Arrangement and a Method for measuring a Radar Cross Section

Abstract

A method for measuring a Radar Cross Section, RCS, of an object (200), wherein the method comprises: acquiring (S1000), by a first antenna (100), observations of the object (200) by performing two dimensional near field frequency scans; computing (S1100), by a processor, the downrange profiles of the acquired observations; computing (S1200), by the processor, the three dimensional Inverse Synthetic Aperture Radar, ISAR, images by superimposing the downrange profiles; extracting (S1300), by the processor, the scattering center of the object (200); and computing (S1400), by the processor, the RCS of the object (200).

Claims

1. A method for measuring a Radar Cross Section, RCS, of an object, wherein the method comprises: acquiring, by a first antenna, observations of the object by performing two dimensional near field frequency scans; computing, by a processor, the downrange profiles of the acquired observations; computing, by the processor, the three dimensional Inverse Synthetic Aperture Radar, ISAR, images by superimposing the downrange profiles; extracting, by the processor, the scattering center of the object; and computing, by the processor, the RCS of the object.

2. The method according to claim 1, wherein after acquiring the observations of the object, the observations are calibrated by the processor.

3. The method according to claim 2, wherein the calibration uses an echo received from a calibration target, TC, located in a known position on a first axis parallel to an imaginary axis, wherein the imaginary axis lies in a straight line between the object and a carrier, and wherein the TC is displaced in a lateral direction along a second axis in relation to the object, wherein the first axis and the second axis are not parallel to each other.

4. The method according to claim 1, wherein after computing the downrange profiles, the distortion of the downrange profiles are corrected by the processor, wherein the correction comprising correcting the distortion from the antenna pattern and/or the transient of a hardware gating equipment, HGE, and/or mechanical deviations.

5. The method according to claim 1, wherein after the extraction of the scattering center of the object, the effects of the ground plane are filtered out by the processor.

6. The method according to claim 1, wherein the downrange profiles are computed by using a Fast Fourier Transform, FFT.

7. The method according to claim 1, wherein the scattering center of the object is extracted by a CLEAN algorithm.

8. A Radar Cross Section, RCS, measuring arrangement for measuring the RCS of an object, wherein the arrangement comprises: a first antenna configured to acquire observations of the object by performing two dimensional near field frequency scans; and a processing unit comprising a processor, wherein the processor is configured to: compute downrange profiles of the acquired observations; compute a three dimensional Inverse Synthetic Aperture Radar, ISAR, image by superimposing the downrange profiles; extract a scattering center of the object; and compute the RCS of the object.

9. The arrangement according to claim 8, further comprising a vector network analyzer, VNA, configured to act as a radar.

10. The arrangement according to claim 8, further comprising a hardware gating equipment, HGE, configured to minimize noise, clutter and a number of frequency samples required by the arrangement to measure the RCS of the object.

11. The arrangement according to claim 8, further comprising a carrier, wherein the carrier comprises a motor configured to move the carrier, and wherein the first antenna, is located on the carrier.

12. The arrangement according to claim 8, further comprising a calibration target, TC, located in a known position on a first axis parallel to an imaginary axis, wherein the imaginary axis lies in a straight line between the object and the carrier, and displaced in a first lateral direction along a second axis in relation to the object, wherein the first and the second axis are not parallel to each other.

13. The arrangement according to claim 12, wherein the positioning of the TC is configured to avoid the TC shadowing the object.

14. The arrangement according to claim 8, further comprising a lateral target, TL, displaced in a second lateral direction along a third axis in relation to the carrier and a second antenna configured to transmit radio signals towards the TL and to receive radio signals reflected off of the TL, wherein the first axis and the third axis are parallel to each other.

15. The arrangement according to claim 8, wherein the first antenna is a vertical antenna array.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0048] These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying figures, wherein like reference numerals refer to like parts, and in which:

[0049] FIG. 1 shows a schematic diagram of the carrier and the components on and around the carrier according to an embodiment described herein;

[0050] FIG. 2 shows a schematic diagram of an object and a calibration target according to an embodiment as described herein;

[0051] FIG. 3 shows a perspective view of the arrangement according to an embodiment as described herein; and

[0052] FIG. 4 shows a block diagram of a method of measuring the Radar Cross Section (RCS) of an object according to an embodiment as described herein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0053] FIG. 1 shows a schematic diagram of the carrier and the components on and around the carrier according to an embodiment described herein.

[0054] In this example, the carrier assembly 50 comprises a first antenna 100, a housing 110 comprising the VNA and the HGE, the carrier 120 containing a motor, a plurality of wheels 130, two second antennas 140 and a rail 150. The first antenna 100 may alternatively be an antenna pair or an antenna array.

