Watching Over Our Planet - FAQ
- Is remote sensing always done using satellites?
Remote sensing is simply sensing things from a distance. You do "remote sensing" whenever you look, hear or smell. Remote sensing can be done for business and scientific research using helicopters, airplanes, rockets, or balloons. Even kites have been tried, but satellites are definitely the most popular platform for carrying remote sensing equipment.
- Is there a difference between a Landsat satellite and a RADARSAT satellite?
Landsat carries a "Thematic Mapper" scanner that uses the visible and infrared parts of the electromagnetic spectrum to make images. As humans, we can only see the visible light part of the spectrum (the colours of red, orange, yellow, green, blue and violet), but we can't see the infrared parts as Landsat can. RADARSAT, however, carries a radar instrument that uses the radar or microwave part of the spectrum to make images. We can't see that part of the spectrum either. These are the same microwaves that are used in many other ways such as in microwave ovens.
- What is the difference between an ACTIVE and a PASSIVE sensor?
A camera provides an excellent example of both passive and active sensors. It is the film of the camera that is the sensor. It records the light that is reflected from the object that is being photographed.
If the illumination for the scene is coming not from the camera but from another source (say, the sun) then the camera is a PASSIVE sensor. On a cloudy day or inside a room or at night, there may not be enough light. When the camera also has to provide the illumination for the scene (using a flash) it becomes an ACTIVE sensor.
- How does radar work?
A short pulse of energy is sent out by the radar antenna at an angle, towards the ground. The pulse bounces off targets on the ground (houses, trees, grass, telephone poles, etc.) and some of the energy is reflected back to the antenna. This is called "backscatter". The more energy a target backscatters, the brighter it will be shown on the radar image.
- Is there one remote sensing satellite that is the best?
There is no "best". The best choice of satellite data depends upon the application.
Some satellites (and the sensors they carry) are designed for looking at fine detail so that small targets can be imaged. Other satellites specialize in covering very large areas all at once, or perhaps in revisiting the same area often. Radar-carrying satellites are chosen for use at night, for penetrating clouds or for mapping special targets like ice. Other satellites carry sensors that are particularly good at imaging in colour, to help in the "spectral" identification of targets.
- How high up are these satellites?
The earth observation satellites such as Landsat and RADARSAT are about 900 km above the Earth. This is much higher than the international space station (about 200 km) but not as high as the communication satellites (in geostationary orbit) that are used for TV and telephone (about 32,000 km).
- How many remote sensing satellites are there?
RADARSAT is Canadian. Other satellites belong to different countries such as U.S., Europe, Japan, France and India. Private companies are now launching remote sensing satellites too, because they have realized that this technology is very useful and profitable.
- How do the remote sensing satellites "cover" the Earth?
The Earth Observation satellites move in a "near polar" orbit. As the Earth spins west to east beneath them, they orbit from the North Pole down to the South Pole, back up to the North Pole, etc., each time passing close to, but not exactly over the poles. These two motions (the satellite orbit and the rotation of the Earth) make it possible for them to see almost the entire surface of the Earth.
- How long does it take for a satellite to "cover" the Earth?
One Landsat satellite, which looks straight down, takes 16 days to cover the whole of the Earth's surface. The NOAA satellite, which also looks straight down but covers a much wider area takes much less time. The RADARSAT and SPOT sensors, on the other hand, can be steered to point at a sideways angle at a target area from several neighbouring orbits. In this way, in a limited fashion, it is possible to get daily views of an area for several days.
- How is the satellite data sent from way up there, down to us?
There are two receiving stations in Canada. One is in Gatineau, Quebec, the other in Prince Albert, Saskatchewan. Together these two receiving stations can pick up all the data transmitted by satellites passing over any part of Canada. Other ground stations have been set up around the world to similarly capture data from a variety of satellites when they are overhead.
Most of the time the satellites re-transmit the data that they receive directly to the ground station below them, using radio waves. At other times when the satellite is not within line-of-sight of a receiving station, it will store the data on board temporarily, and then transmit to the ground station when it passes overhead.
- How long does it take the data to reach Earth?
The data is transmitted instantaneously (well . . . . to be accurate, it's transmitted at the speed of light).
- Why are some satellite images in black and white and others in colour?
Some sensors record images from just one part of the electromagnetic spectrum, showing the image in shades (usually 256) of grey making what's called a "black and white" image. This is how RADARSAT works.
When an image is recorded simultaneously in several parts of the spectrum, then three of those spectral "bands" are shown as shades of red and green and blue. Landsat and SPOT images are often displayed this way. From those three primary additive colours, one can make any of the other colours such as orange, brown, turquoise, etc. That is also how your TV and your computer monitor work - when the three images in red, green and blue are superimposed on the screen, a full range of colour results.
- Why do we get such strange colours in many of these satellite images?
Remote sensing uses parts of the spectrum that people can't see by eye: infrared, ultraviolet, radar, etc. If we want to display (on a photo or a computer monitor) one or more of these bands, we must use one or more of the three primary colours that people can see: red, green and blue. Therefore, you could get some strange combinations, like: infrared information shown as blue, red information shown as green and green information shown as blue !! The resulting colours will be nothing like what we experience using just our eyes.
- Can sensors .see. underground or underwater?
Under very special circumstances (using long wave radar over an area that is extremely dry) it is possible to see a few metres into the ground. In Canada, where usually the ground has lots of moisture, we are limited to seeing what's on the surface only. Some of the visible wavelengths, like blue for instance, penetrate water quite well and if the water is clear, we can see down several metres.
- What is a "spectral fingerprint"?
It's a way to try to identify objects in a satellite image. By using many parts of the spectrum, including the visible colours and perhaps parts of the infrared band, we try to find how an object reflects light. The way that an object reflects different parts of the spectrum is its "spectral fingerprint". There are different spectral fingerprints for different kinds of trees, crops, soil, etc.
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