North-south Baseline Research Articles

Optimal VLBI baseline geometry for UT1-UTC Intensive observations

One of the main tasks of Very Long Baseline Interferometry (VLBI) is the rapid determination of the highly variable Earth’s rotation expressed through the difference between Universal Time UT1 and Coordinated Universal Time UTC (dUT1). For this reason, dedicated one hour, single baseline sessions, called “Intensives”, are observed on a daily basis. Thus far, the optimal geometry of Intensive sessions was understood to include a long east–west extension of the baseline to ensure a dUT1 estimation with highest accuracy. In this publication, we prove that long east–west baselines are the best choice only for certain lengths and orientations. In this respect, optimal orientations may even require significant inclination of the baseline with respect to the equatorial plane. The basis of these findings is a simulation study with subsequent investigations in the partial derivatives of the observed group delays tau with respect to dUT1 partial tau /partial dUT1. Almost 3000 baselines between artificial stations located on a regular 10 times 10 degree grid are investigated to derive a global and generally valid picture about the best length and orientation of Intensive baselines. Our results reveal that especially equatorial baselines or baselines with a center close to the equatorial plane are not suited for Intensives although they provide a good east–west extension. This is explained by the narrow right ascension band of visible sources and the resulting lack of variety in the partial derivatives. Moreover, it is shown that north–south baselines are also capable of determining dUT1 with reasonable accuracy, given that the baseline orientation is significantly different from the Earth rotation axis. However, great care must be taken to provide accurate polar motion a priori information for these baselines. Finally, we provide a better metric to assess the suitability of Intensive baselines based on the effective spread of partial tau /partial dUT1.

A strategic independent VLBI network for Europe to accurately determine Earth Orientation Parameters needed for precision positioning and navigation on Earth and in space

Earth Orientation Parameters (EOP) are required to link the International Celestial Reference Frame (ICRF) to the International Terrestrial Reference Frame (ITRF). The ITRF is required for modern mapping (cadastral surveying, topographical, geological and hydrographical) and is used in all activities where precise positioning in a modern reference frame is required; this includes precise satellite orbit determination and high accuracy navigational requirements. These EOP cannot be modelled to high accuracy, due to the complicated and stochastic nature of global mass transport on Earth, and therefore must be measured using the space geodetic technique of Very Long Baseline Interferometry (VLBI). In a strategic sense, the USA, China, the Russian Federation and the European Union (EU) have recognised the need to develop independent Global Navigation Satellite Systems (GNSS) constellations, resulting in the GPS, Beidou, Glonass and Galileo constellations, respectively. Of these, only the EU does not have the landmass to enable VLBI networks with sufficiently long North-South and East-West baselines to allow EOP measurements with high accuracy. Here we report on our results to create a strategic EOP network for the EU, based on VLBI stations in Europe (Wettzell) and South Africa (HartRAO), and German owned stations in South America (Chile) and Antarctica (O’Higgins). The Vienna VLBI Software (VieVS) is used to analyse the strategic network. We find that these long baselines will provide independent EOP of sufficiently high accuracy. It is essential that the German VLBI station in South America, which has now moved from Chile to Argentina, be equipped with a modern VLBI antenna meeting Global Geodetic Observing System (GGOS) standards; this will ensure also providing the EU with capacity to maintain satellite orbits and be non-reliant on other superpowers for advanced navigational requirements.

Two-dimensional homography-based correction of positional errors in widefield MRT images

A steradian of the southern sky has been imaged at 151.5 MHz using the Mauritius Radio Telescope (MRT). These images show systematics in positional errors of sources when compared to source positions in the Molonglo Reference Catalogue (MRC). We have applied two-dimensional homography to correct for systematic positional errors in the image domain and thereby avoid re-processing the visibility data. Positions of bright (above 15-{\sigma}) point sources, common to MRT catalogue and MRC, are used to set up an over-determined system to solve for the homography matrix. After correction the errors are found to be within 10% of the beamwidth for these bright sources and the systematics are eliminated from the images. This technique will be of relevance to the new generation radio telescopes where, owing to huge data rates, only images after a certain integration would be recorded as opposed to raw visibilities. It is also interesting to note how our investigations cued to possible errors in the array geometry. The analysis of positional errors of sources showed that MRT images are stretched in declination by ~1 part in 1000. This translates to a compression of the baseline scale in the visibility domain. The array geometry was re-estimated using the astrometry principle. The estimates show an error of ~1 mm/m, which results in an error of about half a wavelength at 150 MHz for a 1 km north-south baseline. The estimates also indicate that the east-west arm is inclined by an angle of ~40 arcsec to the true east-west direction.

