Measuring tropospheric water vapor by normalized differential power measurements: an adaptive approach

The NDSA (normalized differential spectral absorption) method is a novel differential measurement way for estimating the total content of water vapor (IWV, integrated water vapor) along a tropospheric propagation path between two low Earth orbit (LEO) satellites. NDSA is based on the simultaneous measurement of the total attenuation at two relatively close frequencies in the Ku/K bands, and on the estimate of a "spectral sensitivity parameter" that can be directly converted into IWV. NDSA is potentially able to emphasize the water vapor contribution, to cancel out all spectrally flat unwanted contributions and to limit the impairments due to tropospheric scintillation. NDSA performance may change with the altitude at which the radio path is located with respect to the Earth. Therefore, after having examined through some simulations accounting for thermal noise at the receiver and tropospheric scintillation effects its measurement performance, we propose here an adaptive approach to the NDSA method in the case of two counter-rotating LEO satellites, based on the estimation of the signal to noise ratio and the change of the central frequency.