SPACE

INSAT-2E on course

INSAT-2E has been manoeuvred into its circular geosynchronous orbit and the satellite's performance described as excellent.

R. RAMACHANDRAN

THE 2.55-tonne multi-purpose INSAT-2E, the fifth and final of the indigenously built INSAT-2 series of geostationary satellites, is now firmly in its orbital slot at 83oE. This task of guiding the satellite to its designated position in the circular geosynchronous orbit, after being injected into an elliptical 36,155 km x 250 km Geosynchronous Transfer Orbit (GTO) by the Ariane 42P launcher on April 3, was completed on April 19. All the payload functions, both meteorological and telecomm- unications, have been switched on and the satellite's performance has been described as excellent.

After the initial glitch that led to a premature abortion of the first firing of the liquid apogee motor (LAM) - which appears to have been an entirely random on-board computer processor-related event - the satellite was taken to its drift orbit by a sequence of four LAM firings (on April 4, 6, 8 and 10), instead of the usual three (originally scheduled for April 4, 5 and 7). According to Dr. S. Rangarajan, Director of the Master Control Facility at Hassan, Karnataka, this had its benefits although it caused a two-day delay. The shorter first firing helped the scientists evaluate the performance of the engine as well as the flow characteristics of the fuel from the newly designed single-tank (instead of the usual double-tank) configuration. Further, it also allowed a marginal saving on the fuel because the drift orbit achieved was more favourable than it would have been otherwise.

All the 17 transponders (12 in the normal C-band and five in the lower extended C-band) have been switched on to full power and their performances are good. Systems for the up-down links in the lower extended C band have been specially installed at Hassan. According to Dr. Rangarajan, the satellite is likely to be declared operational by April 26. By May 1, the Indian Space Research Organisation (ISRO) plans to hand over the transponders to INTELSAT to whom 11 transponders have been leased. Three INTELSAT engineers are already in Hassan, evaluating the transponders' performance.

An image sent by the second meteorological instrument on board INSAT-2E, the Charge Coupled Device (CCD) camera. Capable of providing images with a high resolution of one km from a geostationary orbit, the camera was switched on on April 16. It is for the first time that a CCD camera has been flown in a geostationary satellite.

The visible imaging function of the on-board Very High Resolution Radiometer (VHRR), which is designed to provide earth images for meteorological purposes in three spectral bands - visible, thermal infrared (IR) and water vapour - at 2 km resolution, was switched on on April 14. The other spectral bands were switched on on April 19. The VHRR is equipped to obtain imagery in the new 'water vapour band' in the 5.7-7.1 micrometre wavelength region. This is expected to provide data on cloud water vapour content useful for rain forecast. While water vapour (WV) data is bound to be a useful analytical tool, it is not clear how the Indian Meteorological Department (IMD) plans to use them or to distribute them for research. According to ISRO, the IMD may have to supplement (both in terms of hardware and software) the existing data reception and processing system to handle WV data.

The VHRR cloud cover image of the entire globe in the visible region, which was released on April 15, is, in terms of resolution as well as the operative frequency band, no different from the meteorological images from the earlier INSAT series. The half-hourly images and raw data that are currently used as well as broadcast over the world network, and supplied to U.S. researchers under an agreement between ISRO, the IMD and the National Aeronautics and Space Administration of the United States, are from INSAT-2A and 2B.

What is entirely new and spectacular in some sense is the imagery by the Charge Coupled Device (CCD) imaging system, which is actually an array of opto-electronic imaging elements. The system on INSAT-2E is a 900-element system arranged in three strips of 300 elements each, essentially covering the entire earth from the GSO. The image shown on page 121 is the first ever CCD image from a geostationary satellite but taken of a region over the subcontinent that one strip (300 elements) can scan. It is a false colour composite image of the area imaged in the three operative bands - visible (in bands slightly different from the VHRR), near infrared and shortwave IR (SWIR). In the processing of CCD imagery, the basic software that needs to be implemented is the matching and stitching of the images by the three strips. Such a stitched and processed image should soon become available from ISRO.

Dr. R.N. Agarwal, Mission Director, at the Mission Control Room in Hassan, Karnataka.

It is, of course, known that the remote-sensing satellites, which are polar orbiting (at altitudes of 800-900 km), do carry CCD cameras and IRS multispectral CCD images (of resolution 23.5 in V/NIR and 70.5 m in SWIR) are world class. But since they are not stationary over a given region throughout the day, a lot of radiometric corrections and considerable processing (as conditions change by the time satellite returns over the same region) have to be done to obtain the final images.

In case of geostationary satellites at 36,000 km above, once the matching and stitching software is implemented, very little radiometric corrections are required and the processing is much simpler as the satellite is constantly looking at the same region of the earth. The flip side, of course, is that these bands provide images only during the day and also provide a high resolution of 1 km from a GSO. Yet the first images indicate that the quality is extremely good and can be a useful tool in meteorological analysis.

According to Rangarajan, sending a CCD camera as part of the meteorological payload in all INSAT satellites is being considered if there is a domestic demand for it. However, the IMD will have to install an entire chain of new equipment and processing software to make use of CCD images. Also, it is yet to be decided whether these will be put on the world network. Other operational meteorological satellites of the world such as GMS, Meteosat, NOAA and GOES do not take CCD pictures because certain standards for worldwide use of weather images and data have already been set and are being followed. These images would require evolving new standards. However, since the memorandum of understanding with the U.S. is a broad one (covering data from the entire meteorological payload), this data as well as the WV data will be given to U.S. researchers if they evince interest.

Other new elements of the satellite, like the Ni-H batteries and the Gallium-Arsenide/Germanium solar array, the new processor element in the on-board computer system (not the one that developed a glitch and does not come into use again) are all stated to be performing to their design levels.