[0055] The first antenna 100 is a vertical array antenna which will be described in more detail below. The vertical array antenna allows for diversity to filter out the ground plane effects in the postprocessing which will also be described in more detail below. The first antenna 100 comprises a transmission unit and a receiving unit which transmits signals towards the object (see FIG. 2) for which the RCS is being measured and a receiving unit which receives the signals reflected from the object. Alternatively, these units can be combined into a single transceiver unit. The carrier assembly 50 also comprises a power supply for powering the transmission unit. The power supply may alternatively be contained in the housing 110 and/or be coupled to the first antenna 100. The transmission unit and/or receiver unit and/or transceiver unit may comprise a processor and a memory to control the units and to store preset parameters and/or store results. Each of these units may also communicate with an external device at a remote location where the external device can be used to control each of these units. The first antenna 100 scans one or both dimensions of the two dimensional near field frequency scan at one location before being moved to a second location. In some examples, the first antenna 100 scans one dimension of the two dimensional near field frequency scan and is then moved laterally by the motor of the carrier assembly 50 where the first antenna 100 then scans the second dimension of the two dimensional near field frequency scan.

[0056] The housing 110, which encloses the VNA, the HGE, a switching matrix and the processing unit, is made of a weatherproof material which protects these components from external factors. Alternatively, the processing unit may be at an external location not shown. The housing 110 may also be made of a material which allows for the transmission and receipt of radio signals in relation to the components within the housing 110. A VNA is known in the prior art as an instrument which measures the network parameters of electrical networks. The VNA preferably generates the signal transmitted by the first antenna 100. The VNA receives the signals received by the first antenna 100 which have been converted into electrical signals. The VNA comprises a signal generator, a test set, a receiver and a processor. The VNA then outputs these signals to the processing unit via a wired and/or a wireless connection. The HGE is configured to, inter alia, minimize noise, clutter and a number of frequency samples required by the arrangement (see FIG. 3) to measure the RCS of the object. The HGE is connected between the VNA and the first antenna 100. The HGE shuts the receiving channel to the VNA when energy scattered from the object is not received by the first antenna 100. The HGE shuts the transmitting channel to the first antenna 100 to generate the transmitted pulse, being shut when the receiving channel is open. The switching matrix is configured to select an antenna for the transmission and an antenna for the receipt of the radio signals. It may be the same antenna. The switching matrix may be configured to receive signals from more than one antenna consecutively and/or simultaneously, obtaining, in the latter, more than one observation at the same time with multiple receivers The switching matrix may comprise power amplifiers for the transmission to illuminate the object with more energy. The switching matrix may comprise low noise amplifiers for the receipt of the radio signals with a reduced level of noise. The processing unit comprises a processor and a memory, wherein the processor is configured to compute the RCS of the object 200. The computation will be described in further detail below.

[0057] In some embodiments, there are multiple housings 110, wherein each housing 110 comprises only the VNA and/or only the HGE and/or the switching matrix and/or the processing unit and/or only certain components of each of these devices.

[0058] The carrier 120 is a simple carriage which is configured to carry the equipment of the carrier assembly 50. The carrier 120 can be any sort of device and can be any shape which is suitable for carrying the equipment of the carrier assembly 50.

[0059] In this example, the carrier 120 comprises three wheel sets 130 however; the number of wheels and wheel sets can be altered depending on the dimensions of the carrier 120 and the weight of the carrier assembly 50. The wheel sets 130 are configured to run on rails 150. The number of rails 150 can also be altered depending on the dimensions of the carrier 120 and the weight of the carrier assembly 50. The motor propels the carrier assembly 50 along the rail 150 via a drive train as is well known in the field of vehicles.

[0060] In this example, the carrier assembly 50 further comprises two second antennas 140 which are installed on the carrier 120 to measure echoes produced by lateral targets (see FIG. 3), located in the lateral direction i.e. the direction of motion of the carrier assembly 50. The lateral targets and the two second antennas 140 are configured to electrically measure the sizes of the lateral steps of movement of the carrier assembly between scans of the object by the first antenna 110, and to measure the hardware gating transient response. This information is used for correction purposes in the postprocessing. This information is also used in order to create a three dimensional ISAR image of the object 200. This process is described below.

[0061] FIG. 2 shows a schematic diagram of an object and a calibration target according to an embodiment as described herein.

[0062] The object 200 in this example is an aircraft however, the object 200 may be any object 200 for which the RCS can be measured.

