Fundamentally, atmospheric radars are designed to transmit an electromagnetic (EM) wave and to observe the effects that the atmosphere has on the scattered wave. These interactions may take the form of bending of the radiowave path, or reflection and scattering. In the simplest case, a transmit antenna and a receive antenna are required, which may be located at separates sites. More complex systems might involve multiple receivers and even multiple transmitters. Most commonly in MST atmospheric work, the transmitter and receiver are co-located; in these cases, refraction of the ray paths is not generally significant. Reflection and scattering are the primary phenomena that need to be considered in MST studies.
The radar targets in MST studies
Atmospheric reflection and scattering occur due to the interaction of the EM wave with changes in the refractive index. As discussed in Chapter 3, these refractive-index changes may be caused by a variety of phenomena. We will quickly revisit some of these processes here, because they help us to understand the different modes of radar analysis that we will discuss. Of course, aircraft and missiles are perhaps the most obvious examples of targets that spring to mind when we talk of radar, but these are not the primary targets when it comes to atmospheric studies. One simple example that is relevant is water droplets embedded in the air. In this case, the refractive index inside the water droplets is very different to that of the surrounding air, so each water droplet may scatter a small amount of incident radiation. In this case, scatter from a large number of water droplets is required before a detectable scattered signal can be produced. Insects and birds contain water, so they too can act as radiowave scatters. Indeed some radars use insects as tracers of atmospheric motions. Another example is the ionized trail of plasma left behind when a meteoroid enters the atmosphere. Meteoroids are generally small grains of dust (with diameters from micrometers to centimeters, though larger ones can occur) which enter the atmosphere at high speed (typically 10 to 70 km/s), creating large levels of frictional heating and thereby ionizing the air around them. As a result, a long trail of plasma (typically a few km in length) exists behind the meteoroid, and this so-called “meteor trail” can reflect radiowaves.