Mirror movement and laser shot pattern compensation for frequency modulated continuous wave LIDAR

Abstract

Embodiments of the present disclosure relate to mirror movement and laser shot pattern compensation for frequency modulated continuous wave (FMCW) LIDAR. A scanning system includes a transmitter, a scanning structure, and a controller. The transmitter is configured to transmit a frequency modulated continuous wave (FMCW) beam comprising a plurality of frequency ramps, the plurality of frequency ramps comprising up-conversions and down-conversions matched into up-and-down frequency conversion pairs. The scanning structure is configured to oscillate about a scanning axis such that a deflection angle of the scanning structure varies continuously over time within an angular range between two maximum deflection angles. The controller is configured to segment the angular range into a plurality of sub-angular ranges and assign each up-and-down frequency conversion pair to a different sub-angular range of the plurality of sub-angular ranges. Each up-and-down frequency conversion pair includes an up-conversion transmitted in the assigned sub-angular range during a first scanning movement of the scanning structure and a down-conversion transmitted in the assigned sub-angular range during a second scanning movement of the scanning structure.

Description

Technical Field

[0001] Embodiments of the present disclosure relate to light detection and ranging (LIDAR), and more particularly, to mirror movement and laser shot pattern compensation for frequency modulated continuous wave (FMCW) LIDAR. Background Art

[0002] Light Detection and Ranging (LIDAR) is a remote sensing method that uses light in the form of pulsed lasers to measure the range (variable distance) to one or more objects in a field of view. Specifically, a micro-electromechanical system (MEMS) mirror is used to scan light across the field of view. A photodetector array receives reflections from objects illuminated by the light, and the time it takes for the reflections to reach the individual sensors in the photodetector array is determined. This is also known as measuring time of flight (ToF). The LIDAR system forms a depth measurement and performs distance measurements by calculating the distance mapped to the object based on the time of flight. Therefore, the time of flight calculation can create a distance and depth map, which can be used to generate an image.

[0003] For continuous wave modulation, such as that used for frequency modulated continuous wave (FMCW) beams, the wave detected after reflection has a shifted frequency and / or phase, and this shift is proportional to the distance to the reflecting object or surface. Thus, the distance can be determined based on the measured shift. This is in contrast to pulse modulation, in which the system measures the distance to a 3D object by measuring the absolute time it takes for a light pulse to travel from the source into the 3D scene and back after reflection.

[0004] However, due to the change in beam frequency (wavelength) combined with the continuous scanning motion of the MEMS mirror, problems arise when achieving continuous scanning of the FMCW beam. The continuously firing laser (FMCW) and the mirror / scanner moving in rapid succession will result in a reduction in resolution that depends on the movement speed of the MEMS mirror. In addition, some FMCW LIDAR systems require both up-converted and down-converted FMCW beams. Both the up-converted and down-converted chirps must hit the same target to calculate the target's speed and distance. One chirp can take 5us to transmit its full frequency range (also known as a frequency sweep or wavelength sweep). As a result, when considering the need for up-converted and down-converted chirps, it is necessary to hit the target for at least 10us. Therefore, the faster the scanning frequency, the lower the resolution of the FMCW system.

[0005] Therefore, there is a need for an improved FMC WLIDAR system that can compensate for the movement speed (eg, scanning frequency) of the MEMS mirror. Summary of the Invention

[0006] One or more embodiments provide a scanning system comprising: a transmitter configured to transmit a frequency modulated continuous wave (FMCW) beam into a field of view, wherein the FMCW beam comprises a plurality of frequency ramps having a variable frequency modulation conversion rate; a scanning structure configured to rotate about a scanning axis so that a deflection angle of the scanning structure changes continuously over time, wherein the scanning structure is configured to scan the field of view using the FMCW beam; a measurement circuit configured to measure the deflection angle of the scanning structure as the deflection angle changes over time and to generate position information based on the measured deflection angle; and a controller configured to change the frequency modulation conversion rate of the plurality of frequency ramps based on the position information.

[0007] One or more embodiments provide a method for compensating a scanning system, comprising: emitting a frequency modulated continuous wave (FMCW) beam into a field of view, wherein the FMCW beam includes multiple frequency ramps with variable frequency modulation conversion rates; driving a scanning structure to rotate around a scanning axis so that a deflection angle of the scanning structure continuously changes over time, wherein the scanning structure is configured to scan the field of view with the FMCW beam; measuring a deflection angle of an oscillator structure as the deflection angle changes over time; generating position information based on the measured deflection angle; and changing the frequency modulation conversion rates of the multiple frequency ramps based on the position information.

[0008] One or more embodiments provide a scanning system comprising: a transmitter configured to transmit a frequency modulated continuous wave (FMCW) beam into a field of view, wherein the FMCW beam comprises a plurality of frequency ramps, the plurality of frequency ramps comprising up- and down-frequency conversions matched into up- and down-frequency conversion pairs, each up- and down-frequency conversion pair comprising an up- and down-frequency conversion; a scanning structure configured to oscillate about a scanning axis so that a deflection angle of the scanning structure varies continuously over time, wherein the scanning structure is configured to oscillate within an angular range between a first maximum deflection angle and a second maximum deflection angle; and a controller configured to segment the angular range into a plurality of sub-angular ranges and assign each up- and down-frequency conversion pair of the FMCW beam to a different sub-angular range of the plurality of sub-angular ranges, wherein each up- and down-frequency conversion pair comprises an up-frequency conversion emitted in the assigned sub-angular range during a first scanning movement of the scanning structure, and a down-frequency conversion emitted in the assigned sub-angular range during a second scanning movement of the scanning structure.

[0009] One or more embodiments provide a method for compensating a scanning system, comprising: emitting a frequency modulated continuous wave (FMCW) beam into a field of view through a transmitter, wherein the FMCW beam comprises a plurality of frequency ramps, the plurality of frequency ramps comprising up-conversions and down-conversions matched into up-conversion pairs, each up-conversion pair comprising an up-conversion and a down-conversion; driving a scanning structure to oscillate around a scanning axis so that a deflection angle of the scanning structure continuously changes over time, wherein the scanning structure is configured to oscillate within an angular range between a first maximum deflection angle and a second maximum deflection angle; segmenting the angular range into a plurality of sub-angular ranges; segmenting each up-conversion pair of the FMCW beam into a different sub-angular range of the plurality of sub-angular ranges, wherein each up-conversion pair comprises an up-conversion emitted within an assigned sub-angular range during a first scanning movement of the scanning structure and a down-conversion emitted within an assigned sub-angular range during a second scanning movement of the scanning structure; and controlling the transmitter to emit each up-conversion pair within the sub-angular range to which each up-conversion pair is assigned. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Embodiments are described herein with reference to the accompanying drawings.

