Launch day ends a long countdown: review boards clear the rocket, propellants are loaded, computers run the final minutes, and the rocket lifts off at the planned second.
A launch does not start at lift-off. Weeks of assembly and testing come first. The last stretch is the countdown, which lets many teams carry out their tasks in the right order and at the right time.
At ISRO, two bodies give the go-ahead. The Mission Readiness Review (MRR) committee checks that the rocket and satellite are ready. The Launch Authorisation Board (LAB) then clears the countdown and the launch time.
For example, before GSLV-F08 carried GSAT-6A, the MRR and LAB cleared a 27-hour countdown. It began at 13:56 IST on 28 March 2018 for a launch at 16:56 IST on 29 March. GSLV-F10's launch in August 2021 followed a 26-hour countdown. In each case the count runs backwards to the planned lift-off instant, called T-zero.
As the clock runs down, the teams work through their checks in order, and the spacecraft is put into its launch state. NASA notes that activity steps up sharply about a day before launch, and describes the last few minutes as controlled mostly by computers. At T minus zero, if nothing has stopped the count, the rocket lifts off.
The launch time is set to the minute or even the second because of the launch window. PSLV-C61 was scheduled for 05:59 IST on 18 May 2025; PSLV-C62 for 10:17 IST on 12 January 2026. Both were set to lift off from the First Launch Pad at Sriharikota.
Once the rocket clears the pad, the mission team follows each stage event: booster separation, fairing jettison, stage burnouts and finally injection. On LVM3 this whole sequence takes about 974 seconds, just over 16 minutes. After that, the satellite's own team takes over.
The team also watches for trouble. On GSLV-F10, the cryogenic stage misbehaved at 297.3 seconds after lift-off, and the onboard computer aborted the mission at 307 seconds.
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A launch timeline is a list of events relative to T0, the lift-off instant. ISRO's published LVM3 sequence is a clean example:
Each event is checked against its planned time and conditions. The GSLV-F10 data shows how fast a fault can develop: the cryogenic stage deviated at T+297.3 s and the onboard computer aborted the mission at T+307 s — under 10 seconds later.
The countdown is backwards from T0. With a 27-hour countdown and a launch at 16:56 IST on 29 March, the clock must start at 16:56 − 27 h = 13:56 IST on 28 March, which is exactly what ISRO announced for GSLV-F08.
Why fix the second? Earth turns at about 0.465 km/s at the equator (2π × 6,378 km ÷ 23.9345 h). In one minute a launch site near the equator moves about 28 km relative to an orbital plane fixed in space. For missions that must meet a target orbit precisely, that drift has to be made up with extra propellant, so the window and the lift-off time are planned tightly.
One lesson from GSLV-F10 went into future launch days: ISRO's failure committee recommended automatic monitoring of additional cryogenic-stage parameters before lift-off clearance is given.
The Mission Readiness Review (MRR) committee and the Launch Authorisation Board (LAB).
At 13:56 IST on 28 March.
The rocket must launch inside its launch window so the site is lined up with the target orbital plane or destination; outside it, the rocket would need extra propellant or could not reach the target.