FR2948198A1 - Radar system for carrying out low and very low altitude coverage of area of airspace located above field of wind turbines, has element comprising antenna directed towards zenith arranged to present radiation diagram in azimuth - Google Patents

Abstract

The present invention relates to an ancillary radar system that can be associated with an existing radar system, responsible for the very low coverage or low altitude, and for ensuring the detection of objects flying over a field of wind turbines, that is to say an area on which wind turbines are implanted in such a number as to alter the detection capabilities of the existing system above this area of land. The system according to the invention comprises a plurality of transmitting and / or receiving elements. These elements are placed on some of the wind turbines and are arranged and configured to cover the airspace above the wind turbine field so that objects flying over this space at low or very low altitude can be detected. The system according to the invention comprises means for transmitting the information corresponding to the detections to the global management unit which ensures the exploitation of the data acquired by the entire coverage system with which it is associated.

Description

COMPLEMENTARY RADAR SYSTEM FOR DETECTION OF LOW ALTITUDE EVOLVING TARGETS ABOVE A WINDWATER FIELD The invention relates to the general field of low altitude radar detection of airspaces. It generally concerns the location of objects moving at low altitude over areas with high reflectivity may hide the airspace above them. It relates more particularly to low-altitude detection systems intended to cover an area of the space in which is located a group of wind turbines forming an area commonly called "field" of wind turbines.

The invention deals with the problem of the coexistence between radars and wind turbines. The generalization of the installation of wind turbines constitutes an increasing inconvenience for the systems, radar systems for example, in charge of the coverage of determined zones of territories, in particular in the case of a medium or low altitude cover. Among these systems, it is possible to cite, for example, the low-altitude coverage radars used to monitor the space in which aircraft move when approaching an airport. For such systems, the existence of wind turbine fields in the monitored area constitutes a major inconvenience which strongly conditions the radar detection performance in the area under consideration. Generally involving 2D radars performing a detection simply on the bearing and distance, the zone disturbed by a wind turbine field then extends from the ground up to the maximum altitude covered by the radiation pattern of the radar. As a result, the radar track of an aircraft flying at low altitude in an approach zone of an airport can in such a context be lost during the time interval during which the aircraft passes over the field of flight. wind turbines, the echo reflected by the aircraft is then covered by the high power signal reflected by the wind turbines constituting the field. Similarly, since the radar is rendered blind in the space sector encompassing a wind turbine field, a sector densely populated in aircraft as an airport approach area encompassing a wind turbine field becomes a high space. risk of collision.

The negative effects of the presence of wind turbines on the detection capabilities of a low level surveillance radar in a sector of space encompassing an area in which these wind turbines are installed are thus mainly: - Desensitization caused by the increase of the average level of the reflectivity of the zone considered, this increase being due to the presence of the wind turbines which constitute as many sources of parasitic reflections, - a saturation of the means of reception of the radar, due to the strong equivalent surface of the parts fixed (mast, nacelle) wind turbines, - an increase in false alarms caused by the presence of moving parts (blades),

To prevent these drawbacks, the solution currently implemented consists in limiting the implantation of wind turbines in sensitive areas requiring good radar coverage, particularly at low altitude.

Another type of solution consists in taking into account the negative effects mentioned above when designing wind turbines in order to limit them, for example by reducing the radar equivalent area of wind turbines. Another type of solution is still to adapt the radars intended to operate in such a context, for example by adding them complementary processing means, so as to make them less sensitive to the inconvenience caused by the presence of wind turbines. These last two types of solutions lead to increased development costs and equipment realization both in the realization of wind turbines and the modification of existing detection means.

An object of the invention is to propose an alternative solution to the solutions mentioned above. Another aim is to develop means applicable to both the existing detection equipment park and the equipment to come.

For this purpose the invention relates to a radar system, to achieve the low and very low altitude coverage of the area of the space above a field of wind turbines. The system according to the invention comprises a plurality of elements, transmitters and / or receivers, mounted on some of the wind turbines, configured to jointly perform the coverage of the portion of the airspace considered above the field of wind turbines. According to the invention, the number and arrangement of the elements are a function of the number of wind turbines and the area occupied by them.

According to a preferred embodiment, each element comprises a zenith directed antenna configured and arranged to present an omnidirectional azimuth radiation pattern.

