They are ISRO's standard satellite platforms, named for their mass class — about 1,000, 2,000 and 3,000 kg — onto which different payloads are fitted.
Building a new satellite from scratch every time would be slow and risky. So ISRO, like most space agencies, uses a family of standard buses: proven platforms with power, propulsion, pointing and radios already designed. A new mission picks the bus that fits its size and adds its own payload.
The U R Rao Satellite Centre (URSC), which builds ISRO's satellites, names the classes by mass: I-1K is the 1,000 kg class, I-2K the 2,000 kg class, I-3K the 3,000 kg class and I-4K the 4,000 kg class.
The same bus can carry very different jobs: I-1K has flown both navigation satellites and a Mars probe.
What comes with the bus? GSAT-19 is a good look inside an I-3K. ISRO describes a cuboid main structure built around a carbon-fibre reinforced polymer central cylinder, two solar arrays of ultra triple-junction cells, an attitude and orbit control system, and a liquid apogee motor with chemical thrusters. The payload — here, Ka/Ku-band transponders — is fitted onto that.
At the small end ISRO has the Indian Mini Satellite buses. IMS-1 has a mass of about 100 kg and carries a 30 kg payload; it flew on IMS-1, Youthsat and Microsat-2D. ISRO has transferred the IMS-1 bus technology to private industry.
URSC says it builds satellites from nano-satellites under 20 kg up to the I-4K class at about 5,000 kg.
The IRNSS satellites in the list are built on I-1K buses and the NVS satellites on I-2K buses — the newer, heavier bus carries the extra L1 signal. Open the NavIC tracker
A bus family trades optimisation for reuse. Each class fixes the hard, slow-to-qualify parts — structure, propulsion layout, power regulation, attitude and orbit control, onboard computer — and leaves room for the payload.
Compare the published numbers for satellites on each bus:
Power per kilogram of launch mass: IRNSS ≈ 1,600 ÷ 1,425 ≈ 1.1 W/kg; GSAT-9 ≈ 3,500 ÷ 2,230 ≈ 1.6 W/kg; GSAT-19 ≈ 4,500 ÷ 3,136 ≈ 1.4 W/kg. Within one class the number shifts with the payload, so a bus defines a range rather than a single figure.
The NavIC story shows the step up. The first-generation IRNSS satellites used I-1K; the second-generation NVS series uses I-2K, adds an L1 navigation band and, from NVS-01, flies an indigenous rubidium clock.
Launchers follow the mass: IRNSS-1A flew on PSLV-C22; NVS-01 and NVS-02 on GSLV (F12 and F15); the 3,136 kg GSAT-19 on the first developmental GSLV Mk III.
The mass class in thousands of kilograms: I-1K is the 1,000 kg class, I-2K the 2,000 kg class and I-3K the 3,000 kg class.
I-1K: IRNSS-1A (or the Mars Orbiter Mission). I-2K: NVS-01 (or GSAT-9). I-3K: GSAT-19.
A proven bus cuts design time, cost and risk; only the payload and mission-specific parts need to be new.