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.
The ultimate aim of any radar experiment is of course to determine information about the structures which backscatter the radio waves, and the environment in which they exist. For example, it might be of interest to study the wind speeds associated with the scatterers, or the shape of the scatterers, or to differentiate types of scatterers or reflectors. It might be of interest to determine the radar cross-section of the scatterers, or their spatial distribution over the sky. Other desired information might include the spatial and temporal variation of the scatterer velocities as a function of time and height. If the radio scatter is due to turbulence, it might be desirable to measure the intensity of the turbulence, and/or its spatial distribution. It might be of interest to determine the relative percentages of turbulent to non-turbulent scatter. The list can go on.
In the preceding chapters, we concentrated on: (i) the principles of radar (Chapters 2 to 6); (ii) signal processing procedures (Chapters 3 to 5); and (iii) the nature of the scattering mechanisms (especially Chapter 3). Now is the time to bring all this information together and look more closely at the interaction between the radar and its scattering environment. In particular, we want to determine how the radar may be used to deduce information about the scatterers themselves. This information could include all sorts of spatial scales, right down to the radar wavelength (often indirect information at such small scales), and a wide variety of temporal scales, from fractions of a second to many years.
The purpose of this chapter is therefore to discuss ways that relevant atmospheric parameters can be determined and then interpreted, in order to give new insights into the nature of the scatterers. We will re-examine some of the parameters already discussed, like spectral characteristics, and we will also introduce new ones, like the turbulence anisotropy, amplitude distributions, phase distributions, turbulence strengths, tropopause height, and so forth. Some of the approximations used in determining these parameters are also critically examined. Some consideration will be given to experimental design, and then interpretation of the results. Studies of the parameters evaluated over long periods of time can give a considerable amount of additional information, over and above that which can be determined from a few discrete observations, but discussion of this aspect will not be considered in great detail, due to lack of space.
As discussed in Chapter 2, the MST technique began in part with Woodman and Guillen (1974) after discovery of atmospheric echoes from the troposphere using the Jicamarca incoherent scatter radar in Peru. Following this, recognizing the potential for meteorological applications, several groups set about building specialized radars for low altitude (less than 20 km) application, based broadly on the Jicamarca system. Primary groups who followed this course included a NOAA group in Boulder, Colorado, and a group at the Max Planck Institut für Aeronomie in Northern Germany. The NOAA Aeronomy group developed the so-called “Sunset radar” which was installed close to Boulder, and later a larger system at Poker Flat in Alaska. The Poker Flat system was in part also designed for mesospheric studies. The Max Planck group built a radar in the Harz Mountains, near Katlenburg-Lindau. These were the first VHF instruments designed specifically for meteorological studies.
Later, similar radars were developed by other groups in the UK, Japan, Australia, and various other countries, eventually leading to large networks of such radars. The term “windprofiler” was adopted to describe such radars when used for tropospheric and lower stratospheric (meteorological) wind measurements. One notable development was the construction of the large MU (middle-upper) radar near Shigaraki in Japan. At the time this was a state-of-the-art instrument, and had many important developments incorporated into it.
In this chapter, we will describe some of the details of three radars. One will be the German SOUSY radar in the Harz Mountains, one the MU radar, and the third a lowcost radar (CLOVAR) built in the 1990s in Canada. The objective is not so much to discuss the history of these radars (that was considered in some detail in Chapter 2), but to give more detail about technical developments through the course of evolution of the MST technique. The SOUSY radar was built at a time when personal computers were just starting to be developed, but were of very slow speed. Instruments like the Data General PDP-8 and NOVA mini-computers were just under development; the PDP-8 was developed around 1965, and the NOVA came into being in the late 1960s and early 1970s.
In this chapter, we discuss various extended, and in some cases unusual, applications of MST radar. These may be special cases of general MST techniques, or specific applications of the technique applied to special cases, or even quite unusual applications which are a substantial deviation from “normal” MST standard practices. If such a topic fits well in another chapter, it may appear there – if it is somewhat of an exception, or has a sightly unusual methodology, or is not really an operational technique, it may appear here. Polar mesosphere summer echoes are an example of an “extended” application. While the techniques used to study these unusual echoes are really the same as for other MST studies, the unusual physics associated with the scatterers that produce these echoes makes them of particular interest. Lightning study is an example of a slightly “miscellaneous” application, in that the techniques are a little unusual (high PRFs, and the events are very short lived). Meteor study is an example of a slightly non-standard application that has grown into a substantial field all of its own. Differential absorption is a technique developed early in the days of radar in the 1960s and 1970s which has had a rebirth in the last decades, and deserves a brief mention here. Precipitation study with MST radars is a relatively mature field, but is still a secondary application, so is also included here.
Each of these fields has a significant role in its own right, but extended discussion of them would simply take up too much space, and would spread the intended application of this book beyond its original goals. Hence the topics are summarized briefly in this chapter – maybe too briefly for some, but we have tried to give sufficient references that interested readers may expand their knowledge through these references.
This book is intended to concentrate on experimental and analysis techniques, and the underlying processes (both geophysical and technical) that guide the experiments and their design. Examples of the latter include the basic theory of turbulence, and the theory behind gravity waves (see the next chapter, and also some small discussion in Chapter 2).
