High demands are placed on spacecraft navigation systems, particularly concerning accuracy. This is especially true during highly dynamic flight phases, namely ascent and entry, descent, and landing. To meet this challenge, measurements from different sensors are combined by taking advantage of the positive characteristics of each instrument. For most applications, an Inertial Measurement Unit (IMU), composed of individual inertial sensors, is used to directly measure the accelerations and rotation rates the vehicle experiences. These sensors are generally capable of very high data rates, allowing to measure rapid changes in velocity, position or attitude. However, inertial sensors are not able to perform direct measurements of these properties of the vehicle state. Instead, the measured accelerations and rotation rates have to be numerically integrated to obtain the vehicle’s velocity, position and attitude. During integration, any measurement errors will accumulate over time and the calculated values will drift away from the real values.
An approach for reducing these accumulated errors is to combine the inertial measurements with information from other sensors capable of directly measuring position, attitude or velocity. The combination of these different types of sensors using data fusion algorithms is referred to as an integrated or hybrid navigation system.
For terrestrial applications, IMUs are often combined with an additional sensor for position determination based on the Global Positioning System (GPS). For space applications close to Earth, GPS may also be used. In this case, data fusion is often augmented by information from other sensors, for example star trackers.
The Department of Guidance, Navigation and Control Systems has developed a hybrid navigation system for vehicles that launch out of Earth’s atmosphere or reenter into the atmosphere (e.g. rockets). This system combines measurements of an IMU with information from a GPS receiver and an experimental star tracker into a navigation solution. It was tested on the SHEFEX II sounding rocket mission, which was launched in June 2012. The flight results showed that the system behaved as designed and the performance met the expectations. The system is currently being adapted for DLR’s upcoming ReFEx mission. For this mission, the SHEFEX II navigation experiment has to be evolved into a flight critical, highly reliable navigation system.