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Why is the standard deviation of the BESTPOS log larger than the spec on the data sheet? The standard deviation field in the BESTPOS log contains the instantaneous (single epoch) estimated standard deviations of the position and NOT the standard deviation of the actual position error. The estimated standard deviations refer to a confidence range where the receiver estimates its antenna is in real time, relative to the truth position. The position error RMS in the data sheet refers to the receivers actual position accuracy given truth position.
The estimated standard deviation is a measure on how confident about our estimated position based on the consistency of the measurements, the geometry of the satellites (a.k.a. DOP) and estimates of the measurement errors. The estimated standard deviation is usually slightly pessimistic because outperforming a slightly higher estimated standard deviation is more acceptable than underperforming an optimistic estimated standard deviation. In some applications, including DGPS, information about the quality of the data is missing because of the data transmission format. In these applications a more pessimistic estimated standard deviation is reported as a usual worst case given the missing information.