Changes between Version 12 and Version 13 of Sensors/LeicaLIDAR/MikesNotes


Ignore:
Timestamp:
Aug 28, 2008 11:50:58 AM (11 years ago)
Author:
mggr
Comment:

--

Legend:

Unmodified
Added
Removed
Modified
  • Sensors/LeicaLIDAR/MikesNotes

    v12 v13  
    3737Torsion (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.
    3838
     39MPiA 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.
     40
     41
    3942== Mission planning considerations ==
    4043
     
    6366=== How it works ===
    6467
    65 The 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.
    66 
    67 The 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.
    68 
    69 There 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).
     68The 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 R1-R4") running for a pulse, so up to 4 returns can be detected.  The system has 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].  To measure this, there are actually two banks of timing cards (bank A and bank B, both with R1-R4 cards), so there are 8 timing cards in total.
     69
     70A 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.
     71
     72The intensity of a return is measured only for the first 3 returns (R1-R3), and is an 8 bit value (0=dark (water), 255=bright) relating 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.
    7073
    7174The 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).
     
    151154   * '''We need to measure this one with a survey'''
    152155
    153 Range offset correction.
    154  * Correction for the slightly different timing of the 4 range cards in the system.
     156Range offset correction (+range card calibration).
     157 * Correction for the slightly different timing of the 4 range cards in both banks in the system.
    155158 * At a set distance, the range cards should all return the same result.
    156159 * '''Measured by Leica but also measured and verified in calibration procedure.'''  (see below)
     
    207210
    208211== Range offset calibration ==
    209  * Two datasets required:
    210    * BIT (Built-In Test) mode data, where the range cards are all fed with identical fake data representing the same distance.  All cards should give the same result, so differences are used to calibrate each card against the others.
    211    * A real dataset with
     212
     213Range offset correction (+range card calibration).
     214 * Correction for the slightly different timing of the 4 range cards in the system.
     215 * At a set distance, the range cards should all return the same result.
     216 * Measured by checking the first return pulse against a known distance and by computing the timing errors for each range card so as to calibrate them against the first pulse.
     217 * '''Measured by Leica but also measured and verified in calibration procedure.'''  (see below)
     218
     219Two datasets are required:
     220 1. BIT (Built-In Test) mode data, where the range cards are all electronically fed with identical fake data representing the same distance.  All cards should give the same result, so differences are used to calibrate each card against the others.
     221 1. A real dataset with a well known distance (in the factory this will be a target, in the world it'll be a site with accurate GCPs)
     222
     223First, we need to determine the timing differences between the 4 range cards (R1-R4).  To do this, we use BIT mode data, where all range cards receive the same pulse at the same instant.  Averaging these numbers gives the timing offsets between the cards