Changes between Version 5 and Version 6 of Sensors/LeicaLIDAR/FWMikesNotes


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Timestamp:
Feb 1, 2010 3:37:13 PM (10 years ago)
Author:
mggr
Comment:

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  • Sensors/LeicaLIDAR/FWMikesNotes

    v5 v6  
    1212}}}
    1313
    14 The FWD is implemented as two sampling boards
    15 
    16 The FWD digitises the incoming intensity level and nothing else.  It records either 64, 128 or 256 samples at 1 or 2 nanosecond resolution (1ns is approximately 15cm). The samples are recorded from the time of the first return, with a small buffer before that (the "pre-trigger samples").  This buffer allows the capture of the lead-in to the first pulse (otherwise it would start at the first detected peak) and is specified in terms of meters to the operating software (default 5m).
     14The FWD is implemented as two sampling boards (A & B) that operate digitise the incoming intensity level and nothing else.  It records at 1 or 2 nanosecond resolution (1ns is approximately 15cm of distance travelled).  The laser pulse in the ALS50 system has a 4ns pulse duration (or 9ns if using <100kHz pulse rate mode), so the FWD must have a <= 2ns digitisation to avoid aliasing (Nyquist).
     15
     16The FWD records either 64, 128 or 256 samples.  These samples are recorded from the time of the first return, with a small buffer before that (the "pre-trigger samples").  This buffer allows the capture of the lead-in to the first pulse (otherwise it would start at the first detected peak) and is specified in terms of meters to the operating software (default 5m).  This will also help capture (for example) sparse tree tops prior to the first return.
     17
     18The intensity is digitised to an 8 bit value.  This is nominally the same value as shown in the discrete system and is measured after the AGC amplification has taken place.  The AGC value for the matching discrete return is the value used for all the related FWD samples - i.e. the AGC value is constant for a laser pulse.
     19
     20Similarly, the range data (measured to 1.5cm range / 100 picoseconds) is still part of the discrete system and FWD ranges are derived by association with the first return's range.
    1721
    1822Processing note: the trigger from the discrete system appears to have a small delay (order of 7ns?), that is different for the two possible laser pulse rates and also may be dependent on temperature (unproven).  This delay manifests as an offset between the discrete returns and the matching waveform peaks.  At the time of writing, it is corrected by a manual / statistical approach (see below).
    19 
    2023
    2124Some other important points:
     
    2326  - requires a non-peak gap after a peak is found, so double peaks not spottable without  fwd
    2427
    25 laser pulse is 4ns pulse duration or 9ns (<100khz digitisation), so must have a <2ns intensity digitisation to avoid aliasing (nyquist), normally prefer 1ns
    26 
    27 1ns = 15 cm
    28 
    2928intensity is digitised to 8bit
    30  - no range gate yet but may be added (phil)
    31  - "pre-trigger" samples = buffer before first return peak to ensure you get lead in to pulse, defaults to 5m
    32   - especially important if there's low signal before first real peak, e.g. sparse tree top
    33 range data is measured independently to achieve 1.5cm range (100 picosec).
    34  - fwd data goes to RawWFD, structure identical to RawLaser, files named WFDYYMMDD_HHMMSS_XXXXXXXXXXX{AB}.LWV
    35   - A or B indicates which board digitised the signal
    36 .LWV files
    37  - are always up to 62501KB
    38  - always in A&B pairs
    39  - can't link single scn files to lwv files within a flightline
     29
     30FWD filestructure:
     31 * FWD data is recorded to the RawWFD directory, with a folder structure identical to RawLaser (flightline based)
     32 * files named WFDYYMMDD_HHMMSS_XXXXXXXXXXX{AB}.LWV
     33   * A or B indicates which board digitised the signal
     34   * always in A&B pairs (if not, there has been corruption)
     35   * are always up to 62501KB (64MB?)
     36 * the .LWV files cannot be easily linked to the .SCN (discrete files) - there is no direct linkage between LWV file 2 and SCN file 2
     37
    4038flightline log file says if WFD on/off
    4139 
    42 can turn processing of waveform data off
    4340always produces las 1.3 file (despite currently saying 1.2)
    4441 - filename ends _4.las O_o