  This paper presents the most thorough study to date of vehicular
carrier-phase differential GNSS (CDGNSS) positioning performance in a deep
urban setting unaided by complementary sensors. Using data captured during
approximately 2 hours of driving in and around the dense urban center of
Austin, TX, a CDGNSS system is demonstrated to achieve 17-cm-accurate 3D urban
positioning (95% probability) with solution availability greater than 87%. The
results are achieved without any aiding by inertial, electro-optical, or
odometry sensors. Development and evaluation of the unaided GNSS-based precise
positioning system is a key milestone toward the overall goal of combining
precise GNSS, vision, radar, and inertial sensing for all-weather
high-integrity high-absolute-accuracy positioning for automated and connected
vehicles. The system described and evaluated herein is composed of a
densely-spaced reference network, a software-defined GNSS receiver, and a
real-time kinematic (RTK) positioning engine. A performance sensitivity
analysis reveals that navigation data wipeoff for fully-modulated GNSS signals
and a dense reference network are key to high-performance urban RTK
positioning. A comparison with existing unaided systems for urban GNSS
processing indicates that the proposed system has significantly greater
availability or accuracy.
