The unit remained in Northern Ireland till the end of April, 1944, when it was moved to the advanced landing ground (ALG) at Horne some 30 miles south of London. See the orders. Over there it joined the 142 Fighter Wing. Suitable airstrip covered with wire netting known as the Sommerfield Track made up for the facilities. Everything the Wing needed to operate was under canvas, as part of the plan to prepare units for field operations from the airstrips in Europe. On the field airports intensive work was to ensure the maximum mobility and self-sufficiency, andmany RAF fighter units went through that run-through before the Invasion.

From Horne, the unit began flying escort sorties across the channel, mostly against V-1 sites. On 21 May, the squadron led byS/Ldr Koc strafed targets near Lille loosing two pilots in the process: F/O Brzeski and Sgt Kempka were shot down and taken prisoners. The next day in similar sortie Sgt Bartkowiak was lost. He evaded capture and returned to the unit four months later.

RAF Horne, June3, 1944. The ceremony of presentig three American pilots who flew with Kosciuszko Squadron during 1919/20 war, with the squadron's badge. From right: Captain Elliot Chess, Major Thomas Cumings and General Victor Strahm (signing). On D-Day, the 303 took active part in operations over the landings. Three full strength sorties were flown without incidents that day, and for the next week the squadron continued to fly patrols in the designated areas over Normandy.

During the invasion, the Germans began using its V-1 weapon on