ISRO launched a PSLV Thursday carrying a replacement satellite for the Indian Regional Navigation Satellite System (IRNSS) constellation. The IRNSS-1H satellite launched on schedule from the Second Launch Pad (SLP) at the Satish Dhawan Space Centre at 19:00 local time (13:30 UTC). However, the launch profile was off-nominal and the fairing failed to separate leading to an apparent failure of the mission.



PSLV Launch:

The IRNSS-1H spacecraft was to be the eighth to be launched as part of India’s Indian Regional Navigation Satellite System (IRNSS), or NavIC, program. One of two ground spares built for the seven-satellite constellation, IRNSS-1H is being pressed into service after the failure of all three atomic clocks aboard the first IRNSS satellite, IRNSS-1A.

The second ground spare, IRNSS-1I, is also expected to launch later this year. Two further ground spares are under construction. IRNSS satellites are built and operated by the Indian Space Research Organisation, ISRO.

IRNSS uses a constellation of seven spacecraft in geosynchronous orbit to provide a navigation service covering most of Asia, the Middle East, east Africa and parts of western Australia. The program gives India a dedicated navigation system, serving military and civilian applications, reducing the country’s reliance on systems controlled by foreign governments.

Satellite navigation relies on precision timing provided by several highly-accurate atomic clocks aboard each spacecraft – three rubidium clocks in the case of an IRNSS satellite. The time of transmission is encoded in signals broadcast by a satellite, which receivers can use to work out the time taken for the signals to reach it – and hence the receiver’s distance from the spacecraft.

By triangulating the distance of the receiver from four satellites, whose precise locations in space are known through orbital ephemeris broadcast along with the timing data, a location can be computed in three dimensions.

The problems aboard IRNSS-1A began in mid-2016 with the failure of one of its three atomic clocks. The remaining two failed over the following six months.

Without the clocks in operation the satellite cannot produce a sufficiently accurate signal to be used for precise navigation, although it is continuing to broadcast system messages. At launch in July 2013, the spacecraft was expected to provide ten years of service.

The clocks aboard IRNSS satellites were built by Swiss company SpectraTime. Several satellites in the European Galileo navigation constellation, which also uses rubidium atomic clocks built by the same contractor, have experienced similar failures. IRNSS-1H will use modified versions of the same clocks as its predecessors.

IRNSS-1H is a 1,425-kilogram (3142 lb) satellite which is expected to provide ten years of service. It will be stationed in an inclined geosynchronous orbit at longitude of 55 degrees East, where it will replace IRNSS-1A.

The satellite is of the same design as its predecessor, based around ISRO’s I-1K bus, with the same ten-year design life.

Four of the seven satellites in the IRNSS constellation are deployed in inclined orbits, with two each at 55 and 111.75 degrees East. The remaining three satellites are in equatorial geostationary orbits, at 34, 83 and 129.5 degrees East.

Deployment of IRNSS began in July 2013, with the launch of IRNSS-1A, while the April 2016 launch of IRNSS-1G completed the initial constellation. All seven satellites in orbit were launched by ISRO using PSLV-XL vehicles, the same type of rocket that was tasked with delivering IRNSS-1H to orbit on Thursday.

The Polar Satellite Launch Vehicle, or PSLV, is the workhorse of India’s space program. First flown in September 1993, it has completed thirty-eight successful missions from forty launches prior to Thursday. The rocket’s last thirty-six consecutive launches, spanning almost twenty years, have been successful. This apparent failure would be a rare loss for ISRO in its recent successful era.

PSLV is a four-stage rocket, using a combination of solid and liquid-fuelled stages. In its standard configuration, the PSLV-G, the rocket is augmented at liftoff by six strap-on solid rocket motors, although the lower-capacity PSLV-CA version omits these. The PSLV-XL configuration, first flown in 2008, uses six more powerful strap-ons to achieve a higher payload capacity.

Thursday’s launch used PSLV C39, a PSLV-XL vehicle which will fly from the Second Launch Pad (SLP) at the Satish Dhawan Space Centre on Sriharikota Island. The Satish Dhawan Space Centre, previously named the Sriharikota High Altitude Range (SHAR), has been the point of departure for all of India’s orbital launches.

The Second Launch Pad was built in the early 2000s to support launches of ISRO’s PSLV rocket, as well as the larger Geosynchronous Satellite Launch Vehicle (GSLV), alongside the nearby First Launch Pad (FLP). Unlike the First Launch Pad, where rockets are assembled at the pad, rockets launched from the Second pad are integrated atop a mobile platform in the complex’s Vehicle Assembly Building.

PSLV C39’s launch began with ignition of the first stage once Thursday’s countdown reached zero. The first stage, or PS1, uses an S-138 solid rocket motor. The first four PS0M-XL strap-on boosters ignited 0.42 and 0.62 seconds later, as the PSLV began its ascent. The final pair of boosters are air-lit, beginning their burn 25 seconds after liftoff.

Each booster consists of an S-12 solid rocket motor. After burnout, the two pairs of ground-lit boosters separated from the rocket at 69.9 seconds and 70.1 seconds mission elapsed time, while the air-lit boosters separated 92 seconds into the flight.

Burnout and separation of the PSLV’s first stage occurred one minute and 50.26 seconds after liftoff, with the second stage igniting its Vikas engine two tenths of a second later. The second stage, designated PS2 or L-40, is liquid-fuelled and burns UH25 propellant oxidized by dinitrogen tetroxide.

The Vikas engine is a license-built derivative of the French Viking engine, which powered the Ariane 1, 2, 3 and 4 rockets. It burned for about two-and-a-half minutes. The payload fairing – or “heat shield” in ISRO parlance – was to separate from the nose of the rocket 92.8 seconds into the second stage burn – however, this never occurred, pointing to the first issue for the launch.

Four minutes and 23.08 seconds after liftoff, the second stage separated from the vehicle. The PS3 third stage ignited its solid-fuelled S-7 motor 1.2 seconds later, for a burn of about seventy seconds. A coast phase followed third stage burnout. At this point the overlaps on the mission control center showed misalignment, pointing to an issue.

At ten minutes, 6.72 seconds mission elapsed time, the spent third stage was jettisoned. Ten seconds later, fourth stage ignition took place. The PS4, or L-2.5, fourth stage is liquid-fuelled burning monomethylhydrazine (MMH) and mixed oxides of nitrogen (MON). It burned for eight minutes and 31.66 seconds.

Thirty-seven seconds after the fourth stage completed its burn, IRNSS-1H appeared to have separated, as images showed the satellite moving around inside what was an unintentional fairing prison.

The satellite was aiming to be separated into a subsynchronous transfer orbit, with a perigee of 284 kilometers (177 miles, 153 nautical miles), an apogee of 20,650 km (12,831 mi, 11,150 nmi) and 19.2 degrees inclination.

The margin of error for the launch is five kilometers (3.1 miles, 2.7 nautical miles) for the perigee, 675 km (419 mi, 365 nmi) for the apogee and 0.2 degrees for the inclination. IRNSS-1H was to use its onboard propulsion system to maneuver to its final geosynchronous orbit.

Thursday’s launch was the fifth of 2017 for ISRO, and the third of the year for the PSLV, following June’s successful deployment of CartoSat-2E aboard another PSLV-XL. ISRO’s next launch is expected to occur in November, with PSLV-XL C40 delivering IRNSS-1I into orbit. However, the impact on this failure on the schedule is currently unknown.

(Images via ISRO).