ION GNSS 2012
Session E4: Software Receivers

Title: A Flexible and Portable Multiband GNSS front-end System
Author(s): A. Ruegamer, F. Foerster, M. Stahl, G. Rohmer, Fraunhofer IIS, Germany
Date/Time: Thursday, September 20, 2012, 4:46 p.m.
Room: Grand Ballroom West (Renaissance)

In a few years, at least four independent but interoperable GNSS - GPS, GLONASS, Galileo, and COMPASS - will be available on several frequency bands. Consequently, multiband GNSS reception is getting more and more popular both in the academic world and for commercial applications. The upcoming GNSS span over most of the L-band. For GPS, Galileo, and GLONASS alone, signals are broadcast on 8 different frequency bands (E5a/L5, G3, E5b, L2/L2C, G2, E6, E1/L1, G1) with center frequencies ranging from 1176 MHz (Galileo E5a, GPS L5) to 1609 MHz (GLONASS G1) and with main-lobe bandwidth varying from 2 MHz (GPS L1 C/A, L2C) to over 52 MHz (Galileo E5 AltBOC). The benefits of and demand for multiband GNSS reception and processing are beyond question, but a flexible, portable, and affordable front-end recording solution that can easily be adapted to the reception of all these bands is not yet available on the market.
To close this gap, Fraunhofer previously developed a front-end, called the L125 Triband USB front-end, which allows a fixed frequency recording of L1/E1, L2 and L5/E5a via an USB 2.0 data stream with up to 40 MHz sampling rate, a 2 or 4 bit analog-to-digital converter (ADC) resolution and one antenna input. This USB front-end has been used and validated in numerous scientific and industrial projects. However, increasing customer requests for flexible frequency band selection, adjustable sampling and intermediate frequencies, and multi-antenna support led to a complete redesign of this USB front-end concept.

In this paper, the latest version of the Fraunhofer front-end development, called the Flexiband, is presented. Thanks to its new modular concept, the Flexiband not only supports a set of pre-selected configurations but can also be set up for multi-antenna inputs, user selectable bandwidth, intermediate frequencies, and customized ADC sampling rates and resolutions.

The Flexiband comprises four different blocks: The RF-modules, a baseband board, an interface board, and a software component.

The Flexiband housing can host up to three RF-modules that can process any L-band signal with up to 80 MHz bandwidth. Both intermediate frequencies and filter bandwidths can be customized. The three RF-modules can be fed from a single antenna or use separate antenna inputs, making the Flexiband very attractive for applications such as reflectometry or for antennas´ comparison in real-world measurement campaigns.

The baseband board carrying the RF-modules provides three dual-channel ADC, a clock distribution, an FPGA for a flexible signal conditioning, the system power supply, and various debugging interfaces. All received signal channels are sampled simultaneously with up to 80 MHz sampling rate and up to 14 bit resolution. The ADC outputs are directly connected to the FPGA where the data streams can be digitally filtered, down-sampled, bit width reduced, and finally multiplexed including an error protection protocol. All parameters, like the reception bandwidth, the sampling rate, and the intermediate frequency can be configured either directly by the user software or, upon request, as a firmware feature.

The interface board connects the FPGA output either to a computer, a GNSS digital baseband receiver, or a data recorder. In the standard configuration the interface board transfers the baseband output data via two high speed USB 2.0-interfaces to a standard PC. As the USB interface provides only low level error detection, additional information is embedded in the USB-stream. The USB 2.0 interface is capable of transferring up to 480 Mbit/s but, keeping in mind that data integrity is critical (e.g. to ensure that no sample gets lost), a value of 320 Mbit/s was selected as the fastest rate that still works reliably in practice. Instead of using a USB 2.0 connection the interface board can be replaced by other interface modules providing a completely different physical interface. For instance, a USB 3.0 data interface, featuring over 600 Mbit/s of transferable data rate, is in preparation. Similarly, interface modules to a logic analyzer or to a GNSS digital baseband processing board are possible as well as customized ones. The software component of the Flexiband is twofold: an USB driver with an application programming interface (API) and a visualization and recording software.

The supplied API for both Windows and Linux operation systems gives the user the possibility to realize a complete, real-time software-defined GNSS receiver with multiband and multisystem capability.

The Flexiband visualization and recording software uses the same API and features e.g. a manual or automatic gain control (AGC) and the selection of different configurations. The raw samples can be stored as a multiplexed data stream, in an 8 bit/sample format, or directly as a .mat file for MATLAB. Moreover the front-end GUI software features a real-time spectrum and histogram monitoring. This can be used to check if the antenna is working properly and to directly identify interference sources like continuous wave (CW) or pulsed ones.

To playback the recorded data later on, the support of digital-I/O devices is currently under development. This will enable the exact replay of digital samples to reproduce deterministically the data collection conditions in a software receiver. Moreover it allows the replay of the recorded digital data stream starting right at the challenging position or some samples before within the recorded data stream. This enables a direct and fair comparison of algorithms and identification of performance settings.

The Flexiband housing dimensions are approximately 175x125x50 mm^3 and lightweight. Thanks to an energy saving design the Flexiband front-end can be powered by the USB interface only making it perfectly suited for mobile recording campaigns with e.g. a notebook as a data recording device.

Different Flexiband front-ends can be synchronized among each other. Either the internal reference oscillator or an external high quality frequency reference can be used, e.g. a reference clock from an IMU or from a reference trajectory system.

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