When the Germans began attacking the British coastline by plane in 1940, the British already had a land Radar Detection system. Their land radar system, called Chain Home, combined with a somewhat primitive Visual detection system, the Royal observer corps, made up what was called the Dowding System. The basic idea of the Dowding system was that the radars would track the enemy planes from long range and once they were close enough the job would also be done by the Royal Observer Corps. Both sides would relay information through a private telephone line to get the most accurate picture of the location of the Enemy planes. The Dowding system was very effective but it still had its flaws. One of the flaws was that there was a point in the interception process where the enemy planes were to close for the land radars to read accurately, but to far away to be seen. Another flaw was that effective as the Dowding system was during the day it was almost useless at night. They knew they needed something different a team led by Edward "Taffy" Bowen was chose to lead the experiment.

Why did they need Airborne Radar?

The chain home defense system.

http://en.m.wikipedia.org/wiki/Chain_Home