The push-broom sensors in bands meant to study oceans, in general suffer from residual non uniformity even after radiometric correction. The in-orbit data from OCM-2 shows pronounced striping in lower bands. There have been many attempts and different approaches to solve the problem using image data itself. The success or lack of it of each algorithm lies on the quality of the uniform region identified. In this paper, an image based destriping algorithm is presented with constraints being derived from Ground Calibration exercise. The basis of the methodology is determination of pixel to pixel non-uniformity through uniform segments identified and collected from large number of images, covering the dynamic range of the sensor. The results show the effectiveness of the algorithm over different targets. The performance is qualitatively evaluated by visual inspection and quantitatively measured by two parameters.
The deformation of the optical surfaces of a large aperture high-resolution space-borne optical system induced by earth’s gravity on the ground, which is not present during in-orbit operations, necessitates the evaluation of its performance in terms of wavefront error at various stages of development of the earth observation system. A direct method of evaluation for an optical system at an integrated electro-optical module based on a Shack–Hartmann wavefront sensor (SH WFS) is proposed. Design and analysis of the wavefront sensor that are tailored to meet the requirements of the high-resolution optical system are described. We show that the procedure followed for the development of the SH WFS not only addresses the parameters of the wavefront sensor that are critical to its performance, but also aides in the wavefront sensor alignment and calibration. The performance of the developed SH WFS is demonstrated by testing a simulated telescope which is in situ verified in a test configuration using a standard Fizeau interferometer; a close match of the coefficients of Zernike modes between them is established.
We have proposed a method to determine the focal length of microlens array (MLA) based on the measurement of transverse displacement of image spot in the focal plane for a change of angle of incidence of plane wavefront. An existing interferometer test setup, meant for the surface figure measurement of MLA substrate, along with a charge-coupled device (CCD) is used for this purpose. The interferometer generates as well as measures the angle of incidence of plane wavefront at the MLA, and the transverse displacement of the image spot is determined from images recorded with the CCD. We have also discussed the theory of estimation of the focal length of MLA with spherical wavefront. Error analysis is carried out for both methods and is compared. The proposed plane wavefront method is experimentally demonstrated with an off-the-shelf MLA, and the measured focal length is within 1% of catalogue value.
The Resourcesat-2 (RS2), launched on April 20, 2011 is a follow-on mission to the successfully operational Resourcesat-1 (RS1). Similar to the RS1, RS2 carries 3 multispectral imagers in its platform: the Advanced Wide-
Field Sensor (AWiFS), the Linear Imaging Self-Scanner (LISS 3) and the high-resolution multi-spectral scanner
LISS-4. This study focuses on assessment of the radiometric calibration stability of RS2 AWiFS sensor by comparing near-simultaneous measurements of Terra MODIS acquired over CEOS reference standard targets. The AWiFS sensor operates four distinct spectral bands: B2 (0.52-0.59 μm), B3 (0.63-0.69 μm), B4 (0.77-0.86 μm) and B5 (1.55-1.7 μm) with a spatial resolution of 56 m. Only those bands of the Terra MODIS spectrally matching to AWiFS bands are compared after basic corrections for variations due to the sun and satellite angles with reference to scene center and the atmospheric transmittance on the given day of acquisition. Synchronous acquisitions of these
sensors over the desert regions in Libya, Algeria and Egypt at CEOS recommended geographic coordinates were
acquired and processed to compare the top-of-atmosphere (TOA) reflectance from both sensors. Preliminary results and future efforts are also discussed in this paper.
A methodology for calibrating multispectral IRS Wide-Field Sensors (WiFS), which cover typically about 700 km swath is suggested. Two available bands B3 and B4 of IRS-1C and -1D WIFS and respective bands from four of IRS-P6 Advanced WiFS (AWiFS) are employed. Data products acquired over the Thar Desert within 5 days (near-synchronous condition) were utilized. Linear regression model is assumed valid to obtain the calibration coefficients (CCs), after compensating for intrinsic variations of these bands due to spectral response functions and solar zenith angles. It is found that IRS-1C and -1D WIFS cameras show a strong linearity relationship, as evident from their data samples with goodness of fit of better than 0.99. The application of the CCs may hence provide the desired integrated information when their data products are used in tandem. Analysis of the IRS-P6 AWIFS and IRS-1D WIFS image pairs, however, shows a large variability in their output, especially for the Band B4 datasets. The CCs derived for these sensors' pair needs to be used cautiously, Variation in their relative spectral responses demands further investigation, probably with inclusion of the in-situ measurements to account for variations of the target reflectance and the atmosphere while attempting cross-calibration analysis.
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 -5o and the other, FORE, at +26o 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.
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