1. A method of processing a set of GNSS signal data derived from signals having at least two carrier frequencies and received from two or more satellites at two or more reference stations over multiple epochs, comprising: a. For each satellite, forming a geometry-free combination of GNSS signal data derived from two carrier frequencies received at the reference stations during an epoch; b. For each satellite, applying a filter to the geometry-free combination to obtain state values representing (i) a carrier-phase ambiguity for each reference station and (ii) ionospheric advance at a reference point and state values representing variation of ionospheric advance relative to the reference point across a network area, along with error estimates for the state values; and c. Repeating a, and b, for each of multiple epochs to update the state values and error estimates.