OVERVIEW In recent years, the reduction of multipath has become an essential part of the Federal Aviation Administration’s (FAA) local area augmentation system (LAAS) Ground Facility (LGF) architecture. Multipath is the dominant source of error in a carrier- smoothed code-based differential system, contributing to over 90 percent of the system’s error budget if not mitigated. In late 1996, a development effort was ini- tiated with Ohio University and dB Systems Incorporated to develop a multipath-limiting antenna (MLA) that would reduce ground-induced multipath to a level that would support category (CAT) I/II/III approach and landing. This development effort yield- ed a 14-element dipole array antenna that reduced the multipath error (using narrow-correlator GPS receivers) to approximately 0.2 m. This 14-element MLA covered 0 – 35 deg in elevation angle. The MLA also provided a high-performance airport pseudolite (APL) transmit antenna capability with excellent multipath rejection at typical aircraft glideslope angles (+2.5 – 5 deg) and for LGF installation (from APL transmit to MLA receive antenna). Additionally, the MLA required a high-zenith antenna (HZA) to cover elevation angles above 35 deg. The combined MLA and HZA formed the integrated multipath-limiting antenna (IMLA). Flight testing performed at Ohio University and the FAA William J. Hughes Technical Center (WJHTC) confirmed that CAT I/II/III performance levels were achieved with the MLA. The HZA, however, fell short of providing the necessary multipath rejection at some elevation angles. In early 1998, under a Federal Systems Integration and Management Center (FEDSIM)-sponsored pro- gram through PRC/Litton and Raytheon Systems Company, dB Systems Incorporated was awarded another contract to improve the performance of both the HZA and MLA. The objective was to improve the HZA’s performance to a level commensurate with that of the MLA and to improve the performance of the MLA in the area of the horizon. These objectives were met with an upgraded HZA that provided sig- nificantly improved multipath rejection, and an improved MLA that was more resistant to terrestrial interference sources and had better carrier-to-noise ratios for satellites at low elevation angles. This paper summarizes the IMLA design and per- formance data, multipath rejection performance, and physical design. It also presents installed antenna performance data from Ohio University using the improved HZA and MLA. In summary, with the use of the IMLA, the FAA’s LAAS architec- ture is able to achieve the accuracy and satellite availability (augmented with low-multipath APLs) necessary to meet CAT I/II/III approach and landing requirements with sufficient margin. HISTORICAL BACKGROUND Multipath has long been a major error source for GPS and has been studied for many years. Multiple approaches have been taken to limit multipath in 117 LAAS Integrated Multipath-Limiting Antenna D. BRYCE THORNBERG and DEAN S. THORNBERG dB Systems Inc., Hurricane, Utah MICHAEL F. DIBENEDETTO, MICHAEL S. BRAASCH, FRANK VAN GRAAS, and CHRIS BARTONE Ohio University, Avionics Engineering Center, Athens, Ohio Received January 2003; Revised June 2003 ABSTRACT: A significant portion of the reduction of multipath in the Local Area Augmentation System (LAAS) ground system is accomplished by means of an integrated multipath-limiting antenna (IMLA). Many years of analysis, design, and development have been applied to the creation of a dual-beam IMLA that provides the per- formance necessary to meet the demanding requirements of accuracy, availability, and integrity for the LAAS Ground Facility (LGF). Ground multipath rejection on the order of 35 to 40 dB is achieved with the IMLA through- out the entire satellite hemispherical coverage volume. Flight testing of the LAAS utilizing the IMLA at both the Federal Aviation Administration’s (FAA) William J. Hughes Technical Center and Ohio University has shown that the IMLA rejects multipath to levels demonstrated to allow full compliance with the LGF specifications. NAVIGATION: Journal of The Institute of Navigation Vol. 50, No. 2, Summer 2003 Printed in the U.S.A.