New precision techniques for I.L.S. parameter measurement

The accuracy and simplicity of measurement of the four main parameters of the received I.L.S. signal, i.e. centring accuracy, off-course sensitivity, mean modulation depth and relative tone phase, has been greatly improved by the introduction of a new technique based on phaselocked detection of the 90 and 150 Hz tones using a 120 Hz reference frequency. The paper states the principles of the phase-locked system with the aid of vector diagrams. Basic block diagrams are shown for the various practical systems according to the measurement application, e.g. test equipment calibration,course line monitoring, field use, etc. The particular requirements for measurements in the presence of noise and large unwanted signals are also considered. The performance requirements for the main circuit blocks show that the a.f. circuitry is basically simple and non-critical and is adaptable to micro-miniature techniques. Finally a comparison is made between phase-locked detection and previous filter methods. In particular a centring accuracy of 01 /xA, equivalent to 0.05% difference in tone amplitudes, is easily obtained, and this represents an order of magnitude improvement over filter methods taken to the limit of present-day techniques.