IRS-1C User's Handbook

Referencing Scheme

Introduction

Referencing scheme which is unique for each satellite mission, is a means of conveniently identifying the geographic location of points on the earth. This scheme is designated by Path and Rows. The Path-Row concept is based on the nominal orbital characteristics. This section describes the referencing scheme and related information.

Path

An orbit is the course of motion taken by the satellite in space and the ground trace of the orbit is called a 'Path'. In a 24 day cycle, the satellite completes 341 orbits with an orbital period of 101.35 minutes. This way, the satellite completes approximately 14 orbits per day. Though the number of orbits and paths are the same, the designated path number in the referencing scheme and the orbit number are not the same. On day one (D1), the satellite covers orbit numbers 1 to 14, which as per the referencing scheme will be path numbers 1, 318, 294, 270, 246, 222, 198, 174, 150, 126, 102, 78, 54 and 30, assuming that the cycle starts with path 1. So orbit 1 corresponds to path 1, orbit 2 to path 318, orbit 3 to path 294 etc.. The fifteenth orbit or first orbit of day two (D2), is path 6 which will be to the east of path 1 and is separated from path 1 by 5 paths.

Path number one is assigned to the track which is at 29.7 deg West longitude. The gap between successive path is 1.055 deg. All subsequent orbits fall westward. Path 1 is so chosen, that the pass with a maximum elevation greater than 86 deg for the data reception station of NRSA at Shadnagar can be avoided. This is due to the limitation of antenna drive speed, since it is difficult to track the satellite around zenith. In fact, above 86 deg elevation, if a pass occurs, the data may be lost for a few seconds around zenith. Hence, the path pattern is chosen such, that the overhead passes over the data reception station is reduced to a minimum. To achieve this, path 1 is positioned in such a manner that the data reception station is exactly between two nominal paths, namely 99 and 100. During operation, the actual path may vary from the nominal path pattern due to variations in the orbit by perturbations. Therefore, the orbit is adjusted periodically, after certain amount of drift, to bring the satellite into the specified orbit.

The path pattern is controlled within ±5 km about the nominal path pattern. Due to this movement of actual paths within ±5 km about the nominal path, it is not possible to totally avoid above 86 deg elevation passes for Hyderabad. However, with this approach, the number of passes above 86 deg elevation is reduced to almost one in a 24 days cycle.

Row

Along a path, the continuous stream of data is segmented into a number of scenes of convenient size. While framing the scenes, the equator is taken as the reference line for segmentation. The scenes are framed in such a manner that one the scenes' centre lies on the equator. For example, a LISS-III scene, consisting of 6000 lines, is framed such that the centre of the scene lies on the equator. The next scene is defined such that its centre lies exactly 5,703 lines from the equator. The centre of next scene is then defined 5,703 lines northwards and so on. This is continued upto 81 deg North latitude. The lines joining the corresponding scene centres of different paths are parallel to the equator and are called Rows. The uniformly separated scene centres are such that same rows of different paths fall at the same latitude. The row number 1 falls around 81 deg North latitude, row number 41 will be near 40 deg North and row number of the scene lying on the equator is 75. The Indian region is covered by row numbers 30 to 90 and path numbers 65 to 130.

Scene Definition

The camera scans the ground track line by line continuously. The satellite motion along the track provides continuous imaging of the ground. This continuous stream of data is segmented to convenient sizes. These segments are called scenes. The camera system takes certain amount of time to read and register the CCD array data. This integration time is chosen prior to launch and is fixed throughout the mission. The integration time for each camera is so chosen, that it is equivalent to the time taken by the satellite in nominal orbit to traverse the scan line distance of the respective cameras. The across track width is limited by the swath of the respective cameras. Due to the line-by-line mode of scanning, the along track scan is a continuous strip and is divided into a number of uniform scenes.

Each line of the camera consists of a fixed number of CCD elements in the form of an array. The image obtained by one CCD element is a pixel. The pixel size on ground is equal to the resolution of the respective cameras. The across track length of the scan (swath) is determined by the pixel size and number of elements in a line. Each imaging sensor scans line by line during its integration time, which is fixed for each camera. Thus, each camera scans a fixed number of lines in fixed intervals of time. Therefore, the along track length of a scene is based on the number of lines used to constitute that scene.

Use of Referencing Scheme

The Path-Row referencing scheme eliminates the usage of latitude and longitudes and facilitates convenient and unique identification of a geographic location. It is useful in preparing accession and product catalogues and reduces the complexity of data products generation.

Using the referencing scheme, the user can arrive at the number of scenes that covers his area of interest. However, due to orbit and attitude variations during operation, the actual scene may be displaced slightly from the nominal scene defined in the referencing scheme. Hence, if the user's area of interest lies in border region of any scene, the user may have to order the overlapping scenes in addition to the nominal scene.

Comparision between IRS-1A/1B and IRS-1C Referencing Scheme

The referencing scheme of IRS-1C is different from that of IRS-1A/1B. In the IRS-1C referencing scheme, the adjacent path occurs after five days and not on the next day as in the case IRS-1A/1B. This type of referencing scheme has been chosen keeping in view the PAN sensor, so that the revisit capability of 5 days can be met. The following table gives the major differences in terms of referencing scheme pattern of IRS-1C from IRS-1A/1B.

The huge overlaps between the WiFS scenes of adjacent paths results in repeated coverage of the same area in a given cycle. A given scene can be covered completely on its day of pass and also by a combination of two scenes acquired on different days during the cycle.