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$^\circ$.
The satellites used in Indian Navigational System or so called Indian Regional Navigation Satellite System are not polar satellites.Three of them are Geostationary satellites and other four are Geosynchronous satellites.
Polar satellites revolve around the earth in North-South direction, where as these satellites revolve around with the same speed as that of the earth's rotation so that they appear to be fixed on sky for observers on earth. This is exactly what is needed for navigation systems.