By radio: antennas on the satellite and on the ground send and receive signals that carry the satellite's data down and commands up.
Everything a satellite does reaches us by radio. Pictures, weather data, TV channels and navigation signals come down on the downlink. Commands from engineers go up on the uplink.
Radio waves travel at the speed of light, so even a signal from geostationary height arrives in about an eighth of a second.
Distance weakens the signal. Radio spreads out as it travels, so its strength falls with the square of the distance. A geostationary satellite at 35,786 km is about 72 times farther away than one 500 km overhead, so the same transmitter's signal arrives about 5,000 times weaker.
Most satellites run two separate radio links.
GSAT-19 tested a Ku-band TTC transponder as a new technology.
ISRO's Telemetry, Tracking and Command Network, ISTRAC, in Bengaluru provides the ground segment for ISRO's low-orbit and interplanetary missions, its navigation satellites and its launches. ISRO lists ISTRAC ground stations at Bengaluru, Lucknow, Mauritius, Sriharikota, Port Blair, Thiruvananthapuram, Brunei and Biak in Indonesia, plus deep-space stations.
For far-off missions, ISRO's Indian Deep Space Network at Byalalu uses 18 m and 32 m dishes.
A satellite that cannot talk to Earth is lost to its owners even if it still works. ISRO's Mars Orbiter Mission ended when contact was lost after a long eclipse in April 2022. By contrast, NVS-02, stuck in the wrong orbit, kept its link: ISRO reported that communication with the ground station had been set up and the satellite was healthy.
Pick a low-orbit ISRO satellite and watch it cross India: each time it passes over Bengaluru or Lucknow, ISTRAC can download its data. Open the ISRO satellite tracker
Two numbers dominate a space radio link: delay and loss.
Delay. Light travels about 300 km in a millisecond (NASA's atomic-clock explainer pairs a 1 ms timing error with a 300 km distance error), i.e. c ≈ 300,000 km/s. One way:
Loss. Radio spreads out as it travels, so received power falls with the square of distance. Compare a low-orbit satellite at 500 km with a geostationary one at 35,786 km for the same transmitter and antennas:
(35,786 ÷ 500)² = 71.6² ≈ 5,120 times weaker, which is 10 × log₁₀(5,120) ≈ 37 dB.
To win that back, geostationary links use bigger dishes, more transmit power or lower data rates. Deep space is far worse, which is why ISRO built the 32 m dish at Byalalu and why ISRO says that station later supported the Mars Orbiter Mission.
The standard link-budget equation in decibels is:
P_received = P_transmit + G_transmit + G_receive − L_path − L_other
with free-space path loss L_path = 20 log₁₀(4πd ÷ λ). Higher frequency (smaller λ) increases L_path, but dishes of fixed size gain more at short wavelengths, so the net effect depends on the antennas.
The uplink carries signals from the ground to the satellite, such as commands; the downlink carries signals from the satellite to the ground, such as data and telemetry.
ISRO's Telemetry, Tracking and Command Network provides ground-station support — tracking, telemetry and commanding — for ISRO satellites, launch vehicles and deep-space missions.
About 0.12 seconds: 35,786 km ÷ 300,000 km/s.