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PSLV C-35 launch
An Indian PSLV lifts off September 26 on the PSLV C-35 mission, placing eight satellites in two different orbits. (credit: ISRO)

PSLV C-35: ISRO’s two-orbit mission


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India’s most reliable satellite launch vehicle, the Polar Satellite Launch Vehicle (PSLV), successfully launched eight satellites into low Earth orbit in its 37th flight (mission PSLV C-35) on September 26. The primary payload for this mission was an Indian weather satellite called SCATSAT-1. Five of the other seven satellites launched are commercial payloads from other countries, with the other two developed by Indian university students.

Interestingly, the uniqueness of this mission is not about the type of the satellites it launched, but rather the manner in which these eight satellites got launched.

SCATSAT-1 is expected to assist India’s weather forecasting services. It would also contribute towards improving existing cyclone detection and tracking services. The accuracy of weather forecasting in India has improved significantly particularly during last decade or so, and one of the important factors in that improvement has been inclusion of satellite data in various numerical weather models. The Indian Space ResearchOrganisation (ISRO) has one of the biggest networks of remote sensing satellites in the world and some of them are tasked to provide inputs for ocean studies and meteorology. In May 1999, ISRO launched Oceansat-1, the first dedicated satellite to learn more about oceans and associated aspects of meteorology. Subsequently, Oceansat-2 launched on September 23, 2009, and now SCATSAT-1 is expected to provide inputs similar to Oceansat-2, which technically has a lifetime of five years. This latest satellite is expected to offer data with greater accuracy.

The important payload in all these satellites is the Ku-band pencil beam scatterometer (SCAT) developed by ISRO apart from other payloads like Ocean Colour Monitor (OCM). A scatterometer is essentially a microwave radar sensor that provides inputs for studies of air-sea interaction and ocean circulation, and their effects on weather patterns and global climate. The information gathered from this sensor is also beneficial to know more about El NiƱo activity, sea ice activity in the polar regions and also towards understand long-term effects of deforestation in rain forests. Such information plays a vital role towards monsoon and tropical cyclon forecasting in India. In addition, such inputs are vital for long-term assessment of weather and climate patterns.

Interestingly, the uniqueness of this mission is not about the type of the satellites it launched, but rather the manner in which these eight satellites got launched. This was one of the longest and most complex missions undertaken by ISRO. The key challenge for the mission was to launch these satellites into two different orbits. Today, when the world is talking about the success of Elon Musk’s SpaceX for its ability to reduce launch costs by retrieving the first stage of the rocket that could later be reused, it is also important to take a note of ISRO’s efforts to achieve possibly the task of two missions in a single mission.

Routine satellite launches undertake launches in single orbit. On occasion, organizations do launch multiple satellites on a single mission: ISRO’s PSLV C-34 mission on June 22, for example, launched 20 satellites in a single mission, while a Russian Dnepr rocket launched 37 satellites in a single mission in 2014. These missions, though, involved putting many satellites in the same orbit, although in different locations. This mission was complicated because PSLV had to place payloads in two different orbits. Such twin-orbit maneuver capability has been accomplished by European Space Agency’s Vega rocket as well.

PSLV has been a reliable workhorse for more than two decades. It is this vehicle that won ISRO laurels because it assisted the deployment of a variety of satellite systems, from remote sensing to meteorology to navigational satellites as well as various “glamorous” missions like missions to Moon and Mars.

When the world is talking about the success of Elon Musk’s SpaceX for its ability to reduce launch costs by retrieving the first stage of the rocket that could later be reused, it is also important to take a note of ISRO’s efforts to achieve possibly the task of two missions in a single mission.

The PSLV satellite launching system has a standard configuration of four stages with stages one and three using solid propellants and stages two and four using liquid propellants. This vehicle can carry around less than 2,000 kilograms to sun-synchronous polar orbits of altitude of about 600 to 700 kilometers. In various cases, after the satellite gets put into a particular orbit, it conducts subsequent orbit raising maneuvers to take it to a higher orbit.

