Quantifying the performance of navigation systems and standards for assisted-GNSS - Inside GNSS - Global Navigation Satellite Systems Engineering, Policy, and Design

# Quantifying the performance of navigation systems and standards for assisted-GNSS

Q: What are the differences between accuracy, integrity, continuity, and availability, and how are they computed?

A: These four words describe parameters that quantify the performance of navigation systems. The terms are not unique to satellite navigation, as they have been used for many years with respect to other navigation systems and, with broader definitions, throughout the practice of engineering. This column will describe the use of these terms for navigation and focus on their applicability to GNSS.

Q: What are the differences between accuracy, integrity, continuity, and availability, and how are they computed?

A: These four words describe parameters that quantify the performance of navigation systems. The terms are not unique to satellite navigation, as they have been used for many years with respect to other navigation systems and, with broader definitions, throughout the practice of engineering. This column will describe the use of these terms for navigation and focus on their applicability to GNSS.

Accuracy is the navigation performance parameter that is most commonly used and is the easiest to understand. It is a measure of the error, or the deviation of the estimated position from the unknown true position, of a given navigation tool or system. More precisely, accuracy is a statistical quantity associated with the probabilistic distribution of navigation error.

Depending on the system and its intended application, this quantity can be expressed in somewhat different ways. For example, many military systems express accuracy in terms of “circular error probable” (CEP) in two dimensions or “spherical error probable” (SEP) in three dimensions. This represents the median position error — it exceeds 50 percent of all position errors and falls below the other 50 percent.

More commonly used in civil applications are “1-sigma” and “2-sigma” error limits. In the case of position errors that follow Gaussian distributions, these limits express the 63rd and 95th percentiles of navigation errors, respectively, for a one-dimensional parameter (e.g., altitude). For higher-dimension parameters, such as 2-D horizontal position, the one- and two-sigma limits represent lower percentiles than in the one-dimensional case and can be computed from a chi-squared distribution if the underlying range-domain errors are Gaussian.

Because the Gaussian distribution describes most navigation system error distributions fairly well out to the 95th percentile, many accuracy descriptions use “95%” and “2-sigma” numbers interchangeably. However, this convention should not be taken to mean that the underlying error distribution is actually Gaussian, particularly in the “tails” beyond 2-sigma, where the norm is for variations from the Gaussian distribution to exist.

Accuracy is obviously a value of paramount importance when selecting among candidate navigation systems and deciding what use can be made of their measurements. Because accuracy defines errors under typical conditions, it expresses what users will experience in normal, everyday use.

Q: What is assisted GNSS? What changes are needed in assistance service standards to support the new near-future GNSSs such as GLONASS and QZSS?

A: Assisted GNSS (AGNSS) is a technology that enables faster position determination in an AGNSS-enabled handset than could be achieved using the broadcast GNSS satellite data only.

When a handset positions itself, it requests assistance data from the network. This assistance data includes, among other things: navigation models for ephemerides and clock corrections, reference location, ionosphere models, reference time, and optionally differential corrections for high-accuracy positioning and data bit assistance for high sensitivity.

When the assistance data is delivered over a telecom system architecture’s application layer connection (usually a TCP/IP connection), the typical position fix times are in the order of 10–20 seconds compared to 40–60 seconds using autonomous methods or even longer fix times in weak signal conditions. In the best case, the handset already has the assistance data in its memory — for example, in the form of extended ephemeris — and, as a result, the position fix time can be as short as few seconds.

Currently the multi-GNSS cellular standards in 3GPP GERAN (GSM), 3GPP RAN (UMTS) and OMA SUPL 2.0 (Application Layer) support only GPS L1 C/A-code–based Standard Positioning Service  and all Galileo Open Service signals. The work to include other GPS signals such as L5, L2C, and L1C signals and those from other GNSSs, such as GLONASS, satellite-based augmentation systems (SBASs) and Japan’s regional Quasi Zenith Satellite System (QZSS) is starting in 3GPP and OMA forums this autumn.

Adding new systems is straightforward due to the multi-GNSS framework already in place in the assistance data specifications from earlier releases.

One of the most promising activities is the work towards OMA SUPL 2.1 that promises to bring convergence to the currently very messy situation in application layer–based AGNSS solutions. The current OMA SUPL 2.0 specification is largely a collection of cellular network–specific protocols applied to the application layer that offer very different levels of performance from each other, none of which is optimally suited for the application layer.