[0063] The calibration target TC is an object for which the RCS is already known. This then allows for the VNA and/or HGE and/or the processing unit to be able to calibrate themselves before and/or after measuring the RCS of the object 200. This calibration target TC can be of any shape or size and can be made of any material. In this example, the view as if the user was located at the carrier assembly 50. In this example, the object 200 is head on to the carrier assembly 50 however, the object 200 may be in any orientation in relation to the carrier assembly 50.

[0064] FIG. 3 shows a perspective view of the arrangement according to an embodiment as described herein

[0065] The measurement arrangement 10 locates the object 200 simply on the ground in a volume referred as to target zone TZ. The carrier assembly 50 carrying the equipment needed to measure the RCS of the object 200 is located at a near field distance from TZ. Preferably, the near field distance is a ratio of 1:5, that is to say, if the object 200 has a dimension of 20 meters, the carrier 120 is located 100 meters away. However, any suitable ratio may be used. In this example, the object 200 is located within the target zone TZ and even though it is not shown, it is orientated according to the direction in which the RCS is intended to be measured in relation to the carrier assembly 50. The acquisition system, which comprises the equipment on the carrier 120 as described above, is based on a vertical antenna array which provides diversity to filter out the ground effects by postprocessing. The motor within the carrier 120 to moves the carrier assembly 50 in a lateral direction i.e. along the U axis, in order to perform the azimuth scan and acquire the RCS of the object 200. In this example, the carrier assembly 50 is not on a rail 150 but is free standing. The carrier assembly 50 may be moved in a predetermined manner according to a computer program coupled to the motor or via self-driving technology. Any other suitable method can also be used.

[0066] The calibration target TC is located in front of the target zone TZ i.e. along the X axis, and laterally displaced in relation to the target zone TZ i.e. along the Y axis, to avoid shadowing of the object 200 within the target zone TZ. The echoes produced by the calibration target TC may be used for two purposes: calibration of the system, and estimation of mechanical deviations to be corrected by postprocessing. The first antenna 100 located on the carrier 120 receives these reflected radio signal echoes.

[0067] The 2-D-lateral scan RCS measurement arrangement 10 is, in this example, an outdoors range designed with the purpose of measuring RCS of full-scale target objects 200 located on the ground. The objective is to measure the RCS of an object 200 without the need to lift it from the ground or locate it inside an indoor chamber. This approach drastically reduces the cost of a measuring campaign.

[0068] The measurement of the RCS of the object 200 is performed by near-field NF scans of the object on a 2-D-plane synthetic aperture SA. The synthetic aperture SA is in the vertical direction i.e. Z-axis and in the lateral direction i.e. the U-axis. The vertical scanning across the synthetic aperture SA is carried out, preferably, through switching between the different antenna elements part of the first antenna 100. The lateral scanning across the synthetic aperture SA is carried out through the displacement to adjustable steps of the first 100 and second 140 antennas located on the carrier assembly 50. The first antenna 100 is pointed toward the origin O which is preferably set to be the center point of the object 200. One lateral scan covers a limited equivalent azimuth range. The complete azimuth range can be covered by the first antenna 100 performing several scans with different object 200 orientations. The object can be, preferably, rotated. The object can be, preferably, mounted on a turntable to be easily rotated, or alternatively it could be rotated manually or using its own wheels or any other method. Alternatively, the assembly 50 could be moved to another location to look at the object from another orientation. The different azimuth orientations within one single scan are achieved by moving the carrier assembly 50 along the U-axis. At predetermined positions along this axis, the first antenna 100 performs the required observations across the vertical direction of the object 200. Alternatively, the observations could be taken continuously when the carrier assembly 50 is moving, i.e. at constant speed. These observations are then used in a method to compute the RCS of the object 200 which is described in further detail below. Alternatively, the carrier assembly 50 may be moved in a non-linear path around the object 200. This non-linear path may be a circular path around the object 200 or any other suitable non-linear path.

[0069] For the vertical scanning of the object 200, the receiving antenna element RXA and transmitting antenna element TXA of the first antenna 100 are switched accordingly. In some examples, there are elements that only transmit or only receive. In some examples, there are elements where, in some cases, the elements are transmitting and in other cases, they are receiving i.e. the element alternates between transmitting and receiving. In some examples, the elements can transmit and receive simultaneously. This is performed by the switching matrix. In case a finer sampling than the one provided by the physical separation of the elements of the first antenna 100 is required, multiple scans can be performed at different heights. The system, therefore, can be configured according to the Target Zone TZ size, since larger objects 200 require finer samplings to avoid aliasing. At least one element of the first antenna 100 is transmitting and at least one other element of the first antenna 100 is receiving simultaneously. In case of multiple elements that are receiving simultaneously with different receivers, several observations are taken in parallel. This reduces the scanning time. In some examples, there are a plurality of first antennas 100. There could be several first antennas 100 mounting on the same carrier assembly 50. In this case, several frequency bands could be acquired in parallel.

