I have been using the PX4Flow over I2C. Sometimes the PX4Flow will output a ground distance of 0 mm, most of the time this occurs when the actual distance is less than 300 mm (which seems to be the minimum ground distance the PX4Flow will output). The documentation says a negative ground distance should be output when the distance is unknown, but I have only observed non-negative ground distance values. So does 0 mm mean an unknown distance? And can anyone confirm my observations represent normal operation of the sensor?