Only ten years ago it would have been premature to imagine that ILS was likely to provide the basic means for completed landing in zero visibility. The system, designed in 1939, had then received only technological improvements, and no feature seemed to lend itself to any notable development. A frequency of 110 Mc/s is much too low to obtain a narrow beam; all the rival systems now being developed experimentally use frequencies 10 to 100 times higher. Nor is the measurement of difference in rate of modulation the most accurate operation possible with electrical magnitudes; pulse-systems or phase-comparison systems are better adapted.

In view of the number of installations in use on the ground and in the air it was, however, practically impossible to envisage any other guidance system to replace ILS; for modifications would have to be ‘compatible’, i.e. not involve a change in airborne receivers.