Airborne laser scanner

Abstract

An airborne laser scanner is arranged on an aircraft to survey a target along a flight path and includes a transmitter that emits a plurality of successive laser pulses toward the ground, at least one optical element that deflects the laser pulses along a pulse path toward the target, a motor that moves the optical element to cause a periodically repeating motion of the pulse path, a receiver that receives laser pulses backscattered from the target, and a computer that controls the transmitter, the motor, and the receiver, determines a direction of the pulse path, and triggers the transmitter to emit laser pulses having a first pulse interval variation and triggers the transmitter to emit laser pulses having a second pulse interval variation that overlaps the first pulse interval variation, according to the second pulse interval variation, pulse intervals between pulses emitted in a first cycle of the periodically repeating motion are at least partially modified relative to pulse intervals between pulses emitted in any subsequent cycle of the periodically repeating motion.

Description

Technical Field

[0001] This invention relates to airborne laser scanners. Background Technology

[0002] Using a laser rangefinder carried by an aircraft to scan a target (e.g., the ground surface) for the creation of 3D point clouds and derivatives is typically achieved by moving, particularly rotating or oscillating, optical elements (e.g., mirrors, wedges, or prisms) so that a wider area around the flight path can be surveyed.

[0003] The principle used in this paper essentially involves emitting pulsed electromagnetic radiation onto a target and subsequently receiving the radiation reflected by the target. The distance to the target is determined based on the pulse's travel time (i.e., the time difference between sending the laser pulse (emission pulse) and receiving the echo of that laser pulse returning from the surface). Using the speed of light, this measurement is used to calculate the distance from the surface.

[0004] For example, airborne LiDAR systems (LiDAR: Light Detection and Ranging) use the working principle of Time-of-Flight (ToF) measurements to measure the distance between a scanner sensor in the aircraft and the terrain below. These ranges, combined with knowledge of the aircraft's trajectory and the corresponding pulse transmission direction, can be used to generate 3D point clouds containing terrain such as buildings and vegetation.

[0005] The detected return pulses are sampled very precisely over time. The electrical signal generated by the detector is converted into a digital signal sequence, which is then further processed, typically in real time. The sensor detects both the measurement signal and noise information; if identified as noise, it can be discarded. By using multiple sampling points and / or by synchronously adding the received signal to the emissivity, useful signals can be identified even under adverse conditions, such as enabling measurements at long distances or in noisy or interfered backgrounds.

[0006] One drawback of the described device is that during the pulse transmission period, the sensor is "blind" and cannot detect any received signals. Similarly, for a short period after transmission, the sensor remains blind (or at least it is configured to ignore any incoming signals) due to internal reflections caused by the reflective surfaces of the associated optics. Typically, during surveying, a 3D point cloud containing many blind spots (also known as dropouts) is generated.

[0007] Now, a problem arises when these blind spots appear in regular patterns, as this occurs with existing scanners when an aircraft carrying the scanner flies at a constant altitude over relatively flat surfaces (e.g., parking lots, streets, or sports fields). For example, the regular appearance of these blind spots can create very noticeable stripes or circles in the surveyed clouds, immediately attracting the observer's attention. A reasonable analysis of the problem suggests it is caused by the pulse modulation method.

[0008] Purpose of the invention

[0009] Therefore, the present invention aims to avoid the aforementioned problems and provide an improved airborne laser scanner. The airborne laser scanner according to the present invention allows for more accurate and complete airborne surveys. Summary of the Invention

[0010] This invention relates to an airborne laser scanner configured to be mounted on an aircraft for surveying targets along a flight path. The airborne laser scanner includes: a transmitter configured to emit a plurality of consecutive laser pulses toward the ground; at least one optical element configured to deflect the laser pulses toward the target along the pulse path; a motor configured to move the optical element to cause periodic repetitive motion of the pulse path; a receiver configured to receive laser pulses backscattered from the target; and a computer configured to: control the transmitter, the motor, and the receiver; determine the direction of the pulse path; and trigger the transmitter to emit laser pulses having a first pulse interval variation. The computer is further configured to: trigger the transmitter to emit laser pulses having a second pulse interval variation overlapping the first pulse interval variation, wherein, according to the second pulse interval variation, the pulse interval between those pulses emitted during the first periodic repetitive motion is at least partially modified relative to the pulse interval between those pulses emitted during any subsequent periodic repetitive motion.

