Geodetic Very Long Baseline Interferometry Research Articles

Mitigating the effect of source structure in geodetic VLBI by re-weighting observations using closure delays and baseline-to-jet orientation

An ideal target for geodetic very long baseline interferometry (VLBI) is a strong and point-like radio source. In reality, most celestial sources used in geodetic VLBI have spatial structure. This is as a major source of error in VLBI Global Observing System (VGOS) and also affects legacy S/X observations. Source structure causes a systematic delay, which can affect the geodetic estimates if not modelled or otherwise accounted for. In this work, we aim to mitigate its impact by extending the stochastic model used in the least-squares fitting of the VLBI group delays. We have developed a weighting scheme to re-weight the observations by parameterizing the source structure component in terms of closure delays and jet orientation relative to the observing baseline. It was implemented in the Vienna VLBI Software. To assess the performance of the extended stochastic model, we analysed the CONT17 legacy sessions and generated suitable reference solutions for comparison. The effects of re-weighting were evaluated with respect to the session fit statistics, source-wise residuals, and geodetic parameters. We find that this relatively simple noise model consistently improves the session fit by about 5% with moderate variation from session to session. The geodetic estimates are not affected to a significant level by this new weighting method. Source-wise we see improved post-fit residuals for 63 out of a total of 91 sources observed.

VLBI position variability of AGNs is inversely correlated with their photometric variability

The stability of the International Celestial Reference Frame (ICRF), realized through geodetic very long baseline interferometry (VLBI) positions of thousands of extragalactic objects, is dependent on the individual positional stability of these objects. It has been recently shown that the prevalence of offsets between the VLBI positions of ICRF objects and their Gaia optical positions, which limit the optical-radio reference frame tie, is inversely correlated with optical photometric variability, suggesting that photometrically variable objects may be more positionally stable. In this work, we determine the relationship between VLBI position stability of ICRF objects and optical-radio position offsets as well as optical photometric variability. We created multi-epoch geodetic VLBI solutions for a sample of 520 ICRF sources that have sufficient data to determine the variability in their VLBI positions over time. We compared this position variability with the fractional photometric variability provided by the Gaia extragalactic source catalog, the Gaia-ICRF optical-radio position offsets, the uncertainty-normalized position offsets, and optical $ BP-RP $ color as well as with possible confounders such as optical magnitude, VLBI/Gaia position error, and redshift. We determined the relationship between VLBI position stability and gamma -ray detection by the Fermi Large Area Telescope (LAT), and we determined how the VLBI position and optical flux variabilities correlate with the spectral classification of our sample, considering flat spectrum radio quasars (FSRQs), quasi-stellar objects, BL Lacs, Seyfert, and gigahertz-peaked spectrum radio sources or compact-steep-spectrum radio sources. We found that VLBI astrometric variability is (i) negatively correlated with optical flux variability, (ii) positively correlated with optical-radio offsets, (iii) negatively correlated with optical color index BP-RP, and (iv) negatively correlated with gamma -ray detection. We also found that the most positionally stable sources are among the FSRQ and BL Lac classes. In other words, redder, photometrically variable sources have the most stable VLBI positions, the smallest optical-radio position offsets, and the highest rate of gamma -ray detection, and these sources tend to be spectrally classified as blazars. Our results are consistent with the most positionally stable sources being blazars, a class of object in which the jet is oriented close to the line of sight and where relativistic beaming increases photometric variability and minimizes the projected offset between the optical and radio positions. Our study should therefore orient future geodetic VLBI observing programs preferentially toward sources with high photometric variability because these sources are predicted to have better VLBI position stabilities and smaller optical-radio position offsets, improving the stability of the celestial reference frame axes.

