Radar with range-independent resolution

Abstract

Imaging device, the device comprising: a radar transmitter configured to send a radar signal; a first cell (325) comprising a first antenna (326) and a first delay element, wherein the first cell (325) is configured to receive the radar signal from the radar transmitter and to retransmit the radar signal from the first delay element after a first known time delay; a radar receiver configured to receive a retro-reflective signal based on the radar signal; and a unit for constructing an image (650), wherein the construction of the image (650) comprises determining the absence of a target (390) based on the presence of the newly transmitted radar signal in the retro-reflective signal and the presence of a target (390) based on the absence of the newly transmitted radar signal in the retro-reflective signal.

Description

TECHNICAL FIELD

[0001] The invention generally relates to radar systems. More particularly, it provides an apparatus, method, and system suitable for generating a radar with range-independent resolution. BACKGROUND

[0002] The spatial resolution of radar systems typically depends on the beam angle, the size of the radar antennas, and the distance between the radar and the target. Consequently, there is typically a trade-off between these factors and the achievable radar detection resolution.

[0003] Traditionally, higher spatial resolution could be achieved by increasing the antenna size. For example, as the antenna aperture increases, the antenna beamwidth decreases (i.e., 3 dB attenuation relative to peak). An approximate expression for the inverse relationship between the antenna size (D) and the 3 dB beamwidth (BW) for a horn antenna is BW = 70λ / D, where λ is the wavelength. As a result of the reduced beamwidth, the spot size on the target also decreases, allowing the detection of smaller features on the target. Increasing the antenna size must be done on one or both of the radar transmitter and receiver antennas to improve radar resolution. However, the use of large antennas, such as large parabolic antennas or arrays of antenna elements, can lead to an increase in the cost, size, and weight of the system.Additionally, it may be necessary to mechanically rotate or move the antenna or target to observe various features on the target so that the relevant area on the target is illuminated. Larger antennas can be formed by constructing phased arrays using, for example, digital beamforming or multi-static techniques. However, phased arrays add complexity in the form of individual feeds, transceivers, phase shifters, and / or processing sections.

[0004] Spatial resolution can also be increased by moving the radar system closer to the target. Fig. Figure 1 shows the effect of antenna size, target distance, and beamwidth on the spot size on the target. For a given antenna size (D) and a 3 dB beamwidth (BW), the spot size on the target (X) depends on the distance (R) between the target and the radar antenna. The spot size can be approximately given as X = 2*R*tan[λ / (2D]), where λ is the wavelength. However, moving the antenna closer to the target is not always a practical approach, as the distance between the radar and the target cannot always be controlled. In addition, the spot size does not decrease with the radar-target distance if the distance is shorter than the far-field distance D 2 / λ. In fact, the minimum spot size near the antenna is of the order of magnitude of the antenna aperture itself, as in Fig. 2 shown.

[0005] Super-resolution techniques can also be used to improve the resolution of radar systems. However, super-resolution techniques may require higher processing power and sophisticated algorithms. Furthermore, many super-resolution techniques require a high signal-to-noise ratio for efficient processing. Other techniques, such as monopulse, can also be used to increase resolution.

[0006] A radar system with a resolution independent of the range to the target is required. Therefore, the current state of the art requires addressing the above-mentioned problem.

[0007] DE 10 2014 008 670 A1 describes a radar calibration system for vehicles comprising a target simulator that receives a radar signal through a receiving antenna, delays the received signal through a delay line, and transmits the delayed signal through a transmitting antenna, and a radar device that transmits the radar signal to the target simulator through a transmitting antenna, receives the delayed signal from the target simulator through a plurality of receiving antennas, calculates phase differences between the delayed signals received by the plurality of receiving antennas, and generates and stores phase correction values ​​of receiving channels of the plurality of receiving antennas to correct the calculated phase differences.

