Real Time Implementation of a Non-coherent Deeply Coupled GPS/INS System

Stefan Kiesel, Markus Langer, Gert F. Trommer

Abstract:	In a deeply coupled GPS/INS System the navigation process and the signal tracking process are performed simultaneously by including trajectory dynamics measured by an inertial navigation system. This means that the Kalman filter processing becomes an important part in the control loop for signal tracking. Thus a realtime interface, signal processing inside the Kalman filter, and controlling the NCOs. It will be shown in the next sections that the strapdown algorithm only takes about 100µs and a whole Kalman filter propagation and measurement step takes only about 500µs of processing time. Thus Kalman filtering can easily be done with 50 Hz update rate. Since the IMU data rate is higher, a synchronous task communication was chosen between the IMU and the Kalman filter task, while an asynchronous communication was chosen between the GPS receiver and the Kalman filter task. In the system setup it had to be considered that reading from the GPS receiver interface has the highest priority, since the 1ms correlator results are not buffered. As a consequence, any other task like reading from an IMU or doing Kalman filtering can be interrupted by this task. A non-coherent deeply coupled system design was chosen since it is the optimum architecture for tracking weak signals. Is and Qs are pre-processed in discriminator functions and used in the Kalman filter measurement step. In contrast to pseudorange measurements that are taken synchronously for each channel, discriminator outputs from each channel occur asynchronously. By using these measurements in a central Kalman filter the measurements processed at the same time. It turned out that this inevitable delay does not affect the system stability. With the output of the Kalman filter code and carrier NCOs are controlled. The software concept works for any GPS receiver hardware that provides I and Q samples with a rate of 50Hz or more. Actual tests have been performed with a GP1005 receiver card of GPSCreations proving the validity of the presented approach. The GPS related software uses parts of the open source project OSGPS [5]. The next chapter describes the algorithmic concept. Chapter 2 explains basics of the applied real-time operating system RTAI Linux. Chapter 3 introduces the software design and chapter 4 presents simulation results with a space segment simulator in a HIL test.
Published in:	Proceedings of the 2010 International Technical Meeting of The Institute of Navigation January 25 - 27, 2010 Catamaran Resort Hotel San Diego, CA
Pages:	12 - 21
Cite this article:	Kiesel, Stefan, Langer, Markus, Trommer, Gert F., "Real Time Implementation of a Non-coherent Deeply Coupled GPS/INS System," Proceedings of the 2010 International Technical Meeting of The Institute of Navigation, San Diego, CA, January 2010, pp. 12-21.
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