[0011] In some implementations, the variation in the second pulse interval has a digital pattern.

[0012] In some implementations, the variation in the second pulse interval has a simulated pattern.

[0013] In some implementations, the second pulse interval variation follows a sine pattern, a linear zigzag pattern, a waveform pattern, a sawtooth pattern, a step pattern, or any combination of patterns.

[0014] In some implementations, the pulse interval between those pulses emitted during the first cycle differs by a constant value relative to the pulse interval between those pulses emitted during any subsequent cycle, depending on the variation of the second pulse interval.

[0015] In some implementations, the pulse interval between those pulses emitted during the first cycle differs by a proportional value relative to the pulse interval between those pulses emitted during any subsequent cycle, depending on the change in the second pulse interval.

[0016] In some implementations, the pulse interval between those pulses emitted during the first period differs by a random value relative to the pulse interval between those pulses emitted during any subsequent period, depending on the variation of the second pulse interval.

[0017] In some implementations, the pulse interval emitted during subsequent periods switches between at least two different frequency profiles, depending on the change in the second pulse interval.

[0018] In some implementations, the periodic repetitive motion is zigzag motion, circular motion, or stroke motion.

[0019] In some embodiments, the optical element is a plane mirror, a wedge lens, a prism, or a polygonal mirror.

[0020] In some embodiments, the optical element is configured to deflect laser pulses backscattered from the target toward the receiver.

[0021] In some embodiments, the motor is configured to rotate the optical element about a first axis of rotation to produce a tapered laser pulse emission pattern, wherein the airborne laser scanner further includes an angle encoder configured to provide the position of the optical element.

[0022] In some implementations, the motor is configured to oscillate the optical element about a swing axis, thereby producing a fan-shaped laser pulse emission pattern.

[0023] In some implementations, the airborne laser scanner includes a wobbling sensor configured to provide the position of the optical elements.

[0024] In some implementations, the computer is configured to determine the direction of the pulse path based on the position of the provided optical element.

[0025] In some implementations, the computer is configured to: determine the current of the motor; and based on the current, determine the direction of the pulse path.

[0026] In some embodiments, the motor is configured to rotate the optical element about a second rotation axis, and the optical element is implemented as a multifaceted mirror, the deflection of which generates a fan-shaped laser pulse emission pattern.

[0027] In some embodiments, the optical element is arranged relative to the transmitter in such a way that the optical element deflects the laser pulse at a defined constant angle relative to the axis of rotation or relative to the axis of oscillation.

[0028] The present invention also relates to a computer-implemented method for reducing ranging bias and measurement point loss caused by internal and near-range reflections in an airborne laser scanner, the airborne laser scanner being deployed on an aircraft for surveying targets along a flight path, the method comprising: triggering a transmitter of the airborne laser scanner to emit laser pulses having a first pulse interval variation; deflecting the laser pulses toward the target along the pulse path using at least one optical element of the airborne laser scanner; moving the optical element using a motor of the airborne laser scanner to induce a periodic repetitive motion of the pulse path, wherein triggering the transmitter to also emit laser pulses having a second pulse interval variation overlapping the first pulse interval variation, wherein, according to the second pulse interval variation, the pulse interval between those pulses emitted during the first period of the periodic repetitive motion is at least partially modified relative to the pulse interval between those pulses emitted during any subsequent period of the periodic repetitive motion; receiving the laser pulses backscattered from the target using a receiver of the airborne laser scanner; and determining the direction of the pulse path.

[0029] In some embodiments of the computer implementation method, depending on the change of the second pulse interval, the pulse interval between those pulses emitted during the first period differs from the pulse interval between those pulses emitted during any subsequent period by one of the following: a constant value; a proportional value; and a random value. Attached Figure Description