VLBI celestial and terrestrial reference frames VIE2022b

Context. We present the computation of global reference frames from very long baseline interferometry (VLBI) observations at the Vienna International VLBI Service for Geodesy and Astrometry (IVS) Analysis Center (VIE) in detail. We focus on the celestial and terrestrial frames from our two latest solutions VIE2020 and VIE2022b. Aims. The current international celestial and terrestrial reference frames, ICRF3 and ITRF2020, include VLBI observations until March 2018 (at the standard geodetic and astrometric radio frequencies 2.3 and 8.4 GHz) and December 2020, respectively. We provide terrestrial and celestial reference frames including VLBI sessions until June 2022 organized by the IVS. Methods. Vienna terrestrial and celestial reference frames are computed in a common least squares adjustment of geodetic and astro-metric VLBI observations with the Vienna VLBI and Satellite Software (VieVS). Results. We provide high-precision celestial and terrestrial reference frames computed from 24 h IVS observing sessions. Our latest celestial reference frame solution VIE2022b-sx provides positions of 5407 radio sources at the frequency of 8.4 GHz. In particular, the positions of sources with few observations at the time of the ICRF3 calculation are improved. The frame also includes positions of 870 radio sources not included in ICRF3. The additional observations beyond the data used for ITRF2020 provide a more reliable estimation of positions and linear velocities of newly established VLBI Global Observing System (VGOS) telescopes.

Hartebeesthoek Radio Astronomy Observatory (HartRAO) antenna axis offset determined by geodetic VLBI analysis and ground survey

In the Very Long Baseline Interferometry (VLBI) space geodetic technique, various station-specific error sources corrupt the observable VLBI delay. An antenna axis offset (AO) model is applied in the VLBI data analysis for antennas with non-intersecting rotational axes, such as the 26-m and 15-m antennas for the Hartebeesthoek Radio Astronomy Observatory (HartRAO). The a priori AO values recommended by the International VLBI Service for Geodesy and Astrometry (IVS) for use in geodetic VLBI data analysis are taken, where possible, from values measured in ground surveys. The a priori AO values used for the HartRAO antennas in geodetic VLBI analysis have been identified as possible sources of error. The a priori AO value of 6695.3 mm for the 26-m antenna originates from a 2003 co-locational ground survey, conducted before a major bearing repair in 2008, which could have changed the AO. The a priori AO value of 1495.0 mm for the 15-m antenna was determined in 2007 in only a preliminary GPS survey. In this study, the respective AO values of the HartRAO 26-m and 15-m antennas were estimated from a VLBI analysis using the Vienna VLBI and Satellite Software (VieVS) and compared with measurements from co-locational ground surveys. It was found that the VLBI estimated values do not agree within the formal margins of error with the ground survey values, in that they differ by up to eight millimetres (8 mm) for the 26-m antenna and up to five millimetres (5 mm) for the 15-m antenna. As the ground survey values are considered to be more accurate than the VLBI estimated values, a further investigation of the site-specific error sources that may be contaminating the accuracy of VLBI results is required.

Bridging astronomical, astrometric and geodetic scheduling for VGOS

Very Long Baseline Interferometry (VLBI) experiments are organized within the geodetic, astrometric and astronomical communities for different applications, requiring different observation strategies adopted in scheduling. Currently, the next-generation geodetic and astrometric VLBI Global Observing System (VGOS) is being established. Over the last years, evidence was presented that the delays introduced by the angular structure of the geodetic radio sources contribute significantly to the VGOS observable error budget. Consequently, correcting these structure delays through imaging will play an important part in the future, requiring a different scheduling approach. Within this work, a new source-centric VLBI scheduling approach is presented for improved imaging capabilities of geodetic observations. The algorithm is tested for a seven- and nine-station network and compared with classical geodetic schedules. Monte Carlo simulations are utilized to determine the expected geodetic and astrometric parameter precision, and two independent processing pipelines are used to assess the potential for astronomical source imaging. Based on the simulation results, it is revealed that with the new scheduling approach twice as many sources can be properly imaged. Furthermore, the precision of the Earth orientation parameter estimates is improved on average by {15}{%}, while the source position coordinate estimates are on average improved by {50}{%}. Tests with two VGOS networks of twelve and 29 antennas further reveal that the scheduling approach is also applicable to future VGOS networks.

