Airborne lidar pulse rate modulation

Abstract

An airborne laser scanner configured to be arranged on an aircraft for surveying a target along a flight path. The airborne laser scanner comprises an emitter configured for emitting a plurality of consecutive laser pulses towards the ground surface, at least one optical element configured for deflecting the laser pulses along pulse paths towards the target, a motor configured for altering the pulse paths by moving the optical element, a receiver configured for receiving the laser pulses backscattered from the target, and a computer configured for controlling the emitter, the motor, and the receiver, for determining directions of the pulse paths, and for triggering the emitter to emit the laser pulses with a varying pulse spacing based on the directional component of the pulse paths in a horizontal direction perpendicular to a direction of the flight path.

Claims

1. An airborne laser scanner configured to be arranged on an aircraft for surveying a ground surface along a flight path of the aircraft, wherein the airborne laser scanner comprises: an emitter configured for emitting a plurality of consecutive laser pulses towards the ground surface from the aircraft; at least one optical element configured for deflecting the laser pulses along pulse paths towards the ground surface; a motor configured for altering the pulse paths by moving the optical element; a receiver configured for receiving the laser pulses backscattered from the ground surface toward the aircraft; and a computer configured for: controlling the emitter, the motor, and the receiver, collecting time stamps of the emitted laser pulses from the aircraft and the received laser pulses from the ground or collecting distance values calculated based on time stamps of the emitted laser pulses from the aircraft and the received laser pulses from the ground, and determining directions of the pulse paths, wherein: surveying the ground surface is for generating a three-dimensional point cloud based on the time stamps and on the directions of the pulse paths, the computer is configured for triggering the emitter to emit the laser pulses with a varying pulse spacing based on the directional components of the pulse paths in a horizontal direction perpendicular to a direction of the flight path.

2. The airborne laser scanner according to claim 1, wherein the pulse spacing is gradually varied between a minimum pulse spacing and a maximum pulse spacing, and wherein: the minimum pulse spacing is set when the directional component is minimal, and the maximum pulse spacing is set when the directional component is maximal.

3. The airborne laser scanner according claim 1, wherein the pulse spacing is gradually varied according to a sinusoidal characteristic, a linear zig-zag characteristic, a wave characteristic, a saw tooth characteristic, a step characteristic, or any combination of said characteristics.

4. The airborne laser scanner according to claim 1, wherein the optical element is a prism or a mirror.

5. The airborne laser scanner according to claim 1, wherein the motor is configured for rotating the optical element around a rotation axis, resulting in a cone-shaped laser pulse emission pattern.

6. The airborne laser scanner according to claim 5, wherein the optical element is arranged relative to the emitter in such a way that the optical element deflects the laser pulses in a defined constant angle relative to the rotation axis or relative to the oscillation axis.

7. The airborne laser scanner according to claim 1, further comprising an angle encoder configured for providing positions of the optical element.

8. The airborne laser scanner according to claim 1, wherein the motor is configured for oscillating the optical element around an oscillation axis, resulting in a fan-shaped laser pulse emission pattern.

9. The airborne laser scanner according to claim 8, further comprising an oscillation sensor configured for providing positions of the optical element around the oscillation axis.

10. The airborne laser scanner according to claim 1, wherein the computer is configured for calculating a time-based sequence of the varying pulse spacings of the laser pulses to be emitted from the emitter.

11. The airborne laser scanner according to claim 1, wherein the computer is further configured for: determining a current of the motor, and determining the directions of the pulse paths based on the current.

12. The airborne laser scanner according to claim 1, wherein the computer is further configured for receiving flight data from the aircraft, said flight data comprising a direction of the flight path of the aircraft.

13. The airborne laser scanner according to claim 1, further comprising an Inertial Measuring Unit (EV1U), wherein the computer is further configured for receiving heading data from the EVIU and for determining a direction of the flight path based on said heading data.

14. The airborne laser scanner according to claim 1, wherein the computer and the emitter are further configured for providing the laser pulses with constant pulse energy.

15. The airborne laser scanner according to claim 1, wherein the optical element is further configured for deflecting the laser pulses backscattered from the ground surface towards the receiver.

