A navigation message is modulated onto both of the L1 and L2 carrier frequencies. What is the frequency of the navigation message, and what is its significance?

• A. 2 Hz, conveying timing only
• B. 50 Hz, containing status information on satellites and system
• C. 10 Hz, carrying ephemeris data only
• D. 100 Hz, used for clock synchronization

Answer: (B)

The navigation data rate is 50 bits per second, broadcast on both L1 and L2. This data stream carries the essential information the receiver needs to know where satellites are and what their clocks look like, including satellite health/status, ephemeris (precise orbit and clock corrections), and almanac information for all satellites. The 50 bps rate is chosen so the full set of navigation data can be transmitted continuously and updated as needed, enabling accurate timing and positioning. Without this data stream, a receiver cannot convert pseudorange measurements into precise positions. The content and steady 50 bps rate are what make reliable GNSS positioning possible.