First Results with Two-Colour Satellite Laser Ranging at the Fundamentalstation Wettzell
Atmospheric refraction is a factor which is limiting the accuracy in laser ranging. Models such as that from Marini and Murray [Marini, Murray, 1973] have to be applied for the calculation of the atmospheric correction. These models usually assume rotational symmetric athmospheric layers which are characterized by functions of temperature, pressure and humidity. The corrections are dependant on the length of the propagation path through the atmosphere and on the wavelength of the transmitted laser pulse. The propagation path can be parameterized by the elevation of the propagation direction. The accuracy of such models is limited to several centimeters. This is due to the model itself and of course to the accuracy of the input values. The insitu observations of pressure, temperature and humidity do not represent the true situation of the atmospheric layers. Improvements are expected by synchronous ranging at two or more frequencies (two- or multicolour laser ranging). The dispersion of the atmosphere causes slight differences in the propagation time of the various wavelengths. Such differencies can be used for deriving the atmospheric refraction corrections. At 90 degree elevation for the looking angle the refraction causes a delay of about 16 ns for the wavelength of 532 nm, the dispersion effect between 532 nm (green) and 1064 nm is about 0.8 ns. To derive a refraction correction with an accuracy of only 2 cm the seperation between both frequencies has to be measured with an accuracy of better than 1 mm. The provision of such precise dual-or multicolour ranges still suffers from the technical realization.
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