According to the invention, the receiving means may comprise means for ensuring the elimination of ground echoes and multiple reflections caused by the presence of wind turbines.

According to the invention, each element may comprise means of communication with the system central management system which performs the overall exploitation of the acquired information. The communication means are then configured to use the communication link that equips the wind turbine on which is mounted the element.

According to the invention, each element may comprise means for drawing its power supply from the electrical energy produced by the wind turbine.

In a particular embodiment of the system according to the invention, the signals emitted by the transmitting elements are periodic pulses.

In another particular embodiment of the system according to the invention, the signals emitted by the emitting elements are continuous waves.

The features and advantages of the invention will be better appreciated thanks to the description which follows, description which sets forth the invention through a particular embodiment taken as a non-limiting example and which is based on the appended figures, figures which represent:

- Figures 1 to 3, illustrations relating to a first embodiment of the system according to the invention; FIGS. 4 to 6, illustrations relating to a second embodiment of the system according to the invention;

To solve the difficulty for existing radars to properly deal with the problem of detecting targets flying over wind turbine fields (because wind turbines cause both false echoes and a loss of detection of useful targets), and this, even for high-altitude targets when the radar is of the 2D type (such as almost all "en-route" radars, airport radars and coastal radars), the invention consists in implementing a surveillance system short span tender, installed at the field of wind turbines. According to the invention, the system consists of emitting and / or receiving elements placed on some of the wind turbines constituting the field and whose radiation patterns are defined so as to cover the airspace above the field of wind turbines. . According to a preferred embodiment, the system according to the invention consists of separate transmitters and receivers, each element being placed on a separate wind turbine, the assembly being arranged to form a short-range multistatic radar capable of providing desired coverage vertical to the wind turbine field. The transmitter and receiver elements are equipped with fixed antennas whose radiation patterns are omnidirectional in azimuth, these antennas being oriented at the zenith. These elements are for example mounted on the nacelles of wind turbines considered.

According to the invention, each element of the system advantageously comprises means for communicating with the central management unit of the system. This organ performs in particular the global exploitation of the information acquired by the different receivers. It can also transmit to the various elements of the system orders indicating the mode of operation to be adopted, and in particular the waveform that must use a particular transmitter. These communication means are here advantageously configured to use the communication link which equips the wind turbine on which is mounted the transmitter and / or receiver considered element.

According to the invention, each of the elements of the system comprises means for drawing its power from the electrical energy produced by the wind turbine. Being a short-range system, the consumption of an element, transmitter or receiver, can be easily supported by a wind turbine. In addition, each element comprises means for its power supply from a reserve of energy, battery type for example, during periods when, lack of sufficient wind, the rotor of the wind turbine remains at a standstill . Such an energy reserve can for example be located on the ground, at the foot of the wind turbine.

In a preferred embodiment, the system according to the invention is connected by the existing network to the operating center of the radar or radars already installed and for which additional coverage is required.

The following description focuses on two particular embodiments of the system according to the invention, presented by way of non-limiting examples.

In a first embodiment, the system is a multistatic system, continuous wave ("cw"), consisting of separate emitter elements (Tx) and receivers (Rx), each element being disposed at the top of a wind turbine. Transmitters use antennas with omnidirectional azimuth radiation. They implement continuous waveforms (cw) which make it possible to guarantee the desired range with a reduced peak power. Furthermore, in order to allow each of the receivers to identify the origin of a received signal, the transmitters are configured to emit distinct waveforms while occupying globally an acceptable frequency band, thanks, for example, to the use of orthogonal codes. At the same time, receivers are configured to cover the entire global frequency band. Thus each receiver can determine the origin of the signals it receives.

The receivers making up the system according to the invention are also provided with omnidirectional antennas in azimuth. They further comprise means for implementing a FFC type processing (beamforming by calculation) on the received signals. In this way, it is advantageously possible for them both to suppress the direct signals coming from the transmitters and to have an angular selectivity making it possible to optimize the detection of useful targets.

According to the invention, each receiver comprises means for processing the echoes originating from the reflection, on the objects moving in the covered area, of the signals transmitted by all the transmitters. The fusion of the bistatic detection information provided by each receiver allows a precise 3D location of the targets detected by one or more receivers. The precise location is also achieved, for example, by means of a multilateration method (intersection of location ellipsoids corresponding to the same object detected by several bistatic bases (transmitter / receiver).

