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Geoinformatics_06-2009
article figure 2: measurement principle of oceanpal sensor: the direct signal coming from gnss satellites is gathered as well as the reflected signal coming from the sea/water surface. customers. nevertheless, our research efforts continue in order to improve the precision and accuracy of the algorithms. the next sections review the fundamentals of both techniques. 3.1. phase altimetry algorithm the phase of the icf contains information on the geometrical difference of the direct and reflected signal paths, which is the fundamental parameter used in this algorithm. after a phase unwrapping process and an ambiguity resolution method, the phase information is linked to the altimetry information through a geometrical relationship. the oceanpal experiment at the labaells reservoir (situated near berga, in the north of catalonia (spain), and managed by the catalan water authorities) was undertaken to study the possibility of accurate altimetry measurements with this technique in inland waters. in this campaign, one week of data was gathered in early march 2008 to compare oceanpal gnss-r phase altimetry measurements with the labaells in-situ sensor (a pressure bubbler known to have centimeter precision). after bias removal, the obtained differential standard deviation is lower than 2 centimeters (after 5 minute integration time). even though phase altimetry is a very precise technique, in order to be able to retrieve profitable phase information out of the reflected signal, the signal needs to be coherent. that is, the signal needs to be reflected from a moderately calm and flat surface. for this reason, the phase altimetry algorithm is applied to inland waters, such as lakes and water reservoirs, where the height of waves is moderate, and where the required precision is very high. 4. soil moisture and vegetation monitoring as with other remote sensing techniques, soil moisture observations in gnss-r are based on the variability of the soil s dielectric properties with soil moisture. the relative power of direct and reflected gnss signals gives a measurement of the soil reflectivity, which is then used to estimate the soil s dielectric constant through a simplified scattering model. the volumetric soil moisture (vsm) can be calculated with a semi-empirical model that establishes a quadratic relationship at l-band between dielectric constant and vsm. nevertheless, some constraints need to be taken into account when estimating land biogeophysical parameters. first, the soil roughness introduces a coupled effect with soil moisture, which can bias the estimation. and second, variations in background temperature can lead to variations in the waveform peak s power, which can also lead to error in the determination of our target parameters. in order to be able to cope with these handicaps, some modifications were introduced in the oceanpal instrument. the most important ones were the integration of a calibration chain in order to be able to account for small power mismatches among receiving channels, and the introduction of an additional antenna for the reflected signal. the two reflected antennas are right and left hand circular polarised, rhcp and lhcp, allowing for polarimetric analysis of the reflected signal. in order to test the capabilities of soil moisture detection with a gnss-r receiver, we performed an airborne campaign in june 2008 over los monegros, a semi-arid area near zaragoza, spain, covering several square kilometers. soil samples were taken from the observation areas september 2009 3.2. code altimetry algorithm the code altimetry algorithm derives altimetric information from the displacement of the reflected waveform with respect to the direct one. such a displacement can be directly related to the direct and reflected signals delay (i.e. the lapse), and is used, in a similar way to the previous method, to extract the altimetry information of the water surface being monitored. despite the fact that the code altimetry algorithm is not as precise as the phase altimetry algorithm, it is not subjected to the coherence requirement for the reflected signal. therefore it can be applied in rough, dynamic surfaces such as open ocean and coastal areas. the use of code altimetry in rough water conditions results in a clear observation of tide dynamics, but as expected with a higher error range compared to the situations where phase altimetry can be applied. this was demonstrated comparing oceanpal altimetry measurements in scheveningen pier with the collocated radar. an error standard deviation of 12 centimeters was obtained with tidal amplitudes close to 2 meters. figure 3: oceanpal and radac swh measurements in the scheveningen pier, the hague, the netherlands. october 2008. latest news? visit www.geoinformatics.com 41
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