  High-precision vehicle positioning is key to the implementation of modern
driving systems in urban environments. Global Navigation Satellite System
(GNSS) carrier phase measurements can provide millimeter- to centimeter-level
positioning, provided that the integer ambiguities are correctly resolved.
Abundant code measurements are often used to facilitate integer ambiguity
resolution (IAR), however, they suffer from signal blockage and multipath in
urban canyons. In this contribution, a lidar-aided instantaneous ambiguity
resolution method is proposed. Lidar measurements, in the form of 3D keypoints,
are generated by a learning-based point cloud registration method using a
pre-built HD map and integrated with GNSS observations in a mixed measurement
model to produce precise float solutions, which in turn increase the ambiguity
success rate. Closed-form expressions of the ambiguity variance matrix and the
associated Ambiguity Dilution of Precision (ADOP) are developed to provide a
priori evaluation of such lidar-aided ambiguity resolution performance. Both
analytical and experimental results show that the proposed method enables
successful instantaneous IAR with limited GNSS satellites and frequencies,
leading to centimeter-level vehicle positioning.
