ISRO’s Polar Satellite Launch Vehicle (PSLV) has successfully deployed the SCATSAT-1 ocean research satellite as part of an eight-satellite payload due to launch from the Satish Dhawan Space Centre early on Monday. Liftoff of the Indian rocket was on schedule at 09:12 local time (03:42 UTC).
Monday’s eight-satellite launch, for which SCATSAT is the primary payload, saw the Polar Satellite Launch Vehicle (PSLV) conduct its most complex mission to date lasting two and a quarter hours from liftoff to deployment of the final payload.
The Scatterometer Satellite 1 – or SCATSAT-1 – spacecraft is designed to serve as a stopgap to help measure ocean winds following the failure of the OSCAT instrument aboard the OceanSat-2 satellite in February 2014.
Built in part from spares leftover from earlier missions, SCATSAT-1 is designed for five years of service, providing an overlap with the OceanSat-3 spacecraft currently scheduled for launch in 2018.
SCATSAT’s scientific payload consists of Ocean Scatterometer 2, or OSCAT-2, an enhanced replacement for the OSCAT instrument aboard OceanSat-2. The instrument, a pencil-beam wind scatterometer, uses radar to collect data on wind speeds and directions near to the sea surface from orbit.
Operating in the Ku-band at a frequency of 13.515 gigahertz, OSCAT-2 transmits signals towards the surface of the Earth and detects backscatter resulting from these signals reaching the ocean surface. Rough seas reflect more energy than calm seas, allowing sea surface conditions to be analysed.
OceanSat-2 was India’s first satellite to carry a scatterometer for sea wind research. Launched in September 2009, the satellite was approaching the end of its design life when the travelling wave tube amplifier (TWTA) in the OSCAT instrument malfunctioned, leaving the instrument unusable.
SCATSAT’s OSCAT-2 instrument has been built from the same designs as OSCAT and incorporates spare TWTAs built for the earlier mission, although the digital processing system behind the instrument has been redesigned to take advantage of improvements in technology since the original instrument was constructed.
SCATSAT will be able to measure wind speeds between three and thirty metres per second (6.7-67 miles per hour), to within 1.8 m/s (4 mph).
It will measure wind direction to within twenty degrees. OSCAT will rotate at a rate of 20.5 revolutions per minute, with two beams scanning the Earth’s surface.
Its inner beam uses horizontal polarisation and has a scan radius of 700 kilometres (435 miles, 378 nautical miles) while the outer beam is vertically-polarised with a scanning radius of 920 km (572 mi, 497 nmi).
ISRO constructed SCATSAT-1 around the IMS-2 bus, which was previously used for 2013’s SARAL joint mission with France. It has a mass at launch of around 377 kilograms (831 lb).
In addition to its primary payload, the PSLV is carrying seven smaller satellites for various organisations from India, Algeria, Canada, the United Kingdom and the United States.
Three satellites, ALSAT-1B, 1N and 2B, will be operated by the Algerian space agency, Agence Spatiale Algerienne (ASAL). Manufactured by Surrey Satellite Technology Ltd (SSTL) in partnership with Algerian engineers, ALSAT-1B is a 103-kilogram (227 lb) spacecraft based on the SSTL-100 bus. It carries an Earth imaging payload with panchromatic and multispectral cameras.
ALSAT-2B also carries an imaging payload, the New AstroSat Optical Modular Instrument (NAOMI). This also offers panchromatic and multispectral imaging modes, however at a higher resolution than ALSAT-1B.
The spacecraft itself is based on the Airbus AstroSat-100 platform – a commercial version of the Myriade series – although it was constructed in Algeria under partnership with Airbus and its predecessor EADS Astrium. The 110-kilogram (243 lb) ALSAT-2B follows the same design as ALSAT-2A, which was launched in July 2010 aboard another PSLV, a secondary payload to India’s CartoSat-2B.
ALSAT-1N – also known as ALSAT-Nano – is a three-unit CubeSat which carries a technology demonstration payload as part of a partnership between ASAL and the UK Space Agency (UKSA).
The satellite’s SpaceMag-PV boom experiment which will test the deployment of a two-metre (6.6-foot) boom with a magnetometer at the end to validate the retractable boom’s ability to host research payloads.
The satellite also carries a CubeSat camera system – C3D2 – and an experiment to study the performance of a thin film solar cell in space. The spacecraft bus was assembled by SSTL, with payload integration managed by ASAL.
