Date of Award


Document Type


Degree Name

Master of Science


Department of Electrical and Computer Engineering

First Advisor

Stuart L. DeVilbiss


This thesis proposes a new direct sequence spread spectrum (DS/SS) code phase tracking loop which mitigates the effects of multipath interference on code phase tracking error; such errors can translate to significant range measurement errors in DS/SS ranging systems such as Global Positioning System (GPS). The new code tracking loop, called the modified RAKE delay lock loop (MRDLL), uses maximum likelihood (ML) signal parameter estimation to determine the amplitude, carrier phase, and relative propagation delay of both a direct path and a reflected signal; a multiple correlator code phase tracking loop then exploits these ML signal estimates to remove the tracking error introduced by the reflection. A preliminary analysis showed that the MRDLL's linear tracking region varied with the reflected signal parameters; therefore, an adaptive loop controller (ALC) was introduced to allow the loop designer to fix dynamic specifications such as loop natural frequency. Analysis and computer simulations demonstrated that, when multipath was present, the MRDLL exhibited a significantly lower steady-state code phase tracking error than that of the standard noncoherent delay lock loop (NCDLL), which is typically used in GPS receivers. In an ideal multipath-free environment, the NCDLL is still the best choice for code phase tracking. This GPS receiver design technology will benefit the entire aviation community by eliminating or reducing the dominant source of error in differential GPS-based instrument landing systems, resulting in improved ILS safety, reliability, and integrity. In relative GPS applications, such as precision guided munitions, lethality is improved via elimination of multipath contribution to targeting error. In all GPS applications, the (no longer dominant) error contribution of multipath can be eliminated, yielding enhanced positioning accuracy.

AFIT Designator


DTIC Accession Number