Changes between Version 9 and Version 10 of Sensors/LeicaLIDAR/MikesNotes


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Timestamp:
Aug 26, 2008 5:49:10 PM (11 years ago)
Author:
mggr
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  • Sensors/LeicaLIDAR/MikesNotes

    v9 v10  
    2121Automatic 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.
    2222
     23Laser 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
    2325Water absorbs IR, so expect poor returns from wet surfaces.  Ideally one should wait for a whole dry day after rain.
    2426
     
    3739Consider terrain and minimum target size to determine required point density.  Steep terrain may cause shadowing effects due to perspective.
    3840
    39 Consider reflectivity of surface - lighter surfaces need less laser power (too high a power means overflows on the reflection, though AGC should deal with this for the most part..).
     41Consider reflectivity of surface - lighter surfaces need less laser power.  Too high a power means intensity overflows on the reflection, which mess up the range - AGC should deal with this for large areas but it should be considered in planning, particularly for small bright targets on a generally darker background.
    4042
    4143Recommended to do a figure 8 loop at the start and end of the data acquisitions to make the IMU happy.  Need one at the end for reverse navigation processing.
     
    5355=== How it works ===
    5456
    55 The LIDAR works by firing a laser pulse downwards and measuring the roundtrip time for the light pulse to return, then converting this to a distance.  The pulse isn't modulated by a carrier - it's just an on/off pulse.  There are four timing cards ("range cards"), so up to 4 return pulses can be detected, with the intensity of the return measured only on the first 3 returns.  A minimum time separation between two returns means the minimum distance between two returns must be at least 2.7m for them to be counted as independent.  The expectation for the number of returns is 1 return ~100%, 2 returns ~10%, 3 returns ~1%, 4 returns ~0.1% of points - obviously this varies with the terrain.
     57The LIDAR works by firing a (4ns or 9ns) laser pulse downwards and measuring the roundtrip time for the light pulse to return, then converting this to a distance.  The pulse isn't modulated by a carrier - it's just an on/off pulse.  There are four timing cards ("range cards"), so up to 4 return pulses can be detected, with the intensity of the return measured only on the first 3 returns.  A minimum time separation between two returns means the minimum distance between two returns must be at least 2.7m for them to be counted as independent.  The expectation for the number of returns is 1 return ~100%, 2 returns ~10%, 3 returns ~1%, 4 returns ~0.1% of points - obviously this varies with the terrain.
    5658
    57 There is also a "MPiA" mode (Multiple Pulses in the Air), which fires two pulses evenly separated, rather than waiting for the first to come back (SPiA mode) before firing another [times out in case the pulse is eaten].  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 it merging into the ground).
     59The intensity of a return is measured as an 8 bit value (0=dark (water), 255=bright) and relates to the reflectivity of the illuminated surface.  The value is amplified by an automatic gain controller, and is not related to a physical measure (can it be?).  The intensity can be used in various processing algorithms to help distinguish transitions between surfaces.  The AGC tries to keep the intensity in the range 100-150 or so.
     60
     61There is also a "MPiA" (Multiple Pulses in the Air) mode, which fires two pulses evenly separated, rather than waiting for the first to come back before firing another [SPiA mode, times out in case the pulse is eaten].  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 it merging into the ground).
    5862
    5963The laser is scanned across a (up-to) 75 degree swath by an oscillating mirror.  Due to the acceleration/deceleration of the mirror, this produces a sinusoidal pattern to the trace on the ground, with the highest density of points at the peak and trough of the sine wave (i.e. at the edges of the swath).  If the swath width is set to less than 75 degrees (45 degrees recommended), there's a roll compensation mechanism that tries to smooth out small roll movements by using the remaining freedom of motion.  The scan rate of the mirror is dependent on the FOV angle (36Hz for 40 degree FOV, 24Hz for a 75 degree FOV, calculate with 412.33 * FOV^-0.6548).
     
    111115   * find altitude over ground (measured GPS alt - geoid-spheroid height) and see if it's within the min & max ranges
    112116
     117Laser 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.
     118 * filter out incorrect points (based on high intensity and height difference from locality?)
     119 * also, if you observe odd spikes, check for intensity over 150 - this may indicate a two-peaked response with the first peak being 150 (and being the detected one) and the second being 255 (overflow), causing the effect above
     120
    113121--------------------------
    114122= Calibration =
     
    124132   * '''We need to measure this one with a survey'''
    125133
     134Range offset correction.
     135 * Correction for the slightly different timing of the 4 range cards in the system.
     136 * At a set distance, the range cards should all return the same result.
     137 * '''Measured by Leica but also measured and verified in calibration procedure.'''  (see below)
     138
     139=== Factory calibrated ===
    126140Intensity Based Range Correction (IBRC).
    127  * 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 - ie. takes longer for enough photons to come back to exceed the detection threshold).
     141 * 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).
    128142 * Measured in factory and provided by Leica.
    129143
    130 Range offset correction.
    131  * Correction for the slightly different timing of the 4 range cards in the system
    132  * At a set distance, the range cards should all return the same result
    133  * '''Measured by Leica but also measured and verified in calibration procedure.'''  (see below)
     144Transitional Pulse Rate.
     145 * 2 pulse widths are used (4ns and 9ns) depending on the pulse frequency (higher frequencies require a shorter pulse time), with the system switching over to a shorter pulse at a set frequency (around 120KHz?).  This parameter measures the time difference for the pulses' return.
     146 * In our system, this distance is of the order of 11cm.
     147 * Measured in factory and provided by Leica.
    134148
     149Encoder Offset (scan angle correction).
     150 * The rotating mirror has
    135151
    136152== Calibration site requirements ==