Description

DESCRIPTION OF THE DRAWINGS

(1) In the following, the invention will be described in detail by referring to exemplary embodiments that are accompanied by figures, in which:

(2) FIG. 1: shows an aircraft flying over a ground surface (target), a flight path (trajectory) of the aircraft, an airborne laser scanner according to the invention arranged on the aircraft, a point cloud of the swath of the aircraft generated by the airborne laser scanner, and laser pulses emitted by the airborne laser scanner;

(3) FIG. 2a,b: show two embodiments of the airborne laser scanner;

(4) FIG. 3a,b: show the point clouds of two exemplary laser pulse emission patterns according to prior art;

(5) FIG. 4a,b: show the point clouds of two exemplary laser pulse emission patterns according to the invention;

(6) FIG. 5a,b: show two further embodiments of the airborne laser scanner;

(7) FIG. 6a-d: show four graphs of embodiments of the pulse modulation according to the invention;

DETAILED DESCRIPTION

(8) FIG. 1 shows an aircraft 1 flying over a ground surface as target 3, a flight path 6 (trajectory) of the aircraft 1, an airborne laser scanner 2 according to the invention arranged on the aircraft 1, laser pulses 4 emitted by the airborne laser scanner 1, and a point cloud 5 of the swath of the aircraft 1 generated based on said laser pulses. The laser pulse emission pattern 4 is symbolically shown as a pyramid. In reality however it rather appears as a cone or fan.

(9) FIGS. 2a and 2b each symbolically show an embodiment of an airborne laser scanner 2 according to the invention. FIG. 2a shows the airborne laser scanner 2 comprising a computer 26, an emitter 21 and a receiver 25, which in this example are combined in one unit, but may however also be arranged separately. A prism 22, in particular a wedge prism, as optical element is in operative connection with a motor 24, such that it is rotatable around the rotation axis R. The movement of the optical element 22 induced by the motor may be a continuous or a partial rotatory movement. A continuous rotatory movement is to be understood as at least one full 360°-rotation, in particular a permanent sequence of full rotations, and a partial rotatory movement is to be understood as a rotation, in particular a rotation of less than 360°, which is reversed in a defined scheme. For example, after a 180°-rotation, the motor changes the rotational direction and goes back the 180°, and so on.

(10) During the movement of the optical element 22, the emitter 21 emits a plurality of consecutive laser pulses 211 towards the target. The pulse path is altered by the movement of the optical element. The pulses 251 backscattered from the target are received by the receiver 25.

(11) The computer 26 is connected to the emitter 21, the receiver 25, and the motor 24, and it is configured for controlling these components.

(12) In an embodiment, the computer 26 may additionally be configured for determining a distance from the airborne laser scanner to the ground surface for emitted and received laser pulses based on the time of flight method. Since the angle by which the optical element is deflecting the pulses and the direction of the pulse path are known and/or determinable (e.g. by an angle encoder), the TOF-distance value can be associated to the direction (e.g. at least one coordinate such as angle(s)) of the current pulse path at a specific measurement time.

(13) Particularly, a three-dimensional point cloud based on these associations (point measurements) can be generated by an external computer in a post-processing. In this case, the internal computer 26 is merely configured to collect the data. The data may comprise time stamps of pulse transmission and pulse reception or distance values already calculated by means of said stamps, and transmission/reception direction.

(14) Alternatively, the computer 26 can be configured for generating said point cloud, in particular in real-time.

(15) According to the invention, the laser pulses (or respectively: the laser pulse rate, or the laser pulse spacings) are modulated based on the directional component of the current pulse path in a horizontal direction perpendicular to a direction of the flight path.

(16) The invention allows generating a point cloud which has a more steady (or: even) point distribution and which has less point cluster.

(17) A second embodiment of the airborne laser scanner 2 is shown in FIG. 2b, wherein the construction is similar to the one shown in FIG. 2a but the optical element is a mirror 23 instead of a prism. Also, the motor 24 does not perform full rotations as positioning, but performs oscillations around an oscillation axis O. In particular, the oscillation axis runs along (or: parallel to) the surface of the mirror. The oscillation axis can however also run elsewhere. In particular, the oscillation can also be understood as a partial rotatory movement.

(18) By the oscillating positioning of the mirror 23, the emitted laser pulses 211 are deflected towards the target and back along a pulse path. Said pulse path pivots laterally to the flight path.

(19) FIGS. 3a and 3b show the unfavourable point distribution in point clouds 5 of two exemplary laser pulse emission patterns according to prior art. The paper plane is the ground surface 3. These point clouds 5 have been generated by laser pulses which have been emitted at a constant pulse rate or constant pulse spacing. It can be recognised that the point density in the area of a reverse position 7 of the motor is relatively high compared to the area between the reverse positions 7. At this reverse position 7 the directional component of the pulse path in a horizontal direction perpendicular to a direction of the flight path is maximal.