USING MILLIMETER VLBI TO CONSTRAIN RIAF MODELS OF SAGITTARIUS A*

The recent detection of Sagittarius A* at λ = 1.3 mm on a baseline from Hawaii to Arizona demonstrates that millimeter wavelength very long baseline interferometry (VLBI) can now spatially resolve emission from the innermost accretion flow of the Galactic center region. Here, we investigate the ability of future millimeter VLBI arrays to constrain the spin and inclination of the putative black hole and the orientation of the accretion disk major axis within the context of radiatively inefficient accretion flow (RIAF) models. We examine the range of baseline visibility and closure amplitudes predicted by RIAF models to identify critical telescopes for determining the spin, inclination, and disk orientation of the Sgr A* black hole and accretion disk system. We find that baseline lengths near 3 Gλ have the greatest power to distinguish amongst RIAF model parameters, and that it will be important to include new telescopes that will form north-south baselines with a range of lengths. If an RIAF model describes the emission from Sgr A*, it is likely that the orientation of the accretion disk can be determined with the addition of a Chilean telescope to the array. Some likely disk orientations predict detectable fluxes on baselines between the continental United States and even a single 10-12 m dish in Chile. The extra information provided from closure amplitudes by a four-antenna array enhances the ability of VLBI to discriminate amongst model parameters.

The Taiwan-America Occultation Survey for Kuiper Belt Objects

AbstractThe purpose of the TAOS project is to directly measure the number of Kuiper Belt Objects (KBOs) down to the typical size of cometary nuclei (a few km). In contrast to the direct detection of reflected light from a KBO by a large telescope where its brightness falls off roughly as the fourth power of its distance to the sun, an occultation survey relies on the light from the background stars thus is much less sensitive to that distance. The probability of such occultation events is so low that we will need to conduct 100 billion measurements per year in order to detect the ten to four thousand occultation events expected. Three small (20 inch), fast (f/1.9), wide-field (3 square degrees) robotic telescopes, equipped with a 2,048 × 2,048 CCD camera, are being deployed in central Taiwan. They will automatically monitor 3,000 stars every clear night for several years and operate in a coincidence mode so that the sequence and timing of a possible occultation event can be distinguished from false alarms. More telescopes on a north-south baseline so as to measure the size of an occultating KBO may be later added into the telescope array. We also anticipate a lot of byproducts on stellar astronomy based on the large amount (10,000 giga-bytes/year) of photometry data to be generated by TAOS.

Cool Giant Stars are Bigger at 712 nm than at 754 nm

We used the Mark III Optical Interferometer on Mt. Wilson in 1991 and 1992 to measure the diameters of 42 stars with 10 nm wide filters centered at 712 nm and at 754 nm. These filters probe the stellar atmosphere in a strong TiO band (712 nm) and in a “continuum” band relatively free of TiO absorption (754 nm). The data were taken on a North-South baseline that could be configured to lengths between 3.0 m and 31.5 m. Observations of the target stars were interleaved with frequent measurements of calibrator stars. The square of the calibrated visibility amplitude V2 was determined as a function of baseline length, and a uniform disk model fitted to these data (for details see Mozurkewich et al. 1991). The average formal error of the diameter measurements - derived from the χ2 of the model fits - is 2.5%. Data for a subset of 12 stars have already been published (Quirrenbach et al. 1993).

