Patent Application
20200256979
Aspects of the disclosure relate to radar and, more particularly, to a radar that utilizes a multi-transmit antenna frequency modulated continuous wave architecture,
Synthetic-aperture radar (SAR) is a form of radar that is used to create two or three-dimensional images of objects. Some SAR devices use the motion of a radar antenna over a target region to provide its spatial resolution. SAR is typically mounted on a moving platform such as an aircraft or spacecraft. Successive pulses of radio waves are transmitted to “illuminate” a target scene, and the echo of each pulse is received and recorded. As the SAR device on board the aircraft or spacecraft moves, the antenna location relative to the target changes with time, Signal processing of the successive recorded radar echoes received from the target allows for the combining of the recordings from the multiple antenna positions to create a SAR image. Thus, the distance the SAR system travels over a target in the time taken for the radar pulses to return to the antenna creates the synthetic antenna aperture, which may have an effective aperture size orders of magnitude larger than. the actual physical antenna used, The larger the aperture, the higher the image resolution, which allows SAR to create relatively high-resolution images with comparatively small physical antennas.
However, many SAR devices have distinct drawbacks, For example, a SAR device typically must be in constant motion (e.g., mounted to aircraft or spacecraft) in order to create its synthetic aperture. Moreover, the time needed to generate a high resolution SAR image is often directly tied to how fast the SAR device traverses through the target area while mounted onto its carrier (e.g., aircraft or spacecraft).
Phased array radar devices use a computer-controlled array of antennas to create a beam of radio waves that can be electronically steered to point in different directions without moving the antennas. In such systems, the radio frequency (RF) current from the transmitter is fed to the individual antennas with the correct phase relationship so that the radio waves from the separate antennas add together to increase the radiation in a desired direction, while cancelling to suppress radiation in undesired directions. In a phased array, the power from the transmitter is fed to the antennas through phase shifters that are controlled by a computer system, which can alter the phase electronically, thus steering the beam of radio waves to a different direction.
However, phased array radar devices can also have significant disadvantages. First, such devices may be prohibitively expensive for certain applications due to the large number (e.g., tens, hundreds, even thousands) of costly antennas that may be needed. Second, each antenna of a phased array may require a phase shifter, which further compounds problems associated with cost, size, and complexity. Third, targets within range of the phased array radar typically cannot be detected until the narrow beam formed by antennas cycles through and reaches the target during a sweep. Fourth, the antenna components in the phase array radar should be located at subwavelength distance one from the other, which is hard to achieve in high-frequency devices.
There is a need for radar devices that provide high quality images and fast image acquisition with reduced cost and design complexity. Such devices would ideally require less costly components and eliminate the need for moving components.
This document provides, among other features, methods and apparatus that implement an interferometric radar by using multiple antennas to transmit frequency modulated continuous wave signals, The described radar is not a synthetic aperture radar as it does not require any motion of the transmitter and/or receiver modules. The described device also is not a phased array antenna based radar as the antennas of the device can be located at an arbitrary distance each from the other.
In one aspect, a radar device includes: a signal source configured to provide a transmission signal; a first transmitter configured to transmit the transmission signal as a first frequency modulated continuous wave to a target; a second transmitter configured. to transmit the transmission signal as a second frequency modulated continuous wave to the target, the first and second transmitters spaced apart by a distance greater than a wavelength of the transmission signal; a receiver configured to receive a reflected signal from the target, wherein the reflected signal is a reflection of the transmission signal from the target; and an analyzer configured to determine a target distance and/or an angle of the target with respect to the first and/or second transmitter based on the received reflected signal and the transmission signal provided by the signal source.
In another aspect, a method includes: providing a frequency modulated continuous wave transmission signal to first and second transmitters that are spaced apart a distance greater than a wavelength of the transmission signal; transmitting the transmission signal from the first and second transmitters to a target; receiving a reflected signal from the target at a receiver, the reflected signal a reflection of the transmission signal from the target; and determining a target distance and/or an angle of the target with respect to the first and/or second transmitters based on the received reflected signal and the transmission signal.
In yet another aspect, an apparatus includes: means for generating a frequency modulated continuous wave transmission signal; means for transmitting the frequency modulated continuous wave transmission signal to a target using first and second transmitters that are spaced apart a distance greater than a wavelength of the transmission signal; means for receiving a reflected signal from the target at a receiver, the reflected signal a reflection of the transmission signal from the target; and means for determining a target distance and/or an angle of the target with respect to the first and/or second transmitters based on the received reflected signal and the transmission signal
In the following description, specific details are given to provide a thorough understanding of the various aspects of the disclosure. However, it will be understood by one of ordinary skill in the art that the aspects may be practiced without these specific details. For example, circuits may be shown in block diagrams in order to avoid obscuring the aspects in unnecessary detail. In other instances, well-known circuits, structures and techniques may not he shown in detail in order not to obscure the aspects of the disclosure.
