UTC is the world's reference time, built from atomic clocks; Indian Standard Time is UTC plus 5 hours 30 minutes.
Satellites cross every time zone in an hour and a half. If each country logged their positions in its own local time, nobody's data would match. So the space world uses one shared clock: Coordinated Universal Time, or UTC.
UTC is maintained by the BIPM, an international body based in France that no single country controls. It combines more than 400 atomic clocks in over 80 laboratories across more than 60 nations. NIST calls UTC a "paper" time scale: it is a calculated average and does not tick out signals itself.
Each national lab therefore runs its own real-time copy. In India that lab is CSIR-National Physical Laboratory in New Delhi. It keeps UTC(NPLI) using five caesium atomic clocks and three hydrogen masers. Indian Standard Time is defined as UTC(NPLI) plus 5 hours 30 minutes.
To turn UTC into IST, add 5 h 30 min. Watch out for the date.
Orbital data uses universal time too. CelesTrak explains that a satellite data set's "epoch" — the moment it describes — is given in UT. Its example, epoch 98001.00000000, means 00:00 UT on 1 January 1998. In India that was 05:30 IST the same morning.
SatelliteTracker.in works in UTC internally, because the public orbital data does. Times you are meant to act on — such as when a satellite will pass over your city — are converted and shown in IST.
Pass start times are listed in IST; subtract 5 h 30 min to match them against UTC timestamps in orbital data. See tonight's passes
Why 5 h 30 min? Earth turns 360° in 24 h, or 15° per hour. A 5.5-hour offset corresponds to 5.5 × 15 = 82.5° of longitude east of Greenwich. A single offset is used across the country, so local solar noon in the far east of India comes well before 12:00 IST and in the far west well after.
Why timing precision matters. At the ISS's speed of about 7.7 km/s, an error of one second in the time puts the computed position about 7.7 km off along the track. A millisecond error is still about 8 metres.
Reading a TLE epoch. CelesTrak's format puts the epoch as YYDDD.dddddddd: two-digit year, day of the year, and fraction of the day, in UT. Day 1 is 1 January (not day 0).
Example: epoch 26310.75000000.
If the fraction had been 0.9, the UT time would be 21:36, and IST would be 03:06 on 7 November — a date change that is easy to miss.
Sidereal versus solar time. Orbit software also needs the angle Earth has turned relative to the stars (Greenwich sidereal time), which advances about 3 minutes 56 seconds per day faster than solar time, since a sidereal rotation takes 23.9345 h.
Add 5 h 30 min: 01:45 IST on 15 August.
CSIR-National Physical Laboratory (NPL), New Delhi, which keeps IST as UTC(NPLI) plus 5 hours 30 minutes.
Satellites cross all time zones within one orbit, and data are shared worldwide, so a single universal time avoids confusion between zones and dates.