by Gurbir Singh
The journey towards SAC had begun in 1966 with the Experimental Satellite Communication Earth Station, a 14-m dish antenna, established in Ahmedabad by Sarabhai. Located at the centre of SAC, the antenna remains in operation today. The Earth Station was established with the assistance of the International Telecommunications Union of the UN, and at inception, its primary function was to train engineers from India and beyond. It continues that tradition today by hosting a 9-month post-graduate diploma course in satellite meteorology and communication run by the Centre for Space Science and Technology Education in Asia and the Pacific for students from the Asia-Pacific region.
SAC has been at the heart of most space-borne applications ISRO has built since its inception. Prior to building space-borne instruments, SAC conducted remote sensing from high-altitude balloons. In 1975, for example, a platform housing multiple Hasselblad cameras with a variety of films was carried by a huge balloon (around 6840 m3) to 27-km altitude.[445] The following year, ISRO’s first multispectral scanner was tested within a modified aircraft prior to deployment in space.
Much of the groundwork for the SITE programme in 1975 was done at SAC. After SITE, SAC initiated the Satellite Telecommunications Experiments Project (STEP). STEP used a transponder on the Franco-German satellite called Symphonie that was moved to 49° over the Indian Ocean for two years between 1977 and 1979. ISRO’s first experimental communication satellite in GEO called the Ariane Passenger Payload Experiment (APPLE) was fabricated at SAC.
While Ford Aerospace and Communication Corporation in the US built the first four satellites (INSAT 1A to INSAT 1D) of INSAT, engineers at SAC designed and the five satellites in the second series (INSAT 2A to INSAT 2E). SAC has been in the vanguard of building sensors used by all ISRO’s Earth observation (EO) satellites. The first experimental EO satellite, Bhaskara-1, carried a 1-km resolution TV camera and a three-channel Satellite Microwave Radiometer called SAMIR. SAC introduced multiple charge-coupled devices (CCD) to deliver high-resolution images of 5 m for Indian Remote Sensing IRS-1C. It was the first time that such a high resolution was made available in a satellite for civilian use. The cameras that ISRO used in recent missions to image the Moon and Mars, like SAMIR and the high-resolution CCD, have evolved from this early technology developed at SAC.[446] In addition to the hardware, SAC has been driving the development of applications that can best exploit the data generated by ISRO’s now extensive space-based infrastructure. Some of these applications are summarised below.
Communication: Communication transponders are the primary communication devices that facilitate the telecommunication connection between Earth and space. This may be telephone calls, direct-to-home transmission (satellite TV), search and rescue, or Global Positioning System (GPS)-Aided GEO-Augmented Navigation or satellite navigation (or satnav) service. SAC designs and develops the subsystems that comprise communication transponders. It also undertakes R&D of software applications and models for monitoring coastal erosion, desertification, coastal sediment transport and shoreline vulnerability to storm surges and tsunamis.
Agriculture: For over two decades, SAC has been making Crop Acreage and Production Estimation forecasts for important crops using satellite remote-sensing data for the Ministry of Agriculture. In 2007, it moved to a more inclusive model forecasting agricultural output using space, Agro-meteorology and Land-based Observations. Through these data, India recognised the impact of climate change in the air quality of its major cities, as well as rural communities. SAC has been involved in a series of projects that attempt to use remote-sensing data to help develop long-term ecological records of Indian Himalayas, establish a geospatial database and identify and characterise mangrove ecosystems.
Environment and Meteorology: Satellite data continues to be used to develop models for understanding the connection between the polar environment and Indian monsoon. Specifically, ISRO’s Satellite with ARgos and ALtiKa (SARAL), a joint Indo-French mission, collects high-resolution altimetric measurements of the sea-surface elevation. These data help generate surge forecasts and inform monsoon prediction models. INSAT-3D data provides regular and systematic information on the state of the marine ecosystem of Indian exclusive economic zone at synoptic scales to policymakers, as well as those who rely on this region for their livelihood. Sea-surface wind measurements, along with Oceansat-2’s Ocean Colour Monitor, have helped SAC refine models used to generate the Potential Fishing Zone advisories generated daily for fishermen along the substantial Indian coastline. This technique has now been transferred from SAC to the Indian National Centre for Ocean Information Services.
