Version 8 (modified by mark1, 9 years ago) (diff)

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Full Waveform upgrade

Concepts and Hardware

  • Discrete point recording remains unchanged - full waveform digitiser is an extra.
    • Nothing has changed for the AGC/mainboard of the ALS so the raw laser .SCN files will be the same as before.
    • Extra files will be created that contain the waveform data. Waveform files (.LWV) will be in the raw_wfd directory, SCN files in the raw_laser directory.
    • Waveform files have A or B in filename depending on which data board (A or B) acquired it. When processing both files are taken together.
  • The digitiser starts recording from the first returned peak for the number of required samples (256/128 @ 1ns or 256/128/64 @ 2ns)
    • If first return is noise (cloud/seagull) then the digitiser will start recording there and not at the target.
    • measured heights / distances for different sampling rates:
Sample rate equivalent height
256 @ 1ns 38.4m
128 @ 1ns 19.2m
256 @ 2ns 76.8m
128 @ 2ns 38.4m
64 @ 2ns 19.2m
  • Benefits of full waveform
    • Better idea of the nature of the return e.g. tree canopy height is more accurate than from 1 discrete point
    • Extraction of points from the waveform missed by the discrete measuring
    • (a future benefit) pulse stretching at swath edges compared to nadir: indicating sloped terrain, improved classification
  • Maximum rate of acquiring digitised full waveform data is 120KHz. If a higher rate is used then the waveform data will record for every other emitted pulse. But the SCN files will still be recorded at the high rate.
  • Pulse length (sampling rate) should be chosen based on the maximum height of targets in the area. E.G. if all low lying targets then there is no point using 256 samples at 1 ns - better to use 128 at 1 ns.
  • No changes to the calibration flights except to collect FW data. Only change to the processing is to find the timing offset between FW and discrete points.
  • Pre-trigger - like a buffer zone to record digitised samples before the first return. Default is 5m.
  • Waveform and discrete data use different scalers for the intensity values and so will result in different intensities.
  • AGC is constant for a pulse - so the same value is used for the entire waveform

Software

  • ALSPP outputs in LAS 1.3 format which supports full waveform data (and also LAS 1.0)
    • To output in LAS v1.0 you must also select "LAS file with time stamp" in output options else no files will be output
  • ALSPP takes 10 times as long to process the data to produce a LAS file when using FWD
  • Includes a simple waveform viewer
    • Displays waveform and discrete points
    • There is a "constant" timing offset between the waveform and discrete points - thought to be due to processing time. This should be removed at calibration
  • New version of Terrascan capable of displaying full waveform data and "creating extra" points from the waveform

Something

  • When processing data collected after using FCMS v3.15 (anything after Jan 2010 ?)
    • ALS system refers to a virtual rotation axis (assumes a PAV80 stabilised mount even if you do not have one)
    • we need to update GPS lever arms in FCMS v3.15.
    • .SUP file will automatically offset to the mirror centre in ALSPP processing (Need IPAS version 1.35 for this)
    • Unsure as to whether leverarms need to be updated in IPAS or not - also how do we know IMU offsets to apply?

Changes to processing

  • To process full waveform data in ALSPP - Select "process waveform data" from the inputs menu and tick the box. Enter the "Trigger Delay" - this is the time offset between discrete and waveform data.
  • ALSPP looks for the "RawWfd" folder - this may imply that renaming the directory structure could cause problems.
  • ALSPP can limit the number of points it puts in a LAS file. This allows long flight lines to be split up into more manageable files.
  • Trigger delay needs to be derived - hopefully it remains constant throughout each flight line and whole flying season - this needs to be checked and monitored. Temperature may also affect it.
    • Will have 2 values - one for 4ns pulse width ("over TPR") and one for 9ns pulse width ("under TPR")
    • Use waveviewer tool with lines at 4ns to get the 4ns offset, and lines at 9ns to get the 9ns offset
    • Use points throughout flight line not just at start to derive the offset.
    • Procedure
      • In waveviewer "Ret point loc" is the discrete return time (from emit to receive) for the first return
      • intensity is the value for each discrete return
      • Measure the distance between discrete and full wave peak by zooming into the image and counting the time difference along the x-axis.