IMPROVEMENT OF SPATIAL RESOLUTION WITH STAGGERED ARRAYS
AS USED IN THE AIRBORNE OPTICAL SENSOR ADS40
R. Reulke®, U. Tempelmann®, D. Stallmann®, M. Cramer", N. Haala®
* Institute for Photogrammetry (ifp), Stuttgart University, Germany - ralf.reulke@ifp.uni-stuttgart.de
? Leica Geosystems GIS & Mapping GmbH, Heerbrugg, Switzerland - udo.tempelmann@gis.leica-geosystems.com
© Stuttgart, Germany - dirk.stallmann@o2online.de
Commission I, WG 1/4
KEY WORDS: photogrammetry, geometry, calibration, modeling, aerial sensor, digital, three-line, pushbroom
ABSTRACT:
Using pushbroom sensors onboard aircrafts or satellites requires, especially for photogrammetric applications, wide image swaths
with a high geometric resolution. One approach to satisfy both demands is to use staggered line arrays, which are constructed from
two identical CCD lines shifted against each other by half a pixel in line direction. Practical applications of such arrays in remote
sensing include SPOT, and in the commercial environment the Airborne Digital Sensor, or ADS40, from Leica Geosystems.
Theoretically, the usefulness of staggered arrays depends from spatial resolution, which is defined by the total point spread
function of the imaging system and Shannon's sampling theorem. Due to the two shifted sensor lines staggering results in a
doubled number of sampling points perpendicular to the flight direction. In order to simultaneously double the sample number in
the flight direction, the line readout rate, or integration time, has to produce half a pixel spacing on ground. Staggering in
combination with a high-resolution optical system can be used to fulfil the sampling condition, which means that no spectral
components above the critical spatial frequency 2/D are present. Theoretically, the resolution is as good as for a non-staggered line
with half pixel size D/2, but radiometric dynamics should be twice as high. In practice, the slightly different viewing angle of both
lines of a staggered array can result in a deterioration of image quality due to aircraft motion, attitude fluctuations or terrain
undulation. Fulfilling the sampling condition further means that no aliasing occurs. This is essential for the image quality in quasi -
periodical textured image areas and for photogrammetric sub-pixel accuracy. Furthermore, image restoration methods for
enhancing the image quality can be applied more efficiently. The panchromatic resolution of the ADS40 optics is optimised for
image collection by a staggered array. This means, it transfers spatial frequencies of twice the Nyquist frequency of its 12k sensors.
First experiments, which were carried out some years ago, indicated already a spatial resolution improvement by using image
restitution the ADS 40 staggered 12k pairs. The results of the restitution algorithm, which is integrated in the ADS image
processing flow, has now been analysed quantitatively. This paper presents the theory of high resolution image restitution from
staggered lines and practical results with ADS40 high resolution panchromatic images and high resolution colour images. created
by sharpening 12k colour images with high resolution pan-chromatic ones.
1. INTRODUCTION found e.g. in (Holst, 1998). For staring arrays, the sampling
frequency is created by the detector center-to-center spacing.
1.1 High Resolution Digital Cameras The effective sampling frequency can be increased with
microscan or dither technique in one or two directions. To
increase the sampling rate, the scene can be mechanically or
optically dithered. By these means the image is augmented by
an additional set of samples with an offset relative to the initial
set by half the width of the detector elements.
With linear scanning systems, the detector output in scan
direction can be electronically digitized at any rate. The only
limitation results from the sensor sensitivity required to
achieve an appropriate signal to noise ratio (SNR). In cross
MZ scan direction, the sampling rate for single linear arrays is
applications in airborne and close range photogrammetry. defined by the detector size. Using an additional array and
In the last few years first commercial available digital mapping shifting the detectors, any vertical Nyquist frequency can be
cameras like Leica ADS40 (Sandau, 2000), Zl/Imaging DMC created (Holst, 1998). Figure | shows this situation for an ideal
C d and Vexcel Ultracam (Leberl, 2003) become undisturbed scan using single and a staggered array. For a
available.
Recent technology developments provide new solutions for
high-resolution image acquisition for photogrammetric
applications. These cameras are based on large CCD-matrices
as well as CCD-line sensors and allow for a large freld of view
and a high spatial resolution. Single- or multi-line cameras
have been designed for spaceborne, airborne and terrestrial
scanners to provide high-resolution wide angle or even
panoramic imagery. Such cameras allow new approaches of 3D
scene visualization. and reconstruction as well as new
single linear array the detector size Apix is equal to the
. : ; sampling distance Opix. The sampling limit or Nyquist
1.2 CCD-line cameras and staggered CCD-line arrays a. ee ; ; m
: frequency is 1/26, In the case of staggered arrays the
sampling distance is half of the pixel size and therefore the
Resolution improvement is a scientific topic for imaging since er ae
sampling limit is extend to 1/8pi.
more than 20 years. Sampling and resolution issues can be
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