Radar Tracking Systems
Radar systems can send out thousands of pulses per second. Using a rapid sequence of pulses, a radar system can not only determine the range of a target, but it can also track target motion. Ranging can be performed with an omnidirectional antenna, but target location and tracking require a more sophisticated system with knowledge of the antenna elevation (vertical) angle and azimuthal (horizontal) angle with respect to some fixed coordinate system. Land-based systems generally define true north as the azimuthal reference and the local horizontal as the elevation reference. The azimuthal reference for air and sea systems is the bow of the ship, but elevation reference varies depending on the pitch and roll stabilization of the ship or plane. When you are driving a car down the street, you might characterize other cars as to your left, to your right, or behind you; you define the location of the cars in terms of your own coordinate system. Similarly, when a radar system receives the reflection from a target, it checks the orientation of the receiving antenna with respect to the coordinate axes to determine the object location. Moreover, just as you can use a roadmap to determine the absolute location of an object, so a radar system can be used to locate a target in terms of longitude and latitude. Multiple pulses are required to track the motion of a target. The pulses must be spaced far enough apart that a pulse can be sent out and return before the next pulse is sent, but this is quite feasible when you consider that a radar pulse can travel 100 mi (161 km), strike a target, and return in less than 1/1000 of a second.
Air Traffic Control uses radar to track and direct the courses of the many planes in civilian airspace. Civilian and military craft generally carry a beacon, or transponder, known as the Air Traffic Control Radar Beacon System (ATCRBS). An Air Traffic Control interrogator system sends a signal to the transponder that prompts it to reply with identification and altitude information. In this way, air traffic controllers can monitor the courses of planes in their region. A military version of the beacon, known as Identification, Friend or Foe (IFF) uses coded signals to identify aircraft.