Changes between Version 14 and Version 15 of Sensors/LeicaLIDAR/MikesNotes


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Timestamp:
Aug 28, 2008 6:11:35 PM (10 years ago)
Author:
mggr
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  • Sensors/LeicaLIDAR/MikesNotes

    v14 v15  
    6666=== How it works ===
    6767
    68 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 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.
     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 (R4 actually detects the last return rather than the 4th?).  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.
    6969
    7070A 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.  When there are 4 returns, each range card measures the time of the return pulse.  When there are less than 4 returns, R4 is a second measurement (not a copy of) of the last pulse - i.e. if there are 2 returns, you will have R1, R2 and R4 (= re-measurement of R2).
     
    144144 * 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.
    145145 * Measured in calibration procedure
     146   * Correct pitch error (at nadir) first
    146147   * '''We need to do this'''
    147148
     
    189190The features required for a perfect cal site are:
    190191 * Need a source of multiple returns - tallish (15m) trees in a forest are best.  Try to include a treed/forested area in some parts of the flight lines (doesn't have to be in all, nor in the central area).  The multiple returns are required for the range card calibration (see below).
    191  * Straight, flat areas made of a hard substance that'll generate only one return pulse.  Roads or runways ;)  These are used in the boresight and range calibrations.
    192  * Sloping peaked areas (house rooftops are ideal) with the peak cutting across the line of flight ( --->  /\ ).  These are used to detect pitch and yaw errors in the boresight calibration.
     192 * Straight, flat areas made of a hard substance that'll generate only one return pulse.  Roads or runways ;)  These are used in the boresight (roll) and range calibrations.
     193 * Sloping areas are used to detect pitch and heading errors in the boresight calibration.
     194   * A road running up a hill is good for pitch (vertical change in surface easily found in the image).  These are good for measuring along-track shifts.
     195   * Slopes with peaks (house rooftops are ideal) with the peak cutting across the line of flight ( --->  /\ ).  Again good for measuring along-track shifts.
    193196 * An accurate ground survey (see below).
    194197 * Slow overflight for maximum point density.
     
    219222 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.
    220223
    221 First, we need to determine the timing differences between the 4 range cards (R1-R4) in each bank.
    222 
    223   When there are 4 returns, each range card measures the time of the return pulse.  When there are less than 4 returns, R4 is a second measurement (not a copy of) of the last pulse - i.e. if there are 2 returns, you will have R1, R2 and R4 (= re-measurement of R2).
     224First, we need to determine the timing differences between the 4 range cards (R1-R4) in each bank.  To do this, we use a dataset with multiple varying returns present - we need combinations of 2-4 returns (forests are good for this, being tall and porous enough to give multiple returns).  When there are less than 4 returns, R4 is a second measurement (not a copy of) of the last pulse - i.e. if there are 2 returns, you will have R1, R2 and R4 (= re-measurement of R2).  This is exploited to compute the difference in timing between R2 and R4 (averaging many 2-returns).  Similary 1-return and 3-return pulses are used to measure R1-R4 and R3-R4 differences.  The end result is a set of timing differences between all the cards in a bank.
    224225
    225226Second, we need to establish the timing differences between bank A and bank B.  As with the first step, we need the timing cards to measure exactly the same instant.  To do this, we use BIT mode data, where R1 in both bank receives the same electronically generated pulse at the same instant and are thus measuring the same event.  Averaging these numbers gives the timing offsets between the R1 cards, which can be combined with the first measurements to establish timing between all cards.
     
    228229 * run the timing estimate program on the two datasets above, using a nominal range error of 0
    229230   * ''hopefully Mark wrote down the details on how to do this - it's somewhere off the ALS preprocessor menus, but looked pretty straightforward.  Resulting numbers need to transcribed to ALS processor afterwards''
     231   * '''do we need to turn off "use average of last returns?"'''
    230232 * process a 14 degree strip (+/-7 degrees of nadir) over the dense GCP region (need ~30-40 GCPs @ 1cm vertical accuracy)
    231233   * in TerraScan, compute the average error between the GCPs and surface generated from the point cloud (''didn't write this procedure down either'')
    232234 * enter this error as the new nominal range error into the timing estimate program and re-run
    233235 * reprocess the strip and verify against the GCPs - error should be dz = ~0 (<1cm), stddev ~5cm.
     236
     237= Boresight calibration =
     238TBD.
     239
     240Boresight parameters (pitch, roll, heading).
     241 * Angle between straight-down and what the sensor thinks is straight-down, as it's mounted in the plane.
     242   * 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?).
     243 * Measured in calibration procedure
     244   * '''We need to do this'''
     245
     246Pitch error slope.
     247 * 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.
     248   * 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)
     249 * 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.
     250 * Measured in calibration procedure
     251   * Correct pitch error (at nadir) first
     252   * '''We need to do this'''
     253 * Straight, flat areas made of a hard substance that'll generate only one return pulse.  Roads or runways ;)  These are used in the boresight (roll) and range calibrations.
     254 * Sloping areas are used to detect pitch and heading errors in the boresight calibration.
     255   * A road running up a hill is good for pitch (vertical change in surface easily found in the image).  These are good for measuring along-track shifts.
     256   * Slopes with peaks (house rooftops are ideal) with the peak cutting across the line of flight ( --->  /\ ).  Again good for measuring along-track shifts.
     257
     258
     259--------------
     260= Random snippets =
     261
     262SCN file, DC=delta counter (time in ms since last GPS-second tick), ANG=angle in ticks, RI = return? (in m?).