Multi-wavelength airborne laser scanning

1. Multi-wavelength airborne laser scanning ILMF 2011, New Orleans Dr. Andreas Ullrich CTO, RIEGL LMS GmbH

2. introduction: components of ALS systems • full waveform analysis vs. online waveform processing • primary and secondary ALS data products • discussion multi-spectral, hyper-spectral, multi-wavelength • selection criteria for laser wavelength • availability of laser sources • target properties • signal attenuation, background radiation • laser safety • classification of multi-wavelength data / systems • conclusions contents

3. RIEGL LMS-Q680i RIEGLVQ-820-G RIEGL VQ-580 RiACQUIRE DA42-MPP RIEGL DR-680 RiPROCESS IMU & GPS Flight Guidance components of ALS systems

4. state-of-the-art echo waveform digitizing systems RIEGL VQ-580 A RIEGL VQ-820-G dev A W R R Q-560/Q-680i Full Waveform analysis range: R[m] amplitude: A[LSB and linearized] echo width: W [ns] On-Line Waveform Processing range: R[m] calibrated amplitude: A[dB]calibrated reflectance: r [dB] pulse shape deviation: dev [1]

5. RIEGL LMS-Q680i, wavelength 1550 nm dry conditions wet snow primary data: point cloud

6. RIEGL VQ-580 wavelength 1064 nm amplitude in dB above detection threshold RIEGL VQ-580 wavelength 1064 nm reflectance in dB above white diffusely reflecting target RIEGL VQ-580 wavelength 1064 nm pulse shape deviation from expected pulse shape primary data: point cloud

7. 1064 nm visible visible 532 nm 1064 nm 1550 nm 1550 nm 532 nm images at different wavelengths

8. actual geometric cross-section of target interacting with laser beam directivity of backscatteredreflection reflectance Laser Radar Cross Section (LRCS) • cross section  in [m²] • area-normalized cross section values in [m²m-2] or [dB] • by laser footprint area:  • by illuminated object area: 0 Radiometric calibration of small-footprint airborne laser scanner measurements: Basic physical concepts, Wagner, W.,ISPRS Journal of Photogrammetry and Remote Sensing, 65, 2010. radiometric calibration

9. radiometric calibration

10. multispectralimaging hyperspectralimaging multi-wavelengthlidar 532 nm 905 nm 1064 nm 1550 nm hyperspectrallidar supercontinuum laser (500 nm – 2400 nm) array of receiver channels and ROIC 400 nm 800 nm 1200 nm 1600 nm multispectral/hyperspectral imaging vs. multi-wavelength ALS

11. pulsed time-of-flight laser ranging: best performance wrt maximum range, measurement speed, ranging precision and accuracy • selection of wavelength • availability of suitable laser and detector • reflectance of objects • attenuation of atmosphere and background radiation • laser safety • laser requirements • short pulse width (multi-target resolution, high precision) • high peak power (maximum range) • good beam quality (beam divergence, spatial resolution) • high pulse repetition rate (point density) • narrow spectral width (background rejection) • detector requirements • high bandwidth (corresponds to pulse width) • high sensitivity (maximum range) • low noise (high precision) wavelength selection criteria for ALS sensors

12. 200 400 600 800 1000 1200 1400 1600 1800 2000 diode lasers, 905 nm solid state lasers (fundamental wavelength), Nd:YAG, 1064 nm solid state lasers (harmonics), Nd:YAG, 532 nm, (355 nm) fiber lasers, Er-doped, 1.55 µm fiber lasers, Yt-doped, 1.06 µm fiber lasers, Ho-doped, 2.05 µm frequency-doubled fiber lasers, 532 nm UV INFRARED diode 905 nm solid state 355 nm 532 nm 1064 nm fiber 532 nm 1064 nm 1550 nm 2050 nm suitable laser sources

13. 532 nm 905 nm 1064 nm 1550 nm relative reflectance [%] wavelength [µm] target reflectance versus wavelength

14. 532 nm 1064 nm 905 nm 1550 nm solar spectral irradiance at zenith sun angle 60° at sea level 1400 corresponds to spectrum of sun light absorption due to ozone (O3) , water vapor (H2O), oxygen (O2), carbon dioxide (C02) 1200 1000 800 solar irradiance [W/m²µm] 600 400 200 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 wavelength [µm] background radiation versus wavelength

15. atmospheric transmission 20 km, one wayvisibility 23 km, 10 km, 5 km 905 nm 1064 nm 532 nm 1550 nm transmittance of 1000 feet horizontal air path (sea level) transmittance [%] wavelength [µm] atmospheric attenuation versus wavelength

16. ultraviolet visible infrared 10 000 1 000 100 100 dB 50 dB 10 10 dB 1 100 dB 1 dB absorption coefficient [cm-1] 53 dB 0.53 dB 0.1 10 dB 0.1 dB 0.01 100 dB 1 dB 0.01 dB 0.001 10 dB 0.1 dB 0.0001 3.8 dB 0.038 dB 1 dB 0.01 dB 0.1 0.2 0.4 0.6 0.8 1.0 2.0 4.0 6.0 8.0 10 wavelength [µm] absorption coefficient of clear seawater attenuation at depth10 m attenuation at depth0.1 m attenuation at depth1 mm attenuation in water versus wavelength

17. 1550 nm 1064 nm 905 nm 532 nm 355 nm MPE:maximum permissible exposure parameter: exposure duration / pulse width laser safety considerations

18. NOHDeNOHD NOHD, eNOHD NOHD eNOHD RIEGLLMS-Q680i @ 80kHz RIEGLVQ-580 @ 50kHz RIEGLVQ-820-G @ 100kHz Laser Safety Standards 0m 1.5m 15m 80m 10m 105m 500m 1600m 1600m 1600m max. range @ reflectance 20% max. range @ reflectance 80% 2000m 2000m 2000m Range [m] NOHD (nominal ocular hazard distance): distance beyond which exposure becomes less than maximum permissible exposure (MPE) extended NOHD: includes the possibility of optically-aided viewing Laser Classes / NOHD / ENOHD

19. increasing sensor/system complexity increasing flexibility classification of multi-wavelength ALS

20. select scanner model (wavelength) according to target characteristics, mission requirements, laser safety requirements, ... wide variety of applications covered by eye-safe 1550 nm ALS scanners (e.g., RIEGL LMS-680i and RIEGL VQ-480) • for special applications, e.g., forest health investigations integrate two or more scanners with different wavelength on a single platform  providing flexible “multi-wavelength” system (e.g., RIEGL VQ-480 at 1550 nm and RIEGL VQ-580 at 1064 nm) • for hydrography, ad 532 nm LIDAR • regardless of wavelength: echo-digitizing pulsed time-of-flight systems provide utmost accuracy, multi-target resolution and calibrated (calibratable) amplitudes and target’s cross-section conclusions