An introduction to RADAR technology
We perceive objects around us because they reflect light into our eyes. At night, we might use a flashlight to see where we’re going and identify any obstacles. The light beam travels out from the flashlight, reflects off objects in front of us, and bounces back into our eyes. Our brain promptly calculates this information, determining the distance to objects and adjusting our movements to avoid collisions. Similarly, bats in the dark use echolocation to navigate and locate their prey. This involves emitting high-frequency ultrasound waves from their mouth or nose (the transmitter), which bounce off various objects and are then detected by their finely tuned ears (the receivers). This enables the bat to detect prey as small as a mosquito.
Radar works in much the same way. The word “RADAR” stands for Radio Detection and Ranging and that gives a pretty big clue as to what it does and how it works. Let us just imagine an aircraft flying at night through thick fog. The pilot of the aircraft can’t see where they are going, so the radar helps them to fly safely. A radar is a bit like a torch that uses radio waves in place of light. The radar transmits radio waves with specific frequency and the starts listening or receiving any reflection of that beam from nearby objects. If reflections are detected, the radar knows something is present nearby and it can use the time taken for the reflection to arrive to figure out how far away the object is. In other words, radar is bit like the echolocation system that bats use to see and fly in the dark.
The radio waves used by the radar are produced by the transmitter which uses magnetron, travelling wave tubes (TWT) etc. The high frequency waves produced by the transmitter are actually microwaves, similar to ones generated by a microwave oven. Once radio waves have been generated, an antenna is used to hurl them to air. The radar antennas typically rotate so that they can detect movements over a large range. The radio waves travel outward from the antenna at the speed of light and keep going until they hit something. The same antenna can act as receiver also to receive the reflected waves by using a very important component named Duplexer. The antenna picks up reflected waves during a break between transmissions. The reflected radio waves picked up by the antenna are directed into an electronic equipment like computer that processes the data and displays them in a meaningful form on a television like screen. The receiving equipment filters out useless reflections from the ground, buildings and so on, displaying only significant reflections on the screen itself.
Radars come in two basic types with variations within the types
· Pulsed, where the radar transmits a sequence of pulses of radio frequency (RF) energy.
· CW (Continuous Wave), where the radar transmits a continuous signal.
Since a CW radar transmits a continuous signal, it requires the use of separate transmit and receive antennas because it is not (usually) possible to simultaneously receive while the radar is transmitting. Pulsed radars get around this problem by using what we might think of as time multiplexing. Specifically, during the time the pulse is transmitted the antenna is connected to the transmitter. After the transmit phase is completed the antenna is connected to the receiver. In the radar there is a device that is called a circulator that effectively performs this switching function. Pulsed radars are the most common type of radar because they only require one antenna.
Another variation on radar is type is whether it is monostatic or bistatic.
· In a monostatic radar, the transmitter and receiver, along with their antennas, are located together. This is the most common type, usually employing the same antenna for transmission and reception in pulsed radar, and separate antennas with a shield between them in CW radar.
· In a bistatic radar, the transmitter and receiver are separated. This type might be used in applications like missile seekers, with the transmitter located on the ground or in an aircraft and the receiver in the missile.
Radar operates in the radio frequency band between about 100 MHz (VHF) and 100 GHz (Ka or millimeter wave (mmw))
· Search radars operate at VHF to C band
· Track radars operate in X and Ku bands, and sometimes in K band
· Instrumentation radars and short-range radars sometimes operate in the Ka band.
While selecting operating frequency of the RADAR, we have to consider few things like the following:
· Low frequency radars require large antennas or have broader beams (broader distribution of energy in angle space — think of the beam of a flashlight). They are not usually associated with accurate angle measurement.
· Low frequency radars have limitations on range measurement accuracy because fine range measurement implies large instantaneous bandwidth of the transmit signal. This causes problems with transmitter and receiver design because the bandwidth could be a significant percentage of the transmit frequency.
· Doppler measurement accuracy is also affected, as it’s related to transmit frequency.
· High power is easier to generate at low frequencies.
· For search, we want high power, but we don’t necessarily need fine range or angle measurement. Thus, search radars tend to use lower frequencies.
· For track, we need fine range and angle measurement, but we don’t necessarily need high power. Thus, track radars tend to use higher frequencies.
· Modern radars often combine search and track functions into one radar, leading to frequency and function trade-offs
During world war-II, the radar technology was used for the first time to detect approaching enemy aircraft. But now a days, this technology is used in many aspects. It is used in weather forecasting to figure out how fast storms are moving and when they are likely to arrive in a particular area. Archaeologists point radar down into the ground to study the composition of the earth and find buried deposits. Traffic police are also using this technology in the form of radar guns by the roadside to detect people who are driving too fast.
