This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. IEEE GEOSCIENCE AND REMOTE SENSING LETTERS 1 Snow Thickness Monitoring Using GNSS Measurements Nereida Rodriguez-Alvarez, Student Member, IEEE, Albert Aguasca, Member, IEEE, Enric Valencia, Student Member, IEEE, Xavier Bosch-Lluis, Student Member, IEEE, Adriano Camps, Fellow, IEEE, Isaac Ramos-Perez, Student Member, IEEE, Hyuk Park, Member, IEEE, and Merce Vall-llossera, Member, IEEE Abstract—Global navigation satellite system reflectometry has already shown its potential to perform retrievals of a number of geophysical parameters, including soil moisture, vegetation height, etc. This letter focuses on the study of snow-covered soils using the interference pattern technique (IPT) with the Soil Moisture Interference-pattern GNSS Observations at L-band (SMIGOL) reflectometer, a ground-based instrument. Snow effects are an- alyzed, and an algorithm has been developed for this type of surfaces. From November 2010 to May 2011, a long-term field experiment was carried out at the Pyrenees (Val d’Aran, Lleida, Spain) to test the IPT and the retrieval algorithms on snow- covered soils. Index Terms—Global navigation satellite systems (GNSS), reflectometry, snow, thickness monitoring. I. I NTRODUCTION G LOBAL navigation satellite system reflectometry (GNSS-R) has demonstrated to be useful in many geophysical parameter retrievals [1]–[3]. In the past years, the interference pattern technique (IPT) was developed and tested to retrieve land geophysical parameters [4]–[6]. Other GNSS-R techniques have already been used to retrieve snow [7], [8] and ice [9], [10] geophysical parameters. During this time, the main IPT theoretical aspects have been developed and implemented in retrieval algorithms for soil moisture, surface topography, and vegetation height. Based on the experience gained on veg- etation height retrievals, and realizing that the vegetation layer could be simply modeled by a single layer with a specific di- electric constant, the concept was extended to the observation of snow-covered surfaces. This letter presents the measurements of the SMIGOL reflectometer, located at a meteorological station in the Pyrenees during the winter season of 2011. II. THEORETICAL ASPECTS The IPT mainly consists of the measurement of the inter- ference power pattern between the GNSS (a particular GPS) Manuscript received December 2, 2011; revised February 6, 2012; accepted February 26, 2012. This work was supported in part by research projects ESP2007-65567-C04-02 (Spanish National Research fellowship with refer- ence BES-2008-001902), by AYA2008-05906-C02-01/ESP, and by AYA2010- 22062-C05-05/ESP. The authors are with the Remote Sensing Laboratory, Department of Signal Theory and Communications, and the Institut d’Estudis Espacials de Catalunya (IEEC), Aerospace and Research Center (CRAE), Universitat Politècnica de Catalunya, 08034 Barcelona, Spain. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LGRS.2012.2190379 Fig. 1. Theoretical interference powers for a snow-covered flat soil with (a) 5- and (b) 40-cm snow thicknesses. The snow model used follows the work in [11]. direct and reflected signals, after impinging over the snow- covered soil, which has been modeled using [11]. The received power can be expressed as a function of the elevation angle due to the fact that satellites are moving and changing their position (elevation and azimuth coordinates). The interference power has an oscillating pattern in which notches (minimum amplitude oscillations) are relevant information points about the layer thickness (Fig. 1). These notches are related to the interaction of the GPS signal with the finite layer (snow layer); the thicker the layer is, the larger the reflectivity change (a greater number of notches). As previously observed in [5] and [6], where the vegetation height was directly linked to the number of notches and their positions in the interference power pattern, over the snow layer, the snow thickness can also be inferred from the number and position of the notches. Theoretical simulations similar to the ones in Fig. 1 1545-598X/$31.00 © 2012 IEEE