by johan

Each tower on a given route was assigned a letter in the sequence WUVHRKDBGM, flashing out its message using a simple electronic system. Since towers were spaced every ten miles, the first — the W tower — was at the ten-mile mark. Tower U was at 20 miles, V at 30, until the plane passed the M tower at 100 miles. Then the sequence repeated. All a pilot had to remember was the phrase When Undertaking Very Hard Routes Keep Direction By Good Methods, and his current place in it.