International VLBI Service for Geodesy and Astrometry
The International VLBI Service for Geodesy and Astrometry (IVS) regularly produces high-quality Earth orientation parameters from observing sessions employing extensive networks or individual baselines. The master schedule is designed according to the telescope days committed by the stations and by the need for dense sampling of the Earth orientation parameters (EOP). In the pre-2011 era, the network constellations with their number of telescopes participating were limited by the playback and baseline capabilities of the hardware (Mark4) correlators. This limitation was overcome by the advent of software correlators, which can now accommodate many more playback units in a flexible configuration. In this paper, we describe the current operations of the IVS with special emphasis on the quality of the polar motion results since these are the only EOP components which can be validated against independent benchmarks. The polar motion results provided by the IVS have improved continuously over the years, now providing an agreement with IGS results at the level of 20–25 \(\upmu \)as in a WRMS sense. At the end of the paper, an outlook is given for the realization of the VLBI Global Observing System.
KeywordsVLBI Polar motion Product quality VLBI Global Observing System
Since March 1999, the International VLBI Service for Geodesy and Astrometry (IVS, Schuh and Behrend 2012) has been operating a truly global infrastructure for the determination of highly precise Earth orientation parameters (EOP) and celestial and terrestrial reference frames (CRF/TRF). As it is stated in its Terms of Reference, the IVS is an international collaboration of organizations which operate or support very long baseline interferometry (VLBI) components. As such, the IVS in itself does not possess any financial assets but relies purely on the goodwill of its member organizations for developing and maintaining reliable state-of-the-art components. In this paper, we will assess the current status of the IVS using the example of its polar motion results and its path for future development. We will provide a quantitative evaluation of steps in the quality of the IVS’s products brought about by significant changes in hardware and procedures and the evolution of the number of radio telescopes and observations. At the same time, we will also point at biases between the polar motion results of the IVS and those of the International GNSS Service (IGS).
The technique of astronomical VLBI has encountered its first development steps in the mid-1960s (e.g., Matveenko et al. 1965; Broten et al. 1967; Moran et al. 1967; Bare et al. 1967) with applications to geodesy and astrometry a few years later (e.g., Cohen and Shaffer 1971; Hinteregger et al. 1972; Shapiro et al. 1974). High-precision geodetic and astrometric VLBI started in 1979 with the first observing sessions of the newly developed Mark-3 system (Clark et al. 1985). With a much wider bandwidth than before, precision of centimeters or even a few millimeters were reached on baselines of several thousand kilometers.
The technique has been engineered ever further since then, making use of growing capabilities of modern electronics, new hardware components, and enhanced analysis software including geophysical models. Efforts to exploit VLBI in a structured manner led to the foundation of the IVS in 1999 resulting in plenty of synergies in the geodetic and astrometric VLBI community as a whole. Many publications on the general concepts of VLBI and IVS have appeared, too numerous to cite only a fraction of them, and the reader should be pointed only at a few review papers by Sovers et al. (1998), Schlüter et al. (2002), or Schuh and Behrend (2012) and the many references therein.
From an operational perspective, geodetic and astrometric VLBI always was a technique which was restricted by the necessity of active observations and correlations requiring considerable human resources. For this reason, continuous around-the-clock operations were never feasible and the observations always had to be organized in dedicated sessions predominantly of 24-h duration for subsets of the radio telescopes. Responding to the need of daily monitoring of the highly variable Earth’s phase of rotation, UT1-UTC, the gaps between multi-telescope network sessions (in the early days 1–2 per week) were filled with daily single-baseline sessions of 1 h duration, so-called Intensives. The sole purpose of these short-duration sessions was the determination of UT1-UTC requiring long east–west baselines for dedicated sensitivity (Robertson et al. 1985). This concept is still in operation for the time being.
Geodetic and astrometric VLBI is unique not only for the determination of UT1-UTC but also for nutation components and for the positions of compact extra-galactic radio sources such as quasars. However, for none of these an independent validation is possible. For this reason, we will concentrate on the IVS polar motion results for proving that the developments within the IVS have indeed led to tangible improvements.
