A system comprising a monitoring system for a ground speed determination system in an aircraft and the ground speed determination system, the ground speed determination system comprising a first speed measurement system and a second speed measurement system , the first speed measurement system arranged to determine a first time-varying ground speed measurement based on global positioning system data, the second speed measurement system arranged to determine a second time-varying ground speed measurement based on accelerometer data, the second speed measurement system comprising an integrator and being associated with a predetermined behaviour characteristic comprising an error value that is amplified over time by the integrator, the error value being a first value of a rate of change of acceleration; the monitoring system comprising a processor arranged to: receive ground speed data provided by the ground speed determination system ; perform a correspondence determination process comprising processing the received ground speed data to determine a second value of a rate of change of acceleration indicated by the received ground speed data, and determining whether a correspondence condition is satisfied, the correspondence condition comprising a correspondence between the first value and the second value; and in response to determining that the correspondence condition is satisfied, determine that the first speed measurement system is in an error condition. the ground speed determination system comprising a first speed measurement system and a second speed measurement system , the first speed measurement system arranged to determine a first time-varying ground speed measurement based on global positioning system data, the second speed measurement system arranged to determine a second time-varying ground speed measurement based on accelerometer data, the second speed measurement system comprising an integrator and being associated with a predetermined behaviour characteristic comprising an error value that is amplified over time by the integrator, the error value being a first value of a rate of change of acceleration; the monitoring system comprising a processor arranged to: receive ground speed data provided by the ground speed determination system ; perform a correspondence determination process comprising processing the received ground speed data to determine a second value of a rate of change of acceleration indicated by the received ground speed data, and determining whether a correspondence condition is satisfied, the correspondence condition comprising a correspondence between the first value and the second value; and in response to determining that the correspondence condition is satisfied, determine that the first speed measurement system is in an error condition. receive ground speed data provided by the ground speed determination system ; perform a correspondence determination process comprising processing the received ground speed data to determine a second value of a rate of change of acceleration indicated by the received ground speed data, and determining whether a correspondence condition is satisfied, the correspondence condition comprising a correspondence between the first value and the second value; and processing the received ground speed data to determine a second value of a rate of change of acceleration indicated by the received ground speed data, and determining whether a correspondence condition is satisfied, the correspondence condition comprising a correspondence between the first value and the second value; and in response to determining that the correspondence condition is satisfied, determine that the first speed measurement system is in an error condition. <EOS>