A processing module to monitor a horizontal delay gradient in satellite signals (450(1-N)), the processing module comprising: a dual-processing-satellite-differencing module operable to: receive carrier phase measurements for at least two satellites from at least two reference receivers, the at least two satellites including a monitored satellite and at least one other satellite, wherein the at least two reference receivers have a known geometric relationship to each other; implement a first processing mode to normalize first satellite differences between +λ/2 and -λ/2; characterized in that the dual processing-satellite-differencing module is further operable to: implement a second processing mode configured to normalize second satellite differences between 0 and λ; and select for further processing one of: data indicative of the first satellite differences processed according to the first processing mode; and data indicative of the second satellite differences processed according to the second processing mode; the processing module further comprising a double differencing module configured to form double-differences between one or more pairs of the at least two reference receivers based on the selected data input from the dual-processing-satellite-differencing module; and a gradient estimator module configured to estimate a magnitude of the horizontal delay gradient based on averaged compensated double-differences for the monitored satellite. a dual-processing-satellite-differencing module operable to: receive carrier phase measurements for at least two satellites from at least two reference receivers, the at least two satellites including a monitored satellite and at least one other satellite, wherein the at least two reference receivers have a known geometric relationship to each other; implement a first processing mode to normalize first satellite differences between +λ/2 and -λ/2; characterized in that the dual processing-satellite-differencing module is further operable to: implement a second processing mode configured to normalize second satellite differences between 0 and λ; and select for further processing one of: data indicative of the first satellite differences processed according to the first processing mode; and data indicative of the second satellite differences processed according to the second processing mode; the processing module further comprising a double differencing module configured to form double-differences between one or more pairs of the at least two reference receivers based on the selected data input from the dual-processing-satellite-differencing module; and a gradient estimator module configured to estimate a magnitude of the horizontal delay gradient based on averaged compensated double-differences for the monitored satellite. receive carrier phase measurements for at least two satellites from at least two reference receivers, the at least two satellites including a monitored satellite and at least one other satellite, wherein the at least two reference receivers have a known geometric relationship to each other; implement a first processing mode to normalize first satellite differences between +λ/2 and -λ/2; implement a second processing mode configured to normalize second satellite differences between 0 and λ; and select for further processing one of: data indicative of the first satellite differences processed according to the first processing mode; and data indicative of the second satellite differences processed according to the second processing mode; the processing module further comprising a double differencing module configured to form double-differences between one or more pairs of the at least two reference receivers based on the selected data input from the dual-processing-satellite-differencing module; and a gradient estimator module configured to estimate a magnitude of the horizontal delay gradient based on averaged compensated double-differences for the monitored satellite.