S.V. Ermak1, V.V. Semenov2, A.A. Baranov3, R.K. Lozov4

1–4 Peter the Great St. Petersburg Polytechnic University (St. Petersburg, Russia)

Rubidium atomic clocks are widely used as reference frequency sources in telecommunications and navigation systems. In onboard applications (for example, in satellite navigation systems), their accuracy is affected by variations in the magnitude and direction of the geomagnetic field.

The reasons for the occurrence of frequency shifts of the 0-0-radio-optical resonance (0-0-resonance) in vapors of alkali atoms, due to both the orientation dependence of the light shift of the 0-0-resonance frequency and the quadratic dependence of the 0-0-resonance frequency in the magnetic field, were studied. The investigations were made in the presence of differences in the magnitude of the longitudinal (along the pumping light) and transverse screening coefficients of the magnetic screen of rubidium atomic clocks.

The results of numerical simulation of the orientation frequency shift contribution of 0-0- resonance, as applied to atomic clocks located onboard satellites, to the total error of the ephemeris of the Galileo and GPS satellite global positioning systems are presented. The obtained numerical values of the correlation coefficients of the calculated and experimental dependences of the orientation frequency shift (from 0.3 to 0.75) depended significantly on the magnitude of the operation magnetic field of the atomic clock (0.8 A/m and 8 A/m) and the time shift of the reading of the orientation shift relative to the beginning of the time scale of the experimental dependence of the satellite orientation error.

The results of an experiment comparing the frequencies of 0-0-resonances in a system of two rubidium atomic clocks placed in an external magnetic field slowly changing in magnitude and direction, simulating the dynamics of changes in the magnetic field in the satellite's orbit, are presented. It is shown that in such a system, a significant suppression of fluctuations in the weighted average frequency of two rubidium atomic clocks (by a factor of 4.9 in terms of the Allan deviation) associated with variations in the magnitude and direction of the external magnetic field is realized. The parameter is introduced, which is interpreted as the coefficient of suppression of magnetic variations of the weighted average frequency in the system of two rubidium atomic clocks.
For the cases of induction of 0-0-resonance in a gas cell with laser pumping, the orientation shifts of its frequency in the alkali metal vapors 87Rb, 85Rb, and 133Cs are obtained by calculation. It is concluded that laser tuning to the short-wavelength component of the D2 line is preferable, regardless of the type of working substance. The obtained relative frequency shift of the 0-0-resonance at the level of 10-14 when the angle between the magnetic field vector and the optical axis varies by one degree may not be taken into account in the case of onboard atomic clocks at double radio-optical resonance in rubidium atoms when determining the total orientation error of satellite ephemeris.

The results of the work help to determine the nature of the orientation dependence of the onboard rubidium atomic clock frequency, due to the influence of the geomagnetic field, and to choose methods for suppressing this dependence.

Ermak S.V., Semenov V.V., Baranov A.A., Lozov R.K. Orientation frequency shifts of rubidium onboard atomic clock. Radiotekhnika. 2022. V. 86. № 12. P. 58−67. DOI: https://doi.org/10.18127/j00338486-202212-05 (In Russian)

Ermak S.V., Semenov V.V., Baranov A.A., Lozov R.K. Orientation frequency shifts of rubidium onboard atomic clock. Radiotekhnika. 2022. V. 86. № 12. P. 58−67. DOI: https://doi.org/10.18127/j00338486-202212-05 (In Russian)