Parking radar K3502 Sensors polarity?

Can someone tell me how am I supposed to determine the polarity of the sensors? + and - are printed on the board but the sensors have no markings except T and CTD on them.

‘T’ is the transmitter.
The ‘+’ has w small plastic circle around its pin.

Great it works!

But tell me why wasn’t the polarity of the sensors explained in the manual in the first place? :unamused:
Polarity wasn’t indicated in the manual I got nor is it indicated in the online manual.

You are right, maybe we should indicate it in the manual, for people with less experience.