 Giffen 's force was being tracked by Japanese submarines , who reported on Giffen 's location and movement to their naval headquarters units . Around mid @-@ afternoon , based on the submarine 's reports , 16 Mitsubishi G4M " Betty " from the 705 Air Group and 16 Mitsubishi G3M " Nell " bombers from the 701 Air Group took off from Rabaul carrying torpedoes to attack Giffen 's force , now located between Rennell Island and Guadalcanal . 
