"Downward" message on RC2 controller

[riterb97]

I have never seen these messages on my RC2. After the last upgrade they seem to appear at times. What do they mean

Thx

 

[Cafguy]

Clean the downword sensor lens, if error persists
Use the Assistant to recalibrate the sensors, if error persists
Re-Flash the Drones firmware, if error persists
Replace the sensor.
(The recal should clear the error.)