[0030] By way of example only, preferred embodiments of the invention will be described more fully below with reference to the accompanying drawings, in which: Figure 1 An aircraft carrying an airborne laser scanner according to an embodiment of the present invention is shown schematically; Figures 2 to 4 An airborne laser scanner according to an embodiment of the present invention is illustrated schematically; Figures 5 to 9 The scanning pattern used in an airborne laser scanner according to an embodiment of the present invention is shown schematically; Figure 10 and Figure 11 This illustration schematically shows the errors (losses) in the scanning results of a prior art airborne laser scanner. Figure 12 An exemplary first pulse interval variation used in an airborne laser scanner according to an embodiment of the present invention is illustrated schematically; Figure 13 An exemplary variation of the second pulse interval used in an airborne laser scanner according to an embodiment of the present invention is illustrated schematically; Figure 14 An exemplary variation in the total pulse interval for an airborne laser scanner according to an embodiment of the invention is illustrated schematically; and Figures 15 to 17 The diagram schematically illustrates the blind moment of the receiver of an airborne laser scanner due to the transmission of pulses. Detailed Implementation

[0031] Figure 1 The image shows aircraft 1 flying over the ground, which serves as target 2, while following a flight path 3 (trajectory). The aircraft carries an onboard laser scanner 4, which emits laser pulses 5. These laser pulses are used to generate a point cloud 6 of stripes (scanned areas) of aircraft 1. The laser pulse emission pattern is symbolically shown as a pyramid; however, in reality, it is more like a cone or fan shape.

[0032] Figures 2 to 4 Three exemplary operating principles applicable to airborne laser scanners according to the present invention are shown. Figure 2 An airborne laser scanner 7 is shown, comprising a computer 8, a transmitter 9, and a receiver 10. In this example, the computer, transmitter, and receiver are combined into a single unit, but they can also be configured separately. A prism 11 (particularly a wedge prism) is operatively connected as an optical element to a motor 12, enabling the prism to rotate about a rotation axis A1. The movement of the optical element 11 caused by the motor 12 can be a continuous rotational motion or a partial rotational motion. A continuous rotational motion should be understood as at least one complete 360° rotation, particularly a permanent sequence of full rotations, while a partial rotational motion should be understood as a rotation, particularly less than 360°, which is reversed in the defined scheme. For example, after a 180° rotation, the motor changes direction and returns to 180°, and so on.

[0033] During the movement of optical element 11, transmitter 9 emits multiple consecutive laser pulses T1 toward target 2. The direction of the pulse path is changed by the movement of optical element 11. Pulses R1 backscattered from target 2 are received by receiver 10. Computer 8 is connected to transmitter 9, receiver 10, and motor 12, and is configured to control these components.

[0034] Typically, the computer in some implementations can be additionally configured to determine the distance from the airborne laser scanner to the ground based on the emitted and received laser pulses (i.e., by calculating the time of flight (TOF)). Since the direction of the optical element deflecting the pulse (i.e., the direction of the pulse path) is known and / or can be determined (e.g., by an angle encoder that detects the rotational position of a motor), the determined TOF distance value can be assigned to the direction of the current pulse path at a specific measurement time (e.g., at least one coordinate, such as an angle).

[0035] Specifically, a 3D point cloud based on these data (3D point measurement results) can be generated in post-processing by an external computer. In this case, the internal computer 8 is configured only to collect data. The data may include timestamps of pulse transmission and reception or distance values ​​calculated using the timestamps, as well as the transmission / reception directions. Alternatively, the computer 8 may be configured to generate the point cloud, particularly to generate the point cloud in real time.

[0036] Figure 3 A second exemplary implementation is shown, wherein the construction is similar to Figure 2 The configuration shown is different, but the optical element is a mirror 14 instead of a prism. Furthermore, the motor 15 does not rotate fully for positioning, but rather oscillates about an oscillation axis A2. Specifically, the oscillation axis extends along (or parallel to) the surface of mirror 14. However, the oscillation axis A2 may also extend elsewhere. In particular, the oscillation can also be understood as a partial rotational motion.

[0037] Positioned by the swinging motion of mirror 14, the transmitted laser pulse T2 is deflected toward the target and returns along the pulse path R2. The pulse path pivots laterally to the flight path. Similar to the previous example, scanner 13 has a transmitter unit 16 / receiver unit 17 and a computer 18.

[0038] Figure 4 A third example is shown. Scanner 19 has computer 20, transmitter 21, and receiver 22. In this case, the optical element is a polygon mirror 23, which rotates about axis A3 via motor 24. As a result, the pulse path always stroking from one side to the other. As the polygon mirror 23 continues to rotate, whenever the edge of the mirror reaches the transmitted beam T3, the deflection quickly jumps back to the starting point, and the pulse path starts drifting again. The echo R3 returns to receiver 22 in the same manner.

