Changes between Version 6 and Version 7 of Sensors/LeicaLIDAR/FWMikesNotes


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Timestamp:
Feb 8, 2010 12:29:02 PM (10 years ago)
Author:
mggr
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  • Sensors/LeicaLIDAR/FWMikesNotes

    v6 v7  
    1212}}}
    1313
    14 The 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).
     14The FWD is implemented as two sampling boards (A & B) that operate digitise the incoming intensity level and nothing else.  Each digitisation board is capable of recording at up to 60kHz and interleave the sampling - hence, the maximum sampling rate is 120kHz.  If the laser pulse rate is set higher than 120kHz, the system will FWD one in two pulses (i.e. 120kHz pulse rate = 120,000 FWDs/sec ; 121kHz = 605,000 FWDs/sec).  The FWD boards sample 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).
    1515
    1616The 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.
    1717
    18 The 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.
     18The intensity is digitised to an 8 bit value.  This is measured after the AGC amplification has taken place and is nominally the same measurement as in the discrete system (though the discrete value is accumulated over a longer time period so will typically have a higher value).  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.
    1919
    2020Similarly, 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.
     
    2525 - normal lidar returns based on threshold, then look for peaks
    2626  - requires a non-peak gap after a peak is found, so double peaks not spottable without  fwd
    27 
    28 intensity is digitised to 8bit
    2927
    3028FWD filestructure:
     
    113111  - minimised L/R imbalance, improved radiometric using ccd border area (hrm)
    114112
    115 AGC setting constant for a single pulse (ie. only need the one recorded in SCN)
    116  - confusing slide on intensity scale relating to voltage..  not quite sure what value is recorded (raw or agc scaled)
    117  - note intensity scales are different for standard returns and fwd
    118 
    119113things one can detect.. maybe..
    120114 - pulse stretching (return pulse longer than laser pulse
     
    125119
    126120angle of laser may make a difference  (flat ground at nadir is a slope at edge of fov, or cutting through vegetation)
    127 
    128 hardware changes
    129  - new data logger (cope with higher io)
    130   - hdd -> 160GB SSD, works up to 7620m altitude
    131    - should be ok to use spinning disk for bigger datasets @ low altitude
    132    - can't swap disks in flight without full shutdown of logger, but can keep ipas going without recording
    133  - fwd boards, 2 of them at 60kHz each
    134121
    135122software changes
     
    141128 - alspp outputting in las 1.3 type 4 (option to output 1.0 if needed for compat)
    142129 - version list in presentation
    143 
    144 if pulse rate > 120kHz, then FWD on every second pulse..  e.g. 150kHz SCN file = 75kHz FWD
    145130
    146131where did gain come from in waveform viewer
     
    168153 - drag and drop a log file into it to get nicely parsed info
    169154
    170 data rate / height stuff
    171  - 19.2 / 38.4 / 76.8m for various heights
    172  - 1h04 recording at max rate on 160GB SSD
     155
    173156
    174157terrascan
     
    230213    || 128 @ 2ns   || 38.4m ||
    231214    || 64 @ 2ns    || 19.2m ||
     215
     216data rate / height stuff
     217 - 19.2 / 38.4 / 76.8m for various heights
     218 - 1h04 recording at max rate on 160GB SSD
    232219
    233220 * Benefits of full waveform