An Adaptive Multipath Mitigation Architecture for GPS System WEI-LUNG MAO 1 , JIUN-SHIAN DU 2 , YU-TANG LEE 2 , YING-REN CHIEN 3 Department of Electronic Engineering, National Formosa University 1 Department of Electrical Engineering, National Taiwan University 2 , Data Communication Business Group, Chunghwa Telecom Co., Ltd 3 wlmao@nfu.eud.tw , dujiunshian@hotmail.com , azthomas@gmail.com , curtis.chien@gmail.com Abstract: - In this paper, a multipath mitigation tracking system is presented for static GPS applications. It is comprised of four function blocks, those being (1) adaptive path estimator (APE), (2) multipath interference reproducer (MPIR), (3) Rake-based delay locked loop (RB-DLL), and (4) Rake-based phase locked loop (RB-PLL). Only the short delay condition with delay less than 1.5 PN chip is considered here, because GPS pseudorange error envelope decreases to zero for delay time greater than 1.5 PN chip. In order to estimate reflection profile in the correlation domain, the FFT-based circular correlation and block average method (BAM) are utilized to offer significant savings in computational complexity. The APE estimates the delayed profiles and coefficients of the reflection signals. With the path parameters from APE, the corresponding multipath arms are activated to accomplish the multipath reproduction. These replica profiles are used for subtracting the reflection components from carrier and code discriminators before sending it into the Rake-based carrier/code tracking loops. Simulation results shows that our proposed method provides a better performance in terms of multipath error envelop and carrier-phase error. Key-Words: - Global Positioning System (GPS) Receiver, adaptive path estimator (APE), multipath interference reproducer (MPIR), Rake-based delay locked loop (RB-DLL), and Rake-based phase locked loop (RB-PLL) 1 Introduction The global positioning system (GPS) provides accurate positioning and timing information useful in many applications. The GPS satellites broadcast ranging codes and navigation data with the technique of direct sequence spread spectrum (DS-SS). A wide variety of error sources affect the GPS measurement of pseudorange (also known as code-phase) and integrated Doppler (also known as carrier-phase). Among these are satellite user range error, ionospheric delay, tropospheric delay, receiver dynamic tracking error, multipath and thermal noise. The use of differential techniques theoretically eliminates all error sources which are common to both receivers. The error which remains is multipath, and it becomes the dominant error source in high precision GPS applications. Multipath errors are not identical to the GPS reference station and remote receivers. Thus, it becomes the significant error source in differential GPS. In this paper, a multipath mitigation tracking system is presented for static GPS applications. It is comprised of four function blocks, those being (1) adaptive path estimator (APE), (2) multipath interference reproducer (MPIR), (3) Rake-based delay locked loop (RB-DLL), and (4) Rake-based phase locked loop (RB-PLL). Only the short delay condition with delay less than 1.5 PN chip is considered here, because GPS pseudorange error envelope decreases to zero for delay time greater than 1.5 PN chip. These four sections perform together to achieve the carrier-phase and code-phase multipath cancellation while maintaining a fine synchronization of the incoming spread spectrum signals. The purpose of APE is to detect the fractional-value delay coefficients ( ˆ m L ) and I/Q-phase coefficients ( , ˆ Im b , , ˆ Qm b ) in the correlation domain based on adaptive filtering scheme. Instead of directly computing the correlation sum, an FFT-based periodic convolution method is applied to offer significant savings in computation. Due to the multipath delayed signals, the zero-crossing point in conventional DLL is usually shifted lagging from the correct position. Therefore, the S-curve can be severely distorted and result in pseudorange errors. To obtain the characteristics of multipath channels, the recursive least squares (RLS) algorithm is adopted to estimate the reflection delay portraits. With the path parameters from APE, the corresponding multipath arms are activated to accomplish the multipath interference reproduction (MPIR). These estimated MPI profiles are further utilized for subtracting the reflection components of received signals at carrier and code discriminators before entering the carrier and code tracking loops, respectively. 2 Multipath system description GPS satellites broadcast civil navigation data on channel L1. The transmitted signal for the considered Recent Advances in Signals and Systems ISSN: 1790-5109 80 ISBN: 978-960-474-114-4