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The groundwork describing the atmospheric environment and the types of flows that radar can study in the Earth's atmosphere has been laid in the previous chapter. We now turn to a brief history of how radars came to be involved with studies of this type.
While most of this book is about MST radar, it is important that MST radar be seen in a broader context. We therefore begin this section on the history of the development of MST radar by looking not at MST radar itself, but rather at the development of meteorological radar. As indicated earlier, the period following World War II saw various developments of radar. Two primary streams were (i) ionospheric studies for world-wide communication, and (ii) studies of contaminants in radar detection for military and civil applications. The first stream of development led to extensive studies of the upper atmosphere and ionosphere, and the second led to more intensive investigations of the troposphere. Only with the development of MST radar did the two streams once again really merge.
Initially, there were two main aspects to radar detection – determination of range and, if possible, direction. Directional determination was achieved by using large antennas which concentrated the radar directionally, and range was generally found using timeof- flight delays.
The atmospheric radar principle for range-detection is basically fairly straightforward. A short pulse of an electromagnetic wave of typically several microseconds duration is transmitted from the radar antenna, whereupon it eventually may strike a target. It is then scattered back from the target to the radar antenna. The receive signal is called an echo, by analogy with the sound heard when your voice echoes from a distant object. Multiple radar echoes can be detected if there are multiple targets.
Echo samples are examined at consecutive time steps. Using early radars, this was done visually, whereas with more recent ones, digital sampling is used. Since the radar signal propagates with the speed of light c, the time t elapsed from the transmission of the pulse to the reception corresponds to a given range r = ct/2. We find, as an example, that echoes from backscattering targets at a range of, say 15 km, are received 100 microseconds after the pulse was transmitted. Echo samples are taken at a series of successive delays, called range gates.