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Geoinformatics_06-2009
article a space asset for non-space applications gnss-r starlab barcelona is the developer of oceanpal, a fully operational gnss-r system for water-oriented services (see figure 1). this article provides a brief description of this instrument and the range of water applications it offers: sea state monitoring and sea level monitoring, as well as inland water (i.e. lake) applications. moreover, recent research has evidenced the applicability of this instrument for soil moisture related applications and measurement of land bio-geophysical parameters. by alejandro egido, miquel garcia and marco caparrini measurement of the sea state and water surface roughness to soil moisture, salinity and vegetation monitoring. the following sections of this paper describe the applications that a gnss-r instrument like oceanpal offers. 2. sea state monitoring ocean significant wave height (swh), a typical parameter used to monitor sea state, can be calculated from measurements of the interferometric complex field (icf). the icf is defined as the ratio of the direct and reflected complex waveform peak time series. essentially, in icf the common components of the direct and reflected signals cancel out, leaving only the contribution due to the scattering of the sea/water surface. the presence of waves produces a temporal de-correlation of the sea surface, which turns into a de-correlation of the reflected signal. the parameter measuring this de-correlation is the surface coherence time, which is then linked to the swh through a semi-empirical model. over the last few years, this application has reached an operational maturity level providing quasi-real-time sea state information. a long-term comparison of the swh measurements with a co-located microwave radar (radac, data from which is publicly available on the internet) took place from the scheveningen pier (the netherlands) from december 2007 to november 2008. an rms error of 15 centimeters, with swh ranging from 0.5 to 3 meters, was obtained for the whole campaign. figure 3 shows two weeks of data (october 2008); the overall rms in the differential swh estimation of the two systems is 14 centimeters. figure 1: image of the oceanpal radio frequency unit, and the antenna rig. 1. gnss-r fundamentals back in 1993, the use of signals of opportunity from the gnss systems for earth observation, based on the analysis of the gnss signal reflected from the earth's surface, was proposed by m. martin-neira of the european space agency [1]: gnss reflectometry (gnss-r) was born. the fundamentals behind this concept are to use not only the direct gnss signals coming from the transmitting satellites, but also to process the same gnss signals reflected from the earth s surface (see figure 2 for the on-ground case). the direct and reflected signals are compared in order to extract some relevant geophysical information, especially related to water surface (inland or sea). in order to exploit this concept, a gnss-r instrument (oceanpal in particular) features a zenith-looking antenna that gathers the direct signal, and a nadir-looking antenna that collects the reflected signal. note from figure 2 that the arrival time difference between the two signals (i.e. lapse) conveys information on the distance from the antenna to the sea surface (i.e. height h). additionally, by analyzing the phase coherence level of the reflected signal's phase (i.e. coherence time), it is possible to infer the roughness of the water surface: a perfect specular reflection would preserve the coherence of the reflected signal, while a rough sea (with respect to the signal wavelength) would destroy the coherence of the reflected signal (i.e. the resulting reflected phase would be a random process that is not related at all to the phase of the direct signal). the passive nature of this concept allows for the production of cost- and resource-effective instruments. gnss makes use of l-band radiation and is thus highly interactive with the natural scattering medium but impervious to atmospheric conditions. potential applications exploiting these signals are numerous: from 3. altimetry applications water/sea level monitoring is based on the estimation of the altitude of the oceanpal antennas above the water/sea reflecting surface. this altitude is retrieved by the comparison of the delay (in time or distance) between the reflected and the direct signals. for this purpose, two techniques have been developed based on different characteristics of the reflected signals. the first is known as the phase altimetry, and has its application in inland, calm waters, while the second technique, code altimetry, is used for sea altimetry applications, in rougher waters. both altimetry algorithms have reached a maturity level that allows the provision of operational services to september 2009 40
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