A seven-satellite, indigenous Indian Regional Navigation Satellite System, created by the Indian Space Research Organisation to service the global positioning data needs of the defence forces, has run into problems with a number of rubidium atomic clocks crucial to providing data failing or developing inherent problems.
ISRO had announced in July last year that all three atomic clocks on IRNSS-1A, the first of the seven satellites that was launched on July 1, 2013, had malfunctioned, rendering that satellite ineffective. Now, sources associated with ISRO’s satellite navigation programme say four more atomic clocks on the other six satellites are not performing as required.
Timekeeping is vital to GPS because it pinpoints a location on Earth by measuring the travel time of radio signals between the location and satellites in space. Since radio signals travel at the speed of light — 300,000 km/second — inaccuracy of one millisecond can send navigation information awry by a distance of 300 km on Earth.
ISRO plans to launch a replacement satellite called IRNSS-1H in July-August to compensate for the loss of IRNSS-1A, although it is yet to announce the failure of more atomic clocks, which has not incapacitated the clock systems on the other six satellites.
The failure of seven of the 21 clocks in the constellation has, however, raised concerns. “Six of the seven satellites are still working,” said a senior ISRO official associated with the programme. “There are, however, inherent hardware problems on the rubidium atomic clocks in some of these. These clocks developed the same problems when used in the Galileo satellite system by the European Space Agency.”
The European Space Agency reported last January that anomalies had occurred in three of 36 Rubidium Atomic Frequency Standard (RAFS) clocks in the 18-satellite Galileo system, although none of the satellites were affected. ESA had said, “These failures all seem to have a consistent signature, linked to probable short circuits, and possibly a particular test procedure performed on the ground…”
Nuts & bolts
ISRO has nine satellites indented for IRNSS. While seven satellites make up the Indian regional navigation constellation, the other two were indented as backup in the event of failure. Each satellite has three atomic clocks, one the primary timekeeper and the other two acting as backup.
ISRO launched IRNSS-1A to 1G between July 1, 2013, and April 28, 2016, a Rs 1,420-crore-project. One of the reasons for providing an indigenous GPS over India, despite the existence of several international systems, was the reliability it offers when used for defence purposes. ISRO had started work on the programme in 1999, after the Kargil war where Indian forces could not use GPS to identify the location of its soldiers.
IRNSS, also called NavIC (Navigation Indian Constellation), is intended to provide two services — Standard Positioning Service for general users and an encrypted Restricted Service for authorised users like the defence forces. NavIC is designed to provide a position information service to users within India and up to 1,500 km from its borders.
As many as 45 rubidium atomic clocks were reportedly procured. They have been used in IRNSS despite warnings by senior officials within the system over their efficacy, said a source associated with the satellite navigation programme over the years. “The IRNSS clocks have been out of spec since the launch of the first satellite. It is a serious situation for the satellite navigation system known as Indian GPS. ISRO has glossed over this because only experts can find out when information from a satellite is incorrect,” the source said.
In July last year, shortly after the seventh satellite was launched, ISRO reported that the three clocks on IRNSS-1A had failed. “Three atomic clocks of IRNSS-1A have stopped working. But the rest of the satellite components are functioning perfectly. In fact, we are using this satellite for messaging activity. The stopping of these atomic clocks has not affected the overall performance of our navigation system. We are planning to launch a replacement satellite,” ISRO chairman A S Kiran Kumar had stated.
The subsequent malfunction of four more atomic clocks , sources have indicated, has not incapacitated any other satellite. “Measures are being taken to correct the problems caused by the clocks in the launch of future satellites. The atomic clocks to be used in the other satellites have been modified to prevent malfunction,” a senior official in the programme said.
ISRO chairman Kumar has indicated the number of satellites could go up from the originally envisaged seven to 11 but it is not clear if this is a consequence of the failing clocks. “We are set to launch more navigational satellites. They are in the process of approvals and clearances,” he said recently, and added efforts were on to revive the IRNSS-1A clocks.
Asked about the failure of the additional clocks, an official ISRO spokesperson said, “We have not been told of any failures other than what has been stated by the ISRO chairman, which is three clocks on IRNSS-1A.”
In Europe, the European Space Agency and industrial partner-supplier SpectraTime have come to agree that “some refurbishment is required on the remaining RAFS clocks” to be used in new Galileo satellites.
“We are also in contact with India concerning this interaction and it looks like the behaviour is different on different satellites. It is really very difficult, but we are exchanging information,’” ESA general director Jan Woerner said last January. “… Because they are so important, there is more than one clock on board. They are necessary because if you don’t have the right clocks and even if you don’t consider the relativity theory of Einstein, the error would be more than 500 m in one hour.”