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Galileo Open Service (OS)
|Title||Galileo Open Service (OS)|
|Year of Publication||2011|
Galileo is the European Global Navigation Satellite System, under civil control, that provides satellite positioning services to European citizens and worldwide. The Galileo Open Service (OS) enables free of charge, global ranging, positioning and timing, by means of the Galileo OS Signal-In-Space (SIS).
The freely accessible Galileo Open Service (OS) targets the mass market and it is intended for motor vehicle navigation and location-based mobile telephone services. Free to the user, it provides positioning and synchronization information intended for high-volume satellite radio navigation applications.
The Galileo Open Service is available since December 2016, when the Galileo Initial Services were declared. The services will evolve with the infrastructure deployment until the Full Operational Capability (FOC) is achieved.
Galileo will include in the short term an authentication mechanism through Galileo OS-NMA.
The Galileo Open Service (OS) provides positioning, velocity and timing information that can be accessed free of direct user charge. This service is suitable for mass-market applications, such as in-car navigation and hybridisation with mobile telephones. The Open Service is accessible to any user equipped with a receiver, with no authorisation required. While up to three separate signal frequencies are offered within the Open Service, cheap single-frequency receivers can be used for applications requiring only reduced accuracy. In general, Open Service applications use a combination of Galileo and GPS signals, which improves performance in severe environments such as urban areas. The Open Service could also be used by professionals who require high precision but not integrity, complementing this service with GNSS Augmentation techniques.
The timing service is synchronised with UTC when used with receivers in fixed locations. This timing service can be used for applications such as network synchronisation or scientific applications.
The Open Service does not offer integrity information, and the determination of the quality of the signals is left entirely to the users. There is no service guarantee or liability from the Galileo Operating Company on the Open Service. However, the Open Service will include in the short term an authentication mechanism to called Galileo OS-NMA (Navigation Mechanism). Galileo is the first constellation which will include an NMA mechanism in an Open Service signal.
The following services are supported by Galileo OS SIS:
- Single-Frequency and Dual-Frequency Ranging
- Single-Frequency and Dual-Frequency Positioning
- UTC Time Determination
During the Galileo Initial Services phase, the Galileo OS comprises the Ranging and the UTC Time Determination Services:
- Gal OS Single-Frequency and Dual-Frequency Ranging Service: this service is provided in both SF usage mode and DF usage mode and it allows users to continuously estimate their distance to the satellite. DF usage mode is characterized by higher accuracy of the range measurement. The use of two frequencies allows better compensation of the disturbing effect of the ionosphere on the SIS, thus providing better performance to the end user. The Galileo OS Ranging Service is interoperable with GPS ranging services and therefore it provides a direct benefit to users who are able to exploit both Galileo and GPS constellations, by increasing the number of available SiS.
- Gal OS UTC Time Determination Service: this service provides Galileo users worldwide with direct and accurate access to the UTC reference. The precise atomic clocks in the Galileo System allow very accurate timing data delivery to users. This service provides both the integer offset between the Galileo System Time (GST) and UTC, and the fractional GSTUTC difference at nanosecond level. This allows users to time-stamp their data in UTC or local time and to precisely synchronise to UTC their clocks, which can be situated at very different locations in the world. Since the service only requires visibility of one Galileo OS SIS, availability is already very high globally, even during the initial OS Phase,
Performance and features
The Galileo Open Service is accessible through the signals at L1, E5a and E5b, whether data or pilot. Several combinations are also possible, such as a dual frequency service based on using L1 and E5a (for best ionospheric error cancellation) or single frequency services (at L1, E5a, E5b or E5a and E5b together) in which case the ionospheric error is removed using a model, and even triple frequency services using all the signals together (L1, E5a and E5b), which can be exploited for very precise, centimetre level applications. 
The performance objectives in terms of position accuracy and availability are competitive with respect to existing GNSS and further planned evolutions. In addition, the Open Service is interoperable with other GNSS, in order to facilitate the provision of combined services. The following table presents the expected Galileo Open Service performance once the Full Operational Capability is achieved. 
|Galileo Open Service (positioning & timing (requirements))|
|Single Frequency (SF)||Dual Frequency (DF)|
|Accuracy (95%)||Horizontal: 15 m||Horizontal: 4m|
|Vertical: 35 m||Vertical: 8m|
|Timing Accuracy wrt UTC/TAI||30 ns|
|Ionospheric Correction||Based on SF Model||Based on DF Measurements|
For the Minimum Performance Values expected until this Full Operational Capability is declared, please refer to details in Galileo Performances article.
Next table presents the expected evolution of the horizontal and vertical position accuracy for the Galileo OS Positioning versus the number of operational healthy satellites. The values provided are global values, taking into account the constraint of PDOP ≤6. Values corresponding to average user location (AUL) and worst user location (WUL) are presented.
With regard to the evolution of the availability performance, the next table summarize the expected figures for 18, 22 and 24 satellites.
Galileo Signal Frequencies
Galileo transmits several signals and codes on four different carrier frequencies within the 1.1 and 1.6 GHz band, namely as:
- E1, centred at 1575.42 MHz;
- E5a and E5b, at 1176.45 and 1207.14 MHz respectively, multiplexed together by an AltBOC scheme and transmitted at the E5 carrier frequency centred at 1191.795 MHz;
- E6, centred at 1278.75 MHz.
The Galileo SF OS is provided by three of these signals: E1, E5a and E5b, whereas the Galileo DF OS is provided only by each of the following combinations broadcasted by the same satellite:
- E1 and E5a;
- E1 and E5b.
Following image shows the Galileo Signal Frequencies. See further details on Galileo Signal Plan article
Edited by GMV, using information from ESA, GSA and European Union as indicated through the references.