LIDAR processing notes

Notes from visit to Leica, Switzerland on 11/Aug/2008.

Contact email:

• sensors_emea@…
• Various support engineers at Leica. The one that trained us was Barbara Haebler.

FTP site (for updates, etc):

• ​ftp://airborne:LsA36GmW@ftp.leica-geosystems/airborne/
• problem uploads to go to incoming/ dir

Things that may go wrong in flight

Flying too low - if the LIDAR detects that the laser power may be too high for eye safety, it will cut out the laser automatically (if you're a seagull looking up, bad luck, it only accounts for ground height).

Flying outside the "range gate" (acceptable ranges of distances) may cause similar effects. Going too high will tend to make the edges of the swath drop out first (due to path length).

Automatic gain control weirdness - the measured intensity is returned via an AGC which may step up or down depending on the returns from the ground. Measured return intensity should only be used as a guideline rather than a real measurement.

Water absorbs IR, so expect poor returns from wet surfaces. Ideally one should wait for a whole dry day after rain.

Mission planning considerations

Water absorbs IR, so expect poor returns from wet surfaces. Ideally one should wait for a whole dry day after rain.

For differential GPS, aim to have a base station nearby (within 20km) and ideally in center of the scene. Base station data should be recorded at 2Hz

Ideally choose a time with a good GPS constellation (lots of satellites, PDOP >= 4).

Consider terrain and minimum target size to determine required point density. Steep terrain may cause shadowing effects due to perspective.

Overall system design and comments of note

System is a Leica ALS50 (phase II) LIDAR. There is an accompanying 39 megapixel [7216x5412, 12bit] digital camera, referred to as the "RCD".

A GPS receiver and IMU (brand?) are included. The LIDAR, RCD and IMU, are mounted on a shock plate to protect it from strong movements - this will also isolate the IMU from the hyperspectral imaging sensors.

The IPAS controller (name correct?) allows for event recording, amongst other things. Phil plans to use this to record frame sync pulses from Eagle and Hawk.

There are also some associated control and display devices for operator and pilot usage.

How it works

The LIDAR works by firing a laser pulse downwards and measuring the roundtrip time for the light pulse to return, then converting this to a distance. The pulse isn't modulated by a carrier - it's just an on/off pulse. There are four timing cards ("range cards"), so up to 4 return pulses can be detected, with the intensity of the return measured only on the first 3 returns. A minimum time separation between two returns means the minimum distance between two returns must be at least 2.7m for them to be counted as independent. The expectation for the number of returns is 1 return ~100%, 2 returns ~10%, 3 returns ~1%, 4 returns ~0.1% of points - obviously this varies with the terrain.

There is also a "MPiA" mode (Multiple Pulses in the Air), which fires two pulses evenly separated, rather than waiting for the first to come back (SPiA mode) before firing another [times out in case the pulse is eaten]. If a seagull gets in the way of the second pulse before the first pulse has returned, things will mess up ;) (on an edge of a very unluckily placed cloud, this would look a bit like it merging into the ground).

The laser is scanned across a (up-to) 75 degree swath by an oscillating mirror. Due to the acceleration/deceleration of the mirror, this produces a sinusoidal pattern to the trace on the ground, with the highest density of points at the peak and trough of the sine wave (i.e. at the edges of the swath). If the swath width is set to less than 75 degrees (45 degrees recommended), there's a roll compensation mechanism that tries to smooth out small roll movements by using the remaining freedom of motion. The scan rate of the mirror is dependent on the FOV angle (36Hz for 40 degree FOV, 24Hz for a 75 degree FOV, calculate with 412.33 * FOV^-0.6548).

The laser is an 8W class 4 laser, operating in the infrared range. The divergence results in approximately a 22cm spot on the ground when fired from 1000m up.

Controllable parameters:

• laser intensity (0 -> 8W output), controlled by operator as a percentage output. Has safety cutouts if the light level at the ground could cause eye damage.
• altitude (kinda a parameter ;) ) - minmium of ~650 up to ~2000m (after 2km, you start getting poor returns on forests, etc, the real limit is up to about 6km in ideal conditions)
• ... pulse frequency, scan angle, etc [TBD]

Required measurements for processing:

• GPS and IMU data (including a basestation if not using PPP)
• Pressure and temperature measurements at the plane position above the site [this affects how long it takes the laser light to move through the air]

Calibration site requirements

Flight pattern 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.

The features required for a perfect cal site are:

• Need a source of multiple returns - tallish (15m) trees in a forest are best. Try to include a treed/forested area in some parts of the flight lines (doesn't have to be in all, nor in the central area). The multiple returns are required for the range card calibration (see below).
• Straight, flat areas made of a hard substance that'll generate only one return pulse. Roads or runways ;) These are used in the boresight and range calibrations.
• Sloping peaked areas (house rooftops are ideal) with the peak cutting across the line of flight ( ---> /\ ). These are used to detect pitch and yaw errors in the boresight calibration.
• An accurate ground survey (see below).
• Slow overflight for maximum point density.

For range offset calculation

GCPs required, ideally 1cm vertical accuracy (between all measurements rather than absolute?).

Around 30-40 GCPs should be within the area covered by a 14 degree swath (+/- 7 degrees either side of nadir) taken at an altitude of ~750m [= ground swath width of about 180m].

General accuracy measurements

Other GCPs (number?) should be scattered around a wider area within the full swath width - typically run at 45 degrees [=620m wide on ground @ 750m alt] or to a max of 75 degrees [=1150m], though 75 degrees will introduce more errors..

Quality, accuracy, etc

A nominal quality of 5-10cm (vertical) is suggested as a reasonable output. If one has GCPs, it should be possible to do better (EA claims 1-2cm).

Recommendations

Perform calibrations frequently at the start of the period to get a feel for how they hold. Reduce down to fit circumstances over time.