- Open Access
Fully automated VLBI analysis with c5++ for ultra-rapid determination of UT1
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB 2010
Received: 12 October 2010
Accepted: 20 November 2010
Published: 3 February 2011
VLBI is the only space-geodetic technique which gives direct access to the Earth’s phase of rotation, i.e. universal time UT1. Beside multi-baseline sessions, regular single baseline VLBI experiments are scheduled in order to provide estimates of UT1 for the international space community. Although the turn-around time of such sessions is usually much shorter and results are available within one day after the data were recorded, lower latency of UT1 results is still requested. Based on the experience gained over the last two years, an automated analysis procedure was established. The main goal was to realize fully unattended operation and robust estimation of UT1. Our new analysis software, named c5++, is capable of interfacing directly with the correlator output, carries out all processing stages without human interaction and provides the results for the scientific community or dedicated space applications. Moreover, the concept of ultra-rapid VLBI sessions can be extended to include further well-distributed stations, in order to obtain the polar motion parameters with the same latency and provide an up-to-date complete set of Earth orientation parameters for navigation of space and satellite missions.
Very Long Baseline Interferometry (VLBI) is the only space geodetic technique which allows a determination of all components of Earth rotation. The daily Earth rotation phase UT1 is the most variable quantity which is only partly predictable due to its complicated physical nature. Since the early 1980s, routine experiments have been carried out in order to determine this quantity, using a network of globally well-distributed antennas. In the recent years dedicated one-hour single baseline sessions, up to 7 times a week, have been established, with the goal of providing estimates of UT1 with much lower latency. Although the turn-around time of these so-called Intensive experiments has been improved greatly, there are still bottlenecks in the processing chain which prevent access to UT1 within a few minutes after the last scan has been observed. Thus dedicated ultrarapid UT1 sessions were conducted in order to demonstrate that real-time determination of UT1 becomes possible when automated processing routines are applied.
2.Ultra-rapid UT1 Sessions
Sekido et al. (2008) demonstrated that the usage of highspeed internet connections allows the determination of UT1 within an hour after the last scan has been recorded. Based on this technology, Matsuzaka et al. (2008) reported the fastest determination of UT1, achieved in less than four minutes after the session was completed. Such low-latency results were not only possible because of the excellent network infrastructure but also because the geodetic analysts were transforming the correlator output into observational files and then conducted the parameter estimation. Tasks like ambiguity fixing and ionospheric correction had to be done manually, and the UT1 estimation process itself had to be started afterwards. Two different analysis packages, namely CALC/SOLVE (Baver, 2010) and OCCAM (Titov et al., 2004) were used in parallel to evaluate their usefulness for automated processing of single-baseline UT1 experiments. CALC/SOLVE is capable of resolving ambiguities via a built-in module, but requires user-interactions to carry out this step. The user has to identify the ambiguities via a graphical user interface and shift them to a common level. The OCCAM software does not have the capability to carry out ambiguity resolution, but Koyama et al. (2008) have developed a variety of scripts which overcome this drawback, by separate analysis of X- and S-band data, before fixing the ambiguities and computing the ionosphere corrections. Although this quite cumbersome solution allows partial automation of the analysis, it does not provide a straightforward way to access the correlator output nor is it capable of outputting results in a format that can be submitted directly to the International Earth Rotation and Reference Systems Service (IERS).
3. c5++ and VLBI
Otsubo and Gotoh (2002) have developed an analysis software package based on Java named CONCERTO4 which enabled the user to consistently process SLR, GPS and other satellite tracking data. Driven by the need to update the software and replace the existing Java code, VLBI was added as an additional module by this analysis package c5++. Other than single technique analysis packages, c5++ also provides state-of-the-art modules for a variety of geodetic, mathematical and geophysical tasks that can be combined to a stand-alone VLBI application. Although many of these modules can be used for any of the space geodetic techniques, a couple of technique specific solutions (like relativity, antenna deformation, etc.) had to be coded exclusively for VLBI.
