This is a small and affordable way to add position hold functionality to your UAV without the need for GPS.  It has a working range of up to 2m, and uses a VL53L0X Lidar rangefinder along with a PMW3901 optic flow module.  It connects to your FC via UART and it currently supported only by INAV, but other firmware projects like betaflight will support this in the future.

Specifications

• Brand Name: Matek System
• Item Name: Optical Flow Lidar Sensor 3901-L0X Module
• Optical Flow: PMW3901Lidar: VL53L0X
• UART
• Protocol: INAV MSP
• Working Range: 8cm~200cm
• Field of view: 42 degree(PMW3901), 27 degree(VL53L0X)
• Minimum Illumination >60Lux
• Input voltage: 4.5~5.5V
• Power Consumption: 40mA
• Size: 36x12mm
• Weight: 2g

Tips

• Be sure Optical Flow lens is more than 2cm way from anything for optic flow to initialise correctly while FC starting up.
• Maximum range of VL53L0X is 2m, Altitude hold 0~2m @ throttle 0~100% if VL53L0X is enabled.
• Any unused UART of FC supports it
• Sonar connection (Trig & Echo) has not been implemented yet

Included

• 1x Optical Flow Lidar Sensor 3901-L0X Module