Changes between Version 25 and Version 26 of Sensors/LeicaLIDAR/MikesNotes


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Timestamp:
Sep 5, 2008 4:04:47 PM (11 years ago)
Author:
mggr
Comment:

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  • Sensors/LeicaLIDAR/MikesNotes

    v25 v26  
    1111 * problem uploads to go to incoming/ dir
    1212
    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.
    4413
    4514
     
    150119
    151120See other pages on navigation processing as they cover this already.
     121
     122= QC =
     123
     124== Things that may go wrong ==
     125
     126=== In flight ===
     127
     128Flying 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
     130Flying 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
     132Automatic 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
     134Laser 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
     136Water 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
     140Most 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
     142Streaky 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
     144Poor accuracy at edges - consider trimming the swath width (reduce processed angle) to cut off bad edges.
     145
     146=== Other error sources ===
     147
     148Atmospheric 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
     150Range 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
     152Torsion (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
     154MPiA 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.
    152155
    153156