Airborne measurements of ground reflectance at 1.6 μm

摘要

Active remote sensing using lidar appears to be very attractive for the measurement of atmospheric greenhouse gases like carbon dioxide from spaceborne platforms. Feasibility studies are currently being performed to demonstrate the required measurement performance. Due to the high precision required (less than 0.3 %) for climate studies, space-borne IPDA (Integrating Path Differential Absorption) Lidar is preferred over the range resolving DIAL technique which uses atmospheric backscatter. This is due to the larger Lidar echoes from hard target when using systems of comparable size. Applying the IPDA Lidar method, magnitude and variability of the ground reflectance becomes an important issue in terms of instrument sizing and pointing requirements of space-borne systems. Because of the stringent sensitivity requirements, even small gradients of the ground reflectance could introduce noticeable retrieval errors in the CO2 column content, when the laser transmitter does not point on the same ground spot for the on- and off-line measurement. However, the current knowledge on the variability of the ground reflectance both in the appropriate wavelength range and on small spatial scales is insufficient for an accurate error assessment. In order to address these deficiencies, airborne lidar measurements at 1.6 μm wavelength were performed. The wavelength range around 1.6 μm provides suitable absorption lines for the measurement of carbon dioxide. A pulsed optical parametric oscillator (OPO) system (5 mJ at 1573 nm, 10 Hz pulse rate) was deployed on the DLR Cessna Caravan aircraft to measure the variations of the ground return. In order to simulate a satellite system, statistical analyses on the data including upscaling to a larger ground spot size of a space-borne system and different averaging ranges are being performed. The focus of this study is on the investigation of the characteristics of typical surface types including the open sea.