British fighter aircraft tried to prevent German artillery-observation aircraft from operating by dominating the air from the British front line to the German balloon line, about beyond. Better aircraft like the Bristol Fighter, S.E.5a and the Royal Naval Air Service (RNAS) Sopwith Triplane had entered service since Arras and matched the performance of German Albatros D.III and Halberstadt D.II fighters. For the week before the attack, the barrage line was patrolled all day by fighters at with more aircraft at in the centre of the attack front. No British corps aircraft were shot down by German aircraft until 7 June, when aircraft were able to direct artillery fire simultaneously over the three attacking corps. Behind the barrage line lay a second line of defence, which used wireless interception to take bearings on German artillery-observation aircraft to guide British aircraft into areas where German flights were most frequent. By June 1917, each British army had a control post of two aeroplane compass stations and an aeroplane intercepting station, linked by telephone to the army wing headquarters, fighter squadrons, the anti-aircraft commander and the corps heavy artillery headquarters.