Polar satellites for Global Navigation Systems Why do we need 7 polar satellites for a navigation system like the Indian Regional Navigation Satellite System (IRNSS)?
IRNSS
is intended to serve an area spanning 100 degrees in longitude by 80 degrees in latitude.
Anywhere in that service area, at any time, a receiver needs to compare distances to four of the satellites.
To determine latitude, the satellites can't all be above the equator, so some of the orbits are inclined 29$^\circ$. Those satellites oscillate north-south in pairs. Polar orbits aren't needed; global systems serve the poles with orbits inclined only 55$^\circ$.