A system for monitoring an exhaust gas purification system which is functionally connected to an emission source, in particular to a ship's engine, wherein the system comprises: an emission sensor which acquires an emission signal, a plurality of further sensors (23, 24) which output respective measurement signals related to the exhaust gas purification system, and an electronic processing unit which is configured to determine a process state of the exhaust gas purification system based on the measurement signals of the further sensors (23, 24), characterized in that the emission signal acquired by the emission sensor indicates emissions of the emission source on the exiting from the exhaust gas purification system, and the electronic processing unit is further configured to assess the emission signal and the process state in each case with respect to an anomaly in that: the electronic processing unit determines whether the emission signal of the emission sensor fulfills a predetermined validity criterion or a combination of predetermined validity criteria, and the electronic processing unit then, if a sufficient number of valid process parameters can be determined based on the measurement signals of the further sensors (23, 24), further determines whether the process parameters determined based on the respective measurement signals of the sensors (23, 24) each fulfill predetermined validity criteria, and to determine and to output a compliant emission state, which indicates a correct functionality of the exhaust gas purification system, when no anomaly of the emission signal is present and when an anomaly of the emission signal is present while at the same time no anomaly of the process state is present or the anomaly of the emission signal satisfies an exception condition. an emission sensor which acquires an emission signal, a plurality of further sensors (23, 24) which output respective measurement signals related to the exhaust gas purification system, and an electronic processing unit which is configured to determine a process state of the exhaust gas purification system based on the measurement signals of the further sensors (23, 24), the emission signal acquired by the emission sensor indicates emissions of the emission source on the exiting from the exhaust gas purification system, and the electronic processing unit is further configured to assess the emission signal and the process state in each case with respect to an anomaly in that: the electronic processing unit determines whether the emission signal of the emission sensor fulfills a predetermined validity criterion or a combination of predetermined validity criteria, and the electronic processing unit then, if a sufficient number of valid process parameters can be determined based on the measurement signals of the further sensors (23, 24), further determines whether the process parameters determined based on the respective measurement signals of the sensors (23, 24) each fulfill predetermined validity criteria, and to determine and to output a compliant emission state, which indicates a correct functionality of the exhaust gas purification system, when no anomaly of the emission signal is present and when an anomaly of the emission signal is present while at the same time no anomaly of the process state is present or the anomaly of the emission signal satisfies an exception condition. the electronic processing unit determines whether the emission signal of the emission sensor fulfills a predetermined validity criterion or a combination of predetermined validity criteria, and the electronic processing unit then, if a sufficient number of valid process parameters can be determined based on the measurement signals of the further sensors (23, 24), further determines whether the process parameters determined based on the respective measurement signals of the sensors (23, 24) each fulfill predetermined validity criteria, and to determine and to output a compliant emission state, which indicates a correct functionality of the exhaust gas purification system, when no anomaly of the emission signal is present and when an anomaly of the emission signal is present while at the same time no anomaly of the process state is present or the anomaly of the emission signal satisfies an exception condition.