2 International VLBI Service for Geodesy and Astrometry
2.1 IVS concepts
In metrology in general and in local-scale surveying in particular, measurements are carried out under the premise of traceability, i.e., the deductibility from scales of higher accuracy. This is unfortunately not possible (yet) for measurements of global scale as is the case in space-geodetic techniques. As an alternative, the four geometric techniques VLBI, Satellite Laser Ranging (SLR), Global Navigation Satellite Systems (GNSS) and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) form an observing system of systems, which controls each other by comparisons and combination. One of the key elements of this concept are the regular computations in conjunction with the International Terrestrial Reference Frame (ITRF), which is composed of the ground markers and the coordinate results of all four techniques linked through local tie measurements (Richter el al. 2005). These computations are carried out at three special combination centers of the International Earth Rotation and Reference Systems Service (IERS) (e.g., Altamimi 2011; Seitz et al. 2012; Abbondanza et al. 2016). Through this process, remaining systematics and random errors of the individual techniques can be identified and help in their elimination. The same applies to the two polar motion components of the variable Earth rotation, which are routinely combined by the IERS Earth Orientation Product Center at the Paris Observatory (Bizouard and Gambis 2009; IERS 2015) and the IERS Rapid Service/Prediction Center (Wooden et al. 2010; IERS 2015).
In contrast to these results, VLBI and, thus, the IVS produce a few unique products which cannot be controlled by other techniques, namely UT1-UTC, the two components of the unmodeled contributions of nutation (originating from free core nutation), and the positions of compact extra-galactic radio sources forming the celestial reference frame. These can only be determined by VLBI and have to be evaluated and validated from within the IVS.
The concept of the IVS is laid down in its Terms of Reference and has been described in several publications (e.g., Schlüter et al. 2002; Schlüter and Behrend 2007; Schuh and Behrend 2012). One of its key facts is that IVS components always operate on a best-effort basis which has fostered continuous progress in technology developments mostly guided by compatibility standards. However, this also bears the danger that no long-term commitment exists by any agency for continued operations and funding of its IVS component(s). For this reason, the IVS has experienced a few withdrawals of major contributors to its operations. On the other hand, the voluntary nature of the IVS’s operations is a key factor for new components to dare a trial contribution for an undetermined period.
In contrast to this situation, the global political arena has seen a major step forward in acceptance of the needs for geodetic infrastructure. This has come into effect through the passing of the United Nations General Assembly Resolution “A global geodetic reference frame for sustainable development” on February 26, 2015. The resolution, for the first time in international political circles, recognized the “\(\ldots \) importance of and the growing demand for an accurate and stable global geodetic reference frame for the Earth \(\ldots \)” (UN 2015). The next step will be a roadmap for further development of the resolution and hopefully clear advice for member states on how to support the realization of the Global Geodetic Reference Frame (GGRF). In view of the challenges of Global Change and the need for a reliable framework for decisions, it is quite clear that a best-effort basis will not be sufficient anymore. In this context, serious commitments of the UN member states are needed, which will also have an effect on the IVS and its contributing agencies since the IVS will be an important component for the realization and maintenance of the GGRF.
2.2 Evolution of the IVS
Before the IVS was founded in 1999, the global geodetic VLBI observing activities were driven by initiatives of individual groups. Most noteworthy are the NASA Crustal Dynamics Project (CDP) in the years 1979–1991 (Smith and Baltuck 1993; Coates et al. 1985) and the International Interferometric Survey (IRIS) programme (Carter and Robertson 1986) as well as regional initiatives in Japan (e.g., Yoshino 1999) and in Europe (e.g., Campbell and Nothnagel 2000). Besides those, also smaller research and development activities evolved. Here, scientists had not only to organize the radio telescopes participating but also that they had access to correlation capacity.
The advent of the IVS led to a more structured global organization of the observing programme with an IVS Coordinating Center and multiple IVS Operations Centers. For the first time, a global master schedule fixing the participation of the observatories and allocating scheduling, correlation and first-stage analysis resources was organized for a full year. Since then the master schedule for the following year has always been available some months before the end of the preceding year. Since some of the telescopes are used primarily for astronomical applications, painstaking arrangements have to be made with astronomical organizations like the European VLBI Network (EVN) or with the telescope owners to make the best use of the observing days committed to the IVS.
