However sophisticated the instruments used for remote sensing are, without the means to leave the Earth's surface it is not possible to acquire a reasonable overview of the planet. This is why remote sensing is a relatively new technology. Before the invention of hot air balloons and aircraft, it was not possible to take systematic vertical photographs or images of the Earth's surface.
West London aerial photograph
An obvious way to 'take a picture' of the Earth from a distance is flying aloft. To be able to take the pictures, planes are equipped with cameras. The particularity of planes, from a remote sensing point of view, is that they fly at a relatively low altitude, just a few kilometres above the Earth, and can therefore only take individual photographs of a limited extent, although many details are visible on such photographs, like cars, people, trees, etc.
For surveying aircraft to fly, the weather needs to be good enough in order to have a reasonable chance of acquiring many photographs; the photographs taken by cameras on such aircrafts are therefore usually fairly clear, without many clouds. On the other hand, planes cannot fly during the night or when it is foggy or raining, which may be when an image is required.
Above is an example of an aerial photograph. Can you see all the details on the photograph? You should see cars, lines on the roads, windows on buildings, double-deckers buses, etc.
Launch of Ariane 5 flight 112
In 1957, a major event marked the beginning of a new era in Earth observation: Sputnik, the first artificial satellite, was launched by the Soviet Union.
So what is a satellite? Any idea? Could you name some satellites?
In astronomy, a satellite is understood as a celestial body orbiting around a planet or another celestial body, for example a moon. One satellite that everyone knows about is the Earth's Moon.
Generally, it is said that the Moon is a 'natural satellite' because it is not made by humans. By comparison, 'artificial satellites' are made by us, and orbit around celestial bodies.
So when scientists talk about remote sensing and satellites, what type of satellites are they referring to? Natural or artificial? Why?
* See Answer 1 at the bottom of the page
Note: From now on, when the term 'satellite' is used, it will refer to artificial satellites.
A satellite cannot 'fly' per se, and is unable to leave the Earth’s surface on its own. It needs to be launched by a rocket.
An example of a rocket is Ariane, Europe's satellite launcher.
Satellites do not fly, they move following an orbit. The orbit is the path followed by any celestial body moving around a bigger celestial body.
Usually the orbit is of a quasi-circular shape.
Examples of orbits are those of the Earth around the Sun and of the Moon around the Earth.
Today, satellites are very common platforms used in remote sensing; they carry a great diversity of sensors, often specialised to observe specifically the weather, landscapes or natural disasters, and vegetation. Some are even capable of 'seeing' through clouds or acquiring imagery at night. Two great advantages of satellites with respect to planes are that large areas can be covered with their images and the same area can be observed systematically every time a satellite passes over it.
For example, it would be a huge undertaking for an aircraft to fly over an entire ocean to spot oil slicks potentially left by a boat. With a satellite, the task is much easier. The sensor on board a satellite takes regular images of the ocean and an expert just needs to look at the satellite images to determine if the ocean is clean.
ERS-1 image of polluted sea
The satellite image here shows, in black, a very polluted sea. This image was taken by ERS-1 in December 1992, ten days after the Greek oil tanker Aegean Sea ran aground near the Spanish coast. At that time, the dedicated surveillance aircraft only observed pollution near the coast, but the satellite image revealed that the pollution was much more extensive.
Can you think of another example where it would be preferable to use a satellite rather than a plane to observe phenomena on Earth? Why?
* See Answer 2 at the bottom of the page
However, satellites do have their limits. Clearly, when a satellite is above Australia, it cannot take an image of Europe. If an image of Europe is required, there are two solutions; either wait for the satellite to pass over Europe (but this can take a few days) or have another satellite. This is one of the reasons why so many satellites are needed in order to have a more or less complete coverage of the Earth.
* Answer 1: Artificial because they are 'objects' designed by humans to observe our planet while orbiting it.
* Answer 2: Forest fire monitoring, as it would be dangerous with a plane, as well as tropical cyclones.