Indian PSLV rocket launches RISAT-2B

by William Graham

India upgraded its reconnaissance capabilities through the launch of a RISAT-2B radar imaging satellite Wednesday. Liftoff from the Satish Dhawan Space Centre aboard a PSLV rocket occurred at 05:30 local time (00:00 UTC), with the launch lasting fifteen and a half minutes from liftoff to spacecraft separation.

The satellite launched on Wednesday was the first in a fleet of RISAT-2B satellites that will succeed India’s ten-year-old RISAT-2 spacecraft. Equipped with X-band radar imagers, RISAT-2B will monitor the Earth day and night, in any weather conditions. Two or three RISAT-2B spacecraft are expected to launch this year, while another satellite, the larger RISAT-2A, will join the constellation in 2020.

The RISAT constellation is operated by the Indian Space Research Organisation (ISRO). While ISRO state the satellites’ applications as supporting agriculture, forestry and disaster management, their primary purpose is military surveillance.

RISAT uses a technique called Synthetic Aperture Radar (SAR) to build images of the Earth below it. Signals transmitted from the satellite are reflected from the surface and its echo is recorded when it reaches back to the satellite. These signals can then be processed to build a profile of the ground below.
Radar imaging is important for surveillance applications, as it does not require sunlight or clear skies to be able to observe its target. Optical imaging satellites are only able to see points of interest when they are illuminated by the sun and not hidden by cloud, whereas a spacecraft equipped with SAR can still observe at night and its radio waves can propagate through cloudy skies.

India is not the only country to use radar imaging for military reconnaissance – other systems currently in operation include the United States’ TOPAZ constellation, Japan’s IGS Radar and Italy’s COSMO-SkyMed. SAR has also been used on civilian scientific and commercial satellites and on interplanetary probes.

The satellite aboard Wednesday’s launch will be a replacement for RISAT-2. The original RISAT-2 was built for ISRO Israel Aerospace Industries (IAI), and is based on the Polaris satellite (also known as TecSAR or Ofeq 8) which ISRO had launched for Israel in 2008.

India’s partnership with Israel on this satellite boosted its radar imaging technology, and provided an initial capability ahead of the launch of India’s indigenously-developed RISAT-1. Newer satellites, including RISAT-2B, have been developed by ISRO.

An apparent artistic impression of the satellite

At least two RISAT-2B satellites are being launched, aboard separate rockets, to modernize and enhance India’s radar imaging fleet. The RISAT-2BR1 and RISAT-2BR2 satellites have been announced and despite all official publications surrounding Wednesday’s launch referring to the satellite as only “RISAT-2B”, it is likely that this satellite is RISAT-2BR1.

With recent military launches, ISRO has typically used the generic name for the class of satellite – such as CartoSat-2, instead of naming the specific satellite (e.g. CartoSat-2E). Alternatively, Wednesday’s payload could be RISAT-2B, with the RISAT-2BR1 and 2BR2 launches both to come later in the year. Because of the confusion, this article refers to the satellite as RISAT-2B in the generic sense of the designation.

With a mass of 615 kilograms (1,356 pounds) each, the RISAT-2B satellites are about twice as heavy as RISAT-2, although like their predecessor they are designed to operate for at least five years. RISAT-2 has already surpassed this, having been in orbit since its launch in April 2009.

India’s radar imaging program includes satellites operating in the X-band – the RISAT-2 series – and also the lower-frequency C-band. RISAT-1 was the first C-band satellite ISRO operated. It launched in April 2012 but reportedly failed after four and a half years in orbit. A replacement, RISAT-1A, is scheduled for launch towards the end of 2019.

ISRO used a Polar Satellite Launch Vehicle (PSLV) to conduct Wednesday’s launch, placing its RISAT-2B payload into low Earth orbit. The four-stage PSLV flew in its Core Alone (PSLV-CA) configuration, the lightest of several versions of the rocket.

First flown in September 1993, the PSLV is the workhorse of India’s space program. Using a mixture of solid and liquid-fuelled stages, it was designed to carry satellites to polar and sun=synchronous low Earth orbits, however over the last two and a half decades it has proven its versatility, sending satellites towards geosynchronous transfer orbit and deploying probes bound for the Moon and Mars. Wednesday’s launch marks PSLV’s forty-eighth flight.

PSLV can fly in different configurations depending on the mass of its payload and the target orbit it is aiming to achieve. These configurations vary the number and type of solid rocket boosters attached to the rocket’s first stage, while the four core stages remain the same across all configurations.

The Core Alone (CA) version of the rocket, which will perform Wednesday’s launch, does not use additional boosters, while the PSLV-DL, PSLV-QL and PSLV-XL use two, four and six PS0M-XL boosters respectively. The PSLV-G configuration, which was used for all launches prior to 2007 but has flown with a decreasing frequency since and not at all since 2016, uses six of the smaller PS0M boosters.

