Table of Contents

RADARSAT-1 Climate Change

A Science Unit for Grades 11 and 12

Collecting Energy

Sunlight

In its infancy, environmental surveillance was performed using visible light radiation from the sun. Images were captured on photographic film from high altitude aircraft.

Films, lenses, and colour filters were designed to capture images covering a wide range environmental features and environmental disciplines ranging from agriculture to mineral prospecting.

Because the sun only illuminates one half of the Earth at a time and since aircraft can only over-fly a very tiny fraction of the Earth each day, a better surveillance technology was required.

For phenomena such as weather, which changes rapidly in relatively short periods of time, and for remotely located phenomena, satellite surveillance works extremely well.

To overcome the illumination problem two solutions are available.

1. Imaging instruments can capture infrared radiation (thermal radiation emitted by the heat of the Earth's surface).
   The NOAA series of weather satellites do this.

2. The spacecraft can illuminate the surface of the Earth and image the reflected radiation.
   The RADARSAT-1 uses this technology.

Illuminating the Earth

RADARSAT creates images by illuminating the Earth's surface with pulsed microwaves, and then capturing the reflected radiation. It's somewhat like shining a flashlight on the ground on a dark night. The main difference is that the microwave beam is monochromatic (single wavelength) and coherent (wave fronts are "in-step" like laser beam) radiation. Also the beam is pulsed so that the echo strength and time-delay can be used to reconstruct a radar image of the Earth's surface.

The figure above illustrates this. The long structure across the "bottom" of the spacecraft is its antenna, which transmits the radar pulse and receives the echo.

Physicists and engineers have discovered a way to combine the signals received by two widely separated radar dishes so that the combined resolution of the signal is effectively the same as a single aperture equal to the separation of the two smaller dishes.

Aperture Synthesis

In the illustration to the left, it is shown that two small 15 m diameter radar receiving-dishes are separated by 105 m. Their signals can be combined to yield the resolution of a single huge dish which is 105 m in diameter. This process is called aperture synthesis, and the system is called a synthetic aperture.

Two Satellites Sounds Good But...

Scientists reasoned that two radar satellites, in a polar orbit around the Earth could be used for high resolution, synthetic aperture observations of the Earth's surface.

But the cost of two satellites is very large, furthermore, the difficulty in keeping them precisely synchronized is somewhat daunting!

It is well known that satellites in low earth orbit travel very fast. Canadian scientists asked "Is it possible, with the aid of high speed electronics and very accurate signal timing to make one satellite "look" as if it were two satellites?
The answer they discovered was "yes!"

Aha! One (Very Fast) Satellite

This is the key to RADARSAT's high resolution capability. RADARSAT is a Synthetic Aperture Radar system.

In a sense, RADARSAT creates the electronic illusion of being in two places at once. This is only possible because of the time delay created by the radar echo (from the Earth's surface), and because extremely accurate time and frequency measurements of the returning signal are recorded. With accurate time, frequency and signal strength information, computers can create high resolution ground images from the synthetic aperture radar.