GPS covers the whole world from about 20,200 km up; NavIC is India's own regional system, covering India and about 1,500 km beyond from geosynchronous orbit.
GPS and NavIC do the same job: they broadcast signals that let a receiver work out its position and time. They differ in who runs them, where they work and how high their satellites fly.
GPS is run by the US Space Force. NavIC (Navigation with Indian Constellation, earlier called IRNSS) is run by ISRO. Having its own system means India does not depend entirely on another country for navigation and timing.
GPS is global. Its satellites fly in medium Earth orbit about 20,200 km up, in six planes, so that at least four are visible anywhere on Earth.
NavIC is regional. ISRO says it covers India and a region up to 1,500 km beyond its borders. Its base constellation is seven satellites. Three are geostationary, at 32.5° E, 83° E and 129.5° E. Four are in geosynchronous orbits tilted by 29°, crossing the equator at 55° E and 111.75° E. All seven stay over roughly the same region, so from India they never set.
First-generation NavIC satellites broadcast in the L5 band (1176.45 MHz) and the S-band. The second-generation NVS series adds L1 (1575.42 MHz), the same frequency as GPS's main civil signal. NavIC's L5 frequency is also the same as GPS L5.
NavIC offers a Standard Position Service for everyone and a Restricted Service for strategic users. ISRO quotes position accuracy better than 20 m (2σ) in the main service area. The International Maritime Organization recognises NavIC as part of the worldwide radio navigation system, and IRNSS-1A sends alerts about cyclones, tsunamis and high waves to fishermen.
Watch the inclined NavIC satellites drift north and south over days while the geostationary ones stay put. Track NavIC satellites
Orbit geometry explains the difference.
GPS satellites at 20,200 km take about 12 hours per orbit, so each one passes over a given place for a few hours and then disappears below the horizon. Global coverage needs many satellites spread around the planet.
NavIC uses period-matched orbits instead. Both its geostationary and inclined geosynchronous satellites take one sidereal day (about 23 h 56 min) per orbit, at r ≈ 42,164 km from Earth's centre.
So all seven NavIC satellites hover over the longitudes from 32.5° E to 129.5° E. Seen from India, they never set.
The cost of this choice is that NavIC cannot help a user in, say, South America. The benefit is that seven satellites give permanent regional coverage.
Timing: NavIC system time began at 00:00 UT on 22 August 1999, and its first week-number rollover happened on 6 April 2019, according to ISRO.
NavIC covers only India and nearby regions, and its satellites are in geosynchronous orbits that stay over that region; GPS must cover the whole planet from lower, faster orbits.
The Standard Position Service for civilian users and the Restricted Service for strategic users.
NavIC's L5 signal at 1176.45 MHz uses the same frequency as GPS L5.