A new family of inertial sensors

: Studies were initiated in 2000 in connection with the Pleïades development programme with co-funding from ESA, CNES and Astrium. The studies were aimed at enhancing satellite performance by providing a greater degree of precision in image location (scale factor stable at 10 ppm), while guaranteeing excellent stability during image acquisition (Angular Random Walk at 0.0001°/√h). The utilisation of fibre optics eliminates perturbations induced by rotation of the gyro wheel, resulting in substantially enhanced performance and extended lifetime. The system achieves a level of precision such that the rotation of the Earth round the sun can be measured with just one gyroscope, requiring no other input data.

“Astrium has been developing, producing and testing the vast majority of gyroscopes used in its own satellites, and those of our competitors and in the export market for the last 25 years. This new technology further strengthens our leading position in this domain in Europe. The only competitor for Astrix™ 200 in this domain is Northrop Grumman in the USA. However, the latter’s products cannot be exported, while the Astrium gyroscope is subject to far fewer constraints as it incorporates no ITAR (International Traffic in Arms Regulations) components,” explains Xavier Calmet, who is in charge of the Sensors Division.

A level of performance never previously achieved: Based on fibre optics technology, the gyroscope has two main parts. The GEU (Gyro Electrical Unit) provides command and control functions, also incorporating a light source transmitted to the optical core, the second part of the sensor. This optical beam is split into two light waves, each following an optical fibre path several kilometres long but in opposite directions. To obtain the targeted information, the phase shift between these two backward travelling waves is measured on their arrival. This phase shift is directly proportional to the rotational speed of the satellite containing the gyroscope. This corresponds to direct application of the Sagnac effect (discovered in 1913) and the principle of causality revealed by Einstein’s restricted relativity theory. Xavier continues: “We measure phase shift to within 10-7 radians, while the coil accumulates several billion billion radians (10+10). This is like measuring the distance between Earth and the sun to the nearest thousandth of a millimetre.”

The first Astrix gyroscope will fly in 2008 on ESA’s Planck satellite, and later on Astrium’s COMS, Pleïades, Aeolus, Galileo IOV and other satellites.

Sensors and actuators: the ears and muscles of the satellite: The inertial sensors, gyroscopes and accelerometers are the ‘ears’ of the satellite and control its equilibrium. The actuators are the muscles of the satellite which enable it to manoeuvre, and include the CMG (control momentum gyro) that gives the satellite its agility.

Based in Toulouse, the Sensors and Actuators Department comprises a team of 40, led by Gérard Muller. An area of 1,250 m² is devoted to the production of sensors and actuators. Apart from mechanical testing, the department is self-dependant for the execution of all equipment acceptance tests. A thermal vacuum chamber designed for testing equipment at temperatures between -150°C and +150°C has recently been added to the department’s test facilities.