The making of Astrosat

Interview with M. Annadurai, Director, ISRO Satellite Centre, Bengaluru.

IN the context of the Indian Space Research Organisation’s (ISRO) Astrosat mission, which is India’s first space observatory, Frontline met M. Annadurai, Director, ISRO Satellite Centre (ISAC), Bengaluru, where Astrosat was built. He elaborated on the challenges involved in building Astrosat, how its construction was different from that of the numerous other satellites that ISAC had built so far, the extraordinary cleanliness required for Astrosat, the features of the special transportation chamber that was built for transporting the satellite from Bengaluru to the launch port at Sriharikota, and so on.

Annadurai was Programme Director, Indian Remote-sensing Satellites and Small Satellite Systems of ISRO before he became the Director of ISAC on April 1, 2015. Between 1992 and 2005, he was the lead member of ISRO’s satellite mission team and managed eight INSATs (Indian National Satellite) as Mission Director and brought about efficient ground automation for satellite operations. As Project Director of Chandrayaan-1, India’s spacecraft to the moon, he made several crucial contributions to its realisation. Chandrayaan-1 won many awards, including the prestigious Space Pioneer Award. His another major contribution related to the realisation of India’s Mars Orbiter Mission.

He graduated in Electronics and Communication Engineering from the Government College of Technology, Coimbatore, and obtained his postgraduate degree in Applied Electronics from PSG College of Technology, Coimbatore. He earned his doctoral degree from Anna University. He joined ISRO in 1982.

Excerpts from the interview given before the launch:

What was ISAC’s involvement in the Astrosat project? What elements of the spacecraft was ISAC involved in and what inputs did you, as a space organisation, provide to the scientific payloads of Astrosat?

Basically, if you look at the satellite, the entire bus system is the creation of our engineers’ understanding of what were the payloads that were expected to go into it. Centring on that, the spacecraft was constructed. This is the first satellite that we have fabricated centring on the payloads. The way the satellite is constructed, once you put the payloads inside [the bus], the satellite is ready.

Secondly, having done that, ISAC, being the lead centre in building Astrosat, did hand-holding even with the instrument-makers. They are scientists and they knew what they wanted. But “how to” is the portion where our people showed the way. Initially we let them be [on their own], but we realised that we cannot leave that entirely to them. Maybe that is one of the reasons why the project got slightly delayed. By ISRO’s standards, this has been delayed. But if you look at any other mission of this calibre, of this nature, we are still better off from the schedule point of view. Still we felt at some point that some hand-holding was required from the engineering point of view, without spoiling the science.

Thirdly, though we made the satellite, we learnt from them [astronomer-scientists] the amount of cleanliness required for this satellite. For Chandrayaan-1 this was required to some extent and we had a clean room. But the clean room required for Astrosat called for one more order of cleanliness. The scientists posed that requirement to us and we met that requirement—the way in which the clean room was done, the way in which dusting of the satellite was done and the way in which even the satellite harnessing wires, or whatever, which come in bundles, was done. You know when these things go in the vacuum, degassing can happen from solder, plug settings, etc. Normally, we do not greatly worry about these things. But in this satellite, you have such [highly sensitive] instruments sitting there, you cannot afford to have systems which can cause such things to happen. This calls for a totally different way of doing things even to make the final harness clean. No component which can degas and contaminate the systems can be there.

Over and above that there were many other aspects, including the way in which the solar panels would be deployed in Astrosat versus the way in which it was normally done for other satellites. When all the systems were in place, the solar panels were attached. The solar panels have to be tested to deploy properly. [There is a difference in] the way in which the solar panels were deployed earlier in other satellites and the way in which it is done now—at zero G [the parts of] the deploying mechanism can also contaminate the instruments. Even the launch-phase activities call for a high degree of cleanliness. Until the satellite goes into orbit, we are not supposed to contaminate it. This called for a totally different work culture in our people. We were not supposed to use even a pencil inside the clean room. We cannot take a sheet of paper inside. This was some sort of good learning for our people—how we should work with scientists for real science instruments.

We had earlier worked on the Chandrayaan and Mars orbiters. But this is real science in a totally different context. Astrosat has sophisticated systems and even a small level of contamination will spoil what we are going to look for. Considering this, Astrosat called for a different level of our way of working, our way of making instruments, the way of integrating them, the way of testing the integrated satellite systems and even the transportation of the satellite. We specifically made a transportation system for the satellite.

What did you actually do in the special transportation system?

One is air conditioning. Also, the vehicle has clean-air filters. While going [when the satellite was transported from ISAC in Bengaluru to the launch port at Sriharikota], all the instruments were purged [with gaseous nitrogen of good quality]. That is, a constant positive pressure was ensured. We should not allow anything to enter [the transportation container]. We also ensured that we had enough redundant systems available. So, if any system should fail during transportation, it should not create a problem.

When was the last purging of the instruments?

The last purging happens when the satellite is mated with the launch vehicle, 14 days before the launch. We ensure that the spacecraft as a whole has positive air flow. There were discussions on whether we can go all along with the entire spacecraft in positive flow so that nothing can get in, and that is how positive pressure is ensured.

For previous satellites, we used to do this kind of air-flow mechanism only for specific systems, particularly the batteries. Even for short durations, temperatures can go even up to 35-40 C. We cannot afford to let the battery temperature go beyond 25 C. Otherwise its life will be affected. So, cool air is punched only into the battery segment. But there we were not very much particular how clean the air was. Today [for Astrosat], more than the cooling, it is the cleanliness we are particular about. So the mechanism is very different. A positive flow of clean air is maintained for the entire spacecraft. In any case, the satellite has a final covering. The cover opens up only when the satellite reaches the orbit, ensuring that everything is clean.