Considerations of VLBI transmitters on Galileo satellites

For directly linking the dynamical reference frame of satellite orbits to the quasi-inertial reference frame of extra-galactic radio sources, observations of satellites with the Very Long Baseline Interferometry (VLBI) technique are the only conceivable method. Hence, the satellite observations should be embedded in VLBI network sessions during which also natural radio sources are observed. For this reason, it would be most practical if the artificial signal generated at the satellite for VLBI observations covers the same frequency bands as regularly observed by VLBI radio telescopes and should have a similar flux density across the observed bandwidth as these natural sources. The use of satellites of Global Navigation Satellite Systems (GNSS) such as the Galileo system is advisable because they are well monitored in terms of precise orbit determination and the altitude allows common visibilities of many VLBI telescopes. So far, signal generation on a GNSS satellite dedicated to VLBI observations has not been realized yet, partly because suitable signal generation equipment has not been considered in depth. In addition, many aspects, such as legal implications and technical complications, have not yet been addressed. In this publication, we compiled various aspects of generating an artificial VLBI signal on a GNSS satellite. We describe the legal and technical aspects of generating and emitting an artificial signal on a Galileo satellite suitable for VLBI observations including a design study for the necessary equipment on the satellite. Since geodetic VLBI is currently in a transition period from traditional observations at S and X band to the broadband VLBI Global Observing System (VGOS), the proposed equipment generates a signal suitable for both frequency setups. We have also considered the restrictions for installation on a satellite, such as power consumption, weight, and size. The equipment mainly consists of three devices: noise source, amplifier, and antenna. A diode is used as the noise source. This noise is amplified by a set of low noise amplifiers and then radiated by a spiral antenna. The diode and the amplifiers were chosen from the market, but the antenna was newly designed and simulated. The output signal of this chain was tested using a VLBI baseband data simulator, then correlated and fringe-fitted for validation. The instrumentation proposed here is easy to be constructed, but will still have to be tested in the laboratory together with the instruments on the actual satellite.

Short-baseline interferometry local-tie experiments at the Onsala Space Observatory

We present results from observation, correlation and analysis of interferometric measurements between the three geodetic very long baseline interferometry (VLBI) stations at the Onsala Space Observatory. In total, 25 sessions were observed in 2019 and 2020, most of them 24 h long, all using X band only. These involved the legacy VLBI station ONSALA60 and the Onsala twin telescopes, ONSA13NE and ONSA13SW, two broadband stations for the next-generation geodetic VLBI global observing system (VGOS). We used two analysis packages: nu Solve to pre-process the data and solve ambiguities, and ASCOT to solve for station positions, including modelling gravitational deformation of the radio telescopes and other significant effects. We obtained weighted root mean square post-fit residuals for each session on the order of 10–15 ps using group-delays and 2–5 ps using phase-delays. The best performance was achieved on the (rather short) baseline between the VGOS stations. As the main result of this work, we determined the coordinates of the Onsala twin telescopes in VTRF2020b with sub-millimetre precision. This new set of coordinates should be used from now on for scheduling, correlation, as a priori for data analyses, and for comparison with classical local-tie techniques. Finally, we find that positions estimated from phase-delays are offset sim +3 mm in the up-component with respect to group-delays. Additional modelling of (elevation dependent) effects may contribute to the future understanding of this offset.

Correcting for site displacements at different levels of the Gauss-Markov model – A case study for geodetic VLBI

In a previous paper, Glomsda et al. (2020) revisited the impact of distinct parts of non-tidal loading in the analysis of geodetic Very Long Baseline Interferometry (VLBI). The loading is represented by displacements of the reference positions of the observing VLBI antennas, which are variables of a corresponding Gauss-Markov model for estimating various geodetic target parameters. These displacements were applied at two different levels of the model, the observation and the normal equation level, and quite similar results were obtained for both cases. In this paper, the authors provide a more detailed theoretical discussion of the application of site displacements at the distinct levels, which also comprises the a posteriori application at the solution level. For each case, the respective formulas and implications for the Gauss-Markov model are derived, and equations for assessing the differences between the estimated parameters are established. In this way, the authors aim to create a deeper understanding of the results of the previous paper, which show the capability of the normal equation level to approximate the application of site displacements at the observation level with less effort and prerequisites, and to provide evidence for the claims made in that paper (for VLBI only): (i) the chosen reference frame of the site displacements is basically irrelevant except for the solution level; (ii) the Jacobi matrix does not change substantially; (iii) the loss of temporal resolution of the site displacements is more important than the linear approximation of the functional model at the normal equation level; and (iv) the station coordinate estimates for all three levels strongly depend on the regularizing (datum-) conditions of the model. The theoretical results are substantiated with numerical examples, which consider site displacements generated from non-tidal atmospheric loading by the Earth-System-Modelling group of the Deutsches GeoForschungsZentrum (GFZ). However, the results are valid for site displacements of any source.