Observation statistics separated by session type
The evolution of the IVS observing program can best be depicted in graphical form. For completeness, the period since the start of high-precision geodetic VLBI since the end of 1979 (Mark-3 era) is added. Starting in 2014, the number of individual group delay observations has increased considerably (Fig. 2). This is a direct consequence of the sharp increase in the average number of observations in the 24-h sessions (Fig. 3). Another noticeable fact in this plot, although in the pre-IVS era, is the steep increase in the average number of observations from 1993 to 1996. The reason is a change in the paradigm of creating observing schedules. Earlier on, the observations were selected from a list of 20–24 radio sources while in that period the list of candidates was increased to 50 and beyond. Through this, the slew times of the radio telescopes reduced and the number of observations increased.
Since the recorded bandwidth strongly affects the signal-to-noise ratio (SNR) and thus the precision of the group delay determinations, it has always been the goal in geodetic and astrometric VLBI to increase the bandwidth both in terms of the receiving system and the recording system. Between 2003 and 2007, tape recording was replaced by hard disk recording and from 2008 onward only disk units such as the Mark5s were operated by all IVS stations.
2.3 Current IVS operations
Today and certainly for many years to come, the IVS operates on the basis of the predefined annual master schedules. The key observing sessions for EOP determinations are the so-called IVS-R1 and IVS-R4 sessions, which are set for Mondays and Thursdays for a rapid determination of all five components of Earth orientation. The remaining 261 days of a year are filled irregularly with about another 70 dedicated 24-h sessions for the maintenance of the terrestrial and celestial reference frames, and daily 1-h sessions for the determination of UT1-UTC.
EOP: Two rapid turnaround sessions each week (IVS-R1 and IVS-R4), mostly with eight stations, some with nine or ten stations depending on station availability. These networks were designed with the goal of having comparable x pole and y pole results. Data bases are available no later than 15 days after each session. Daily 1-h UT1-UTC Intensive measurements for 5 days (Monday through Friday, Int1) on the baseline Wettzell (Germany) to Kokee Park (Hawaii, USA), on weekend days (Saturday and Sunday, Int2) on the baseline Wettzell (Germany) to Tsukuba (Japan), and on Monday mornings (Int3) in the middle of the 36-h gap between the Int1 and Int2 Intensive series on the network Wettzell (Germany), Ny Ålesund (Norway), and Tsukuba (Japan).
TRF: Bi-monthly global TRF sessions (IVS-T2) with 14–18 stations using all stations at least two times per year as well as regional network sessions (e.g., EURO and AUS).
CRF: Bi-monthly sessions using the Very Long Baseline Array (VLBA) and up to eight geodetic stations (RDV), plus astrometric sessions (IVS-CRF) to observe mostly southern sky sources.
Monthly R&D sessions to investigate instrumental effects, research of EOP biases from networks observing simultaneously, and study ways for technique and product improvement.
Triennial 2-week continuous VLBI observing campaigns (the last one being CONT14) to produce continuous VLBI time series and to demonstrate the best results that VLBI can offer, aiming for the highest sustained accuracy.
Although certain sessions have primary goals, such as CRF, all sessions are scheduled so that they contribute to all geodetic and astrometric products. On average, a total of about 1400 station days per year are used in around 180 geodetic sessions during the year keeping the average days per week which are covered by VLBI network sessions at 3.5. Hence, a typical observing week with the currently used S/X system can be described as depicted in Fig. 6.
At this point let us briefly mention a VLBI peculiarity. Unlike other space-geodetic techniques, VLBI needs a correlation process and, even more important, has to cope with the fact that the actual measurement process is not instantaneous. To determine a single group delay, the two radio telescopes forming a VLBI baseline have to track the extra-galactic radio source for a certain period of time, e.g., between 10 and 200 s. Depending on a number of parameters like the strength of the radio source or the sensitivity of the radio telescopes, this time period is needed to gather a sufficient level of signal from the very weak intensity of the radio source relative to the noise encountered in the whole process. Since the Earth rotates and the telescope-source geometry changes constantly, interferometry takes effect and has to be taken care of in the correlation and so-called fringe-fitting process. For more details on this, see, e.g., Whitney (2000) and Thompson et al. (2007).
