Performance of an L1/E5 GNSS Receiver using a Direct Conversion Front-End Architecture

With the modernisation of the GPS system well on its way and Galileo transmitting its first signals, there will be a number of new GNSS signals widely available in the near future and dual frequency operation is directly publically available. Some of the signals are very wideband; they are intended to supply a superior tracking accuracy and improved multipath performance. To allow reception of such wideband signals the RF-front-end and the digital parts of a receiver must be able to operate at a sufficient bandwidth with a correspondingly high sample rate. These demands are challenges facing the implementation of a mass-market dual-band GNSS receiver. On the other hand such a receiver would outperform a single frequency narrow bandwidth heritage GPS L1 receiver in many aspects. Direct-conversion receiver architecture simplifies the construction of a wideband GNSS receiver. The main advantage is the reduction of analogue elements and hence costs, which is highly desirable for a future dual-frequency GNSS receiver for the mass market. This paper assesses the suitability of applying direct conversion receiver architecture to wideband GNSS signals. The introduction summarises the new wideband signals, outlines the challenges in designing direct conversion receiver architectures and presents a dual frequency prototype receiver created for performance assessment. The prototype receiver is composed of a direct conversion tuner intended for flexible down conversion in the L-band and a heritage L1 GNSS frontend. The output of both front-ends is stored simultaneously for post processing using a software receiver. Common difficulties of direct conversion receivers are analysed regarding their impact on GNSS reception, in particular the DC offset and flicker noise problems. The satisfactory functioning, especially for BOC modulated signals, of a direct conversion receiver is presented, alongside a performance comparison with the heritage front-end. Dual band signal tracking of Galileo in orbit validation element (GIOVE) signals is used to provide an assessment of pseudorange and carrier phase relations.