ground-based - airborne - moving
The portable Lidar POLIS has been developed at the Meteorological Institute at the University of Munich (MIM) in collaboration with the Ground Truth Center Oberbayern (GTCO). The lidar is designed in a modular way and consists of three parts that can be carried by one person. The set-up of POLIS is done at the Meteorological Institute. The sturdy design of the lidar allows a trouble-free utilisation during field campaigns at the ground as well as onboard an air-plane. Despite its small size, the lidar is powerful enough to cover the range from the overlap at 70 m above ground up to tropopause level.
Telescope
The goals of the telescope design were outstanding mechanical and optical stability and the easiness of alignment. This is achieved by the use of Dall-Kirkham-Cassegrain optics, designed and manufactured at the Optical Development Workshop of the Academy of Sciences of the Czech Republic (VOD), and a mechanical assembly (telescope tubus) developed especially for POLIS at the Meteorological Institute of the University of Munich. The focal length of the telescope (1196 mm), its diameter (200 mm) and the following optics are matched with respect to optimized performance, size, and cost. Together with the close mounting of the laser on the telecope, the lidar nearfield can be observed already from a distance of 70 m from the instrument, still using small bandwidth interference filters (down to 0.5 nm bandwidth).
Detection Channels
Due to the modular design of the detection channels, POLIS can either measure at the two backscatter wavelengths - 355 nm and 532 nm - or at one wavelength and its N2-Raman shifted wavelength - 355 nm and 387 nm -, or the depolarisation ratio at 532 nm or 355 nm. As a first step one channel was implemented and measurements were performed mainly at 355 nm. In the two-channel version depolarisation measurements at 355 nm startet in autumn 2004.
Mobility of the System
POLIS can be driven from ground, from air-plane, and from a driving van. Provisions are made to assure the mechanical stability, the alignment and protection of the optics during such field campaigns. The telescope-optics assembly is watertight with pressure equalisation and dry air purge facility for airborne operation. The secondary mirror is glued directly onto the front window, which protects the optics from pollution. The Dall-Kirkham concept of the telescope makes its alignment very simple and therefore stable. The mechanical parts of the optics are precisely manufactured with very low tolerances and are made of high quality material. The system proved its stability during several field campaigns onboard air-planes and on the street.
Further technical details on POLIS can be found in the publications (further down).
International Laser Radar Conferences: