High demands are placed on spacecraft navigation systems, particularly concerning accuracy. This is especially true during highly dynamic flight phases, namely ascent and entry. To meet this challenge, measurements from different sensors are combined by taking advantage of the positive characteristics of each instrument. Typically this involves combining inertial sensors with absolute measurement sensors.
For most applications, an Inertial Measurement Unit (IMU) is used to directly measure a vehicle’s acceleration and rotation rate at a high data rates. This information is integrated to calculate a vehicle’s position, velocity and orientation. Over time, any errors introduced by inertial sensors build up and must be corrected. For this reason, inertial sensors are combined with absolute sensors, such as a Global Position System receiver (GPS) or star tracker. These sensors provide direct measurements of position, velocity or orientation at a low data rate. Combining sensors to take advantage of each instrument’s positive characteristics is called sensor fusion and is the basis of an integrated navigation system.
The Hybrid Navigation System was developed for ascent and atmospheric entry vehicles and combines an IMU, GPS and an experimental star tracker. It was tested on the SHEFEX2 sounding rocket mission, which was launched in June 2012. The flight results showed that the system behaved as designed and the performance was as expected. This system will be further developed in the SHEFEX3 mission as a flight critical component. Another system currently under development is the SINPLEX miniaturized system, which combines an IMU, laser ranger finder, navigation camera and star tracker.