GPS Performances

GPS
Title	GPS Performances
Author(s)	GMV
Level	Basic
Year of Publication	2011

The GPS is a space-based global navigation satellite system (GNSS) that provides reliable positioning, navigation, and timing services to civilian and military users on a continuous worldwide basis.

GPS provides two levels of service, Standard Positioning Service and the Precise Positioning Service:

1. The Standard Positioning Service (SPS), is a positioning and timing service provided on GPS L1 frequency and available to all GPS users. The L1 frequency contains a coarse acquisition (C/A) code and a navigation data message.
2. The Precise Positioning Service (PPS), is a highly accurate military positioning, velocity and timing service broadcasted at the GPS L1 and L2 frequencies. Both frequencies contain a precision (P) code ranging signal with a navigation data message that is reserved for authorized use by the use of cryptography.

The levels of performance that the user can expect from GPS are specified in the Standard Positioning Service Performance Standard, and the Precise Positioning Service Standard. However, the values provided by these documents are very conservative, being the actual performances usually better than these official values.

Moreover, the performance obtained with GPS depends strongly on the mode of operation. For instance, a stand-alone receiver that uses only the signals received from the satellites, the levels of performance are:

• C/A-code receivers ~ 5 -10 m.
• P/Y-code receivers ~ 2 -9 m

In case of using GPS in a differential mode, the performances that can be expected are:

• C/A-code DGPS receivers ~0.7 -3 m.
• P/Y-code DGPS receivers ~0.5 -2.0 m.

Combined and Future Services Performances

Combining GPS with other GNSS constellations: GLONASS, GALILEO.

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

• 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, 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.