Changes between Version 11 and Version 12 of Sensors/LeicaLIDAR/MikesNotes


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Timestamp:
Aug 26, 2008 6:59:32 PM (11 years ago)
Author:
mggr
Comment:

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  • Sensors/LeicaLIDAR/MikesNotes

    v11 v12  
    3333Atmospheric 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.
    3434
    35 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 (one can only get "late" measurements?).
    36 
     35Range 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?).
     36
     37Torsion (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.
    3738
    3839== Mission planning considerations ==
     
    103104 * look out for data gaps (listed in log)
    104105
     106
    105107Most 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.
    106108
     
    127129
    128130== Parameters needing calibration ==
     131
     132Boresight parameters (pitch, roll, heading).
     133 * Angle between straight-down and what the sensor thinks is straight-down, as it's mounted in the plane.
     134   * Angle between a line from the sensor head (mirror centre) to the point on the ground at the centre of the swath and a line from the sensor head to the centre of the spheroid (or the reference frame's Z axis?).
     135 * Measured in calibration procedure
     136   * '''We need to do this'''
     137
     138Pitch error slope.
     139 * The mirror will not be mounted exactly flat to the laser so, as the mirror moves, the pitch of the beam will change by a small amount.
     140   * Roll and yaw either have no error slope (laser position central? geometry means no effect?) or a negligible effect (presumably, as there's no parameter)
     141 * Measure pitch at nadir and at swath edges to determine how the pitch changes - the first order for this is is the pitch error slope.
     142 * Measured in calibration procedure
     143   * '''We need to do this'''
    129144
    130145Lever arm measurements:
     
    141156 * '''Measured by Leica but also measured and verified in calibration procedure.'''  (see below)
    142157
     158Mirror torsion parameter.
     159 * This measures how much the mirror flexes at the edges of the swath, when under high acceleration, putting it out of position.
     160 * Later Leica mirrors (including ours) are much stiffer and apparently do not flex.
     161 * Fixed very high value (100,000 N?) meaning "no measurable flex".
     162 * Can be measured during the calibration procedure, but should never be necessary.
     163
    143164=== Factory calibrated ===
     165
    144166Intensity Based Range Correction (IBRC).
    145167 * A range correction based on the reflectivity of the target object ("reflections are slower on darker objects"?  perhaps this is more that a lower intensity return may take longer to be detected - i.e. takes longer for enough photons to come back to exceed the detection threshold).
     
    152174
    153175Encoder Offset (scan angle correction).
    154  * The rotating mirror has a (optical?) shaft encoder attached that tells it the position of the mirror.  The mirror will not be mounted exactly at nadir relative to the shaft encode and this value measures the offset between the centre of the scan pattern and the centre point of the encoder.  The value is measured in "ticks" and will appear quite high as the shaft encoder has a high resolution (order of millions of ticks).
     176 * The rotating mirror has a (optical?) shaft encoder attached that tells it the position of the mirror.  The mirror will not be mounted exactly at nadir relative to the shaft encode and this value measures the offset between the centre of the scan pattern and the centre point of the encoder.
     177 * The value is measured in "ticks" and will appear quite high as the shaft encoder has a high resolution (order of millions of ticks).
    155178 * Measured in factory and provided by Leica.
     179
     180
    156181
    157182== Calibration site requirements ==