[0070] The lateral and vertical steps are, hereafter, referred to as U and Z channels respectively in accordance with the axes shown in FIG. 3. The combination of a Z channel and a U channel identifies a position where a frequency sweep of S.sub.21 parameter is registered in the Vector Network Analyser VNA, with channel 1 connected to a TXA and channel 2 to an RXA. The S.sub.21 parameter is a scattering parameter which is well known to the skilled person. A multiport VNA can be used, which contains multiple receivers. Each receiver can be connected to different RXA. In this case, other S parameters such as S.sub.31 or S.sub.41 are also registered.

[0071] Nu*Nz (number of lateral steps (Nu), number of vertical steps (Nz)) functions are referred to as the down-range DR measured profiles. The Nu*Nz function preferably results in a quasi-Fourier Transform of the frequency scan S.sub.21. This results in a calculation for each frequency scan position which are then transformed into their respective DR profiles. In between the VNA, and the RXA and TXA, a hardware-gating equipment HGE is connected. The HGE shuts the channel 2 of the VNA when energy scattered from the object is not received. The HGE shuts the channel 1 of the VNA to generate the transmitted pulse, being shut when the receiving channel is open.

[0072] FIG. 3 shows the calibration target TC located in front of the target zone TZ, and is laterally displaced to said target zone TZ in order to avoid any first-order interference, otherwise known as shadowing, with the object 200. The location of the calibration target TC is an input in the postprocessing process and fixes the origin O of the target zone TZ. The echoes produced by the calibration target TC are used for two purposes: calibration of the Z channels, which comprise the complete radiofrequency chains and all of the radiofrequency chains of the specific U channel that is used for this calibration, and the estimation of mechanical deviations of the hardware in the direction of the V-axis. The latter calibration is used for the correction of those deviations during the postprocessing.

[0073] The size of the lateral steps are electrically measured using the two second antennas 140 positioned on the carrier 120 and are pointed to the lateral targets TL located in the lateral direction i.e. the U-axis. Therefore, an additional frequency sweep is taken each lateral step using these second antennas 140. By identifying the echoes produced by the lateral targets TL, the size of the lateral steps between sweeps performed by the first antenna 100 can be measured, as well as the transient of the hardware gating equipment. This information is used for correction purposes in the postprocessing and also to compute the downrange profiles and the three dimensional ISAR images.

[0074] Through postprocessing of the data obtained from the first 100 and second 140 antennas and measured by the VNA and/or transferred to the processing unit, 3-D-ISAR images of the object 200 are computed. The vertical resolution of the first antenna 100, which is related to its size, defines the arrangement's 10 capability to suppress the ground plane effects. Therefore, larger vertical apertures improve the antenna performance and give a more accurate RCS result but also increase the number of samples, requiring more antennas, testing time, and hardware complexity.

[0075] Scattering centers SCs are extracted from the NF 3-D-ISAR Images, which are used to compute the RCS of the object 200.

[0076] Different scattering mechanisms can also be identified in the SC representation of the object 200. The RCS of the object 200 can be computed considering different spatial zones, which enables to filter out the effects of the ground plane or to evaluate the RCS contribution of a part, area, zone or specific feature of the object. The complete postprocessing scheme in described below in relation to FIG. 4.

[0077] FIG. 4 shows a block diagram of a method of measuring the Radar Cross Section, RCS, of an object according to an embodiment as described herein.

[0078] The measurement method is based on the following five main steps: [0079] (i) acquisition (S1000) of the observations by performing two dimensional (vertical-lateral) near field frequency scans; [0080] (ii) computation (S1100) of the downrange profiles; [0081] (iii) computation (S1200) of 3-D Inverse Synthetic Aperture Radar, ISAR, images by superimposing the downrange profiles; [0082] (iv) extraction (S1300) of the scattering centers; and [0083] (v) computation (S1400) of the RCS.

[0084] This method is carried out in the VNA and/or the HGE of the arrangement 10. The method may alternatively be carried out in a separate dedicated processing device in the arrangement 10. The separate dedicated processing device may be located on and/or in the carrier 120 and coupled to the VNA and/or the HGE and/or the first antenna 100 and/or the second antenna 140 via a wired and/or a wireless connection. Alternatively, the separate dedicated processing device may be located in an external location not associated with the carrier 120.

