A positioning system comprising a first GNSS receiver and a second GNSS receiver, the first GNSS receiver and the second GNSS receiver being mounted on a train: - the first GNSS receiver being configured to receive a spatial position, code measurements and carrier phase measurements of GNSS signals computed by the second GNSS receiver through a communication link; - the first GNSS receiver comprising a processing logic configured to: ∘ calculate a position of the first GNSS receiver from code measurements and carrier phase measurements of GNSS signals, and from the spatial position, the code measurements and the carrier phase measurements of GNSS signals received from the second GNSS receiver; and to ∘ estimate one or more parameters representative of multipath at the position of the first GNSS receiver, the estimated one or more parameters being a residual difference between pseudo ranges measured by the first GNSS receiver and pseudo ranges predicted using said position of the first GNSS receiver, after deduction of modeled errors, wherein modeled errors comprise satellite clock errors, receiver clock errors, ionospheric errors and tropospheric errors; wherein the processing logic is further configured to communicate to the second GNSS receiver said one or more parameters representative of multipath estimated at the position of the first GNSS receiver through the communication link, and wherein the second GNSS receiver is configured to use said one or more parameters representative of multipath estimated by the first GNSS receiver, to improve its own GNSS position computed when the second GNSS receiver is within a specified range and within a specified timeframe of said position of the first GNSS receiver. - the first GNSS receiver being configured to receive a spatial position, code measurements and carrier phase measurements of GNSS signals computed by the second GNSS receiver through a communication link; - the first GNSS receiver comprising a processing logic configured to: ∘ calculate a position of the first GNSS receiver from code measurements and carrier phase measurements of GNSS signals, and from the spatial position, the code measurements and the carrier phase measurements of GNSS signals received from the second GNSS receiver; and to ∘ estimate one or more parameters representative of multipath at the position of the first GNSS receiver, the estimated one or more parameters being a residual difference between pseudo ranges measured by the first GNSS receiver and pseudo ranges predicted using said position of the first GNSS receiver, after deduction of modeled errors, wherein modeled errors comprise satellite clock errors, receiver clock errors, ionospheric errors and tropospheric errors; ∘ calculate a position of the first GNSS receiver from code measurements and carrier phase measurements of GNSS signals, and from the spatial position, the code measurements and the carrier phase measurements of GNSS signals received from the second GNSS receiver; and to ∘ estimate one or more parameters representative of multipath at the position of the first GNSS receiver, the estimated one or more parameters being a residual difference between pseudo ranges measured by the first GNSS receiver and pseudo ranges predicted using said position of the first GNSS receiver, after deduction of modeled errors, wherein modeled errors comprise satellite clock errors, receiver clock errors, ionospheric errors and tropospheric errors;