Figures 1 to 3 illustrate an embodiment of this first embodiment. The system taken as an example comprises four transmitters cw, represented by circles 11, and a receiver, represented by star 12, distributed in the manner illustrated in FIG. 1. Transmitters 11 and receiver 12 are placed at the top of wind turbines, which places them at a height of about 60 m above the ground. The CW emission is also performed in L-band, on an instantaneous frequency band of 2.5 MHz, the peak power emitted by each transmitter being about 5 watts.

The representations 2-a and 2-b of FIG. 2 give an account of the performances obtained by means of the system according to the invention, in terms of altitude coverage, for the detection of large objects of the "airliner" type whose radar equivalent area (SER) is of the order of 100 m2. The targets are here considered as flying at a speed of 75 m / s and the performances are calculated in the presence of parasitic reflections on the ground. Figure 2 considers a median vertical plane 21, passing through the receiver 12, such as that illustrated by the 2-a representation. The curve 22 of the representation 2-b shows the location of the points delimiting the domain in which the coverage made by the system taken as an example corresponds to an instantaneous detection probability of 90%. It is specified here that the "instantaneous" character is here evaluated with regard to the integration time that is necessary for the radar to filter the signals and obtain the necessary signal-to-noise ratio. This duration is typically of the order of a few tens to a few hundred milliseconds. This duration also corresponds to the rate of renewal of the information (for example "pads") that can be transmitted to the operating system.

Thus, as can be seen, the delimited area advantageously extends from an altitude of about 200 m to an altitude of about 10,000 m, so that an airliner is detected in the area above the aircraft. wind turbines up to an altitude of at least 10000m. The coverage thus achieved extends over a wide area at altitude and distance. For an altitude of 2000 m, for example, this coverage is performed omnidirectionally in azimuth, over a range of distances having a radius greater than ten kilometers.

The representations 3-a and 3-b of FIG. 3 give an idea of the performances obtained by means of the system according to the invention, with regard to altitude coverage, for the detection of small objects of the type. "drone" (UAV) whose radar surface area (SER) is of the order of 1 m2. The targets are again considered as flying at a speed of 75 m / s. Like FIG. 2, FIG. 3 considers a median vertical plane 31 passing through the receiver 12, such as that illustrated by FIG.

8 a. The curve 32 of the representation 3-b shows the location of the points delimiting the domain in which the coverage performed by the system taken as an example corresponds to an instantaneous detection probability of 90%, the performances being calculated in the presence of reflections on the ground.

Thus, as can be seen, the delimited area advantageously extends from an altitude of about 200m to an altitude of about 4000m so that a UAV (UAV) is detected in the area above the wind turbines in an area ranging from an altitude of 200m to an altitude of at least 3000m. A UAV (UAV) operating in this area is therefore detected.

In a second embodiment, the system is a multistatic pulse system, consisting of elements both transmitters and receivers, each element being disposed at the top of a wind turbine. According to the invention, each of the elements constituting the system uses a simple omnidirectional antenna, for example a dipole implanted above a ground plane, used alternately for transmission and for reception.

As in the case of the continuous multistatic radar described above, the receivers of the pulsed multistatic radar process the echoes from the reflection on the targets of all the transmitters. The fusion of the information provided by each receiver makes it possible to precisely locate the targets using a multilateration method.

The transmit and receive antennas are as previously oriented at the zenith. However, unlike the multistatic continuous wave system described above, the suppression of the signals from the direct path transmitters is ensured by the very use of the pulsed mode and does not require any specific means.

Figures 4 to 6 illustrate an example of this second embodiment. The system taken as an example comprises four transceiver elements 41 distributed in the manner illustrated in FIG.

9 receivers are placed at the top of wind turbines, which places them, as in the previous example, at a height of about 60 m above the ground. The emission is also performed in L-band, on an instantaneous frequency band of 2.5 MHz, the peak power emitted by each transmitter being about 100 watts. The duration of the transmitted pulses is 0.2 ps and the average repetition period of the pulses is 50 ps. The targets are here, as before, considered as flying at a speed of 75 m / s and the performances are calculated in the presence of reflections on the ground.

