Let's take a closer look at the photo. The path from the satellite to the car roof and then to the antenna is 4 cm longer than the direct path from the satellite to the antenna. If the strengths of the direct and reflected signals were equal, their combination would extend the path by 2 cm. However, the car roof is not flat, and the direct and reflected signals are not equal. As a result, some satellites are received more by the direct signal, and others are received more by the reflected signal. This is different for all three antennas, causing the vector between the antennas to move as it pleases.
An even worse situation occurs when the direct signal is not captured, but the reflected signal is. For example, if the satellite is low in the sky and the direct signal is blocked by a building, the reflected signal arrives from a building opposite. In this case, the error in measuring the signal is already in the hundreds of meters, and the error in measuring coordinates across all satellites can be tens of meters.
That's why receivers use an "elevation mask", which blocks the reception of satellites located low above the horizon. For high-precision receivers, the typical mask is 15 degrees. This value is chosen because signals from low satellites travel a much longer path through the troposphere, and their tropospheric distortions are too large and unpredictable.
© Eltehs SIA 2023
