Intercontinental Baselines Research Articles

First Very Long Baseline Interferometry Detections at 870 μm

The first very long baseline interferometry (VLBI) detections at 870 μm wavelength (345 GHz frequency) are reported, achieving the highest diffraction-limited angular resolution yet obtained from the surface of the Earth and the highest-frequency example of the VLBI technique to date. These include strong detections for multiple sources observed on intercontinental baselines between telescopes in Chile, Hawaii, and Spain, obtained during observations in 2018 October. The longest-baseline detections approach 11 Gλ, corresponding to an angular resolution, or fringe spacing, of 19 μas. The Allan deviation of the visibility phase at 870 μm is comparable to that at 1.3 mm on the relevant integration timescales between 2 and 100 s. The detections confirm that the sensitivity and signal chain stability of stations in the Event Horizon Telescope (EHT) array are suitable for VLBI observations at 870 μm. Operation at this short wavelength, combined with anticipated enhancements of the EHT, will lead to a unique high angular resolution instrument for black hole studies, capable of resolving the event horizons of supermassive black holes in both space and time.

Evaluation of BeiDou time transfer over multiple inter-continental baselines towards UTC contribution

A road map for the implementation of the BeiDou Navigation Satellite System (BDS) time transfer in coordinated universal time (UTC) has been drawn up, including a pilot experiment to evaluate BDS time transfer on multiple baselines. In the pilot experiment, several laboratories contributing to UTC have been equipped or are planned to be equipped with BDS time and frequency transfer receivers made by the National Institute of Metrology (NIM, Beijing). In the first phase, concentrating on the evaluation for the global capacity of BDS time transfer, the experiments have been implemented on multiple inter-continental baselines, involving NIM, Bureau International des Poids et Mesures (BIPM, Sevres), other institutes in China and the Czech Republic, and stations from the International Global Navigation Satellite System (GNSS) Service (IGS) network. The satellite signal coverages at various sites were characterized for satellite number, satellite elevation and satellite azimuth. Stability and accuracy of time transfer by BDS have been evaluated, based on common clock difference and multiple inter-continental baselines experiments, concluding agreement between BDS time transfer and GPS time transfer and a time stability of less than 1 ns at some thousand seconds averaging time at the present satellite coverage of BDS.

Combining GPS and VLBI for inter-continental frequency transfer

For decades the global positioning system (GPS) has been the only space geodetic technique routinely used for inter-continental frequency transfer applications. In the past very long baseline interferometry (VLBI) has also been considered for this purpose and the method’s capabilities were studied several times. However, compared to GPS current VLBI technology only provides few observations per hour, thus limiting its potential to improve frequency comparisons. We therefore investigate the effect of combining GPS and VLBI on the observation level in order to draw the maximum benefit from the strength of each individual technique. As a test-bed for our study we use the CONT11 campaign observed in 2011. First we review the frequency transfer performance that can be achieved with independent technique-specific analyses, both with individual software packages and with the multi-technique software c5++. With this analysis approach both techniques, GPS and VLBI, show similar frequency link instabilities at the level of 10−14 to 10−15 (MDEV) on inter-continental baselines for averaging times of one day. Then we use the c5++ software for a combined analysis of GPS and VLBI data on the observation level. We demonstrate that our combination approach leads to small but consistent improvements for frequency transfer of up to 10%, in particular for averaging periods longer than 3000 s.

Higher-order ionospheric effects in GPS time and frequency transfer

In high-precision geodetic time and frequency transfer, which requires precise modeling of code and carrier phase GPS data, the ionosphere-free combinations P 3 and L 3 of the codes and carrier phases, made on the two GPS frequencies, are used to remove the first-order ionospheric effect. We quantify the impact of the residual second- and third-order ionospheric effects on geodetic time and frequency transfer solutions for continental and intercontinental baselines. All time transfer computations are done using the ATOMIUM software, developed at the Royal Observatory of Belgium. In order to avoid contamination by some imperfect modeling of the second- and third-order ionospheric effects in the satellite clock products, only single-difference, common-view processing is used, based on code and carrier phase measurements. The results are shown for weak and strong solar activity, as well as for particular epochs of ionospheric storms. Second-order ionospheric delays can lead to corrections up to about 10 ps in the common-view clock solution of intercontinental baselines with very different longitudes. However, realistic values of the geomagnetic field in the ionosphere are required to assess the amplitude of second-order ionospheric effects in time and frequency transfer during an ionospheric storm.

