GNSS Performances

Fundamentals
Title	GNSS Performances
Edited by	GMV
Level	Basic
Year of Publication	2011

Performance of a navigation system is most usually measured in terms of the solution error (accuracy) and of the portion of the time it can be used for the intended application (availability). However, other performance parameters can become important for some applications. For instance, when a navigation system is used for air or maritime navigation, an unwarned large solution error can seriously increase the risk of an accident, possibly causing damage of goods, injuries to people or even death. Such errors can occur without violating the accuracy specification, and this is why the civil aviation community has defined the concept of integrity as a measure of the probability that such hazardous situations can take place. Also, since some flight phases (such as approach and landing) are of special criticality, it is important that the navigation system remains available without interruptions during such periods although actually all flight phases have certain needs at this respect. That is why the civil aviation community has defined a fourth performance parameter called continuity.

Performance Parameters

The civil aviation community has put the greatest effort in the rationalization and standardization of positioning (navigation) performance parameters and requirements, thus specifying the so-called Required Navigation Performance (RNP) that an airborne navigation system must accomplish:

RNP: A statement of the navigation performance accuracy, integrity, continuity and availability necessary for operations within a defined airspace.

The four parameters used to characterize GNSS performance are based on the RNP specification are summarized hereafter:

Accuracy: The accuracy of an estimated or measured position of a craft (vehicle, aircraft, or vessel) at a given time is the degree of conformance of that position with the true position, velocity and/or time of the craft. Since accuracy is a statistical measure of performance, a statement of navigation system accuracy is meaningless unless it includes a statement of the uncertainty in position that applies.

Availability: The availability of a navigation system is the percentage of time that the services of the system are usable by the navigator. Availability is an indication of the ability of the system to provide usable service within the specified coverage area. Signal availability is the percentage of time that navigation signals transmitted from external sources are available for use. It is a function of both the physical characteristics of the environment and the technical capabilities of the transmitter facilities.

Continuity: The continuity of a system is the ability of the total system (comprising all elements necessary to maintain craft position within the defined area) to perform its function without interruption during the intended operation. More specifically, continuity is the probability that the specified system performance will be maintained for the duration of a phase of operation, presuming that the system was available at the beginning of that phase of operation.

Integrity: Integrity is the measure of the trust that can be placed in the correctness of the information supplied by a navigation system. Integrity includes the ability of the system to provide timely warnings to users when the system should not be used for navigation.

Furthermore, an additional performance indicator, the Time To First Fix (TTFF), is also used by some receiver manufacturers. The TTFF is a measure of performance of a GNSS receiver that accounts for the time elapsed from the GNSS receiver switch-on until the output of a navigation solution within a certain performance (e.g. in terms of accuracy).

Uses of GNSS Performance Parameters

The performance parameters described above measure the performance of the GNSS system. These performance parameters can be used in different ways, with different purposes and in different conditions.

One of the uses of the GNSS Performance Parameters is to define requirements that a GNSS System should have in specific conditions. All GNSS systems had such requirements during the specification phase of the development of the system. These requirements are one of the drivers of the system design and development. During the operational phase these requirements are kept and sometimes updated as operational requirements against which actual performance will be verified. Such performance requirements for Galileo can be found in the Galileo Mission High Level Definition coming from the Galileo System Requirement Document. The GPS Performance requirements can be found in the GPS Performance Requirements Documents.

These performance requirements can be used also to define the performance that an application will need from a GNSS system in order to be able to perform its functionality adequately.

In the same way that these parameters are used for the definition of requirements of a GNSS system, they are use to evaluate its actual performance under typical and well defined conditions. The actual performance of GPS is monitored by several entities such as the US Federal Aviation Administration in its GPS Performance Analysis Reports.

The increase in performance given by space-based augmentations systems (EGNOS, WAAS, MSAS) and of other GNSS Augmentations is measured also using these parameters.

Besides being used to evaluate the performance of the GNSS systems and its augmentation systems and technologies these performance parameters are used to measure the performance of the receivers that use these systems. This performance evaluation can be made for generic conditions (such as open sky locations) or for the evaluation of the receiver on specific conditions since the receiver performance can be influenced by local factors such as:

• Location of the user - Different locations yield different performances since different satellites will be visible at different positions in the sky.
• Time of day - Despite the fact that different views of constellation will be visible at different times of day, some errors (such as the errors caused by Ionospheric Delay) have different impacts during different times of the day.
• Surrounding environment - Surrounding buildings and vegetation can lead to masking of the sky leading to less visible satellites, signal attenuation effects coming from tree foliage and multi-path effects coming from the reflection of the GNSS signal on buildings or other landmarks.