Indian space programme comes of age

The 1,200-kilogram remote-sensing satellite IRS-1D's successful injection with the indigenous PSLV-CI launch vehicle marks the maturing of the Indian Satellite Launch Programme.

India, which already has the world's largest civil remote-sensing satellite constellation, made a quantum jump with the IRS-1D's augmentation on Monday.

A companion to one of the world's most sophisticated remote-sensing satellites, IRS-1C, put in orbit in December, 1995, the IRS-1D will give the highest resolution imageries of any civil remote-sensing satellite.

Like the IRS-1C, the IRS-1D carries three imaging sensors and an on-board tape recorder to collect data when the satellite is not visible to ground stations.

An additional feature incorporated in the IRS-1D is the satellite positioning system for determining precisely the position of a spacecraft in orbit.

Like its counterpart-in-space, the IRS-1D is planned for global coverage. It will simultaneously cater to requirements from various users, particularly for panchromatic data.

The panchromatic camera on board has a spatial resolution of 5.8 metres and off-nadir viewing capability to provide stereo imaging.

Three charge coupled devices employed in the IRS-1D will provide a total coverage of 70 km and the off-nadir viewing capability, if within plus-minus 26 degree (plus or minus 398 kms), with a revisit frequency of five days.

The Linear Imaging Self-Scanner, a multi-spectral camera operates in visible and near-infrared spectral bands with a spatial resolution of 23.5 metres and a short wave IR band. It has a resolution of around 70.5 metres. LISS-III uses refractive optics and the sensor covers a swath of 141 km for the visible and near IR band, and 148 km for SWIR band.

The Wide Field Sensor, operating in visible and near-IR region, has a spatial resolution of 188.3 metre and a wide swath of 810 km. The total swath is covered using two optical heads, two lenses and two CCDS. Because of the wide swath, the camera will be able to observe the same region once every five days.

Injected at 27 DEG south and 75 DEG east into the polar sunsynchronous orbit at 817 km, the IRS-1D will continue to further enhance data services. The satellite will take 101.35 minutes to complete one revolution around the earth. Thus, it will complete about 14 orbits a day.

The entire earth will be covered in 341 orbits during a 24-day cycle. IRS-1D and IRS-1C will be positioned to ensure combined repitivity in 12 days' time.

The new satellite will boost the efforts of Antrix Corporation, the marketing arm of the Department of Space, to popularise the use of IRS data in other countries. The Antrix Corporation has signed a deal with the US-based EOSAT, giving it rights for commercial reception and distribution of data worldwide from IRS satellites for 10 years.

The EOSAT's earth station at Norman has already been upgraded to receive IRS data. In addition, new earth stations have been established in Germany, Thailand and Taiwan.

The payload data from the IRS-1D, received at the National Remote-Sensing Agency's Shadnagar station, Hyderabad, is recorded in high density digital tapes and processed through networking. A variety of data products will be available to users from IRS-1D. The raw data recorded at earth stations will be connected to various levels of processing. The products will be supplied to users on both photographic and digital media.

While the Hyderabad-based NRSA's data centre will cater to the Indian users, the American SI-EOSAT incorporated would collect requirements from global user community and transmit the consolidated prioritised requests to the data centre.

The IRSS was commissioned with the launch of IRS-1A in March 1988.

The IRS system now has five ISRO-built ones -- IRS-1B (launched in August, 1991), IRS-1C (December, 1995) IRS-P2 (October, 1994) and IRS-P3 (March, 1996).

IRS-1B, like its predecessor IRS-1A, carried two types of cameras, LISS-I and LISS-11, with spatial resolution of 72.5 metres and 36.25 metres respectively. LISS-1 has a swath of 148 kms.

The IRS-1C had incorporated more advanced features than its preceding 'twins', in terms of spatial resolution, additional spectral bands, stereoscopic imaging, wide field coverage and more frequent revisit capability.

Both IRS-P2 and IRS-P3 were launched by Indian Polar Satellite Launch Vehicles during the second and third development launches. While IRS-P2 has LISS 11 cameras (similar to that of IRS-1A and IRS-1B), IRS-P3 carried two remote-sensing payloads WIFS similar to IRS-1C, but with an additional short wave infrared band and a modular opto-electronic scanner of the German Space Agency. IRS-P3 also carried an x-ray astronomy payload and a C-band transponder for radar calibration.

The ISRO has plans to put IRS-P4, carrying ocean colour monitor and a multifrequency scanning microwave radiometre for launch on board PSLV-C2 during 1998-99.

Another two satellites -- IRS-P5, with very high resolution pan camera for cartographic application, and IRS-P6 for agricultural applications -- have been planned for year 1999-2000.

UNI