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Galileo Performances
| GALILEO | |
|---|---|
| Title | Galileo Performances |
| Edited by | 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 Public Regulated Service (PRS), the availability of the service should be 99.5%.
Galileo Service Level Perfomances
From Galileo Mission High Level Definition, the performances requirements for each service are the following:[1]
Galileo Open Service Performances
| Galileo Open Service (positioning & timing) | ||
|---|---|---|
| Single Frequency (SF) | Dual Frequency (DF) | |
| Coverage | Global | |
| Accuracy (95%) | Horizontal: 15 m | Horizontal: 4m |
| Vertical: 35 m | Vertical: 8m | |
| Availability | 99.8 % | |
| Timing Accuracy wrt UTC/TAI | N/A | 30 ns |
| Ionospheric Correction | Based on SF Model | Based on DF Measurements |
| Integrity | No | |
In the scope of the IOV tests campaign, the dual-frequency open service revealed average positioning accuracy of 8 m horizontal and 9 m vertical, 95% of the time. The average timing accuracy was in the order of 10 nano seconds which does not meet the requirement. However this results will be improved as more satellites are launched and ground stations come on line[2].
Galileo Public Regulated Service Performances
| Galileo Public Regulated Service (PRS) | |||
|---|---|---|---|
| Dual Frequency (DF) | |||
| Coverage | Global | ||
| Accuracy (95%) | Horizontal: 6.5 m | ||
| Vertical: 12 m | |||
| Availability | 99.5 % | ||
| Timing Accuracy wrt UTC/TAI | 100 ns | ||
| Ionospheric Correction | Based on DF Measurements | ||
| Integrity | Computes | Yes | |
| Alarm Limit | H:20 m -V:35 m | ||
| Time-To-Alarm | 10 s | ||
| Integrity Risk | 3.5x10e-7/150 s | ||
| Continuity Risk | 10e-5/15 s | ||
Galileo Commercial Service
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.
Segment Performance Drivers
To fulfil 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: GNSS based integrity systems and techniques, such as SBAS, ARAIM, RAIM and GBAS, would benefit from the addition of new constellations, including Galileo, in terms of lower achievable protection levels and/or integrity risk.
- 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 Works, And Works Well, ESA, February 2014
- ^ 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.