Introduction

Many instruments have been used to study the atmosphere, both by in-situ and remote methods. From anemometers to satellites, chemical sensors to balloons and rockets, the array of tools is broad. Since the early 1900s, a key instrument for such studies has been radar. RADAR stands for Radio Detection And Ranging. Radars operating in a variety of frequency bands, from wavelengths of kilometers to wavelengths of millimeters, have all found application. They have been used to study the upper ionosphere and the neutral atmosphere, right down to ground level.

In this book, we will concentrate on a class of radar generally referred to as MST radar. In this description, M stands for Mesosphere, S for Stratosphere, and T for Troposphere, where these three “spheres” refer to different height-regimes of the atmosphere which collectively cover the region from ground level up to about 90 km altitude. More exact definitions will be given shortly. For now, consider the troposphere as the region from the ground to 12 km altitude, the stratosphere as the region from 12 to 50 km altitude, and the mesosphere the region from 50 to 90 km altitude. Under the narrowest definition, the term MST radar was originally used primarily to refer to radars operating in the VHF (very high frequency) band, with special emphasis on frequencies around 50 MHz, which could probe (at least in part) all three regions. More generally it has come to refer to any radars that can be used for studies of any of these three regions of the atmosphere. These radars include MF (medium frequency), HF (high frequency), VHF, and UHF (ultra-high frequency). They also include so-called meteor radars. Generally, precipitation radars (referred to as “Doppler radars” by the meteorological community) are not considered to be MST radars, although we will discuss them a little in this book. (As an aside, we will generally refer to these radars as precipitation radars in this book. The phrase “Doppler radar” is not a good one to describe these radars, since they are most certainly not the only Doppler radars! The term “Doppler radar” arises from the fact that these radars can measure the Doppler frequency-shift of reflected signals.