System and method for an airborne mapping lidar
A system for generating an image of a region of interest (ROI) from a flying aircraft using a light detection and ranging (lidar) system that includes a transmitter, a receiver, an inertial navigation system, a scanning system, and a system control and a data processing computer. The transmitter includes a pulsed laser. The receiver includes a sensor to detect light scattered and reflected from the ROI. The scanning system uses a field-of-view (FOV) of the transmitter and receiver to interrogate a field-of-regard (FOR) during a flight over the ROI, and the FOV angle is narrower than the FOR angle. The system also includes a software application that divides the ROI into collection unit (CU) areas, and for each CU determines area characteristics, estimates one or more flight paths required to collect CU area data, and determines settings of the sensor for collecting the CU area data.
1 . A method for generating an image of a region of interest (ROI) from a flying aircraft comprising:
providing a light detection and ranging (lidar) system comprising
a transmitter comprising a pulsed laser;
a receiver comprising a sensor to detect light scattered and reflected from the ROI;
an inertial navigation system;
a scanning system that uses a field-of-view (FOV) of the transmitter and receiver to interrogate a field-of-regard (FOR) during a flight over the ROI, wherein a FOV angle is narrower than a FOR angle;
a system control and a data processing computer comprising at least a memory storing computer-executable instructions of a software application and a processor coupled to the memory and configured to execute the computer-executable instructions; and
wherein the software application comprises computer executable instructions that divide the ROI into collection unit (CU) areas, and for each CU determine area characteristics, estimate one or more flight paths required to collect CU area data, and determine settings of the sensor for collecting the CU area data;
wherein the scanning system comprises a first scanning mirror with a single axis of rotation and a second scanning mirror with a single axis of rotation and wherein the single axis of rotation of the first scanning mirror is non-parallel to the single axis of rotation of the second scanning mirror, and wherein the scanning system orients the lidar system in any direction inside the FOR;
wherein the lidar system further comprises a scan controller and wherein the scan controller determines positions of the first and second scanning mirrors and sends command voltages and receives readback voltages to and from an input/output (I/O) device, respectively, in blocks;
wherein the I/O device samples readback voltages indicating at least one of positions, velocities, and error signals of the first and second scanning mirrors at a sampling rate and saves the readback voltages in blocks and wherein the blocks of the readback voltages are provided to the system control and data processing computer at a lower rate than the sampling rate; and
wherein the sampling rate is chosen based on an acceleration capability of the first or second scanning mirrors so that an angular error between the first or second scanning mirrors' actual position and a calculated position is less than a threshold value.
2 . The method of claim 1 , wherein the scanning system points the lidar toward any direction within the FOR by moving the first scanning mirror and the second scanning mirror by angles m 1 and m 2 , respectively, and wherein the processor executes the computer executable instructions of the software application that calculate the angles m 1 and m 2 using:
m
1
=
-
arctan
(
u
x
+
-
w
x
2
+
u
x
2
+
2
u
y
2
w
x
-
u
y
2
)
m
2
=
-
ξ
+
arctan
(
w
y
+
-
u
z
2
+
w
y
2
+
w
z
2
u
z
+
w
z
)
wherein u x , u y , u z are coordinates of a vector of incoming line of sight of the sensor and w x , w y , w z are coordinates of a vector of outgoing line of sight; and
wherein ξ is the zero position angle of the second scanning mirror.
3 . The method of claim 1 , wherein the lidar system further comprises a scan queue that is loosely coupled to a scheduling application and tightly coupled to a scan controller and wherein the scan queue comprises an ordered data structure of the CU areas and for each CU area comprises at least one of a 3D-polygonal boundary in geographic coordinates, a number of times to image the CU area sequentially before choosing another CU area, a total number of times to image the CU area, a value of a swipe angular speed, and a minimum fraction of the CU's area that should be within the FOR in order for the CU area to be scanned.
