Introduction and historical background

The purpose of aerospace sensors is to provide all the necessary information on the variables and parameters of the flight system for the execution of the mission at hand. A mission is generally defined in three dimensional space. Sometimes, on adding the fourth’dimension’, namely, time, a four-dimensional space is defined. In contrast to the three spatial coordinates which can increase or decrease, time always grows. The recent concept of 4-D guidance has special significance in coordinating flight paths such as in air traffic control or in space rendezvous operations, where the notions of ’here-now’ or ’there-then’ assume paramount importance.