Which system uses 24 satellites in a 12-hour orbit for navigation purposes?

The correct answer is the Global Positioning System (GPS), which utilizes a constellation of 24 satellites in medium Earth orbit. These satellites are arranged in a way that completes two orbits every 24 hours, allowing for a constant coverage of the Earth's surface for navigation purposes. Each satellite continuously transmits signals that allow GPS receivers to triangulate their position based on the time it takes for the signals to travel from the satellites to the receivers.

The GPS is renowned for its accuracy and reliability, making it the standard system for global navigation and positioning. It enables users to determine their exact location anywhere on the planet, which is essential for various applications, including aviation, marine navigation, and outdoor activities.

In contrast, the other options do not utilize satellite-based systems. The Tactical Air Navigation System (TACAN) is an aircraft navigation aid that provides distance and bearing information but relies on ground-based stations. The VHF Omnidirectional Range (VOR) is also a ground-based radio navigation system that provides azimuth information to aircraft. The Marker Beacon is a device that provides distance information while flying over a runway approach but operates within a limited range and does not utilize satellites for navigation.