Software versus FPGA based realtime kinematic ...

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Software versus FPGA based realtime kinematic GNSS receiver - a comparison Thorsten L¨uck† , Michael Bodenbach† Thomas Pany‡ † IfEN GmbH, Poing, Germany ‡ University FAF, Neubiberg, Germany

BIOGRAPHY ABSTRACT On the background of the upcoming European global navigation system GALILEO and the modernization of GPS many new applications in the ”high-end” of satellite based positioning and navigation are feasible. Especially the availability of a second and/or third frequency for the general purpose user allows for higher precision and faster acquisition for kinematic applications. In the effort of implementing new algorithms for real time kinematics, two different approaches for the baseband processing unit has been developed. One approach implements a totally PC based ”software receiver”, which processes the digitized intermediate frequency within the CPU of the personal computer. As the correlators as well as the loop discriminators use the microcontroler of the computer, a high performant computer is required. However using a software solution has the advantage of maintaining the full control over the baseband process and the opportunity of testing different algorithms side by side. In an other approach, the whole baseband processing is performed within a field programmable gate array (FPGA) thus reducing the load of the microprocessor considerably. In addition, using a FPGA has the advantage of processing the digitized IF samples in real time whereas retaining full flexibility and controle over the correlation and loop closure process. Implementing different algorithms or channel configurations often requieres a time consuming compilation process of the FPGA configuration file. However, charging with different configuration, total reconfiguration of the FPGA takes only a few seconds. Both methods open the flexibility to implement new technologies with respect to the GPS modernization and the upcoming GALILEO by software updates and easily scales in terms of number of correlators and channels as the limiting factor is either CPU performance or the number of available gates within the FPGA. For instance the po-

sition of any correlator with respect to prompt is adjustable during runtime and is configurable even within the FPGA based receiver with microchip resolution or even better. While the PC based software receiver uses a standard COTS digitizer to sample the IF signal the FPGA based receiver processes digital samples with six bit resolution at 40 MHz sample rate. However, both units use the same analog IF signal from a L1/L2 high frequency frontend. By using almost standard components within the same PC, both approaches can be used simultaneously which allows direct comparison of either method. This paper introduces the different methods of base band processing - PC based and FPGA based - and compares the performance of both. Using different test environments, the precise code and phase measurements which are essential for real time kinematics is proven for both approaches. INTRODUCTION The presented work has been performed on behalf of the German Aerospace Center (DLR) with the major goal, to not only develop an innovative design for a realtime kinematic (RTK) system, based on component of the shelf (COTS) technologies but create a system with potential for future Galileo and modernized GPS with the ability to connect to internet services and geographic infomation systems (GIS). It must be pointed out, that - while several up to date RTK receiver are available on the american and canadian marked - only little developments in these direction were undertaken in germany by the time of application of this project. The receiver presented here thus serve as a demonstrator for different techniques on software and hardware level to fulfill the requirements of a GNSS RTK receiver for. It has been designed to receive and process the GPS coarse acquisition (CA) code on both, L1 and L2 carrier frequency. While there were plans to implement algorithms for L2CS tracking, these had to be postponed due to the de-

lay of the launches for GPS block IIR-M sattellites. RTK DEMONSTRATOR Two major signal pathes are realized in the receiver - a pure software and a hardware accelerated base band processing block. Figure 1 demonstrates the overall concept of the RTK demonstrator. On hardware level, the receiver consists of a standard GPS antenna (Novatel GPS-702), a proprietary radiofrequency frontend and a standard personal computer (PC). The RF frontend delivers an intermediate frequency (IF) signal at approx. 8MHz center frequency and 12MHz band width as either an analog or digital signal with 40MHz. sample rate. In addition, the PC consists of an analog to digital converter and a fpga based configurable computer, both connected to the main processor using the PC internal PCI bus. All components instead of the RF frontend are commercial parts (COTS). On software level, a top level application with graphical user interface (GUI) is used for central commanding of all hardware and software modules. Two algorithms are implemented within the navigation processor to realize single point and realtime kinematic positioning. A standard Novatel receiver (Millennium OEM3) acts as reference. RF Frontend Hardware based base band processing Software based base band processing VERIFICATION Signal Processing Navigation Solution CONCLUSION REFERENCES [1] Kaplan, E. D. (editor), “Understanding GPS: Principles and Applications”, Artech House, Boston, 1996. [2] Teunissen, P. J. G., and Kleusberg, A., “GPS for Geodesy”, 2nd ed., Springer, New York, 1998.

Fig. 1 Overall concept of the developed RTK demonstrator