3C 279 is an archetypal blazar with a prominent radio jet that show broadband flux density variability across the entire electromagnetic spectrum. We use an ultra-high angular resolution technique – global Very Long Baseline Interferometry (VLBI) at 1.3 mm (230 GHz) – to resolve the innermost jet of 3C 279 in order to study its fine-scale morphology close to the jet base where highly variableγ-ray emission is thought to originate, according to various models. The source was observed during four days in April 2017 with the Event Horizon Telescope at 230 GHz, including the phased Atacama Large Millimeter/submillimeter Array (ALMA), at an angular resolution of ∼20 μas (at a redshift ofz = 0.536 this corresponds to ∼0.13 pc ∼ 1700 Schwarzschild radii with a black hole massMBH = 8 × 108 M⊙). Imaging and model-fitting techniques were applied to the data to parameterize the fine-scale source structure and its variation. We find a multicomponent inner jet morphology with the northernmost component elongated perpendicular to the direction of the jet, as imaged at longer wavelengths. The elongated nuclear structure is consistent on all four observing days and across different imaging methods and model-fitting techniques, and therefore appears robust. Owing to its compactness and brightness, we associate the northern nuclear structure as the VLBI “core”. This morphology can be interpreted as either a broad resolved jet base or a spatially bent jet. We also find significant day-to-day variations in the closure phases, which appear most pronounced on the triangles with the longest baselines. Our analysis shows that this variation is related to a systematic change of the source structure. Two inner jet components move non-radially at apparent speeds of ∼15 cand ∼20 c(∼1.3 and ∼1.7 μas day−1, respectively), which more strongly supports the scenario of traveling shocks or instabilities in a bent, possibly rotating jet. The observed apparent speeds are also coincident with the 3C 279 large-scale jet kinematics observed at longer (cm) wavelengths, suggesting no significant jet acceleration between the 1.3 mm core and the outer jet. The intrinsic brightness temperature of the jet components are ≲1010K, a magnitude or more lower than typical values seen at ≥7 mm wavelengths. The low brightness temperature and morphological complexity suggest that the core region of 3C 279 becomes optically thin at short (mm) wavelengths.

The Event Horizon Telescope (EHT) is a very long baseline interferometry (VLBI) array that comprises millimeter- and submillimeter-wavelength telescopes separated by distances comparable to the diameter of the Earth. At a nominal operating wavelength of 1.3 mm, EHT angular resolution (lambda/D) is 25 micro-as, which is sufficient to resolve nearby supermassive black hole candidates on spatial and temporal scales that correspond to their event horizons. With this capability, the EHT scientific goals are to probe general relativistic effects in the strong-field regime and to study accretion and relativistic jet formation near the black hole boundary. In this Letter we describe the system design of the EHT, detail the technology and instrumentation that enable observations, and provide measures of its performance. Meeting the EHT science objectives has required several key developments that have facilitated the robust extension of the VLBI technique to EHT observing wavelengths and the production of instrumentation that can be deployed on a heterogeneous array of existing telescopes and facilities. To meet sensitivity requirements, high-bandwidth digital systems were developed that process data at rates of 64 gigabit/s, exceeding those of currently operating cm-wavelength VLBI arrays by more than an order of magnitude. Associated improvements include the development of phasing systems at array facilities, new receiver installation at several sites, and the deployment of hydrogen maser frequency standards to ensure coherent data capture across the array. These efforts led to the coordination and execution of the first Global EHT observations in 2017 April, and to event-horizon-scale imaging of the supermassive black hole candidate in M87.

We report results from very long baseline interferometric (VLBI) observations of the supermassive black hole in the Galactic center, Sgr A*, at 1.3 mm (230 GHz). The observations were performed in 2013 March using six VLBI stations in Hawaii, California, Arizona, and Chile. Compared to earlier observations, the addition of the APEX telescope in Chile almost doubles the longest baseline length in the array, provides additional uv coverage in the N–S direction, and leads to a spatial resolution of ∼30 μ as (∼3 Schwarzschild radii) for Sgr A*. The source is detected even at the longest baselines with visibility amplitudes of ∼4%–13% of the total flux density. We argue that such flux densities cannot result from interstellar refractive scattering alone, but indicate the presence of compact intrinsic source structure on scales of ∼3 Schwarzschild radii. The measured nonzero closure phases rule out point-symmetric emission. We discuss our results in the context of simple geometric models that capture the basic characteristics and brightness distributions of disk- and jet-dominated models and show that both can reproduce the observed data. Common to these models are the brightness asymmetry, the orientation, and characteristic sizes, which are comparable to the expected size of the black hole shadow. Future 1.3 mm VLBI observations with an expanded array and better sensitivity will allow more detailed imaging of the horizon-scale structure and bear the potential for a deep insight into the physical processes at the black hole boundary.