[0011] Figure 1 is a schematic block diagram of a LIDAR scanning system according to one or more embodiments;

[0012] Figure 2A shows the frequency dependence of an FMCW beam on a deflection angle θ of a micro-electromechanical system (MEMS) mirror according to one or more embodiments;

[0013] Figure 2B shows further dependence of the frequency of an FMCW beam on a deflection angle θ of a microelectromechanical system (MEMS) mirror according to one or more embodiments;

[0014] Figure 3 shows a diagram of an FMCW beam launched into a segmented FOV as up-converted (forward ramp) and down-converted (backward ramp) according to conventional technology;

[0015] Figure 4 shows a diagram of an FMCW beam transmitted into a segmented FOV as an up-conversion (forward ramp) and a down-conversion (backward ramp) according to one or more embodiments; and

[0016] Figures 5 to 7 Additional illustrations are shown of an FMCW beam transmitted into a segmented FOV as up-converted (forward ramp) and down-converted (backward ramp) in accordance with one or more embodiments. DETAILED DESCRIPTION

[0017] Hereinafter, various embodiments will be described in detail with reference to the accompanying drawings. It should be noted that these embodiments are for illustrative purposes only and should not be interpreted as limiting. For example, although an embodiment may be described as including a plurality of features or elements, this should not be interpreted as indicating that all of these features or elements are required to implement the embodiment. On the contrary, in other embodiments, some of these features or elements may be omitted, or these features or elements may be replaced by alternative features or elements. In addition, other features or elements other than those explicitly shown and described may be provided, such as conventional components of a sensor device.

[0018] Unless otherwise specifically stated, features from different embodiments may be combined to form additional embodiments. Variations or modifications described with respect to one embodiment may also be applicable to other embodiments. In some instances, well-known structures and devices are shown in block diagram form rather than in detail to avoid confusing the embodiments.

[0019] In addition, identical or similar elements or elements having identical or similar functions are denoted by identical or similar reference numerals in the following description. Since identical or functionally equivalent elements are given the same reference numerals in the drawings, repeated descriptions of elements having the same reference numerals may be omitted. Therefore, the descriptions provided for elements having the same or similar reference numerals are interchangeable.

[0020] Unless otherwise specified, the connections or couplings between elements shown in the drawings or described herein may be based on wired or wireless connections. Furthermore, such connections or couplings may be direct connections or couplings without additional intervening elements, or indirect connections or couplings with one or more additional intervening elements, as long as the general purpose of the connection or coupling (e.g., for transmitting a certain signal or for transmitting a certain information) is substantially maintained.

[0021] In the present disclosure, expressions including ordinal numbers such as "first," "second," etc. may modify various elements. However, these elements are not limited by the above expressions. For example, the above expressions do not limit the order and / or importance of the elements. The above expressions are only used to distinguish elements from other elements. For example, a first box and a second box indicate different boxes, although both are boxes. For other examples, without departing from the scope of the present disclosure, a first element may be referred to as a second element, and similarly, a second element may also be referred to as a first element.

[0022] Embodiments relate to optical sensors and optical sensor systems, and to obtaining information about optical sensors and optical sensor systems. A sensor may refer to a component that converts a physical quantity to be measured into an electrical signal (e.g., a current signal or a voltage signal). The physical quantity may include, for example, but is not limited to, electromagnetic radiation, such as visible light, infrared (IR) radiation, or other types of illumination signals, current, or voltage. For example, an image sensor may be a silicon chip within a camera that converts photons of light from a lens into a voltage. The larger the active area of ​​the sensor, the more light that can be collected to create an image.

[0023] As used herein, a sensor device may refer to a device that includes a sensor and other components (e.g., a bias circuit, an analog-to-digital converter, or a filter). The sensor device may be integrated on a single chip, but in other embodiments, multiple chips or components external to the chip may also be used to implement the sensor device.

[0024] In a frequency modulated continuous wave (FMCW) light detection and ranging (LIDAR) system, a light source continuously emits a FWCM light beam into the field of view, and the light is reflected from one or more objects by backscattering. Specifically, FWCM LIDAR is an indirect time-of-flight (ToF) system during which the frequency or wavelength of the emitted light beam is continuously swept between a minimum and a maximum value defining a predefined frequency / wavelength range or band. For example, the frequency or wavelength of the emitted light beam can be modulated according to a triangular wave modulation pattern consisting of a series of frequency or wavelength ramps.

[0025] Scanning, such as an oscillating horizontal scan (e.g., from left to right and right to left of the field of view), an oscillating vertical scan (e.g., from bottom to top and from top to bottom of the field of view), or a Lissajous scan (e.g., according to a Lissajous scan pattern using two scan axes) can illuminate the scene in a continuous scanning manner. By emitting a continuous frequency modulation while continuously injecting laser light in different scan directions, an area called the field of view can be scanned, and objects within the area can be detected and imaged. The field of view thus represents a scan plane with a center of projection. Raster scanning or omnidirectional scanning can also be used.

[0026] Figure 1is a schematic block diagram of a LIDAR scanning system 100 according to one or more embodiments. The LIDAR scanning system 100 includes a transmitter unit 21, which is responsible for the transmitter path of the system 100, and a receiver unit 22, which is responsible for the receiver path of the system 100. The system also includes a system controller 23, which is configured to control the components of the transmitter unit 21 and the receiver unit 22, and is configured to receive raw data from the receiver unit 22 and process it (e.g., via digital signal processing) to generate object data (e.g., point cloud data). Thus, the system controller 23 includes at least one processor and / or processor circuitry (e.g., a comparator, a TDC, an ADC, and a digital signal processor (DSP)) for processing the data, as well as control circuitry, such as a microcontroller, configured to generate control signals. The LIDAR scanning system 100 may also include a sensor 26, such as a temperature sensor, that provides sensor information to the system controller 23.

[0027] The emitter unit 21 includes an illumination unit 10, a MEMS mirror 12, and a MEMS actuator 25 configured to drive the MEMS mirror 12. Specifically, the MEMS actuator 25 actuates and senses the rotational position of the mirror, and provides position information of the mirror (e.g., the tilt angle or degree of rotation about the rotation axis) to the system controller 23. The system controller 23 can use the position information to process the reflected / received light to generate image data.

