An aircraft navigation system and navigation method. The system comprises a civil-certified GPS-receiver for determining first position information based on C/A-GPS-signals, and a military-type GPS-receiver for determining a second position information based on P(Y)-GPS-signals. A monitoring unit detects a first abnormal condition during jamming conditions within the C/A-GPS-signals. The unit compares the first position information with the second position information and detects a second abnormal condition when a difference between these position information is larger than a threshold value for more than a threshold duration. Upon detecting an abnormal condition, an alert procedure is initiated. In remotely piloted aircraft, Command & Control link-loss is also monitored and during link-loss, a transponder code automatically indicates communication failure. When in such situation an abnormal condition is additionally detected, a transponder code indicates airborne emergency and, subsequently, the aircraft navigation system is switched to navigating based on position information from the military-type GPS-receiver.