The Event Horizon Telescope (EHT) is a very-long-baseline interferometry (VLBI) experiment that aims to observe supermassive black holes with an angular resolution that is comparable to the event horizon scale. The South Pole occupies an important position in the array, greatly increasing its north-south extent and therefore its resolution. The South Pole Telescope (SPT) is a 10-meter diameter, millimeter-wavelength telescope equipped for bolometric observations of the cosmic microwave background. To enable VLBI observations with the SPT we have constructed a coherent signal chain suitable for the South Pole environment. The dual-frequency receiver incorporates state-of-the-art SIS mixers and is installed in the SPT receiver cabin. The VLBI signal chain also includes a recording system and reference frequency generator tied to a hydrogen maser. Here we describe the SPT VLBI system design in detail and present both the lab measurements and on-sky results.

Centaurus A (Cen A) is a bright radio source associated with the nearby galaxy NGC 5128 where high-resolution radio observations can probe the jet at scales of less than a light day. The South Pole Telescope (SPT) and the Atacama Pathfinder Experiment performed a single-baseline very-long-baseline interferometry (VLBI) observation of Cen A in 2015 January as part of VLBI receiver deployment for the SPT. We measure the correlated flux density of Cen A at a wavelength of 1.4 mm on a similar to 7000 km (5 G lambda) baseline. Ascribing this correlated flux density to the core, and with the use of a contemporaneous short-baseline flux density from a Submillimeter Array observation, we infer a core brightness temperature of 1.4 x 10(11) K. This is close to the equipartition brightness temperature, where the magnetic and relativistic particle energy densities are equal. Under the assumption of a circular Gaussian core component, we derive an upper limit to the core size phi = 34.0 +/- 1.8 mu as, corresponding to 120 Schwarzschild radii for a black hole mass of 5.5 x. 10(7) M-circle dot.

The Galactic Center black hole Sagittarius A* (Sgr A*) is a prime observing target for the Event Horizon Telescope (EHT), which can resolve the 1.3 mm emission from this source on angular scales comparable to that of the general relativistic shadow. Previous EHT observations have used visibility amplitudes to infer the morphology of the millimeter-wavelength emission. Potentially much richer source information is contained in the phases. We report on 1.3 mm phase information on Sgr A* obtained with the EHT on a total of 13 observing nights over 4 years. Closure phases, the sum of visibility phases along a closed triangle of interferometer baselines, are used because they are robust against phase corruptions introduced by instrumentation and the rapidly variable atmosphere. The median closure phase on a triangle including telescopes in California, Hawaii, and Arizona is nonzero. This result conclusively demonstrates that the millimeter emission is asymmetric on scales of a few Schwarzschild radii and can be used to break 180-degree rotational ambiguities inherent from amplitude data alone. The stability of the sign of the closure phase over most observing nights indicates persistent asymmetry in the image of Sgr A* that is not obscured by refraction due to interstellar electrons along the line of sight.

The Event Horizon Telescope (EHT) is an earth-size aperture synthesis radio astronomy array capable of making high-resolution measurements of submillimeter emission near the event horizon of supermassive black holes. The EHT uses existing standalone submillimeter radio telescopes which are retrofitted to serve as VLBI stations. Current instrument development goals include increasing the number of stations in the array and increasing their sensitivity. We have developed a 4GHz bandwidth digital backend (DBE) unit, based on the CASPER (Collaboration for Astronomy Signal Processing and Electronics Research) open source ROACH2 (Reconfigurable Open Architecture Computing Hardware) platform. The ROACH2 digital backend, which we call the R2DBE, has dual channels each sampling at a rate of 4096MSps (megasamples-per-second), a factor of 4 improvement over the previous generation system. Recording 2-bits per sample, the bandwidth is equivalently stated as 16 gigabits-per-second (Gbps). This paper includes system design of the R2DBE, discusses laboratory test results of the system using correlated noise input, and presents field test results. The R2DBE was distributed to seven sites in early 2015, enabling the EHT campaign in 2015 March to collect data with 2GHz bandwidth in each polarization. The 16 gigabit-per-second (Gbps) R2DBE can be scaled to create a 64Gbps system using four R2DBEs in parallel. Thus, it enables a clear path to the EHT's goal of 4GHz dual-polarization and dual-sideband across the array.

