A variety of technological navigation aids all help pilots achieve a smooth and safe landing. Early pilots didn't worry about keeping away from other aircraft and landed in any direction on an open field that gave them the best angle relative to the wind. As traffic grew and more aircraft began to use airports rather than farms or fields, landings became limited to certain directions, and descending aircraft, competing for the same course, were in danger of colliding. The earliest landing aids were people-powered. Flagmen provided aircraft separation and direction control. They stood on the field waving red, green, or white cloths that told pilots if they were approaching a clear field at the correct angle. Green flags would indicate a clear field or the proper direction; red flags meant danger or told the pilot to circle until further instruction. More often, flagmen were replaced by a system of red and green airport lights to show the runway threshold and sides. When fields were marked with rotating lights so they could be found after dark. In the early 1930s, airports installed the earliest forms of approach lighting, which indicated the correct angle of descent and whether the pilot was right on target. These were called the glide path or glide slope. Gradually, the colors of the lights and their rates of flash became standard worldwide based on International Civil Aviation Organization (ICAO) standards. Radio navigation aids also assisted in landing. One type, introduced in 1929, was the four-course radio range, where the pilot was guided by the strength of Morse code signals. Another type that was tried experimentally was the low-frequency radio beam. These radio beams flared outward from the landing point like a "v," so at the point farthest from the runway, they were wide, and it was easy for the pilot to fly between the arms of the "v." But near the landing point, the space between the beams was extremely narrow, and it was often easy for the pilot to miss the exact center point that he had to hit for landing. Another new method had a pilot tune into a certain frequency at a checkpoint far from the airport, then use a stopwatch to descend at a precise rate to the touchdown area of the runway. This method also proved difficult. The development of RADAR at the beginning of World War II allowed the military to use a new landing aid called ground control approach (GCA). GCA consisted of two six-inch (15-centimeter) radar displays using cathode ray tubes (CRTs). One CRT displayed the approaching aircraft while the radar operator directed pilots into a waiting position using voice radio. The other CRT display helped determine how the pilot should steer to find the proper heading and approach angle to land. Then a controller literally talked the pilot down. GCA used mobile trailers that could roll to a new runway or even a new airport when needed. Today's military still uses an advanced version of this system, called precision approach radar. GCA proved its worth during the Berlin Airlift in 1948-1949 when as many as a thousand flights landed each day. The introduction of the slope-line approach system was a first in landing aids. Developed in the 1940s, the aid consisted of lights in rows that showed the pilot a simple funnel of two rows that led him to the end of the runway. Other patterns showed him when he was off to the right or left, or too high or low. The system was inexpensive to build and operate although it had some limitations and was not suitable for certain airports.