Yes, RADAR is a Scrabble word!

Letter values in Scrabble US: R 1, A 1, D 2, A 1, R 1.

Is it a valid word?

RADAR is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

measuring instrument in which the echo of a pulse of microwave radiation is used to detect and locate distant objects

Also: microwave radar, radio detection and ranging.

Not playable in Scrabble US: RADIOLOCATION.