University of Hannover
Currently, IKONOS, QuickBird and OrbView-3, are the three commercially used very high resolution (VHR) optical satellites with ground sampling distances (GSD) of 1m and better.
The other VHR satellites in the making, are – IRS Cartosat-2 from India, Kompsat-2 from South Korea, EROS-B from Israel and Pleiades from France having a GSD between 0.7m and 1m. In addition, the resolution will be improved by WorldView-1, WorldView-2 and OrbView-5 from USA down to 0.5m. The competition is improving the order conditions and making the data acquisition for geo-information products more economic. As a result, the number of imaging satellites as well as their imaging capacity is becoming increasingly important. The imaging capacity is dependent on the storing and download capabilities and more importantly, the agility and the requirement for a slow down mode. With insufficient sampling rate or the requirement for collecting more energy because of missing transfer delays and integration, the satellites have to rotate during imaging to reduce the angular speed; this of course leads to a reduced imaging capacity.
Today, nearly all original space images are projected to a plane with constant height, such as done by IKONOS Geo and Quick- Bird OR Standard. The tendency goes towards images being projected to a plane, named by SPOT level 1B. The scene orientation has to respect the image product. All imaging satellites are equipped with a positioning system like GPS, gyros and star sensors. Based on this, the full orientation of each image line can be determined. The now available VHR sensors do allow a standard deviation of the ground coordinates better than 10m without control points. This may be sufficient for some purposes, but usually it has to be verified or improved. Following are the different orientation procedures in use:
A. Rational polynomial Coefficients (RPC) – the direct sensor orientation from the satellite vendors do allow the determination of the relation between the image and the ground coordinates by a polynomial as function of the geographic ground coordi- nates X, Y, Z divided by another. Third order polynomials are in use, so with 80 coefficients the orientation information can be expressed. This can be improved by means of control points named bias corrected RPC method.
B. For the centre of the scene or the start of the scene, the view direction from the ground to the satellite is given in the header data distributed together with the images. Together with the information about the satellite orbit and the image progress, this allows the geometric reconstruction of the imaging for any ground point. Like with the preceding described method, this has to be improved by means of control points. For original images, the ephemeris is given, allowing a similar scene orientation.
C. The field of view is very small, allowing also some approximations. With the 3D-affine transformation, the mathematical model of parallel projection may be used. It is not using any of the available orientation information; so at least 4 three-dimensional well-distributed control points have to be used.