Which system requires an unobstructed line of sight to four or more satellites?

The Global Positioning System (GPS) requires an unobstructed line of sight to at least four satellites to accurately determine a user's position on Earth. This is because GPS relies on the triangulation method, which involves measuring the time it takes for signals from multiple satellites to reach the receiver. By having signals from at least four satellites, the system can calculate not only the two-dimensional position (latitude and longitude) but also the three-dimensional position (including altitude) of the receiver.

The necessity for an unobstructed line of sight is crucial, as physical barriers like buildings, trees, or mountains can block satellite signals, leading to inaccurate positioning. The ability to access signals from multiple satellites improves reliability and accuracy, which is foundational to GPS functionality.

Other systems like Galileo and GNSS share similarities with GPS, but the specific requirement for four satellites for precise positioning is particularly highlighted in the context of GPS. The Indian Regional Navigation Satellite System (IRNSS) has its own unique specifications and may function differently in terms of satellite requirements compared to GPS.