Space technology for national development: A meeting in New Delhi identified 170 projects that include a wide range projects to exploit space technology that include real-time forest fire alert system, support the development of 100 Smart Cities across India, coral reef mapping to monitor coastal pollution, support safety and planning for Indian railways & roads and river water quality and river bank erosion management.[447]
Climate Change: Using data from multiple remote-sensing satellites, SAC has developed state-of-art techniques for the analysis and forecasting of weather and climate on a regional and global level. In the backdrop of worsening greenhouse gas emissions and increasing global temperature, India has shown its willingness to cooperate at an international level. During a meeting of 11 of the world’s largest space agencies in New Delhi in April 2016, India proposed a “virtual remote-sensing satellite constellation for the BRICS nations” to help them jointly meet the Paris Agreement on Climate Change.[448]
ISRO Satellite Centre
ISRO Satellite Centre (ISAC) is a modern hi-tech facility for building, integrating and testing communication, remote-sensing and science satellites. It emerged from the site used by Professor U.R. Rao's team to build India's first satellite, Aryabhata, in 1975. All satellites share a basic design for subsystems for electrical power, communication, temperature control, attitude control and navigation sensors. Each satellite is a 3-D jigsaw puzzle constructed by integrating multiple systems by several teams belonging to disparate establishments based potentially in different cities working to a single overall design.
It is at ISAC that these components and subsystems come together and are tested as a single integrated spacecraft for the first time. ISAC has grown, and today, it incorporates the ISRO Satellite Integration and Test Establishment and the Laboratory for Electro-Optics Systems. Also located within ISAC is a Comprehensive Assembly, Test and Thermo-Vacuum Chamber used to test a fully assembled satellite in space-like conditions. The large chamber can physically accommodate a satellite in its final integrated state and then subject it to a space-like environment.
The air in the chamber can be removed, and temperature increased or decreased to simulate the ambient conditions of space, where the satellite will operate. Typically, the temperature in LEO fluctuates between +150°C on the day-side and -150°C 45 minutes later on the night-side of the orbit. Most spacecraft rotate in orbit to minimise the variation in temperature. A spacecraft is tested rigorously in this environment to qualify components and systems that will operate in the extremely fluctuating conditions that prevail in space.
A spacecraft’s power supply is the key factor that determines its capacity to fulfil its mission objectives. ISAC, in cooperation with Bengaluru-based Bharat Heavy Electricals Ltd, has streamlined the production of solar panels used on ISRO spacecraft. Almost all modern spacecraft operate on a battery recharged by modern lightweight solar panels that are folded during the launch but unfurled once in space.[449] It is this battery that provides the power for maintaining the satellite’s subsystems and the payload, which may be transponders for communication satellites (Geosynchronous Satellite (GSAT) series), atomic clock and radio transmitters (Indian Regional Navigational Satellite System (IRNSS), or camera and sensors (Mars Orbiter Mission). Each satellite’s power system is designed to meet its unique requirements. Its solar panel efficiency and battery capacity must be suffi
cient to meet the power requirements for its mission objectives. The solar panels must be large enough to recharge the battery while the spacecraft is in the sun to sustain it through the period when it is not.
Typically, most of the mass of a rocket at launch is the propellant necessary to get the payload into orbit.[450] The final orbit does not remain stable but is subject to minor gravitational fluctuations, potential east-west variations as a result of solar radiation or a reduction in altitude as a result of atmospheric drag. For example, the ISS is located in an orbit of 400 km, but over time the tenuous atmosphere causes drag and lowers the orbit (by around 50 m per day) to such an extent that its orbit has to be boosted a few times every year. Navigation sensors and control algorithms developed at ISAC allow ground operators at the Master Control Facility (MCF) to determine where a spacecraft is and calculate with precision any adjustments that may be required. These critical manoeuvres require short engine burns using the onboard attitude control thrusters to reposition the satellite.[451] Efficient algorithms minimise the propellant required for station-keeping (maintaining the desired orbit) and thus maximise the operational lifetime of a spacecraft.
