Final answer:
A navigator uses two radar ranges to triangulate a ship's position, an essential skill despite the prevalence of GPS, which provides highly accurate location data using satellite signals. (option c)
Step-by-step explanation:
A navigator fixing a vessel's position by radar needs at least two radar ranges. This allows for a technique known as radar triangulation, where the navigator uses the radar to measure the distances to two separate landmarks or other fixed points. By plotting these distances on a nautical chart, the intersection of the two ranges provides the ship's location. This is necessary because using only one radar range would offer multiple possible positions along a circle of that radius, and more information is required to pinpoint the exact location.
Modern navigation often relies on the Global Positioning System (GPS), which uses signals from a network of 24 satellites equipped with atomic clocks. These satellites broadcast their position, and a GPS receiver on the ship detects these signals to calculate its position with a high degree of accuracy, generally within at least 15 meters or about 50 feet. Despite the availability of GPS, traditional radar navigation remains an essential skill for navigators, especially in situations when GPS may not be available or reliable.