Changes between Version 15 and Version 16 of Sensors/LeicaLIDAR/MikesNotes

Timestamp:

Aug 28, 2008, 6:28:44 PM (16 years ago)

Author:

mggr

Comment:

--

Sensors/LeicaLIDAR/MikesNotes

@@ -50 +50 @@
 Consider terrain and minimum target size to determine required point density.  Steep terrain may cause shadowing effects due to perspective.
 
-Consider reflectivity of surface - lighter surfaces need less laser power.  Too high a power means intensity overflows on the reflection, which mess up the range - AGC should deal with this for large areas but it should be considered in planning, particularly for small bright targets on a generally darker background.
+Consider reflectivity of surface - lighter surfaces need less laser power.  Too high a power means intensity overflows on the reflection, which mess up the range - AGC should deal with this for large areas but it should be considered in planning, particularly for small bright targets on a generally darker background.  Assume 10% reflectivity is typical.
 
 Recommended to do a figure 8 loop at the start and end of the data acquisitions to make the IMU happy.  Need one at the end for reverse navigation processing.
@@ -183 +183 @@
 
 
-
 == Calibration site requirements ==
 
@@ -193 +192 @@
  * Sloping areas are used to detect pitch and heading errors in the boresight calibration.
    * A road running up a hill is good for pitch (vertical change in surface easily found in the image).  These are good for measuring along-track shifts.
-   * Slopes with peaks (house rooftops are ideal) with the peak cutting across the line of flight ( --->  /\ ).  Again good for measuring along-track shifts.
+   * Slopes with peaks (house rooftops are ideal) with the peak cutting across the line of flight ( `--->  /\` ).  Again good for measuring along-track shifts.
  * An accurate ground survey (see below).
  * Slow overflight for maximum point density.
@@ -236 +235 @@
 
 = Boresight calibration =
-TBD.
-
-Boresight parameters (pitch, roll, heading).
- * Angle between straight-down and what the sensor thinks is straight-down, as it's mounted in the plane.
-   * Angle between a line from the sensor head (mirror centre) to the point on the ground at the centre of the swath and a line from the sensor head to the centre of the spheroid (or the reference frame's Z axis?).
- * Measured in calibration procedure
-   * '''We need to do this'''
-
-Pitch error slope.
- * The mirror will not be mounted exactly flat to the laser so, as the mirror moves, the pitch of the beam will change by a small amount.
-   * Roll and yaw either have no error slope (laser position central? geometry means no effect?) or a negligible effect (presumably, as there's no parameter)
- * Measure pitch at nadir and at swath edges to determine how the pitch changes - the first order for this is is the pitch error slope.
- * Measured in calibration procedure
-   * Correct pitch error (at nadir) first
-   * '''We need to do this''' 
+
+The basic method is similar to the imaging instruments, but exploits the measurement of range.  A set of flight lines and data points are chosen, in a particular order, such that specific calibration parameters are isolated.
+
+The flight pattern for boresight calibration is a cross made of 4 opposing flight lines in the 2 directions (e.g. N->S v S->N + W->E v E->W), and one set of parallel lines with 50%(?) overlap.
+
+
+== Manual procedure and QC of automatic procedure ==
+
+=== Pitch ===
+
+Take opposing flightlines.  If there is a pitch error, the lidar will be sensing pulses ahead/behind (along track) the nadir point.  There will be a small overall range error, but the main effect will be that the data is shifted backwards/forwards of where it should be.
+
+Search for areas close to nadir where the height of the surface changes in an clear and predictable fashion, with reasonable sampling density (e.g. a road leading up a hill, or rooftops with their peaks going across-track).
+
+Take along-track profiles on both opposing flightlines.
+ * If there is no pitch error, there will be no offset and the resulting profiles will coincide (the hill profiles will overlay and look like `/`).
+ * If there is a pitch error, the profiles will be offset along track and the resulting profiles will not coincide (the hill profiles won't overlay and will look like `/  /`).  By measuring the along-track offset, one can estimate the pitch error.
+
+One must use profiles close to nadir to avoid being affected by the pitch-error-slope parameter (see below).
+
+Repeat until the pitch error has been removed.
+
+=== Roll ===
+
+
+
+=== Pitch error slope ===
+
+The mirror will not be mounted exactly flat to the laser so, as the mirror moves, the pitch of the beam will change by a small amount.  The resulting errors are along-track offsets just like the pitch errors, but varying, with no error at nadir and maximum error at the edges of the swath.
+
+To correct this, first correct the pitch error at nadir (the overall pitch error), then determine the pitch error at the edges of the swath.  This will let you compute an error slope (first order linear).
+
  * Straight, flat areas made of a hard substance that'll generate only one return pulse.  Roads or runways ;)  These are used in the boresight (roll) and range calibrations.
  * Sloping areas are used to detect pitch and heading errors in the boresight calibration.