Liquid Propulsion Systems Centre
The Liquid Propulsion Systems Centre (LPSC) is responsible for the R&D for control and management of liquid and cryogenic propellants, control valves, transducers, liquid engines and complete launch vehicle stage. It is also responsible for developing the launch vehicle stages for launch from Earth to space and the propulsion systems used by a spacecraft’s onboard engines once in space. LPSC originally consisted of three sites: LPSC Valiamala, LPSC Mahendragiri and LPSC Bangalore. On 1 February 2014, LPSC Mahendragiri was renamed as a separate entity, the ISRO Propulsion Complex (IPRC). The other two sites remain within the LPSC.
Part of VSSC’s integration team is based within the LPSC campus at Valiamala to support subsystem level integration of launch vehicle stages at VSSC. Integration of stages with liquid engines is coordinated both by VSSC and LPSC. LPSC Bangalore develops the thrusters that are fitted to the spacecraft (IRS, GSAT and INSAT) as part of its onboard propulsion system. Once the launch vehicle has delivered a satellite to space, its onboard propulsion system takes over. This system is responsible for manoeuvring the satellite to its final operational orbit, an operation that is typically overseen by ISRO’s MCF in Hassan. In support of the Mars Orbiter Mission (MOM), a duplicate Liquid Apogee Motor (LAM) in Valiamala was used to test commands on Earth before they were sent to for Mars. This was particularly important for the critical Mars Orbit Insertion manoeuvre. This testing provided confidence that the MOM LAM would fire after the long pause between leaving Earth and arriving at Mars.
LPSC is tasked with designing, building and delivering liquid engine propulsion systems for complete rocket stages. It is also responsible for developing engines, propellant tanks and the associated control units that regulate the combustion of propellant during the launch. Combustion requires fuel, oxygen and ignition. On Earth, the atmosphere provides the oxygen, but rockets that operate in space must take the oxygen supply with them. A variety of rocket fuels and oxidisers are used, and each has associated attributes of performance, handling, cost, manufacture, transport, storage and efficiency.
As a launch date approaches, propellants matching the launcher requirements are made available at Sriharikota. For instance, the four stages of the Polar Satellite Launch Vehicle (PSLV) alternate between solid (stages 1 and 3) and liquid (stages 2 and 4) propellants. The same solid propellant is used in the first and third stages, while the second and fourth stages use different liquid propellants.[452] The spacecraft within the launch vehicle also requires a propellant to reach and maintain orbit. The solid propellant is produced onsite at Sriharikota. All other propellants are manufactured at three geographically separated sites.
ISRO Propulsion Complex
IPRC is located in Mahendragiri, 350 km south-west from Sriharikota, and is responsible for leading-edge research, development and testing facilities for liquid, cryogenic and semi-cryogenic propulsion technologies.
Figure 8‑6 ISRO Propulsion Complex, Mahendragiri. Credit ISRO
Development work on semi-cryogenic technology is conducted at IPRC. Although a late starter, ISRO is expecting to test semi-cryogenic engines by the end of 2017. IPRC undertakes a series of activities, which include the production and supply of liquid and cryogenic propellants for launch vehicles and spacecraft. The testing activities at IPRC include:
Structural testing in a pressure chamber that can accommodate hardware up to 5 m in diameter to simulate space-like environment
Testing of launch vehicle subsystems, such as turbopumps, injectors, gas generators, gas bottles and umbilicals
Development of semi-cryogenic engines and production of cryogenic propellants
High-altitude testing of cryogenic engines
Modelling thermal structures and computational fluid dynamics analysis of components and subsystems.
