FAA researching Advanced RAIM for GPS + Galileo approaches

January 30, 2023  - By

Quarterly reports evaluate ARAIM progress for FAA

Photo: jpgfactory/iStock/Getty Images Plus/Getty Images

Photo: jpgfactory/iStock/Getty Images Plus/Getty Images

An official evaluation of Advanced RAIM (ARAIM), a GPS technique used in aviation receivers for safer landings and take-offs, is being conducted by the William J. Hughes Technical Center (WJHTC) of the U.S. Federal Aviation Administration (FAA).

The WAAS Test Team at the technical center has begun to monitor the Integrity Support Data (ISD) parameters of ARAIM using evaluation tools and methods developed by both the center and Stanford University. Results of this monitoring will be published in a quarterly report on the WAAS Test Team website.

The Need for Advanced RAIM

ARAIM addresses various weaknesses of Receiver Autonomous Integrity Monitoring (RAIM).

To assure the integrity of GPS, aviation receivers implement RAIM, which detects any GPS satellite fault, and can then isolate and remove it from the navigation solution.

However, RAIM provides integrity only for horizontal operations, such as enroute and non-precision approach. Additional integrity is needed to allow advanced capabilities, such as vertically guided approaches. Other integrity systems, including the Wide Area Augmentation System (WAAS), provide the integrity needed to permit these additional operations.

Since RAIM’s debut, GPS and other GNSS have evolved to improve their performance and upgraded to add an additional civilian signal, making possible ARAIM architecture.

ARAIM increases the geometric diversity and integrity availability by using two core GNSS constellations (such as GPS and Galileo). ARAIM takes advantage of the second civilian signal by specifying dual-frequency processing so that the ionospheric error from GNSS signals is directly measured by the user equipment.

The ionosphere — in most cases, the largest source of error in a GNSS signal — can also reduce the integrity of GNSS signals. Data provided for ARAIM use can include improved performance commitments from the GNSS constellation. RAIM uses static values for those performance commitments.

Enabling LPV-200 Approaches

The dual-frequency multi-constellation ARAIM seeks to allow LPV-200 approaches worldwide. LPV-200 (localizer performance with vertical guidance) delivers accurate information on an aircraft’s approach to a runway with the use of GNSS positioning technology. The result is lateral and angular vertical guidance without the need for visual contact with the ground until an aircraft is 200 feet above the runway.

ARAIM is considered an aircraft-based augmentation system: the algorithm to determine GPS integrity is in the aircraft receiver, just like RAIM. ARAIM could use both GPS and Galileo to achieve the worldwide LPV-200 service goal, with the integrity needed available from satellites of both constellations.

Integrity Support Data. An important aspect of ARAIM is the integrity support message, which contains the ISD that describe a GNSS constellation’s accuracy and reliability. Each GNSS constellation service provider generates and updates its ISD data, while the receiver manages and uses each GNSS constellation message.

The specific ISD parameters for ARAIM have not yet been finalized, but candidate data includes the probabilities of satellite and constellation failure (for instance, more than two satellites fail due to a common cause), user range error,  user range accuracy data, and other candidate data.

The ISD will be finalized when the International Civil Aviation Organization (ICAO) Standards and Recommended Practices (SARPs) with the ARAIM requirements are completed.

To ensure that the data provided in the ISM remains valid, external monitoring is needed. The external monitoring ensures the satellite and constellation failure probabilities provided in the ISM continue to be valid. External monitoring also characterizes the user range accuracy and user range error in the ISM.

Read more about the project in the FAA’s SatNav News, Fall/Winter issue.

About the Author: Tracy Cozzens

Senior Editor Tracy Cozzens joined GPS World magazine in 2006. She also is editor of GPS World’s newsletters and the sister website Geospatial Solutions. She has worked in government, for non-profits, and in corporate communications, editing a variety of publications for audiences ranging from federal government contractors to teachers.