Canadian Geographic
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Canada from Space


Two basic types of sensor are used in Earth-observation satellites: active and passive. An active sensor, such as a radar, sends an electromagnetic signal from the satellite down to the ground, and measures how much bounces back. That allows the sensor to collect information about the characteristics of the surface, in much the same way that a bat can “see” by making noises and listening for the echo. Passive sensors include simple digital cameras, which detect sunlight reflected off the ground, and thermal infrared sensors, which detect heat radiated from the ground. Sensors are often carefully tuned, like a radio, to particular frequencies that correspond to the signals from plants, clouds, water and ice, or other features of interest.

Satellites can travel in either geostationary or polar orbit. Geostationary satellites are located at very high altitudes of about 36,000 kilometres, and they orbit at the same speed that the Earth rotates, meaning that they stay above the same location on the planet at all times. Many weather satellites are geostationary. Polar-orbiting observation satellites are typically found at a much lower altitude of 500 to 1,000 kilometres, passing near the North and South poles on each orbit. This allows them to pass over much of the planet after a series of orbits. RADARSAT-1, for example, covers most of Canada every 72 hours, and the entire earth every 24 days. Once data is collected by the satellite, it is transmitted to a ground receiving station where it can be analyzed within a few hours. The Canada Centre for Remote Sensing operates two receiving stations, in Prince Albert, Sask. and Gatineau, Que.



This interactive animation depicts two types of sensors found on satellites, as well as two different types of orbits. Clicking on buttons below brings up animations that illustrate how information is received by active or passive sensors, and the flight-paths followed by satellites in a geosynchronous or polar orbit.


On the next page:

Ice and water

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Quiz :

How small is the smallest feature that RADARSAT-1 can clearly resolve in its imaging?

8 m
21 m
3 m