[0028] A drive voltage (i.e., an actuation or drive signal) is applied to the actuator structure of the MEMS mirror 12 by the MEMS driver 25 to drive the oscillation of the MEMS mirror 12. The drive voltage may be referred to as a high voltage (HV). The actuator structure may include interdigitated electrodes made of a cross-mirror comb and a frame comb to which the drive voltage (i.e., an actuation or drive signal) is applied by the MEMS driver 25. The drive voltage applied to the actuator structure generates a driving force between, for example, the cross-mirror comb and the frame comb, thereby generating a torque on the mirror body about the axis of rotation. The drive voltage may be switched or toggled between on and off (HV on / off) to generate an oscillating drive force. The oscillating drive force causes the mirror to oscillate back and forth between two extreme values ​​on its axis of rotation. The drive voltage may be a constant drive voltage, meaning that the drive voltage is the same voltage when actuated (i.e., turned on). However, it will be understood that the drive voltage is switched between on and off to generate the mirror oscillation. Depending on the configuration, the actuation may be regulated or adjusted by adjusting the drive voltage off time, the voltage level or the duty cycle of the drive voltage.

[0029] In other embodiments, an electromagnetic actuator or a piezoelectric actuator can be used to drive the MEMS mirror 12. For electromagnetic actuators, a drive current (i.e., an actuation or drive signal) can be used to generate an oscillating drive force. Thus, it will be understood that drive voltage (drive / driving voltage) and drive current (drive / driving current) can be used interchangeably herein to refer to an actuation signal or a drive signal, and both can generally be referred to as a driving force.

[0030] As the mirror oscillates, the capacitance between the interdigitated electrodes changes depending on the mirror's rotational position. MEMS actuator 25 is configured to measure the capacitance between the interdigitated electrodes and, from this, determine the rotational or angular position of MEMS mirror 12. By monitoring the capacitance, MEMS actuator 25 can detect zero-crossing events and their timing, and can determine the tilt angle of MEMS mirror 12 at any given moment. MEMS actuator 25 can also use the measured capacitance to determine the mirror frequency and record the information in a memory at MEMS actuator 25 or at system controller 23.

[0031] Sensing the position of the MEMS mirror 12 is performed based on a detector configured to measure capacitance. For example, as the MEMS mirror moves, the geometry of the finger structures changes, resulting in a change in the geometry of the capacitance. As the geometry of the capacitance changes, the capacitance itself changes. Therefore, a specific capacitance directly corresponds to a specific position (i.e., tilt angle) of the MEMS mirror. By sensing the capacitance of the finger structures, the MEMS actuator 25 can monitor and track the oscillations of the mirror and determine the specific position of the MEMS mirror, including zero crossings.

[0032] One way to measure capacitance is to measure the current flowing through the finger-like structure, convert the measured current into a voltage, and then further relate the voltage to the capacitance and / or rotation angle. However, any method of measuring capacitance can be used. The direction of rotation (e.g., positive or negative, from left to right or from right to left, clockwise or counterclockwise, etc.) is also detected by measuring the change in capacitance over time, where a positive or negative change indicates the relative direction of rotation. The MEMS actuator 25 can also record the current and voltage measured during the capacitance measurement. Therefore, increasing the accuracy of the position sensing of the reflector can improve the overall accuracy of the LIDAR system.

[0033] Since the mirror is driven at a scanning frequency (e.g., 2 kHz), when the mirror rotates in a first rotational direction (e.g., from left to right or clockwise), it will pass through the zero position (i.e., 0°) at some point in time. Similarly, when the mirror rotates in a second rotational direction (e.g., from right to left or counterclockwise), the mirror will pass through the zero position at some point in time. These instances of passing through the zero position can be referred to as zero-crossing events occurring at zero-crossing times.

[0034] The MEMS mirror 12 is a mechanically movable mirror (i.e., a MEMS micro-mirror) integrated on a semiconductor chip (not shown). The MEMS mirror 12 according to this embodiment is configured to rotate about a single scanning axis (i.e., a 1D MEMS mirror) or about two scanning axes that are generally orthogonal to each other (i.e., a 2D MEMS mirror). As a 2D MEMS mirror, the MEMS mirror 12 may be a Lissajous scanner configured to steer a laser beam in two dimensions (e.g., horizontally and vertically).

[0035] It will be further appreciated that the LIDAR scanning system can include multiple scanning mirrors 12 in a Lissajous scanning system (i.e., a 2x1D system), wherein a first 1D MEMS mirror has a single scan axis for steering the beam in a horizontal scan direction, and a second 1D MEMS mirror has a single scan axis for steering the beam in a vertical scan direction. Thus, the two MEMS mirrors in the Lissajous scanning system are, for example, mounted at the same location in a vehicle and configured to scan the same field of view.

[0036] Thus, the transmission technique involves transmitting a light beam into the field of view from a transmitting mirror that continuously oscillates about one or more scan axes, such that the light beam is projected into the field of view and moves across the field of view as the transmitting mirror changes its direction of transmission. One or more embodiments may use two resonant scan axes. In the case of a line scanner, the scan pattern may not be a Lissajous pattern, but rather a different type of pattern may be generated.

[0037] Upon impacting one or more objects, the emitted light is reflected back to the LIDAR scanning system 100 as reflected light through backscattering. As will be described in more detail below, the MEMS mirror 12 receives the reflected light and directs it via a beam splitter device onto a photodetector 15. The photodetector 15 receives the reflected light and is configured to generate an electrical measurement signal. The electrical measurement signal can be used by the system controller 23 to generate a 3D map of the environment and / or other object data based on the reflected light (e.g., via FFT calculation and processing).

[0038] The receiver unit 22 includes a MEMS mirror 12, a photodetector 15, and a receiver circuit 24, which includes an analog readout circuit configured to read out the measurement signal received from the photodetector 15. The transmitter and receiver can share the same MEMS mirror. Alternatively, the transmitter and receiver can use different MEMS mirrors. Alternatively, only the transmitter has a MEMS mirror, while the receiver is non-scanning.