The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any implementation or aspect described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other aspects of the disclosure. Likewise, an aspect is an implementation or example. Reference in the specification to “an aspect,” “one aspect,” “some aspects,” “various aspects,” or “other aspects” means that a particular feature, structure, or characteristic described in connection with the aspects is included in at least some aspects, but not necessarily all aspects, of the present techniques. The various appearances of “an aspect,” “one aspect,” or “some aspects” are not necessarily all referring. to the same aspects. Elements or aspects from an aspect can be combined with elements or aspects of another aspect.
In the following description and claims, the term “coupled” may mean that two or more elements are in direct physical or electrical contact. However, “coupled” may also mean that two or more elements are not in direct contact with each other, but yet still co-operate or interact with each other.
Not all components, features, structures, characteristics, etc. described and illustrated herein need be included in a particular aspect or aspects. if the specification states a component, feature, structure, or characteristic “May,” “might,” “can” or “could” be included, for example, that particular component, feature, structure, or characteristic is not required to be included. If the specification or claim refers to “a” or “an” element, that does not mean there is only one of the element. If the specification or claims refer to “an additional” element, that does not preclude there being more than one of the additional element. it is to be noted that, although some aspects have been described in reference to particular implementations, other implementations are possible according to some aspects. Additionally, the arrangement and/or order of circuit elements or other features illustrated in the drawings and/or described herein need not be arranged in the particular way illustrated and described. Many other arrangements are possible according to sonic aspects.
In each figure, the elements in some cases may each have a same reference number or a different reference number to suggest that the elements represented could be different and/or similar. However, an element may be flexible enough to have different implementations and work with some or all of the systems shown or described herein. The various elements shown in the figures may be the same or different. Which one is referred to as a first element and which is called a second element is arbitrary.
The radar device 100 may include a voltage controlled oscillator (VCO) 102, a ramp generator 104, a mixer 106, a processing circuit 108 (e.g., spectrum analyzer, network analyzer, processor, etc.), a first transmitter 110, a second transmitter 112, and a receiver 114, The first and second transmitter 110, 112 antennas are separated from one another a known and fixed distance D, which is significantly longer than the wavelength of electromagnetic radiation transmitted by the transmitters 110, 112. The radar device 100 generally operates using an FMCW scheme where the ramp generator 104 provides one or more ramping voltage signals 152 to the VCO 102, which the VCO 102 uses to generate a frequency modulated transmission signal 154 that is fed to the first transmitter 110 via a first path 156a and the second transmitter 112 via a second path 156b. In one aspect of the disclosure, the path length of the first and second paths 156a, 156b is equal. The ramp generator 104 is also coupled to the processing circuit.
Referring to both
Since the path lengths 154, 156a, 156b from the VCO 102 to the first and second transmitters 110, 112 are substantially equal, the instantaneous frequency of the signals transmitted out from the transmit antennas 110, 112 at any given point in time are also equal. Consequently, signals transmitted out from the first and second transmitters 110, 112 reach a target located along the zero-angle azimuth plane 160 at the same time since the zero-angle azimuth plane is equidistant to both the first and second transmitters 110, 112.
By contrast, if a target, such as the target 101 shown in
This difference in frequency between the first and second transmission signals 172, 174 is directly proportional to the angle θ of the target 101. With a larger θ, the path length difference is greater between the first and second transmission signals 172, 174 to reach the target 101, and thus it takes a greater amount of time for one of transmission signals to cover the distance versus the other. That is, the greater the θ, the greater the time difference between the two transmission signals to the target 101.
