A satellite is an object in space that revolves around a larger object. There are two types of satellites: natural satellites (like the Moon orbiting the Earth) and artificial satellites (such as the International Space Station orbiting the Earth).

I Communication Satellites

1.Indian National Satellite (INSAT) system

• With 9 operational communication satellites in Geo-stationary orbit, the INSAT system is one of the largest domestic communication satellite systems in the Asia-Pacific region.
• Applications: With over 200 transponders in the C, Extended C, and Ku bands, the INSAT system supports telecommunications, television broadcasting, satellite newsgathering, societal applications, weather forecasting, disaster warning, and search and rescue operations.

2.GSAT Satellites:

• Later in the 2000s, ISRO started developing GSAT satellites to augment the capacity of the INSAT system and to provide new telecommunications services, such as broadband internet and mobile telephony.

II Earth Observation Satellites

• Earth observation satellites are satellites that observe various Earth phenomena from space.
• ISRO has launched numerous operational remote sensing satellites since IRS-1A in 1988. India now operates one of the largest constellations of remote-sensing satellites.
• Applications: Agriculture, water resources, urban planning, rural development, mineral prospecting, the environment, forestry, ocean resources, and disaster management.
• Example-EOS-07,Cartosat-3,HysIS

III Satellite Navigation

1.GPS Aided GEO Augmented Navigation (GAGAN):

This is a Satellite Based Augmentation System (SBAS) implemented jointly with Airport Authority of India (AAI). The main objectives of GAGAN are to provide Satellite-based Navigation services with accuracy and integrity required for civil aviation applications and to provide better Air Traffic Management over Indian Airspace. The system will be interoperable with other international SBAS systems and provide seamless navigation across regional boundaries. The GAGAN Signal-In-Space (SIS) is available through GSAT-8 and GSAT-10.

2.Indian Regional Navigation Satellite System (IRNSS) : NavIC

This is an independent Indian Satellite based positioning system for critical National applications. The main objective is to provide Reliable Position, Navigation and Timing services over India and its neighbourhood, to provide fairly good accuracy to the user. The IRNSS will provide basically two types of services

• Standard Positioning Service (SPS)
• Restricted Service (RS)

To date, ISRO has built a total of nine satellites in the IRNSS series; of which eight are currently in orbit Three of these satellites are in geostationary orbit (GEO) while the remaining in geosynchronous orbits (GSO) that maintain an inclination of 29° to the equatorial plane. The IRNSS constellation was named as “NavIC” (Navigation with Indian Constellation) by the Honourable Prime Minister, Mr. Narendra Modi and dedicated to the nation on the occasion of the successful launch of the IRNSS-1G satellite.

IV Experimental Satellites

• ISRO has launched many small satellites mainly for the experimental purposes. This experiment include Remote Sensing, Atmospheric Studies, Payload Development, Orbit Controls, recovery technology etc.
• Example-INS-2TD a technology demonstrator satellite from ISRO, which is a precursor to India-Bhutan Joint Satellite (INS-2B).

V Small Satellites

• Sub 500 kg class satellites – a platform for stand-alone payloads for earth imaging and science missions within a quick turnaround time.

VI Student Satellites

• ISRO’s Student Satellite programme is envisaged to encourage various Universities and Institutions for the development of Nano/Pico Satellites.
• Example–Kalamsat-V2,AzaadiSAT(Indian Earth observation 8U Cubesat weighing around 8 kg developed by the Space Kidz)

VII Scientific Spacecrafts/Mission

1.AstroSat

AstroSat is the first dedicated Indian astronomy mission aimed at studying celestial sources in X-ray, optical and UV spectral bands simultaneously. The payloads cover the energy bands of Ultraviolet (Near and For), limited optical and X-ray regime (0.3 keV to 100keV). One of the unique features of AstroSat mission is that it enables the simultaneous multi-wavelength observations of various astronomical objects with a single satellite.

AstroSat with a lift-off mass of 1515 kg was launched on September 28, 2015 into a 650 km orbit inclined at an angle of 6 deg to the equator by PSLV-C30 from Satish Dhawan Space Centre, Sriharikota. The minimum useful life of the AstroSat mission is expected to be 5 years.