An attempt to detect polarization effects in the envelope of gamma Cassiopeiae with the GI2T interferometer

We report on the results of an observing run of Cassiopeiae using the optical long baseline interfer- ometer Grand Interf erom etre a 2T elescopes (GI2T) in the southern France equipped with lm-sheet polarizers. Four interferometric baselines ranging from 20 to 40 m were obtained on this star during 1994 on September 5th and 6th, with Cephei as a calibrator. We aimed at detecting possible variations of the angular diameter of Cas as a function of polarization directions parallel and orthogonal to the north-south baseline of the GI2T. We do not detect any variation of the visibility as a function of Hour Angle on Cephei, neither on Cas. We nd an upper limit of 1.3 on the ratio of the angular diameters of the envelope of Cas seen through polarizers parallel and perpendicular to the North-South direction of the GI2T's baseline. We discuss how this limit could constrain density laws in the wind of this Be star. We clearly resolve Cas for increas- ing baselines in the red continuum whatever the polar- ization. Assuming the continuum flux originating from a photosphere surrounded by an elliptical gaussian envelope we estimate the extent of the latter as (2:9 0:5) mas. This diameter agrees well with the H emitting envelope as obtained by other interferometers.

VLBI Astrometry of the Millisecond Pulsar B1937+21

view Abstract Citations (9) References (13) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS VLBI Astrometry of the Millisecond Pulsar B1937+21 Dewey, Rachel J. ; Ojeda, Maria Rose ; Gwinn, Carl R. ; Jones, Dayton L. ; Davis, Michael M. Abstract We report the results of astrometric VLBI observations of PSR B1937+21. Observations of the pulsar and a nearby quasar were made at 1.67 GHz, using four antennas: Arecibo, the very large array, and the DSN 70 m antennas in California and Spain. We determine a position of α=19h39m38s.5611±0s.0003 and ρ=21°34'59".118±0.016 for the pulsar in the lERS extragalactic reference frame at epoch 1990.2. The accuracy of our result in right ascension is limited by systematic uncertainties in the ionospheric delay calibration, and larger-than-expected random scatter in the pulsar delays related to interstellar scintillation. The accuracy in declination is limited by the lack of a long north-south baseline in our experiment and the uncertainty of the location of the Arecibo antenna in the lERS celestial reference frame. When compared with the pulsar's timing position [Kaspi, et al., ApJ, 428, 713 (1994)] in the planetary ephemeris reference frame, our result yields a measurement of the offset of the lERS reference frame and the DE200 planetary ephemeris frame of Δα(IERS-DE200)=0.0008±0s0.0003 and Δρ(IERS-DE200)=-0.024±0.016, which is in reasonable agreement with the frame-tie rotation determined by Folkner et al. [ARA&A, 297, 279, (1994)]. Publication: The Astronomical Journal Pub Date: January 1996 DOI: 10.1086/117783 Bibcode: 1996AJ....111..315D Keywords: PULSARS: INDIVIDUAL: B1937+21; ASTROMETRY full text sources ADS | data products SIMBAD (3)

The 6m mm-VLBI telescope at Kagoshima, Japan

VLBI observations of millimeter wavelengths can probe the broad line emission and jet forming regions of quasars, and the scale of an accretion disk around massive black holes in nearby active galaxies, which are self-absorbed at longer wavelengths. Therefore, the extension of the mm-VLBI network is timely and urgent problem. We are now planning to move Nobeyama 6 m mm VLBI telescope to Kagoshima. The telescope will be placed at Kinkohwan park in Kagoshima City in late 1992 and will be operated in 1993. The frequencies for VLBI observations will range from 22 GHz to 100 GHz.To date, there are two mm-VLBI facilities in Japan: Nobeyama 45 m and Kashima 34 m, However, the longest baseline is only about 200km EW which provides a fringe separation of 17 mas. This is not sufficient for high resolution mapping. A 10 m telescope which is under construction at Mizusawa, 400 km north from Kashima which will be usable up to 43 GHz and will add a north-south baseline, which is very important for astrometrie measurement.