[0008] US 7,298,318 B2 discloses a microwave imaging system comprising: a plurality of microwave antennas acting as microwave sources for providing microwave illumination of electromagnetic radiation having a wavelength between 0.3 mm and 30 cm to illuminate a target object, and as microwave receivers for receiving reflected microwave illumination reflected from the target object; four scanning arrays each comprising a plurality of reflective antenna elements, each reflective antenna element comprising a planar patch antenna formed on one side of a printed circuit board substrate and a surface-mounted field-effect transistor mounted on an opposite side of the printed circuit board substrate, a ground plane positioned between the planar patch antenna and the surface-mounted field-effect transistor,wherein the reflective antenna elements are capable of being programmed with a respective phase delay to direct the microwave illumination from one of the microwave sources to a position on a target object in a target object region, wherein the reflective antenna elements are further capable of being programmed with a respective phase delay to receive reflected microwave illumination reflected from the position on the target object and to direct the reflected microwave illumination to one of the microwave receivers, wherein two scanning fields are arranged on one side of the target object region and two scanning fields are arranged on an opposite side of the target object region such that a coverage area of ​​each scanning field covers a quadrant of the target object; and a processor operable to measure an intensity of the reflected microwave illumination,to determine a value of a pixel within an image of the target object constructed by the processor. SUMMARY OF THE INVENTION

[0009] The objects underlying the invention are achieved by the features of the independent patent claims. Embodiments of the invention are the subject of the dependent patent claims.

[0010] Viewed from a first aspect, the present invention provides an apparatus for imaging, the apparatus comprising: a radar transmitter configured to transmit a radar signal; a first cell having a first antenna and a first delay element, the first cell configured to receive the radar signal from the radar transmitter and retransmit the radar signal after a first known time delay from the first delay element; and a radar receiver configured to receive the retransmitted radar signal from the first cell.

[0011] Viewed from a further aspect, the present invention provides a method of imaging, the method comprising: transmitting a first radar signal by a radar transmitter; receiving at least a portion of the radar signal at a first antenna of a first cell; generating a first time-delayed signal by a first delay element based on the received at least a portion of the radar signal; retransmitting the first time-delayed signal by the first antenna of the first cell; receiving the at least a portion of the first time-delayed signal by a radar receiver; and constructing an image, wherein constructing the image comprises determining the absence of a target based on the presence of the first time-delayed signal and the presence of a target based on the absence of the first time-delayed signal.

[0012] Viewed from a further aspect, the present invention provides an imaging system, the system comprising: a radar, the radar comprising: a radar transmitter configured to generate a radar signal, and a radar receiver; and a cell array, the cell array comprising: a first cell, the first cell comprising a first antenna and a first delay element connected to the first antenna, the first antenna and the first delay element configured to receive the radar signal, generate a first time-delayed radar signal through the first delay element, and transmit the first time-delayed radar signal such that at least a portion of the first time-delayed radar signal arrives at the radar receiver, and a second cell, the second cell comprising a second antenna and a second delay element connected to the second antenna,wherein the second antenna and the second delay element are configured to receive the radar signal, generate a second time-delayed radar signal through the second delay element, and transmit the second time-delayed radar signal such that at least a portion of the second time-delayed signal arrives at the radar receiver.

[0013] The present invention relates to improving the performance of radar, spectroscopy and imaging systems by (i) performing single or multi-pixel imaging with a single fixed-beam radar regardless of the size of the radar antenna, and (ii) providing super-resolution of target edges on the order of a wavelength or less that is independent of the target range.

[0014] In one embodiment, the invention further relates to a novel technique for radar imaging that achieves super-resolution of target profile edges by placing an array of cells containing true delays or lumped delays behind the target. In one embodiment, the resolution of the radar is determined in part by the size of the cells. Typical cells may be on the order of a wavelength, and the resolution of the radar is of the same order. This resolution can be achieved without considering the size of the illuminating and receiving radar antennas and without considering the distance between the radar and the target. The cell structure, in one embodiment, consists of an antenna (e.g., a horn, open waveguide, or patch antenna) coupled to a lumped delay element (such as a phase shifter, capacitors, inductors, etc.) with, for example,a short circuit after the lumped element or the true delay element (such as a waveguide, a printed microstrip line, a stripline, a coaxial cable, a coplanar waveguide, etc.) with generally varying length or delay for each cell, e.g., by shorting the delay element to the desired length. The cells are placed behind the target, and the overall design does not require significant changes to the radar system itself. In the described device, the spatial resolution of the radar, together with the cells, is determined by the size of the cells and not by the size of the radar antennas or the radar-target range. The cell structure can be used with any radar system, such as continuous wave, pulsed, or other radar.

