Galileo Frequency & Signal Design
June 1, 2003 By: Tony Pratt, Rafael Lucas-Rodriguez, Philippe Erhard, Jeremie Godet, Jean-Luc Issler, Jean-Christophe Martin, Gunter W. Hein GPS WorldThis article describes the Galileo frequency structure and signal design as developed by the European Commission's Galileo Signal Task Force (STF).The European Commission (EC) established the task force in March 2001. Chaired by the EC, the STF consists of experts nominated by European Union (EU) member states, official representatives of the national frequency authorities, and experts from the European Space Agency (ESA).
The Signal Task Force is playing a major role in determining the Galileo frequency and signal design. The STF reports to the EC Galileo Steering Committee through the EC for approval. One task of the STF is to look for ways to optimize the interoperability of Galileo with the Global Positioning System (GPS). The Signal Task Force also contributes to the preparation of the next World Radio Conference (WRC) to be held by the International Telecommunications Union (ITU) in 2003 (WRC2003).
This article presents the most recent proposal for the Galileo frequency and signal structure. It first discusses the relevant requirements for the system. The discussion then describes the mapping of Galileo services to signals, followed by detailed considerations (noise and multipath) of the relevant frequency bands. We discuss the results from interference analyses as well as the matter of interoperability and compatibility with GPS in terms of signal structure and geodetic and time reference frame.
Signal Requirements
The European Union intends for the Galileo system to provide four navigation services and one search and rescue (SAR) service. The primary signals of Galileo are intended to provide an "Open Service" (OS) of a high quality, consisting of six different navigation signals on three carrier frequencies. OS performance will at least equal that expected from the "follow-on" generation (Block IIF) of GPS satellites scheduled to begin launching in 2005 and the future GPS III system architecture currently being investigated.
The GPS IIF/III satellites will offer wideband signals on three civil (open) frequencies: one high-chipping rate signal (L5 centered at 1176.45 MHz) and two low-chipping rate signals (L1 at 1575.42 MHz, L2 at 1227.60 MHz). Moreover, the GPS modernization program will offer additional civil and military code structures on L2.
Compatible, Independent. Among the leading goals of the STF's efforts was ensuring compatibility and interoperability with other satellite navigation systems, particularly GPS, and other uses of the portions of the RF spectrum in which Galileo will operate. The EC policy paper that lead to the Galileo Resolution at the Transport Council Meeting on June 17, 1999, stated this objective as follows: "Galileo must be an open, global system, fully compatible with GPS, but independent from it."
At its 25–26 March 2002 meeting (during which the development phase of Galileo was finally decided), the Transport Council of the European Union again underlined its desire that compatibility and interoperability with GPS should be one of the key drivers for Galileo. The present Galileo signal plan achieves a maximum of interoperability to GPS, while still reducing vulnerability when using one system as a back-up of the other.
Independence means preventing or reducing vulnerability to simultaneous failures of GPS and Galileo. This can be achieved in part by maintaining separate space and ground infrastructures and control systems, and in part by implementing distinct signal designs and separate frequencies.
In order to discuss the term "fully compatible with GPS" in more detail, we must consider the general users of a Global Navigation Satellite System (GNSS). For their work, these users want to be able to track as many satellites as possible in order to increase positioning performance and have redundancy for signal availability, integrity, and continuity. The best way to achieve this is through use of an "all-in-view," combined (Galileo/GPS) receiver, which can be manufactured cheaply only when the design is as simple as possible.
This can be achieved well if GPS and Galileo signals use the same centre frequencies, because use of multiple frequencies by GNSS receivers would require several front-ends (antenna elements, RF integrated circuits, and low noise amplifiers) and a more complex signal processing design. Multiple frequencies also introduce frequency biases in the receiver that have to be solved either by calibration (if possible) or by incorporating extra observations into the positioning algorithm in order to determine and eliminate these biases.
 (Click on image for larger view.) Figure 1: Galileo frequency spectrum |
Security Aspects. A global satellite navigation system, even a civilian one such as Galileo, must also consider global security aspects. Satellite navigation is nowadays - and even more in the future - used in critical infrastructure where an uninterrupted GNSS service is absolutely vital. Examples can be found in telecommunications, electrical energy distribution, banking and financial transactions, and other sectors where, in particular, GNSS time is used.
In these sectors an interrupted service would provoke a chain of other malfunctioning services. Disruption of GNSS services is a potential threat for national and world economic systems as well as safety and security-related applications. Misuse of GNSS services can threaten national security. Consequently, spectral separation and secure, controlled-access services are ways to protect GNSS users against interruption. A good example is the decoupling of military and civil services in the modernized GPS by using different waveforms. In times of a crisis, civil services must be "jammable" without affecting the military and/or security signals.
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