The Asymmetric Envelope of gamma Cassiopeiae Observed with the MK III Optical Interferometer

We present interferometric observations of the Be star γ Cas in the light of the Hα emission line. Northsouth baselines ranging from 4.2 to 23.6 m have been used over a large range of hour angles. With the resulting coverage of the u-v plane, we have been able for the first time to measure the asymmetry of the Hα emitting region directly. The brightness distribution on the sky can be fit by an elliptical Gaussian model with an axial ratio of 0.74 and an angular diameter (FWHM of the major axis) of 3.2 milliarcsec (mas). The radial brightness distribution is not well-constrained by our data; elliptical uniform disk or ring models also give acceptable fits

Radiointerferometric polar motion observations with high temporal resolution

For several years polar motion has been routinely monitored with the technique of geodetic Very Long Baseline Interferometry (VLBI) at five-day intervals. Here we present the results of the first two series of VLBI measurements for polar motion monitoring observed on a quasi daily basis employing only a single baseline. The high sensitivity of the long north-south baseline between radio telescopes at Wettzell in Germany and Hartebeesthock in South Africa for both components of polar motion permitted relatively short and inexpensive measurements of only two hours duration per session. The results of this series agree very well with the pole path determined with the IRIS network using 4 observatories in each observing session of 24 hour duration. With the polar motion results of the two series spanning about 35 days each, spectral analyses were performed which have shown a fortnightly period with a high degree of probability. These measurements demonstrate the potential of long north-south VLBI baselines for monitoring polar motion with very high temporal resolution for studies of short period fluctuations.

Preliminary measurements of star positions with the Mark III stellar interferometer

view Abstract Citations (19) References (5) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Preliminary Measurements of Star Positions with the Mark III Stellar Interferometer Mozurkewich, D. ; Hutter, D. J. ; Johnston, K. J. ; Simon, R. S. ; Shao, M. ; Colavita, M. M. ; Staelin, D. H. ; Hines, B. ; Hershey, J. L. ; Hughes, J. A. ; Kaplan, G. H. Abstract Preliminary astrometric measurements with the Mark III optical stellar interferometer using a 12.005 m north-south baseline were made during seven nights in November 1986. A total of 22 stars were observed over large angles in a 3000-A wide band centered at 7000 A. As expected, the estimated precision of the relative positions increases with hour-angle coverage. For the ten stars observed over at least 90 deg of hour angle, the right ascensions are consistent at the 5 ms of time (1sigma) level, while the uncertainties in declination are at the 22 mas (1sigma) level. With this small data set, the question of systematic errors cannot be fully examined. A discussion of observational techniques, errors, limitations of this system, and future plans is presented. Publication: The Astronomical Journal Pub Date: April 1988 DOI: 10.1086/114723 Bibcode: 1988AJ.....95.1269M Keywords: Astrometry; Astronomical Coordinates; Interferometry; Star Trackers; Optical Thickness; Refractivity; Astronomy; ASTROMETRY full text sources ADS | data products SIMBAD (21)

Milliarcsecond Imaging with a Large Optical Array

Considerable progress has been made over the last few years in ground based optical interferometery. The Mark III optical interferometer at Mount Wilson which at this time is operating with a 12 meter north south baseline is capable of measuring amplitude and phase. Experience gained from this facility will allow for the development of a large optical array. Preliminary plans are described for a facility that will be capable of milliarcsecond imaging of celestial objects at wavelengths from 0.4 to 10 microns. If funded this array will be in operation by 1993.

Radiointerferometric polar motion determination using a very long north-south baseline

The first high accuracy VLBI measurements with the Hartebeesthoek Radio Astronomy Observatory (HartRAO) at the southern end of the African tectonic plate were made possible at the beginning of 1986 through the loan of a MARK III DAT to HartRAO by the US National Geodetic Survey. Six twenty-four hour experiments spread over thirty-three days were used to precisely determine the HartRAO station position and to measure baseline lengths to Europe and North America. Interleaved between these multi-station experiments, a single baseline from Wettzell to HartRAO was used for two hours on a daily basis in order to measure pole positions. The formal errors of the x and y pole component determinations for each day are about ±2 mas and ±1 mas respectively, but an offset of about 6 mas from the IRIS values remains to be investigated.

The Nancay multifrequency radioheliograph: New observations

The Nançay radioheliograph provides images of the sun on a north-south baseline at up to five frequencies between 150 and 450 MHz with time resolution up to 0.1 s. East-west images are also obtained at 164 MHz with a time resolution up to 0.02 s. This paper briefly presents the performances of the instrument and the observing modes. As an illustration preliminary results on noise storm continua and type III bursts are reported.

