Changes between Version 13 and Version 14 of Sensors/LeicaLIDAR/MikesNotes


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Timestamp:
Aug 28, 2008 12:47:06 PM (11 years ago)
Author:
mggr
Comment:

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  • Sensors/LeicaLIDAR/MikesNotes

    v13 v14  
    6868The 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.
    6969
    70 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.
     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.  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).
    7171
    7272The 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.
     
    211211== Range offset calibration ==
    212212
    213 Range 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)
     213Range offset correction (+range card calibration) is to correct for the slightly different timing of the 4 range cards (R1-R4) in both banks (A & B) in the system, and to correct any overall ground offset.
    218214
    219215Two datasets are required:
     216 1. A real dataset with:
     217   1. a source of multiple returns, such a forest (for step 1 below)
     218   1. a strip with well known distances (for verification)
    220219 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 
    223 First, 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
     220
     221First, 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).
     224
     225Second, 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.
     226
     227Procedure:
     228 * run the timing estimate program on the two datasets above, using a nominal range error of 0
     229   * ''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''
     230 * process a 14 degree strip (+/-7 degrees of nadir) over the dense GCP region (need ~30-40 GCPs @ 1cm vertical accuracy)
     231   * in TerraScan, compute the average error between the GCPs and surface generated from the point cloud (''didn't write this procedure down either'')
     232 * enter this error as the new nominal range error into the timing estimate program and re-run
     233 * reprocess the strip and verify against the GCPs - error should be dz = ~0 (<1cm), stddev ~5cm.