RADAR and SAR technology

This article explains the basics of RADAR and SAR technology.

The word RADAR stands for Radio Detection And Ranging which is an electronic device to measure or track objects at distances using electromagnetic waves (radio waves) or ultrasonic sound. The RADAR is built on the simple principle of wave reflection. When a wave is transmitted it tends to reflect when it hits any other surface. The reflected wave can be used to measure the distance of the object. In ancient times, people navigating in the ship used sound to find the nearby land. They used to blow the horn in the middle of the ocean if the sound reflects back as an echo after hitting the cliff, the land is near, if not the sound would disappear and they need to keep rowing. 

 

Figure 1: RADAR principle 

 

Figure 1 above shows how the RADAR system works. The antenna transmits the pulse and it reflects back when it hits the target and the antenna again picks up the reflected pulse to calculate the exact position, speed, direction, shape, and size of the object. 

Synthetic Radar System (SAR), which is a physically moving small instrument over an area, uses the same principle as explained above to take the map of the surface using electromagnetic waves (radio waves) which are capable of traveling through space at the speed of light.  SAR measures the distance between the sensor and the surface of the earth and stores the reflected signal in a two-dimensional array which later transformed into a picture. Since its constantly moving it is mounted in satellites and airplanes which helps to cover more area in a short time. 

For e.g., to get the image of the terrain the SAR sensor moves at a uniform speed and altitude scanning along the way which is called azimuth direction, and the direction perpendicular to the flight path is the across-track or range direction. SAR sensor receives thousands of backscattered waves which are collected and the data is processed to get the 2-d image of the surface. The intensity of the reflected waves depends on the characteristics of the illuminated terrain. For e.g., the calm water and smooth surfaces look black whereas the rough surfaces appear bright.  

 

Figure 2: Image of the San Diego 

SAR uses a very wide range of frequency bands so it can be used to visualize different surfaces at any time and any moment. In recent years, constellations of SAR satellites have been deployed in space to monitor the surface of the earth. It can be very useful against natural disasters, climate change, and human error.