strength, and initiating at a predetermined received signal strength, the electronic squinting to track the source.