Probing the microarcsecond structure of the Circinus megamasers through diffractive interstellar scintillation

We have used the Long Baseline Array to observe the Circinus Galaxy H2O megamasers on two epochs separated by 49 d. The masers were observed to be highly variable, consistent with previous studies. We observe rapid variations in the linewidth and central velocity of the maser's spectral profile. The characteristics of these spectral variations suggest that the masers have an anisotropic structure, with axial ratios ≲5, and display velocity gradients in their microarcsecond structure. We also find a strong anticorrelation between the linewidth and the modulation index of the maser features and a relationship between the modulation index and characteristic time-scale of the variations. These various phenomena can all be simply explained through a model where the rapid variations in the masers are produced by quenched diffractive scintillation in the presence of modest velocity gradients in the maser emission. The very long baseline interferometry (VLBI) images are consistent with previous high-resolution observations and show maser emission arising both in a circumnuclear accretion disc, and in a low-velocity outflow. We find no systematic differences in the variability properties of the disc and outflow masers. The VLBI images reveal that there is considerable spectral blending in single-dish spectra of the Circinus megamasers. In addition to the VLBI imaging we also used these observations to look for a time delay in the intensity variations observed at the different telescopes in the array, as expected for scintillation-induced variability. No time delay was detected, which allows us to place a lower limit on the velocity of the interstellar medium during these observations 22 km s−1 for the second epoch. We conclude that any future attempts to measure a time delay in this source should utilize intercontinental baselines to increase the chances of success.

Extragalactic water masers, geometric estimation of H0, and characterization of dark energy

High precision estimation of the equation of state of dark energy depends on constraints external to analyses of Cosmic Microwave Background fluctuations. A geometric estimation of the local expansion rate, H 0, would provide the most direct and robust constraint. Traditional techniques to estimate H 0 have depended on observations of standard candles for which systematic effects can be 10% or more. Observations of water maser sources in the accretion disks that feed the central engines of active galaxies enable simplified, robust, and largely geometric analyses. Many thousand maser sources will be discovered in studies with the SKA, owing to its great sensitivity. Spectroscopic monitoring and interferometric mapping – with intercontinental baselines – will allow estimation of H 0 to ∼1% and possibly better.

Evaluation of GPS‐based ionospheric TEC map by comparing with VLBI data

Precise ionospheric total electron content (TEC) map of the Earth is useful, if it is available for calibration of ionospheric dispersive delay for space measurement techniques using microwave such as GPS, VLBI, and spacecraft navigation. Recent rapid development of GPS techniques is making it more realistic that Earth's ionosphere TEC map measured by GPS observation is practically applicable in those space measurements. For the purpose of evaluating the accuracy of the ionospheric TEC map produced from GPS measurements, two cases of TEC maps were compared with dual band VLBI TEC measurements. In one case, local TEC maps produced from observation data using TECMETERs, which are a kind of GPS receiver for TEC measurements, were compared with VLBI data. As the second case, Global Ionosphere Maps (GIMs) generated by the Center for Orbit determination in Europe (CODE) were compared with VLBI data from short baseline to intercontinental baseline. The ionospheric group delay derived from the local TEC map of the first case had about 80 % correlation with VLBI data on 109 km short baseline. Also the group delay computed by using the GIM data of the CODE (GIM/CODE) had about 90 % correlation with VLBI data on that baseline. In comparisons on intercontinental baselines, correlations between GIM/CODE data and TEC measured by VLBI indicated almost unity. Then it was found that more than 90 % of ionospheric TEC could be predictable with that TEC map. Through further statistical analysis of TEC comparison data, the error spectrum of GIM/CODE data was computed. Comparing the obtained error spectrum with error of the spherical harmonics component of the GIM/CODE data, the latter was always smaller than the former, and the error of GIM/CODE data is suspected to be underestimated, especially at low spatial frequency. It was inferred from the spectrum that more than 0.8 TECU of ionosphere perturbations remain in the higher spatial frequency region, which is not covered by the GIM/CODE model. Total accuracy of GIM/CODE data was evaluated around 3.7 – 3.9 TECU. Also phase delay rates derived from the GIM/CODE were compared with VLBI data. It indicated correlation around 0.6 – 0.8 on intercontinental baseline, but it is not enough accuracy for practical use in phase delay rate correction in VLBI observation. The reason for low coincidence is understood by the lack of small scale and short timescale TEC variation information in that TEC map model.

The Influence of Propagation through the Irregular Interstellar Plasma on VLBI Observations

It has been known since pulsars were discovered that the interstellar medium (ISM) can cause angular broadening of radio sources. However, the amount of scattering along typical lines of sight to extra-galactic sources is not large enough to cause any loss of visibility, even on intercontinental baselines, unless the frequency is below about 100 MHz. Recent studies have shown that interstellar scattering (ISS) can have considerably stronger effects; this paper addresses their relevance to VLBI.

NASA/Crustal Dynamics Project Results: Tectonic Plate Motion Measurements with Mark-III VLBI

The NASA Crustal Dynamics Project (CDP) has been using VLBI on intercontinental baselines to measure tectonic plate motions since 1979. We report on measurements between sites on the North American plate (Haystack/Westford, MA; Owens Valley and Mojave, CA; Ft. Davis, TX and Gilmore Creek, AK), the Eurasian plate (Onsala, Sweden; Wettzell, West Germany, and Shanghai, China), the Pacific plate (Kauai, HI; Kwajalein in the Marshall Islands, and Vandenberg AFB, CA), the African plate (Hartebesthoek, RSA), and Japan (Kashima).

