Radar Signal Processing
Written by:
-Jayant Meshram, Ajinkya Mondhe, Akash Nachan, Tejas Nilangekar, Aditya Patil
RADAR?
RADAR — RAdio Detecting And Ranging, is a detection system that uses Radio Waves to determine the distance, angle, and radial velocity of an object with respect to a site. RADARs are used in military, law enforcement, space exploration, remote sensing, aircraft navigation, ship Navigation, and air traffic controller.
RADAR works similar to the concept of an echo. When we shout in well or near a wall, the sound is reflected back and bounces up to us. The radar transmits a focused pulse of microwave energy (yup, just like a microwave oven or a cell phone, but stronger) at an object, most likely a cloud. Part of this beam of energy bounces back and is measured by the radar, providing information about the object. Radar can measure precipitation size, quantity, speed, and direction of movement, within about a 100-mile radius of its location.
Basic RADAR usually has these four components:
- A transmitter
- A transmit/receive switch
- An antenna
- A Receiver
Transmitters' Job is to create the energy pulse. Switch controls when to transmit and receive the pulses. The antenna sends these pulses out into the atmosphere, and a Receiver processes the received signals.
Signal Processing in RADAR
Signal processing focuses on analyzing, modifying, and synthesizing signals such as sound, images, and scientific measurements. Signal processing techniques can be used to improve transmission, storage efficiency, and subjective quality and to also emphasize or detect components of interest in a measured signal.
In RADAR systems as well, Signal Processing is an important aspect. Signal processing is employed in the RADAR systems for various applications such as interference reduction to ensure good signal transmission, and to fulfill some practical applications such as gaining insight into the distance and speed of a targeted object.
Range of the object — Time Domain Processing
One way to obtain the distance of an object using RADAR systems is a time of flight.
We transmit a short pulse of radio signal (electromagnetic radiation) and measure the time it takes for the reflection to return. The distance is one-half the round trip time multiplied by the speed of the signal. The factor of one-half comes from the fact that the signal has to travel to the object and back again.
Mathematically, if we have an object situated at Distance R, then,
R = cTr/2
where c = velocity of light and Tr = Round Trip time.
Another way to obtain distance is Frequency Modulation. We change the Transmission frequency over time. Signals take a finite time to travel to and from the target. Naturally, since we are changing the frequency over time. when the received signal receives at the receiver end, we observe it to be a different frequency than what the transmitter is emitting at the moment. By comparing the frequency of the two signals the difference can be easily measured.
Accurate Distance Measurement is important as RADAR finds this use case in sensitive fields such as the Military. For example fighter aircraft for finding enemy aircraft and controlling air-to-air missiles, rockets, and guns.
Speed of an object — Frequency Domain Processing
RADAR exploits Doppler Effect to measure a radial velocity(true velocity component in the direction of radar beam e.g. projection of true velocity on the vector connecting target to radar). Doppler effect is defined as the increase (or decrease) in the frequency of sound, light, or other waves as the source and observer move towards (or away from) each other.
The Doppler effect is only able to determine the relative speed of the target along the line of sight from the radar to the target. Any component of target velocity perpendicular to the line of sight cannot be determined by using the Doppler effect alone.
So we have Doppler Frequency Shift (Fd) as the Difference between Tx frequency and Rx echo frequency = Δf
fo = original Transmitted Frequency
Then the speed of an object is given by,
Apart from Military applications and normal car speed detection on road using police RADAR guns, speed calculation is used in weather RADAR to find precipitation, determine its motion and intensity, and identify the precipitation type such as rain, snow or hail.
Supressing Unwanted Radar Signal
Unwanted interference removal in RADAR systems is an important aspect of many technologies. This is to achieve acceptable performance in hostile environments involving weather, terrain, and electronic countermeasures.
Signal processing is employed in radar systems to reduce the radar interference effects. Signal processing techniques include moving target indication, Pulse-Doppler signal processing, moving target detection processors, correlation with secondary surveillance radar targets, space-time adaptive processing, and track-before-detect. Constant false alarm rate and digital terrain model processing are also used in clutter environments. The Process usually is called RADAR Clutter processing.
One prominent example is Pulse-Doppler signal processing. It includes frequency filtering in the detection process. Pulse-Doppler begins with coherent pulses transmitted through an antenna or transducer. There is no modulation on the transmit pulse. Each pulse is a perfectly clean slice of a perfect coherent tone. The coherent tone is produced by the local oscillator. There can be dozens of transmit pulses between the antenna and the reflector. In a hostile environment, there can be millions of other reflections from slow-moving or stationary objects. Transmit pulses are sent at the pulse repetition frequency.
Energy from the transmit pulses propagates through space until they are disrupted by reflectors. This disruption causes some of the transmit energy to be reflected back to the radar antenna or transducer, along with phase modulation caused by motion. The same tone that is used to generate the transmit pulses is also used to down-convert the received signals to the baseband. The reflected energy that has been down-converted to the baseband is sampled. Sampling begins after each transmit pulse is extinguished. This is the quiescent phase of the transmitter. The quiescent phase is divided into equally spaced sample intervals. Samples are collected until the radar begins to fire another transmit pulse. The pulse width of each sample matches the pulse width of the transmit pulse. Enough samples must be taken to act as the input to the pulse-Doppler filter.
Final Thoughts
Signal Processing plays an important role in RADAR systems. As discussed, it helps to improve the transmission and gain insight into very practical applications such as distance calculation and speed measurement. Not only that, using frequency domain and fast convolution, signal processing also helps RADAR to achieve the Desired Range Resolution. Altogether, this all is very important as RADAR is used quite hostile and prone to interference systems such as weather and military areas. Signal processing techniques help such RADAR systems to work efficiently.
