For our measurements, we use roughness function f(gl) = exp (?2(2��gl/��)2), which is a modification factor for the Fresnel ref
Barometric altimeters can be used for measuring the height of an object above a given reference level, e.g., the sea level��a popularly used term for this height is pressure altitude. Common applications are, e.g., in avionics, where barometric altimeters can help in stabilizing the vertical position and velocity of a flying object [1,2]. Recently, barometric altimeters have been considered for interesting human-centric applications, such as personal navigation and human motion tracking and monitoring. Beside their use as wristop computers for leisure and sports, barometric altimeters were integrated within multi-sensor pedestrian navigation systems and activity monitors, for improving performance of dead-reckoning algorithms or classifiers of human motion patterns: e.

g., they were used to detect the moving styles of going up/down the stairs or in an elevator [3]; to determine the correct floor of a user in a multi-storey building [4]; to detect stair ascent-descent in ambulatory monitors designed for estimating the energy expenditure incurred during activities of daily living, including stair-walking [5,6]. More recently, it was proposed that the measured pressure altitudes might help improving accelerometry-based fall detection systems [7�C10]. The rationale behind this application is that, when a person is standing, the difference in atmospheric air pressure between the waist and the feet can be monitored for sudden changes that would indicate a fall event [7].

However, rapid changes in pressure uncorrelated with altitude can be generated, outdoors, by unpredictable atmospheric conditions, and, indoors, by frequent opening/closing of windows or doors and by air conditioning systems [11]. The consequence is that barometric altimeter data tends to be very noisy, and their accuracy is rather poor.Recent advances in MEMS sensing technologies have led to the availability of barometric altimeters that are well suited for integration in mobile and portable systems. The most popular barometric altimeters for human-centric applications GSK-3 are manufactured by Bosch Sensortec (Reutlingen, Germany) [8�C12] and VTI Technologies (Vantaa, Finland) [6,7,9]. These sensors can be used in few measurement modes, which differ in the resolution offered for a given sampling rate.

Published recipes for pressure altitude signal conditioning and pre-processing do not go beyond generic prescriptions for low-pass filtering, e.g., in order to obtain an accuracy of 0.1 m using the barometric altimeter alone, several seconds of averaging would be necessary [12].In this paper we develop a method for capturing salient statistical properties of the noise that affects barometric altimeters (stochastic approach to noise modeling).