"int pixhawk's terminal you can run below commands
nsh>px4flow test then you can get below information about sonar
px4flow: ground_distance: 0.63 m
px4flow: time since last sonar update [us]: 9524010
px4flow: quality integration average : 255…"

"There are many chinese vendors as you can see below https://s.taobao.com/search?q=px4flow&s_from=newHeader&ssid..."

"Because MS5611 has kilometer-long range and tens of meters' error over time, while sonar has meter level range and centimeter level error."

"I have just tried the V3.3.2 copter for pixhawk, and get below result
1.opt_flow enabled, EKF_GPS_TYPE=3.
2.opt_m_x, opt_m_y,opt_qua is not zero in flying data->status
3.EKF5.meaRng is between (0,3), EKF5.FIX goes up step by step from 0 to 20,…"

"Can the image built from px4flow source code work with APM:Copter 3.3.1 that downloaded by MissionPlanner?"