| 13 | | == Things that may go wrong == |
| 14 | | |
| 15 | | === In flight === |
| 16 | | |
| 17 | | Flying too low - if the LIDAR detects that the laser power may be too high for eye safety, it will cut out the laser automatically (if you're a seagull looking up, bad luck, it only accounts for ground height). |
| 18 | | |
| 19 | | Flying outside the "range gate" (acceptable ranges of distances) may cause similar effects. Going too high will tend to make the edges of the swath drop out first (due to path length). |
| 20 | | |
| 21 | | Automatic gain control weirdness - the measured intensity is returned via an AGC which may step up or down depending on the returns from the ground. Measured return intensity should only be used as a guideline rather than a real measurement. |
| 22 | | |
| 23 | | Laser power too high - if the intensity overflows (reaches 255), the intensity based range correction will probably be wrong. For example, a freshly asphalted road with bright white reflective (overflowing) strips may appear with the strips appearing to float up to 20cm above the road surface. |
| 24 | | |
| 25 | | Water absorbs IR, so expect poor returns from wet surfaces. Ideally one should wait for a whole dry day after rain. |
| 26 | | |
| 27 | | === In processing === |
| 28 | | |
| 29 | | Most Leica systems are mounted "laser backwards" (cables will be at the rear if this is so) - ensure that z=180 in the IPAS Pro aircraft tab. |
| 30 | | |
| 31 | | Streaky intensity images - "bad, contact support for guidance". These will probably cause range errors due to the intensity variation, no idea what the cause would be as yet.. |
| 32 | | |
| 33 | | Poor accuracy at edges - consider trimming the swath width (reduce processed angle) to cut off bad edges. |
| 34 | | |
| 35 | | === Other error sources === |
| 36 | | |
| 37 | | Atmospheric effects; the light path may bend due to atmospheric refraction, density effects, etc causing the laser to hit the ground earlier or later than expected, or in a different position. This will be most noticeable at the edges of the swath where there are longer path lengths (and more atmosphere to pass through) and may look like the swath curls up or down at the edges (path length error), or may look like compression or stretching of the edges (if there is horizontal divergence). The error was claimed to be a fraction of a meter at 6km altitude (i.e. not well bounded). Measurement of temperature may help with this effect, but was said to be a minor value. |
| 38 | | |
| 39 | | Range correction error; if the range correction is wrong, the electronics will measure the path length incorrectly. Points at nadir will be in error vertically only and points at the edge will have a vertical and horizontal error. The error will make a flat piece of land look like a smile and the nadir point will be below the expected land surface (one can only get "late" measurements?). |
| 40 | | |
| 41 | | Torsion (of mirror) error; the mirror may be out of the expected position at the edges of the swath due to it bending under high acceleration. There will be no error at nadir (no acceleration) and errors at the edges, inducing a smile effect again, but with the nadir at the correct height. Range errors should be corrected before working on the torsion error, and the nadir point should be used for the range correction as there will be no mirror torsion effect there. |
| 42 | | |
| 43 | | MPiA mode errors: if a seagull gets in the way of the second pulse before the first pulse has returned, things will mess up. On an edge of a very unluckily placed cloud, this would look a bit like the cloud merging into the ground. Presumably rare or minor. |
| | 121 | |
| | 122 | = QC = |
| | 123 | |
| | 124 | == Things that may go wrong == |
| | 125 | |
| | 126 | === In flight === |
| | 127 | |
| | 128 | Flying too low - if the LIDAR detects that the laser power may be too high for eye safety, it will cut out the laser automatically (if you're a seagull looking up, bad luck, it only accounts for ground height). |
| | 129 | |
| | 130 | Flying outside the "range gate" (acceptable ranges of distances) may cause similar effects. Going too high will tend to make the edges of the swath drop out first (due to path length). |
| | 131 | |
| | 132 | Automatic gain control weirdness - the measured intensity is returned via an AGC which may step up or down depending on the returns from the ground. Measured return intensity should only be used as a guideline rather than a real measurement. |
| | 133 | |
| | 134 | Laser power too high - if the intensity overflows (reaches 255), the intensity based range correction will probably be wrong. For example, a freshly asphalted road with bright white reflective (overflowing) strips may appear with the strips appearing to float up to 20cm above the road surface. |
| | 135 | |
| | 136 | Water absorbs IR, so expect poor returns from wet surfaces. Ideally one should wait for a whole dry day after rain. |
| | 137 | |
| | 138 | === In processing === |
| | 139 | |
| | 140 | Most Leica systems are mounted "laser backwards" (cables will be at the rear if this is so) - ensure that z=180 in the IPAS Pro aircraft tab. |
| | 141 | |
| | 142 | Streaky intensity images - "bad, contact support for guidance". These will probably cause range errors due to the intensity variation, no idea what the cause would be as yet.. |
| | 143 | |
| | 144 | Poor accuracy at edges - consider trimming the swath width (reduce processed angle) to cut off bad edges. |
| | 145 | |
| | 146 | === Other error sources === |
| | 147 | |
| | 148 | Atmospheric effects; the light path may bend due to atmospheric refraction, density effects, etc causing the laser to hit the ground earlier or later than expected, or in a different position. This will be most noticeable at the edges of the swath where there are longer path lengths (and more atmosphere to pass through) and may look like the swath curls up or down at the edges (path length error), or may look like compression or stretching of the edges (if there is horizontal divergence). The error was claimed to be a fraction of a meter at 6km altitude (i.e. not well bounded). Measurement of temperature may help with this effect, but was said to be a minor value. |
| | 149 | |
| | 150 | Range correction error; if the range correction is wrong, the electronics will measure the path length incorrectly. Points at nadir will be in error vertically only and points at the edge will have a vertical and horizontal error. The error will make a flat piece of land look like a smile and the nadir point will be below the expected land surface (one can only get "late" measurements?). |
| | 151 | |
| | 152 | Torsion (of mirror) error; the mirror may be out of the expected position at the edges of the swath due to it bending under high acceleration. There will be no error at nadir (no acceleration) and errors at the edges, inducing a smile effect again, but with the nadir at the correct height. Range errors should be corrected before working on the torsion error, and the nadir point should be used for the range correction as there will be no mirror torsion effect there. |
| | 153 | |
| | 154 | MPiA mode errors: if a seagull gets in the way of the second pulse before the first pulse has returned, things will mess up. On an edge of a very unluckily placed cloud, this would look a bit like the cloud merging into the ground. Presumably rare or minor. |