It had been suspected this system would be of little use in practice. When that turned out to be the case, the RAF turned to an entirely different system that was also being planned. This consisted of a set of tracking stations using HF/DF radio direction finders. Their aircraft radios were modified to send out a 1 kHz tone for 14 seconds every minute, allowing the stations ample time to measure the aircraft's bearing. Several such stations were assigned to each "sector" of the air defence system, and sent their measurements to a plotting station at sector headquarters, who used triangulation to determine the aircraft's location. Known as "pip-squeak", the system worked, but was labour-intensive and did not display its information directly to the radar operators. A system that worked directly with the radar was clearly desirable.