The fringe-fitting process then provides a single (time dependent) group delay observable for each pair of telescopes observing simultaneously for a certain integration time. Subsequently, each group delay observable is then just a snapshot of a certain observing epoch and interferometry has served its purpose. For completeness, it should be mentioned that the fringe-fitting process also produces a phase delay and a phase delay rate for each scan, which are, however, of minor importance.
The interferometry part with correlation and fringe-fitting is a domain of only a small group of scientists working in this field and developing new routines for enhancements of the technique, needed for example for the development of the VLBI Global Observing System (VGOS, Petrachenko et al. 2012). This makes the IVS very vulnerable to loss of expertise. The majority of IVS associates working for analysis-related component deals with group, phase and phase delay rates alone, seldom being aware of the intricacies of interferometry.
3 Current quality of IVS EOP products
As Malkin (2009) stated already, the formal errors of the IVS EOP strongly depend on the volume of the polyhedron spanned by the observing network. Using the complete IVS data set from 1979 to early 2016 (Nothnagel et al. 2015), this is clearly visible in Figs. 7 and 8, where the standard deviations of x pole and y pole components are plotted against the volume of the network polyhedron. To give a hint of the scale of the network volumes it should be mentioned that the Earth has a volume of about 1000 Mm\(^3\) (cubic megameters).
The plots distinguish between four of the most important series, the IVS-R1, IVS-R4, IVS-T2 and the CONT sessions (see Sect. 2.3). While the CONT sessions always show the best performance, the R1 and R4 mainly differ by their maximum network volume. In addition, a slight advantage is discernible for the R1 sessions. Interestingly, the T2 sessions, which often form large volumes, produce slightly increased standard deviations. Most probably, the reason for this is the IVS-T2 recording bandwidth, which is restricted to 128 Mb/s due to limitations of a few less-used telescopes. In terms of median standard deviations, the CONT sessions produce 12 and 8 \(\upmu \)as, the IVS-R1 22 and 21 \(\upmu \)as, the IVS-R4 30 and 29 \(\upmu \)as and the IVS-T2 37 and 35 \(\upmu \)as for the x pole and y pole component, respectively.
Much better suited for documenting the improvements of the IVS than the standard deviations are comparisons with independent EOP determinations of equivalent accuracy of other techniques. Since the GPS polar motion results of the International GNSS Service (IGS) currently provide the best series available, they can serve as a good independent benchmark. The differences between the IVS combination results and the IGS solution are depicted in Figs. 9 and 10, comprising of all IVS sessions. Considering that quite a number of them are not so well suited for EOP determinations because of network size or special purpose, it is no wonder that the scatter appears rather large. As we will see later (Figs. 11 and 12), the scatter of the sessions designed for EOP determinations (R1 and R4) is rather below the ±250 \(\upmu \)as threshold. If we consider only this band in the difference plots, we see that from 2012 onward many more data points condense near the running median line (±35 days), which is a consequence of the larger networks.
WRMS differences of IVS Combi w.r.t. IGS at selected intervals after a period-specific bias were subtracted
x pole (\(\upmu \)as)
y pole (\(\upmu \)as)
Looking at the differences with respect to network volume (Figs. 13, 14) the larger networks again show a slightly better agreement w.r.t. IGS. The small positive bias of the y pole differences as in Fig. 12 is again rather obvious.
4 The future of the IVS
With the advent of the VGOS network (VLBI Global Observing System) and its growing number of fast telescopes a new concept of observations will be phased in based on new technology developments (Petrachenko et al. 2009, 2012). At the beginning, these observations will predominantly aim at rapid determinations of UT1-UTC with up to four 1-h sessions per day (Petrachenko et al. 2013). At a later stage and a successful period of pilot observations, continuous observations with varying network constellations are foreseen. The 24 h-7 days/week operation does not imply that each and every telescope has to operate continuously but that at any time an IVS network is in operation guaranteeing continuous availability of EOP results. With these not only UT1-UTC but also polar motion and nutation will be monitored continuously. For both, and especially for polar motion, a significant improvement in accuracy is expected making the IVS a strong competitor of the IGS for being the primary source of polar motion information.
