India will begin deploying its second generation of indigenous navigation satellites Monday, with the launch of a GSLV Mk.II rocket lofting the NVS-01 satellite. Liftoff, from the Satish Dhawan Space Centre, is scheduled for 05:12 UTC (10:42 a.m. local time).
The Navigation with Indian Constellation, or NavIC, system is a regional satellite navigation network developed by the Indian Space Research Organisation (ISRO). Also known as the Indian Regional Navigation Satellite System (IRNSS), the system’s first generation consists of seven satellites in geosynchronous orbit – although a total of nine spacecraft have been launched, including replacements.
IRNSS-1A, the first satellite in the series, was launched in July 2013, with the constellation reaching its full complement following the launch of IRNSS-1G in April 2016. After three years in orbit, the atomic clocks aboard IRNSS-1A began to develop faults, and IRNSS-1H was launched as a replacement in August 2017. This did not reach orbit after the payload fairing of its carrier rocket failed to separate, so IRNSS-1I was deployed instead in April 2018, becoming the final first-generation IRNSS satellite to launch.
While IRNSS-1A is the only satellite in the constellation that ISRO has confirmed to have suffered failures of all its clocks, the agency has confirmed the failure of several other clocks across other satellites in the fleet. ISRO’s annual report for 2020-21 noted that the IRNSS-1G satellite had also been moved to a secondary role, but did not identify the reason for this. NVS-01 is expected to take IRNSS-1G’s slot in the constellation.
Accurate timekeeping provided by atomic clocks is vital to the operation of navigation satellites, as the signals they broadcast contain timestamps. Receivers use these timestamps, and the knowledge that radio signals propagate at the speed of light, to calculate distance to the satellites and to triangulate their position based on the signals and relative positions of multiple satellites.
Beginning with NVS-01, the second-generation satellites will fly with Indian-developed atomic clocks in place of the units manufactured by Swiss company SpectraTime which were flown on previous spacecraft.
With a mass of 2,232 kilograms, NVS-01 is also over 900 kilograms heavier than the first-generation satellites and incorporates other enhancements to expand the capabilities of the NavIC constellation. These include broadcast of a new L1 signal, in addition to the L5 and S-band signals broadcast by the existing satellites, which will improve interoperability with other satellite navigation systems such as the US Global Positioning System (GPS) network.
The new second-generation spacecraft, including NVS-01, are based on ISRO’s I-2K satellite bus instead of the I-1K platform used on the first-generation spacecraft. Powered by a pair of solar arrays generating up to 2.4 kilowatts, the satellites have a design life of at least 12 years.
The increased mass of the second-generation satellites renders them too heavy for the Polar Satellite Launch Vehicle (PSLV) rockets that were used to deploy the first generation. Instead, they will be carried aboard the more powerful Geosynchronous Satellite Launch Vehicle, or GSLV, using the Mk.II version of this rocket.
NVS-01 being encapsulated in GSLV F12’s payload fairing (credit: ISRO)
GSLV is a three-stage rocket which first flew in April 2001, with its first and second stages being derived from the PSLV. Four liquid-fuelled strap-on boosters are attached to the first stage, in place of the solid boosters used on PSLV, and it has a high-energy cryogenic propellant third stage. The GSLV Mk.I and Mk.II differed in the use of a Russian-engined third stage on the now-retired Mk.I, while the Mk.II introduced an Indian-built engine on the cryogenic stage. The larger GSLV Mk.III is of a separate, unrelated, design.
Monday’s launch will be the first GSLV Mk.II mission since August 2021, and marks the rocket’s return to flight after failing to deploy the EOS-03 satellite in its previous outing. During that launch, which ended a string of six successful launches, the pressure in the rocket’s third stage hydrogen tank failed to build up as expected in flight, and therefore the stage’s fuel booster turbopump (FBTP) was unable to supply sufficient propellant to the engine. An investigation determined that a vent and relief valve had leaked after damage to its soft seal, possibly from the stress of operation, cryogenic operating conditions, or contamination.
As a result of the failure, several changes have been made to GSLV’s third stage to prevent a repeat. The vent and relief valve has been strengthened, but to ensure sufficient pressure in the third stage propellant tanks, a pressurization system has been installed rather than relying on the natural build-up of pressure as the hydrogen expands. Additional monitoring equipment has also been installed on the stage.
