This article developed a barometric altimeter with high precision to solve the problem that it is difficult for vehicles positioning in multiple occlusion environments. The GNSS receiver can achieve navigation and positioning in the case of three visible satellites and improve positioning accuracy in the vertical direction aided by the barometric altimeter. Barometric altimeter is controlled by microcontroller STM32, by using pressure sensor to collect the atmospheric pressure and temperature, and sent data to the host computer using a serial port. The benefit of barometric altimeter is demonstrated with the two contrast experiments. The experimental results show that the height resolution measured by the barometric altimeter is 1 m and the RMSE of the height is 0.81 m at 500 m altitude, and satisfy the requirements of vehicles positioning.