Due to its unpredictable nature, UT1-UTC requires a daily monitoring cycle or even a higher time resolution (Luzum and Nothnagel 2010). At present, the IERS Bureau for Rapid Service and Predictions updates its data sets every 6 h (Hackman, pers. comm.). For this reason, the IVS will also endeavor to meet this update frequency as soon as a basic VGOS network is in operation. As stated above, there will be an initial 1-h session four times daily. However, since product quality improves with more observations, continuous observations will soon become the standard observing mode. In addition, block-wise observations spread over a day are not very practical in an operational sense for its frequent setup and telescope stow procedures. From a conceptual point of view, continuous observations are not strictly necessary for monitoring of the other EOP since they vary only in a rather slow manner. However, continuous observations and permanent analysis processes are necessary for a constant monitoring of neighboring and subsequent data points. This guarantees constant robustness of the results and allows for immediate reactions when mistakes may appear.
VLBI is one of the space geodesy techniques that jointly provide the most accurate terrestrial and celestial reference frames and EOP solutions for geodesy and astronomy, as well as for related sciences and manifold practical applications. However, the VLBI technique has special capabilities, which makes it the only modern highly accurate technique for establishing and maintaining the celestial reference frame, Universal Time, and movement of the Earth’s rotation axis in space (precession and nutation). Furthermore, VLBI is the only technique that can provide alone a full TRF–EOP–CRF solution (the tie to the geocenter is still to be provided independently, but VLBI observations of the Earth orbiting spacecrafts can solve this task too).
A precise celestial reference frame realized by a set of accurate positions of celestial objects is needed for numerous astronomy, navigation, time and other measurements. For millennia, the CRF was based on optical star positions. With the establishment of VLBI, much more accurate CRF realizations became available. In 1998, the International Celestial Reference Frame (ICRF) based on the positions of extra-galactic radio sources has been adopted by the International Astronomical Union (IAU) as the fundamental celestial reference frame, replacing the FK5 optical frame (Ma et al. 1998). Currently, the third ICRF version is under development to be completed in 2018 (Jacobs et al. 2014).
Monitoring of the precessional–nutational motion of the Earth’s rotation axis plays an extremely important role in refining and verifying geophysical models (e.g., Dehant et al. 2012 and papers cited therein). The VLBI EOP series became longer with time and thus the precession and long-period nutation parameters are better determined. Improvement in the VLBI EOP accuracy allows to investigate much finer details in the Earth’s variable rotation such as free core nutation (FCN), and even smaller nutation terms. Analysis of the VLBI EOP series allow to provide new insights into the Earth’s interior, such as better understanding and modeling of the processes on the core–mantle boundary and inner core boundary, core shape, impact of the processes in atmosphere and ocean on the Earth’s rotation.
A successful realization of the VLBI observing programs essentially requires both inter-institutional and international cooperation, which is realized through the International VLBI Service for Geodesy and Astrometry. For more than 17 years, the IVS has been providing high-quality Earth orientation parameters as well as celestial and terrestrial reference frames. During this time, the accuracy has improved steadily. As has been explained in the text, technical improvements, such as the recorded bandwidth or correlation capacity, as well as network extensions together with organizational optimizations and larger observing networks have led to a much better overall quality of the results.
In the paper, we concentrated on the polar motion results because these are the ones which can be validated against benchmark results from the IGS while the other EOP products are unique to VLBI. Today the agreement in both polar motion components is on the order of 20–25 \(\upmu \)as. Even though the VLBI observing sessions are non-continuous, they are the only control for GNSS-based polar motion determinations providing the long-term stabilization, which is needed in the presence of frequent system changes in GNSS analyses.
In the intermediate future, the IVS will change its EOP monitoring to the VLBI Global Observing System (VGOS) with a considerable number of dedicated telescopes. These will provide the basis for a quantum leap in accuracy but also for continuous operations of the IVS network and daily provision of all components describing the Earth’s variable rotation.
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