Data bank

Print edition : December 05, 2008

At the Spacecraft Control Centre of the ISRO Telemetry, Tracking and Command Network, at Peenya in Bangalore.-K. MURALI KUMAR

The Indian Space Science Data Centre will ingest, archive and disseminate payload data for all space science missions.

A WEEK before the launch it looked like the most unlikely place to house the all-important Indian Space Science Data Centre (ISSDC) for the Chandrayaan-1 lunar orbiter mission. The ISSDC is intended to be, according to the website of the Indian Space Research Organisation (ISRO), the primary data centre for the payload data archives of Indian space science missions. This data centre will be responsible for the ingestion, archiving and dissemination of the payload data and related ancillary data for all space science missions.

The centres building, located next to the building on which stands the massive deep space antenna DSN-32 on the Indian Deep Space Network (IDSN) campus at Byalalu near Bangalore, was under construction and one wondered how it would be ready in time for the launch. Of course, the data from the 11 on-board experiments would start coming in only after the satellite reached the final lunar pole-to-pole orbit, which would be around November 15. Even then the ISSDC building was far from a state requiring only the final touches.

One had to make ones way past piled-up stones, cement, tiles, bricks and water puddles, and construction workers who were busy obviously working against time. But inside the under-construction building, you were in for a surprise. It seemed that, given the fast-approaching launch schedule, it had become necessary to get the data infrastructure ready first and then the building shell around it. To turn around what is usual in construction business must have required some ingenuity on the part of the civil engineers and the building contractors.

To the right of what would eventually be the main entrance to the building is a corridor. To the left of that is a big hall housing massive computer systems and allied electronics. To the right of the corridor are several smaller rooms with scores of PCs at which people were busy poring over tables and graphs displayed on large desktop monitors. To the left of the main entrance is another huge hall with several video monitors, much like a satellite control room. It is indeed what one may call the instrument control room. Here were the computers for the principal investigators of the various experiments and their scientist-engineer teams to sit and monitor the performance of their respective instruments via both the video images on the monitors and the data streams in the computers on their desks once the spacecraft reached its designated final 100 km circumpolar orbit around the moon.

The downlink signal that will be received at the antenna control centre at Byalalu will be both in S-band and X-band. The former is meant for communications between the Spacecraft Control Centre (SCC) of the ISRO Telemetry, Tracking and Command Network (ISTRAC) headquarters at Peenya in Bangalore for orbit control and housekeeping operations and the latter for data recorded by the on-board experiments. The signal received by DSN-32 (which is built in what is known as the Cassegrain configuration) is reflected on to a sub-reflector situated at the focus of the large 32-metre-diameter dish, which in turn reflects it towards the centre of the dish where there is an opening. From here the signal is carried by a waveguide comprising a series of seven oval-shaped reflectors into the antenna control room situated under the base of the antenna.

As L. Srinivasan, the head of operations at Byalalu, explained, the sixth mirror is what is known as dichroic mirror in honeycomb configuration, which is transparent to one band and reflects the other. This serves to separate the S- and X-band data streams by transmitting the higher frequency X-band through and reflecting the S-band. After separation, the X-band data will be reflected by the seventh mirror on to an antenna feed and digital down converter and then on to a series of cryo-cooled low-noise amplifiers (LNAs). The insides of these LNAs are maintained at sub-zero temperatures of 15 K (-258oC). Similarly, the S-band data will also be sent separately through the feed-down converter-LNA combination of devices.

The signal received by the 32-metre-diameter antenna is reflected onto a sub-reflector situated at the focus of the dish, which in turn reflects it towards the centre of the dish where there is an opening.-K. MURALI KUMAR

Both the unscrambled and amplified data streams are sent to the SCC (interview with S.K. Shivakumar, Director, ISTRAC, Frontline, November 21). Here a basic check is first done on the X-band or science data to ensure that it is in the expected format. This is then checked for quality by ensuring that it is frame-synchronised; that is to say the data received are proper and can, therefore, be used for further processing. The quality-assured data are sent directly to the ISSDC at Byalalu.

The science data received at the ISSDC will first be segregated into data streams for each payload. The segregated data will be processed to generate Level-0 and Level-1 data products at Byalalu itself for distribution to the various science users. Level-0 data are basically raw data. This level corresponds to converting the zeros and ones the binary representation of the data received to some basic format. For example, if the data correspond to some imagery, they will be converted to the corresponding pixel representation.

All the orbit-related and attitude data at the time of data gathering will be added on at Level-1 so that the exact satellite position is known for any corrections that may be required to be made to the basic data, according to Srinivasan. The Level-1 product is thus a gross product; a quick-look product, to which no corrections to any parameter say, geometric corrections such as geographical or coordinate corrections because of the tilt of the camera or vibrations have been made. These will be done at Level-2. Upwards of Level-3, products would be application-specific.

Raw payload data/Level-0 data and Level-1 data will be available at the ISSDC in the respective servers of different payloads. Through the different means of dissemination Internet or dedicated links that have been established for access to all users (Frontline, November 21), these will be transferred to the respective Payload Operations Centres (POCs) for further processing, analysis and generation of higher-level data products.

Each POC is co-located at the respective institutions of the Principal Investigators (PIs) of the various on-board experiments, which will generate the higher-level products depending upon their requirement and various applications. For such purposes the data may have to be band-separated because a particular application may depend on a particular frequency, say, visible or infra-red, which would be carried out at the users end, Srinivasan pointed out. The PI will also coordinate the science to be done with the data with other investigators in a given experiment, who could be from different institutions.

The higher-level data products generated by the POCs will be transferred to ISSDC archives for storage and dissemination. The data archives for Level-0 and higher products will be organised following the international Planetary Data System (PDS) standards. The ISSDC has been designed with a mindboggling storage capacity of 400 petabytes (400 x 1015) or 400 million billion bytes of data to be archived for 20 years, according to Bangararaju, a senior engineer associated with the IDSN.

An important part of the mission from the perspective of ISRO, the launch contractor of these payloads, is the availability of data from foreign experiments to Indian researchers. While the payloads CIXS (Chandrayaan-1 X-ray Spectrometer) and SARA (Sub-keV Atom Reflecting Analyser) have collaborating Indian institutions the ISRO Satellite Centre (ISAC), Bangalore, and the Space Physics Laboratory (SPL), Thiruvananthapuram and associated co-investigators, Indian researchers have apparently shown interest in data from other experiments as well.

After considerable negotiation, it has apparently been agreed to share raw and calibration data of each of the payloads. Accordingly, a nodal Indian investigator for each of the foreign payloads has been identified with whom the PIs of the non-Indian experiments will coordinate the sharing of these data. Of course, all the processed data from the six non-Indian experiments (except for CIXS and SARA) will be the property of the respective PIs.

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