Galileo Navigation Message

Fundamentals
Title	Galileo Navigation Message
Author(s)	J. Sanz Subirana, JM. Juan Zornoza and M. Hernandez-Pajares, University of Catalunia, Spain.
Level	Basic
Year of Publication	2011

Introduction

Each Galileo satellite transmits ten navigation signals in the frequency bands E1, E6, E5a and E5b, right-hand circularly polarised. These signals are designed to support four different services: Open Service (OS), Safety of Life (SoL) service, Commercial Service (CS) and Public Regulated Service (PRS). A brief description of these Galileo services is provided as follows:

• OS: The Open Service is free of charge for worldwide users. Up to three separate signal frequencies are offered within OS. Single-frequency receivers will provide performance similar to GPS C/A. In general, OS applications will use a combination of Galileo and GPS signals, which will improve performance in severe environments such as urban areas.
• SoL: The Safety-of-Life Service provides timely warnings to the users when them fails to meet certain margins of accuracy (integrity). The service will be certified for SoL users requiring integrity information (civil aviation...). Two signals are provided in the frequency bands reserved for Aeronautical Radio-Navigation Services (E1 and E5).
• CS: The Commercial Service provides access to two additional signals protected through commercial encryption (ranging data and messages). Higher data rates (up to 500 bps) for broadcasting data messages are provided.
• PRS: The Public Regulated Service is for security authorities (police, military, etc) requiring a high continuity of service, with controlled access. Under governmental control. Enhanced signal modulation/encryption is introduced to provide robustness against jamming and spoofing. Two PRS navigation signals with encrypted ranging codes and data will be available. The Galileo satellites will also be able to detect and report signals form Cospas- Sarsat Search and Rescue (SAR) beacons.

As in the case of GPS, all satellites share the same frequencies, and the signals are differentiated by the code division multiple access CDMA technique. These signals can contain data and pilot channel. Both channels provide ranging codes, but the data channels include also navigation data. Pilot channels (or pilot tones) are data-less signals, so that no bit transition occurs, helping the tracking of weak signals. A diagram of the different Galileo signals is shown in figure 1 (from [Powe, M., 2006]) where the data and pilot channels are plotted in orthogonal planes to highlight that they are shifted by 90 degrees in phase, which allow for their separation in the receivers.

Figure 1: Galileo Signals in Space

A brief description of each signal is provided as follows

• E1 signal supports the OS, CS and SoL service and PRS. It contains three navigation signal components in the L1 band. The first one, E1-A, is encrypted and only accessible to authorised PRS users, and it contains PRS data at 50 bps. The other two components, E1-B and E1-C, are open access signals with unencrypted ranging codes accessible to all users. E1- B is a data channel and E1-C a pilot (or data-less). The E1-B data stream, at 125 bps, also contains unencrypted integrity messages and encrypted commercial data. Note: the band E1 is shared with GPS L1.
• E6 signal is a dedicated signal for supporting the CS service and PRS. It provides three navigation signal components transmitted in the E6 band. E1, the first one, E6-A, is encrypted and only accessible to authorised PRS users, carrying PRS data. The other two, E6-B and E6-C, are commercial access signals and include a data channel E6-B and a pilot (or data-less) channel E6-C. Its ranging codes and data are encrypted. A data rate of 500 bps allows the transmission of added-value commercial data.
• E5a signal supports OS. It is an open access signal transmitted in the E5a band that includes two signal components. A data channel, E5a-I, and a pilot (or data-less) channel, E5a-Q. The E5a signal has unencrypted ranging codes and navigation data, which are accessible by all users. It transmits the basic data to support navigation and timing functions, using a relatively low 25 bps data rate that enables more robust data demodulation. Note: the band E5a is shared with GPS L5.
• E5b signal supports the OS, CS and SoL services. It is an open access signal transmitted in the E5b band that includes other two signal components: The data channel E5b-I and the pilot (or data-less) channel E5b-Q. It has unencrypted ranging codes and navigation data accessible to all users. The E5b data stream also contains unencrypted integrity messages and encrypted commercial data. The data rate is 125 bps.

Note: the band E5b will be shared with GLONASS G3.

The E5a and E5b signal components are modulated onto a single E5 carrier frequency 1191.795 MHz using a technique known as AltBOC. The composite of the E5a and E5b signals is denoted as the E5 signal and can be processed as a single large bandwidth signal with an appropriate user receiver implementation, which results in a low multipath and tracking noise signal. Next table 1 shows a summary of the Galileo signals, frequencies and applied modulations. The ranging Code Rate and Data Rate are also given in the table.

Table 1: Galileo Navigation Signals. The two signals located in the E5a and E5b bands respectively are modulated onto a single E5 carrier frequency 1191.795 MHz using the AltBOC technique: AltBOC(15,10).