The CanX-7 satellite is being carried for the University of Toronto’s Institute of Aerospace Studies (UTIAS). Its launch has been designated Nanosatellite Launch Services 19 (NLS-19) by UTIAS, who have also contracted the launch of other university and commercial satellites on past missions.
A three-unit CubeSat, CanX-7 carries four deployable sails which are designed to expedite the decay of the satellite’s orbit once deployed. The satellite’s primary mission is to demonstrate whether this sail is a practical way to reduce the orbital lifetime of small satellites, helping to mitigate debris in low Earth orbit.
The satellite also carries an Automatic Dependent Service Broadcast (ADS-B) receiver intended to collect identification and tracking data from aircraft, demonstrating the use of such receivers in space. Like ALSAT-1N, CanX-7 adheres to the three-unit CubeSat form factor.
Pathfinder 1 is a commercial imaging satellite which will be operated by US company BlackSky Global. The spacecraft is one of two which BlackSky intends to launch in order to validate their design prior to deploying a constellation of up to sixty spacecraft.
A second satellite is expected to fly aboard a Falcon 9 launch from Vandenberg Air Force Base, however the launch date for this mission is currently uncertain after a Falcon 9 exploded during fuelling for a static fire test ahead of the planned Amos 6 launch earlier this month.
The PES Institute of Technology Imaging Satellite (PISat) is a 5.3-kilogram (12 lb) satellite which will be operated by the Crucible or Research and Innovation Centre (CoRI) at India’s People’s Education Society (PES) University.
PISat carries an off-the-shelf NanoCam C1U imaging system developed by Danish aerospace company GomSpace. The three-megapixel imager offers a spatial resolution of 80 metres (262 feet) and a frame rate of up to twelve frames per second.
Pratham, which is being carried for the Indian Institute of Technology at Bombay, was built primarily to give students experience of working on a satellite mission.
The spacecraft will measure the total electron count at points within Earth’s ionosphere. The ten-kilogram (22 lb) satellite was designed and built by students at the institute and is expected to operate for four months in orbit.
Within the PSLV’s payload fairing, SCATSAT-1 is mounted atop a dual launch adaptor. ALSAT-1B and 2B, Pathfinder, PISat and Pratham are mounted underneath this adaptor, while the CubeSats, ALSAT-1N CanX-7, will be deployed from dispensers mounted on opposing sides of a platform near the base of the fourth stage.
Monday’s launch used India’s Polar Satellite Launch Vehicle (PSLV), making its thirty-seventh flight. Introduced in 1993. The PSLV has a record of thirty-four successful launches, one partial failure and one outright failure, which occurred on its first launch.
The PSLV has made thirty-two consecutive launches successfully over a period of almost nineteen years since placing the IRS-1D satellite into a lower than planned orbit during its fourth flight in September 1997.
The rocket wasPSLV C35. Although ISRO publications ahead of the launch state that the rocket will fly in the PSLV-XL configuration, images of the rocket under assembly and at the launch pad have shown that it instead flew in the PSLV-G configuration, which differs from the XL through its use of smaller booster rockets augmenting the first stage. Monday’s launch was the twelfth to use the PSLV-G configuration.
The PSLV-G is the “standard” PSLV configuration, used for all launches prior to the introduction of the and less powerful PSLV-CA configuration, which omits the first stage boosters altogether, and the more powerful PSLV-XL, in 2007 and 2008 respectively.
Monday’s launch was the first flight of a PSLV-G since April 2011 and only its second since January 2007 – prior to the PSLV-CA’s maiden flight.
ISRO budget documents for the 2016-17 financial year indicated that only the CA and XL versions were still in service; it is unclear whether this was incorrect or whether C35 is using up leftover booster components, in which case Monday’s launch may be the final flight of a standard PSLV.
The PSLV is a four-stage rocket, with a mixture of solid and liquid fuelled stages. The first stage, or PS1, is powered by an S-138 solid motor. Attached are six PS0M boosters, using S-9 motors derived from the first stage of India’s earlier Satellite Launch Vehicle (SLV) and Augmented Satellite Launch Vehicle (ASLV) rockets.
The liquid-fuelled PS2, or L-40, second stage is powered by a Vikas-2B engine – derived from the European Viking-4B engine used on the Ariane 2, 3 and 4 rockets. The Vikas engine burns UH25 propellant, a mixture consisting of one-part hydrazine hydrate and three parts unsymmetrical dimethylhydrazine (UDMH), oxidised by dinitrogen tetroxide.
The third stage, or PS3, uses an S-7 solid motor, with the fourth stage PS4 – or L-2.5 – powered by a pair of liquid-fuelled engines burning monomethylhydrazine (MMH) and mixed oxides of nitrogen (MON).
