Coordinated Universal Time

Abstract: A method for creating time-stamped frames sent between computers includes receiving and generating a universal coordinated time reference signal using a global positioning system. Clocks independent of operating systems of the sending and receiving computer are synchronized with the reference signal. A test frame is created including a tag having reserved fields for transmit and receive time stamps. The transmit time stamp is inserted into the reserved transmit time stamp field, without intervention of the sending computers central processing unit, that corresponds to the time on the synchronized clock at the instant the test frame is sent. The test frame having the transmit time stamp is received by the receiving computer, and a receive time stamp is inserted into the reserved receive time stamp field corresponding to the time on the synchronized clock of the receiving computer when the test frame was received.

Abstract: The clock products of the International Global Navigation Satellite Systems (GNSS) Service (IGS) are used to characterize the timing performance of the GPS satellites. Using 5-min and 30-s observational samples and focusing only on the sub-daily regime, approximate power-law stochastic processes are found. The Block IIA Rb and Cs clocks obey predominantly random walk phase (or white frequency) noise processes. The Rb clocks are up to nearly an order of magnitude more stable and show a flicker phase noise component over intervals shorter than about 100 s. Due to the onboard Time Keeping System in the newer Block IIR and IIR-M satellites, their Rb clocks behave in a more complex way: as an apparent random walk phase process up to about 100 s and then changing to flicker phase up to a few thousand seconds. Superposed on this random background, periodic signals have been detected in all clock types at four harmonic frequencies, n × (2.0029 ± 0.0005) cycles per day (24 h coordinated universal time or UTC), for n = 1, 2, 3, and 4. The equivalent fundamental period is 11.9826 ± 0.0030 h, which surprisingly differs from the reported mean GPS orbital period of 11.9659 ± 0.0007 h by 60 ± 11 s. We cannot account for this apparent discrepancy but note that a clear relationship between the periodic signals and the orbital dynamics is evidenced for some satellites by modulations of the spectral amplitudes with eclipse season. All four harmonics are much smaller for the IIR and IIR-M satellites than for the older blocks. Awareness of the periodic variations can be used to improve the clock modeling, including for interpolation of tabulated IGS products for higher-rate GPS positioning and for predictions in real-time applications. This is especially true for high-accuracy uses, but could also benefit the standard GPS operational products. The observed stochastic properties of each satellite clock type are used to estimate the growth of interpolation and prediction errors with time interval.

Abstract: For more than two decades, NASA deep space network (DSN) frequency and timing metrology has been a driving application for remote transfer of stable radio-frequency signals over fiber-optic cables. Precise, accurate, and stable signals are essential for deep-space communication and tracking, and syntonized and synchronized reference signals from atomic clocks calibrated to coordinated universal time must often be distributed over large distances. Fiber-optic technologies developed at the jet propulsion laboratory have resulted in several operational signal transport capabilities that enable precise spacecraft navigation and sensitive radio science experiments. These techniques are now finding further applicability in metrology applications to remotely compare ultra stable microwave and optical atomic clocks and for antenna array X- and Ka-band signal transport applications where temporal phase stability and alignment are critical. The pioneering DSN photonic link developments and capabilities are summarized, and a stabilized multiphotonic link architecture for ultrastable signal transport in antenna arrays is described.

Abstract: This invention relates generally to a method and apparatus for timely forwarding, discarding, and delivering data packets over the network and to their destination nodes and the optimization of data transfer throughput through the network. The timely forwarding and discarding are possible thanks to the standard global common time reference (CTR) that is known as UTC (Coordinated Universal Time). UTC is available from GPS (Global Positioning System), Galileo, and GLONASS (Global Navigation Satellite System). Data transfer throughput optimization is pursued by taking advantage of the timely forwarding and discarding properties to improve the data packets transfer flow control mechanisms, such as the sliding window re-sizing algorithm implemented by the widely deployed Transmission Control Protocol (TCP).