After my opinion pieces on the state of NavIC, a couple of friends asked me some pertinent questions. These are good questions, and I thought of explaining the same for larger audiences.
1. Why four satellites? What makes India’s approach different?
Yes, four satellites are the least number needed for accurate three-dimensional navigation. A receiver must figure out four things at once: its own clock error, latitude, longitude, and altitude. Four signals, four equations. If you get below that, the maths just doesn’t work anymore. But this is where India’s design philosophy is considerably different from that of the other major navigation systems. GPS, GLONASS, Galileo, and BeiDou are all global systems. They are built so that at least four satellites can be seen from anywhere on Earth at any given moment. That global goal needs big groups of twenty or more satellites in Medium Earth Orbit that are always circling the Earth. India made a different choice that made perfect sense. NavIC is a regional system that covers India and a 1,500 km area outside its borders. For that limited goal, ISRO used a smarter orbital strategy that combined Geostationary (GEO) satellites, which stay above the equator and look like they’re not moving in the sky, with Geosynchronous (GSO) satellites, which are tilted at an angle and move in a slow figure-eight pattern over India every day. This combination of seven satellites gives constant, high-elevation coverage over India without having to chase worldwide coverage. It was a nice solution in theory. In practice, it works well, but only when all the satellites are working.
2. How accurate is NavIC when it works on its own?
This is where the idea of “Dilution of Precision” (DOP) becomes important to comprehend. At its heart, it’s an issue in geometry. Indeed, taking back memory to my university days two decades ago we studied this first. It is necessary to get four satellite signals, but that is not enough for acceptable precision. Where those satellites are in the sky compared to each other is also important. The geometry is bad if all four signals come from satellites that are close together in one quadrant of the sky. The ranging lines are almost parallel, and slight time errors lead to big location errors. To get good precision, satellites need to be spread out across the whole sky, with some high above and others in different directions and heights. This spread makes triangulation geometry crisp and fixes positions tightly. A bad spread causes what is officially known as high DOP, which signifies bad accuracy no matter how strong the signals are. Even with all seven satellites working, 24 hours NavIC’s DOP performance was not the same over all of India. In southern India, across Kerala, Tamil Nadu, and Karnataka, satellites were well spread out over the sky and were very accurate. The geometry was relatively weaker throughout northern and northeastern Indias. The DOP readings were at levels that were problematic for operations. These are, most importantly, the areas that are closest to India’s most important strategic borders. Over the past few years, satellites have stopped working because their atomic clocks, which are the precise timekeepers that every navigation satellite needs, have broken. This has caused the constellation to shrink, the geometry to get worse, and the accuracy to drop a lot. When combined with other navigation systems like GPS or Galileo, NavIC may still be able to help with placement. A smartphone that uses more than one constellation will always work better than one that only uses one. The accuracy gains from combining constellations are not small; they are very large. But this is where the strategic conflict comes in: the whole point of NavIC was to make India less dependent on foreign systems that may be refused or degraded in a geopolitical crisis. An indigenous navigation system that only works when American or European satellites support it is not genuinely independent. India’s goal should be to build a NavIC constellation that is both accurate and strong enough to stand on its own.
3. Can’t ISRO just launch three more satellites quickly?
This is the most prevalent mistake, and it’s important to be clear: no, you can’t just launch three navigation satellites whenever you want. You can’t just install NavIC satellites in a low orbit and start using them right away since they aren’t Earth observation satellites. To provide the correct amount of geometric variation to the constellation, each NavIC satellite must be moved into a very specific position in geosynchronous orbit, with a specific longitude, inclination, and altitude. The whole system is a carefully planned arrangement of how things move in space. This implies that the three new satellites can’t all be sent into space on the same rocket. To get the constellation back to its original shape, they need to be launched at separate times and into different orbits. Every launch needs its own planning, mission design, and sequencing for getting into orbit. There are no shortcuts; it takes time.
What next? It’s not enough for NavIC to just deploy one additional satellite into space to fill a gap. To restore the constellation’s geometric integrity, not merely its headcount, at least three satellites need to be added in a planned and ordered way. The engineers and mission planners who are working on this are completely aware of these limits, and you can be sure that work is being done. However, NavIC should not be taken for granted while the rest of India’s infrastructure becomes more reliant on it. Today, GPS-based positioning is used to build precision agriculture, utility monitoring, infrastructure surveys, fleet management, and a lot of other civilian applications. As these dependencies increase, the necessity for a resilient, autonomous indigenous navigation system intensifies — not diminishes. The military aspects of this dependency, which have their own serious effects, need to be talked about in more length and separately. The bottom line is clear: NavIC is a strategically important system that is not yet ready for use. It is not an option to restore it and make it so that it can work reliably without external constellations. It is a national necessity.