Geometric and Radiometric Correction of MOS-1 Imagery in a Canadian Processing System
2 Image Characteristics
MOS-1 was designed to observed and monitor land, ocean and atmospheric processes. The satellite carries three sensors which observe the earth over a wide range of spatial resolutions in the visible, near-infrared, thermal infrared, and microwave regions of the electromagnetic spectrum. In this senses, MOS-1 1aqffords he remote sensing community with an ideal opportunity to study and compare data from a diverse set of sensors. As data is simultaneously acquired by all three sensors, the traditional problems associated with comparing multi sensor datasets taken at different times and under different environmental conditions are eliminated.
2.1 MESSR Imagery
The multi spectral Electronic Self-Scanning Radiometer 9MessR) was designed to observe the earth using four spectral bands in the visible and near infrared spectrum. The sensor contains two radiometers, systems 1 and 2, which point at fixed angles 9nominally 2.73o0 to either side of nadir. under normal operating conditions, imagery may be acquired from only a single system at a given time. Each MESSR is a push broom sensor incorporating a single 2048 lament CCD linear array in each spectral band. it has a nominal ground resolution of 50 meters acquiring data over a 100 kilometer swath to one side of the satellite track. Radio metrically, the MESSR’s four spectral bands are similar to those of the Lands at MSS sensor. This, together with the similar geometric resolutions, has sparked interest in comparing the quality of data acquired by the two sensors.
Evaluation of raw MES imagery has shown that the data contains a number of radiometric and geometric artifacts which hamper its analysis (Manore 89, Henry 89 a ). Significant band mis registration, caused by misalignment of the for detector arrays, is present in imagery acquired by both systems. The magnitude of this mis registration varies between system 1 and system 2, and may be as large as 6 pixels. In many applications such as land use classification, accurate band-to-band registration critical and any misalignment can introduce significant misclassification errors.
An analysis of six MESSR images was undertaken to characterize the magnitudes of the detector array displacements and their dependence on temperature. Using Band 1 as a reference, the spectral bands were correlated against one another at uniformly distributed grid points over image. The mean shifts in the along and across track directions were then calculated, and are shown in Table 1. These results, which are consistent with other5s reported in the literature (Manore 89, Henry 89 a), illustrate the magnitude of the band mis registration errors in the raw data.
Table 1. Mean band mis registration in raw MESSR data in the along and across track directions. The vales are reported in units of pixels.
| MESSR System |
Misregistration From Band 1 |
|
Band 2 | Band 3 | Band 4 |
1
2 | 1.07/0.72
-0.12/0.58 | 2.25/-0.28
4.87/1.38 | 0.80/-0.86
1.05/-1.32 |
Analysis of the radiometric quality of raw MESSR data has identified artifacts introduced by the electronics of the sensor. Vertical striping, typical of linear array sensors, has been observed in all MESSR images studied thus far. This striping takes two forms: random striping caused by differences in the gains and dark current offsets of adjacent CCD elements, and periodic striping caused by the even-odd shift registers of the CCD. The random striping may be removed to a large extent by absolutely calibrating the data using NASDA supplied calibration coefficients. The periodic even-odd striping is not as easily removed.
2.2 VTIR Imagery
The MOS-1 Visible and Thermal Infrared Radiometer (VTIR) is designed to observe the temperature of earth surfaces (sea surfaces) and cloud tops, and ice and clod distribution using one band in the visible region and three bands in the infrared region of the electromagnetic spectrum. The VTIR is most similar to the NOAA AVHRR sensor , although it does have a number of distinct characteristics. It is mechanical scanning radiometer consisting of a single detector in each band with ground resolution of nominally 900 meters in the visible band, and 2.7 kilometers in the infrared bands. The VTIR provides 16 gain modes in each band. The imagery may be calibrated using data obtained while observing a built-in black body and deep space when the scanning mirror is not sweeping the earth.
The image quality of raw VTIR data is relatively good with only a few problems being reported in the literature. While the band mis registration is much better than for MESSR, there is still a 1 to 2 pixel misalignment between the different detectors. Radiometric artifacts may also be observed in the data in the form of horizontal striping particularly noticeable in VTIR Band 4,, as well as a small vertical striping noise almost periodic in nature.
2.3. MSR Imagery
The MOS-1 Microwave Scanning Radiometer (MSR) is designed to designed to observe snow conditions, water vapor in the atmosphere over oceans, and the amount of water in clod and sea ice by receiving microwave noise signals emitted from ground surfaces and oceans. The MSR is comprised of a conical scan system which employs an offset casse grain antenna common to both bands. Half the time required for one rotation of the antenna is used for earth observation while the remaining half is used for recording the calibration data of low and high reference temperature sources. The MSR produces data in two bands (23 GHz and 31 GHz) at two different integration times (47 msec and 10 msec) as a radiometric resolution of 10 bits. the ground resolution is nominally 32 kilometers in the two 23 GHz bands and 23 kilometers in the two 31 GHz bands.
Because of the conical scan system, the ground scanning pattern of the MSR is semicircular, leading to large geometric distortions in the raw imagery. Analysis of the radiometric quality of this data has shown the presence of horizontal typing in the imagery as well as random pixel dropouts, particularly prevalent near the top and bottom portions of the imagery. As the processing unit of MSR data is a full satellite pass, these dropouts are most likely caused by cross in communication with the satellite when it is near the horizon.
