Doppler Radar - History

History

Doppler radar tends to be light weight because heavy pulse hardware is eliminated. The associated filtering removes stationary reflections while integrating signals over a longer time span, which improves range performance while reducing power. These advantages were applied by the military during the 1940s.

FM radar was developed during World War II for the use by United States Navy aircraft to support night combat operation. Most used the UHF spectrum and had a transmit Yagi antenna on the port wing and a receiver yagi antenna on the starboard wing. This allowed bombers to fly an optimum speed when approaching ship targets. This also allowed escort fighter aircraft to train guns on enemy aircraft during night operation. These strategies were adapted to semi-active radar homing. Some form of Doppler radar has remained in use for night and all weather operation, although weight imposed limitations until the 1970s.

Doppler systems are light enough for mobile ground surveillance associated with infantry and surface ships. These detect motion from vehicles and personnel for night and all weather combat operation. Modern police radar are a smaller more portable version of these systems.

• AN/PPS-4 Radar
• AN/PPS-5 Radar
• AN/SPG-51 Gun and Missile Fire Control Radar
• MK 74 Guided Missile Fire Control System
• MK 92 Guided Missile Fire Control System CORT variants

Early Doppler radar sets relied on large bulky analog filters to achieve acceptable performance. Analog filters, waveguide, and amplifiers pick up vibration like microphones, so bulky vibration damping is required. That extra weight imposed unacceptable kinematic performance limitations that restricted aircraft use to night operation, heavy weather, and heavy jamming environments until the 1970s.

Digital fast fourier transform filtering became practical when modern microprocessors became available during the 1970s. This was immediately connected to coherent pulsed radars, where velocity information was extracted. This quickly proved useful in both weather and air traffic control radars. The velocity information provided another input to the software tracker, and improved computer tracking. Because of the low Pulse Repetition Frequency (PRF) of most coherent pulsed radars, which maximizes the coverage in range, the amount of doppler processing is limited. The doppler processor can only process velocities up to ±1/2 the PRF of the radar. This is not a problem for weather radars. Velocity information for aircraft cannot be extracted directly from low PRF radar because of sampling restricts measurements to about 75 mile per hour.

Specialized radars quickly were developed when digital techniques became light weight and more affordable. Pulse-Doppler radars combine all the benefits of long range and high velocity capability. Pulse-Doppler radars use a medium to high PRF (on the order of 3 to 30 kHz). This medium PRF allows for the detection of either high speed targets or high resolution velocity measurements. Normally it is one or the other; a radar designed for detecting targets from zero to Mach 2 does not have a high resolution in speed, while a radar designed for high resolution velocity measurements does not have a wide range of speeds. Weather radars are high resolution velocity radars, while air defense radars have a large range of velocity detection, but the accuracy in velocity is in the 10's of knots.

Antenna designs for the CW and FM-CW started out as separate transmit and receive antennas before the advent of affordable microwave designs. In the late 1960s traffic radars began being produced which used a single antenna. This was made possible by the use of circular polarization and a multi-port waveguide section operating at X band. By the late 1970s this changed to linear polarization and the use of ferrite circulators at both X and K bands. 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.