What is the main task of the user segment (receiver) of the satellite navigation system NAVSTAR/GPS?

The main task of the user segment, or receiver, in the NAVSTAR/GPS satellite navigation system revolves around selecting the appropriate satellites for tracking, acquiring their signals, and accurately measuring the time it takes for those signals to travel from the satellites to the receiver. This process is fundamental to determining the receiver's position on the Earth's surface.

To achieve positioning, the receiver uses signals transmitted by multiple satellites. By knowing the speed of light (the speed at which the signals propagate) and the time taken for each signal to reach the receiver from each satellite, the receiver can calculate its distance from each one. This distance measurement, combined with data from at least four satellites, allows the receiver to triangulate its precise location through a process known as trilateration.

The other options touch on aspects of satellite functionality or monitoring but do not accurately describe the primary role of the user segment. For instance, broadcasting signals is not a task performed by the receiver, nor is it responsible for monitoring satellite status or orbital planes, which fall under the domain of the control segment or the satellite itself.