4 . A system for generating an image of a region of interest (ROI) from a flying aircraft comprising:
a light detection and ranging (lidar) system comprising:
a transmitter comprising a pulsed laser;
a receiver comprising a sensor to detect light scattered and reflected from the ROI;
an inertial navigation system;
a scanning system that uses a field-of-view (FOV) of the transmitter and receiver to interrogate a field-of-regard (FOR) during a flight over the ROI, wherein a FOV angle is narrower than a FOR angle;
a system control and a data processing computer comprising at least a memory storing computer-executable instructions of a software application and a processor coupled to the memory and configured to execute the computer-executable instructions; and
wherein the software application comprises computer executable instructions that divide the ROI into collection unit (CU) areas, and for each CU determine area characteristics, estimate one or more flight paths required to collect CU area data, and determine settings of the sensor for collecting the CU area data;
wherein the scanning system comprises a first scanning mirror with a single axis of rotation and a second scanning mirror with a single axis of rotation and wherein the single axis of rotation of the first scanning mirror is non-parallel to the single axis of rotation of the second scanning mirror, and wherein the scanning system orients the lidar system in any direction inside the FOR;
wherein the lidar system further comprises a scan controller and wherein the scan controller determines positions of the first and second scanning mirrors and sends command voltages and receives readback voltages to and from an input/output (I/O) device, respectively, in blocks;
wherein the I/O device samples readback voltages indicating at least one of positions, velocities, and error signals of the first and second scanning mirrors at a sampling rate and saves the readback voltages in blocks and wherein the blocks of the readback voltages are provided to the system control and data processing computer at a lower rate than the sampling rate; and
wherein the sampling rate is chosen based on an acceleration capability of the first or second scanning mirrors so that an angular error between the first or second scanning mirrors' actual position and a calculated position is less than a threshold value.
5 . The system of claim 4 , wherein the lidar system further comprises:
a scheduling application that runs in real time during the flight;
a scan controller; and
a scan queue that is loosely coupled to the scheduling application and tightly coupled to the scan controller.
6 . The system of claim 4 , wherein the scanning system points the lidar system toward any direction within the FOR by moving the first scanning mirror and the second scanning mirror by angles m 1 and m 2 , respectively, and wherein the processor executes the computer executable instructions of the software application that calculate the angles m 1 and m 2 using:
m
1
=
-
arctan
(
u
x
+
-
w
x
2
+
u
x
2
+
2
u
y
2
w
x
-
u
y
2
)
m
2
=
-
ξ
+
arctan
(
w
y
+
-
u
z
2
+
w
y
2
+
w
z
2
u
z
+
w
z
)
wherein u x , u y , u z are coordinates of a vector of incoming line of sight of the sensor and w x , w y , w z are coordinates of a vector of outgoing line of sight; and
wherein ξ is the zero position angle of the second scanning mirror.
7 . The system of claim 4 , wherein the I/O device receives and saves the blocks of command voltages from the scan controller and generates drive voltages for the first and second scanning mirrors one sample at a time at a fixed rate.
8 . The system of claim 4 , wherein the inertial navigation system is used for time synchronization between the first and second scanning mirror position data and data streams within the system via a time-stamped waveform recorded on a channel of a data acquisition system.
9 . The system of claim 4 , wherein the collection units comprise one of rectangles, polygonal areas, or 3D polygonal volumes.
10 . The system of claim 4 , wherein the area characteristics comprise at least one of terrain elevation profiles, lower envelope of a scene's height, upper envelope of a scene's height, spatial resolution, angular diversity, sampling density, or land cover.
11 . The system of claim 4 , wherein the settings of the sensor for collecting the CU area data comprise at least one of laser power, system photonic link, camera settings, swipe speed, or swipe overlap.
12 . The system of claim 4 , wherein the lidar system further comprises a scan queue that is loosely coupled to a scheduling application and tightly coupled to a scan controller and wherein the scan queue comprises an ordered data structure of the CU areas and for each CU area comprises at least one of a 3D-polygonal boundary in geographic coordinates, a number of times to image the CU area sequentially before choosing another CU area, a total number of times to image the CU area, a value of a swipe angular speed, and a minimum fraction of the CU's area that should be within the FOR in order for the CU area to be scanned.
13 . The system of claim 5 , wherein the scan controller is programmed to search the scan queue for a selected CU area that is in view and to send a command to the scanning system to scan the selected CU area for a selected number of scan times and to remove the selected CU area from the scan queue when the selected number of scan times is reached or to add the selected CU area back to the scan queue when the selected number of scan times is not reached.
14 . The system of claim 4 , wherein the scan controller calculates a specific motion of the first and second scanning mirrors for scanning a selected CU area by dividing the CU area into subsection areas (swipes) which are scanned with substantially uniform sampling density, next computing geographic coordinates of a longitude, a latitude, and an altitude, representing beginning and end points of each swipe, next calculating first and second scanning mirror commands to aim at the geographic coordinates, next moving the first and second scanning mirrors to the beginning point of the swipe and moving the first and second scanning mirrors at a substantially constant speed along the swipe and at the end point transitioning to a different swipe until the entire CU area has been scanned.
15 . The system of claim 14 , wherein the swipes comprise one of concentric arcs or substantially parallel lines.