TABLE OF CONTENTS The radar antenna beam pattern typically covers only a small portion of the angular region of interest. The antenna must couple the energy between the transmitter and space. To observe targets over a portion of space larger than covered by the main beam, the antenna beam is moved to observe different angles at different times. Often the spatial coverage is thought of as requiring a certain number of beam positions. For hemispheric coverage, there are approximately N bp independent beam positions required:
where:
? AZ = 3-dB azimuth beamwidth (rad)
? EL = 3-dB elevation beamwidth (rad)
The motion of the beam (angular position versus time) is called scanning. The scanning pattern is designed to cover the entire volume of interest to the radar system. The size of this volume can vary depending on the purpose of the radar at the time. If searching a hemisphere is necessary to acquire a target, a smaller search volume can subsequently be used to refine its location. If the radar can track the target, scanning may cease altogether. Such constant illumination of the target by the radar can be a warning to the target that tracking is occurring and an attack may soon occur. In the case of an electronically steered array (ESA), tracking may be indicated by repeated bursts occurring at the same beam position (or having the same amplitude when observed by an ELINT receiver).
In mechanically scanned radar systems, the scanning pattern is typically periodic.