[0039] Figures 5 to 7 The resulting sampling pattern is shown, where each black dot represents a measured 3D point, or a point on target 2 that reflects a measurement pulse. Figure 5 Scan 24 in the middle is Figure 2The result of the configuration of the scanner 7. This spiral pattern is caused by the conical pulse path motion induced by the rotating wedge prism 11 and the flight motion. Figure 6 The pattern 25 shown follows Figure 3 The structure of scanner 13. This zigzag shape is generated by the forward and backward movement and flight movement of mirror 14. Figure 7 The strokes 26 shown are generated by, for example Figure 4 This is caused by configuration 19.

[0040] Figure 8 and Figure 9 The corresponding scan patterns 27 and 28 are shown, which have an improved dot distribution as disclosed in EP3517998A1. Due to the nature of the pulse path motion, scanners with lateral return motion and deflecting optics (such as scanners 7 and 13) accumulate too many dots at the edges of the strip and too few dots at the center (roughly: the nadir). This is not the case with scanner 19, where the dot distribution is already uniform. The improvement of EP3517998A1 to the scanner is that the pulse rate should depend on how much the direction of the pulse path is instantaneously shown to the side (relative to the direction of the flight path). That is, the closer the pulse path is to the edge of the scan area, the lower the pulse rate will be to pick up fewer dots, and in particular, the closer the pulse path is to the center (flight path), the higher the pulse rate may be, or at least reach its nominal value. This will also be explained below. Figure 12 As shown in the image.

[0041] The present invention aims to solve the problems associated with all of the above examples.

[0042] Figure 10 and Figure 11 The image symbolically illustrates a 3D point cloud of the corresponding scanned area, highlighting systemic defects within the point cloud. These defects are likely to occur when an aircraft carrying an airborne laser scanner flies over terrain with virtually no outlines (e.g., parking lots, sports fields, streets, flat roofs, lakes, farmland, etc.). Because the scanner sends laser pulses so rapidly, at least one more pulse (or even multiple pulses) is emitted before the reflected pulse returns to the scanner. This technique is also known as Multipulse In Air (MPIA). Now, when the returning pulse arrives at the receiver simultaneously with the transmitted pulse, the receiver cannot sample the returning pulse. The receiver is "blind" for the time it takes to emit a new pulse (and for a short period after emission due to internal reflections). Therefore, some points are missed.

[0043] For example, Figure 10Scan 29 has a line 30 at its center, essentially along the flight path, which symbolizes the absence of measurement points. This scan has been recorded (e.g., using data from...). Figure 3 or Figure 4 The device), that is, the scanning pattern is as follows Figure 6 or Figure 9 or Figure 7 As shown. These losses always occur at the same lateral location and propagate along the flight direction. The reason is that the aircraft flies at a constant altitude and the ground is flat, as described above. Now using the scanning pattern mentioned above (zigzag): Figure 6 and Figure 9 ;stroke: Figure 7 Points on the outside are farther from the aircraft, while points at the center have the shortest distance. The distance increases symmetrically to both sides from the centerline (flight path). Therefore, in Figure 10 In the middle, it happens to be the center line of the point that is a certain distance away from the aircraft, and as mentioned above, the scanner cannot capture these points. Figure 11 An example of a point cloud 31 with two gap lines 32 and 33 is shown, as they are both some distance from the aircraft where the aforementioned loss phenomenon occurred.

[0044] In such Figure 2 The scanner device shown generates a device with the following characteristics: Figure 5 In the case of the 3D point cloud of the scanned pattern shown, the points on the circle are (essentially) equidistant from the laser scanner. This fact makes the scanner particularly vulnerable to loss of data, because if the aircraft is flying at the critical altitude, not just some points, but all points will be affected. Take a quick look... Figures 15 to 17 This shows how it happens: Figure 15 The first plot shows the transmitted pulse T, with its intensity I plotted on the y-axis relative to time t on the x-axis. The pulse is emitted at a constant pulse rate. Figure 16 The second drawing in the image shows the reflected pulse R. int The intensity I is plotted on the y-axis relative to time t on the x-axis. These reflections are caused by optical elements inside the scanner, and they terminate at the receiver very close to the transmission time period. These reflections prolong the receiver's blind time window 34, as it is virtually impossible to distinguish these reflections from any real received pulse from the target. Figure 17 The third drawing shows the received pulse R. ext The received pulse R returns from the target. ext The intensity I is plotted on the y-axis relative to time t on the x-axis. They fall into blind window 34 and therefore cannot be sampled by the receiver.