[0039] The photodetector 15 can be any of a variety of photodetector types, including avalanche photodiodes (APDs), photocells, and / or other photodiode devices. An imaging sensor such as a charge-coupled device (CCD) can be a photodetector. In the examples provided herein, the photodetector 15 can be a single-pixel photodetector (e.g., a single APD), a 1D APD array comprising an array of APD pixels arranged in one dimension, or a two-dimensional (2D) APD array comprising an array of APD pixels arranged in two dimensions. As described above, the photodetector 15 can be a 1D array comprising a single column of photodiodes.

[0040] The photodetector 15 receives the reflected light pulse as a receive line RL and generates an electrical signal in response thereto. The distance to an object can be calculated via the time difference between the transmitted and received laser pulses or the laser frequency modulation. A depth map can plot distance information.

[0041] Receiver circuit 24 may receive analog electrical signals from the plurality of photodetectors of photodetector 15 and send the electrical signals as raw analog data to system controller 23 for processing. Receiver circuit 24 may also receive a trigger control signal from system controller 23 that triggers activation of one or more photodetectors. Receiver circuit 24 may also receive a gain setting control signal for controlling the gain of the one or more photodetectors.

[0042] The system controller 23 uses the received data to calculate time-of-flight information for each field position within the field of view to generate object data (eg, point cloud data), and generates a 3D point cloud.

[0043] Figure 2A The dependence of the frequency of an FMCW beam on the deflection angle θ of a microelectromechanical system (MEMS) mirror according to one or more embodiments is shown. The deflection angle θ can be a tilt angle, a rotation angle, a mirror position, a rotational position, or any other reference to the position of the MEMS mirror relative to its scan axis. Therefore, these position terms are used interchangeably herein.

[0044] Figure 2A Includes top and bottom views. Figure 2AThe top diagram shows the frequency ramp of an FMCW beam used for FMCW ranging in one or more embodiments. Also shown are the corresponding wavelength components that change as the frequency of the ramp changes. The frequency ramp includes a forward ramp (upward ramp) portion and a backward ramp (downward ramp) portion. Thus, different frequencies / wavelengths are emitted at different times.

[0045] Figure 2A The bottom diagram shows an example of the time-varying deflection angle θ of the MEMS mirror during a scanning operation, where the MEMS mirror moves from right to left (i.e., counterclockwise) during the forward ramp of the frequency ramp and from left to right (i.e., clockwise) during the backward ramp of the frequency ramp. Both the frequency of the FMCW beam and the deflection angle θ of the MEMS mirror vary continuously over time. Therefore, FMCW beams of different frequencies / wavelengths are incident on the MEMS mirror at different emission times and / or at different deflection angles θ.

[0046] The length L of the frequency ramp is equal to the amount of time (i.e., duration) it takes for the frequency to change from the minimum frequency to the maximum frequency, or the amount of time it takes for the frequency to change from the maximum frequency to the minimum frequency. The length L of the forward ramp (upward ramp) portion and the backward ramp portion can be equal. Thus, the duration of one triangular wave interval is 2L. It should also be noted that the length L of the frequency ramp, and therefore the length of the triangular wave interval, can be adjusted by the controller of the FMCW LIDAR system. For example, the system controller 23 can adjust the length L of the ramp to cover a certain segment of the entire FOV. In this way, the length L of the ramp can be synchronized with the MEMS mirror motion, specifically with an angular sub-range of the full range of motion of the MEMS mirror 12 about its scan axis. For example, each frequency ramp of the FMCW beam can be mapped to a specific angular sub-range, and the continuous change of the ramp's frequency is synchronized with the continuous change of the deflection angle within the angular sub-range. Sensing circuitry can further be provided to sense the rotational or deflection position (e.g., the rotation angle θ of the MEMS mirror) to provide further feedback information to the controller to assist in synchronization.

[0047] Figure 2B shows further dependence of the wavelength (frequency) of the FMCW beam on the deflection angle θ of a micro-electromechanical system (MEMS) mirror according to one or more embodiments, Figure 2B Includes top and bottom views.

[0048] Figure 2BThe top diagram shows a series or multiple frequency ramps of an FMCW beam used for FMCW ranging in one or more embodiments. The frequency ramps include a series of N forward ramps (upward ramps) and a series of N backward ramps (downward ramps), where N is an integer greater than zero, and the total number of frequency ramps over a full cycle of the MEMS mirror motion is 2N. Thus, different wavelengths are emitted at different times.

[0049] Figure 2B The bottom diagram shows the time-varying deflection angle θ of the MEMS mirror during a scanning operation, where the MEMS mirror moves from right to left (i.e., counterclockwise) during the forward ramp of the frequency ramp and from left to right (i.e., clockwise) during the backward ramp of the frequency ramp. Both the wavelength of the FMCW beam and the deflection angle θ of the MEMS mirror vary with time. Therefore, FMCW beams of different wavelengths are incident on the MEMS mirror at different emission times and / or at different deflection angles θ.

[0050] Specifically, multiple ramps can be adapted to fit into a half-cycle HP of the MEMS mirror's motion. In this case, a series of N consecutive forward ramps is generated during the first half-cycle of a cycle of the MEMS mirror's motion, and a series of N consecutive backward ramps is generated during the second half-cycle of the same cycle of the MEMS mirror's motion. The two series of N consecutive ramps can also be reversed, so that a series of N consecutive backward ramps is generated during the first half-cycle and a series of N consecutive forward ramps is generated during the second half-cycle. However, within a cycle, the series of N consecutive forward ramps and the series of N consecutive backward ramps are continuous, so that the last ramp in the first series and the first ramp in the second series meet at the maximum deflection angle of the MEMS mirror to form a triangle.

[0051] The MEMS mirror changes its direction of motion at the maximum deflection angle. In other words, the beam transmitter changes the direction of the ramp from forward to backward or vice versa at the maximum deflection angle of the MEMS mirror. The pattern of forward and backward frequency ramps then repeats for the next MEMS mirror motion cycle.

[0052] Alternatively, as will be explained below, the direction of the frequency ramp can be changed while the mirror moves in the same direction. For example, the backward ramp can be continuous with the forward ramp when the MEMS mirror moves from left to right, or the forward ramp can be continuous with the backward ramp when the MEMS mirror moves from right to left. Thus, the direction change of the ramp need not occur at the maximum or minimum deflection angle.

[0053] In addition, each frequency ramp has a length L, which is equal to the amount of time (i.e., duration) it takes for the wavelength to change from a minimum wavelength to a maximum wavelength, or equal to the amount of time it takes for the wavelength to change from a maximum wavelength to a minimum wavelength. Here, the mirror period can be defined as 2N*L.

