  Robust, high-precision global localization is fundamental to a wide range of
outdoor robotics applications. Conventional fusion methods use low-accuracy
pseudorange based GNSS measurements ($>>5m$ errors) and can only yield a coarse
registration to the global earth-centered-earth-fixed (ECEF) frame. In this
paper, we leverage high-precision GNSS carrier-phase positioning and aid it
with local visual-inertial odometry (VIO) tracking using an extended Kalman
filter (EKF) framework that better resolves the integer ambiguity concerned
with GNSS carrier-phase. %to achieve centimeter-level accuracy in the ECEF
frame. We also propose an algorithm for accurate GNSS-antenna-to-IMU extrinsics
calibration to accurately align VIO to the ECEF frame. Together, our system
achieves robust global positioning demonstrated by real-world hardware
experiments in severely occluded urban canyons, and outperforms the
state-of-the-art RTKLIB by a significant margin in terms of integer ambiguity
solution fix rate and positioning RMSE accuracy.