Characterizing the astrometric instability of extragalactic radio source positions measured with geodetic VLBI

Context. Geodetic very long baseline interferometry (VLBI) has been used to observe extragalactic radio sources for more than 40 yr. The absolute source positions derived from the VLBI measurements serve as a basis to define the International Celestial Reference Frame (ICRF). Despite being located at cosmological distances, an increasing number of these sources are found to show position instabilities, as revealed by the accumulation of VLBI data over the years. Aims. We investigate how to characterize the astrometric source position variations, as measured with geodetic VLBI data, in order to determine whether these variations occur along random or preferential directions. The sample of sources used for this purpose is made up of the 215 most observed ICRF sources. Methods. Based on the geodetic VLBI data set, we derived source coordinate time series to map the apparent trajectory drawn by the successively measured positions of each source in the plane of the sky. We then converted the coordinate time series into a set of vectors and used the direction of these vectors to calculate a probability density function (PDF) for the direction of variation of the source position. For each source, a model that matches the PDF and that comprises the smallest number of Gaussian components possible was further adjusted. The resulting components then identify the preferred directions of variation for the source position. Results. We found that more than one-half of the sources (56%) in our sample may be characterized by at least one preferred direction. Among these, about three-quarters are characterized by a unique direction, while the remaining sources show multiple preferred directions. The analysis of the distribution of these directions reveals an excess along the declination axis that is attributed to a VLBI network effect. Whether single or multiple, the identified preferred directions are likely due to source-intrinsic physical phenomena.

VieRDS: A Software to Simulate Raw Telescope Data for very Long Baseline Interferometry

The digital data produced by each telescope within the observation process of Very Long Baseline Interferometry (VLBI) can be referred to as raw data. The raw telescope data represents the initial data stage (Level 0 data) of the VLBI processing chain, consisting of correlation, fringe-fitting, and geodetic/astrometric parameter estimation. Any systematic effects which are compensated and used for parameter estimation within this chain, are present in the raw telescope data streams. The group delay, although the primary geodetic VLBI observable, can be considered to be the most prominent effect. In this publication, we present a new software package implemented in MATLAB that can simulate raw telescope data for VLBI. The software is called VieRDS (https://github.com/TUW-VieVS/VieRDS) and features tools to simulate a delay, a delay rate, a phase offset, and a frequency response of arbitrary magnitude for each channel of the telescope data stream. The signal model consists of a radio source component of selectable flux density, a system noise component, and a phase calibration signal. Furthermore, a tool for one and two bit quantization and a module to store the simulated data in VDIF data format is implemented. For the first time, the simulation of a dispersive group delay for a VGOS broadband frequency setup, and the simulation of the characteristic station frequency response of two VGOS telescopes are shown for demonstration purposes. The simulated data can be correlated and fringe-fitted by commonly used VLBI software correlators and fringe-fitting programs. VieRDS is thus an ideal tool for testing hypotheses of adverse observational effects in a controlled environment by deliberately causing these effects and studying the responses in the correlator output. In this article, we focus on the description of the key characteristic of the simulation concept, the digital signal creation, the digital signal processing algorithms, and the software architecture.

Geodetic VLBI Experiments Using Two NTSC Stations

Asian Pacific Space Geodynamics (APSG) geodetic Very Long Baseline Interfero-metry (VLBI) observing sessions are being regularly performed to improve the accuracy of the geocentric coordinates of the participating stations in the Asia-Pacific region. Two small antennas, located at Jilin and Sanya, have been built and operated by the National Time Service Center (NTSC). They have a wide frequency coverage of 1.2-9.0 GHz and no standard geodetic recording mode available. We conducted mixed-mode observations with participation of those two NTSC stations in APSG40 and APSG46 experiments. The observational data have been correlated and analysed at Shanghai. For the first time we derived the coordinates of JILIN and SANYA at 1 cm level by means of geodetic VLBI technique. The difference of coordinates of JILIN station between two APSG experiments is about 2 cm if the comparison is made on the same epoch of observations. Here we present the experiment setup, data processing method and analysis results.