(20) The pattern of FIG. 3a corresponds to a laser scanner of FIG. 2a, wherein continuous full rotations are performed while the aircraft is moving along a flight path 6.

(21) The pattern of FIG. 3b corresponds to a laser scanner of FIG. 2b, wherein the motor oscillates the mirror within the reverse positions 7 while the aircraft is moving along a flight path 6.

(22) The reverse position may in some embodiments also be defined as a position of the motor in which a laser pulse is deflected by the optical element at the largest angle with respect to a plumb-line of the aircraft and in a plane perpendicular to a flight path of the aircraft. In other words, in the reverse position of the motor, an emitted laser pulse measures the fringe of the swath of the scanner, i.e. the most lateral areas with respect to the trajectory of the aircraft. At this reverse position, the motor reverses the deflection to the respective other direction (towards the other edge of the swath) with respect to an axis perpendicular to the flight path 6 in FIGS. 3a and 3b.

(23) FIGS. 4a and 4b show the result of pulse modulation according to the invention. The larger the direction component of the pulse path in a horizontal direction perpendicular to a direction of the flight path 6, the larger the pulse spacing is. In other words, the closer the points of the cloud 5 are to the reverse lines 7, the lower the pulse rate is. Accordingly, a more even point distribution is achieved.

(24) In further embodiments, an airborne scanner according to the invention can achieve various scan patterns. FIG. 5a shows a double prism arrangement configured to deflect the laser path twice—once by the prism 22 and once by the prism 22′. Both prisms may be motorised by motor 24 and motor 24′ respectively. In particular, said motors are configured for rotating the prisms in opposite directions.

(25) FIG. 5b shows a double optical element arrangement configured to deflect the laser path twice—once by the mirror 23 and once by the prism 22. Both the mirror 23 and the prism 22 may be motorised by motors 24 and 24′. The oscillation of the mirror 23 may be superimposed by the rotation of the prism 22.

(26) Possible scan patterns resulting from the arrangements shown in FIGS. 5a and 5b (or other combinations of mirrors and/or prisms) may have the shape of a flower or a spiral following a zigzag path. By varying the respective rotation/oscillation speeds and/or directions, various different patterns are achievable.

(27) Be the pattern as it may, according to the invention, the pulse rate is decreased the more the pulses approach the reverse lines 7, or in other words, the more the pulse paths are directed towards the edges of the swath.

(28) FIGS. 6a, 6b, 6c, and 6d each show exemplary pulse modulations provided by the computer to the emitter. The modulations all have in common that they are depending on the pulse path direction.

(29) If the lateral (relative to the flight direction) component of the pulse path direction reaches a maximum, i.e. the laser pulses arrive at a reverse position 7, the pulse spacing 8 reaches its maximum (or respectively: the pulse rate has a minimum).

(30) Accordingly, if the motor reaches a position so as to send a laser pulse at the smallest angle, in particular 0°, with respect to a plumb-line of the aircraft and in a plane perpendicular to a flight path of the aircraft, the pulse spacing 8 reaches its minimum (or respectively: the pulse rate reaches its maximum). In this case, the laser pulse is directed essentially onto the flight path 6 on the ground surface, i.e. said directional component of the pulse paths in a horizontal direction perpendicular to a direction of the flight path is minimal, in particular zero.

(31) The pulse spacing may be gradually varied according to a sinusoidal characteristic (FIG. 6a), a linear zig-zag characteristic (FIG. 6b), a wave characteristic (FIG. 6c), or a saw tooth characteristic (FIG. 6d). In another embodiment (not shown), the pulse spacing is varied according to a step characteristic, wherein the pulse spacing is kept constant for a step period and then jumps up step by step. In particular, any combination of the above mentioned pulse sequences may be applied.

(32) The sequences of pulses 4 indicated by the arrows in FIGS. 6a-d are qualitative illustrations. In fact, the sequences of pulses 4 differ slightly based on the according pulse spacing characteristic 8. Said courses of pulse spacings 8 are also to be understood as qualitative illustrations, which may appear exaggerated. However, the graphs 8 are intended to show that there may be various optional mathematical principles behind the way the pulses are modulated according to the invention.

(33) Although the invention is illustrated above, partly with reference to some preferred embodiments, it must be understood that numerous modifications and combinations of different features of the embodiments can be made. All of these modifications lie within the scope of the appended claims.