The signal 176 reflected back from the target 101 is proportional to sin(2πfit)+sin(2πf2t). The distance D between the transmitters 110, 112 and the difference between f2 and f1 is analyzed by the processing circuit 108 to determine the angle θ. The difference between the frequency f1 and fpresent (currently transmitted signal's frequency) or the difference between the frequency f2 and fpresent may be used to determine the distance d between the target 101 and the transmitters 110, 112. The azimuth angular resolution is equal to c/(2*D*BW) and is thus unrelated to the carrier frequency of the transmitted signals. As with
Referring
In the examples described above, it was assumed that the path lengths 156a, 156b that lead from the VCO 102 to the transmitters 110, 112 are equal or that any differences are known so as to compensate for frequency output differences at the transmitters 110, 112. In one aspect, the VCO 102 may output an RE signal directly to the transmitters 110, 112. However, in such an implementation, the paths 156a, 156b, may introduce too much loss into the transmission signal 202 unless high quality, and expensive components are used. Thus, transmitting an RE signal from the VCO 102 to the transmitters 110, 112 may be possible but may be impractical.
A portion of the modulated light carrier wave is input to another photodetector 1307 that supplies the RF signal to the mixer 1306. The mixer 1306 also receives the reflected RF signal from one or more targets to generate the beat signal for processing by the processing circuit 1308 as described above with respect to
In one example, each vertical MIMO array 1410 and 1412 includes N transmitters arranged vertically (i.e. orthogonal to transmission axis 1400 and parallel with azimuth axis 1402). Hence, 2*N versions of the FMCW transmission signals are sent to a target, such as exemplary target 1406, and 2*N versions of the reflected signal are received by a receiver 1408. The received signals are routed into a photonic network analyzer 1414, which determines the different target distances and angles to each of 2*N different points on the surface of the target 1406 that is facing the radar. From these signals, the network analyzer 1414 determines the shape (or profile) of one vertical slice of the surface of the target facing the transmitters. The shape of the slice may consist, for example, of 2*N points, each represented by a distinct distance and azimuth. As the target moves relative to the transmitters (or the transmitters move relative to the target, or both) along an axis that is orthogonal to both axis 1400 and axis 1402, additional vertical slices of the target are scanned. The network analyzer 1414 uses a set of slices obtained over a period of time (e.g. M slices obtained over T seconds) to generate a 3D image of the surface of the target facing the transmitters, with the number of points in the resulting 3D image equal to 2*N*M. In various examples, N may be in the range of 5-100 (e.g. 5, 10, 50, or 100), and M also may be in the range of 5-100 (e.g. 5, 10, 20, 50, or 100). These values are merely illustrative. The greater the number of transmitters N in each MIMO, the higher the resolution. T may be, e.g., in the range of 0.1 seconds to 1 second. If the target is rotating relative to the transmitters, or the transmitters move around the target, then similar 3D images may be generated for other surface portions of the target, such as its rear surface or its top or bottom surfaces so as to provide a more complete 3D image of the entire surface of the target.
As shown in
Some advantages of at least sonic implementations of the radar methods and devices described herein include:
High aperture: In one aspect, the radar is practical because by using a low phase noise, broadband, tunable photonic VCO, the high frequency optical signal of photonic VCO can be transmitted with very low loss and very low crosstalk between Tx elements over large distance via photonic link such as an optical fiber.
High resolution: The photonic VCO creates a broadly tunable RF signal by demodulating two or more optical tones on a fast photodiode photodetector). The resolution of the configuration presented is proportional to the bandwidth of VCO. Broad bandwidth of the VCO allows for high resolution. For instance an 80 GHz VCO bandwidth leads to 0.2 deg resolution if placed on a front face of an apparatus. The VCO tuning range can exceed an octave.
To achieve high spectral purity of the optical tones lasers may be locked to high quality factor tunable optical resonators. The tunability of VCO frequency is achieved because of the relative tunability of the optical tones. The tuning of the optical tones can be achieved electro-optically, or mechanically, or by temperature variations, applied to the high-Q factor optical resonators used to increase the spectral purity of the optical tones.
An example of a VCO that may be used in radar devices described herein is a VCO represented by two lasers locked to two high-Q independently tunable resonators (e.g., see
Another example of a VCO that may be used in radar devices described herein is a VCO represented by locking two lasers to two different modes of the same tunable optical resonator. The relative frequency of the lasers can be tuned by tuning the frequency difference between the resonator modes.
An example of a VCO that may be used in radar devices described herein is a VCO represented by a tunable opto-electronic oscillator. The broad frequency tuning enabled by the photonic VCO is important for large range and angular resolution. The broad frequency tuning of the VCO makes the system stable with respect to clutter.
In some aspects of the disclosure are described frequency-manipulation/sweep SAR systems that consists of two or more spatially separated ordinary or MIMO transmission antennas forming a sequence of complex beams in the far field. The Rx antenna receives the reflected signals from the targets and a mathematically processed record of the signal forms the far field.
