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Galileo Performances
| GALILEO | |
|---|---|
| Title | Galileo Performances |
| Author(s) | GMV |
| Level | Basic |
| Year of Publication | 2011 |
Galileo system is a space-based global navigation satellite system (GNSS) that provides reliable positioning, navigation, and timing services to users on a continuous worldwide basis freely available to all.
The Galileo performances are different for each service. For the Galileo Open Service (OS) no specific requirements of integrity are applicable. The performances for horizontal positioning accuracy at 95% for a dual-frequency receiver are 4 m (8 m for vertical accuracy), with an availability of the service of 99%.
In the case of the Galileo Safety of Life (SoL) and the Galileo Public Regulated Service (PRS), the performance requirements include horizontal and vertical accuracy, integrity, continuity and time to alert for different service levels. The availability of the service should be 99.5% for both services.
Galileo Service Level Perfomances
From Galileo Mission High Level Definition, the performances requirements for each service are the following: [1]
| Parameter | Galileo Service Level | |||||||
|---|---|---|---|---|---|---|---|---|
| OS | SoL | PRS | ||||||
| Single-Frequency User (SF) | Dual-Frequency User (DF;E5a/L1) | DF (E5b/L1) | SF | DF (L1/E6) | ||||
| E5a | E5b | L1 | L1 | E6 | ||||
| Lateral accuracy (95%, m) | 24 | 24 | 15 | 4 | 4 | 15 | 24 | 6.5 |
| Vertical accuracy (95%, m) | 35 | 35 | 35 | 8 | 8 | 35 | 35 | 12 |
| Velocity accuracy (95%, m/s) | 0.2 | 0.2 | 0.2 | |||||
| Timing Accuracy wrt UTC/TAI (ns) | N/A | 30 | N/A | N/A | 100 | |||
| Horizontal Alarm Limit (m) | N/A | 12 | N/A | 20 | ||||
| Vertical Alarm Limit (m) | N/A | 20 | N/A | 35 | ||||
| Time-To-Alarm (s) | N/A | 5.2 | N/A | 10 | ||||
| Integrity Risk | N/A | 2.10-7 / 150s | N/A | 2.10-7 / 150s | ||||
| Continuity Risk | N/A | 1-8.10-6 / 15s | N/A | 1-8.10-6 / 15s | ||||
| Availability of Accuracy (%) | 99.8 | 99.8 | 99.8 | |||||
| Availability of Integrity (%) | N/A | 99.5 | N/A | 99.5 | ||||
| Coverage | Global | Global | Global | |||||
The Galileo Commercial Service will be a controlled access service operated by Commercial Service Providers, who will make decisions on the offered services: e.g. integrity data, differential corrections for local areas, etc. which will depend on the final characteristics of the other services offered by Galileo.
The Galileo services performances result from the processing of a combination of Galileo signals, by the user terminal, under certain nominal environmental conditions (no intentional interference, low multi-path, ...). It is envisaged that a guarantee will be offered for all applications for which a disruption of service would have significant Safety of Life or economic impacts, this will be donde by guaranteeing the quality of the Signal In Space with certain specifications defined in a Interface Control Document [2], in order to achieve the specified service performance at end-user level.
It has been tested the Open Service performance with GIOVE satellites, the two experimental Galileo satellites. Applying the specified ranging accuracy performance (i.e. 130 cm 95%) and the related nominal and degraded segment constellations, the following global Galileo Open Service Dual-Frequency (E5a-L1) Availability performance is achieved over system lifetime. The service is declared available if both accuracy requirements are temporally and locally met, i.e. 4 m horizontal and 8 m vertical positioning performance.
Segment Performance Drivers
To fulfill the Galileo Services requirements, especially with respect to integrity and continuity, key performance drivers have been defined and flown down to Segment level, i.e. Space Segment (SSEG), Ground Mission Segment (GMS), Test User Segment (TUS), and Ground Control Segment (GCS).
The Orbit Determination & Time Synchronisation (ODTS) performance is also driving the signal performance quality indicator, SISA, which needs to be ensured in all available GMS states. Such ensuring of signal quality directly drives Galileo's integrity services. Furthermore the ground monitoring performance indicator, SISMA, is required for the integrity services, and two GMS states exist that are defined via the related SISMA upper bound performance. The final SSEG and GMS key performance requirements are summarized below and are driving OS as well as SOL service performance. The nominal state is if all Galileo Sensor Stations (GSS) and 27 Signal in Space (SIS) from the operational satellites are available. [3]
| Nominal | Degraded | Service | |
|---|---|---|---|
| ODTS /Ranging Accuracy (1 sigma) | 65 cm | OS | |
| GMS Availability of Navigation | 99.95 % | OS | |
| SISA | 85 cm | SoL | |
| SISMA | 70 cm | 130 cm | SoL |
| GMS Availability of Integrity | 94.73 % | 5.23 % | SoL |
| SSEG State Probability | 94.09 % | 5.66 % | OS,SoL |
Such segment key performances drivers have been independently consolidated through the early GIOVE experimentation activities, which show great consistency with the "designed" performance status.
Combined services performances
Galileo is being designed to be interoperable with other systems and, therefore, it will, in a great many instances, be used as part of a combined service. The identification of combined services is necessary to:[1]
- Meet the most demanding user applications.
- Reduce satellite navigation system weaknesses.
- Provide robust solutions for applications requiring system redundancy for safety and/or security reasons.
- Access future GNSS market.
- Enable and expand new market opportunities.
The most obvious systems to be combined with Galileo are the other existing GNSS systems, GPS,GLONASS, SBAS and GBAS as they share with Galileo many characteristics that facilitate a combination at user level. By combining Galileo with other GNSS systems, improved performance in the following domains can be expected:
- Availability: Using as an example Galileo in combination with GPS and SBAS systems, the number of operational satellites will be in the region of 60. In normal urban environments this would result in an increased availability for 4 satellites from 40% to more than 90%.
- Position Accuracy: Allied to an increased availability in restricted environments (urban) is a better geometry of spacecraft or enhanced positioning performance.
- Integrity: SBAS systems, in addition to generating ranging signals, provide integrity information on GPS and GLONASS. Thus if an application requires the broadcast integrity information of two systems this can be achieved using SBAS. Typically, Safety of Life applications would benefit from this additional service.
- Redundancy: By combining services from separate and fully independent systems full redundancy can be achieved. This is particularly important for Safety of Life applications that require full system backup.
References
- ^ a b Galileo Mission High Level Definition, v3, September 2002.
- ^ Galileo OS SIS ICD Issue 1 Revision 1 September 2010e
- ^ Veit Oehler, Jan M. Krueger, Tanja Beck, Michael Kirchner, Hans L. Trautenberg, Jörg Hahn, Daniel Blonski, “Galileo System Performance Status Report,” Proceedings of the ION GNSS 2009, Savannah, GA, USA, Sept. 22-25, 2009.