[0054] The length L of the frequency ramp can be adjusted by the controller of the FMCW LIDAR system. For example, the system controller 23 can adjust the slope and thus the length L of the ramp according to the scanning frequency of the MEMS mirror. In this way, the length L of the ramp can be synchronized with the motion of the MEMS mirror. Specifically, the system controller 23 can allocate (i.e., map) each ramp and synchronize each ramp with a sub-range or segment of the full angular range of the MEMS mirror, such as Figure 2B . This angular sub-range or segment further corresponds to a sub-range or segment of the full field of view (FOV). Thus, the FOV area within the full FOV is mapped to an angular sub-range, and the angular sub-range is mapped to a frequency ramp (i.e., the FOV area is mapped to the frequency ramp). For example, the first frequency ramp is synchronized with the first angular sub-range of the MEMS mirror movement, the second frequency ramp is synchronized with the second angular sub-range of the MEMS mirror movement, and the frequency ramp N is synchronized with the Nth (last) angular sub-range of the MEMS mirror movement in the clockwise direction. In each angular sub-range, the transmitter 21 changes (i.e., ramps) the frequency (or wavelength) synchronously with the change in the deflection angle of the MEMS mirror. Therefore, the continuous change in frequency is synchronized with the continuous change in the deflection angle. Therefore, the frequency changes step by step with the deflection angle of the MEMS mirror.

[0055] As described above, the resolution of the FMCW LIDAR system 100 is affected by the movement speed of the MEMS mirror 12. The movement speed depends not only on the scanning frequency, but also on the angular position to which the MEMS mirror is moved. For example, the movement speed of the MEMS mirror 12 is highest at zero angle (i.e., zero crossing) and lowest at the two maximum deflection angles where the mirror changes direction. Therefore, it can be said that the movement speed depends on the angular position of the MEMS mirror 12 about the scanning axis.

[0056] The MEMS actuator 25 is configured to measure the angular position and transmit the angular position information to the system controller 23. Furthermore, the system controller 23 is configured to adjust the slew rate of the frequency ramp (i.e., the change in the instantaneous slope of the frequency ramp) based on the angular position of the MEMS mirror 12. In this case, the maximum slew rate of the frequency ramp is mapped to the zero tilt angle, while the minimum slew rate of the frequency ramp is mapped to the maximum tilt angle. This applies to both the up-slew (forward ramp) and the down-slew (backward ramp). The system controller 23 is configured to generate slew rate configuration information based on the measured mirror position and transmit the slew rate configuration information to the illumination unit 10. The system controller 23 may further transmit the slew rate configuration information to the receiver signal processing circuitry for use in compensating the processing of the sensor data. The illumination unit 10 is configured to adjust the slew rate based on the received slew rate configuration information.

[0057] Specifically, the slew rate is adjusted in direct relation to the movement speed based on the angular position of the MEMS mirror 12 on its scan axis. Thus, the slew rate is increased to a maximum slew rate (maximum slope) at zero crossings and is decreased to a minimum slew rate (minimum slope) at the two maximum deflection angles.

[0058] Thus, as the angular position of the MEMS mirror changes from zero angle, where the mirror is flat, to one of the maximum deflection angles, the slew rate can continuously decrease. Similarly, the slew rate can continuously increase as the angular position of the MEMS mirror changes from one of the maximum deflection angles to zero angle. The change in slew rate can result in a frequency ramp that is triangular or parabolic in shape.

[0059] It will also be understood that the slew rate can be changed dynamically during a scanning operation. The change in slew rate during the currently active slew can be based on position measurements taken during a previous slew (i.e., the most recent slew before the slew was transmitted) or based on real-time position measurements taken during the currently active slew. The goal is to have each slew cover the same sub-range of the FOV (i.e., the same angular range) as the MEMS mirror 12 moves. Thus, the resolution of the LIDAR system 100 remains the same across the full FOV.

[0060] In the former case of using a position measurement acquired during a previous FM transition, the previous FM transition and the current FM transition are transmitted consecutively. The system controller 23 can determine the rate of change of the MEMS mirror position during the previous FM transition to use in calculating the FM transition rate variance to be applied to the current FM transition.

[0061] Another benefit is that the varying slew rate provides increased resistance to interference from other FMCW LIDAR transmissions from other systems or other transmitters. Thus, the LIDAR system 100 is more robust to interference from other LIDAR beams.

[0062] As further noted above, some FMCW LIDAR systems require both an up-converted and a down-converted FMCW beam. Both the up-converted and down-converted FMCW beams must hit the same target in order to calculate the target's velocity and range. For example, while one FM can be used to determine the range of a target, velocity determination is impossible without both the up-converted and down-converted beams hitting the target. However, an angular range that includes a first angular sub-range (i.e., a first sub-FOV) for one FM and a second angular sub-range (i.e., a second sub-FOV) for another FM may result in one of the FMs missing the target.

[0063] Figure 3 An illustration of an FMCW beam emitted into a segmented FOV as an up-conversion (forward ramp) and a down-conversion (backward ramp) according to conventional technology is shown. Directional arrows are used to indicate whether the ramp is an up-conversion with an increasing frequency f or a down-conversion with a decreasing frequency f. In this example, the full FOV is segmented into three sub-FOVs, including FOV1, FOV2, and FOV3. The scene projected in the full FOV includes large objects and small objects that are targets of the LIDAR system. As time increases, the MEMS mirror 12 rotates from left to right about its scan axis to scan the full FOV. Therefore, each sub-FOV corresponds to a different angular sub-range of the MEMS mirror 12 about its scan axis. In addition, each forward ramp and backward ramp corresponds to a different angular sub-range and does not overlap in the angular range over which they are emitted.

[0064] Here, to scan a sub-FOV (e.g., FOV1, FOV2, or FOV3), two consecutive frequency conversions (up-conversion and down-conversion) are used. If a large object is located in FOV1, both the up-conversion and down-conversion can hit the object. However, if a small object is located in FOV2, the up-conversion misses the object, and only the down-conversion hits it. As a result, the system controller 23 cannot calculate the velocity of the small object. Since the velocity is used to calculate the trajectory of the object, the trajectory cannot be calculated either.

