A ground station is an antenna site on Earth that tracks satellites, receives their data and sends them commands.
A ground station is the satellite's partner on Earth. At its heart is a dish antenna on a mount that can swing to follow a satellite across the sky. Behind it sit radio receivers, transmitters and computers linked to a mission control centre.
A satellite in low Earth orbit crosses the sky in minutes and is then out of reach until a later lap. To talk to it more often, agencies spread stations across the globe.
ISRO's ISTRAC network has stations at Bengaluru, Lucknow, Sriharikota, Port Blair and Thiruvananthapuram in India, and abroad in Mauritius, Brunei and Biak in Indonesia. Stations far apart see the satellite on different parts of its orbit.
Worked example: a satellite 500 km up is above a station's horizon for at most about 11 to 12 minutes on one pass (see the calculation below). It makes about 15 laps a day, but Earth turns beneath it, so only a few of those laps come within range of any one station.
ISRO describes several kinds of terminal:
ISRO built the deep-space network for Chandrayaan-1 and says its capabilities later served missions such as the Mars Orbiter Mission.
ISTRAC's work goes beyond talking to spacecraft. It also builds the ground segment for India's NavIC navigation system — one-way and two-way ranging stations and a precise timing facility — and designs weather radars. ISRO adds search-and-rescue and disaster-management services, and support for telemedicine and tele-education.
Watch a low-orbit ISRO satellite cross the map and time how long it takes to pass over southern India — that window is all a Bengaluru dish gets on that lap. Open the ISRO satellite tracker
How long can one station see a low-orbit satellite? Use the geometry of the horizon.
A satellite at altitude h is visible (at 0° elevation) when it is within an Earth-central angle λ of the station, where
cos λ = R ÷ (R + h)
For h = 500 km and R = 6,371 km (NSSDCA): cos λ = 6,371 ÷ 6,871 = 0.9272, so λ ≈ 22.0°.
On an overhead pass the satellite sweeps 2λ ≈ 44° of its orbit while in view. Its period comes from Kepler's third law with GM = 398,600 km³/s² (NSSDCA):
T = 2π √(r³ ÷ GM) = 2π √(6,871³ ÷ 398,600) ≈ 2π × 902 s ≈ 5,670 s ≈ 94.5 min
Pass length ≈ (44 ÷ 360) × 94.5 ≈ 11.5 minutes, horizon to horizon. Real passes are shorter: most are not overhead, and buildings and haze block the lowest few degrees, so stations set a minimum elevation below which they do not track.
A satellite makes about 1,440 ÷ 94.5 ≈ 15 laps a day, but Earth turns beneath it, so a given station sees only a few of them. That is the case for a global network: stations at Bengaluru, Mauritius, Brunei and Biak sit under different parts of the same orbit.
Geostationary satellites are the opposite: they hang over one longitude, so a single station with a fixed dish sees them all the time. Deep-space craft move slowly across the sky but are so far away that only the biggest dishes, like Byalalu's 32 m, can hear them.
A low-orbit satellite is visible from one place for only a few minutes per pass. Stations spread across the globe see it on different parts of its orbit, giving more contact time.
At Byalalu, near Bengaluru; it has 18 m and 32 m dish antennas.
About 11–12 minutes horizon to horizon: the satellite is in view over about 44° of a 94.5-minute orbit.