4. Automated Analysis
4.1 Ambiguity resolution
4.1.1 Optimum choice of the functional model
4.2 Ionosphere correction
Once all ambiguities have been fixed, X- and S-band data can be combined and an ionosphere correction for each observation can be determined. Since the choice of the ambiguity reference is arbitrary for single baseline sessions, the ionosphere correction will be affected by this choice. Nevertheless, as this constant will later be absorbed in the clock-offset it does not harm the estimation of the target parameters.
4.3 UT1 computation
Based on the ionosphere-free X-band observations, one can estimate UT1 from the single baseline VLBI observations. Station coordinates are kept fixed to the ITRF20082 nominal values and the theoretical delays are computed in accordance with the latest IERS Conventions (McCarthy and Petit, 2004). Wet troposphere delays, a quadratic clock model (similar to the one described in Eq. (1)) as well as a UT1 offset are parametrized for the least-squares adjustment. The latter value represents an average difference between the Earth orientation’s phase as measured by VLBI and the one based on a-priori information. Thus, adding the estimated offset to an UT1 value based on a-priori information for the middle of the session, gives the final estimate of UT1 for that session.
5. Software Validation
Fully automated processing and analysis of UT1 experiments has become reality. The VLBI module of c5++ has been adopted for this purpose by adding the functionality for automated ambiguity resolution which remained as one of the large hurdles for unattended operation since this processing step usually requires human interaction by the analyst. Because the results agree well with those obtained from another software package, c5++ automated UT1 processing can be applied for routine operations like the Intensive sessions. Although with the current choice of the functional model (see Section 4.1.1) ambiguities could be resolved for all INT2 sessions without human interaction, it is still possible that in a future data-set the algorithm will not converge. Thus, c5++ will be extended with functionality to try several other approaches for the ambiguity resolution step if the suggested algorithm does not converge after a user-defined number of iterations.
Currently, the fully automated analysis scheme is tested with INT2 sessions on a semi-routine base. GSI used the c5++ solution to estimate UT1 directly after the correlation has finished and put the results on a FTP server. Additionally, the IERS has agreed to use this output in order to test their impact on the generation of daily UT1. Thus, if these new near-real time UT1 measurements are acceptable for the routine UT1 product, it is anticipated that GSI can provide their solutions based on automated processing with c5++. As suggested by Luzum and Nothnagel (2010) other Intensive sessions could also be automated, providing UT1 on a daily base in near-real time. The Intensive experiments operate with long East-West baselines that give high sensitivity for UT1 monitoring, but these sessions are insensitive to any of the wobble parameters. Adding a third station, that shares a North-South baseline with one of the two sites, as well as extending the length of the session by a few hours may give enough stability to decouple the three parameters and obtain a meaningful set of all three Earth orientation parameters. Extension of the INT2 experiments would either require a station in Southern Africa (for a NS baseline w.r.t. Wettzell) or using one of the Australian telescopes to obtain the North-South baseline with a Japanese antenna. The latter configuration would be preferable as most of the Australian VLBI sites are connected with optical fiber, which enables fast data streaming via international high-speed networks. Since for such a scenario three baselines need to be correlated, moderate upgrades at the correlation centers might be required, whereas hardly any modification in the post-processing chain are necessary. Given that such extended Intensive (eINT) experiments are operated similar to the recent ultra-rapid sessions, users would be provided with a complete and consistent set of all three Earth orientation parameters and the IERS would be able to improve their products. Moreover, experience gained from the automated processing of such session might be valuable for establishing the next generation VLBI network (VLBI2010) as described by Niell et al. (2007).
The Geospatial Information Authority of Japan (GSI) is acknowledged for carrying out the INT2 sessions and providing observational data. The authors are very grateful to Ms. Lucia Plank for enabling us to validate our software within the IVS software comparison campaign, as well as the IERS and the IVS are thanked for providing products and data. We want to thank Dr. Luzum and one anonymous reviewer for the valuable comments that led to significant improvements of our paper.
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