Patent Application
20110187581
The present invention relates to a radar device for detecting or tracking aerial targets fitted to an aircraft. It applies notably in the field of airborne radars, in particular radars with large angular coverage and short range that are necessary for example for carrying out a function of the “see and avoid” type on drones, which function is also commonly called “Sense & Avoid”.
The systems provided for carrying out the “Sense & Avoid” function on board drones use conventional radars operating in the millimetre band, for example in the Ku or Ka bands, which are derived from radars provided for other functions. The current state of regulations does not permit the flight of automatic craft within the general air traffic.
These conventional systems are equipped with a rotary mechanical antenna. They have a naturally slender beam because of their high operating frequency. They thus allow precise tracking, on the other hand, their scan speed must be very high to cover an angular domain suited to a function of “Sense & Avoid” type at a sufficient rate. Typically, such a function ought to cover an angular field comparable to a pilot's angle of vision, i.e. about ±110° in azimuth and 40° in elevation. The angular coverage constraint is therefore only partially satisfied. Moreover, these conventional systems require a protuberance of the drones since they cannot be placed directly on their structure, thus posing aerodynamism or bulkiness problems.
Conventional electronic scanning, using a slender formed beam, can solve this fast coverage problem, but it exhibits the drawbacks of the complexity of the time allocated to the processing of a given direction, which becomes extremely small on account of the angular field to be covered in a given timescale. One solution for solving these problems is to use a computational beamforming system, termed CBF. Indeed, a CBF system makes it possible to carry out continuous observation of a plurality of direction by means of slender beams formed simultaneously by computation, and makes it possible to overlay one or more antenna systems directly onto the structure of a drone, or of any other aircraft, without significant additional bulkiness, since it is no longer necessary to provide for antenna rotation.
An exemplary simple CBF system operates in the following manner:
The range R of a radar monitoring a certain solid angle is given by the following proportionality relation:
where PE represents the power emitted, AR the reception antenna surface area, λ the wavelength, σ(λ) the equivalent radar cross section of the target, denoted RCS, which may be dependent on the length λ, T the repetition period of surveillance of the domain of solid angle Ω and L the microwave frequency losses. This relation assumes that the emission antenna pattern exactly covers the domain Ω. This condition fixes the gain of the emission antenna.
It is apparent that if the RCS of the target is independent of wavelength, so also is the surveillance range.
However, this simple solution exhibits a drawback as soon as a tracking and precise angular location function is necessary. Indeed:
A good compromise is to work in the S band. Moreover the constraints related to spectral congestion are less critical than in other neighbouring bands. The choice of this band is more favourable than the L band notably from an angular precision point of view, but the location precision problem remains. Indeed, the coverage of the emission antenna is well suited to the search domain but is unsuited to the domain necessary when tracking. When tracking, the majority of the energy emitted is lost in directions where the target cannot be present since its position is then known a priori. This results in a needless loss of the signal-to-noise ratio, S/N, in relation to conventional radar systems.
An aim of the invention is notably to allow the embodiment of an emission device for CBF radar suited at one and the same time to the surveillance phase and to the tracking phase, and economical to implement. For this purpose, the subject of the invention is a radar device for detecting and/or tracking aerial targets fitted to an aircraft, comprising at least:
The given angular sector Ω is for example the search domain to be covered by the said device.
An antenna system comprises for example at least:
The electronic scanning system is for example composed of phase shifters with n states disposed in the emission pathways, the pathways being fed simultaneously through one and the same microwave-frequency signal, the orientation of the beam is dependent on the combination of the phase shifts on the various phase shifters.
Advantageously, the phase shifters are for example phase shifters with one state. In this case, the orientation of the beam is for example obtained by binary switching of two microwave-frequency paths of different lengths upstream of power amplifiers.
A phase shifter comprises for example a delay line and a switch in parallel.
Other characteristics and advantages of the invention will become apparent with the aid of the description which follows offered in relation to appended drawings which represent:
where θR is the aperture of the lobe formed during reception in the direction of measurement, for example in elevation. This aperture is related to the size of the antenna, T is the integration time, for example equal to one second, which is also the rate of renewal of the measurements in a conventional DBF (Digital Beam Forming) solution. GE is the gain on emission, AR is the surface area of the reception antenna and PE the power emitted.
