UERE (user equivalent range error)

User Equivalent Range Error (UERE) is a term used in the context of Global Navigation Satellite Systems (GNSS) and other positioning technologies to quantify the accuracy of position measurements for a user or receiver. UERE represents the error that can be expected in the calculated position of a user when using a positioning system, and it takes into account various factors that contribute to the overall positioning error.

Importance of User Equivalent Range Error (UERE):

In GNSS and similar positioning systems, the accuracy of user positions is essential for a wide range of applications, including navigation, surveying, timing, and location-based services. UERE provides valuable information to users and system operators about the reliability of the position estimates and helps manage expectations for different use cases.

Components of User Equivalent Range Error (UERE):

The UERE value is typically represented as a standard deviation or root mean square (RMS) error. It combines various error sources, including:

1. Satellite Clock Errors: GNSS satellites have onboard atomic clocks to provide precise timing information, but even these clocks have small inaccuracies that can affect position calculations.
2. Satellite Orbit Errors: GNSS satellites move in predetermined orbits, but these orbits are not perfectly known. Errors in the predicted satellite positions can result in positioning inaccuracies.
3. Ionospheric and Tropospheric Delays: GNSS signals can be delayed as they pass through the Earth's ionosphere and troposphere, leading to range measurement errors.
4. Multipath Interference: In urban environments or areas with reflective surfaces, GNSS signals can bounce off buildings or obstacles before reaching the receiver, causing signal distortions and errors.
5. Receiver Noise: The receiver's electronics and processing algorithms introduce some level of noise into the position calculations.
6. Atmospheric Conditions: Environmental factors like temperature, pressure, and humidity can affect the speed of GNSS signals, leading to position errors.
7. Selective Availability (SA): In the past, GNSS systems intentionally introduced errors (SA) into the signals for civilian users to limit accuracy. However, selective availability has been turned off for most GNSS systems like GPS.
8. Multipath Interference: Signal reflections from surrounding objects may cause the receiver to misinterpret the signal, leading to position errors.

Quantifying User Equivalent Range Error (UERE):

To calculate the UERE, it is necessary to consider a combination of all the above error sources. The UERE value is usually represented as a distance in meters, which corresponds to the radial error (horizontal or vertical) between the true position and the estimated position with a given confidence level (e.g., 95% confidence).

For example, if the UERE is specified as 5 meters with 95% confidence, it means that the estimated position provided by the GNSS receiver could differ from the true position by up to 5 meters with 95% certainty.

Improving Positioning Accuracy:

To improve positioning accuracy, various techniques are employed, such as differential GPS (DGPS), Real-Time Kinematic (RTK) positioning, and augmentation systems like Wide Area Augmentation System (WAAS) or European Geostationary Navigation Overlay Service (EGNOS). These methods use additional reference stations and correction data to enhance the accuracy of position estimates and reduce UERE.

Conclusion:

User Equivalent Range Error (UERE) is a crucial metric for understanding the accuracy and reliability of positioning systems like GNSS. By quantifying the expected errors in user position calculations, UERE helps users and system operators make informed decisions about the suitability of positioning technology for different applications. Minimizing UERE is an ongoing goal in the development and improvement of positioning systems to provide more accurate and reliable location information for users worldwide.