GPS World, September 2010

INNOVATION Remote Sensing Friendly Reflections L akes and water reservoirs are the worlds most important sources of accessible fresh water Despite its paramount importance not only for a large variety of human activities but also for the sustainability of ecosystems fresh water is already scarce in many regions The problem is envisaged to become worse in the coming decade In addition in climatological studies surface water storage is a critical element of the water cycle since the analyses integrate all hydrologic processes precipitation runoff evapotranspiration and so on over a given basin and for hydroelectric companies it is the main parameter to be kept under observation for efficient energy production All of these concerns make the monitoring of fresh water resources a prime activity for a wide variety of stakeholders including governments climate research organizations and hydroelectric production companies Coastal management is also a wideranging issue with large social and economic impacts Care of our coasts includes dealing with threats such as storm surges and flooding coastal erosion and conflicting land use issues Coastal areas support the greatest concentration of living resources and people on the planet In the past few decades these regions have experienced a population density increase which is envisioned to grow steadily Furthermore conflicts between commercial interests recreational activities infrastructure development environment conservation and exploitation of natural resources will become increasingly important and contentious In fact the coastal zone is a peculiar environment in which terrestrial oceanic atmospheric and human inputs of energy and matter converge Storm surges and coastal flooding events have caused considerable damage and economic loss on European coasts in par Monitoring Water Level with GNSS Alejandro Egido and Marco Caparrini WHY IS THE SKY BLUE This is an age old question interesting to anyone with a curiosity about his or her surroundings But what has it got to do with global navigation satellite systems Believe it or not there is a connection Some of you might remember the explanation of the skys color from your Physics 101 course but to bring everyone up to the same level lets review Everything we see is the result of the interaction of light and matter And by matter we mean the atoms molecules and particles making up matter Light causes matter to vibrate And vibrating matter due to its electrical charges in turn emits light which combines with the original light But matter not only reemits light in the forward direction it re emits light in all other directions This is called scattering Now the light from the sun includes all colors and so if look directly at the sun when it is high in the sky dont try this at home it looks white or slightly yellowish We are seeing the light propagating directly toward our eyes When we look at the sky away from the sun we are seeing scattered light And this scattered light is predominantly blue Why It turns out that scattering is proportional to the fourth power of frequency Light that is of a higher frequency say a factor of two is sixteen times more intensely scattered So blue light which has about twice the frequency of light from the INNOVATION INSIGHTS with Richard Langley Scattered signals reveal characteristics of their source red end of the visible spectrum is scattered much more than red light Violet light is scattered even more but our eyes are not as sensitive to violet light as they are to blue light Hence the sky looks blue So what has this got to do with GNSS As we know for the best positioning and navigation results we need the satellite signals to travel along a direct path to the receivers antenna There may be slight changes in the speed and direction of propagation of these direct path signals caused by the interaction of the electromagnetic waves with the matter making up the ionosphere and the neutral atmosphere but these are readily accounted for in the position fixes However once they reach the Earths surface the signals can be reflected by buildings vegetation the ground water surfaces and so on The signals are actually being scattered by the matter they encounter A receiver can selectively acquire the scattered signals and the resulting measurements can be interpreted to reveal certain characteristics of the source of the scattering In this months column we learn about the design and application of a GNSS instrument that uses scattered signals for monitoring the level and roughness of inland and coastal water surfaces yet one more use of GNSS signals for the betterment of planet Earth GPS World September 2010 www gpsworld com 50