GPS Space Segment – Number of NAVSTAR satellites The GPS space segment (NAVSTAR constellation) nominally consists of how many operational satellites?

Introduction / Context:
GPS (NAVSTAR) provides global coverage using a medium-Earth-orbit satellite constellation. Understanding the nominal constellation size is fundamental to dilution of precision (DOP), satellite visibility, and mission planning for surveying and navigation.

Given Data / Assumptions:

• Nominal, textbook constellation (legacy design).
• Focus on operational satellites required for full global coverage.

Concept / Approach:
The classic NAVSTAR design specifies 24 satellites arranged in 6 orbital planes (4 per plane), with appropriate RAAN spacing and 55° inclination to ensure at least 4 satellites are visible worldwide at most times.

Step-by-Step Solution:
Recall the standard: 6 planes, 4 satellites each.Total = 6 * 4 = 24 operational satellites (not counting on-orbit spares).

Verification / Alternative check:
Although modern GPS often operates with more than 24 spacecraft (including spares), the nominal constellation in textbooks/exams remains 24.

Why Other Options Are Wrong:

• 8, 12, 16: too few for continuous global 3D positioning.
• 30: can be seen in practice but exceeds the nominal design.

Common Pitfalls:
Confusing nominal constellation size with the actual number currently on orbit; exams typically ask for the nominal 24.

Final Answer:
24