[0065] According to another embodiment described herein, the system controller 23 is configured to modify the up-converted and down-converted FMCW laser shot patterns so that the up-converted and down-converted shots from different scanner movements form a pair and match their FOVs. In other words, the up-converted shot is paired with the down-converted shot from different mirror scans so that they both cover (i.e., overlap) the same sub-angular range of the full FOV. Furthermore, unlike conventional approaches, the up-converted and down-converted shots in the up-converted and down-converted pairs are not fired consecutively in the same scan, but rather are fired during different scans.

[0066] Scanning is defined as a scanning movement of the MEMS mirror 12 in one direction about its scan axis. Thus, a first scan can be defined as a movement of the MEMS mirror from left to right (i.e., clockwise) occurring during a first scan period, and a second scan can be defined as a movement of the MEMS mirror from right to left (i.e., counterclockwise) occurring during a second scan period. The first scan period and the second scan period can be continuous or non-continuous with each other. However, to implement non-continuous scanning, more memory may be required, and processing time may be increased due to the delay caused by the time interval between the added pair of up-conversion and down-conversion.

[0067] Thus, an up-down frequency conversion pair, comprising an up-down frequency conversion and a down-down frequency conversion, is formed by the up-down frequency conversion emitted when the MEMS mirror 12 rotates about its scan axis in a first scan direction, and the down-down frequency conversion emitted when the MEMS mirror 12 rotates about its scan axis in a second scan direction opposite to the first scan direction. Furthermore, as described above, both the up-down frequency conversion and the down-down frequency conversion of the pair cover the same sub-angular range. Thus, the two frequency conversions are excited at the same position range of the MEMS mirror 12 about its scan axis and cover the same portion of the full FOV (i.e., they are excited in the same emission direction corresponding to the sub-angular range of the MEMS mirror 12).

[0068] Figure 4 A diagram of an FMCW beam transmitted into a segmented FOV as an up-conversion (forward ramp) and a down-conversion (backward ramp) according to one or more embodiments is shown. Two consecutive mirror scans are shown, with a left-to-right scan increasing in time from left to right and a right-to-left scan increasing in time from right to left. FOV segments FOV1 to FOV6 each correspond to the length of the frequency ramp. In addition, each FOV segment includes an up-conversion and a down-conversion forming an up-and-down-conversion pair.

[0069] In this case, the first scan period of the first scan and the second scan period of the second scan are continuous with each other (ie, the second scan immediately follows the first scan). However, in another embodiment, the scans may be non-continuous with each other.

[0070] In addition, each FOV segment corresponds to a specific angular range of the MEMS mirror 12 about its scan axis. Therefore, the up-down frequency conversion pairs are assigned by the system controller 23 to the same angular range of the MEMS mirror 12 about its scan axis. For example, if the MEMS mirror 12 rotates between + / -15°, the up-down frequency conversion pairs can be assigned to the angular range of -15° to -10°, -10° to -5°, -5° to 0°, 0° to 5°, 5° to 10°, or 10° to 15°. The width of the angular range is fully configurable and is not limited to 5°, and in practice, the width of the angular range can be 1° or less. Thus, the smaller the resolution, the more FOV segments are used and the more ramps are used. It will be understood that each FOV segment can be referred to as a sub-FOV or a region of interest (ROI) in the FOV, and that these terms can be used interchangeably.

[0071] In this example, alternating up-conversions and down-conversions are generated during each scanning movement. Thus, the up-conversions are continuous with the down-conversions, and the down-conversions are continuous with the up-conversions. Each successive frequency conversion of the same scan covers a different FOV segment and is assigned to a different angular range of the MEMS mirror 12 around the scanning axis.

[0072] Thus, for a 1D scanner, as the MEMS mirror 12 moves from left to right, an up-conversion of FOV_n is fired, and as the mirror moves from right to left and reaches the same FOV_n (i.e., the same angular range about the scan axis), a down-conversion of FOV_n is fired, where n is an integer.

[0073] For a 2D Lissajous scanner, an up-conversion is fired at a certain FOV_n, and a corresponding down-conversion is fired when the Lissajous pattern crosses the same FOV_n (from the opposite direction, diagonally, etc.) or crosses the same FOV_n in the next Lissajous frame. Since a 2D scanner uses two orthogonal scanning axes, the FOV segment is defined by both the angular range of the first axis and the angular range of the second axis, where the two angular ranges define the FOV of interest within the full FOV. When the angular ranges on the two axes are met at non-consecutive moments, a pair of up-conversions and down-conversions is fired. That is, just like in the 1D scanning method, a pair of up-conversions and down-conversions is generated during different scans of the 2D scan.

[0074] The system controller 23 is configured to process the reflected light beams from the up-and-down frequency conversion pairs together to generate both velocity information and direction information of the target struck. Thus, the system controller 23 tracks the angle information received from the MEMS actuator 25 and matches the reflected signals from the up-and-down frequency conversion pairs corresponding to the same angle range for processing based on the tracked angle information.

[0075] This allows for an FMCW laser shot pattern that transmits both up-converted and down-converted signals to certain FOVs. The result is a doubling of the LIDAR’s resolution at the expense of frame rate and memory.

[0076] Figures 5 to 7 Additional illustrations of FMCW beams launched into segmented FOVs as up-converted (forward ramp) and down-converted (backward ramp) frequency according to one or more embodiments are shown. Specifically, Figures 5 to 7 Follow the reference Figure 4 Similar principles are described using up and down frequency conversion pairs assigned to the same angular range, but implemented using different laser shot patterns based on different system requirements.

[0077] Scans shown as continuous can also be reconfigured to be non-continuous. However, to implement non-continuous scanning, more memory may be required, and processing time may increase due to the delay caused by increasing the time interval between a pair of up-conversion and down-conversion.

[0078] For example, in some cases, the FMCW laser cannot be started or stopped arbitrarily and / or only supports certain dead times between ramps. In these embodiments, the system controller 23 sets and adapts the FMCW laser shot pattern in such a way that the up-conversion and down-conversion pairs of the FOV of interest overlap with each other to accommodate the dead times.

[0079] For example, in Figure 5 In FIG. 1 , dead time is implemented in FOV segments FOV3 and FOV6. Dead time is provided after successive up-conversions and down-conversions of a scan, and each dead time of a scan movement is matched and aligned with the corresponding dead time of the relative scan movement. Thus, system controller 23 assigns dead time to a specific angular range of MEMS mirror 12 about its scan axis.

