Development of a Dual Frequency Software-based GNSS receiver

Throughout the last decade, a trend towards the development of software-based Global Navigation Satellite System (GNSS) receivers has evolved in the fields of scientific research. Software-based GNSS receivers feature a high flexibility for the adaptation to several applications, serve as a development platform for studying new algorithms and techniques, and especially have the advantage of requiring only few hardware parts. The main focus of current developments is the single frequency approach, aiming at mass market applications. The resulting position accuracy is insufficient for many applications, e.g., for the automotive domain. Modern driver assistance systems require position accuracies in the range of one lane width and even better (< 1m). Also safety relevant applications pose strong demands on higher position accuracy. Beside the errors introduced by 1967 23rd International Technical Meeting of the Satellite Division of The Institute of Navigation, Portland, OR, September 21-24, 2010 the satellites (satellite orbit, satellite clock), the troposphere, and the receiver clock, the ionosphere is one of the biggest error sources, when neglecting local effects like multipath for the moment. Adding a second measurement of a signal of a different carrier frequency, the error due to the ionosphere can be eliminated. Furthermore the integrity can be increased, which is essential for a wide range of applications. The current project work aims at developing a dual frequency software-based GNSS receiver to fulfil the above mentioned accuracy requirements. In its present implementation the receiver is capable of GPS code measurements making use of both available civil signals (L1 C/A and L2C). Efforts in modernizing the GPS satellites are still ongoing, but there are already (as at August 2010) eight Block IIR-M satellites and one IIF satellite in orbit, transmitting the second civil signal L2C on the L2 carrier. By using this second frequency it is possible to reduce/eliminate the ionospheric effect. After a brief introduction on the overall concept, the paper mainly focuses onto the implementation of the dual frequency software part. Afterwards, the main components of the receiver, i.e., the Digital Signal Processing (DSP) module, the Position, Velocity, and Time (PVT) module, and the acquisition aiding module are discussed in more detail. Concluding, some results of testing and validating the software-based receiver are presented. Thereby, the special emphasis is placed on the comparison between the single frequency and the dual frequency approach.