Scanner

The primary goal of the scanning technique is to create a wide swath withconsistent along- and across-track point spacing, and reliable and accurate elevations for the entire swath. Several scanning techniques have been used in airborne LiDAR systems. In theory there are no special reasons why one scanning technique is preferable to another, although scan patterns that facilitate constant incident angle on the terrain can reduce data voids related to dynamic range of the receiving optics. The most common scanning techniques are the Oscillating Mirror and Rotating Mirror.

(1) Oscillating Mirrors. In systems using an oscillating mirror, the mirror rotates back and forth between limited extents, producing a zigzag (i.e. sinusoidal pattern) line on the surface of the target area (Figure 1-2). The mirror is always pointing downwards towards the ground so data collection can be continuous and theoretically all pulses of the laser can be used. The field of view and scan rate can be set by the operator prior to acquisition. Changing the field of view provides additional flexibility as it allows laser pulses to be collected over a shorter span (denser data ) or a wider span (sparser data). Although the oscillating mirror is the most widely used scanning mechanism for airborne LiDAR systems, there are inherent disadvantages of using the oscillating mirror principle. The changing velocity and acceleration of the mirror as it oscillates from one end to the other causes unequal spacing of the laser pulses on the target. The point density increases along the edges of the scan where the mirror slows down, and decreases along the center in the along-scan direction. The forward motion of the aircraft causes the zig-zag pattern with varying point spacing along the edges of the scan in the cross-scan direction. Manufacturers have solved these problems by essentially ignoring the outlier points on a scan and modeling the distortions caused by changing speed using a computer algorithm.

(2) Rotating Mirrors. The rotating mirror is another commonly used scanning mechanism for airborne LiDAR systems. In this approach, the mirror is rotated continuously at a constant velocity in one direction producing a parallel line scan (Figure 1-3). The constant velocity ensures that there are no acceleration type errors encountered in the oscillating mirror scanner. The point spacing is also more regular both along and across the scan. However, the biggest disadvantage is that observations cannot be taken during a significant time during each mirror rotation when the mirror is pointing away from the target. Typically, 30-40% of the emitted laser pulses are not aimed at the target area and are essentially lost to the scanning mechanism.

(3) Other Scanning Patterns. Otherscanning mechanisms less commonly used include the push broom (fiber scanning) pattern where the laser pulsed energy is transmitted into one of the fibers arranged in a circle producing a nutating scan pattern (Figure 1-4) and the Palmer scanner that produces an elliptical scanning pattern with redundant data that can be used for calibration or to get a forward and aft view of the same target (Figure 1-5).