Telemetry is the stream of health readings a satellite sends down; telecommands are the instructions engineers send up to control it.
Engineers can never touch a satellite after launch. Everything they know about it comes from telemetry: thousands of readings of temperatures, voltages, battery charge, wheel speeds, valve states and more, sent down by radio.
Everything they do to it is by telecommand: coded instructions sent up to switch equipment on or off, fire thrusters or change settings. Together with tracking — measuring where the satellite is — this is called TT&C.
The first telemetry after launch is watched closely. Within days of NVS-02's launch, ISRO could report from telemetry that its solar panels had deployed, power generation was normal and communication with the ground station was established.
ISRO describes how its network handles this. Telemetry received at ground stations is passed in real time to the mission control centres. Commands written at a control centre are sent to whichever station can see the satellite, and uplinked from there. Contacts are scheduled across the stations so that every satellite is checked and maintained regularly.
ISTRAC also provides TT&C during launches, from lift-off until the satellite is released into orbit, and makes the first estimate of the new satellite's orbit. Operators work through one common interface and do not need to manage the network of stations behind it.
When NVS-02 failed to raise its orbit in 2025, telemetry was the evidence. ISRO's Apex Committee studied the satellite's telemetry, along with simulations, and found that the firing signal had not reached the pyro valve on the engine's oxidiser line. The likely cause was a connector that lost contact in both the main and backup paths.
That finding led to changes that ISRO applied to CMS-03, launched in November 2025.
Commands can also rescue a spacecraft. In January 2017 ISRO commanded its Mars Orbiter Mission to fire its thrusters and dodge an 8-hour eclipse that would have drained its battery.
Watch a low-orbit ISRO satellite's track: it can only send telemetry or receive commands while it is in view of a ground station. Open the ISRO satellite tracker
Telemetry is a budget problem: a satellite generates data continuously but can send it only while a ground station is in view.
Worked example (illustrative). A satellite logs 2,000 health parameters, each 16 bits, once per second:
2,000 × 16 = 32,000 bit/s = 32 kbit/s, or about 2.8 Gbit per day.
If it sees a ground station for 10 minutes on an S-band link running at 15 Mbit/s (a rate NASA reports demonstrated on a 5 MHz S-band channel), one pass can move
15 × 10⁶ × 600 = 9 × 10⁹ bit ≈ 1.1 GB.
So one pass a day clears a full day of housekeeping data with room to spare; payload data, such as images, is far larger and usually goes on a separate, faster link.
Commands need the opposite qualities: a low data rate but near-perfect reliability, because a wrong command can end a mission.
Tracking is the third leg. Ranging measures the round-trip time of a signal; ISRO's NVS-02 even carries a dedicated C-band ranging payload. Distance = c × (round-trip time) ÷ 2, so a 1 μs timing error gives a 150 m range error. Repeated range measurements over a pass let ISTRAC estimate the orbit, as it does right after every launch.
Telemetry is data sent from a satellite to the ground about its health and status. Telecommand is coded instructions sent from the ground to control the satellite.
Telemetry, Tracking and Command (sometimes written Telemetry, Tracking and Control).
ISRO's Apex Committee used telemetry and simulations to show that the firing signal never reached the oxidiser-line pyro valve, most likely due to a loose connector contact in both the main and backup paths.