Broadly, PSLV can be viewed to have two functional models, one which allows a standard launch and other with strap-on motors connected to first stage with an aim to increase the thrust provided by the first stage. PSLV-C35 used strap-on motors and had a pay;oad of eight satellites, with a total weight of 675 kilograms. Normally, a standard PSLV mission lasts for approximately 20 minutes. The total time taken after the lift-off by PSLV-C35 was 2 hours, 15 minutes, and 33 seconds.

Of the eight satellites launched, seven were placed in one orbit. Within first 17 minutes of the launch, the primary satellite SCATSAT-1 deployed in a sun-synchronous orbit of 724 kilometers. Subsequently, the mission continued with seven remaining satellites. As announced by ISRO, after the release of SCATSAT-1, the fourth stage of PSLV coasted over the south polar region and then started ascending towards the northern hemisphere. A safe distance between the orbiting SCATSAT-1 and PSLV-C35 fourth stage was made certain by suitably maneuvering this stage. After 1 hour and 5 minutes, two engines of PSLV fourth stage were reignited and fired for 20 seconds, which allowed it to enter into an elliptical orbit. About 50 minutes later, the PSLV fourth stage fired its engines again for another 20 seconds. This second firing made the fourth stage to enter into a circular orbit of 669 kilometers and, within next two to three minutes, the remaining seven satellites were deployed in a predetermined sequence.

In comparison with the PSLV, the Vega launcher differs somewhat. Broadly, the way ISRO has exploited the capabilities of PSLV has made it into what could be called a multipurpose vehicle. Vega is a vehicle specifically designed to launch payloads in the 1,500-kilogram class to an altitude of 700 kilometers. Vega is European Space Agency (ESA) program with major investments by Italy and operated by Arianespace. Vega has been developed specifically as a small satellite launcher and, in general, is expected to carry payload from 300 to 2,500 kilograms, depending on the type and altitude of the orbit. This launcher offers the ability to place multiple payloads into orbit. The first launch by Vega took place in February 2012 and all seven missions of Vega to date has been successful (although not all of them were multi-orbit missions). By coincidence, just ten days before ISRO’s launch, Arianespace launched a Vega rocket which inserted five high-resolution Earth observation satellites into different orbits, thus demonstrating the flexibility of its upper stage. This mission lasted for approximately one hour and 45 minutes.

Developing capabilities to launch satellites into two different orbits in a single mission offers ISRO more flexibility to manage their order book.

Currently, it is important to understand the relevance of the development of Vega vehicle and ISRO’s “flirting” with PSLV towards developing multiple and multi-orbit payload delivery systems from a commercial perspective. It is a bit early to predict with certainty that twin-orbit maneuver capability will be in great demand in the immediate future. However, it is becoming increasingly clear that both nation-states and private players value small satellites, being low-mass and low-volume platforms that can be sent into orbit for less cost. Today, many smaller states are keen to launch their own satellites, and small satellites offer them viable options. Internet service providers are making significant investments in this field. Also, the strategic utility of such systems is obvious, and all this is pushing various space agencies into developing an exclusive new generation of small satellite launch vehicles.

Presently, ISRO does not have a major share of the global small satellite launch market; however, they are found making steady progress. The PSLV C-35 mission carried five satellites for foreign customers: three for Algeria and one each for Canada and the United States. The recent seventh mission of Vega had clients like Google subsidiary Terra Bella. Interestingly, a few months back ISRO has also launched satellite for the same client and is scheduled to carry their payload to space in the future as well. All this demonstrates the enhancement of ISRO’s global credibility. ISRO has launched 79 satellites for the foreign customers to date, but, their order book indicates that the demand for ISRO’s launching services is sharply increasing.

Developing capabilities to launch satellites into two different orbits in a single mission offers ISRO more flexibility to manage their order book. It also demonstrates to prospective clients that ISRO has multiple launching options available and hence they could get their satellites launched more quickly if they want to work with ISRO.


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