The emergence of low-cost micro-electro-mechanical system inertial measurement units, cameras and global navigation satellite system (GNSS) receivers has promoted the research of multisensor fusion positioning. With the rapid development of chip technology, it has become a trend for low-cost GNSS chips to provide multi-frequency carrier phase observations while supporting multiple constellations. To enhance the positioning performance of multi-frequency and multi-system precise point positioning (PPP) in the difficult urban environment, we propose a tightly coupled system of monocular visual-inertial odometry (MVIO) and uncombined PPP (UC-PPP) based on factor graph optimization. The initialization of MVIO and UC-PPP adopts a coarse-to-fine approach to correct the transformation of local and global frames online. Moreover, the sliding window and marginalization methods are adopted to retain the constraints between adjacent observations and eliminate useless observations in the window. The pedestrian and vehicle tests in urban environments verify the performance of the proposed method. Compared with open-source software GVINS, the positioning accuracy of the proposed method has been further improved by using carrier phase observations with higher measurement accuracy. Compared with PPP alone, the improvement of the proposed method for the low-speed and short-distance pedestrian test in the east, north, and up directions is 73.3, 54.8 and 62.7%, respectively, while the improvement for the high-speed and long-distance vehicle test is 63.0, 59.3 and 70.5%, respectively. Experiment results show that the proposed method has better positioning accuracy and continuity in difficult urban environments.

低成本微机电系统惯性测量单元、相机和全球导航卫星系统（GNSS）接收机的出现推动了多传感器融合定位的研究。随着芯片技术的快速发展，低成本GNSS芯片在支持多星座的同时提供多频载波相位观测已成为趋势。为了提高多频多系统精密单点定位（PPP）在困难的城市环境中的定位性能，我们提出了一种基于单目视觉惯性里程计（MVIO）和非组合PPP（UC-PPP）的紧耦合系统因子图优化。 MVIO和UC-PPP的初始化采用从粗到细的方式在线修正局部和全局帧的变换。此外，采用滑动窗口和边缘化方法来保留相邻观测值之间的约束并消除窗口中无用的观测值。城市环境中的行人和车辆测试验证了所提出方法的性能。与开源软件GVINS相比，通过使用测量精度更高的载波相位观测，该方法的定位精度进一步提高。与单独PPP相比，该方法对东、北、上方向低速短距离行人测试的改进分别为73.3%、54.8%和62.7%，而高速和短距离行人测试的改进分别为73.3%、54.8%和62.7%。长途汽车测试分别为63.0、59.3和70.5%。实验结果表明，该方法在困难的城市环境中具有更好的定位精度和连续性。