GSAT communication satellite costing Rs 508 crore unused for half its lifetime: CAG report

GSAT communication satellite costing Rs 508 crore unused for half its lifetime: CAG report

Subject: Science and Technology

Context:

• A lack of coordination between the Department of Space (DoS) and the Defence Research and Development Organisation (DRDO) led to the underutilisation of the GSAT-6 satellite, according to the Comptroller and Auditor General of India (CAG) report released December 23, 2022.

About the GSAT-6 satellite:

• The communication GSAT-6 satellite was launched in 2015 at a cost of Rs 508 crore.
• It was designed to provide communication through the S-band and a national beam in the C-band for strategic users, according to the Indian Space Research Organisation (ISRO).
  • The S and C bands are part of the microwave spectrum, which includes frequencies ranging from 1 gigahertz to over 100 GHz.
• GSAT-6 had two main components:
  • The space segment and the ground segment.
  • DOS was responsible for realising and launching the satellite, while DRDO was responsible for the ground segment.
  • Under the revised utilisation plan, it was proposed to utilise the space segment capacity for meeting the communication needs of the strategic and societal sectors.
  • GSAT-6 was expected to last for 12 years from 2015. But it was left unused for half its lifetime.

Communication satellites:

• A communications satellite is an artificial satellite that relays and amplifies radio telecommunication signals via a transponder.
• It creates a communication channel between a source transmitter and a receiver at different locations on Earth.
• Communications satellites are used for television, telephone, radio, internet, and military applications.
• Orbit used for communication satellite:
  • Many communications satellites are in geostationary orbit 22,300 miles (35,900 km) above the equator, so that the satellite appears stationary at the same point in the sky; therefore the satellite dish antennas of ground stations can be aimed permanently at that spot and do not have to move to track the satellite.
  • Others form satellite constellations in low Earth orbit, where antennas on the ground have to follow the position of the satellites and switch between satellites frequently.
• The frequency used in communication satellites:
  • Communications satellites use a wide range of radio and microwave frequencies.
  • To avoid signal interference, international organizations have regulations for which frequency ranges or “bands” certain organizations are allowed to use.
  • This allocation of bands minimizes the risk of signal interference.

Satellite orbits:

• Geostationary orbit (GEO):
  • It is 22,236 miles (35,785 km) from Earth’s surface.
  • This orbit has the special characteristic that the apparent position of the satellite in the sky when viewed by a ground observer does not change, the satellite appears to “stand still” in the sky.
  • This is because the satellite’s orbital period is the same as the rotation rate of the Earth.
  • The advantage of this orbit is that ground antennas do not have to track the satellite across the sky, they can be fixed to point at the location in the sky the satellite appears.
• Medium Earth orbit (MEO) satellites are closer to Earth. Orbital altitudes range from 2,000 to 36,000 kilometres (1,200 to 22,400 mi) above Earth. Two medium Earth orbits are notable: the semi-synchronous orbit and the Molniya orbit.
  • Semi-synchronous orbit is a near-circular orbit (low eccentricity) 26,560 kilometres from the centre of the Earth (about 20,200 kilometres above the surface).
    • A satellite at this height takes 12 hours to complete an orbit.
    • In 24 hours, the satellite crosses over the same two spots on the equator every day. This orbit is consistent and highly predictable.
    • It is the orbit used by the Global Positioning System (GPS) satellites.
  • Molniya orbit is the second common medium Earth orbit.
    • It was invented by the Russians, the Molniya orbit works well for observing high latitudes.
    • Molniya orbit offers a useful alternative to geostationary orbit, as satellites in a geostationary orbit are parked over the equator, so they don’t work well for far northern or southern locations, which are always on the edge of view of geostationary satellites.
    • The Molniya orbit combines high inclination (63.4°) with high eccentricity (0.722) to maximize viewing time over high latitudes.
    • Each orbit lasts 12 hours, so the slow, high-altitude portion of the orbit repeats over the same location every day and night. Russian communications satellites and the Sirius radio satellites currently use this type of orbit.
• The region below medium orbits is referred to as low Earth orbit (LEO) and is about 160 to 2,000 kilometres (99 to 1,243 mi) above Earth.
  • Because of their low altitude, these satellites are only visible from within a radius of roughly 1,000 kilometres (620 mi) from the sub-satellite point.
  • In addition, satellites in low earth orbit change their position relative to the ground position quickly.
  • So even for local applications, many satellites are needed if the mission requires uninterrupted connectivity.
• Polar orbits:
  • Satellites in polar orbits usually travel past Earth from north to south rather than from west to east, passing roughly over Earth’s poles.
  • Satellites in a polar orbit do not have to pass the North and South Pole precisely; even a deviation within 20 to 30 degrees is still classed as a polar orbit. Polar orbits are a type of low Earth orbit, as they are at low altitudes between 200 to 1000 km.
  • Sun-synchronous orbit (SSO) is a particular kind of polar orbit. Satellites in SSO, travelling over the polar regions, are synchronous with the Sun.
  • This means they are synchronized to always be in the same ‘fixed’ position relative to the Sun.
  • This means that the satellite always visits the same spot at the same local time – for example, passing the city of Paris every day at noon exactly.
  • This serves a number of applications; for example, it means that scientists and those who use satellite images can compare how somewhere changes over time.

Comparison between LEO and MEO:

• MEO satellites are similar to LEO satellites in functionality.
• MEO satellites are visible for much longer periods of time than LEO satellites, usually between 2 and 8 hours.
• MEO satellites have a larger coverage area than LEO satellites.
  • A MEO satellite’s longer duration of visibility and wider footprint means fewer satellites are needed in a MEO network than in a LEO network.
• One disadvantage is that a MEO satellite’s distance gives it a longer time delay and weaker signal than an LEO satellite, although these limitations are not as severe as those of a GEO satellite.
• Like LEOs, these satellites do not maintain a stationary distance from the earth.
  • This contrasts the geostationary orbit, where satellites are always 35,786 kilometres (22,236 mi) from the earth.