This article is dedicated to the estimation of the amplitudes of GNSS satellites signals and, consequently, to their Signal to Noise Ratio. The in-phase component calculated by a one bit quantization receiver is a direct observation of the amplitude. In this article we propose a linear model that links 1 ms rate observations of the in-phase component of a GNSS receiver with the amplitudes of the received satellites signals. This model integrates the CDMA code and the carrier of the signals of all the satellites in view, thus considering the signals cross-correlations. A Kalman filter is designed in order to estimate the amplitudes of these signals at high rate. The proposed amplitude estimator is applied to GNSS-Reflectometry for retrieving the reflection coefficient of Earth surfaces. A set of experiments have been conducted for estimating a variety of sand moistures on a beach. It is shown that the proposed high-rate amplitude estimator can be of particular importance for kinematic GNSS-R setups.