Very Long Baseline Interferometry (VLBI) is a primary space-geodetic technique for determining precise coordinates on the Earth, for monitoring the variable Earth rotation and orientation with highest precision, and for deriving many other parameters of the Earth system. The International VLBI Service for Geodesy and Astrometry (IVS, http://ivscc.gsfc.nasa.gov/) is a service of the International Association of Geodesy (IAG) and the International Astronomical Union (IAU). The datasets published here are the results of individual Very Long Baseline Interferometry (VLBI) sessions in the form of normal equations in SINEX 2.0 format (http://www.iers.org/IERS/EN/Organization/AnalysisCoordinator/SinexFormat/sinex.html, the SINEX 2.0 description is attached as pdf) provided by IVS as the input for the next release of the International Terrestrial Reference System (ITRF): ITRF2014. This is a new version of the ITRF2008 release (Bockmann et al., 2009). For each session/ file, the normal equation systems contain elements for the coordinate components of all stations having participated in the respective session as well as for the Earth orientation parameters (x-pole, y-pole, UT1 and its time derivatives plus offset to the IAU2006 precession-nutation components dX, dY (https://www.iau.org/static/resolutions/IAU2006_Resol1.pdf). The terrestrial part is free of datum. The data sets are the result of a weighted combination of the input of several IVS Analysis Centers. The IVS contribution for ITRF2014 is described in Bachmann et al (2015), Schuh and Behrend (2012) provide a general overview on the VLBI method, details on the internal data handling can be found at Behrend (2013).

Near a black hole, differential rotation of a magnetized accretion disk is thought to produce an instability that amplifies weak magnetic fields, driving accretion and outflow. These magnetic fields would naturally give rise to the observed synchrotron emission in galaxy cores and to the formation of relativistic jets, but no observations to date have been able to resolve the expected horizon-scale magnetic-field structure. We report interferometric observations at 1.3-millimeter wavelength that spatially resolve the linearly polarized emission from the Galactic Center supermassive black hole, Sagittarius A*. We have found evidence for partially ordered magnetic fields near the event horizon, on scales of ~6 Schwarzschild radii, and we have detected and localized the intrahour variability associated with these fields.

A circular quadruple-ridged flared horn achieving almost-constant beamwidth over 6:1 bandwidth is presented. This horn is the first demonstration of a wideband feed for radio telescopes which is capable of accommodating different reflector antenna optics, maintains almost constant gain and has excellent match. Measurements of stand-alone horn performance reveal excellent return loss performance as well as stable radiation patterns over 6:1 frequency range. Physical optics calculations predict an average of 69% aperture efficiency and 13 K antenna noise temperature with the horn installed on a radio telescope.

A relatively inexpensive 16 Gbps data-recording system based on commercial off-the-shelf technology and open-source software has recently been developed. Combining this recorder with the parallel development of broadband Very Long Baseline Interferometer (VLBI) instrumentation is enabling dramatically improved sensitivity for both astronomical and geodetic VLBI. In this article, we describe the VLBI system and the results of a demonstration experiment that illustrates a number of cutting-edge technologies that can be deployed in the near future to significantly enhance the power of the VLBI technique.

Issues related to digital-backend (DBE) systems can be difficult to evaluate in either local tests or actual VLBI experiments. The 2nd DBE intercomparison workshop at Haystack Observatory on 25-26 October 2012 provided a forum to explicitly address validation and interoperability issues among independent global developers of DBE equipment. This special report discusses the workshop. It identifies DBE systems that were tested at the workshop, describes the test objectives and procedures, and reports and discusses the results of the testing.

The recent development of a relatively inexpensive 16-Gbps data-recording system based on commercial off-the-shelf technology and open-source software, along with parallel development in broadband Very Long Baseline Interferometry (VLBI) techniques, is enabling dramatically improved sensitivity for both astronomical and geodetic VLBI. The system is described, including the results of a demonstration VLBI experiment that illustrates a number of cutting-edge technologies that can be deployed in the near future to significantly enhance the power of the VLBI technique.

