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This paper describes special VLF propagation measurements in connection with a feasibility study of a long range navigation system. Round-trip single-frequency measurement of phase stability was made between Hawaii and San Diego on frequencies from 10.2 kc to 18.2 kc in 1 kc increments. During January 15 to 23, 1958, the standard deviation of phase stability on 12.2 kc was 4 Â¿sec daytime and 5 Â¿sec night time. One-way two-frequency transmissions were monitored in San Diego and Washington, D.C., to determine the phase stability of a I kc difference frequency for pairs of frequencies from 10.2 kc to 18.2 kc. Data analyzed at time of submission of this paper (10.2-16.2 kc) indicate limitations of the two-frequency system for lane identification (resolution of cyclic ambiguities corresponding to one period of the carrier frequency). The techniques used to instrument these tests are considered somewhat unique. Data reported herein are general and applicable to any propagation study. The data being collected are leading to a better understanding of the mechanism of VLF propagation.