S Air Force air traffic controllers began routinely using Ground Controlled Approach GCA equipment to help military pilots land safely in poor visibility. A controller observed the precise angle returns from these azimuth and elevation radars and radioed landing guidance to the pilot.
GCA also included a degree rotating radar for surveillance of the entire terminal area. Civil controllers first used military GCA equipment at LaGuardia Airport in , where it helped triple the landing rate to 15 planes per hour.
GCA experienced occasional problems, but it worked well enough so that by it was being used at many airports. ILS uses similar course guidance principles, but uses receivers in aircraft that display course deviation directly to the cockpit.
Thus, as ILS replaced the GCA scanning pencil beams, improved rotating radars with faster scan rates and larger coverage areas also replaced the GCA terminal area surveillance radars.
As air traffic continued to grow it also became important to track planes in high-altitude airspace. Accordingly, the coverage of air traffic radar surveillance grew throughout the s as long-range radars were deployed along important air routes. Initially, these aircraft surveillance radars had no automatic tracking capability. By the s radar surveillance of civil aircraft routinely included automatic aircraft tracking.
Air traffic control radars now track both aircraft and hazardous weather. Modern air traffic control radars use the Doppler effect to discriminate moving aircraft from stationary targets and to measure storm velocities. In the late s it was proposed that IFF technology be used for surveillance of civil aircraft. The use of transponders increases the detection range of the radar, eliminates clutter interference from other reflectors, and provides a means of aircraft identification and altitude reporting.
In the early s the U. S Federal Aviation Administration FAA published a national standard for air traffic control interrogators and transponders. The Surface Movement Radar SMR scans the airport surface to locate the positions of aircraft and ground vehicles and displays them for air traffic controllers in bad weather. Surface movement radars operate in I- to K-Band and use an extremely short pulse-width to provide an acceptable range-resolution. The range is limited to a few kilometers, the antenna rotation speed is 60 revolutions per minute.
Weather radar is very important for air traffic management. There are weather-radars specially designed for air traffic safety. Includes those airborne radar systems for weapons fire-control missiles or guns and weapons aiming. Spaceborne Radar Systems. Considerable effort has been applied to spaceborne radar SBR research for intelligence, surveillance, and reconnaissance missions over the last 30 years.
These include both land-based and shipborne ATC radar systems used for assisting aircraft landing, and supporting test and evaluation activities on test ranges. Simple Pulse Radar: This type is the most typical radar with a waveform consisting of repetitive short-duration pulses. Typical examples are long-range air and maritime surveillance radars, test range radars, and weather radars. There are two types of pulse radars that uses the Doppler frequency shift of the received signal to detect moving targets, such as aircraft, and to reject the large unwanted echoes from stationary clutter that do not have a Doppler shift.
One is called moving-target indication MTI radar and the other is called pulse Doppler radar. Its waveform is a train of pulses with a low PRR to avoid range ambiguities. What this means is that range measurement at the low PRR is good while speed measurement is less accurate than at a high PRR's. Almost all ground-based aircraft search and surveillance radar systems use some form of MTI. Airborne Moving-Target Indication AMTI Radar: An MTI radar in an aircraft encounters problems not found in a ground-based system of the same kind because the large undesired clutter echoes from the ground and the sea have a Doppler frequency shift introduced by the motion of the aircraft carrying the radar.
The AMTI radar, however, compensates for the Doppler frequency shift of the clutter, making it possible to detect moving targets even though the radar unit itself is in motion. Pulse Doppler Radar: As with the MTI system, the pulse Doppler radar is a type of pulse radar that utilizes the Doppler frequency shift of the echo signal to reject clutter and detect moving aircraft. This causes the measurement of the target's radial velocity as derived from the Doppler frequency shift to be highly ambiguous and can result in missing some target detections.
On the other hand, the pulse Doppler radar operates with a high PRR so as to have no ambiguities in the measurement of radial velocity. A high PRR, however, causes a highly ambiguous range measurement. The true range is resolved by transmitting multiple waveforms with different PRR's.
High-Range Resolution Radar: This is a pulse-type radar that uses very short pulses to obtain range resolution of a target the size ranging from less than a meter to several meters across. It is used to detect a fixed or stationary target in the clutter and for recognizing one type of target from another and works best at short ranges. Pulse-Compression Radar: This radar is similar to a high-range resolution radar but overcomes peak power and long-range limitations by obtaining the resolution of a short pulse but with the energy of a long pulse.
It does this by modulating either the frequency or the phase of a long, high-energy pulse. The frequency or phase modulation allows the long pulse to be compressed in the receiver by an amount equal to the reciprocal of the signal bandwidth. Synthetic Aperture Radar SAR : This radar is employed on an aircraft or satellite and generally its antenna beam is oriented perpendicular to its direction of travel.
The SAR achieves high resolution in angle cross range by storing the sequentially received signals in memory over a period of time and then adding them as if they were from a large array antenna. The output is a high-resolution image of a scene. It is usually used to obtain an image of a target. The resolution in cross range is not as good as can be obtained with SAR, but it is simpler than the latter and is acceptable for some applications.
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