[0015] These and other features, aspects and advantages of the present invention will become apparent by reference to the following description, the appended claims and the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a diagram showing the relationship of the spot size of a radar to the range to the target and the beam width according to the prior art; Fig. Figure 2 is a diagram showing the near-field and far-field components of the radar beam according to the prior art; Fig. 3 is a schematic diagram of the range-independent resolution radar system according to an embodiment of the present invention; Fig. 4 is a schematic representation of the cell array with eighteen open waveguides as antennas according to an embodiment of the present invention, wherein each antenna is connected to a waveguide of different length by shortening the waveguide at the desired position; Fig. 5 is a graphical representation of the resulting clock frequencies of a continuous wave conversion of an embodiment of the present invention; Fig. 6A is a schematic representation of a target present in front of the cell array according to an embodiment of the present invention; Fig. 6B is a graphical representation of the clock frequencies available for a frequency modulated continuous wave radar according to an embodiment of the present invention; Fig. 6C is a reconstructed target image according to an embodiment of the present invention; Fig. 7 is a schematic representation of a compact structure with shallow meandering waveguide tunnels according to an embodiment of the present invention; Fig. 7A is a schematic diagram of a top layer of a device according to an embodiment of the present invention having nine antennas; Fig. 7B is a schematic diagram of a center signal distribution layer of a device according to an embodiment of the present invention; Fig. 7C is a schematic diagram of a bottom true delay line layer of a device according to an embodiment of the present invention; Fig. 7D is a schematic diagram of a multilayer device according to an embodiment of the present invention; Fig. 8A is a schematic view of eight open waveguide antennas connected to eight waveguides of different lengths illuminated by an incident field according to an embodiment of the present invention; Fig. 8B is a graphical representation of field strengths of a wave incident on the cell array of the range-independent resolution radar system according to an embodiment of the present invention; Fig. 8C is a graphical representation of directivity of a retransmitted signal according to an embodiment of the present invention; Fig. 9 is a barcode-like device according to an embodiment of the present invention; and Fig. 10 is a range-independent resolution radar system including lenses according to an embodiment of the present invention. DETAILED DESCRIPTION

[0017] In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodiments by which the invention may be practiced. It should be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.

[0018] A schematic representation of the radar system with range-independent resolution according to an embodiment of the present invention is shown in Fig. 3. The range-independent resolution radar system 300 is comprised of a radar 310 and a cell array 320. The radar 310 includes a transmitter and a receiver. The target 390 is placed between the transmitter 310 and the cell array 320. Each cell 325 includes a cell antenna 326 and a true delay 327. The cell antenna 326 may be any type of antenna suitable for transmitting and receiving signals at the radar's operating frequency and may be an open waveguide, horn, patch, dipole, slot, miniaturized, or dielectrically surrounded reduced-size antenna. The true delay 327 may be implemented as a microstrip line, a stripline, a waveguide, a coplanar waveguide, or any other transmission line capable of conducting the incoming energy received by the cell antenna and reflecting it back to the radar.The real delay 327 can be replaced by other delay elements, such as lumped elements. Depending on the radar's signal-to-noise ratio capabilities, the gain of each antenna, the transmission line losses, and the coupling between cells and the transmission line, the antenna size can be reduced and the transmission line occupied without any inherent physical limit imposed by the wavelength.

