Blip, Ping, and Buzz: Making Sense of Radar and Sonar

6. Specialized Applications and Advanced Techniques

We have explored most of the extensive territory that constitutes remote sensing. We have trekked through a morass of technical notes to reach the lofty peaks of signal correlation, Doppler processing, and synthetic apertures . I hope that you have gained an appreciation of the lay of the land. If you wish to explore in more detail, I have referred to other guides that will take you into whatever particular valley or hilltop appeals to you. A few offshore islands remain that I will expand upon in the first part of this chapter , because they represent important applications of our subject, hitherto unexplored, that everyone who claims an interest in remote sensing should know something about. These special applications have evolved into unique and sometimes strange forms that reflect their unique realms. Once I have delineated weather radar, air-traffic control (ATC), fish-finding sonar, and echocardiograms, I will devote the second part of this chapter to an overview of some advanced techniques that, generally speaking, are still on the drawing board or at the experimental stage. When fully developed these techniques promise to continue the improvement of our remote sensing capabilities. Weather Radar: Looking for Clutter Every day on the TV weather forecasts we see large maps of the earth’s surface , superimposed with atmospheric pressure contours, cloud cover, and 6 Special Applications & Advanced Techniques meteorologists’ graphics. These maps are constructed from a wide variety of sources (data from weather balloons, ground stations, satellite images, and radar) compiled during the preceding hours, and present weather forecasts that have been calculated by large number-crunching computers. The physics of weather formation and development is fairly well understood, enabling meteorologists to extrapolate the current measured state of the world’s weather for a day or two into the future. This is what you see on the TV forecasts. My local TV weather channel predicts sunshine, rainfall rate, and temperatures up to two weeks ahead, but this is a joke; meteorology is incapable of accurate predictions more than a few days into the future because of the well-known “butterfly effect.” Atmospheric physics is governed by equations with solutions that are chaotic. This technical term means that, starting with two very slightly different sets of meteorological conditions at, say, midday Tuesday, the equations predict somewhat different weather for midday Wednesday, quite different weather for midday Thursday, very different weather for Friday, and so on. In other words, chaotic systems are characterized by equation solutions that critically depend on the initial conditions. The problem is that we do not know the “initial” conditions of the weather (say at midday Tuesday) since we cannot measure them all. Think of what such measurement would entail: we would need to know the temperature and pressure everywhere in the atmosphere all over the earth at that time, as well as humidity, wind speed, cloud formation, etc. We would also need to know the surface and subsurface conditions of all the world’s oceans, because the interaction of atmosphere with the oceans influences weather significantly . Obviously, our data gathering of meteorological information is incomplete and imperfectly measured, so our knowledge of the state of the weather at midday Tuesday is only approximate. But the chaotic equations of meteorology require exact knowledge to predict reliably into the distant future. Any errors—say by not including the effects of a butterfly flapping its wings somewhere in Asia—and our predictions for weather in Baltimore a week from Friday will be wrong. Good weather forecasting requires good weather measurement. The better we can estimate the current conditions over the earth, the further into the future our weather predictions will be (approximately) valid. This is where weather radar comes into its own: radars provide a great deal of information about the meteorological state of the atmosphere, in a very short time. Weather radars transmit microwave radiation of wavelengths between about 1 and 10 centimeters. The short wavelengths are better at picking out 158 Blip, Ping & Buzz small cloud droplets or drizzle, they provide better angular resolution, and, because short-wavelength radars are generally smaller, they are much less expensive. On the other hand, longer microwave wavelengths penetrate better through the atmosphere—they suffer much less attenuation and so operate at much greater range. Doppler radars detect objects in the atmosphere such as raindrops (or swarms of bugs) that are carried along by the wind, and so tell us about wind speed. Some...