It is apparent from relation (1) that:
In a conventional DBF solution, all the parameters are fixed by the operating conditions during surveillance. To obtain greater measurement precision, it is then necessary to track the target over a certain number k of sequences of duration T. The drawback of this scheme is that the target has approached significantly closer during this time k×T. To obtain the sought-after precision at a given distance R, it is necessary to detect the target at a greater distance R′ given by the following relation:
R′=R+kV
RR
T (3)
VRR being the closing speed of the target.
The ratio R′/R is all the higher as the radar system is short range. Obtaining the range R′ therefore requires that the emission power PE be significantly increased, this being very penalizing notably from the cost and consumption point of view.
A less penalizing solution during tracking consists in splitting the domain to be covered Ω into a certain number m of sub-domains ω, m being greater than or equal to 2. In the surveillance phase, the renewal time of the measurements T is divided into a certain number m of smaller elementary intervals Tp such that
In a simple case, all the sub-intervals Tp are equal and their value is:
Each of the sub-domains ω is observed by introducing a simplified electronic scan on emission. The electronically controlled emission beam corresponds to an increased antenna gain:
It is therefore seen that the product of the antenna gain on emission times the processing gain proportional to the observation time Tp remains constant whatever the partitioning of the domain to be covered Ω into sub-domains ω. The range and the angular precision in the surveillance phase are therefore unchanged with respect to a simple CBF solution.
On the other hand, in the tracking phase, the emission beam can be constantly pointed at the sub-domain (known a priori) where the target is situated. We therefore have TP=T, but the gain on emission is
Equation (2) therefore shows, all other things being equal, that the angular measurement error σ has been reduced by factor
Under these conditions, the required angular precision is achieved by tracking after a number of sequences of duration T
The detection range necessary for establishing a track of required precision is now only
More generally a device according to the invention can comprise a plurality of antenna systems, each antenna system being dedicated to the coverage of a given angular sector Ω.
An antenna system comprises at least:
In a preferential embodiment, the M reception pathways are digitized as far upstream as possible towards the antennas, in particular for reasons of cost and of ease of implementation of the CBF. This signifies notably that the signals received are sampled and then digitized by means of sampling and analogue-digital conversion circuits disposed as close as possible to the antennas. The signals thus digitized are thereafter processed by the digitized reception circuits and then transmitted to computation means, these means performing notably the CBFs and then the associated radar processings.
The emission sources conventionally comprise frequency generating circuits as well as power amplifiers, or emitters, providing the emission signals for the antennas.
Preferably, the emitters are situated as close as possible to the emission antennas and the receivers, notably the low noise amplifiers and the analogue-digital converters, are situated as close as possible to the reception antennas. The emission part is for example situated near the reception part. This makes it possible notably to reduce the losses inherent to the links between the antennas and the microwave-frequency functions in the global energy balance of the device and to obtain a compact system.
The emission and reception circuits as well as the computation and signal processing means are for example disposed inside the aircraft.
It is also possible to include the whole in antennas termed “smart antennas” devised on the surface: the circuits are then physically outside the aircraft.
The antennas are for example radiating sources, several known embodiments possibly being used, such as “patch” sources for example.
The exemplary embodiment of
The device therefore comprises 32 reception pathways and 4 emission circuits.
The two antenna systems can emit in alternation or else, preferably, simultaneously. It suffices for this purpose that the two emissions of the two antenna systems are synchronous and emit orthogonal waveforms.
It should be noted that the spacing between the pathways, whether on emission or on reception, is not necessarily close to λ/2, λ being the length of the emitted wave. The choice of the spacing is notably dependent on the patterns sought as a function of a given application.
This phase-shift system is for example effected by switch means 412, 422, 432, 442 which short-circuits a delay line 413, 423, 433, 443 of around λ/4, the precise value of the delay adjusting the various pointings of the emission beam. The short-circuit can be for example effected simply through a diode turned on or off by a control voltage. Such a phase-shift system is simple and inexpensive. The amplifier of each emission pathway feeds the antennas 35 of the group of antennas 31, 32, 33, 34 associated with this emission pathway.
The common low-level source 40 therefore feeds the Q phase-shift devices through distributions of the same delay, Q being equal to 4 in the example of
In the example of this
In a first combination 51, the phase-shift codes of the four groups of antennas 31, 32, 33, 34 are all in the 0 state, all the switches are therefore closed and no phase shift is applied. The antenna beam formed on the basis of the antenna system is therefore oriented along the axis of the antenna system. This axis passes through the centre of the groups of antennas 31, 32, 33, 34 and is oriented perpendicularly to the phase plane of the set of these antennas.