[0019] It should be noted that the radar transmitting antenna, which illuminates the target, and the radar receiving antenna, which detects the returns from the target, can be different or could be implemented by the same antenna. It should also be noted that in Fig. 3 the target does not necessarily block all cells. The apparatus, system, and method of embodiments of the present invention are applicable to all cases where it is possible to position such cells behind the target. The target may also move in the area in front of the cells, and the imaging system may generate a series of images at different times during the time the target is present. Each cell generates a delay caused by its different length. In continuous wave radars, such as frequency-modulated continuous wave radar, each delay would be converted into a different clock frequency f b translated in the radar receiver. In pulse radars, the different delay causes the reflected signal to be reflected at different times t rarrives at the radar receiver. The cell has a known delay from the true delay 327 by which it delays the radar signal. The radar receiver can determine which cells are blocked based on the presence and / or amplitude of the signal, which correspond to the known time delay of the cell. Starting with a case where the array is illuminated by the radar signal and no target obstructs the path, the radar measures the delay associated with all cells in the array. In the case where one or more cells are blocked by the target, the values ​​for f b or t rassociated with these cells are not present at the radar output. Since the position of the cells can be known and predetermined, a high-resolution image of the target's profile edges or the entire target can be formed. This high resolution is achieved without considering radar antenna size or target-to-radar range, and without the need for conventional super-resolution techniques.

[0020] Fig. Figure 4 shows an embodiment of the cell arrangement 320 according to an embodiment of the present invention with eighteen open waveguides as antennas, each antenna being connected to a waveguide of different lengths by shortening the waveguide at the desired position. For example, when this structure is illuminated by a frequency-modulated continuous-wave radar with a signal whose frequency spectrum has a bandwidth B around f0, eighteen clock frequencies are generated in the radar receiver according to the delay of each cell. A graphical representation of the resulting clock frequencies 510 according to an embodiment of the present invention is shown in Fig. 5. Note that cell array 320 may include any number of cells. This number may be selected, for example, taking into account the typical cell size, the target size, and the operating frequency.

[0021] The range resolution of the radar is given by ΔL = c / (2B), where c is the speed of light and B is the radar bandwidth. Therefore, the length difference between cells must be greater than or equal to ΔL. While the example here refers to a frequency-modulated continuous-wave radar, the same structure can be used with any continuous-wave, pulsed, or other radar type. In the case of a pulsed radar, the length difference between each cell generates different delays in the radar receiver.

[0022] Fig. 6A shows a schematic representation of a target 390 present in front of the cell array 320 according to an embodiment of the present invention. The target 390 blocks some of the cells, and other cells of the cell arrays 320 can remain open. When the target and the cells are illuminated with a beam whose size is equal to or larger than the cell array, only some of the clock frequencies 510 (fb ) or delays (t r ) corresponding to the cells left open, corresponding signals to the radar receiver. The clock frequencies generated for a frequency-modulated continuous wave radar according to an embodiment of the present invention are described in Fig. 6B. Since the positions of the cells are known and predetermined, an image 650 of the target can be formed, as shown in Fig. 6C shown.

[0023] The distance between the target and the cells is not limited to any specific value. However, short distances are preferred to avoid diffraction effects, where cells whose direct line of sight is blocked by the target could reflect some of the energy back to the radar.

[0024] The collective field reflected back by all cells in the structure is actually the aperture field of an antenna array. Therefore, the lengths of the true delays of each cell can be randomly distributed to create a random phase distribution across this aperture. This minimizes the possibility of beam offset, which reduces the power propagated to the radar's receiving antenna.

[0025] Other configurations of the array are also possible. In one embodiment, the straight and long waveguides used in Fig. 4, can be replaced by a more compact structure with flat meandering waveguide tunnels according to an embodiment of the present invention, as shown in Fig. 7D. The top layer 710 of the device is shown in Fig. 7A. This top layer 710 has a four-by-four array of open waveguides that act as the cell antennas. The middle layer 720 of the device is Fig. 7B. This layer has distribution tunnels from the cell antennas to the real delays, which in this embodiment are waveguide tunnels. In this embodiment, the waveguide tunnels are thin tunnels. A bottom layer 730 of real delays is shown in Fig. 7C. Note that the combined delay of a waveguide tunnel of the middle layer 720 and the true delay of the bottom layer 730 results in the known delay value of the true delay of the cell array 320. All combined layers that create the cell array 320 are shown in Fig. 7D can be seen.

