GPS based attitude determination

• Main Author: Thorstensson, Erika
• Format: Others
• Language: English
• Published: Linköpings universitet, Institutionen för teknik och naturvetenskap 2009
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-94447

This paper is the result of a masters thesis performed at Linköping University for Saab Bofors Dynamics in Linköping, Sweden. Attitude is defined as the orientation of a coordinate frame in reference to another coordinate frame. This is often referred to as three consecutive rotations, called roll, pitch and yaw (or heading). Attitude determination is generally performed using inertial navigation systems composed of gyros and accelerometers. These systems are highly accurate but are very expensive and experience a drift when used for a long period of time. The global positioning system, or GPS, was developed by the US military to determine a user’s position, velocity and time. These parameters can all be determined by performing measurements on the GPS satellite signal code that is modulated onto the GPS satellite signal carrier. But the GPS signal can also be use to determine attitude by performing carrier measurements for two or more GPS antennas. When determining the relative position between two antennas, by measuring the phase difference between them, information of the baseline is attained. The calculated baseline will be in a local navigation frame. By rotating it to the known body frame, a rotation matrix known as a direction coordinate matrix, or DCM, will be attained. From this rotation matrix, only two of the three attitude angles can be calculated, since the third rotation is about the baseline’s own axis. Using three or more antennas (two or more baselines), all three attitude angles can be determined from the DCM. This thesis work included development of a hardware platform carrying four NovAtel Superstar II GPS receivers. The platform enables serial communications between the receivers and a PC, as well as providing the supply for the receivers and antennas. The attitude determination algorithm was written and tested using a test platform mounted onto a car. The result shows a one degree deviation from an INS reference system in heading and pitch during both static and dynamic tests. The roll angle was not measured during the test drives because only one baseline was locked. The GPS based attitude determination system performed well when the baselines were locked, but it requires some improvements for full functionality.