Abstract: A tracking system is provided for detecting abnormal drift errors in the outputs of sensors (3) monitoring a plurality of parameters of a gas turbine engine (1). To this end the tracking system comprises a tracking simulator (5) providing a real-time computer model of the engine having control inputs for receiving control signals corresponding to control signals supplied to the engine in operation, and outputs for supplying estimated sensor output values Yest. The system additionally includes a tracking compensator (6) for producing estimated engine performance variation values &dgr;pest for supplying to the simulator (5), a memory (8) containing reference information indicative of the engine performance variation in response to hypothetical changes in the sensor output signals, and a sensor error estimation system (7) for producing estimated sensor output error values zest in dependence on the estimated engine performance variation values &dgr;pest and the reference information.