Your firm has been hired to design a system that allows airplane pilots to make instrument landings in rain or fog. You've decided to place two radio transmitters 52m apart on either side of the runway. These two transmitters will broadcast the same frequency, however out of phase with each other. This will cause a nodal line to extend straight off the end of the runway. As long as the airplane's receiver is silent, the pilot knows she's directly in line with the runway. If she drifts to one side or the other, the radio will pick up a signal and sound a warning beep. To have sufficient accuracy, the first intensity maxima need to be 60m on either side of the nodal line at a distance of 4.4km.
Determine what frequency should you specify for the transmitters?
Express your answer using two important figures.