Approximately 10% of active galactic nuclei exhibit relativistic jets, which are powered by the accretion of matter onto supermassive black holes. Although the measured width profiles of such jets on large scales agree with theories of magnetic collimation, the predicted structure on accretion disk scales at the jet launch point has not been detected. We report radio interferometry observations, at a wavelength of 1.3 millimeters, of the elliptical galaxy M87 that spatially resolve the base of the jet in this source. The derived size of 5.5 ± 0.4 Schwarzschild radii is significantly smaller than the innermost edge of a retrograde accretion disk, suggesting that the M87 jet is powered by an accretion disk in a prograde orbit around a spinning black hole.

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Approximately 10% of active galactic nuclei (AGN) exhibit relativistic jets, which are powered by accretion of matter onto super massive black holes. While the measured width profiles of such jets on large scales agree with theories of magnetic collimation, predicted structure on accretion disk scales at the jet launch point has never been detected. We report radio interferometry measurements at 1.3mm wavelength of the elliptical galaxy M87 that spatially resolve the black hole accretion disk system at the jet base. The derived size of 4.9 +/0.4 Schwarzschild radii is significantly smaller than the innermost edge of a retrograde accretion disk, and implies that the M87 jet is powered by an accretion disk in a prograde orbit around a spinning black hole. One Sentence Summary: Black hole sized structure is detected at the base of a relativistic jet in the center of an active galaxy. Main Text: The compact central regions of some galaxies are so luminous that they outshine the combined output of all other energy sources in the galaxy. The small size and high power output of these active galactic nuclei (AGN) are most plausibly explained by the conversion of gravitational energy through accretion onto a super massive black hole. Many AGN produce powerful collimated jets of relativistic particles that can extend for hundreds and thousands of light-years, providing an important mechanism for redistributing matter and energy on large scales that affect galactic evolution (1). Jets are thought to form through magnetic acceleration processes located within the accretion flow or at the central black hole itself (2-4), but no observations to date have had the angular resolution required to detect and confirm structure on these scales for extragalactic jet sources. High-resolution radio interferometry of these sources at cm wavelengths is limited by optical depth effects that obscure the innermost accretion region. For these reasons, it remains unclear if jet formation requires a spinning black hole (5,6), or if jets are likely to be launched from disks orbiting in the opposite direction (retrograde) of the spin of the black hole (7,8). To address these questions, we have assembled a Very Long Baseline Interferometry (VLBI) array operating at a wavelength of 1.3mm, where AGN become optically thin, and angular resolutions necessary to resolve the inner accretion disks of nearby AGN are obtained. At a distance of 16−1.1 +1.3Mpc (9) and with a mass most recently measured to be (6.6 ± 0.4) × 10 M⊙ (10), the Schwarzschild radius of the M87 black hole (RSCH = 2GM/c = (6.3 ± 0.4) × 10 parsec = (2.0 ± 0.12) × 10 cm) subtends an angle of 8.1 ± 0.8 micro arcseconds, presenting us with the best known opportunity for studying the formation of relativistic jets on scales commensurate with the black hole and accretion disk. Radiating via synchrotron emission, the relativistic jet from M87 extends for hundreds of kilo parsecs and terminates in extended lobes of emission as it slows and interacts with the intergalactic medium. Closer to the galaxy's core, on hundreds of parsec scales, the radio jet is remarkably well collimated with an opening angle of less than 5 degrees (11), and is also clearly seen in the optical, ultra-violet, and x-rays (12,13) where the emission is primarily confined to knots along the central ‘spine’ of the jet. VLBI observations at wavelengths ranging from 3.5mm to 20cm, show that within 10's of milli arcseconds of the core, the jet opening angle, delineated by edge brightening in the outflow, increases to greater than 40 (14-18). This wide opening angle is a signature of the launch point for a magnetohydrodynamically (MHD) powered jet that has not yet had time to collimate (2), and identifies the VLBI core as the most likely site of the central black hole. We observed M87 over three consecutive days with a 1.3mm wavelength VLBI array consisting of four telescopes at three geographical locations: the James Clerk Maxwell Telescope (JCMT) on Mauna Kea in Hawaii, the Arizona Radio Observatory's Submillimeter Telescope (SMT) in Arizona, and two telescopes of the Combined Array for