A system for determining a parameter indicative of an amount of a reducing agent in a selective catalytic reduction module of a vehicle, the system comprising: - the SCR module having at least one passage for transporting a stream of gas and an SCR catalyst for converting NOx emissions, and having an extension in an axial direction, - a first antenna unit configured to transmit microwave signal (S T) and receive a microwave signal (S R), a second antenna unit configured to transmit and receive microwave signals, and characterized in that the system further comprises at least a third antenna unit configured to transmit and receive microwave signals, - the first antenna unit, the second antenna unit and the third antenna unit being arranged spaced apart from each other, and further arranged outside the SCR module, and configured to transmit microwave signals towards the SCR catalyst and to receive microwave signals reflected within the SCR module, wherein the first antenna unit, the second antenna unit and the third antenna unit are further configured to transmit and receive signals intermittently between each other, wherein the system further comprises: - processing circuitry connected to the first antenna unit, second antenna unit and the third antenna unit, and configured to determine a plurality of differences between the transmitted and received microwave signals (S T, S R, S T2, S R2, S T3, S R3, S T4, S R4) and the received reflected microwave signal (S Ref, R), and to determine a parameter indicative of the amount of a reducing agent in the SCR catalyst based on the determined differences between the transmitted and received microwave signals and the received reflected microwave signal (S Ref, R). - the SCR module having at least one passage for transporting a stream of gas and an SCR catalyst for converting NOx emissions, and having an extension in an axial direction, - a first antenna unit configured to transmit microwave signal (S T) and receive a microwave signal (S R), a second antenna unit configured to transmit and receive microwave signals, and characterized in that the system further comprises at least a third antenna unit configured to transmit and receive microwave signals, - the first antenna unit, the second antenna unit and the third antenna unit being arranged spaced apart from each other, and further arranged outside the SCR module, and configured to transmit microwave signals towards the SCR catalyst and to receive microwave signals reflected within the SCR module, wherein the first antenna unit, the second antenna unit and the third antenna unit are further configured to transmit and receive signals intermittently between each other, wherein the system further comprises: - processing circuitry connected to the first antenna unit, second antenna unit and the third antenna unit, and configured to determine a plurality of differences between the transmitted and received microwave signals (S T, S R, S T2, S R2, S T3, S R3, S T4, S R4) and the received reflected microwave signal (S Ref, R), and to determine a parameter indicative of the amount of a reducing agent in the SCR catalyst based on the determined differences between the transmitted and received microwave signals and the received reflected microwave signal (S Ref, R).