ISRO successfully designed, built and used a cryogenic stage on the Geosynchronous Satellite Launch Vehicle (GSLV) Mk2 that placed India’s communication satellite GSAT-14 into Geosynchronous Transfer Orbit (GTO) on 5 January 2014. All cryogenic engines used up until then on the GSLV Mk1 were acquired from the former USSR. Cryogenic propellants are so far, the most efficient type of propellant available for launch vehicles. Engineers measure propellant efficiency in terms of Specific Impulse, abbreviated to Isp. The Isp increases from solid, liquid, semi-cryogenic to cryogenic propellants.[453] Cryogenic technology introduces extremely low temperatures, and the engineering challenges they bring make rocket science, rocket science. The most efficient propellant is a hydrogen (fuel) and oxygen (oxidiser) combination, but both being gases demand large storage tanks that define the size and shape of the launch vehicle.
However, once cooled, they turn to liquid. In their liquid form, they occupy much less space and are easier to transport and store. Oxygen liquefies at -183°C and hydrogen at -253°C. Nothing can be colder than -273°C (absolute zero). Rocket engines using cryogenic technology present engineering challenges of enormous proportions. Storage tanks need to operate at low temperatures, intricate plumbing is required to transport the fuel from the tanks to the combustion chamber, pumps must operate at 4,200 revolutions per minute, valves must switch on or off with microsecond precision, and the temperature of the combustion chamber must be maintained at 3,000°C without catastrophic destruction. It is at IPRC that these engines are tested and qualified.
ISRO Telemetry, Tracking and Command Network
The significant number of national and international communication links required to support ISRO's space operations come together like the lines of a spider's web in Bangalore at the headquarters of the ISRO Telemetry, Tracking and Command Network (ISTRAC). Initially, established as the ground segment for the IRS satellite in 1982, ISTRAC now incorporates all ISRO's communication resources and continues to grow. It is the hub for communication with spacecraft in orbit or deep space. Over the years, ISRO's communication infrastructure had developed on an ad-hoc basis but is now optimised as a single integrated entity. ISTRAC is responsible for supporting space vehicle launches, guiding satellites into their designated orbits and managing them during their lifetime once there.
Telecommunication transponders are built to high standards to operate at fixed frequencies. Poor design or quality of components may allow frequency drift, which could result in designated recipients being unable to receive data. Worse, radio interference could result in the loss of a service from a nearby satellite. ISTRAC’s telemetry, tracking and command (TT&C) facilities consist of ground stations located throughout India and beyond, including Bengaluru, Lucknow, Sriharikota, Port Blair, Thiruvananthapuram, Mauritius, Brunei, Antarctica Ground Station for Earth Observation Satellites (AGEOS) and Biak (Indonesia).[454] A network of ground-based radar systems is used to track launch vehicles from the moment of launch through to orbit insertion.
; Figure 8‑7 ISRO's 32-m Antenna at Byalalu. Credit Author
In 2008, ISRO launched Chandrayaan-1 its mission to the Moon. ISRO designed and built the spacecraft, which also carried a host of instruments from several international partners, and launched it from Sriharikota. To detect radio signals arriving on Earth from the Moon 380,000 km (236,121.05 miles) away, a large antenna was required. The site was selected from three shortlisted against key criteria.[455] A few days before the launch of Chandrayaan-1, on 17 October 2008, ISRO inaugurated its flagship deep space antenna, a 32-m fully steerable dish at Byalalu close to Bangalore, which is now the nucleus of the Indian Deep Space Network (IDSN), an integral element of ISTRAC.
The Byalalu facility is the centrepiece of IDSN and the ISRO Navigation Centre (INC), a communications hub for the IRNSS, which came into full operation in 2016. In addition to the 32-m antenna, Byalalu also operates a fully steerable 18-m antenna. The two antennae were built and are operated to international standards and can interoperate during periods of collaboration with space agencies of other nations. Byalalu has been instrumental in operating ISRO’s MOM, which has been much more challenging than operating the Moon mission given the increased distance.