A broadcaster beams channels up to a geostationary satellite, which sends them back down to millions of small dishes that point at one fixed spot in the sky.
When you watch a DTH (direct-to-home) channel, the picture has made a trip of more than 70,000 km. It went up from the broadcaster's large dish to a satellite, and came back down to the small dish on your roof or balcony.
The satellite sits in geostationary orbit, 35,786 km above the equator. At that height one orbit takes about 23 hours 56 minutes, the same time Earth takes to spin once. So the satellite seems to hang still in the sky. That is why your dish is bolted in place and never has to move.
ISRO calls INSAT a major catalyst for the spread of television in India, and says satellite TV now covers all of India's area and population. Doordarshan is a major user of INSAT satellites. ISRO adds that high-power Ku-band transponders let DTH work with the smallest dish antennas all over India.
GSAT-24 is a good recent example. It was launched on 22 June 2022 and weighs 4,180 kg. NSIL, the Department of Space's company, leased its entire capacity to Tata Play. Tata Play started using it on 7 August 2023.
GSAT-30, launched on 17 January 2020 on an Ariane 5 from Kourou, French Guiana, replaced INSAT-4A. ISRO lists DTH television, television uplinking and digital satellite news gathering among its jobs.
A satellite this high sees a huge part of the planet at once. ESA notes that just three evenly spaced geostationary satellites can give near-global coverage.
The catch is distance. A signal at the speed of light needs about 0.12 seconds to cover 35,786 km, so every DTH picture arrives a quarter of a second after it left the studio. For TV this does not matter. For video calls and games it does, which is why newer internet satellites fly much lower.
Pick a GSAT or INSAT satellite: its position barely moves and its altitude stays close to 35,786 km, because it sits above the equator. See India's satellites on the globe
Why 35,786 km? A satellite looks fixed only if its orbital period equals one sidereal day, T = 86,164 s. Kepler's third law gives the orbit radius:
r = (GM × T² / 4π²)^(1/3)
With Earth's GM = 398,600 km³/s²:
r = (398,600 × 86,164² / 39.478)^(1/3) = (7.496 × 10¹³)^(1/3) ≈ 42,164 km
Subtract Earth's equatorial radius, 6,378 km, and the height is 35,786 km. NASA's orbit catalogue gives the same 42,164 km from Earth's centre.
The orbital speed is v = 2πr / T = 2π × 42,164 / 86,164 ≈ 3.07 km/s, matching ESA's figure of about 3 km/s.
Delay: a viewer directly under the satellite is 35,786 km away. Light covers that in 35,786 / 299,792 ≈ 0.119 s. The signal goes up and comes down, so the minimum delay is about 0.24 s. Viewers away from the sub-satellite point are slightly farther away, so their delay is a little longer. Broadcasters do not care, because nobody can tell that a TV picture is a quarter-second old.
One more consequence: because the satellite is fixed over the equator, a dish in Srinagar points lower towards the south than a dish in Chennai. The farther you are from the equator, the lower the satellite sits in your sky.
The satellite is in geostationary orbit, where one orbit takes as long as one Earth rotation. It stays over the same point on the equator, so it looks fixed in the sky.
About 35,786 km above the equator.
About 0.24 seconds, because the signal must travel about 35,786 km up and 35,786 km down at the speed of light.