Title: Feasibility of Microsecond Timing with a WWVB-disciplined Rubidium Oscillator
Author(s): Kevin Croissant, Sabrina Ugazio and Frank van Graas
Published in: Proceedings of the 49th Annual Precise Time and Time Interval Systems and Applications Meeting
January 29 - 1, 2018
Hyatt Regency Reston
Reston, Virginia
Pages: 244 - 255
Cite this article: Croissant, Kevin, Ugazio, Sabrina, van Graas, Frank, "Feasibility of Microsecond Timing with a WWVB-disciplined Rubidium Oscillator," Proceedings of the 49th Annual Precise Time and Time Interval Systems and Applications Meeting, Reston, Virginia, January 2018, pp. 244-255.
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Abstract: WWVB timing signals have been broadcast from Fort Collins, Colorado since 1963. Starting in the 1960s, oscillators disciplined by WWVB achieved frequency transfer below the 1×10-11 level in most parts of the United States [1]. The majority of current users of WWVB do not correct for path delay between the WWVB transmitter and the user receiver, resulting in timing errors up to 15 ms. When the path delay is removed, timing accuracies traceable to Coordinated Universal Time maintained by the National Institute of Standards and Technology, UTC(NIST), are provided with an uncertainty of 100 µs. In 2012, a binary phase shift keying (BPSK) modulation was added to the 60 kHz carrier at a rate of 1 bit per second (bps) to increase the robustness and reliability of the timing signal [1]. According to Table 4-14 in [2], the projected precision timing requirements in critical infrastructure sectors do not go below 1 µs for time transfer and 1×10-11 for frequency stability over 24 hours. WWVB has met the frequency requirement using a disciplined oscillator since the 1960s. The feasibility for WWVB to meet the time transfer requirement of 1 µs is the topic of this paper. To evaluate WWVB timing performance, a software-defined radio is used with a Rubidium (Rb) oscillator. The WWVB signal is sampled at 250 kHz using the Rb as the reference. To evaluate the stability of the WWVB signal, the Rb oscillator is disciplined by GPS with a stability better than 10 ns averaged over 24 hours. The WWVB signals were recorded continuously for 21 days. The radio frequency (RF) signal is filtered with a 600-Hz bandpass filter centered at 60 kHz. Next, one-second sample batches are obtained after synchronization of the WWVB signal to the whole second. The time synchronization also enables the removal of the 1 bps data on the carrier. Next, the phase of the carrier is estimated for each 1-s batch, outliers due to interference are removed and the phase of the signal is accumulated to obtain the path delay change as a function of time. The daily repeatability of the path delay is used to evaluate the stability of the WWVB time reference. Next, the daylight portion of the signal that mostly contains the ground path propagation of the WWVB signal is calibrated with GPS as the reference. Following the calibration phase, the Rb is disciplined by the WWVB signal. The results show that 1 µs (1 sigma) timing is feasible with a WWVB-disciplined Rubidium oscillator.