2. Image acquisition
VHR Optical satellites are passive remote sensing systems concerned with imaging the earths surface. The radiation collected by the satellite originates from the sun, which is either reflected, or absorbed by the earth surface and re-emitted. This is oppose to an active system such as Synthetic Aperture Radar (SAR) for example, which collects backscattered microwave radiation which the satellite emits itself. Most VHR satellites collect data in the Visible and Near Infra Red range of the electro-magnetic spectrum, but one MAXAR satellite additionally collects Short-Wave Infra Red.
As an example of orbit characteristics and collection capability, here's some statistics on WorldView-3:
The satellite orbits the earth at an altitude of 670km, with an orbit period of 97 minutes
The width of swath at nadir (when the satellite is point directly downwards) is 13.1km
It is capable of collecting 680,000 sq km's of imagery per day
The satellite is maneuverable in space, such that it can be pointed at targets in the earth surface.
The revisit time is 4.5 days at 40N and with the satellite pointing up to 20 degrees off-nadir
It can re-visit the same point on earth within one day, however this would be at reduced spatial resolution
Once the data is acquired by the satellite, it is transferred to the Ground Segment via downlink stations for post-processing and dissemination.
Stereo image capture
An additional capability of many VHR satellite platforms is the ability to collect in-track stereo imagery.
In-track stereo imagery describes the way in which a satellite will acquire imagery over the same area twice, as it overpasses. The two separate images are normally acquired approximately one minute apart, requiring the satellite to point and target accordingly. The result is two images with different viewing geometries allowing end-users to perform photogrammetric techniques to extract elevation data from the imagery.
MAXAR has this capability to collect in-track stereo. The process of extracting elevation data from this imagery is now a well established method for building elevation datasets of the polar regions.
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