IRS-1C User's Handbook

Orbit And Coverage

The primary objective is to provide systematic and repetitive acquisition of data of the Earth's surface under nearly constant illumination conditions. The satellite operates in a circular, sun-synchronous, near polar orbit with an inclination of 98.69 deg, at an altitude of 817 km in the descending node. The satellite takes 101.35 minutes to complete one revolution around the earth and completes about 14 orbits per day. The entire earth is covered by 341 orbits during a 24 day cycle. The orbital parameters are summarized in the following table

The mean equatorial crossing time in the descending node is 10.30 a.m. ±5 minutes. The orbit adjust system is used to attain the required orbit initially and it is maintained throughout the mission period. The ground trace pattern is controlled within &lusmn;5 km of the reference ground trace pattern.

The sensors collect data with the different swaths. The swath of LISS-III sensor in the visible bands is 141 km while in SWIR band it is 148 km. The swath of PAN and WiFS sensors are 70 km and 810 km respectively. Details of overlaps and sidelaps between scenes of a sensor are given in the following table.

The successive orbits are shifted westward by 2820 km at the equator. The following figure shows a typical Ground trace of the orbits. The entire globe is covered in 341 orbits between 81 deg North and 81 deg South latitudes during the 24 day cycle.

Ground trace pattern

Scene layout

There is an overlap of 7 km between adjacent scenes of LISS-III along a path. Also there is a sidelap of 23.5 km between scenes of adjacent paths at equator. The sidelap has minimum at equator. As we go away from the equator, the sidelap increases because the paths come closer to each other as we move towards the pole. Typically, at 40 deg latitude the sidelap is around 40% of the swath and at 81 deg latitude it is 99%. The Figure depicts the sidelap and overlap in case of LISS-III (SWIR) scenes. It can be seen that the overlap is same as LISS-III (V,NIR) as the lengths of LISS-III (V, NIR) and LISS-III (SWIR) scenes are same. The sidelap is more in case of LISS-III (SWIR) as the swath is also more when compared to LISS-III (V, NIR).

As the swath of WiFS is very large, there is a sidelap of about 85% between WiFS scenes of adjacent paths at equator. But, between the nth and n+6th path, the sidelap is around 105 km at equator. Also there is a overlap of around 80% between adjacent scenes in a path. But the overlap between mth and m+5th scenes along a path is around 130 km. Hence one out of every consecutive five scenes can be downloaded for data products generation.

Revisit capability of PAN

Because of PAN's tilting capacity, a given area can be viewed more than once within one cycle. This is known as revisit due to PAN's steerability. The figure shows a path with three adjacent paths on either side from equator, the tilt angle with which the central path can be viewed from adjacent paths and also the day number on which the adjacent paths occur relative to the central path. From the figure it also can be seen that the maximum wait period to view an area is 5 days only. The maximum tilt angle being ±26 deg, PAN camera can see only three paths on either side at equator. As we go away from equator, paths become closer to each other. Hence, more number of paths can be viewed by PAN at high latitudes.