Common-clock very long baseline interferometry using a coherent optical fiber link

Among the most powerful techniques for the exploration of the Universe is very long baseline interferometry (VLBI), which is based on the simultaneous observation of radio sources in the sky with arrays of distant ground-based antennas. One of the effects currently limiting its ultimate sensitivity is the phase-instability of the reference clocks adopted at each antenna. This term can be made negligible delivering the same clock signal to multiple telescope sites using optical fibers. We realized such an infrastructure by disseminating a coherent optical frequency signal to two distant radio telescopes using a 1739-km-long fiber. We performed a 24 h geodetic VLBI campaign in which the same clock reference was used at both telescopes and analyzed it using standard VLBI procedures. The results were consistent with the expectations, confirming that the proposed approach is feasible and configures as a novel tool for studying the role of clocks, troposphere, and systematic effects in the ultimate VLBI resolution.

Geodetic VLBI for precise orbit determination of Earth satellites: a simulation study

Recent efforts of tracking low Earth orbit and medium Earth orbit (MEO) satellites using geodetic very long baseline interferometry (VLBI) raise questions on the potential of this novel observation concept for space geodesy. Therefore, we carry out extensive Monte Carlo simulations in order to investigate the feasibility of geodetic VLBI for precise orbit determination (POD) of MEO satellites and assess the impact of quality and quantity of satellite observations on the derived geodetic parameters. The MEO satellites are represented in our study by LAGEOS-1/-2 and a set of Galileo satellites. The concept is studied on the basis of 3-day solutions in which satellite observations are included into real schedules of the continuous geodetic VLBI campaign 2017 (CONT17) as well as simulated schedules concerning the next-generation VLBI system, known as the VLBI Global Observing System (VGOS). Our results indicate that geodetic VLBI can perform on a comparable level as other space-geodetic techniques concerning POD of MEO satellites. For an assumed satellite observation precision better than 14.1 mm (47 ps), an average 3D orbit precision of 2.0 cm and 6.3 cm is found for schedules including LAGEOS-1/-2 and Galileo satellites, respectively. Moreover, geocenter offsets, which were so far out of scope for the geodetic VLBI analysis, are close to the detection limit for the simulations concerning VGOS observations of Galileo satellites, with the potential to further enhance the results. Concerning the estimated satellite orbits, VGOS leads to an average precision improvement of 80% with respect to legacy VLBI. In absolute terms and for satellite observation precision of 14.1 mm (47 ps), this corresponds to an average value of 17 mm and 7 mm concerning the 3D orbit scatter and precision of geocenter components, respectively. As shown in this study, a poor satellite geometry can degrade the derived Earth rotation parameters and VLBI station positions, compared to the quasar-only reference schedules. Therefore, careful scheduling of both quasar and satellite observations should be performed in order to fully benefit from this novel observation concept.

Resolving VLBI correlator ambiguity in the time delay model improves precision of geodetic measurements

Abstract The modern Very Long Baseline Interferometry (VLBI) relativistic delay model, as documented in the IERS Conventions, refers to the time epoch when the signal passes one of two stations of an interferometer baseline (selected arbitrarily from the pair of stations and called the ‘reference station’ or ‘station 1’). This model consists of the previous correlation procedure used before the year 2002. However, since 2002 a new correlation procedure that produces the VLBI group delays referring to the time epoch of signal passage at the geocenter has been used. A corresponding correction to the conventional VLBI model delay has to be introduced. However, this correction has not been thoroughly presented in peer reviewed journals, and different approaches are used at the correlators to calculate the final group delays officially published in the IVS database. This may cause an inconsistency up to 6 ps for ground-based VLBI experiments between the group delay obtained by the correlator and the geometrical model delay from the IERS Conventions used in data analysis software. Moreover, a miscalculation of the signal arrival moment to the ‘reference station’ could result in a larger modelling error (up to 50 ps). The paper presents the justification of the correction due to transition between two epochs elaborated from the Lorentz transformation and the approach to model the uncertainty of the calculation of the signal arrival moment. Both changes are particularly essential for upcoming broadband technology geodetic VLBI observations.

