FöRFATTARE
Hedman, Karin

DATUM
2004-05-28

INSTITUTION
Tillämpad fysik, maskin- och materialteknik / Experimentell mekanik

SAMMANFATTNING
Atmospheric refraction bends optical beams resulting in that an object appears in a position different from the real one. This phenomenon is very important in astronomy. Astronomical refraction can only be assumed if the light source is at a very far distance from the observer.

In case of optical free-space communication, the distances are comparatively short and well-known formulas for astronomical refraction are no longer sufficient. An exact knowledge of the structure of the whole atmosphere is required to assess refraction in this case. Due to the complexity of the atmosphere, analytical solutions are not possible. Hence, ray tracing based on the International Standard Atmosphere was used instead. A method of ray tracing applied for astronomical refraction was modified for non- astronomical objects. Two different categories of refraction were examined: 1) the observer is situated on ground and the object is at an altitude of at least 25 km and 2) both object and observer are below 25 km and the link path is mainly vertical. A simple approximation formula included the wavelength, ground temperature, ground pressure and the height of the homogeneous atmosphere was found. This formula is different for altitudes above and below 25 km.

The refraction for horizontal rays in the upper troposphere and lower stratosphere is examined. Since refraction varies with wavelength, rays of different wavelength are refracted differently and separate from each other. The size of the deviation of the rays is examined for three sets of typical laser wavelengths: 1) 800, 1064, 1550 nm and 2) 1470, 1530 and 1550 nm. The results of this part of the work are useful in the development of wavelength diversity. Optical free-space communication is negatively affected by turbulence in the atmosphere and wavelength diversity would reduce these effects.

The results presented in this work are useful for applications like laser beam pointing and satellite tracking, optical free-space communication between HAPs (High Altitude Platforms) and UAV (Unmanned Aerial Vehicles).

ISSN 1402-1617 / ISRN LTU-EX--04/172--SE / NR 2004:172