GPS Carrier-Phase Frequency Transfer.

Global Positioning System Carrier-Phase (GPS CP) is a mission area where the U.S. Naval Observatory (USNO) is developing positioning, time, and frequency transfer products.

Time Differences

In this page will be shown the uncalibrated time and fractional frequency transfer as realized between USNO(MC3) and a good IGS/VLBA station hydrogen maser located in Hawaii (MKEA). The clock time difference data was produced from USNO IGS-style final solutions generated at USNO with points every 7.5 minutes in the form USNO(MC3) minus station clock, see figure 1 for a sample data segment. The data has had daily phase steps (RMS 0.383) and a peak-to-peak accumulated phase error of 1.6 nanoseconds removed to generate the displayed "continuous function".


Figure 1: Uncalibrated continuous function time differences for USNO(MC3) - MKEA both of which have hydrogen masers and are in the IGS network.

Fractional Frequencies

The data in figure 1 was then converted to fractional frequency (parts in ten to the 15th) which is displayed in figure 2.


Figure 2: Fractional frequency of the data shown in figure 1.

Total Deviation Instabilities

A new frequency instability characterization method has been recently developed and is called the Total Deviation (TotDev)[1]. In figure 3 the output of this estimator is presented which allows analysis of the noise types and amplitudes in the data shown above in figure 2. At the fundamental data rate of 7.5 min. the sigma x(TotDev) is 4.7e-14 and improves generally with a slope of -1/2 which is produced by white frequency modulation (white FM). There is evidence of small periodic components in this data seen as the broad bumps. The TotDev instabilities were produced with the Stable/Win version 1.0 softare package[2].


Figure 3: TotDev instablities. The slope of approximately -1/2 indicates a dominant noise source characterized as white FM.


REFERENCES

  1. Howe, D. A., and Greenhall, C. A. (1998). Total Variance: A Progress Report on a New Frequency Stability Characterization", 29th Annual Precise Time and Time Interval (PTTI) Applications and Planning Meeting, 2-4 December, 1997, pp.39-48
  2. Stable/Win (software) Version 1.0, 23 August 1997, Hamilton Technical Services, 195 Woodbury Street, S. Hamilton, MA 01982 USA.


This page is http://tycho.usno.navy.mil/gpscp_rs.html
(11 June 1998)

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