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 performance 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 throughout 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.

GPS multipath has been studied since the early 1970s. Prior to the investigation described in this paper, however, the effects due to the relative Doppler shift between the direct and multipath signal components have received scant attention. The single previous study that did address the issue indicated coherent receivers had significant performance advantages over noncoherent receivers. Specifically, it was stated that under the condition of fast-fading multipath, noncoherent receivers would yield a bias error, whereas coherent receivers would not. After reviewing the background theory, this paper describes a revised model of the phenomenon and shows the results of hardware simulations which validate the existence of the bias in both receiver types and offer support for the new model. A case study of a specific commercial receiver is presented.

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