Abstract: The invention relates to a system for monitoring an airspace for an extensive area, with at least two optical sensors with a passive Fourier transform infrared spectrometer, wherein each optical sensor has an adjustable monitoring range and wherein the monitoring ranges of the at least two optical sensors overlap at least in sections, having a server for evaluating the measurement data and for controlling the at least two optical sensors, the server being set up to monitor the optical sensors for automatic scanning of the monitored areas, wherein the server assigns a respective solid angle to the measurement data on the basis of the position data of the optical sensor, evaluates the measurement data of the optical sensors to derive the spectral intensity distribution of the received IR radiation for each solid angle and, by means of correlation of the intensity distribution with known gas spectra, to identify at least one target substance, in the event of an incident, if a first optical sensor identifies a ta