CS Signals

The Commercial Service (CS) signals will provide the capabilities that the Galileo CS will offer to the users. As described in the Galileo OS SIS ICD, the CS signal is composed of two dedicated signals transmited on the E6 band (1260–1300 MHz), a data (E6-B) component and a pilot (E6-C) component; plus the “Reserved-1” fields in the Open Service I/NAV disseminated through the E1-B. E6 signals are modulated with a binary phase shift keying BPSK(5) at a carrier frequency of 1278.75 MHz, which is used by all satellites and shared through a code division multiple access (CDMA) RF channel access method. Therefore, the signal main lobe and most of the signal power is in the 1273.75-1283.75 MHz band.

The E6-B channel contains the data component, allowing the transmission of 448 bits per second, and the E6-C transmits the pilot component. Both channels allows to encrypt the information at signal-level.

GalE6

The main services foreseen to be provided throught the CS are authentication and high accuracy:

AUTHENTICATION
CSServices

Due to their low power, GNSS signals can be easily jammed, and because of the lack of authentication, they could also be forged or “spoofed” with the appropriate equipment. Therefore, protecting GNSS has become one of the major topics of interest for GNSS community. GNSS information can be protected using two different protection layers: data-level protection also known as Navigation Message Authentication (NMA) and signal-level protection.

A relevant feature of the CS E6 signal is that the primary spreading codes of both components can be either encrypted or disseminated in plain. When encrypted, the spreading codes are replaced by an unpredictable bit-stream generated through a secret key, making the signal indistinguishable from noise for unauthorized receivers.
In addition to other technical and regulatory measures, features in the GNSS signals allowing authentication are undoubtedly a major building block of location security. This capability allows not only to authenticate the information encoded in the signal but also to authenticate the signal time of arrival, at least against certain threats and with a certain confidence level. Both factors are required for a trustworthy position and time estimation. Nevertheless, it presents other challenges as the managerial of crypto keys amongst the users.
With this in mind, Galileo is a good candidate to offer authentication services to civil communities for two main reasons. The first is that Galileo E6-B and E6-C signal spreading codes can be encrypted, which provides spreading code authentication for receivers (or server-receiver architectures) having the encryption keys. The second reason is that the available bandwidth in both E6-B and E1-B Galileo signals permits the transmission of authentication and re-keying data to authenticate the navigation messages while guaranteeing full backward-compatibility.

HIGH ACCURACY

High accuracy is generally understood as a positioning accuracy on the order of a few centimeters. Two primary approaches have been used in the past years to provide high accuracy: real time kinematic (RTK) and precise point positioning (PPP). The main advantage of using PPP instead of RTK is that it provides a global and absolute positioning and timing service without the need for nearby reference stations.
PPP is based on the use of accurate GNSS satellite orbits and clock data to estimate a user position based on carrier phase measurements, where the ionospheric delay is typically removed by performing the iono-free combination.

The most common and optimized technique in terms of bandwidth for real-time PPP is to send orbits and clock corrections to the navigation message, allowing the reconstruction of the accurate values in the receiver. The Galileo E6-B channel is well suited to transmit PPP information. Various analyses have shown that the available rate of 448 bps per satellite allows the transmission of satellite orbits and clock data at an adequate update rate to provide accuracy at the centimeter level.