GPS World, September 2010

Remote Sensing INNOVATION RF receiver 1 RF receiver 2 SW GPS slave SW GPS master Processing Web interface GNSS satellite Sea surface RHCP antenna LHCP antenna FIGURE 1 Basic operation of Oceanpal and the principle of GNSS R based sea surface monitoring Right hand and left hand circularly polarized antennas feed signals to radio frequency RF receiver front ends that in turn feed software SW receiver back ends and subsequent processing algorithms ticular Such events possibly linked to the world climate change are expected to get worse in the near future due to sea level rise and storm activity So close monitoring of both inland waters and coastal regions is necessary for the well being of the planet And since the need is so pervasive monitoring systems should be characterized by a relatively low cost low maintenance and easy deployment to serve the widest possible user community We have developed a patent pending solution using signals from global navigation satellite systems GNSS Called Oceanpal our monitoring system exploits reflected GNSS signals as signals of opportunity for passive remote sensing of the Earths water surfaces These multipath signals are usually considered to be nuisance signals since they reduce the accuracy of GNSS positioning applications But for monitoring various processes affecting the Earths surface they are very beneficial The technique is known as GNSS reflectometry GNSS R and during the past decade its use as a technique for Earth observation purposes has taken root GNSS R is basically a bistatic radar technique While most radar systems such as those used for monitoring air space and harbor approaches and for weather forecasting combine the radar transmitter and receiver at the same site so called monostatic radar bistatic systems use transmitters and receivers separated by a considerable distance Such systems have been used for studying certain atmospheric phenomena and for military applications where simple line of sight reflections from the target of interest are inadequate or insufficient The concept of bistatic radar can be extended to satellite signals Since some of the signal transmitted by a satellite gets reflected off the Earths surface detecting this reflected signal by a separate passive receiver would provide some information about the reflecting surface While any satellite signal could be used in principle GPS and other GNSS turn out to be particularly useful The concept of using GPS signal reflections was initially proposed in 1993 by Manuel Martín Neira working at the European Space Agencys European Space Research and Technology Centre in Noordwijk The Netherlands Since then the technique has been successfully implemented by an increasing number of researchers We could list several reasons for the continuous growing interest in GNSS as a remote sensing tool but two main ones stand out first the global availability and stability of GNSS signals enables their use as reliable signals of opportunity and second GNSS makes use of L band radiation which is highly interactive with the natural scattering medium but relatively impervious to atmospheric conditions Moreover the passive nature of this concept allows for the production of cost and resource effective instruments Navigation signals are sensitive to a wide variety of geophysical parameters including topography surface roughness surface moisture ionospheric electron content tropospheric water vapor water salinity and vegetation Research targeting related geophysical applications has been ongoing for many years and the first pre operational services exploiting reflected GNSS signals are now available In fact while the scientific community is waiting for a dedicated GNSS R space mission to confirm the theoretical predictions about the characteristics of reflected signals observed from space ground based and airborne sensors have already been developed and validated for a number of applications The GNSS R research area that has been most thoroughly investigated concerns the reflection of navigation signals from water surfaces given the highly reflective nature of water However from water the interest has now moved towards ice and land applications more specifically to the detection of sea ice and the monitoring of soil moisture Recently GNSS R has also been proposed as a possible tool to monitor vegetation This article focuses on the presentation of the Oceanpal sensor and the description of the altimetry algorithms for monitoring the levels of sea coastal and inland waters Our Instrument As mentioned above Oceanpal is a GNSS R based sensor designed for operational monitoring of coastal and inland waters The instrument comprises three subsystems a radio frequency RF section an intermediate frequency IF section and a dataprocessing section The RF section features a pair of low gain L band antennas A right hand circularly polarized RHCP zenith facing antenna collects the direct GNSS signals while a lefthand circularly polarized LHCP nadir facing antenna collects the sea or lake surface reflected GNSS signals On reflection the signals become predominantly LHCP Data bursts of some www gpsworld com September 2010 GPS World 51