In some aspects, the radar devices described herein may exploit the advantage of photonic links between Tx transmitters. Doing so may eliminate problems associated with high (if not forbidding) transmission line loss across long signal paths in the mm-band. The systems described herein may have a photonic source of FMCW swept signal that is transmitted to Tx subassemblies practically loss-free and is converted to RF or mm radiation at Tx subassemblies.
In some aspects, the radar devices described herein may exploit a wide bandwidth photonic VCO that provides an improved angular resolution where the angular resolution is c/(2*D*VCO_bandwidth). In some aspects, the radar devices described herein exploit a narrowband and fast-sweeping rate photonic VCO that provides an improved radial resolution since radial resolution is proportional the linewidth of VCO and inversely proportional to a sweep rate of VCO. In sonic aspects, the radar devices described herein do not have to have any moving parts or active phase shifters, All beam manipulations may happen or occur in the frequency domain.
The various components and functions shown in
One or more of the components, steps, features, and/or functions illustrated in
Also, it is noted that the aspects of the present disclosure may be described as a process that is depicted as a flowchart, a flow diagram, a structure diagram, or a block diagram. Although a flowchart may describe the operations as a sequential process, many of the operations can be performed in parallel or concurrently. In addition, the order of the operations may be re-arranged. A process is terminated when its operations are completed. A process may correspond to a method, a function, a procedure, a subroutine, a subprogram, etc. When a process corresponds to a function, its termination corresponds to a return of the function to the calling function or the main function.
Information generated by the methods and apparatus described herein may be stored in a storage medium. A storage medium may represent one or more devices for storing data, including read-only memory (ROM), random access memory (RAM), magnetic disk storage mediums, optical storage mediums, flash memory devices and/or other machine-readable mediums and, processor-readable mediums, and/or computer-readable mediums for storing information. The terms “machine-readable medium”, “computer-readable medium”, and/or “processor-readable medium” may include, but are not limited to non-transitory mediums such as portable or fixed storage devices, optical storage devices, and various other mediums capable of storing or containing instruction(s) and/or data. Thus, the various methods described herein may be fully or partially implemented by instructions and/or data that may be stored in a “machine-readable medium,” “computer-readable medium,” and/or “processor-readable medium” and executed by one or more processors, machines and/or devices.
Furthermore, aspects of the disclosure may be implemented by hardware, software, firmware, middleware, microcode, or any combination thereof. When implemented in software, firmware, middleware or microcode, the program code or code segments to perform the necessary tasks may be stored in a machine-readable medium such as a storage medium or other storage(s). A processor may perform the necessary tasks. A code segment may represent a procedure, a function, a subprogram, a program, a routine, a subroutine, a module, a software package, a class, or any combination of instructions, data structures, or program statements. A code segment may be coupled to another code segment or a hardware circuit by passing and/or receiving information, data, arguments, parameters, or memory contents. Information, arguments, parameters, data, etc. may be passed, forwarded, or transmitted via any suitable means including memory sharing, message passing, token passing, network transmission, etc.
The various illustrative logical blocks, modules, circuits, elements, and/or components described in connection with the examples disclosed herein may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic component, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing components, e.g., a combination of a DSP and a microprocessor, a number of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration.
The methods or algorithms described in connection with the examples disclosed herein may be embodied directly in hardware, in a software module executable by a processor, or in a combination of both, in the form of processing unit, programming instructions, or other directions, and may be contained in a single device or distributed across multiple devices. A software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. A storage medium may be coupled to the processor such that the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor.
Those of skill in the art would further appreciate that the various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the aspects disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system.
The various features of the invention described herein can be implemented in different systems without departing from the invention. It should be noted that the foregoing aspects of the disclosure are merely examples and are not to be construed as limiting the invention. The description of the aspects of the present disclosure is intended to be illustrative, and not to limit the scope of the claims. As such, the present teachings can be readily applied to other types of apparatuses and many alternatives, modifications, and variations will be apparent to those skilled in the art.
This patent document claims the priority of U.S. Provisional Application No. 62/636,023, entitled “METHODS AND APPARATUSES FOR IMPLEMENTING RADAR HAVING A SYNTHETIC APERTURE BY USING TWO OR MORE TRANSMIT ANTENNAS TRANSMITTING FREQUENCY MODULATED CONTINUOUS WAVE SIGNALS,” filed on Feb. 27, 2018, the entire disclosure of which is incorporated by reference herein.
| Number | Date | Country | |
|---|---|---|---|
| 62636023 | Feb 2018 | US |
