Swingate Down, Identification of Crop Mark

I recently came across a very large crop mark on the site of Swingate Down airfield, Dover and beside the later radar station compound. Kent HER have spoken to someone who identified it as "a big identification ‘badge’ with the name of a nearby airfield on it that probably dates to the 1920s". The circle is 460ft (140m) across and there seem to be rings of dark dots both inside and outside of the circle. Does anybody have any evidence that such a thing existed? It seems rather large and I feel that it may be something much older.

Airfield outline here: Boy did this take some finding-hence a month late. From AIR1/453/15/312/2 Vol. 6

Many thanks for the information. I had just decided that the mark probably showed something prehistoric when I came across this 1940 image of Manston which shows concentric circles of the same size. To add to the confusion this one IS recorded on Kent HER as a ring ditch at TR 3360 6610 (monument number TR 36 NW 245). Could they both possibly be connected with very early bombing practice, where the target itself was actually on the airfield? I have no idea what a target of this period might look like. If not then it could just be a coincidence that flat hilltop sites suit both uses.