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Frequency engineering, rather than signal level, determines the usable distance of most of the VORTAC and Instrument Landing Systems. This paper presents the parameters of the system in-being which are determined by the frequency engineer. The levels of radio frequency interference protection afforded the system determine the signal environment for the airborne equipment. It is essential that the airborne equipment be capable of operating within these parameters. Radio frequency interference generated within the system by co- and adjacent-channel frequency assignments is controlled by limiting the usable distance of facilities. Geographical separation criteria have been developed to assure that the specified desired/undesired signal ratios are not derogated within the designated service volumes. A theoretical study was completed by NBS Central Radio Propagation Laboratory. This study included on a statistical probability basis: long- and short-term propagation anomalies, attitude and lobing structure of the airborne antenna, lobing structure of the ground antenna, etc. Measurements of airborne receiver operation in the presence of interfering signals were used to set the allowable desired/undesired signal ratios. The geographical separation criteria then were determined from the prediction curves furnished by CRPL. The VORTAC and ILS system in-being has been engineered by use of these separation criteria. The calculations are empirically verified by special flight inspection aircraft prior to the commissioning of each facility.