Also by mid-1940, the Japanese had employed new tactics to stave-off attacks from defending Chinese fighters; Chinese air raid early-warning net would alert the Chinese fighter squadrons to scramble their fighters towards approaching Japanese bomber formations (ideally with ample time to attain sufficient altitude ahead of the raids), however, they were being monitored by a fast Japanese Navy C5M (IJAAF designation "Ki-15") scouting-attack planes at high-altitude which would radio instructions to the bombers to circle out of range as the Chinese fighters rose to meet the bombers; once the Chinese fighters had run low on fuel and returned to base the scouts would then direct the bombers to attack the Chinese fighters on the ground refueling. While the Chinese soon cracked the code used by the C5M crews relaying messages to the bomber formations, some of the countermeasures used by the Chinese pilots were to launch a pair of I-16s that were almost fast enough to intercept the C5M in pursuit, but enough to force the scouts away, while a second flight of fighters were launched against returning Japanese bombers as the first flight of Chinese interceptors returned to disperse in smaller auxiliary airbases to refuel, although successful interceptions were still limited against the IJNAF bombers as the Polikarpovs struggled in particular against the new Model 22/23 G3M bombers.