The radio relays that make up the main payload of INSAT, GSAT and CMS communication satellites, carrying TV, phone, internet and broadband links across India.
A transponder is a receiver and transmitter joined together. It picks up a signal sent up from a ground station, shifts it to a different frequency, amplifies it and sends it back down over a wide area. One satellite carries many transponders, each handling a slice of radio spectrum.
From geostationary orbit a satellite stays over the same spot, so dishes on the ground can point at it without moving.
C band needs larger dishes but copes well with heavy rain, and has long carried TV distribution and telephone links. Ku band works with smaller dishes and is used for direct-to-home TV and VSAT networks. Ka band allows narrow spot beams for high-throughput broadband. For example, GSAT-10 carries 12 normal C-band, 6 extended C-band and 12 Ku-band transponders.
Every INSAT, GSAT and CMS communication satellite carries transponders. INSAT-3A, for example, carried 24: twelve normal C-band, six extended C-band and six Ku-band.