Gruelling tests

At the Satellite Centre in Bangalore on September 15, scientists perform tests on Chandrayaan-1.-ISRO

The spacecraft, integrated with its payloads, was tested for three months for communication between instruments and with the ground.

CHANDRAYAAN-1 underwent a series of tests for several weeks at the ISRO Satellite Centre (ISAC), Bangalore. It was broiled at temperatures of above 120 degrees Celsius, subjected to freezing conditions below minus 120 degrees Celsius and put in vacuum before it was declared flight-worthy and transported to Sriharikota.

During the thermo-vacuum trials, the spacecraft underwent soak and shock tests. It was subjected to high temperatures for a long duration in the soak test and went through high and low temperatures in quick succession in the shock test. Besides, Chandrayaan-1 faced a high level of noise and vibration tests to check whether its instruments held together. Elaborate tests were done on the solar panel to ensure that it unfolded like an accordion and that its antenna pointed in the right direction.

Of the 11 scientific instruments, five are from India. They were built for the first time in India, said T.K. Alex, Director, ISAC. After the spacecraft was fully integrated with its payloads, it was tested for almost three months for technical interfaces, communication between the instruments, and communication with the ground. For these and the thermo-vacuum tests, special facilities such as a space simulation chamber and an ultra clean room were set up at ISAC.

After ISRO decided to send a spacecraft to the moon, it made an announcement inviting proposals from other countries on the scientific instruments it would like to fly on board Chandrayaan-1. It received 26 proposals and selected six three from the European Space Agency (ESA), two from the National Aeronautics and Space Administration (NASA) and one from Bulgaria. Our instruments and those from abroad complement each other, said Alex.

The six instruments from abroad were tested and calibrated in individual laboratories in those countries Sweden, Germany, the United Kingdom, the United States and Bulgaria and flown to ISAC.

Since every system and subsystem was critical to the spacecraft, for every item, there was a redundant system. If an item or a subsystem did not work, there was a standby, Alex said. There were two star trackers, two gyroscopes, two transmitters, two receivers, and so on. All the 11 instruments were then integrated into the spacecraft bus and tested again for almost three months for technical interfaces and communication between the instruments.

The most important tests were done in the space simulation chamber, which is seven metres deep and four metres wide. We created the vacuum and the temperatures that prevail in space. Chandrayaan-1 will experience minus 150 degrees Celsius to plus 120 degrees Celsius in orbit. We simulated these conditions, using liquid nitrogen and gaseous nitrogen inside a shroud of the space simulation chamber. The spacecraft was tested in varying temperatures for almost 20 days, said Alex.

The spacecraft went through important vibration and acoustic tests. When the launch vehicle lifts off with the spacecraft, it goes through heavy, steady vibrations and shock. The vibration levels are 15 times more than the earths gravity and the frequencies are very large. So the satellite was mounted on a vibration-simulator and subjected to extreme vibrations.

Another important test was in an acoustic simulator developed by the National Aerospace Laboratories (NAL), Bangalore. When a launch vehicle lifts off, its engines generate more than 150 decibels of noise. (An aircraft produces 145 decibels of noise during take-off.). The spacecraft and the instruments on board should be able to withstand this level of noise. There were other tests too on its deployment mechanisms such as the solar array and the antenna. Commands were given to the folded solar panel to open like an accordion and rotate to face the sun in order to generate solar power and supply it to the spacecraft.

M. Annadurai, Project Director, Chandrayaan-1, said no issues emerged when the spacecraft underwent thermo-vacuum tests, including soak and shock tests. He said: During these tests, we found that there was no issue and all the systems were working well.

At the end of these tests, the cumulative results were analysed. The spacecraft again faced a series of tests at the spaceport at Sriharikota after it was transported there by road from Bangalore.

Chandrayaan-1 has a combination of remote-sensing and communication satellites. The complexities of the remote-sensing payloads may be similar to those in a regular remote-sensing satellite, but the comparison ends there. We had to develop prototypes and test them for high levels of endurance in the environment, Alex said.

The lithium-ion batteries, connectors, wires and detectors in the spacecraft were all made in India. The lithium-ion battery supplies power to the spacecraft when its solar panel is not illumined by the sun. You name it and that was made in India. That is the beauty of this mission. Even the space simulation chamber was made in India, he said.

The high-resolution and low-resolution optics in the two cameras of the Indian instruments the Terrain Mapping Camera (TMC) and the Hyper Spectral Imager (HySI) were fabricated at the Laboratory for the Electro-Optic Systems (LEOS) in Bangalore.

Alex said: We get special raw glass, grind it and polish it into mirrors and lenses of very large sizes. We can fabricate at LEOS lenses and mirrors of half a metre to one metre in diameter. This is a world-class facility. It is part of ISAC.