[0085] The two dimensional lateral scans are performed by the first antenna 100 in the carrier assembly 50. The scans are performed by the first antenna 100 making one scan and then the carrier assembly 50 being moved along the rail 150, in the U axis, to a second predetermined position where the first antenna 100 makes a second scan. This process repeats until the entire object 200 within the target zone TZ has been scanned. Alternatively, the observations could be taken continuously when the carrier assembly 50 is moving, i.e. at constant speed. The number of scans may be predetermined by, for example, a processor coupled to the motor of the carrier 120 and/or the carrier assembly 50 may comprise a unit which determines when the object 200 has been completely scanned by monitoring signals received by the first antenna 100, and/or the device by following a sampling criteria. Furthermore, in order to aid the subsequent steps of the method, the second antennas 140 also receive signals echoes from the lateral targets TL. These lateral target measurements allow the processor to process how far apart each scan has been and thereby compute a downrange profile for each scan and resultantly, a three dimensional ISAR image.

[0086] These observations are then passed to a processor and used to compute S1100 the downrange profiles. These downrange profiles are preferably determined by the processing device after receiving signals from the VNA. The VNA is able to measure both the amplitude and phase of the received signals and is therefore able to pass these signals to the processing device which can accurately compute the downrange profiles. The downrange profiles are computed by using a Fast Fourier Transform, FFT. This may allow for a more efficient processor as FFT is a particularly efficient method of calculation. It may also allow for a faster method thereby increasing the speed of RCS computation and/or the number of objects that the RCS can be computed for in a set period of time. The use of a FFT in computing is well known to the skilled person.

[0087] The profiles are then superimposed upon each other in order to compute S1200 a three dimensional ISAR image for the object 200 within the target zone TZ. This allows for an accurate three dimensional radar image of the object 200. The application of three dimensional ISAR imaging is known to the skilled person.

[0088] This three dimensional image is then used to extract S1300 the scattering center of the object 200. Knowing the position of the scattering center is important to the method as only by knowing the position of the scattering center is it possible to compute the RCS of the object.

[0089] The scattering center is then used to compute S1400 the RCS of the object 200 in the target zone TZ. After the RCS has been computed, the user is then able to see how the object 200 would be identified on, for example, a RADAR system. The scattering center of the object is extracted by a CLEAN algorithm. The use of the CLEAN algorithm may allow for a more efficient processor due to the characteristics of the algorithm by, for example, parallelization. It may also allow for a faster computation of the scattering center thereby increasing the speed of the method and the RCS computation and/or the number of objects that the RCS can be computed for in a set period of time.

[0090] There are also three optional steps within the method which improve the accuracy of the RCS measurement of the object 200.

[0091] After the observations of the object 200 have been acquired S1000, the observations are calibrated S1050 by the processor. Preferably, the calibration S1050 uses an echo received from a calibration target TC, by the first antenna 100, located between the object 200 and the carrier 120, and wherein the calibration target TC is displaced in a lateral direction in relation to the object 200. This calibration allows for a more accurate RCS result as has been described above in relation to FIG. 3. The information for the calibration is, therefore, taken at the same time that the measurement itself. The information for the calibration is embedded in the measurement data. This may allow for a more accurate calibration. This may also allow for a simpler and faster complete process.

[0092] After computing S1100 the downrange profiles, the distortion of the downrange profiles are corrected S1150 by the processor, wherein the correction S1150 comprises correcting the distortion from the antenna pattern and/or the transient of a hardware gating equipment HGE and/or mechanical deviations. This correction may lead to a more accurate RCS measurement as various external factors have been removed. The method of removing these distortional factors are known to the skilled person.

[0093] After the extraction S1300 of the scattering center of the object, the effects of the ground plane are filtered out S1350 by the processor. The vertical array first antenna 100 allows for diversity to filter out the ground plane effects. This is done by suppressing the contributions produced and/or located on the ground plane. This results in an improved RCS measurement.

[0094] The technique described herein is an outdoors facility, designed to measured targets located on the ground. The technique described herein may measure in a static configuration, prioritizing the accuracy of the results. The technique described herein may use an antenna, preferably an antenna array, performing a lateral scan to generate the synthetic aperture. The technique described herein may use the lateral scan capability to scan the target in the azimuth direction. The technique described herein may use a near-to-far field transformation technique to electronically measure the target close to target.

[0095] No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and en-compasses modifications apparent to those skilled in the art and lying within the scope of the claims appended hereto.