The representations 5-a and 5-b of FIG. 5 give an account of the performances obtained by means of the pulse system according to the invention, with regard to coverage at altitude, for the detection of bulky objects of "airliner" type. whose radar equivalent area (SER) is of the order of 100 m2. Figure 5 considers a median vertical plane 51 such as that illustrated by the 5-a representation. The curve 52 of the representation 5-b shows the location of the points delimiting the domain in which the coverage made by the system taken as an example corresponds, as in the previous example, to an instantaneous detection probability of 90%.

The representations 6-a and 6-b of FIG. 6 give an account of the performances obtained by means of the system according to the invention, with regard to altitude coverage, for the tracking of small objects of the type. drone (UAV) whose radar equivalent area (SER) is of the order of 1 m2. Figure 6 considers a median vertical plane 61 such as that illustrated by the representation 6-a. The curve 62 of the representation 6-b shows the location of the points delimiting the domain in which the coverage achieved by the system taken as an example corresponds, as in the previous example, to an instantaneous detection probability of 90%.

The previous example shows that, at least in the altitude range of low and mid altitude monitoring, the performances obtained with a multistatic pulse system are similar to those obtained with a multistatic continuous wave system (CW). The choice of one or the other system will therefore be based on criteria such as the complexity of installation or the cost.

Whatever the configuration adopted, the reception means constituting the radar system according to the invention, the radar receivers in the two configurations described above, are preferably provided with two specific functions, intended to address the particularities related to the proximity of the blades. of the wind turbine which carry the elements of the system as well as those of neighboring wind turbines.

The first function is intended to compensate for the deformations of the antenna pattern caused by the displacement of the blades of the wind turbine. The disturbance created by a wind turbine blade with respect to the receiving means positioned on the same blade can be compared to that caused by a metal cylinder 1 m in diameter 15 and located about ten meters from an L-band radar. having a small antenna compared to the size of the cylinder. This disturbance is reflected, in the case of a monostatic radar, by a drop of about 12 dB of the signal-to-noise ratio. This fall finally results in a decrease in the range of two for the targets located in the geometrical shadow of the blade, as well as in the existence of complex phenomena of interference between the incident wave emitted by the means of transmission. emission and the waves reflected by the blade. As the blades rotate, the effects of this phenomenon also vary periodically in space and time. It is therefore necessary to compensate for this effect by any suitable method known. It is for example possible to use the information on the position of the blades of the wind turbine which can also have the radar mounted on the wind turbine and to apply a post-processing on the radar spots produced by the means of extraction of the radar considered.

The second function implements any appropriate method to fight against clutter that may be constituted by the reflection of the signal emitted by an elementary radar on the blades of other wind turbines and in particular wind turbines close to that on which it is placed.

These two functions can be achieved by the implementation of any known method, for example by implementing the method of the French patent application No. 0807211 filed on 18/12/2008 by the applicant.5

Claims (7)

REVENDICATIONS1. Radar system, for achieving the low and very low altitude coverage of the area of the space above a wind turbine field, characterized in that it comprises a plurality of elements, transmitters (11) or receivers (12), mounted on some of the wind turbines, these elements being configured to jointly cover the portion (22, 32) of the airspace considered above the wind turbine field, the number and arrangement of the elements (11, 12) depending on the number of wind turbines and the area occupied by them. 2. Radar system according to claim 1, characterized in that each element comprises an antenna directed towards the zenith configured and arranged to present an omnidirectional radiation pattern in azimuth. 3. Radar system according to claim 2, characterized in that the receiving elements (12) comprise means for ensuring the elimination of ground echoes and multiple reflections caused by the presence of wind turbines. 20 4. Radar system according to any one of claims 1 to 3, characterized in that each element (11, 12) comprises means of communication with the system central management system which performs the overall exploitation of the information acquired, the 25 communication means being configured to use the communication link that equips the wind turbine on which is mounted the element. 5. Radar system according to any one of claims 1 to 4, characterized in that each of the elements (11, 12) comprises means for drawing its power from the electrical energy produced by the wind turbine. Radar system according to one of the preceding claims, characterized in that the signals emitted by the transmitter elements (11) are periodic pulses. 7. Radar system according to any one of claims 1 to 5, characterized in that the signals emitted by the emitting elements (11) are continuous waves.