A transponder is the satellite's relay: it receives a signal from the ground, shifts it to a different frequency, amplifies it and sends it back down.
A communication satellite is mostly a set of radio relays in the sky. Each relay is called a transponder. ISRO describes it simply: a transponder receives a signal on the uplink frequency, translates it to the downlink frequency, amplifies it and beams it back to Earth.
A single satellite carries many transponders. Each one handles its own slice of frequencies, so one satellite can carry dozens of TV channels, internet links and phone circuits at the same time.
ISRO's INSAT and GSAT satellites carry transponders in the S-band, C-band, upper extended C-band, Ku-band and Ka-band. Different jobs suit different bands.
C-band beams can be spread over a very wide area. GSAT-30's C-band coverage reaches the Gulf, many Asian countries and Australia, while its Ku-band beam covers the Indian mainland and islands. Ku-band transponders carry DTH television to small dishes. Ka-band is used by newer high-throughput satellites such as GSAT-N2.
GSAT-31 was launched to add Ku-band transponder capacity in geostationary orbit. ISRO lists its uses as VSAT networks, television uplinks, digital satellite news gathering, DTH television and cellular backhaul.
A satellite can cover a whole country with one wide beam, or split the area into many smaller spot beams. Beams that do not overlap can reuse the same frequencies, which multiplies the total capacity.
GSAT-N2 has 32 user beams: 8 narrow beams over the North-East and 24 wider beams over the rest of India. ISRO says this multi-beam design allows frequency reuse, giving a total throughput of about 48 Gbps.
The electronics can work for many years, but the satellite needs fuel to hold its slot. ISRO says communication satellites last about 7 to 15 years, depending mostly on the propellant they carry. At the end, a geostationary satellite is pushed about 200 to 250 km higher into a graveyard orbit and its transmitters are switched off for good.
Pick a GSAT satellite: its altitude stays close to 35,786 km because it is a fixed relay over the equator. Find India's communication satellites
Capacity is set by how much spectrum a satellite has and how many times it can reuse it.
Take GSAT-N2. ISRO quotes about 48 Gbps across 32 user beams, an average of 48 / 32 = 1.5 Gbps per beam. If the satellite had a single country-wide beam, every user would share one copy of the spectrum. With 32 beams that do not interfere, the same frequencies can be used many times over. That is the whole idea behind a high-throughput satellite.
Why translate the frequency at all? The uplink signal arrives extremely weak after a 35,786 km trip, while the downlink leaves the satellite strong. If both used the same frequency, the satellite's own transmitter would drown its receiver. Shifting the frequency keeps them apart.
How weak is weak? Radio power spreads over a sphere, so the power per square metre falls as 1/(4πd²). At d = 35,786 km = 3.58 × 10⁷ m, the sphere's area is 4π × (3.58 × 10⁷)² ≈ 1.6 × 10¹⁶ m². A 100 W transmitter spread evenly over that sphere would give only about 6 × 10⁻¹⁵ W per square metre. Real satellites focus their energy into beams, which is why beam size matters so much: a beam covering only India puts far more power on each rooftop dish than a beam covering half the planet.
An onboard device that receives a signal on the uplink frequency, translates it to the downlink frequency, amplifies it and sends it back to Earth.
Beams aimed at different areas can reuse the same frequencies without interfering, so the same spectrum is used many times over.
Its propellant. When the fuel needed to hold its slot runs low, it is moved to a graveyard orbit about 200–250 km higher and switched off.