[0080] exist Figure 6 In the embodiment of the present invention, continuous up-conversion is generated during a first scan, and continuous down-conversion is generated during a second scan moving in the opposite direction of the first scan. Up-conversion and down-conversion that overlap in the same sub-FOV (i.e., the same angular range) form an up-and-down-conversion pair, and their corresponding reflected signals are processed together.

[0081] exist Figure 7 In

[15] , dead time is inserted between each ramp, and the dead time of two consecutive scans in opposite directions matches the same sub-FOV. After the first two consecutive scans, the dead time is shifted to a different sub-FOV. Specifically, for the second two consecutive scans, the ramp is generated in the sub-FOV to which the dead time was assigned for the first two consecutive scans, and the dead time is now used in the sub-FOV to which the ramp was assigned for the first two scans. Thus, the ramp and dead time swap positions, making it possible to scan the full FOV while still achieving the dead time and up / down frequency conversion pairs assigned to the same angular range. Therefore, four scans are required to scan the full FOV.

[0082] Given the above, the FMCW laser shot pattern is adapted by assigning up-converted and down-converted shots to the same region of interest (ROI) of the FOV and processing the data of the up-converted and down-converted pairs together to calculate the speed and distance of the object. This doubles the resolution of the LIDAR system at the expense of frame rate and memory.

[0083] Although the embodiments described herein relate to MEMS devices with mirrors, it should be understood that other embodiments may include optical devices other than MEMS mirror devices, including other oscillating structures, including oscillating structures unrelated to LIDAR. In addition, although some aspects have been described in the context of an apparatus, it is clear that these aspects also represent a description of a corresponding method, where a block or device corresponds to a method step or a feature of a method step. Similarly, aspects described in the context of a method step also represent a description of a corresponding block or item or feature of a corresponding apparatus. Some or all of these method steps may be performed by (or using) a hardware device, such as a microprocessor, a programmable computer, or an electronic circuit. In some embodiments, one or some of these method steps may be performed by such an apparatus.

[0084] It should also be noted that the methods disclosed in the specification or claims may be implemented by a device having means for performing each of the corresponding actions of these methods. In addition, it should be understood that the disclosure of multiple actions or functions disclosed in the specification or claims cannot be interpreted as being in a specific order. Therefore, the disclosure of multiple actions or functions does not limit these to a specific order unless such actions or functions are not interchangeable for technical reasons. In addition, in some embodiments, a single action may include or be divided into multiple sub-actions. Unless explicitly excluded, such sub-actions may be included and included as part of the disclosure of this single action.

[0085] The technology described in this disclosure can be implemented at least in part with hardware, software, firmware or any combination thereof. For example, various aspects of the technology described can be implemented in one or more processors, and the one or more processors include one or more microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASICs), field programmable logic arrays (FPGAs), programmable logic controllers (PLCs) or any other equivalent integrated or discrete logic circuits, and any combination of such components. The term "processor" or "processing circuit" can generally refer to any one of the aforementioned logic circuits, or any other equivalent circuits, alone or in combination with other logic circuits. The control unit including hardware can also perform one or more of the technology of this disclosure. The control unit can use electrical signals and digital algorithms to perform its reception, analysis and control functions, which may further include correction functions. Such hardware, software and firmware can be implemented in the same device or in a separate device to support the various technologies described in this disclosure.

[0086] One or more aspects of the present disclosure may be implemented as a non-transitory computer-readable recording medium having recorded thereon a program embodying a method / algorithm for instructing a processor to execute the method / algorithm. Thus, the non-transitory computer-readable recording medium may have an electronically readable control signal stored thereon that cooperates (or is capable of cooperating) with a programmable computer system so as to execute the corresponding method / algorithm. The non-transitory computer-readable recording medium may be, for example, a CD-ROM, a DVD, a Blu-ray disc, a RAM, a ROM, a PROM, an EPROM, an EEPROM, a flash memory, or an electronic storage device.

[0087] Although various embodiments have been disclosed, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention, which changes and modifications will achieve some of the advantages of the concepts disclosed herein. It will be apparent to those skilled in the art that other components performing the same functions may be appropriately substituted. It will be understood that other embodiments may be utilized and that structural or logical changes may be made without departing from the scope of the invention. It should be mentioned that features explained with reference to a particular figure may be combined with features of other figures, even those in figures not explicitly mentioned. Such modifications to the general inventive concept are intended to be covered by the appended claims and their legal equivalents.

Claims

1. A scanning system comprising: a transmitter configured to transmit a frequency modulated continuous wave (FMCW) beam into a field of view, wherein the FMCW beam comprises a plurality of frequency ramps with variable frequency modulation slew rates; a scanning structure configured to rotate about a scanning axis such that a deflection angle of the scanning structure varies continuously over time, wherein the scanning structure is configured to scan the field of view using the FMCW beam; a measurement circuit configured to measure the deflection angle of the scanning structure as the deflection angle varies over time, and to generate position information based on the measured deflection angle; as well as a controller configured to change a slew rate of the plurality of frequency ramps based on the position information; wherein the controller is configured to receive the position information during transmission of the frequency ramp, and the controller is configured to change the chirp rate of the frequency ramp in real time based on the position information; or Wherein the controller is configured to receive the position information during transmission of a previous frequency ramp, and the controller is configured to change the slew rate of a subsequent frequency ramp based on the position information corresponding to the previous frequency ramp.

2. The scanning system of claim 1, wherein the slew rate is the instantaneous slope of a frequency ramp.

3. The scanning system according to claim 1, wherein: The scanning structure is configured to rotate between a maximum deflection angle and a zero deflection angle, The controller is configured to vary the slew rate between a maximum value synchronized with the zero deflection angle and a minimum value synchronized with the maximum deflection angle.

4. The scanning system according to claim 3, wherein: The controller is configured to continuously change the slew rate between the maximum value and the minimum value based on the position information.

5. The scanning system according to claim 1, wherein: The scanning structure is configured to rotate from a first maximum deflection angle to a second maximum deflection angle with a zero deflection angle between the two maximum deflection angles, The controller is configured to vary the slew rate between a maximum value synchronized with the zero deflection angle and a minimum value synchronized with the first maximum deflection angle and the second maximum deflection angle.