Large simultaneous disturbances (LSDs) in the Antarctic ionosphere

A high frequency radio Doppler experiment was deployed in the Antarctic Peninsula region, centred on Argentine Islands (65°15′S, 64°16′W; L = 2.3), to investigate the morphology and sources of ionospheric disturbances. The experiment consisted of a three-transmitter dual frequency network which permits horizontal and vertical propagation velocities to be estimated over a north-south baseline of 200 km and an east-west baseline of 100 km. A new class of ionospheric disturbance has been observed, in the period range 10 min−1 h. These disturbances are characterised by unusually good correlation between perturbations on all available Doppler signals, but are apparently non—propagating and occur simultaneously at each reflection point. Several of these events display large (2 Hz at about 5 MHz transmitted frequency) Doppler shifts, thus we have labelled them Large Simultaneous Disturbances (LSD s). Criteria for identification of LSD s are established and the analysis of one event is described in detail. The occurrence statistics of the LSD s are presented, including their seasonal and diurnal distributions. There is no clear general relationship between LSD s and local geomagnetic field perturbations. However, examination of the magnetic indices AE and I RC indicates that there is a loose association between the occurrence and amplitude of LSD s and magnetic activity. Several possible mechanisms for the generation of LSD s at middle latitudes are reviewed. The most likely explanation is that high latitude electric fields penetrate to magnetic middle latitudes and drive the ionospheric plasma via the E × B drift.

Double Star Measurement with the Cerga Two Telescope Interferometer

AbstractThe CERGA interferometer is made of two 25 cm aperture telescopes with a variable north-south baseline, spanning from 5.5 to 35 meters. In addition to stellar diameters, it has provided binary stars measurements with a 0.5 milliarcsecs precision for separation down to 2.5 milliarcsoconds (Labeyrie 1971).The visual limiting magnitude is presently 3.5. Large separation binaries will be observed with one milliarcsecond precision using a slightly different technique. This should provide a mean of detecting possible planetary induced orbital perturbations.

Tidbinbilla Two-Element Interferometer

A phase-stable two-element interferometer has been formed by linking the 64- and 34-m antennas of the Deep Space Network at Tidbinbilla, Australia. Utilizing the existing first-stage maser receivers at 2.3 GHz to yield a system temperature of approximately 20 K, the system has a 5 sigma detection sensitivity of 50 mJy in 1 s, with a rms confusion of approximately 6 mJy. The 195-m north-south baseline permits positional measurements to approximately 2 arcsec of sources stronger than approximately 100 mJy over the declination range -80 to +30 deg. Such measurements are ideally suited for the optical identification of weak radio sources. It is planned to extend the frequency of operation of the instrument to 8.4 GHz in the near future.

Experience in optical interferometry with two telescopes

Initial stellar interferometers were used by Michelson and Pease from 1920 to 1937. About ten stellar diameters were resolved with 0.008" resolution. Thirty years later, Hanbury Brown and Twiss built the intensity interferometer. Until 1971, thirty-two diameters were measured with 0.0005" resolution. In 1974, interference fringes were obtained with two small telescopes operated as a Michelson stellar interferometer. Since 1976, this instrument designed by Labeyrie has been located at CERGA. Stars up to magnitude 4 are observable with a north-south baseline currently spanning 35 m and giving a 0.003" resolution. Fringes are detected visually is dispersed light. In the near future, the fringe visibility will be measured by a computerized image analyser. In 1977 and 1978, fifteen stars were observed. Six stellar disks were resolved with baselines shorter than 20 m. The two telescope interferometer is the prototype of a new generation of telescopes with high spatial resolution and shows that it is now possible to build coherent arrays with larger individual telescopes.

Hydrogen Line and Continuum Study of Maffei 2 BY Radio Interferometry

The suspected spiral galaxy Maffei 2 has been interferometrically observed in the 21-cm line and adjacent continuum using a north-south baseline having a 6 5 fringe spacing. Velocity resolution was 21 km Positions of the emission centroids at each velocity allowed determination of the orientation and dynamic parameters of the object and a lower limit to the total mass. The fractional hydrogen content of Maffei 2 is less than 0.005D, where D is the distance in Mpc. The continuum source associated with it is shown to be extended.