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Following is a description of main type of Cameras used in satellites.
The Strip Camera
Strip cameras record images by moving film past a fixed slit in the focal plane as the
camera is moved forward. The slit remains fixed, and image is formed on
the film as it moves past the open slit. This camera is used in missions
requiring object height determinations. It is used on a aircraft based
platform. Other uses are airport runaway inspection, highway and rail road
studies, selection of rights of way for pipelines and power lines (Not
in India, though), and for determination of tree types for forestry applications.
The disadvanatge of this type of camera is that there can be a 'banding' effect on the strip
photograph due to cyclic changes of exposure. Also, since the slit is continuously
open, if the film velocity is not steady, as can happen due to aircraft
vibrations, motion blurring will be introduced in the photograph.
The Panchromatic Camera
This is the most widely used camera in satellite imagery applications. Also called the single
lens camera, this camera consists of usual optics which focusses light on a CCD array. The CCD
array converts the light falling on it to voltage, which is then sampled and quantized to get
the actual bitstream, which represents the picture in the digital form.
This camera is also used in aircraft platform, but there instead of CCD arrays, photography film
is usually used. The Panchromatic camera is so called because it can sense the radiation beyond
the visible wavelength(Infra Red ).The main characterstics of a panchromatic camera are :
- Low Geometric distortion and can therefore be used for photogrammetric purposes.
- A low distortion lens system is employed which is held in position relative to plane of the film.
- A Frame of imagery is acquired with each opening of camera shutters which is generally tripped at
a set frequency.
- Focal length usually varies from a few cms to more than a metre,focal lengths of 150mm, 300mm
and 450mm are commonly used.
The panchromatic camera is most widely used camera in remote sensing applications in general. It
finds uses in photogrametry, forests and land cover surveying, and for gathering visual and near
IR band data.
The Panoramic Camera
This is a camera designed to take the photographs of a wide area and therefore it has a lens
having a wide field of view. This enables the camera to take the photograph of a large area,
typically 40 to 50 kilometers in length and (also) breadth. There are different kinds of panchromatic
cameras, and the major types are listed below:
Wide Angle Lens Camera
This camera has a wide angle lens, hence the name. This allows us to photograph a larger area in
a photograph as compared to the normal cameras.
Rotating Lens Camera
This type of camera has the film in a semi-circular assembly, and the lens rotates in an arc,
always keeping the same distance from the film, thus maintaining the focus. As the lens rotates,
It receives reflected light from the surface and focusses on the film through a slit. This allows
the camera to take a picture in an arc of 180 Deg.
Rotating Prism Camera
This is nearly the same as the above type, the only difference is that whilst the lens remains
stationary, a rotating prism is used to focus the light.
The panoramic camera is able to cover a big area in a single photograph with clear details. However,
due to the fact that the image is being taken over a larger area introduces distortion due
to differing conditions of the weather in different parts of the image. Also a geometric
distortion is also introduced due to the constructional features of the camera. So while the
panoramic camera is very useful for preliminary surveys, it cannot be used for photogrammetry.
The Multi Lens Camera
This camera has four lenses each of which focusses light on its own film roll. Each of these
lens assemblies are identical except for the fact that they have different filters. One has a
Red Filter, One has a Green, one has a Blue filter, and one has a Infra Red Filter. We can thus
take photographs of exactly the same area on the ground in four different bands.
These photographs can be viewed in a special viewer in real or false color or various combinations
of filters to view a particular feature in enhanced clarity. This camera is now falling out of
favour mainly because A combination of four identical cameras suitably coupled can perform the
same function, also the technology of multi-spectral scanners have advanced to such an extent that
now they are the preferred instruments for this purpose.
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