L M. Nelson

Assessment of GPS carrier-phase stability for time-transfer applications

We have conducted global positioning system (GPS) carrier-phase time-transfer experiments between the master clock (MC) at the U.S. Naval Observatory (USNO) in Washington, DC and the alternate master clock (AMC) at Schriever Air Force Base near Colorado Springs, Colorado. These clocks are also monitored on an hourly basis with two-way satellite time-transfer (TWSTT) measurements. We compared the performance of the GPS carrier phase and TWSTT systems over a 236-d period. Because of power problems and data outages during the carrier-phase experiment, the longest continuous time span is 96 d. The data from this period show agreement with TWSTT within /spl plusmn/1 ns, apart from an overall constant time offset (caused by unknown delays in the GPS hardware at both ends). For averaging times of a day, the carrier-phase and TWSTT systems have a frequency uncertainty of 2.5 and 5.5 parts in 10/sup 15/, respectively.

First comparison of remote cesium fountains

The frequencies of the cesium fountain primary frequency standards at the National Institute of Standards and Technology and the Physikalisch-Technische Bundesanstalt have been compared. Two-way satellite time and frequency transfer and GPS carrier-phase were the principal frequency-transfer techniques used to make the comparison. For the 15-day interval in which both fountains were in operation the frequencies were compared with an additional uncertainty due to the comparison process of only 6.2 /spl times/ 10/sup -16/. The two standards agree within their stated one-sigma uncertainties of /spl sim/ 1.7 /spl times/ 10/sup -15/.

Relationship of AM to PM noise in selected RF oscillators

We have studied the amplitude modulation (AM) and phase modulation (PM) noise in a number of 5 MHz and 100 MHz oscillators to provide a basis for developing models of the origin of AM noise. To adequately characterize the AM noise in high performance quartz oscillators, we found it necessary to use two-channel cross-correlation AM detection. In the quartz oscillators studied, the power spectral density (PSD) of the f/sup -1/ and f/sup 0/ regions of AM noise is closely related to that of the PM noise. The major difference between different oscillators of the same design depends on the flicker noise performance of the resonator. We therefore propose that the f/sup -1/ and f/sup 0/ regions of AM and PM noise arise from the same physical processes, probably originating in the sustaining amplifier. >

Long-term time comparison between frequency standards at NIST and PTB for a test of the validity of local position invariance

The caesium fountain frequency standard CSF1 of PTB has been compared with a hydrogen maser HMNI operated in NIST, Boulder, for more than two years. Two time comparison methods were used for that purpose, two-way satellite time and frequency transfer (TWSTFT) and GPS carrier-phase based frequency comparisons (GPSCP). Standard GPS common view time comparisons performed in parallel were useful to identify the properties of the two methods. Between TWSTFT and GPSCP variations of as much as 10 ns peak-to-peak during the whole period were found. It was nevertheless possible to determine a new limit of the validity of local position invariance (LPI), tighter by almost a factor of 4 than previously possible. The validity of LPI could be tested by comparing two different kinds of atomic frequency standards in the same time-varying gravitational potential /spl Delta/U(t) caused by the earths annual elliptic orbit around the sun. LPI predicts a null result in such an experiment A frequency variation between CSF1 and HMNI in phase with /spl Delta/U(t) greater than 6.10/sup -6/ of the amplitude of /spl Delta/U(t)/c/sup 2/ can be excluded.

Calibration of carrier phase GPS receivers

With the development of better standards we are investigating the use of GPS carrier phase for time transfer. Currently Two-Way Satellite Time Transfer (TWSTT) can be used as a means to compare two remote clods at an uncertainty of one part in 10/sup 14/. Using TWSTT as a baseline, we can show some of the promising results obtained with the GPS carrier phase method of time transfer. We also discuss receiver reset calibration using 1 pps data and differences in our results when using predicted versus precise ephemerides.