6. A method for compensating a scanning system, the method comprising: emitting a frequency modulated continuous wave (FMCW) beam into a field of view, wherein the FMCW beam comprises a plurality of frequency ramps having a variable frequency modulation rate; driving a scanning structure to rotate about a scanning axis so that a deflection angle of the scanning structure continuously changes over time, wherein the scanning structure is configured to scan the field of view using the FMCW light beam; measuring the deflection angle of the scanning structure as the deflection angle varies with time; generating position information based on the measured deflection angle; as well as varying a frequency modulation rate of the plurality of frequency ramps based on the position information; wherein said position information is received during transmission of a frequency ramp, and said chirp rate of said frequency ramp is varied in real time based on said position information; or wherein the position information is received during transmission of a previous frequency ramp, and the chirp rate of the frequency ramp is changed based on the position information corresponding to the previous frequency ramp.

7. The method according to claim 6, wherein: The scanning structure is configured to rotate between a maximum deflection angle and a zero deflection angle, and Changing the slew rate of the plurality of frequency ramps includes changing the slew rate between a maximum value synchronized with the zero deflection angle and a minimum value synchronized with the maximum deflection angle.

8. The method according to claim 7, wherein: Changing the FM slew rate of the plurality of frequency ramps includes continuously changing the FM slew rate between the maximum value and the minimum value based on the position information.

9. A scanning system comprising: a transmitter configured to transmit a frequency modulated continuous wave (FMCW) beam into a field of view, wherein the FMCW beam comprises a plurality of frequency ramps, the plurality of frequency ramps comprising up-conversions and down-conversions matched into up-conversion and down-conversion pairs, each up-conversion and down-conversion pair comprising an up-conversion and a down-conversion; a scanning structure configured to oscillate about a scanning axis such that a deflection angle of the scanning structure varies continuously over time, wherein the scanning structure is configured to oscillate in an angular range between a first maximum deflection angle and a second maximum deflection angle; as well as a controller configured to segment the angular range into a plurality of sub-angular ranges and assign each up-and-down chirp pair of the FMCW light beam to a different sub-angular range of the plurality of sub-angular ranges, wherein each up-and-down chirp pair includes an up-chirp transmitted in the assigned sub-angular range during a first scanning movement of the scanning structure and a down-chirp transmitted in the assigned sub-angular range during a second scanning movement of the scanning structure; wherein position information indicative of a position of the scanning structure is received during transmission of the frequency ramp, and a chirp rate of the frequency ramp is varied in real time based on the position information; or wherein the position information is received during transmission of a previous frequency ramp, and a chirp rate of the frequency ramp is changed based on the position information corresponding to the previous frequency ramp.

10. The scanning system of claim 9, wherein: The scanning structure is configured to scan the field of view with the FMCW beam using a plurality of scanning movements, during which the scanning structure rotates from the first maximum deflection angle to the second maximum deflection angle, or from the second maximum deflection angle to the first maximum deflection angle.

11. The scanning system of claim 10, wherein: The first scanning movement causes the scanning structure to rotate from the first maximum deflection angle to the second maximum deflection angle in a first scanning period, and The second scanning movement causes the scanning structure to rotate from the second maximum deflection angle to the first maximum deflection angle in a second scanning period. The scanning system according to claim 11 , wherein the second scanning period is continuous with the first scanning period. The scanning system according to claim 11 , wherein the second scanning period is discontinuous with the first scanning period.

14. The scanning system of claim 10, wherein: The first scanning movement causes the scanning structure to rotate from the first maximum deflection angle to the second maximum deflection angle in a first scanning period, and The second scanning movement causes the scanning structure to rotate from the first maximum deflection angle to the second maximum deflection angle in a second scanning period that is discontinuous with the first scanning period.

15. The scanning system of claim 9, wherein the plurality of sub-angular ranges are sized according to a segment size, and the length of each frequency ramp is synchronized with the segment size. 16 . The scanning system of claim 15 , wherein the length of each frequency ramp corresponds to a duration it takes for each of the plurality of frequency ramps to transition between a minimum frequency and a maximum frequency.

17. The scanning system of claim 9, wherein the up-conversion and the down-conversion of an up-conversion pair are transmitted at the same region of interest in a field of view defined by an assigned sub-angular range.

18. The scanning system of claim 16, wherein the lengths of the up-conversion and the down-conversion in the up-conversion and down-conversion pairs are synchronized with the size of the allocated sub-angular range.

19. The scanning system of claim 9, wherein the controller is configured to control the lengths of the up-conversion and the down-conversion so as to synchronize the lengths with their assigned sub-angular ranges.

20. The scanning system of claim 9, wherein: The first scanning movement includes a first dead time, during which the transmitter does not transmit the FMCW light beam, The second scanning movement includes a second dead time during which the transmitter does not transmit the FMCW light beam, and The controller is configured to assign the first dead time and the second dead time to the same sub-angle range.

21. The scanning system of claim 9, further comprising: a measurement circuit configured to measure a deflection angle of the scanning structure as the deflection angle varies with time and to generate position information based on the measured deflection angle, and The controller is configured to synchronize the transmission of the up-conversion and the down-conversion in each up-conversion pair with its assigned sub-angular range based on the position information.

22. A method of compensating a scanning system, the method comprising: transmitting a frequency modulated continuous wave (FMCW) beam into a field of view via a transmitter, wherein the FMCW beam comprises a plurality of frequency ramps, the plurality of frequency ramps comprising up-conversions and down-conversions matched into up-conversion and down-conversion pairs, each up-conversion and down-conversion pair comprising an up-conversion and a down-conversion; driving the scanning structure to oscillate about a scanning axis such that a deflection angle of the scanning structure varies continuously over time, wherein the scanning structure is configured to oscillate within an angular range between a first maximum deflection angle and a second maximum deflection angle; Segmenting the angle range into a plurality of sub-angle ranges; assigning each up- and down-modulation pairs of the FMCW light beam to a different sub-angular range of the plurality of sub-angular ranges, wherein each up- and down-modulation pair includes an up-modulation transmitted in the assigned sub-angular range during a first scanning movement of the scanning structure and a down-modulation transmitted in the assigned sub-angular range during a second scanning movement of the scanning structure; as well as controlling the transmitter to transmit each up- and down-converted pair at its assigned sub-angular range; wherein position information indicative of a position of the scanning structure is received during transmission of the frequency ramp, and a chirp rate of the frequency ramp is varied in real time based on the position information; or wherein the position information is received during transmission of a previous frequency ramp, and a chirp rate of the frequency ramp is changed based on the position information corresponding to the previous frequency ramp.

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