In a second combination 52, the phase shifts of the two groups of antennas at the bottom 33, 34 are coded to 1, the others being coded to 0. A phase shift is then applied to the signals emitted by these bottom antennas. The antenna beam is in this case oriented upwards.
In a third combination 53, the phase shifts of the two groups of antennas at the top 31, 32 are coded to 1, the others being coded to 0. A phase shift is then applied to the signals emitted by these top antennas. The antenna beam is in this case oriented downwards.
In a fourth combination 54, the phase shifts of the two groups of left antennas 31, 33 are coded to 1, the others being coded to 0. A phase shift is then applied to the signals emitted by these left antennas. The antenna beam is in this case oriented rightwards.
In a fifth combination 55, the phase shifts of the two groups of right antennas 32, 34 are coded to 1, the others being coded to 0. A phase shift is then applied to the signals emitted by these right antennas. The antenna beam is in this case oriented leftwards.
A device according to the invention then operates as follows. For a given antenna system 22, 23, the antenna beam on reception is formed by CBF by the computation means on the basis of the signals received by each reception antenna 30. For this same system, the antenna beam on emission, being more slender than the search domain to be covered Ω to be covered by construction, is successively pointed in a number of directions inside this angular sector so as to progressively cover this sector. Each antenna system therefore covers the search domain Ω. Advantageously, the beam is pointed in the various directions by a simplified active electronic scanning system such as illustrated by
In this example a group of antennas 61, 62 is reduced to a single antenna. A common source 41 still feeds the two emission pathways, these pathways having the same structure as the emission pathways of the previous example. In particular, each pathway comprises a power amplifier 611, 621 preceded by a phase-shift system composed of a switch 612, 622 in parallel with a delay line 613, 623.
All the emission or reception antennas have for example the same pattern. The system of
In a first combination 71, the phase-shift codes of the two antennas 61, 62 are all in the 0 state, the switches are therefore closed and no phase shift is applied. The antenna beam formed on the basis of the antenna system is therefore oriented along the axis of the antenna system.
In a second combination 72, the phase shift of the bottom antenna 62 coded to 1, the phase shift of the other antenna 61 being coded to 0. A phase shift is then applied to the signals emitted by this bottom antenna. The antenna beam is in this case oriented upwards.
In a third combination 73, the phase shift of the top antenna 61 coded to 1, the phase shift of the other antenna 62 being coded to 0. A phase shift is then applied to the signals emitted by this top antenna. The antenna beam is in this case oriented downwards.
As indicated previously, the invention uses a method of CBF in which the domain to be covered Ω, covered by an antenna system, is subdivided into a certain number of sub-domains w. A sub-domain ω is covered by the beam of the antenna system, oriented successively in a certain number of directions as described in the exemplary embodiments of
According to the invention, a hybrid system is therefore achieved, composed of a CBF on reception and of an electronic active scan on emission. This electronic scan on emission is achieved at low cost as shown by the exemplary embodiments of
Moreover, the subdivision of the total domain Ω makes it possible to adapt the range during surveillance as a function of the direction. A significant range may, for example, be required along the horizontal axis whereas a lesser range may suffice at large angles of elevation, positive or negative. For example, if the time to explore the total domain Ω is T, it is possible to allocate 0.5 T to the observation of the zone with zero elevation and 0.25 T to the two zones with large elevation. It is thus possible to allocate an optimal allocation of the integration time.
As regards the timing of the surveillance function with respect to the tracking function, as long as there is no detected target, the sub-domains of aperture ω can be explored cyclically with a total cycle time of T. If a target is detected, the surveillance exploration ceases and the emission beam is temporarily pointed for a given duration Tp towards the direction of the detection: a track is then opened. It is noted that if Tp is equal to T, the required angular precision is obtained after a number
of measurements whereas the CBF solution described in relation to
therefore the range R′ necessary for the track aperture is now only:
The constraint on the emission power is therefore reduced, for one and the same precision, since the range required on the first detection is reduced. Stated otherwise, the gain on emission during tracking is increased. This results in a greater signal-to-noise ratio, S/N, for one and the same integration time and consequently increased precision during tracking.
The exemplary embodiments of the emission pathways of
