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What is radar?

An introduction as to how radar is used for remote sensing.

Now we are going to introduce you to remote sensing with radar. Last week we noted that only visible light, infrared radiation and radio waves can pass through the atmosphere, and therefore only these three parts of the electromagnetic spectrum can be used in terrestrial remote sensing. We dealt with visible and infrared light multispectral imagery in Week 3, now we are going to look at radio waves!

atmosphere opacity and imagery diagram Radio waves can pass through the atmosphere and so can be used for remote sensing just like visible and infrared light. Based on an image courtesy of NASA.

In remote sensing, radio wave frequencies are primarily used in radar. In fact, ‘radar’ is an abbreviation for ‘radio detection and ranging’ – technical language for ‘finding things and working out how far away they are’!

Radar systems can be mounted on satellites just like multispectral imaging instruments. However, there is one big difference between these two types of remote sensing. Multispectral instruments are passive – they measure the visible light and infrared radiation that come from the sun and are reflected by the Earth. Radar systems are active – they fire radio waves at the Earth and then measure what comes back!

active vs passive diagram Active (right) and passive (left) remote sensing – radar systems are active. Courtesy of William Deadman.

The part of the electromagnetic spectrum most commonly used for radar is actually in the microwave part of the spectrum. This is a sub-section of the radio region that is often distinguished from other radio waves as they have specific uses. We are going to carry on talking about ‘radio waves’ in their most general sense in this section.

As the radio signal that the radar is measuring comes from the satellite itself, it is possible to collect extra information that is not applicable to passive sensors.

  • The length of time it took for a signal to travel to the Earth’s surface and come back.
  • The strength of the returning signal compared to the original signal.
  • Whether the signal has altered in any way.

The first bit of extra information tells us about the height of the specific section of the Earth’s surface being surveyed. As all radio waves travel at the same velocity – the speed of light – the time any signal takes to arrive back at the satellite is directly related to how close it is to the satellite. A mountain top is clearly higher than the ocean, so the distance between the mountain and the satellite is smaller than that between the sea and the satellite. This means that the satellite will receive the return signal in a shorter time from the mountain than from a patch of ocean.

SAR signal diagram The radio signal that hits the mountain will take slightly less time to reach the satellite than the signal that hits the sea. Courtesy of William Deadman.

The strength of the returning signal, and whether it has been altered in any way, can tell us about the nature of the surface it has hit. Some surfaces (like calm water) reflect the signal very cleanly, while others (like dense vegetation) scatter the signal much more. Imagine bouncing a ball on a tarmac road versus bouncing it on thick, wet grass! The sort of surfaces that can scatter the signal can also alter its physical properties. Although this data needs careful interpretation, it can help us understand what sort of surface the satellite is observing.

Radio signal scatter diagram A radio signal will be reflected in different ways by different surfaces. Based on an image courtesy of NASA (from the NASA SAR Handbook).

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