A Software-Defined Spaceborne GNSS Receiver C. Fern´ andez-Prades Communication Systems Division CTTC/CERCA Castelldefels, Spain carles.fernandez@cttc.cat J. Arribas Communication Systems Division CTTC/CERCA Castelldefels, Spain javier.arribas@cttc.cat M. Majoral Communication Systems Division CTTC/CERCA Castelldefels, Spain marc.majoral@cttc.cat A. Ramos Institut Municipal d’Inform` atica (IMI) Formerly at CTTC/CERCA Barcelona, Spain aramosd@bcn.cat J. Vil` a-Valls Communication Systems Division CTTC/CERCA Castelldefels, Spain jvila@cttc.cat P. Giordano European Space Research and Technology Centre European Space Agency (ESA) Noordwijk, The Netherlands Pietro.Giordano@esa.int Abstract—This paper reports the design, proof-of-concept im- plementation and preliminary performance assessment of a low- cost, software-defined spaceborne GNSS receiver. The presented approach takes advantage of the flexibility of software-defined radio technology and the forthcoming availability of radiation- hardened, space-certified Systems-on-Module to implement a fully customizable receiver with the capability to process GNSS signals in real-time and to deliver GNSS products in standard formats. The core GNSS engine is based on a free and open source software implementation of a multi-band, multi-system GNSS receiver released under the General Public License v3.0 and available in a public source code repository. Index Terms—Aircraft navigation, Software radio, Embedded software, Signal processing, Global navigation satellite system. I. I NTRODUCTION Spaceborne GNSS receivers are currently critical elements of multiple space missions, ranging from science to fully commercial applications, including new navigation concepts and algorithms for space users (e.g., on-board precise orbit determination, radio occultation, GNSS reflectometry, space weather). However, spaceborne GNSS receivers need to operate in scenarios that are quite different from those of ground-based receivers, in particular the higher (albeit predictable) dynamics conditions. In addition, space is a harsh environment for semi- conductor devices. Charged particles and gamma rays create ionization, which can alter device parameters. In addition to permanently damaging complementary metal-oxide semi- conductor (CMOS) integrated circuits, radiation may cause single-event effects, which are caused by ionizing radiation strikes that discharge the charge in storage elements, such as configuration memory cells, user memory and registers. When those effects happen, the system is usually recoverable with a power reset or a memory rewrite, but they also may destroy the device [1]. Until recently, radiation-hardened solutions were limited to application-specific integrated circuits (ASICs) and one-time- This work has been partially funded by the European Space Agency through contract AO/2-1647/17/NL/CRS. programmable solutions. A relevant example of spaceborne GNSS receiver is the AGGA-4 (Advanced Galileo and GPS ASIC, see [2], [3]). However, lately there has been an in- crease in the availability of space-grade FPGAs and memory devices. As examples, we can mention Xilinx’s Virtex-5QV, Microsemi’s RTG4 and Atmel’s ATF80 FPGA processors. Those devices potentially allow the implementation of space- qualified GNSS receivers fully defined by software. Finally radiation tolerant solutions are becoming more and more popular in low cost missions, such as GOMspace’s GOMX-3 commercial software-defined radio platforms. Software-defined receivers bring interesting features for space, such as reprogrammability (or upgradeability) and self- healing (or auto-remediation) capabilities. Examples could be the possibility to upload algorithms yet-to-be-invented at the receiver’s launch time, or the ability to recover from a single- event effect by remotely rewriting damaged functionalities, reducing the need of onboard redundancy. The objective of the work presented in this paper is to demonstrate the feasibility of a software-defined spaceborne GNSS receiver. The proposed design constitutes a new ap- proach to GNSS spaceborne receivers. While current ap- proaches are ASIC-based, the design presented in this paper takes advantage of the flexibility of software-defined radio technology and the forthcoming availability of radiation- hardened, space-certified Systems-on-Module (SoMs) with the capability to receive GNSS signals and to process them in real- time, all in a customized UNIX-like environment providing standard communication protocol interfaces for telemetry and telecontrol. In a further innovation step, the core GNSS engine is based on a free and open source software implementation of a multi-band, multi-system GNSS receiver released under the General Public License v3.0 and available in a public source code repository 1 . This scheme brings some benefits to the development, such as public scrutiny of how the baseband 1 Upstream repository available at https://github.com/gnss-sdr/gnss-sdr