List common GNSS error sources that should be modeled in an EKF.

• A. Satellite clock errors, ephemeris errors, ionospheric and tropospheric delays, multipath, receiver thermal noise, receiver biases.
• B. Satellite clock errors and multipath only.
• C. Ionospheric delays and tropospheric delays only.
• D. Receiver thermal noise only.

Answer: (A)

This item tests which error sources GNSS measurements must be modeled in an EKF to keep estimates accurate and unbiased. In GNSS state estimation, the measured pseudorange (and often carrier phase) are affected by several error terms that propagate into the state if not accounted for. Satellite clock errors and ephemeris errors are essential because inaccuracies in the satellite’s timekeeping and orbital parameters translate directly into range biases that the filter must separate from the receiver’s motion. Ionospheric and tropospheric delays are propagation effects that alter signal travel time as it passes through the atmosphere; modeling them—typically with zenith delays and mapping functions or estimating ionospheric delay—is crucial because they vary with path geometry and frequency. Multipath introduces additional biased components when signals reflect off nearby surfaces, so including a way to handle or mitigate multipath prevents these biases from corrupting the state update. Receiver thermal noise represents the fundamental random noise floor of the measurements, and receiver biases (such as inter-channel or receiver clock biases) account for systematic offsets in the receiver’s measurement chain. Together these sources cover the dominant errors that affect GNSS ranging, enabling the EKF to attribute observed measurement deviations to either motion or error terms and to adapt over time.

Choosing only a subset, like just satellite clock errors and multipath, misses key contributors such as ephemeris and atmospheric delays and would lead to biased or inconsistent estimates.