| | 44 | The boresighting procedure for the LiDAR is similar to the hyperspectral instrument boresighting, except that we also have the z (height) error to help identify the offsets. It is often the case that errors are identified easier by examining vertical profiles of overlapping data as well as horizontal ones. |
| | 45 | |
| | 46 | The angles (pitch, roll and heading) that describe the pointing direction of the laser scanner with respect to the nadir need to be identified along with the 'pitch error slope'. The pitch error slope is an error term that describes the fact that the mirror is not mounted perfectly flat against the laser. Therefore as the mirror moves the pitch of the beam will change by a small amount. The resulting errors are along-track offsets just like the pitch errors, but varying, with no error at nadir and maximum error at the edges of the swath. |
| | 47 | |
| | 48 | To correct for the pitch offset we take opposing flight lines. If there is a pitch error then the lidar will be sensing pulses ahead/behind (along track) of the nadir point. There will be a small overall range error, but the main effect will be that the data is shifted backwards/forwards of where it should be. |
| | 49 | |
| | 50 | If there is a small roll error, surfaces will be tilted up on one side of the swath and down on the other. In opposing flight lines, these errors will be on different sides of the swath. Vertical displacements due to roll error will be most noticeable at the edges of the swath. For small errors, the nadir point will be roughly the same, for large ones, there will be some across-track shift, but that will be much harder to measure than the vertical errors. |
| | 51 | |
| | 52 | If there is a heading error, one side of the swath will appear further ahead (along track) of the other side. Looking at opposing flights will not help with this but overlapping, parallel lines will (where the overlap is ~30-50% across the swath). Examining the overlapping region will show an along track shift if there is a heading error. |
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| | 54 | Note that typically the LiDAR range corrections will be identified during the same procedure, and so using the z errors to identify the boresight means an iterative approach to this is required. |
| | 55 | |