The launch of IRNSS-1H is scheduled in august 2017 by PSLV-C39. The satellite will be the replacement for IRNSS-1A whose atomic clocks were failed. Although the satellite still performs other functions, the data is coarse, and thus cannot be used for accurate measurements. ISRO plans to replace it with IRNSS-1H in August 2017.
Read more about IRNSS mission/NAVIC http://www.indino.in/indian-regional-navigation-satellite-system-irnss-navic/
|Satellite||Launch Date||Launch Vehicle||Orbit||Status||Remarks|
|IRNSS-1A||1 July 2013||PSLV-C22||Geosynchronous / 55°E, 29° inclined orbit||Redundant||Atomic clocks failed.|
|IRNSS-1B||4 April 2014||PSLV-C24||Geosynchronous / 55°E, 29° inclined orbit||Operational|
|IRNSS-1C||15 October 2014||PSLV-C26||Geostationary / 83°E, 5° inclined orbit||Operational|
|IRNSS-1D||28 March 2015||PSLV-C27||Geosynchronous / 111.75°E, 31° inclined orbit||Operational|
|IRNSS-1E||20 January 2016||PSLV-C31||Geosynchronous / 111.75°E, 29° inclined orbit||Operational|
|IRNSS-1F||10 March 2016||PSLV-C32||Geostationary / 32.5°E, 5° inclined orbit||Operational|
|IRNSS-1G||28 April 2016||PSLV-C33||Geostationary / 129.5°E, 5.1° inclined orbit||Operational|
|IRNSS-1H||PSLV-C39||Planned, Aug 2017||To replace defunct IRNSS-1A.|
Clock Failure –
In 2017 it was announced that all three rubidium atomic clocks on board IRNSS-1A had failed, mirroring similar failures in the Galileo constellation. The first failure occurred in July 2016, following which two other clocks also failed. This rendered the satellite somewhat redundant and required replacement. Although the satellite still performs other functions, the data is coarse, and thus cannot be used for accurate measurements. ISRO plans to replace it with IRNSS-1H in July or August 2017.
Two more clocks in the navigational system had started showing signs of abnormality, thereby taking the total number of failed clocks to five.
As a precaution to extend the operational life of navigation satellite, ISRO is running only one rubidium atomic clock instead of two in the remaining six satellites. Each satellite has three clocks, therefore a total of 27 clocks for all satellites in the system (including standby satellites). The clocks of both IRNSS and GALILEO were supplied by SpectraTime. ISRO replaced the atomic clocks in two standby NavIC satellites. The setback comes at a time when IRNSS is yet to start commercial operations.