[0045] As mentioned above, such as Figure 12The pulse interval variation of the triangular signal shown can avoid complete blackout in the case of a cone-shaped scanner. According to this variation, the pulse rate increases as the pulse path approaches its lowest point, and decreases as the pulse path approaches the lateral edge of the scanned area. However, due to the symmetrical distribution of the pulse rate across the cross-section of the scanned area, this will result in... Figure 10 and Figure 11 According to Figure 2 The scanner configuration has the same problem, and it cannot be resolved (at most, the problem is reduced). Figure 3 and Figure 4 The issue concerns the scanner. Note that this pulse interval variation is merely an example and is not essential for applying the invention. Specifically, when this disclosure refers to "first pulse interval variation," it does not necessarily refer to the triangular signal.

[0046] The embodiments of the present invention greatly improve upon the above-mentioned problems, resulting in fewer missing points and / or their distribution in a manner that does not attract the viewer's attention, because they are not as... Figure 10 and Figure 11 The illustration is shown.

[0047] The problem is solved by triggering an emitter to emit laser pulses having a second pulse interval variation that overlaps (or in other words, superimposes) with the first pulse interval variation, wherein, according to the second pulse interval variation, the pulse interval between those pulses emitted during the first cycle of the periodic repetitive motion is at least partially modified relative to the pulse interval between those pulses emitted during any subsequent cycle of the periodic repetitive motion. Figure 13 It shows the superimposed from Figure 12 The potential second pulse interval change on the triangular signal. Then Figure 14 The resulting changes in pulse interval or pulse rate are shown.

[0048] The periodic repetitive motion of the pulse path used in the above examples are respectively the results of a 360° circular conical scan. Figure 5 ), a zigzag pattern with two lines (back and forth) Figure 6 ) and a stroke pattern along a line ( Figure 7 ).exist Figure 14 In this implementation, the first cycle of this periodic repetitive motion is characterized by a change in the pulse interval between it and the next cycle of the periodic repetitive motion. The boundary between cycles is indicated here by dashed lines. If the aircraft flies at a critical altitude above a nearly flat surface, this modification achieves the lost "dithering," making the loss unrecognizable in the 3D point cloud. Of course, other implementations may have the same characteristics as... Figure 13Different variations in the second pulse interval (the flip-flop between two factors) can further improve survey quality.

[0049] Furthermore, this invention provides reduced ranging bias. If the glass return and ground return are very close in time and space, the glass return (internal reflection) can bias the ground return. The solution of this invention also reduces the likelihood of glass return and ground return overlapping, thus resulting in less so-called near-field terrain noise (ranging bias) because the ground layer is less likely to overlap with either the glass return or the near-field return.

[0050] Although the invention has been illustrated above (with reference in part to some preferred embodiments), it must be understood that various modifications and combinations of different features of the embodiments are possible. All such modifications are within the scope of the appended claims.

Claims

1. An airborne laser scanner configured to be mounted on an aircraft for surveying targets along a flight path, wherein, The airborne laser scanner includes: A transmitter configured to emit multiple consecutive laser pulses toward the ground; At least one optical element, the at least one optical element being configured to deflect the laser pulse toward the target along a pulse path; An electric motor configured to move the optical element to cause periodic repetitive motion of the pulse path; A receiver configured to receive the laser pulse backscattered from the target; and Computer, the computer being configured to: Control the transmitter, the motor, and the receiver; Determine the direction of the pulse path; and The transmitter is triggered to emit laser pulses with a first pulse interval variation. The computer is further configured to: trigger the transmitter to emit laser pulses having a second pulse interval variation that overlaps with the first pulse interval variation, wherein, according to the second pulse interval variation, the pulse interval between those pulses emitted during the first cycle of the periodic repetitive motion is at least partially modified relative to the pulse interval between those pulses emitted during any subsequent cycle of the periodic repetitive motion.

