GPS Carrier Phase Time Transfer Research Articles

A three-cornered hat analysis of instabilities in two-way and GPS carrier phase time transfer systems

A three-cornered hat analysis has been performed on three independent time transfer systems between the United States Naval Observatory (USNO) and the National Institute of Standards and Technology (NIST). These include (1) a direct Two-Way Satellite Time and Frequency Transfer (TWSTFT) link provided by the USNO, (2) an indirect TWSTFT link, and (3) GPS carrier phase (GPSCP) links. Time transfer instabilities for individual links, as quantified with the time deviation and TOTAL time deviation, have been measured for the first time for averaging periods between 2 h and 85 days. Time transfer instabilities beyond one day are roughly flicker phase in nature and generally range between 40 ps and 120 ps. Frequency transfer instabilities, as quantified by the modified Allan deviation, are observed to be as low as 1 × 10−16 for a 20 day comparison and into the mid 10−17 range at longer comparison times.

A Study of GPS Carrier-Phase Time Transfer Noise Based on NIST GPS Receivers.

To do a better time comparison between high-precision clocks (such as a Cesium-fountain clock and Hydrogen-maser clock), we want to study and eventually lower the GPS carrier-phase time transfer noise. The GPS carrier-phase time transfer noise comes from four sources: GPS satellite, GPS signal path, ground receiving equipment (receiver and antenna), and data-processing algorithm. This paper focuses on the noise introduced by the ground receiving equipment. At NIST, we have installed seven GPS receivers. All receivers have the same reference time, i.e., UTC(NIST). Three of them are connected to the same antenna. The other four are connected to four different antennas. This architecture enables us to study the time-transfer noise from the ground receiving equipment. We study both long-term (> 100 days) noise and short-term (< 1 day) noise. For the long-term noise, the time-transfer result using one receiver can vary from that using another receiver by up to 1.8 ns, during 1.3 years. To achieve sub-nanosecond GPS timing accuracy, a careful monitoring of the time delays or a more frequent calibration is needed. For the short-term noise, we find that the common-clock difference between receivers using the same antenna is less noisy than that using two different antennas, at an averaging time of less than 0.5 hour. This indicates that the antenna and antenna cable contribute to the super-short-term noise of GPS carrier-phase time transfer significantly. In addition, the response to the GPS receiver's reference-time change is tested in this paper. The variation in the response can be up to ± 350 ps. Last, this paper gives the best carrier-phase time transfer result we can currently achieve with the available equipment at NIST. The best frequency stability is 4.0×10-16 at 3 hours, 1.1×10-16 at 1 day, 4.0×10-17 at 10 days, and 1.3×10-17 at 48 days.

Development of a GPU-Based Two-Way Time Transfer Modem

We have developed a new two-way time transfer modem to improve the time transfer precision of remote clock comparison. As a timing signal, we apply a binary offset carrier, which is similar to those signals used for the next-generation Global Navigation Satellite Systems. We took advantage of versatile A/D and D/A converters, and most of the digital signal processing stages were realized by software, running on an off-the-shelf PC. This enabled us to realize the complete system with cheaper equipment, leading to an affordable low-cost modem. For the real-time digital signal processing stages implemented in software, we relied on a graphics processing unit (GPU) developed for computer game enthusiast. The developed modem can receive four channels at the same time with a single GPU card. We performed two-way satellite time transfer experiments using these modems between Japan and Taiwan. The obtained results are consistent within 200 ps with respect to the results of GPS carrier phase time transfer. As a consequence, we improved the time transfer precision by nearly one order of magnitude as compared to a conventional two-way modem without increasing the connection fees caused by commercial communication satellites.

Real-time formation of a time scale using GPS carrier-phase time transfer network

This paper presents a prototype real-time clock ensemble system that utilizes a Kalman filter-based ensemble algorithm to combine remote clocks linked via a real-time GPS carrier-phase time transfer network. The calculated ensemble clock is realized by the use of an optimal control technique, where emphasis is placed on seeking a balance between the short-term stability of a hydrogen maser clock and the long-term stability of the ensemble clock. The performance of the system is investigated by conducting a five-month long ensemble experiment based on local realizations of UTC from four timing laboratories, and the frequency stability of the real-time ensemble time scale as well as its realizations are assessed with reference to UTC.

A study on the Common-View and All-in-View GPS time transfer using carrier-phase measurements

The performance of carrier-phase based GPS precision time transfer was investigated by conducting a 7-month campaign of time transfer among four national timing laboratories. The Common-View and the All-in-View techniques, which are two typical techniques for the GPS carrier-phase time transfer, are examined in terms of time transfer and frequency transfer together with other independent time transfer systems such as TWSTFT. Comparison of clock estimates with BIPM Circular-T was performed and the modified Allan deviation of clock estimates was calculated to assess the relative frequency instability as a measure of frequency transfer performance. Also an issue of baseline limitation for the carrier-phase Common-View methods has been investigated by conducting a 1-month GPS time transfer to a set of IGS stations with different baselines. Finally, the potential of the GPS carrier-phase time transfer as a real-time synchronization method was assessed by carrying out the GPS time transfer with the IGS Ultra-rapid orbits.

Carrier-phase time transfer

We have conducted several time-transfer experiments using the phase of the GPS carrier rather than the code, as is done in current GPS-based time-transfer systems. Atomic clocks were connected to geodetic GPS receivers; we then used the GPS carrier-phase observations to estimate relative clock behavior at 6-minute intervals. GPS carrier-phase time transfer is more than an order of magnitude more precise than GPS common view time transfer and agrees, within the experimental uncertainty, with two-way satellite time-transfer measurements for a 2400 km baseline. GPS carrier-phase time transfer has a stability of 100 ps, which translates into a frequency uncertainty of about two parts in 10(-15) for an average time of 1 day.