In-Flight Measurements and Correction Model for Rubidium Oscillator Specific Force and Magnetic Field Susceptibility

S. Craig, F. van Graas

Abstract:	Rubidium (Rb) oscillator performance is well-documented for stationary operations, where it exhibits frequency stability on the order of 5 to 10 pHz/Hz over time periods of several hours. However, it is also well documented that Rb oscillators are susceptible to environmentally induced errors, including changes in specific force, vibration, temperature, pressure, and magnetic field. These error sources have been observed to cause frequency excursions in excess of 70 pHz/Hz after only tens of seconds, a significant performance degradation when compared to stationary operation. One solution to reduce in-flight frequency degradation is to use advanced oscillator designs with improved magnetic shielding, low g-sensitive oscillators and temperature control. For applications where the oscillator is integrated with other navigation sensors, frequency performance could be improved through error modeling, which is the focus of this paper. To obtain baseline performance characteristics, specific force and magnetic field errors were first evaluated in the laboratory environment using a Cesium (Cs) oscillator as the reference. The laboratory performance was then used for the design of a correction model. Next, flight dynamics were used to evaluate the correction model by calculating the covariance matrix of the error model coefficients. The model was found to be feasible and applied to measurements from a Rb oscillator connected to a GPS receiver. Frequency errors were estimated using a precise carrier phase velocity/frequency algorithm in combination with attitude and heading angles from a navigation-grade inertial navigation system (INS). For the Rb oscillator under test, a factor of two improvement in standard deviation error performance was obtained by using the coefficients for magnetic field and specific force. Also, the peak frequency deviations during a 33-minute flight segment with benign dynamics were reduced from ±40 pHz/Hz to ±25 pHz/Hz.
Published in:	Proceedings of the ION 2013 Pacific PNT Meeting April 23 - 25, 2013 Marriott Waikiki Beach Resort & Spa Honolulu, Hawaii
Pages:	567 - 576
Cite this article:	Craig, S., van Graas, F., "In-Flight Measurements and Correction Model for Rubidium Oscillator Specific Force and Magnetic Field Susceptibility," Proceedings of the ION 2013 Pacific PNT Meeting, Honolulu, Hawaii, April 2013, pp. 567-576.
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