Normalized Differential Spectral Attenuation (NDSA) Measurements Between Two LEO Satellites: Performance Analysis in the Ku/K-Bands

Normalized differential spectral attenuation (NDSA) is a novel differential measurement method to estimate the total content of water vapor [integrated water vapor (IWV)] along a tropospheric propagation path between two low Earth orbit (LEO) satellites. A transmitter onboard the first LEO satellite and a receiver onboard the second one are required. The NDSA approach is based on the simultaneous estimates of the total attenuation at two relatively close frequencies in the Ku/K-bands and of a "spectral sensitivity parameter" that can be directly converted into IWV. The spectral sensitivity has the potential to determine the water vapor contribution, to cancel out all spectrally flat unwanted contributions, and to limit the impairments due to tropospheric scintillation. In this paper, we focus on the measurement accuracy of the spectral sensitivity parameter. Specifically, we examine this accuracy at three different frequencies and for two models of atmospheric structure. We first provide an approximate expression of the accuracy and then validate this expression through Monte Carlo simulations based on microwave propagation models.