In earlier chapters, we have discussed radars in a general sense, and dealt with some of the techniques available to optimize signal detection. We have discussed the conceptual difference between CW and pulsed systems, and concepts like range resolution and sampling strategies. In this chapter, we will take a closer look at the electronics and engineering that is required to develop a radar, and the associated hardware. Key topics will include antennas, transmitters, receivers, and controllers. Some topics from the previous chapter may be repeated, but generally in greater detail.
One thing that all radars have in common is a need for a transmit antenna and a receive antenna. These may or may not be located at separate sites. The transmitter transmits radiowaves through a transmitter antenna into the air, and receives echoes from a target, or from multiple targets, with the receiver antenna. When the transmitter and receiver are co-located, the radar is referred to as a “monostatic radar,” while the term “bistatic radar” refers to the case that the transmitter and receiver are physically separated. If two or more receivers which detect echoes from a common target are located at different places, the system is called a “multistatic radar.” The degree of separation can be an important factor as well – if the transmitter and receiver are within maybe a few wavelengths of each other, they may be referred to as either monostatic or bistatic, depending on the application, even though, in the strictest sense, they are bistatic/multistatic. An example is the so-called “spaced antenna method” for measuring winds, in which case there are multiple receiver antennas but the theoretical development is often done in a quasi-monostatic sense. Generally, if the separation between the transmitting and the receiving antennas can be neglected compared with the distance to the target, the system is considered as monostatic, although even then the meaning of “small” and “large” distances depends on the objectives of the experiment. Experiments requiring detailed phase information between receivers may need to be considered multistatic, whereas if no phase information is needed the same configuration might be considered as monostatic, for example.
Some of the earliest applications of windprofiler radars were in regard to tropospheric and lower stratospheric studies. The radars developed at the Sunset site near Boulder, Colorado (Green et al., 1979) and in the Harz mountains in Germany (the SOUSY radar (Czechowsky et al., 1976)) were two of the earliest such instruments, and were certainly built with meteorological studies in mind. Some of these radars have already been described in Chapter 2, and the SOUSY radar was extensively discussed in Chapter 6.
The most direct meteorological studies have been in regard to wind motions, but these radars have also been usefully employed in other areas, including studies of turbulence strengths and anisotropy, tropopause height measurements, gravity wave momentum fluxes, precipitation measurements, temperature profile determinations, and various others.
It is impossible to cover all aspects of MST radar applications relating to the troposphere in just one chapter. For this reason, we will concentrate mainly on results, rather than on specific details about techniques. It will be assumed that the techniques have been sufficiently covered in earlier chapters.
The early years of tropospheric studies have been especially well covered in several excellent reviews, including those by Röttger and Larsen (1990), Gage (1990), Larsenand Röttger (1982) and Balsley and Gage (1982). Some of the early parts of this chapter will involve a recap of the main results of those publications.
Röttger and Larsen (1990) discussed the origins of VHF MST radar studies in the context of: (i) developments following the use of high-power X, S, and UHF band radars in the United States of America, as well as FMCW (frequency modulated continuous wave) techniques; coupled with (ii) the detection of tropospheric echo fading observed at Jicamarca (Peru) by Woodman and Guillen (1974); and (iii) the application of phasecoherent techniques. These events in turn led to the first dedicated VHF-ST radars being built at Sunset, near Boulder, Colorado, and in the Harz mountains of Germany (the SOUSY, sounding system radar). Phase coherent detection was especially important in the development of such systems, for without it, detection of useful tropospheric echoes with VHF systems would be nearly impossible.
As we have already discussed, there are many competing factors that must be taken into account in order to optimally investigate the atmosphere through radar observations. One of the more notable examples is the Doppler dilemma. Obviously one would like to select an inter-pulse period (IPP) corresponding to a sufficiently large Nyquist velocity interval. Here sufficiently large means a velocity range that encompasses most of the anticipated radial velocities to be observed. The range of Nyquist velocities is extended by decreasing the IPP. However, decreasing the IPP also reduces the maximum unambiguous range that can be measured. Ideally one would like to maintain a large Nyquist velocity (short IPP) and large maximum unambiguous range (long IPP) – hence the dilemma. Another example involves the disparity between the desire to improve range resolution and improve radar sensitivity. Range resolution can be improved by decreasing the radar pulse width; however, this means a decrease in the amount of power that illuminates a scatterer and corresponding decrease in detectability. That is, the desire to increase the detectability of atmospheric signals by transmitting longer radar pulses is at odds with the need to improve range resolution.
In many cases, techniques have been developed that allow us to work around the compromises that arise in designing radar experiments. For example, pulse compression (discussed in Chapter 4) is used to improve range resolution without compromising the signal-to-noise ratio (SNR) (Schmidt et al., 1979). By and large, such techniques are known to introduce corresponding undesirable side effects. For the case of pulse compression, either the existence of some level of range side-lobes, or a decrease in temporal resolution, are a by-product of complementary codes.
In this chapter, we discuss how the use of multiple-receiver and multiple-frequency techniques can be used in atmospheric remote sensing as a means of improving angular and range resolution respectively. Before proceeding, we should clarify one point of nomenclature. The term multiple-receiver will be used throughout this chapter to describe a radar system that is capable of receiving and recording atmospheric signals on two or more spatially separated antennas or groups of antennas. The myriad names associated with interferometric techniques were discussed in Chapter 2, Section 2.15.6: here, we will discuss in detail just a subset of these, but the points discussed will cover to some extent all the techniques.
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