[0026] Fig. Figure 8A shows a cell array 320 consisting of eight open waveguide antennas connected to eight waveguides of different lengths, each illuminated by an incident field, according to an embodiment of the present invention. The illumination represents the signal transmitted by a radar. The different waveguide lengths are achieved by shorting each waveguide to the desired length. The spatial pattern of the reflected wave is Fig. 8B. The spatial pattern shows that each cell has detected the illuminating wave and re-transmitted a wave according to the length of the true delay. Illuminating a multi-cell structure with a radar signal, such as a pulsed signal or a continuous wave signal, generates the various output signals, such as delayed pulses or clock frequencies, from which the radar image or target profile can be generated. Note that the length differences between the waveguides were selected in one embodiment such that the re-transmitted energy was steered by 30 degrees, as shown in Fig. 8C. This means that a phase shift can be deliberately introduced in each of the cells to change the direction of the newly transmitted signal. However, the cells can be randomly distributed with respect to the phase shifts between the cells to avoid beam steering of the reflected wave. The relatively high stray field strengths behind the waveguides, as seen in Fig. 8B, can be reduced as desired. This reduction can be achieved by placing radar-absorbing materials around the cell openings.

[0027] In addition to the frequencies reflected by the target and the structure, the amplitude of the reflected wave can be used to obtain additional information about the target. In the case of a frequency-modulated continuous wave radar, the received frequency corresponding to a particular cell may have a lower amplitude if the cell is partially blocked. The degree of attenuation can help determine the extent of blockage by the target. The attenuated signal can also be used to further enhance spatial resolution beyond the cell size. Signal attenuation can also be used to detect material properties. For a highly conductive metallic target, the blocked cells will emit little or no energy. If the target is made of other, only partially absorbing materials, cells blocked by the target are able to reflect some of the energy back to the radar.In this way, the system can be used to characterize the material composition of the target, particularly at the radar's operating frequency. Note that a partially transparent object does not obstruct the radar beam, allowing the corresponding returned signal (e.g., clock frequency components in a frequency-modulated continuous wave radar or delayed pulses in pulse radar) to be revealed, albeit in an attenuated form. When amplitude information is taken into account, the image of the target object is still generated, although it is determined that the corresponding frequencies are returned with lower power due to attenuation. The attenuated signal can provide information about the object's transparency, which is determined by the material, its thickness, and its shape.

[0028] The system can also assist in determining target thickness. The characteristics of the returned signal depend on the target thickness for some materials. In the case of cells that are partially blocked or partially transparent, reference frequency signals and additional clock frequency signals are generated, corresponding to energy traveling back and forth between the cells multiple times. This information can be used in these cases to determine target thickness. Furthermore, in the case of continuous-wave radar, partially blocked cells can generate frequencies corresponding to waves traveling back and forth between the cells multiple times before being received by the radar. These frequencies indicate that they are not completely blocked, thus improving the spatial resolution to below the cell size.A spatial resolution better than the wavelength can be achieved when the distance of the blocking object from the delay line aperture is shorter than one wavelength, and when the aperture size of the delay line itself is smaller than one wavelength. The limit of spatial resolution is the aperture dimension, as long as the delay line is capable of guiding a wave to the specified small aperture dimension.

[0029] The cell array 320 can also be configured to create a barcode-like device. The barcode-like device according to an embodiment of the present invention is shown in Fig. 9. This flat device has nine patch antennas and nine printed transmission lines, with each transmission line connected to an antenna. By using a phased array or a focal plane array, the radar beam can be focused or directed onto each of the cells separately. In this case, other applications, such as a wireless barcode, can be implemented. Each cell can be connected to a specific length, and repetitions between actual delay lengths are permitted because the beam is directed onto a specific cell and reads its delay, generating a numerical value based on the delay. These numerical values ​​can be combined into a barcode value. Since the total number of possibilities is very large, even with a small number of cells, a barcode-like device can be implemented.

[0030] A method according to an embodiment of the present invention for improving the resolution for targets smaller than the cell array is described in Fig. 10. When a small target is placed in front of a multicellular structure and blocks only a small number of cells, the resolution of the formed image can be improved by adding a first lens 910 and a second lens 920, placing the target 390 between the lenses, and placing the cell array 320 after the second lens 920, as shown. In this way, the shape of the target projected onto the cells is larger, and more cells are effectively blocked. This lens system results in an improvement in image resolution compared to the case without lenses.