Prior to the 2021 failure, GSLV looked to have turned a corner after an initially-troubled service life. Of the type’s first seven launches, only two reached their planned orbits. Another mission was a partial failure, achieving a lower-than-planned orbit but one that its payload was able to recover from. The maiden flight also reached a lower-than-planned orbit but in that case, the satellite was unable to recover to its planned orbit – although in part due to a design flaw in its own propulsion system as well as the off-nominal launch. Three more launches failed to reach orbit altogether.
GSLV F12 being rolled out to the Second Launch Pad (credit: ISRO)
After a three-year gap between the seventh and eighth GSLV launches, the type returned to flight in 2014 with its first successful launch in almost a decade. Five more successes followed between 2015 and 2018, before the failure in 2021.
The NVS-01 launch marks the fifteenth flight of the GSLV rocket (excluding GSLV Mk.III), and the ninth use of the GSLV Mk.II with its indigenous cryogenic engine. The mission is designated GSLV F12, and will lift off from the Second Launch Pad (SLP) at the Satish Dhawan Space Centre (SDSC).
SDSC has been the site for all of India’s orbital launches to date. It currently has two active launch pads for orbital-class rockets: the First Launch Pad, which is mostly used for PSLV missions, and the Second Launch Pad which supports PSLV, GSLV Mk.II and GSLV Mk.III. Historically, GSLV launches have also occurred from the First Launch Pad – however the most recent of these was in 2004, before the SLP came online.
Rockets launched from SLP are assembled vertically away from the launch pad, before rolling into position on top of a mobile launch platform. GSLV F12 arrived at the Second Launch Pad on May 24, five days before its planned liftoff.
Monday’s launch will begin with ignition of the four High Thrust Vikas Engines (HTVEs) powering the rocket’s quartet of L40H strap-ons, at the T-4.8 second mark in the countdown. At T0, the center S139 solid rocket motor will light, and GSLV F12 will lift off. Initial ascent will be under the power of the four HTVEs engines and the S139, with the liquid-fuelled engines continuing to burn after S139 burnout about 106 seconds into the mission.
GSLV F12 at the Second Launch Pad ahead of the NVS-01 mission (credit: ISRO)
At 148.9 seconds mission elapsed time, the Vikas engines will shut down. Six tenths of a second later, the second stage’s single engine – also an HTVE – will ignite. Stage separation will come 1.6 seconds after second stage ignition, with the interstage being jettisoned from the base of the second stage about 5.8 seconds later.
First flown on GSLV F08 in March 2018, The High Thrust Vikas Engine is an enhanced version of the standard Vikas used on previous GSLV and PSLV missions. Vikas is itself a license-built derivative of the French Viking engine previously used on Ariane rockets, and burns UH25 propellant – a mixture of unsymmetrical dimethylhydrazine and hydrazine hydrate – oxidized by dinitrogen tetroxide.
GSLV F12 is using the new ogive-shaped payload fairing that was introduced with the previous mission, GSLV F10. This protects the satellite as the rocket ascends through the dense lower regions of Earth’s atmosphere, but is no longer required once it reaches space. Three minutes and 57.4 seconds into the flight, with the rocket flying at an altitude of 114.76 kilometers, the fairing will separate and expose NVS-01 to space for the first time.
The mission timeline for the GSLV F12 mission with NVS-01 (credit: ISRO)
The second stage burn will last 141.8 seconds from ignition to shutdown. The second stage will separate about 3.4 seconds after its engine shuts down. The third stage will ignite one second after staging to complete the mission of inserting NVS-01 into orbit. GSLV F12 introduces upgrades to the third stage, including a “V2”, or version 2, engine as well as the new pressurization system developed to prevent a repeat of the GSLV F10 failure. As a cryogenic stage, it burns liquid hydrogen and liquid oxygen.
After firing for 13 minutes and 29.5 seconds, the third stage will shut down, with burnout expected to be complete five seconds later. Ten seconds after this, at 18 minutes and 40 seconds mission elapsed time, NVS-01 will separate. The target orbit for spacecraft separation has a perigee and apogee – the points closest to and farthest from Earth – of 170 and 36,568 kilometers respectively, with an inclination of 19.36 degrees to the equator. NVS-01 will use its own propulsion system to raise itself to its final geosynchronous orbit.
Following on from Monday’s GSLV Mk.II launch, India’s next missions are expected to be the launch of the Chandrayaan-3 lunar lander aboard a GSLV Mk.III in mid-July, and a PSLV launch to deploy the Aditya-L1 solar astronomy satellite shortly afterward.
(Lead image: GSLV F12 at the Second Launch Pad ahead of Monday’s launch. Credit: ISRO)
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