Geometric Potential of IKONOS Images


Wieslaw Wolnieiwcz
Warsaw University of Technology
w.wolniewicz@gik.pw.edu.pl


Introduction
Very often and user taking advantage of the final product in form of orthophotomap, aerial photographs and VHRS can see no difference whatsoever between them. Hence the question arises whether geometrical relations obtained from classical aerial photogrammetry correspond with the contemporary VHRS systems and how much are they different.

We tend to transfer the very well known geometry of aerial photogrammetric cameras to geometry of satellite camera. However, we should be aware of the fact that there are substantial differences between them, so we should have completely different approach to geometry of satellite images, especially the ones of very high resolution. The differences consist in the following:

  1. Satellite scanners, including in particular the VHR ones, can be characterized with a very small angle of view. First of all it results from a very long focal of the optical system (for instance, in Ikonos system, the telescope has a focal 10.2 meters long, and its angle of view is 0.950
  2. Internal geometry is very well defined (camera is calibrated) and the parameters of calibration are fixed in time (stable conditions in space).
  3. Elements of external orientation of images are constantly measured with great accuracy and frequency. Positions with single meters accuracy is measured with a use of GPS, and the elements of angle orientation with precision to single angle seconds by inertial system (INS). Systematic error of inertial system (drift) is adjusted by star tracker.
  4. Imaging is produced in a progressive way by way of dynamic recording.
In result of the above mentioned we hale his option to localize (georeference) the image objects solely on the basis of on-board data (i.e. without the points of ground control) with accuracy to a dozen of meters. Such opportunity revolutionized the new approach in satellite photogrammetry and created the new equations for mathematical relations between image and ground – RFM (Rational Function Models).

Characteristics of IKONOS system
Geometry of orbit of the system is show on figure 1 while figure 2 shows the spectrum range of IKONOS system.


Figure 1. Orbital Geometry of IKONOS.       Figure 2. Spectral range for each bend.


The satellite moves some 7 km/s, what means that the ground pixel 1 m for a single element CCD is exposed within the time of 0.14 ms. This time is too short; for creation of sufficient charge on CCD elements, one needs at least min. 0.7 ms, what corresponds to the distance of 5 m. In order to solve this problem, one uses so called TDI - Time Delay Integration. Every panchromatic line of CCD contains a dozen or more physical lines, thus forming a rectangular table CCD of shape enhanced orthogonally to the direction of flight. The charge is produced on every element of the line CCD and it is – pursuant to the lock tact – with a pace of moving image – moved to element in the next line, what provides for “accumulation” of charges forming a given pixel of image from a number of dozen or so physical elements of CCD. It is said that in the background surface of system VHRS there is a line CCD producing panchromatic images, and 4 lines producing images in the narrow ranges of spectrum. It is a considerable simplification. Actual positioning of lines is far more complicated. The figure does not include the lines of length exceeding dozens of thousands of pixels and for this very reason the physical panchromatic line is composed of several separate segments (for instance: of 3 in system Ikonos). These segments slightly overlap along the line and they are orthogonally slightly separated (in the direction of satellite run). In result of preliminary processing, from images produced of several segments, one generates a single virtual line of image. Similarly, the spectral lines are composed of segments, separated due to physical dimensions from panchromatic segments. This means that in effect the spectral images are produced a moment later, after a panchromatic image.

IKONOS has the digital system design pushes the state-of-the art in with focal plane array consists of multiple. panchromatic and multispectral line arrays (figure 3).


Figure 3. IKONOS Focal Plane Unit (Space Imaging 2001).


  • Unit size: 10in. x 9in. x 9in. (25cm x 23cm x 23cm)
  • Panchromatic sensor: 12 micron pixel pitch, 13,800 pixels
  • Multispectral sensor: 48 micron pixel pitch, 3375 pixels
IKONOS is equipped with the Kodak Space Remote Sensing Camera, producing 13 800 pixels with imaging width 11.3 km in nadir point enabling to obtain a size of ground pixel of 82 cm. Of course, this parameter depends on inclination angle of optical system.

The following table illustrates the influence of the deflection angle of optical system upon the size of ground pixel and upon the width of the imaging area, respectively in the systems IKONOS. Figure 4 illustrates the ground size of pixel in relation to the angle of deflection of the optical system.


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