Monday’s launch took place from the First Launch Pad (FLP) at ISRO’s Satish Dhawan Space Centre at Sriharikota. The oldest of two operational orbital launch complexes at the centre, FLP replaced smaller complexes to the south that were used by the SLV and ASLV rockets.
Rockets launched from FLP are assembled vertically atop the pad, in contrast to the nearby Second Launch Pad where they are assembled away from the pad atop a mobile platform, which then transports them to the launch pad.
PSLV C35’s launch began with ignition of the first stage at the zero-second mark in the countdown.
Four of the six strap-on boosters ignited shortly afterwards – in pairs, 0.42 and 0.62 seconds after core stage ignition respectively – and the rocket climbed away from the launch pad. The final pair of solid rocket motors were air-lit, igniting twenty-five seconds into flight.
The first pair of ground-lit boosters separated from the PSLV at 67.9 seconds into the flight, with the second pair separating two tenths of a second later. Separation of the air-lit motors occurred at ninety seconds mission elapsed time.
The PSLV’s first stage burned for one minute and 52.74 seconds before it separated and the second stage’s Vikas engine ignited.
The second stage powered PSLV C35’s ascent for two minutes and 31.26 seconds, with separation of the rocket’s payload fairing occurring 48.2 seconds into its burn. At four minutes and 24.2 seconds elapsed time, the seconds stage separate.d Third stage ignition took place 1.2 seconds later, beginning an approximately seventy-second burn.
Once its burn was completed, the spent third stage remained attached to the vehicle until the nine-minute, 47.8-second mark in the countdown. Following third stage separation this coast phase continued, with fourth stage ignition timed for 12 minutes and 26.52 seconds after liftoff to begin the first of three planned burns.
Although ISRO have demonstrated the ability of the PSLV’s fourth stage to make multiple burns after spacecraft separation on two previous flights, Monday’s launch was the first time this restart capability was relied upon to complete a mission – allowing the rocket to deploy its cache of payloads into two different orbits.
The fourth stage’s first burn lasted four minutes and 29.32 seconds, establishing a roughly circular orbit at an altitude of around 730 kilometres (454 miles, 394 nautical miles) and an inclination of 98.1 degrees. Thirty-seven seconds after the end of the burn, SCATSAT separated from the PSLV to begin its mission in this orbit.
Following separation of the primary payload, PSLV C35’s mission entered a lengthy coast phase. The next activity was the first restart of the fourth stage, 65 minutes and 5.18 seconds after SCATSAT’s deployment. The second burn lasted 20.54 seconds, lowering the orbit’s perigee.
After a further 48-minute, 47.96-second coast a third burn – lasting 19.44 seconds – lowered the apogee to circularise the orbit. The target orbit at this point in the flight is again roughly circular, at an altitude of around 689 kilometres (428 miles, 327 nautical miles) and an inclination of 98.21 degrees.
The sequence of deploying PSLV C35’s secondary payloads began with the jettison of the rocket’s dual launch adaptor, at two hours, twelve minutes and 42.96 seconds elapsed time; 37 seconds after the end of the third and final fourth stage burn.
Thirty seconds after the adaptor separates, ALSAT-1N was deployed. CanX-7 separated ten seconds after ALSAT-1N, with Pratham and PISat separating after further twenty and ten-second intervals. ALSAT-1B was released ten seconds after PISat and ALSAT-2B will follow fifteen seconds later. The final spacecraft, Pathfinder 1, was deployed after another fifteen-second interval.
Coming less than three weeks after the successful launch of INSAT-3DR via a Geosynchronous Satellite Launch Vehicle, Monday’s launch marked the shortest time between two Indian orbital launches.
The launch was the sixth of the year for India, surpassing the five launches completed successfully in 2015 as the most the country has completed in a calendar year.
Monday’s launch is expected to be the last PSLV launch of 2016, concluding a year in which the rocket has made five launches for the first time. The first three of these, on 20 January, 10 March and 28 April completed the Indian Regional Navigation Satellite System (IRNSS) by deploying the last three members of the initial seven-satellite constellation.
A fourth launch in June deployed twenty satellites including the CartoSat-2C military reconnaissance satellite.
The next PSLV launch is expected in the opening months of 2017, with the ResourceSat-2A remote sensing satellite. ISRO’s next scheduled launch will be the first orbital demonstration flight of the Mark III GSLV, or LVM3, which is expected to occur no earlier than December and will attempt to deploy the GSAT-19 spacecraft.
(Images via ISRO).