Gravitational deformation of ring-focus antennas for VGOS: first investigations at the Onsala twin telescopes project

The receiving properties of radio telescopes used in geodetic and astrometric very long baseline interferometry (VLBI) depend on the surface quality and stability of the main reflector. Deformations of the main reflector as well as changes in the sub-reflector position affect the geometrical ray path length significantly. The deformation pattern and its impact on the VLBI results of conventional radio telescopes have been studied by several research groups using holography, laser tracker, close-range photogrammetry and laser scanner methods. Signal path variations (SPV) of up to 1 cm were reported, which cause, when unaccounted for, systematic biases of the estimated vertical positions of the radio telescopes in the geodetic VLBI analysis and potentially even affect the estimated scale of derived global geodetic reference frames. As a result of the realization of the VLBI 2010 agenda, the geodetic VLBI network is currently extended by several new radio telescopes, which are of a more compact and stiffer design and are able to move faster than conventional radio telescopes. These new telescopes will form the backbone of the next generation geodetic VLBI system, often referred to as VGOS (VLBI Global Observing System). In this investigation, for the first time the deformation pattern of this new generation of radio telescopes for VGOS is studied. ONSA13NE, one of the Onsala twin telescopes at the Onsala Space Observatory, was observed in several elevation angles using close-range photogrammetry. In general, these methods require a crane for preparing the reflector as well as for the data collection. To reduce the observation time and the technical effort during the measurement process, an unmanned aircraft system (UAS) was used for the first time. Using this system, the measurement campaign per elevation angle took less than 30 min. The collected data were used to model the geometrical ray path and its variations. Depending on the distance from the optical axis, the ray path length varies in a range of about pm ,1,hbox {mm}. To combine the ray path variations, an illumination function was introduced as weighting function. The resulting total SPV is about - ,0.5 mm. A simple elevation-dependent SPV model is presented that can easily be used and implemented in VLBI data analysis software packages to correct for gravitational deformation in VGOS radio telescopes. The uncertainty is almost 200,upmu hbox {m} (2sigma ) and is derived by Monte Carlo simulations applied to the entire analysis process.

The correlation process in Very Long Baseline Interferometry

The correlation process is an important element of geodetic Very Long Baseline Interferometry (VLBI). For many decades there has been constant progress in the capabilities of geodetic VLBI correlators following technical development and the need for ever higher precision of the results. In this publication, we describe the concept and some of the important practical aspects of VLBI correlation for a better understanding of the general geodetic readership. Time-lag correlators provide the easiest approach to the concept and to the individual steps necessary for the operations. More modern approaches then use the path where the data streams are Fourier transformed first before the cross correlation is actually performed. These correlators are called FX correlators, in contrast to XF correlators, where the cross-multiplications are done before the Fourier transform. Both concepts lead to the same results, the cross power spectrum. Following the correlation process proper, the VLBI observables, time delay and delay rate, have to be found in a fringe fitting process.

Structure Effects for 3417 Celestial Reference Frame Radio Sources

Abstract Geodetic/astrometric very long baseline interferometry (VLBI) has been routinely observing using various global networks for 40 yr, and it has produced more than 10 million baseline group delay, phase, and amplitude observables. These group delay observables are analyzed worldwide for geodetic and astrometric applications, for instance, to create the International Celestial Reference Frame (ICRF). The phase and amplitude observables are used in this paper, by means of closure analysis, to study intrinsic source structures and their evolution over time. The closure amplitude rms, CARMS, indicating how far away a source is from being compact in terms of morphology, is calculated for each individual source. The overall structure-effect magnitudes for 3417 ICRF radio sources are quantified. CARMS values larger than 0.3 suggest significant source structures and those larger than 0.4 indicate very extended source structures. The 30 most frequently observed sources, which constitute 40% of current geodetic VLBI observables, are studied in detail. The quality of ICRF sources for astrometry is evaluated by examining the CARMS values. It is confirmed that sources with CARMS values larger than 0.30 can contribute residual errors of about 15 ps to geodetic VLBI data analysis and those with the CARMS values larger than 0.4 generally can contribute more than 20 ps. We recommend CARMS values as an indicator of the astrometric quality for the ICRF sources and the continuous monitoring of the ICRF sources to update CARMS values with new VLBI observations as they become available.

