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Emerald Group Publishing Limited
Copyright © 2010, Emerald Group Publishing Limited
Article Type: Editorial From: Sensor Review, Volume 30, Issue 2
Our theme for this issue is “Metrology and 3D measurement” and the papers we include cover this field and concentrate on spatial three dimensional measurements. This basically amounts to the mapping of the physical surface of objects. However, the third dimension does not need to map the position of a physical surface; it can also be represented by light intensity, colour, temperature or chemical composition to name just a few.
With the Copenhagen Climate Conference, “green” issues are getting a lot of attention and it is perhaps not insignificant that the European Space Agency (ESA) recently launched the Soil Moisture and Ocean Salinity (SMOS) satellite carrying the all important Microwave Imaging Radiometer using Aperture Synthesis (MIRAS) instrument (Figure 1).
This satellite is designed to fill in a great hole in our understanding and ability to measure the soil moisture and salinity of our oceans. These are simple measurements when undertaken at a specific location on Earth, but local measurements do not provide an adequate image of the global picture and how water moves around on our planet in the soil, skies and oceans. For this an image from space is required and the “camera” needs to see in the microwave region of the electromagnetic spectrum.
This could be achieved using a single 20 m diameter antenna, however as this would be too large and heavy to launch, MIRAS uses a “Y” shaped array of 69 mini-antennas. With such a sophisticated instrument you do not just launch and press the “ON” button. However, so far all has gone according to plan and the instrument has been deployed correctly and is currently undergoing extensive calibration procedures.
The MIRAS instrument is a good example of how a well designed system can be better than the sum of its parts. Each of the 69 antennas is just 165 mm in diameter and 19 mm high, or about the size of a small plate. If you took 69 of these antenna and looked at their data individually then you would probably end up with something about as useful as a car parking distance sensor. If you took all 69 and averaged (or some other statistical permutation) the data coming from them you would end up with a more reliable parking distance sensor that could survive a good number of individual failures, but it would still basically be a car parking distance sensor.
However, if you do as MIRAS has done and arrange the 69 antennas in a precisely defined array and then do some very fancy processing and correlation of the data you can create an instrument that can measure the moisture content of soil down to a depth of 1-2 m.
This technique is known as synthetic-aperture imaging and was invented in 1952 by Carl A. Wiley and has been widely used for terrestrial and astronomical applications.
It can also be highly beneficial not just to connect multiple versions of the same sensor into a system but also to correlate the data from a variety of different sensors, either arranged spatially or co-located. By combining data from multiple sources while taking into account temporal and spatial variations we can often create a virtual instrument that is both low cost and exceedingly useful.