This paper investigates the estimation of modulation transfer function (MTF) and point spread function (PSF) using onorbit
data of the first dedicated cartographic mission of ISRO, namely, IRS-Cartosat-1. The Cartosat-1 was launched in
May 2005 with a motivation to realize in-track stereo-pair imagery at a ground sampling distance of 2.5 m with one of
its two cameras, AFT, kept to view a ground scene at -5<sup>o</sup> and the other, FORE, at +26<sup>o</sup> with respect to nadir. As with
any high-resolution satellite imagery, several factors viz., stray light, optics aberrations, defocusing, satellite motion,
atmospheric transmittance etc. can have a strong impact on the observed spatial quality of the Cartosat-1 imagery. These
factors are cumulatively accounted by PSF or by the MTF in the spatial frequency domain. The MTF is, thus, of
fundamental importance since it provides assessment of spatial response of the overall imaging performance of the
system. In this paper, estimation of the PSF and MTF was carried out by capturing imagery over airport runway strip as
well as artificial targets laid at two different locations within India. The method adapted here uses a sharp edge from two
adjacent uniform dark and bright fields or targets. A super-resolved edge of sub-pixel resolution was constructed from
the image edge slanted to satellite path to meet the basic requirement that the target width is much smaller than the
spatial resolution width. From the preliminary results, the MTF for the FORE is found to be approximately lesser by
about 2% with respect to AFT; this difference may be attributed to relatively a longer traverse of ground signal through
the atmospheric column in the case of FORE camera.
With ever increasing demand for high spatial and spectral resolutions, high number of bits of multispectral (MS) sensor
imagery from space borne systems, but not compensated by an equivalent increase on onboard data transmission or
memory limits, efficient data compression and/or streaming approaches gain importance. This paper discusses about the
use of JPEG-Like algorithm, for which hardware and software were well proven from the Cartosat-1 spacecraft, to
compress onboard high resolution multispectral imagery for future missions. It studies two possible ways of
compressing the multispectral data: (1). Apply JPEG-like algorithm bandwise for all three bands, and decompress in
ground processing. This would yield compression ratio (CR) of 1:3.31, (2). Combine IRS-Green and Red (since both are
highly correlated bands) in quincunx sampling grid, compress the grid and IRS-NIR data by JPEG algorithm. This
approach would have the advantage of a higher CR of 1:4.97. It was found that the JPEG like algorithm used in
Cartosat-1 could be directly used for MS data onboard as it would still preserve the spatial and spectral contents of the
multispectral information after decompression in ground processing. Further research work is required to improve the
image quality in the latter case despite the fact that it offers a better CR.
With ever-increasing number of spectral channels from space-borne hyperspectral instruments, demand on approaches
for fast search schemes for matching hyperspectral pixel vector with standard spectral library database has increased
proportionately. The present-day methods are tedious and time consuming to meet the above task. We propose a fast
matching scheme based on bivariate short-interval local variance that can be used to capture the essence of reference
materials in the spectral library. The variance of each selected window is computed across the spectral curve data and
the peak variance above a threshold is taken as a spike. The position and linewidth of the spikes are shown to carry
unique signatures of the given material spectral data, which can be stored and used as matching criteria. The choice of
appropriate threshold is important; it has been found that the mean value of background variance signal could be used as
the threshold value. The proposed method was successfully applied to identify some samples of the AVIRIS
hyperspectral imagery to the standard JPL spectral library database.
A CCD based characterization setup comprising of a light source, CCD linear array, Electronics for signal conditioning/ amplification, PC interface has been developed to generate images at varying densities and at multiple view angles. This arrangement is used to simulate and evaluate images by Super Resolution technique with multiple overlaps and yaw rotated images at different view angles. This setup also generates images at different densities to analyze the response of the detector port wise separately. The light intensity produced by the source needs to be calibrated for proper imaging by the high sensitive CCD detector over the FOV. One approach is to design a complex integrating sphere arrangement which costs higher for such applications. Another approach is to provide a suitable intensity feed back correction wherein the current through the lamp is controlled in a closed loop arrangement. This method is generally used in the applications where the light source is a point source. The third method is to control the time of exposure inversely to the lamp variations where lamp intensity is not possible to control. In this method, light intensity during the start of each line is sampled and the correction factor is applied for the full line. The fourth method is to provide correction through Look Up Table where the response of all the detectors are normalized through the digital transfer function. The fifth method is to have a light line arrangement where the light through multiple fiber optic cables are derived from a single source and arranged them in line. This is generally applicable and economical for low width cases. In our applications, a new method wherein an inverse multi density filter is designed which provides an effective calibration for the full swath even at low light intensities. The light intensity along the length is measured, an inverse density is computed, a correction filter is generated and implemented in the CCD based Characterization setup.
This paper describes certain novel techniques of design and implementation of system calibration for effective Imaging to produce better quality data product especially while handling high resolution data.
Conference Committee Involvement (2)
GEOSS, CEOS, and the Future Global Remote Sensing Space System for Societal Benefits