How many satellites are required for a GPS constellation?

How many satellites are required for a GPS constellation?

• A. 24
• B. 28
• C. 20
• D. 32

Answer: (A)

Determining a 3D position and the receiver’s time requires solving for four unknowns: x, y, z, and the receiver clock bias. Each satellite gives one range measurement, so you need at least four independent measurements to solve for those four unknowns. To ensure this can be done anywhere on Earth with reliable accuracy, the GPS system is designed with enough satellites distributed in several orbital planes so that, from any location, at least four satellites are available above the horizon. The standard design places 24 satellites in six orbital planes, with four satellites in each plane, evenly spaced. This configuration guarantees a usable four-satellite geometry globally, providing the necessary measurements for a precise position and time fix. In practice, more satellites can be present (up to 32), but 24 is the nominal constellation required to meet global coverage and accuracy targets.

Determining a 3D position and the receiver’s time requires solving for four unknowns: x, y, z, and the receiver clock bias. Each satellite gives one range measurement, so you need at least four independent measurements to solve for those four unknowns. To ensure this can be done anywhere on Earth with reliable accuracy, the GPS system is designed with enough satellites distributed in several orbital planes so that, from any location, at least four satellites are available above the horizon. The standard design places 24 satellites in six orbital planes, with four satellites in each plane, evenly spaced. This configuration guarantees a usable four-satellite geometry globally, providing the necessary measurements for a precise position and time fix. In practice, more satellites can be present (up to 32), but 24 is the nominal constellation required to meet global coverage and accuracy targets.