Data products can be categorized as:
Standard Products
These are products generated using the information received from the satellite.The raw data recorded at the ground station is corrected for various levels of processing. Products are supplied on either photographic or digital media. Photographic products are supplied as films/paper prints and digital products on CD-ROMs.
Corrections applied to raw data.
Raw data suffers from both geometric and radiometric distortions which have to be corrected. The various corrections applied are as follows:
Radiometric Corrections
Each of the sensors of IRS Satellites, consists of linear arrays of Charge Coupled Devices (CCD) with different number of detector elements. The response of the different elements of any of the arrays will not be exactly uniform. Radiometric correction is the process which normalizes these responses on the basis of the laboratory measured radiometric calibration values.
The steps for performing the radiometric correction are :
1) Detector normalization
2) Framing of required scene
3) Failed/degraded detector correction
4) Line loss correction
Geometric Corrections The data acquired by the satellite, have the following distortions which call for geometric corrections:
- Distortion due to the relative motion of the satellite with respect to the earth
- Distortion due to Earth curvature
- Panoramic distortion arising out of the tilt angles
Corrections are needed for removing the above mentioned distortions and projecting onto the user specified map projection in the desired datum.
The distortions in the data from the satellite are corrected in two steps :
- By establishing a mapping between the output space, as defined by the user.
- By transforming the input data into this defined output space.
Product Formatting
The corrected products containing the video data and the supporting ancillary information are organized in user specified format and written into the desired media.
Major digital product formats are
- GeoTIFF
- Fast Format.
- HDF (Hierarchical Data Format)
- LGSOWG (Landsat Ground Station Operators Working Group), also called Super Structure Format (SS).
Standard Product Types
- Path/Row Based products
- Shift Along Track product
- Quadrant products
- Georeferenced products
- Basic Stereo pair
- Area of Interest (AOI) based products
Path/Row based products
These products are generated based on the referencing scheme of each sensor. The user has to specify the path/row, sensor, sub-scene, date of pass, band number/band combination (for photographic products) and the product code as inputs.
Shift Along Track (SAT) products
If a user's area of interest falls in between two successive scenes of the same path, then the data can be supplied by sliding the scene in the along track direction. These are called Shift Along Track products. These products are available from IRS-1C time frame. In this case, the percentage of shift has to be specified by the user in addition to the inputs specified for path/row based products. The percentage of shift has to be between 10% and 90% in multiples of 10%.
Quadrant products
These products are applicable only to LISS-III and TM sensors. In the case of TM only four quadrants are supplied. In the case of LISS-III, the full scene is divided into four nominal and eight derived quadrants. As seen from the Figure, quadrant numbers 1 to 4 are nominal quadrants. The remaining eight quadrants are obtained by sliding quadrants 1,2,3 and 4 by 25% along and across the scene, within the path. LISS-III photographic quadrant products are generated on 1:125,000 scale. While placing a request for these products, users need to specify the quadrant number and shift if any, in addition to the details specified for path/row based products.
Georeferenced products
These are true north oriented products. These products are supplied on digital media only. The location accuracy of these products is same as that of standard products. The inputs to be supplied are same as that of path/row based products with appropriate product code.
Basic Stereo products
The oblique viewing capability of IRS-1C/1D PAN and IRS-P6 LISS-IV sensors can be used to acquire stereo pairs. the Cartosat-1 mission provides along track stereo images.A stereo pair comprises of two images of the same area, acquired on different dates and from different angles.
One of the parameters from which the quality of a stereo pair can be judged is the base/height (B/H) ratio. B/H ratio is the ratio of distance between two satellite passes or the veiwing angular separation of the camerasand satellite altitude.
Two scenes selected on two different dates, satisfying the required B/H ratio are supplied as a stereo pair. The data is only radiometrically corrected and are supplied on digital media.
Area of Interest (AOI) based products
Area of Interest (AOI) based products cover user-specified geographic area. The scenes covering the AOI can be selected from basic stereo data or from single camera data (Aft camera data is default). The scenes covering the user's area of interest are packaged together as tiled products and provided without mosaicing. The different tiles are not radiometrically matched. The minimum and maximum order quantity is 25x25 sq km for IRS-P5 and 23x23Sq Km for IRS-P6. The maximum order quantity is 10,000 sq km. All AOI based products are provided as digital products (CD/DVD) only.
In the case of Cortosat-1,these products are supplied with different processing levels.
i. Radiometrically corrected
ii. Orthokit
The orthokit products are supplied with radiometric corrections and the required projections. A file consisting of the rational polynomial coefficients is also provided for further processing at user's end. Users can produce their own orthorectified / Georectified products by using commercially available, off the shelf (COTS) software.
Value Added Products 
Value added products are generated by further processing the standard corrected data.Examples are
- Geocoded products
- Merged products
- Ortho products
- Template registered products
Geocoded Products
Geocoding corrects the imagery to a source independent format, whereby multi-date and multi-satellite data can be handled with ease. Geocoded products are generated after applying radiometric and geometric corrections, orienting the image to true north and generating the products with an output resolution, appropriate to the map scale (in the case of photographic products). Both floating and Survey of India (SOI) map sheet based products are supplied. All geocoded products are supplied as one product (no split map sheet data as in the case of IRS-1A/1B).
Merged Products
Merged products are generated by merging multi-spectral data with high resolution Panchromatic/Mono band data. The two data sets are first registered and then merged. The registration is based on an automatic technique for selection of control points based on feature contrast and finding match points through digital correlation. The merging of registered data is performed using a number of techniques.
For example, the high resolution LISS-IV image is wavelet decomposed for two levels using the cubic-spline wavelets to obtain a low resolution image corresponding to the LISS-III resolution and 6 detail components corresponding to high frequency information. The LISS-III image is combined with these detail components and the wavelet recomposed to obtain the merged images. While this is the default merging technique used, other techniques are used whenever required.
Ortho products
Ortho images are geometrically corrected products with corrections for displacement caused by terrain and relief. In other words, it is a picture prepared in such a manner that the perspective aspect of the picture has been removed.
Hence ortho image is an image that shows ground objects in their true map or so called orthographic projection. An orthographic projection is the one in which the projecting rays are perpendicular to the plane of projections. Any part of the object that is parallel to the plane of projection will appear in its proper shape and correct scale. These properties of the ortho image enable it to be used as a map for the measurement of distances, angles, areas etc., with scale being constant everywhere. They can also be used as map layers in geographic information systems or other computer based data manipulation, overlaying, management and analysis or display operations. However, ortho image differs from a map in the manner of depiction of detail. On a map, only selected detail is shown by conventional symbols, whereas on an ortho image all detail appears just as in the original satellite image. On the other hand satellite image without terrain correction differs from the ortho image, in scale variation due to height and tilt distortions.
The basic inputs required for orthoimage generation are (i) Digital Elevation Model (DEM), (ii) Ground Control Points (GCP) (iii) Satellite ephemeris (orbit, attitude information) and (iv) Radiometrically corrected image data.
Template registered products
These products are made available from IRS-P6 AWiFS sensor. The AWiFS sensor is envisaged for extensive use in crop monitoring. This can be facilitated by having data sets belonging to the same area, having similar geometric fidelity and temporally registered. Such data sets are called templates in RESOURCESAT-1 context. These templates are used, as reference images to register the AWiFS scene data. The template space is a fixed spatial reference grid over India. Full India space is overlaid with 1 deg X 1 deg template grid and each grid can be uniquely identified by a number.
These are products generated by further processing / analyzing the data like vegetation indices and sea surface temperature profiles. These products are readily usable by the user. |
