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

Timestamp:

Feb 8, 2010, 12:29:02 PM (15 years ago)

Author:

mggr

Comment:

--

Sensors/LeicaLIDAR/FWMikesNotes

@@ -12 +12 @@
 }}}
 
-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).
+The 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).
 
 The 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.
 
-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.
+The 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.
 
 Similarly, 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.
@@ -25 +25 @@
  - normal lidar returns based on threshold, then look for peaks
   - requires a non-peak gap after a peak is found, so double peaks not spottable without  fwd
-
-intensity is digitised to 8bit
 
 FWD filestructure:
@@ -113 +111 @@
   - minimised L/R imbalance, improved radiometric using ccd border area (hrm)
 
-AGC setting constant for a single pulse (ie. only need the one recorded in SCN)
- - confusing slide on intensity scale relating to voltage..  not quite sure what value is recorded (raw or agc scaled)
- - note intensity scales are different for standard returns and fwd
-
 things one can detect.. maybe..
  - pulse stretching (return pulse longer than laser pulse
@@ -125 +119 @@
 
 angle of laser may make a difference  (flat ground at nadir is a slope at edge of fov, or cutting through vegetation)
-
-hardware changes
- - new data logger (cope with higher io)
-  - hdd -> 160GB SSD, works up to 7620m altitude
-   - should be ok to use spinning disk for bigger datasets @ low altitude
-   - can't swap disks in flight without full shutdown of logger, but can keep ipas going without recording
- - fwd boards, 2 of them at 60kHz each
 
 software changes
@@ -141 +128 @@
  - alspp outputting in las 1.3 type 4 (option to output 1.0 if needed for compat)
  - version list in presentation
-
-if pulse rate > 120kHz, then FWD on every second pulse..  e.g. 150kHz SCN file = 75kHz FWD
 
 where did gain come from in waveform viewer
@@ -168 +153 @@
  - drag and drop a log file into it to get nicely parsed info
 
-data rate / height stuff
- - 19.2 / 38.4 / 76.8m for various heights
- - 1h04 recording at max rate on 160GB SSD
+
 
 terrascan
@@ -230 +213 @@
     || 128 @ 2ns   || 38.4m ||
     || 64 @ 2ns    || 19.2m ||
+
+data rate / height stuff
+ - 19.2 / 38.4 / 76.8m for various heights
+ - 1h04 recording at max rate on 160GB SSD
 
  * Benefits of full waveform