Fringe fitting and group delay determination for geodetic VLBI observations of DOR tones

Extracting the group and phase delays of interferometric observations produced in the Very Long Baseline Interferometry (VLBI) measurement concept requires a special fringe fitting and delay search algorithm for the recorded bandwidth. While fringe fitting is in use routinely for several megahertz wide channels in geodetic and astrometric VLBI with quasar observations, fringe fitting for artificial tones of very small bandwidth of artificial signals for Differential One-way Ranging (DOR) requires a different way of handling. In a project called Observing the Chang’E-3 Lander with VLBI (OCEL), the DOR tones emitted by the Chang’E-3 lander were observed in a standard geodetic VLBI mode with 8 or 4 MHz wide channels to maintain compatibility with the corresponding quasar observations. For these observations, we modified the existing fringe fitting program of the Haystack Observatory Processing Software (HOPS), fourfit, to properly handle narrow band DOR tones. The main motivations are that through this modification, the data of quasars and artificial radio sources can be processed in the existing geodetic analysis pipeline, and that the algorithm can be used for similar projects as well. In this paper, we describe the algorithm and show that the new algorithm produces much more reliable group delay results than using the standard fourfit algorithm. This is done by a simulation test and in particular by processing of real observations. It is shown that in many cases, systematic deviations of several nanoseconds, which are seen with the standard fourfit algorithm, can be avoided. The ultimate benefit of the new procedure is demonstrated by reducing the errors in delay triangle closures by at least a factor of 3, which, in the OCEL case, is from ∼300 to ∼100 ps.

Source Structure and Measurement Noise Are as Important as All Other Residual Sources in Geodetic VLBI Combined

AbstractThe celestial sources observed by geodetic very‐long‐baseline interferometry (VLBI), most of which are quasars, are resolved—they have a finite angular size as measured by the interferometer—yet modern geodetic VLBI analysis techniques still treat these sources as if they are point‐like. The errors introduced by source structure, the angular distribution of emission, have traditionally been believed to be a small fraction of the total measurement error budget. We analyzed the VLBI data of the CONT14 continuous observing campaign using standard geodetic and imaging techniques, along with an analysis of closures, combinations of interferometric measurements where the error contributions from station‐based effects such as tropospheric and ionospheric errors, clock and cable length errors, station position errors, and so on, cancel exactly, leaving only nonclosing errors, principally source structure and measurement noise. Our statistical analysis of the closure information, based on actual VLBI observations revealed that nearly half (40%) of the total geodetic VLBI error budget (in terms of variance) in CONT14 came from source‐structure effects, nearly the same variance as all station‐based error sources (43%). The remaining error due to other nonsource‐structure, nonclosing errors is also significant (17%). Previous studies, which were primarily simulation‐based, did not account for the full range of effects of source structure on real measurements. Source structure is a major contributor to errors in geodetic VLBI, and source structure must be taken into account in the entire VLBI operational chain, from scheduling to analysis, in order to mitigate its effects on astrometric and geodetic measurements.

Observing APOD with the AuScope VLBI Array.

The possibility to observe satellites with the geodetic Very Long Baseline Interferometry (VLBI) technique is vividly discussed in the geodetic community, particularly with regard to future co-location satellite missions. The Chinese APOD-A nano satellite can be considered as a first prototype—suitable for practical observation tests—combining the techniques Satellite Laser Ranging (SLR), Global Navigation Satellite Systems (GNSS) and VLBI on a single platform in a Low Earth Orbit (LEO). Unfortunately, it has hardly been observed by VLBI, so major studies towards actual frame ties could not be performed. The main reason for the lack of observations was that VLBI observations of satellites are non-standard, and suitable observing strategies were not in place for this mission. This work now presents the first serious attempt to observe the satellite with a VLBI network over multiple passes. We introduce a series of experiments with the AuScope geodetic VLBI array which were carried out in November 2016, and describe all steps integrated in the established process chain: the experiment design and observation planning, the antenna tracking and control scheme, correlation and derivation of baseline-delays, and the data analysis yielding delay residuals on the level of 10 ns. The developed procedure chain can now serve as reference for future experiments, hopefully enabling the global VLBI network to be prepared for the next co-location satellite mission.