"At-sea alignment" is a synonym for inertial high technology and computing. An aircraft carrier has an inertial navigation system "hybridized" with GPS information (and even an electromagnetic log). It gathers optimal geographical positioning and speed information with regard to the ground, acceleration and angular velocity. It transmits this information through a digital air link (infrared in this case for security reasons) to the fighter aircraft lined up on the deck (which they receive through an infrared receiver situated on the top of their tail fin). In the aircraft, an inertial system, also hybridized with GPS, receives this information from the aircraft carrier system and uses it to initialize its position, speed and acceleration, despite the "lever arm" which exists with regard to the ship's navigation unit. This is the basic operating principle of "at-sea alignment", first applied by Sagem in the 1970s and continued today in the Rafale fighter aircraft on board the Charles de Gaulle aircraft carrier.
Pioneering application:
At-sea alignment of Super-Etendard aircraft
In the 1970s, Sagem made both the inertial units ("tuned-rotor gyroscope technology") of the Super-Etendard aircraft and their hybrid navigation systems, as well as the hybrid navigation system of the aircraft carrier, which was based on the same type of inertial system. Not to mention the TELEMIR infrared digital link between the ship navigation system and the aircraft navigation systems. This system proved satisfactory for over twenty years; although it was modernised over this period, this concerned more the electronics than the operating principle.
At-sea alignment today:
Rafale on the Charles de Gaulle aircraft carrier
Two Minicins, inertial systems derived from submarine systems, form the core of the hybrid inertial navigation/GPS (Kalman) system of the aircraft carrier. There is still a TELEMIR infrared digital link between the aircraft carrier's navigation system and that of the Rafales. However, the inertial systems on these fighters are now ring-laser gyro SIGMA RL90's, which include GPS (Kalman) "correction", and as long as the aircraft is at stand still on the deck, TELEMIR data for initializing the aircraft navigation is taken into account. The principles and algorithms, which were so new in the 1970s, continue to be used on the whole but they have been extended to a third level of navigation: the AASM missiles on board the aircraft.
Three levels of navigation operate in relay:
Aircraft carrier > aircraft > AASM (air-to-ground modular weapon)
In the same way as the aircraft navigation systems "align at-sea" with the aircraft carrier navigation system, the navigation systems of the on-board armament (AASMs) "align in-flight" with the aircraft navigation system. The same principle applies: data from the reference navigation system (that of the aircraft in this case) is sent in real time to the system to be aligned (the weapon).
For each of the three examples on this page, in addition to the "real-time" software operated by avionics, top mission planning (SLPRM among others) and "debriefing" software is used which is essential for such complex missions.