2. The airborne laser scanner according to claim 1, wherein, The second pulse interval variation has a digital pattern.

3. The airborne laser scanner according to claim 1, wherein, The second pulse interval variation has a simulated pattern.

4. The airborne laser scanner according to claim 1, wherein, The second pulse interval variation follows a sine pattern, a linear zigzag pattern, a waveform pattern, a sawtooth pattern, a stepped pattern, or any combination of the patterns.

5. The airborne laser scanner according to claim 1, wherein, According to the second pulse interval variation, the pulse interval between those pulses emitted during the first cycle differs by a constant value relative to the pulse interval between those pulses emitted during any subsequent cycle.

6. The airborne laser scanner according to claim 1, wherein, According to the second pulse interval change, the pulse interval between those pulses emitted during the first cycle differs proportionally from the pulse interval between those pulses emitted during any subsequent cycle.

7. The airborne laser scanner according to claim 1, wherein, According to the second pulse interval variation, the pulse interval between those pulses emitted during the first period differs by a random value relative to the pulse interval between those pulses emitted during any subsequent period.

8. The airborne laser scanner according to claim 1, wherein, Depending on the change in the second pulse interval, the pulse interval emitted during the subsequent period switches between at least two different frequency distributions.

9. The airborne laser scanner according to claim 1, wherein, The periodic repetitive motion is a zigzag motion, a circular motion, or a stroke motion.

10. The airborne laser scanner according to claim 1, wherein, The optical element is a plane mirror, a wedge lens, a prism, or a polygonal mirror.

11. The airborne laser scanner according to claim 1, wherein, The optical element is configured to deflect the laser pulse backscattered from the target toward the receiver.

12. The airborne laser scanner according to claim 1, wherein, The motor is configured to rotate the optical element about a first rotation axis to generate a conical laser pulse emission pattern, wherein the airborne laser scanner further includes an angle encoder configured to provide the position of the optical element.

13. The airborne laser scanner according to claim 1, wherein, The motor is configured to oscillate the optical element about a swing axis, thereby producing a fan-shaped laser pulse emission pattern.

14. The airborne laser scanner of claim 13, wherein the airborne laser scanner includes a wobbling sensor configured to provide the position of the optical element.

15. The airborne laser scanner according to claim 14, wherein, The computer is configured to determine the direction of the pulse path based on the position of the provided optical element.

16. The airborne laser scanner according to claim 1, wherein, The computer is configured to: Determine the current of the motor; and The direction of the pulse path is determined based on the current.

17. The airborne laser scanner according to claim 1, wherein, The motor is configured to rotate the optical element about a second rotation axis, and the optical element is implemented as a multifaceted mirror, the deflection of which generates a fan-shaped laser pulse emission pattern.

18. The airborne laser scanner according to claim 1, wherein, The optical element is arranged relative to the transmitter in such a way that it deflects the laser pulse at a defined constant angle relative to the axis of rotation or relative to the axis of oscillation.

19. A computer-implemented method for reducing ranging bias and measurement point loss caused by internal and near-range reflections in an airborne laser scanner, said airborne laser scanner being deployed on an aircraft for surveying targets along a flight path, said method comprising the steps of: The transmitter of the airborne laser scanner is triggered to emit laser pulses with a first pulse interval variation; Using at least one optical element of the airborne laser scanner, the laser pulse is deflected toward the target along the pulse path; The optical element is moved by a motor of the airborne laser scanner to cause a periodic repetitive motion of the pulse path, wherein the transmitter is triggered to also emit laser pulses having a second pulse interval variation that overlaps with the first pulse interval variation, wherein, according to the second pulse interval variation, the pulse interval between those pulses emitted during the first cycle of the periodic repetitive motion is at least partially modified relative to the pulse interval between those pulses emitted during any subsequent cycle of the periodic repetitive motion; The receiver of the airborne laser scanner receives the laser pulse backscattered from the target; Determine the direction of the pulse path.

20. The computer-implemented method according to claim 19, wherein, According to the second pulse interval change, the pulse interval between those pulses emitted during the first period differs from the pulse interval between those pulses emitted during any subsequent period by one of the following: Constant value; Proportion value; as well as Random value.

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