Doppler radar
This variation gives direct and highly accurate measurements of the radial component of a target's velocity relative to the radar.
If the pitcher moves at an angle, but at the same speed, the frequency variation at which the receiver catches balls is less, as the distance between the two changes more slowly.
[3] There is no need to invoke Albert Einstein's theory of special relativity, because all observations are made in the same frame of reference.
Radars may be: Doppler allows the use of narrow band receiver filters that reduce or eliminate signals from slow moving and stationary objects.
The advantage of combining Doppler processing with pulse radars is to provide accurate velocity information.
Basically, any target that is heading 90 degrees in relation to the antenna beam cannot be detected by its velocity (only by its conventional reflectivity).
Ultra-wideband waveforms have been investigated by the U.S. Army Research Laboratory (ARL) as a potential approach to Doppler processing due to its low average power, high resolution, and object-penetrating ability.
Return signals from weather, terrain, and countermeasures like chaff are filtered out before detection, which reduces computer and operator loading in hostile environments.
The associated filtering removes stationary reflections while integrating signals over a longer time span, which improves range performance while reducing power.
Continuous-broadcast, or FM, radar was developed during World War II for United States Navy aircraft, to support night combat operation.
Modern Doppler systems are light enough for mobile ground surveillance associated with infantry and surface ships.
[8][9] Early Doppler radar sets relied on large analog filters to achieve acceptable performance.
Velocity information for aircraft cannot be extracted directly from low-PRF radar because sampling restricts measurements to about 75 miles per hour.
Pulse-Doppler radars use a medium to high PRF (on the order of 3 to 30 kHz), which allows for the detection of either high-speed targets or high-resolution velocity measurements.
PD radars operate at too high a PRF to use a transmit-receive gas filled switch, and most use solid-state devices to protect the receiver low-noise amplifier when the transmitter is fired.
Most modern weather radars use the pulse-Doppler technique to examine the motion of precipitation, but it is only a part of the processing of their data.
In June 1958, American researchers David Holmes and Robert Smith were able to detect the rotation of a tornado using the mobile continuous-wave radar (photo to the right).
Norman's laboratory, which later became the National Severe Storms Laboratory (NSSL), modified this radar to make it a pulsed Doppler radar allowing more easily to know the position of the echoes and having a greater power[15] The work was accelerated after such event in the United States as the 1974 Super Outbreak when 148 tornadoes roared through thirteen states.
The reflectivity only radar of the time could only locate the precipitation structure of the thunderclouds but not the mesocyclonic rotation and divergence of winds leading to the development of tornadoes or downbursts.
By directly measuring the movement of the ground with the radar, and then comparing this to the airspeed returned from the aircraft instruments, the wind speed could be accurately determined for the first time.
This system sent a pulsed signal at a very low repetition rate so it could use a single antenna to transmit and receive.
Similar systems were used in a number of aircraft of the era,[17] and were combined with the main search radars of fighter designs by the 1960s.
The aircraft's compass was integrated into the computer so that a desired track could be set between two waypoints on an over water great circle route.
It was generally backed up with position fixes from Loran, VORs, NDBs, or as a last resort sextant and chronometer.
The combination of Doppler offset and reception time can be used to generate a locus of locations that would have the measured offset at that intersects the Earth's surface at that moment: by combining this with other loci from measurements at other times, the true location of the ground station can be determined accurately.
[citation needed] A notable example of utilizing Doppler information is in the detection and classification of small unmanned aerial vehicles.
Radar systems operating at extremely high frequency offer enhanced Doppler resolution for a given coherent processing interval.
