The Wide Area Augmentation System (WAAS) broadcasts information that allows users to bound the positioning error that arises from ionospheric delay of signals emitted by Global Positioning System satellites. A critical objective of WAAS is to make ionospheric grid delays (IGDs) and grid ionospheric vertical errors (GIVEs) available to users as often as possible.

This innovation improves WAAS availability by modifying the methodology employed to protect the user from threats due to the under-sampling of ionospheric irregularities. The technology consists of a methodology for improving WAAS availability over coastal regions, in particular California and Alaska, by making the under-sampled ionospheric irregularity threat model dependent upon the overall level of ionospheric disturbance. To determine the local state of the ionosphere, it is proposed to introduce into WAAS a moderate storm detector.

The current WAAS ionospheric threat model provides terms used to inflate the GIVEs that are independent of the state of the ionosphere. In this technology, the threat model is separated into two branches — one to be used when the ionosphere is quiet, and one to be used when it is disturbed. Threats that cause the moderate storm detector to trip are eliminated from the quiet-time branch of the threat model. Using this quiet-time branch under nominal ionospheric conditions significantly reduces the magnitude of the users’ computed protection limits, thereby dramatically improving WAAS availability.

An analysis of the WAAS data over the past three years indicates that the fraction of time in which the quiet-time threat model branch could have been used throughout the WAAS coverage region exceeds 99.94%. Adopting this branch substantially increases availability at the edges of coverage — the difference in availability achieved by implementing the moderate storm detector is found, at some ionospheric grid points, to be as much as 75%. A subgroup of the WAAS Integrity and Performance Panel has recommended to the FAA that this proposal be implemented in WAAS.

This work was done by Lawrence C. Sparks of Caltech and Eric Altshuler of Sequoia Research for NASA’s Jet Propulsion Laboratory. For more information, contact This email address is being protected from spambots. You need JavaScript enabled to view it..

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

Technology Transfer at JPL
Mail Stop 321-123
4800 Oak Grove Drive
Pasadena, CA 91109-8099
E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.

Refer to NPO-49669.

NASA Tech Briefs Magazine

This article first appeared in the January, 2016 issue of NASA Tech Briefs Magazine.

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