Instrument Rating Checkride Practice Test

What is the minimum number of satellites required for a GPS receiver to perform FDE?

1. Five satellites with baro-aiding

2. Four satellites

3. Six satellites

4. Three satellites

The correct answer is: Six satellites

To understand the minimum number of satellites required for a GPS receiver to perform Fault Detection and Exclusion (FDE), it is important to know how GPS works. A GPS system calculates a receiver's position by using signals from multiple satellites. These satellites provide information about their position and the time the signal was sent. FDE is a critical function that allows a receiver to identify and mitigate the effects of faulty satellite data. For effective FDE to occur, the GPS receiver needs sufficient geometric information, which is achieved by processing data from multiple satellites. In the case of FDE, a minimum of six satellites is necessary. With six satellites, the GPS receiver can apply mathematical algorithms to detect anomalies or faults in the satellite signals. If one satellite is determined to be faulty, the receiver can exclude it from the calculations while still maintaining an accurate position solution with the remaining satellites. If fewer than six satellites are utilized, there may not be enough geometrical diversity for reliable fault detection. Thus, while four satellites can provide a position fix, they do not provide the necessary data redundancy to effectively perform FDE. This understanding of satellite requirements emphasizes the importance of having adequately redundant data to ensure both accuracy and reliability in GPS navigation systems.