GPS World, September 2010

INNOVATION Remote Sensing by our organization in collaboration with Pontificia Universidad Católica de Chile the Pontifical Catholic University of Chile and funded by ENDESA Empresa Nacional de Electricidad S A one of the worlds largest electricity companies An Oceanpal instrument was installed at Lake Laja in the Biobío Region Chile a water reservoir managed by ENDESA Chile The Hydro project aims to use remote sensing assets to predict and monitor water flow in the Laja River basin For that having precise measurements of Lake Lajas level is a must The instrument was installed on the shore of the lake as seen in FIGURE 4 However the high variability of the lakes level more than 10 meters in one year and the abruptness of the terrain results in the number of observed reflections from the water surface being quite low This is especially the case when the level of the lake is low In this situation the number of different GPS satellites observed per hour was calculated to be fewer than two for more than 45 percent of the time and fewer than three for more than 85 percent of the time Given this scarcity of reflections we could not use the phase altimetry algorithm as described above We developed a new phase altimetry algorithm which considers the interferometric phase evolution over time The resulting relative phase parameter can be linked to the height of the antennas over the water surface by means of the same geometrical relationship as before Despite the fact that measuring a relative phase increases the measurement noise with respect to the case in which an absolute phase is used the phase ambiguity and the bias between the direct and reflected receiving channels do not need to be calculated thus reducing the complexity of the algorithm and its convergence requirements A Kalman filter is used to smooth the inherently noisy behavior of the relative phase The Oceanpal measurements were compared to those of a sensor operated by the Dirección General de Aguas DGA the Chilean water management agency An accuracy better than 9 centimeters was achieved in determining the lakes level during the austral winter when the lake is at its minimum level and therefore the satellites reflections from the water surface are scarce The lake level has its maximum during the summer after the melting season During this period of time the achieved accuracy of Oceanpal with the new phase algorithm was better than 5 centimeters A comparison of Oceanpal and DGAs sensor measurements of the water level is shown in FIGURE 5 Measuring Sea Level Sea level is obtained from Oceanpal measurements by means of the code altimetry algorithm due to the inherent roughness of the sea surface This technique derives altimetric information from the displacement of reflected waveforms with respect to the direct ones Such a displacement can be directly related to the delay between the direct and reflected signals the so called lapse and is used in a similar way to the phase based 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 subject to the FIGURE 4 The Oceanpal installation at Lake Laja Chile DGA sensor Oceanpal 13305 13300 13295 13290 13285 13280 13275 13270 13265 Lake height meters a 05 04 2009 05 05 2009 04 06 2009 04 07 2009 03 08 2009 Date dd mm yyyy DGA sensor Oceanpal 13365 13360 13355 13350 13345 13340 Lake height meters b 11 11 2009 09 12 2009 06 01 2010 03 02 2010 Date dd mm yyyy FIGURE 5 A comparison of measurements of Lake Lajas water level by Oceanpal and a water bubbler sensor operated by Dirección General de Aguas DGA for two periods of time corresponding to a the austral winter from late April 2009 until early August 2009 and b the austral summer from late November 2009 until late January 2010 GPS World September 2010 www gpsworld com 54