GNSS Errors and Limitations

Main GNSS Error Sources

• Ionospheric Propagation Delay: The most significant single source of error in GNSS. Variations in the ionosphere's electron density cause signals to slow down, especially for satellites low on the horizon. Dual-frequency receivers can nearly eliminate this error, while single-frequency receivers rely on models that only partially correct it.

• Satellite Clock Errors: Satellite clocks can drift, introducing timing errors that affect range measurements. These are corrected by ground control uploads, but errors may persist until the next update.

• Satellite Orbital (Ephemeris) Errors: The actual position of a satellite may differ from its broadcast position due to gravitational forces (from the sun, moon, and solar wind). This can introduce position errors of up to about 0.5 meters.

• Multipath Effects: GNSS signals can reflect off surfaces like terrain or buildings, causing the receiver to process both direct and reflected signals. This can lead to errors of several meters, especially in mountainous or urban environments.

• Receiver and Instrument Errors: Electrical noise, processing limitations, and antenna design can all degrade accuracy. High-quality aviation receivers minimize these effects, but less sophisticated devices may introduce more error.

Dilution of Precision (DOP)

• Geometric Dilution of Precision (GDOP): The relative positions of satellites in the sky affect the accuracy of position calculations. Poor geometry (e.g., satellites clustered together) increases GDOP and reduces accuracy. More satellites and better distribution improve precision.

User Equivalent Range Error (UERE) and Position Estimation

• The combination of all individual error sources is termed UERE. When UERE is combined with GDOP, it allows for an estimation of the overall position accuracy.

GNSS Limitations and Interference

• Interference and Jamming: GNSS signals are weak and vulnerable to intentional jamming or unintentional interference, which can degrade or block reception.

• System Limitations: GNSS does not correct for all errors in real time, and some errors (like multipath) are difficult to eliminate. Ground-based augmentation systems can help correct certain errors, especially for precision approaches.

Summary Table: Major GNSS Errors

• Ionospheric delay (dominant, especially for single-frequency)
• Satellite clock error
• Satellite orbital (ephemeris) error
• Multipath
• Receiver/instrument error
• Dilution of precision (GDOP)

Understanding these errors and limitations is essential for safe and effective use of GNSS in aviation.