Effects of Source Structure on Astrometry and Geodesy

VLBI hybrid maps of a number of radio sources have been used to make calculations of the effects of source structure on group delay and phase delay for baselines of different lengths. The effects on the two types of delay are of comparable magnitude and have a slightly stronger than linear dependence on baseline length. For sources selected to have only modest structure, the r.m.s. delay error caused by structure at 8.4 GHz is on the order of 30 to 50 picoseconds for intercontinental baselines.

Using delta differential one-way range to determine highly elliptical earth orbits

Acquisition strategies for delta differential one-way range measurements are designed for determining highly elliptical Earth orbits. The mission orbit for the U.S. Charged Composition Explorer spacecraft of the international Active Magnetospheric Particle Tracer Explorers serves to illustrate these acquisition techniques. The study assumes intercontinental baselines formed by Deep Space Network antennas at Goldstone, Canberra, and Madrid. The best performing strategy employs data spanning baseline view periods covering both inbound and outbound legs near the same orbit periapse. Rapidly changing viewing geometry yields both angular position and velocity information but frequently imposes the need for a different reference quasar for each observation. Although limited by an S-band transponder and a small bandwidth, the addition of delta differential one-way range to coherent Doppler and range improves apogee position accuracy by more than an order of magnitude. The measurement noise, highly dependent on acquisition geometry, varies several orders of magnitude across the baseline view periods primarily due to parallax. Careful selection of observation times minimizes the geometric influence on data noise. Additional measurement accuracy improvements, possible with dual frequency calibration, increased spanned bandwidth, and water vapor radiometry, are presented for comparison. Quasar position uncertainties then dominate the measurement noise.

Accurate Positions of 120 Radio Sources with Declinations Above −45°

Development of a catalogue of approximately 100 radio sources having positional accuracy of ~0.005 arcseconds is one goal of the DSN radio reference frame program. This objective is set by the navigation requirements of the Galileo Project for reconstruction of the probe entry angle into the atmosphere of Jupiter. These radio positions are determined by VLBI observations using the antennas of the NASA Deep Space Network on two intercontinental baselines – California-Spain and California-Australia. Since 1978, measurements have been made simultaneously at 2.3 and 8.4 Ghz. Sixteen to twenty-four hours of data on each of the two baselines are usually obtained within a few days of each other, and since mid-1982 these pairs of observations have been made at approximately six week intervals.

Very Long Baseline Interferometry in the Southern Hemisphere

Phase coherent interferometers with intercontinental baselines became possible because of the development of stable frequency standards. With sufficiently stable frequency standards, no connection is necessary between the two ends of an interferometer. The first VLBI experiments were conducted by a group at the University of Florida who used an intensity interferometer with independent tape recorders for observations of Jupiter. Later, they changed to a coherent system using crystal-controlled oscillators. Since then, several interferometer systems have been developed. A Canadian group developed a system using video tape recorders at each end of the interferometer. They recorded the data in analogue form and managed to bring the two tapes together and to synchronize them to an accuracy of better than a microsecond. After synchronization, the outputs were combined and fringes extracted. Their system has a bandwidth of about 4 MHz. No-one else has attempted a wide-band analogue system.

Extragalactic radio sources

More than 10 000 individual discrete sources have been detected by radio surveys made at frequencies from 10 MHz to 5 GHz. Although only a few hundred of these are reliably identified with galaxies or quasars, it is widely believed that the overwhelming majority are extragalactic. As a result of the rapid improvement in the sensitivity and the resolution of radio telescopes, the coordinates of many of the cataloged sources are being routinely measured with an accuracy of about 1" and with special effort to a few tenths of a second of arc. Their angular structure is being mapped with a resolution of a few arc seconds by means of sophisticated multielement interferometer arrays; and for the very bright sources, tape recording interferometers with continental and intercontinental baselines give a resolution three or four orders of magnitude better. The form of the radio frequency spectra is known in some detail between 38 MHz and 10 GHz for hundreds of sources, and for a smaller number from 10 MHz to 100 GHz. Yet, in spite of the many exciting new observational results, or perhaps because of them, many more problems in understanding the extragalactic sources have arisen than have been answered by the more recent observations.

Long Coherence Intercontinental Interferometry

Compact radio sources have been observed over several years at 2.3 GHz by interferometers of the NASA-JPL Deep Space Network with trans- or inter-continental baselines to study the secular behaviour of fine structure appearing in these sources. A comparison of the variation in flux density of components of diameter < 0.001 arcsec with the corresponding variation in total flux density of the source at the same frequency indicated that variations could be wholly ascribed to components of this order of magnitude.

Asymmetries in Compact Sources at 2,298 MHz

SINCE 1967, experiments have been carried out over two intercontinental baselines, Island Lagoon, near Woomera, to Goldstone, California and Island Lagoon to Johannesburg. The purpose of these experiments has been to measure the secular variations on a 0.001 arc s scale of a number of compact sources.