2.Mars Orbiter Mission

Mars Orbiter Mission is ISRO’s first interplanetary mission to planet Mars with an orbiter craft designed to orbit Mars in an elliptical orbit of 372 km by 80,000 km. Mars Orbiter mission can be termed as a challenging technological mission and a science mission considering the critical mission operations and stringent requirements on propulsion, communications and other bus systems of the spacecraft. The primary driving technological objective of the mission is to design and realize a spacecraft with a capability to perform Earth Bound Manoeuvre (EBM), Martian Transfer Trajectory (MTT) and Mars Orbit Insertion (MOI) phases and the related deep space mission planning and communication management at a distance of nearly 400 million Km. Autonomous fault detection and recovery also becomes vital for the mission.

3.Chandrayaan-1

Chandrayaan-1, India’s first mission to Moon, was launched successfully on October 22, 2008 from SDSC SHAR, Sriharikota. The spacecraft was orbiting around the Moon at a height of 100 km from the lunar surface for chemical, mineralogical and photo-geologic mapping of the Moon. The spacecraft carried 11 scientific instruments built in India, USA, UK, Germany, Sweden and Bulgaria.

4.Chandrayaan-2

Chandrayaan-2 will be an advanced version of the previous Chandrayaan-1 mission to Moon.Chandrayaan-2 is configured as a two module system comprising of an Orbiter Craft module (OC) and a Lander Craft module (LC) carrying the Rover developed by ISRO.

5.Chandrayaan-3

Chandrayaan-3 is a follow-on mission to Chandrayaan-2 to demonstrate end-to-end capability in safe landing and roving on the lunar surface. It consists of Lander and Rover configuration. It will be launched by LVM3 from SDSC SHAR, Sriharikota. The propulsion module will carry the lander and rover configuration till 100 km lunar orbit. The propulsion module has Spectro-polarimetry of Habitable Planet Earth (SHAPE) payload to study the spectral and Polari metric measurements of Earth from the lunar orbit.

6.Aditya-L1

Aditya-L1 is a satellite dedicated to the comprehensive study of the Sun. It has 7 distinct payloads developed, all developed indigenously. Five by ISRO and two by Indian academic institutes in collaboration with ISRO.

7.X-ray Polarimeter Satellite (XPoSat) 

• XPoSat is the first dedicated satellite from ISRO to carry out research and measure X-ray emission from celestial sources like black holes and neutron stars.
• It is the second satellite in the world to study X-ray polarization, first being NASA’s Imaging X-ray Polarimetry Explorer (IXPE), launched in 2021.
• ISRO successfully launched the X-ray Polarimeter Satellite (XPoSat) by Polar Satellite Launch Vehicle (PSLV)-C58.
• The spacecraft carries two scientific payloads in a low earth orbit.
  •  The primary payload POLIX (Polarimeter Instrument in X-rays) will measure the polarimetry parameters (degree and angle of polarization) in medium X-ray energy range of 8-30 keV photons of astronomical origin. 
  • The XSPECT (X-ray Spectroscopy and Timing) payload will give spectroscopic information in the energy range of 0.8-15 keV.
• POLIX can be used to study the origin of cosmic rays in the universe, the nature of black holes, and the role of black holes in the evolution of galaxies, among others.
• On the other hand, XSPECT will offer fast timing and excellent spectroscopic resolution in soft X-rays in the range of 0.8-15 keV. It can distinguish between two wavelengths emitted by objects.
• It is designed to observe a variety of sources. These include X-ray pulsars, black hole binaries, low-magnetic field neutron stars, active galactic nuclei (a small region at the centre of some galaxies), and magnetars (young and highly magnetized neutron stars).
• Entirely built by two Bengaluru-based institutes—ISRO’s UR Rao Satellite Centre and Raman Research Institute—XPoSat’s development began in 2008, with a formal agreement signed with ISRO in 2015
• XPoSat is India’s third space-based observatory, following the recently launched solar mission Aditya-L1 and AstroSat.

Other space activities

Thirty Meter Telescope (TMT) 

• It is an ambitious international project coming up at Mauna Kea in Hawaii, involving India, the US, Canada, China, and Japan that aims to advance the understanding of the universe significantly.
• The TMT has been conceived as a 30-metre diameter primary-mirror optical and infrared telescope that will enable observations into deep space.
• Indian Institute of Astrophysics (IIAP) is leading the consortium of Indian institutions that are involved with the TMT project.
• India TMT will be jointly funded by the Departments of Science and Technology and Atomic Energy.
• India expects to be a major contributor to the project and will provide hardware (segment support assemblies, actuators, edge sensors, segment polishing, and segment coating), instrumentation (first light instruments), and software (observatory software and telescope control systems)