In forty-seven launches to date, PSLV has achieved success forty-four times. Despite the failure of its maiden flight, and a partial failure its first operational launch four years and three missions later, PSLV went on to record thirty-six consecutive successful launches from 1999 to 2017.

During August 2017’s PSLV-C39 mission the rocket’s payload fairing failed to separate, leaving the IRNSS-1H satellite in the wrong orbit and unable to deploy, however since then PSLV has completed a string of another six successful launches.

The ISRO fleet – via ISRO

The flight number of the rocket that made Wednesday’s launch was PSLV-C46. It was the fourteenth PSLV to fly in the Core Alone configuration, and the third PSLV to launch in 2019. The rocket departed from the First Launch Pad (FLP) of the Satish Dhawan Space Centre on Sriharikota Island.

Formerly the Sriharikota High Altitude Range, this facility has been the launch site for all of India’s orbital launches beginning with the country’s first launch attempt with a Satellite Launch Vehicle (SLV) and the Rohini Test Payload in August 1979.

The SLV and later Augmented Satellite Launch Vehicle (ASLV) flew from now-disused complexes to the south of the modern launch pads at Satish Dhawan. The First Launch Pad was constructed in the early 1990s for the PSLV, and has also been used by Geosynchronous Satellite Launch Vehicle (GSLV) rockets in the past. PSLV can also fly from the adjacent Second Launch Pad (SLP), which it continues to share with the GSLV.

When launching from the First pad, PSLV rockets are assembled vertically in position on the launch pad using a mobile service tower. Buildup of PSLV-C46 began after the last launch from the pad – January’s PSLV-C44 mission that carried Microsat-R into orbit – with the rocket’s stages being stacked one atop another with the payload fairing – encapsulating the RISAT-2B satellite at the top.

Stages being mated for this mission – via ISRO

PSLV’s first stage is the solid-fuelled PS1, based around an S-138 motor filled with hydroxyl-terminated polybutadiene (HTPB) propellant. This ignited when the countdown reached zero, propelling PSLV-C46 away from its launch pad. PSLV headed downrange over the Bay of Bengal, following an azimuth of 140 degrees.

The first stage burned for just over one minute and 50 seconds, carrying the rocket to an altitude of 51.9 kilometers (32.2 miles, 28.0 nautical miles). Two tenths of a second after the first stage separated from the rocket, PSLV’s second stage ignited to begin its burn.

The second stage, PS2 or L-40, burns liquid propellant: UH25 is a mixture of three parts unsymmetrical dimethylhydrazine (UDMH) and one part hydrazine hydrate, which is oxidized by dinitrogen tetroxide. The second stage engine, Vikas, is a license-built derivative of France’s Viking engine which powered early versions of the Ariane rocket.

About sixty-seven seconds into the two-minute, 32-second burn of the second stage, the payload fairing separated from the nose of the rocket. Termed a “heat shield” by ISRO, the fairing is the nosecone of the rocket, which protects the satellite as it ascends through the atmosphere. Upon reaching space the fairing is no longer needed and can be jettisoned to save weight.

About 1.2 seconds after the second stage shut down and separated, PSLV’s HPS3 third stage ignited. This is another solid-fuelled stage with an S-7 motor burning HTPB, and will fire for about seventy seconds. Once the third stage burned out it remained attached while PSLV coasts towards the highest point, or apogee, of its trajectory. The third and fourth stages separated at nine minutes and 24 seconds into the flight, with the PS4 fourth stage igniting ten seconds later.

Also designated L-2-5, the PS4 is a small liquid-fuelled stage with two engines burning monomethylhydrazine and mixed oxides of nitrogen (MON-3). With a five-minute and eight-second burn, PS4 completed the job of inserting its RISAT-2B payload into a circular orbit 557 kilometers (346 miles, 301 nautical miles) above the Earth’s surface at an inclination of 37 degrees.

Spacecraft separation came forty-seven seconds after the end of this burn, at fifteen minutes, 29.4 seconds mission elapsed time. After deploying its payload the PS4 will make a second burn, deorbiting itself for reentry to the southeast of Australia.

Wednesday’s launch was the third of the year for ISRO, following successful PSLV missions in January and April. India’s next launch is expected to be the PSLV-C47 mission next month, which will deploy the first CartoSat-3 remote sensing satellite.

ISRO’s schedule for the rest of the year includes the Chandrayaan-2 lunar mission aboard a GSLV Mk.III rocket in July, the first demonstration launch of its new Small Satellite Launch Vehicle (SSLV) and the deployment of further RISAT spacecraft via the PSLV.

Related Articles