[0031] Although specific embodiments of the present invention have been described, it will be apparent to one skilled in the art that other embodiments exist that are equivalent to the described embodiments. Accordingly, it should be understood that the invention is not to be limited by the specific illustrated embodiments, but only by the scope of the appended claims.

[0032] While the foregoing written description of the invention will enable one skilled in the art to utilize what is presently considered the best mode thereof, variations, combinations, and equivalents to the specific embodiment, method, and examples herein will be apparent to those skilled in the art. The invention, therefore, should not be limited to the embodiment, method, and examples described above, but should be limited to all embodiments and methods within the scope of the invention. To the extent necessary to understand or complete the disclosure of the present invention, all publications, patents, and patent applications mentioned herein are expressly incorporated by reference to the same extent as if individually incorporated.

[0033] Having described exemplary embodiments of the present invention, it will be apparent to those skilled in the art that the disclosures herein are merely exemplary, and that various other alternatives, adaptations, and modifications may be made within the scope of the present invention. Accordingly, the present invention is not limited to the specific embodiments illustrated herein, but is limited only by the following claims.

Claims

[1] An imaging device, the device comprising: a radar transmitter configured to transmit a radar signal; a first cell (325) having a first antenna (326) and a first delay element, the first cell (325) configured to receive the radar signal from the radar transmitter and retransmit the radar signal from the first delay element after a first known time delay; a radar receiver configured to receive a return signal based on the radar signal; and a unit for constructing an image (650), wherein constructing the image (650) comprises determining the absence of a target (390) based on the presence of the retransmitted radar signal in the return signal and the presence of a target (390) based on the absence of the retransmitted radar signal in the return signal. [2] The apparatus of claim 1, wherein the first delay element is a true delay (327). [3] Apparatus according to any one of the preceding claims, wherein the radar receiver performs the steps of: Determining a first pixel value, the first pixel value based on the first known time delay and an amplitude of the received retransmitted radar signal. Shading an image position of the first pixel value based on the position of the first cell (325). [4] Apparatus according to any one of the preceding claims, wherein the radar transmitter is a continuous wave transmitter, and the first cell (325) is configured to generate a known clock frequency (510) at the radar receiver from the transmitted radar signal and the retransmitted radar signal. [5] Apparatus according to any one of claims 1 to 3, wherein the radar signal transmitter is a pulsed transmitter, and the radar receiver constructs an image (650) of the target (390) by determining the amplitude of the newly transmitted radar signal with the first known time delay. [6] The apparatus of any preceding claim, further comprising a second cell (325), the second cell (325) comprising a second antenna (326) and a second delay element, the second cell (325) being configured to receive the radar signal from the radar transmitter and retransmit the radar signal from the second delay element after a second known time delay. [7] Apparatus according to any one of the preceding claims, wherein the first antenna (326) comprises a waveguide antenna, and the first delay element comprises a waveguide. [8] The apparatus of claim 1, wherein the first antenna (326) comprises a patch antenna, and the first delay element comprises a microstrip line. [9] The apparatus of claim 1, further comprising a first lens (910) and a second lens (920), wherein the first lens (910) and the second lens (920) are located between the first cell (325) and the radar transmitter. [10] A method of imaging, the method comprising: Sending a radar signal by a radar transmitter; Receiving at least a portion of the radar signal at a first antenna (326) of a first cell (325); Generating a first time-delayed signal by a first delay element based on the received at least one portion of the radar signal; retransmitting the first time-delayed signal by the first antenna (326) of the first cell (325); Receiving a return signal based on the radar signal by a radar receiver; and Constructing an image (650), wherein constructing the image (650) comprises determining the absence of a target (390) based on the presence of the first time-delayed signal in the return signal and the presence of a target (390) based on the absence of the first time-delayed signal in the return signal. [11] The method of claim 10, wherein the first delay element is a true delay (327). [12] The method of any one of claims 10 or 11, wherein constructing a radar image (650) by the radar (310) comprises: Determining a first pixel value, wherein the first pixel value is based on the first time-delayed signal; and Shading the image position of the first pixel value based on a first position of the first cell antenna (326).

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