*2.2. Principle of the DBA System*

Single-barometer altimetry affected by atmospheric temperature, seasonal changes, and other factors drifts up to several tens of meters within a day, with poor stability and reliability [35]. Due to the Earth's gravitational field, the space atmosphere pressure and height show a certain regular distribution. Except for the local strong convection zone, the trend of atmospheric pressure variation in the local range of several tens of kilometers shows the same physical characteristics, and the atmosphere is basically in hydrostatic equilibrium in the vertical direction. Usually, the atmospheric pressure is distributed more evenly in the horizontal direction, and the pressure difference is about 1 hPa at a distance of 100 km. Thus, the concept of "difference" can be extended to the field of barometric altimetry with the help of differential GNSS positioning, that is, by setting one (or several) barometric reference points, the barometric altimetry value of the mobile station can be corrected by the precise altitude of the reference station to compensate the influence of atmospheric physical environment changes on the altitude measurement results of the mobile station, thus improving the accuracy of the user altitude of the mobile station.

When the barometric reference station and the mobile station are within a few tens of kilometers, their latitude, gravitational acceleration, and water vapor factors have the same effect on the atmospheric pressure, so the above three errors can be neglected to obtain the simplified DBA formula [36]:

$$h = h\_0 + 18,410(1 + \frac{t\_m}{273.15}) \lg \frac{P\_0}{P} \tag{3}$$

where *h* is the altitude of the mobile station to be found, *h*<sup>0</sup> is the known altitude of the reference station, *P*<sup>0</sup> is the pressure of the reference station, *P* is the pressure of the mobile station, and *tm* is the average Celsius temperature between the reference station and the mobile station.
