Routine relative phase observations made during 1962 at Boulder on VLF signals radiated from NPM, Hawaii, and NBA, Canal Zone, have been analyzed to determine whether they show evidence of a semidiurnal lunar variation. It is found that the daytime observations of NBA show the presence of a significant (at the 5 per cent level) semidiurnal lunar height variation of 0.11 km; the maximum height occurs 2 hours before lunar transit. The nighttime observ[1.tions of NBA and both the NPM observations are not inconsistent with the daytime NBA results. Some possible reasons for the lower significance of the night time NBA and NPM results arc discussed.
The use of VLF signa ls for intercontinental fr equ e ncy compari son has become very popular in r ccent years, and it has been s hown by oth er workers that a precision of about 2 parts in 1011 can be achieved w it h meas ure me nts over a 24-hr per iod. Phase reco rds made at Bou lder, Colo ., of the NPM, Hawaii , 18. 6 k c/s transmi ssion have been studi ed for several p eriods of nearl y two weeks duration in 1962. D ev iations from an ass umed li near fr eque ney differen ce between th e transmitter and receive r oscillato rs s how an attain ab le prec ision of 2.5 parts in 10 11 in a 24-hr obse rvation, exte nding to 3. 1 parts in J01 2 in a J92-hr ob~er v ati o n . \\TitllOut fu rther data on t il(' re ma ini ng differe nces between the oscillators, t here is no evid encc that the propaga tion conditi ons over the path a re limi t ing t his precision. It is poin ted out t ha t the 50° (peak to p eak) semiannual va riation in t he d ay to night chan ge in phase, if attributed entirely to one level of refleetion, would ha ve an efIect on precis ion of fr eq uenc y co mparison of about 1 part in 10 12 •
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