Added to all these, from ISAC’s point of view, given the present schedule of launches, Astrosat was one among many satellites being realised at the same time. When we were building Chandrayaan-I, it had all the priority. But today, we cannot afford to give priority to any one particular satellite. We were building Astrosat along with three other satellites. In three to four years, you will be amazed to see the way in which we will be realising the satellites. Each one of the satellites will have a timetable. To fulfil the timetable, across the constraints, will be a tough situation.

This is akin to an operation theatre. After the operation is done on a patient, another patient comes in. Something similar happens here. We have only limited number of thermo-vacuum chambers and only one acoustic chamber. This means each satellite we are building has an allotted time to undergo the tests in these chambers. So we are working against time. This was another dimension to this mission.

We are building five satellites now—Pancha Ratnas—which will be in prime configuration. They will use all the facilities in ISAC. They have to be taken care of according to the strict time schedule. This is how I am managing all the satellites.

What are the other aspects in which ISAC was critically involved?

The structure [of the satellite] itself was totally different from what we were doing earlier. Our teams worked on the mechanical systems of each of the on-board instruments, including the full telescopes. We realised each one of the doors needed for the telescope and all the mechanisms for them. There were some initial hiccups with regard to the doors, but we were able to solve them. Over and above that, the required filters were done by us. A good number of components were also sourced by ISAC.

What are these filters that ISAC has made for?

These filters are basically for the LAXPC [Large Area X-ray Proportional Counters] payload. They have some xenon pumps, and filtered gas is used for that. We have done the filters. Basically, IISU [ISRO Inertial Systems Unit, Thiruvananthapuram] made them, but we have done the systems engineering. One of the instruments, the SSM [Scanning Sky Monitor], is from ISAC’s Space Astronomy Group. The VSSC [Vikram Sarabhai Space Centre, Thiruvananthapuram] was also involved with some instruments, but we helped from the engineering point of view. ISAC supplied the PCBs [printed circuit boards] and various other components.

After the instruments landed here, they were assembled here [ISAC]. There were some initial failures, but we did not allow them [instrument-makers] to take them [instruments] back and return them. Our people worked here itself. A transit laboratory was built for the first time for Astrosat. Normally, the instruments will arrive here and get mated here [with the satellite]. For example, NASA flew an instrument on Chandrayaan-1. We have an airlock room here. After the payload is transported here, there will be stand-alone tests [in the airlock room] whether everything is okay. These are cursory checks done by them. Then the payload goes to the clean room.

But, [unlike in the case of Chandrayaan-1], here [for Astrosat], tests on the payloads had not been done to that level. So we made another clean room called transit clean room. Each payload came to the transit clean room. It underwent each one of the tests at ISAC only: EMI [electromagnetic interference] tests, thermo-vacuum tests, vibration tests, calibration tests and so on, and any corrections required were done here. Some special fixtures, which are not normally required, had to be made for carrying out both vibration tests and thermovac tests on the payloads. So the payloads stayed on this campus for much more time than they did even in their laboratories, and ISAC provided all the necessary logistics support.

Since all the instruments have to point towards one particular direction, how are you going to organise the solar array pointing towards the sun all the time?

Yes, that requirement is there. Of course, the solar array is capable of rotating in one direction and enough margin is given so that the array need not be perpendicular, but a grazing angle incidence of sunlight is not allowed. We have taken care of that constraint. What is important is that the face [of the satellite] carrying all co-aligned instruments should not face the sun. With whatever axis of rotation that has been given to the solar panel, we are able to manage.

How are you going to schedule the operation of individual payloads in terms of catering to their proposers?

As of now two teams—one from the engineering point of view and the other from the science point of view —have been identified for that. The science team will tell us about whatever proposals that have been received. The first preference during the first few months is for the concerned PIs [Principal Investigators]. Also, the system will scan the sky and they will try to identify a couple of other things. From there, from the time-allotment point of view across the instruments, to start with the PIs will get [time] like any other mission. Having done that, if any other new proposal comes, the science team will look at it and see how operations have to be coordinated. Then the new proposal goes to the engineering team [to see whether] it is plausible from the engineering point of view. One will look at that without affecting the power constraints. Having done that, how quickly we can download. In Astrosat, data can be downloaded even without re-orienting the satellite. Normally, for Cartosat and other satellites, data are collected, the satellite is given proper orientation, and data are downloaded. That is ensured by the two-phased array antennas (PAAs) on two sides of the satellite. They take care of the two hemispheres. That means at a given time, between the two we have to choose the PAA that looks at the ground station. That flexibility exists. Notwithstanding that, we have to take care of the power constraint.

With Astrosat’s launch, and with the earlier launches of Chandrayaan-1 and the Mars Orbiter, do you think there is now a judicious blend of application-oriented satellites such as remote-sensing satellites, communication satellites, weather satellites and navigation satellites and science missions such as Astrosat, Chandrayaan and the Mars Orbiter Mission?

A balanced approach is required. Definitely, after the Astrosat mission, we now know how to handle real-science missions. Both the science and engineering teams have realised that we can work together in a much better way than what we have been used to all along. The thrust on application-oriented satellites continues to be very much there. The way in which I am lining them up, only a few science satellites are there. We are not gearing up in a big way with science satellites. After Astrosat, we have Chandrayaan-II in 2017 and then Aditya, which will take another two years or so.