Changes between Version 2 and Version 3 of Sensors/LeicaLIDAR/MikesNotes
- Timestamp:
- Aug 22, 2008, 7:02:46 PM (16 years ago)
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Sensors/LeicaLIDAR/MikesNotes
v2 v3 17 17 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. 18 18 19 Water absorbs IR, so expect poor returns from wet surfaces. Ideally one should wait for a whole dry day after rain. 20 19 21 == Overall system design and comments of note == 20 22 … … 23 25 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. 24 26 25 The IPAS controller (name correct?) which allows for event recording, amongst other things.27 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. 26 28 27 29 There are also some associated control and display devices for operator and pilot usage. … … 29 31 === How it works === 30 32 31 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. 33 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. 32 34 33 35 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). 34 36 35 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 [physical or in software? probably s/w, cf camera anti-shake].37 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). 36 38 37 The laser is an 8W class 4 laser, operating in the infrared range. 39 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. 38 40 39 41 Controllable parameters: 40 42 * 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. 41 * altitude (kinda a parameter ;) ) - minmium of ~650 up to ~2000m (after 2km, you start getting poor returns on forests, etc )43 * 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) 42 44 * ... pulse frequency, scan angle, etc [TBD] 43 45 … … 68 70 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). 69 71 72 == Recommendations == 73 74 Perform calibrations frequently at the start of the period to get a feel for how they hold. Reduce down to fit circumstances over time.