ISRO gives India its Independent Time Standard

A little before the stroke of midnight on Monday, ISRO hurled its navigation satellite, IRNSS-1A, into space. It’s the first in the series that would constitute India’s regional navigational constellation of seven satellites by early 2015. If in the olden days light from the stars guided the humans towards their destinations, today it’s a bunch of satellites in the sky and a network of earthbound tracking stations.

An earlier article in Forbes India, ISRO’s Very Own GPS is Ready, describes the programme and what it means for the country. Here let’s look at the atomic clock which is at the heart of IRNSS. Now that this clock is ready, India has its own time an independent timing standard which can also be used by its financial institutions like the banks and the stock exchanges, power grids, the transport system and any other service where time synchronization is essential.

All navigation systems, like the GPS, GLONASS and Galileo have their own atomic clocks. These clocks work like conventional clocks except that the time-base is different: instead of an oscillating mass which we see in a pendulum clock (or pulse of a quartz crystal), atomic clocks use the energy release from atoms when they switch from one energy state to another as their input to the counter. (More details on how atomic clocks work here.)

Building this atomic clock for IRNSS was both fun and challenge. Like a highly evolved orchestra, these atomic clocks coordinate ground systems, in different parts of the country, the clocks in space and millions of users in a symphony that’s masterful and accurate to a point that most global ventures rely on unconditionally – from weather forecasters, telecom operators to defense organizations. Why we need such an accurate clock is because an error of the magnitude of a microsecond could translate into a ranging error of nearly 300m. This could further accumulate to a position error of close to a kilometer, which is in no way acceptable for a navigation system, says AS Ganeshan, programme director for satellite navigation at Isro

The clock system works by having an IRNSS Network Timing facility (IRNWT) on ground and a Rubidium Atomic Frequency Standard, or a Rubidium clock, on-board the satellite. The ground facility established at Byalalu, near Bangalore, provides extremely accurate and highly stable IRNSS network time which consists, to use the orchestra analogy, an ensemble of Hydrogen Masers and Caesium clocks. These run a timescale with a stability specification of 5 x 10 ^-15 seconds over a day. (In other words, their drift, which all clocks exhibit, is of the order of a millionth of 5 nanoseconds in a day.)

The IRNSS time is provided with an uncertainty of less than 20 nanoseconds with respect to the UTC (Co-ordinated Universal Time).

This is how the IRNSS group of satellites will beam signals when the constellation is complete

Up in the satellite, the Rubidium clock provides the frequency reference on which the navigation signals are generated and broadcast by the satellite. But placing this clock in the satellite posed a challenge of its own. It required a thermal management of the spacecraft because the stability of a Rubidium clock is highly dependent on the temperature. It has to be maintained within a temperature range of +/- 1 ⁰C, when the satellite itself is subjected to the harsh environment of space where the temperature variation is from +150 ⁰C to – 50 ⁰C depending on the side of the satellite that faces the blazing sun or the freezing outer space.

The engineers had to devise a state of the art thermal control systems, says Ganeshan, using a procedure called ‘Yaw flipping” in which gradual rotation of the satellite on its Yaw (the earth pointing) axis is adopted. This flipping maneuver avoids the sun from directly illuminating the panel where the Rubidium clocks are placed. This ensures the stability of the on-board clock which is continuously monitored from the ground control centres and is synchronized with the IRNSS network time through the navigation software.

Simply put, the navigation system works like this: the satellites transmit their position and a precise time; using this information and the speed of light, a ground receiver calculates its distance from a satellite. By using data from four or more navigation satellites, a typical GPS/GLONASS/Galileo/IRNSS receiver determines its location to within 20 metres.

UPDATE: In response to Gp Capt Sandhu's comment below, this post is corrected to clarify that IRNSS is not India's first time standard (or will not substitute Indian Standard Time, or IST) but an independent timing system which is very close to IST. Other satellite navigation systems like GPS and GLONASS also have their own system time which are different from their respective country's standard time. However, they are very close.

IRNSS system time, which is a free to air service and available always, can also be used by various users who need precise timing information.

The thoughts and opinions shared here are of the author.