How to convert to NED coordinates

Hi I am using Nexonar motion capture system( infrared tracker ). I have installed infrared sensor on the drone. this infrared sensor is tracked by camera which gives relative position of drone from camera. (0,0,0) is at camera.
I want to use drone for indoor purpose. i send coordinates using ATT_POS_MOCAP mavlink packet. But this message takes coordinates in NED format. How can I convert these values into NED format?