It is well known that the GNSS programme has given an important contribution to the research activity in the field of Satellite navigation, positioning and timing. The European Galileo project is going to constitutes an essential contribution to the satellite navigation market, taking into account that a relevant effort is being devoted to the design of Local Elements (LE) for providing local assistance data to users. Looking at the near future, it is well known that the Galileo system will introduce signal properties (e.g. in terms of modulations) that will be different for each channel and band, since they have been designed also with particular attention to the services requirements; the same will happen with the Modernized GPS.

As a direct consequence, the study and design of advanced terminals able to deal with both the Signal-In-Space (SIS) and other classes of signals transmitted by stations and satellite augmentations (EGNOS), become indispensable.

In this scenario, flexibility represents an essential feature for the User Terminals (UTs). The Software Defined Radio (SDR) approach constitutes a satisfactory solution bacause it allows for the development of reconfigurable devices realized on a modular architecture. It is clear that the reconfigurability aspect can play a key role in specific applications where the features of the environment surrounding the UT, change with time. A typical example is a user moving in an indoor scenario, where the performances of GNSS receivers could strongly limited by several factors: acquisition of weak signals is critical, visibility of a sufficient number of satellites is not guaranteed, and the SIS received at the antenna propagates via multiple paths.

Following these considerations in according to the most recent research works on GNSS receiver (Akos, 2003), the paper discusses a receiver architecture based on SDR techniques where functionalities are software-implemented on modular platforms composed by Field Programmable Gate Array (FPGA) and high speed Digital Signal Processor (DSP).

The conceptual scheme of a reconfigurable terminal for positioning applications, based on SDR techniques is shown in Figure 1. Such an architecture represents the functional diagram of a hybrid Navigation/Communication (NAV/COM) user terminal based on FPGA and DSP, as considered for the work presented in this paper.

It is clear that the user terminal functions are distributed among the different functional blocks and modules:1. The Algorithm Control Unit, an advanced routine implemented on DSP allows for the real-time configurability of the platform. It consists in a set of commands for the control of the System Control Unit internal modules.2. The Navigation and Communication Units constitute the core blocks (FPGA- implemented) of the system; in fact, they are able to acquire and tracking signals incoming from satellites and Base Tranceiver Stations (BTS). 3. All modules for the on-fly reconfigurability are software uploaded (on the FPGA) and, during the stand-by phase, can be changed, upgraded and integrated employing the external reconfigurability interface.

In order to understand what are the potentialities of the SDR implementation, in the next session the architecture of the Navigation Unit is analyzed and some performance results are highlighted.

Given the flexibility of the overall GNSS receiver architecture presented in the previous section, the Navigation Unit has been designed and implemented on reconfigurable hardware on the basis of the data processing rate. In particular, the resources have been mapped over a Xilinx FPGA Virtex II XCV2000EBFF896 and a Texas Instruments DSP C6416 (fixed point). Figure 2 shows the platform employed for the implementation, and the communication links between the FPGA, the DSP and a common PC.