Which statement correctly describes the requirement for a 3D GPS fix?

Which statement correctly describes the requirement for a 3D GPS fix?

• A. A 3D position fix requires at least four satellites and is typically about 33 meters accurate
• B. A 3D position fix requires three satellites and is about 10 meters
• C. A 2D fix uses four satellites and is about 50 meters
• D. A 3D fix requires five satellites and is about 100 meters

Answer: (A)

To get a true three-dimensional GPS position, you must solve for three spatial coordinates plus the receiver’s clock bias. Each satellite measurement provides one equation, so you need four independent equations to solve for these four unknowns. That’s why at least four satellites are required for a 3D fix. In typical open-sky conditions, a standalone GPS receiver with four satellites can yield about tens of meters of accuracy, often cited around 33 meters. More satellites and better geometry improve accuracy, but four is the minimum for a 3D fix. With only three satellites you can determine a 2D position (horizontal coordinates) but cannot resolve altitude and the time bias, so a full 3D fix isn’t possible. Five satellites aren’t required as the minimum, they would mainly improve precision.

To get a true three-dimensional GPS position, you must solve for three spatial coordinates plus the receiver’s clock bias. Each satellite measurement provides one equation, so you need four independent equations to solve for these four unknowns. That’s why at least four satellites are required for a 3D fix. In typical open-sky conditions, a standalone GPS receiver with four satellites can yield about tens of meters of accuracy, often cited around 33 meters. More satellites and better geometry improve accuracy, but four is the minimum for a 3D fix. With only three satellites you can determine a 2D position (horizontal coordinates) but cannot resolve altitude and the time bias, so a full 3D fix isn’t possible. Five satellites aren’t required as the minimum, they would mainly improve precision.