Research in Millimeter-wave Astronomy (CARMA, located ~60m apart) in California. On Mauna Kea, the JCMT partnered with the Submillimeter Array (SMA), which housed the Hydrogen maser atomic frequency standard and wideband VLBI recording systems; the SMT and CARMA were similarly equipped. These special-purpose systems allowed two frequency bands of 512 MHz to be sampled at 2-bit precision and recorded at an aggregate rate of 4 Gigabits/second. Data recorded at all sites were shipped to MIT Haystack Observatory for processing on the Mark4 VLBI correlator. Once correlated, data for each VLBI scan (typically 10 minutes) were corrected for coherence losses due to atmospheric turbulence and searched for detections using established algorithms tailored for high frequency observations (see SI). M87 was clearly detected each day on all VLBI baselines, and the interferometric data were then calibrated to flux density units (Figure 1; see SI for details). Clear detections on the long baselines to Hawaii (CARMA-JCMT and SMT-JCMT) represent the highest angular resolution observations of M87 reported in any waveband, and when combined with the CARMA-SMT baseline data they provide a robust means to measure the size of the M87 core, which is unresolved with longer wavelength VLBI. The baseline between the two CARMA antennae corresponds to angular scales of ~4 arcseconds and is sensitive to extended and much larger scale jet structure; these data were thus used to refine calibration of the antennas, but were excluded from analysis of the core component. To extract a size for the core, we fit a two-parameter circular Gaussian model to the 1.3mm VLBI data, deriving a total flux density and full width half maximum (FWHM) size for each day of observations. Sizes and flux densities fit separately for each day are consistent with each other at the 3σ level, indicating no significant variation in the 1.3mm core structure over the three days of observation. Fits on a plot of correlated flux density vs. baseline length are shown in Figure 1. When data from all three days are combined, the weighted least-squares best-fit model for the compact component results in a flux density of 0.98 ±0.04 Jy and a FWHM of 40 ±1.8micro arcseconds (3σ errors). Conversion to units of Schwarzschild radius yields a value of 4.9± 0.4 RSCH (1σ errors) where the errors are completely dominated by uncertainties in the distance to M87 and the black hole mass. We adopt the circular Gaussian size derived using data from all three days for subsequent discussion. Our VLBI observations cannot be used to fix the absolute position of this Gaussian component. However, two separate lines of evidence imply that this ultra-compact 1.3mm emission is in immediate proximity to the central super massive black hole at the jet-launch point. First, the long history of VLBI observations of M87 at many wavelengths can be used to construct a jet width profile that begins ~100 RSCH from the core and extends to core-separations of more than 10 RSCH. Figure 2 shows this profile along with a power law fit to the data that matches the functional form and characteristics exhibited by General Relativistic MHD simulations (18-20) in which the jet opening angle widens as it nears the black hole. The best-fit power law intersects the size of the 1.3mm emission region at a core distance of only 1.25 RSCH. Since the angle of the M87 jet axis to our line of sight is estimated to be within the range 15 – 25 (21), the de-projected distance of this intersection point lies in the range 3 – 5 RSCH. A second method of locating the 1.3mm emission derives from observations of the position shift of the M87 core as a function of wavelength. The core corresponds to the point in the jet where opacity due to synchrotron self-absorption approaches unity, and this τ∼1 surface should shift towards the jet launch point with decreasing observing wavelength. Multi-wavelength astrometric VLBI observations confirm that over a wide range of frequency, the absolute position of the core moves asymptotically towards the central black hole with a ν dependence (21). This relation also places the 1.3mm emission at an apparent distance of just 1.25 RSCH from the black hole. Such strong evidence for associating the M87 core with the central black hole contrasts with the case of blazar sources, in which relativistic jets are closely aligned to our line of sight and the core becomes visible hundreds of thousands of Schwarzschild radii from the central engine (22). In M87, the favourable geometry of a misaligned jet and increased transparency of the synchrotron emission at mm wavelengths (23) allows us direct access to the innermost central engine with 1.3mm VLBI. The most plausible mechanisms for powering extragalactic jets involve conversion of the black hole rotational energy through the Blandford-Znajek (BZ) process (3), whereby magnetic fields lines cross the black hole event horizon or become locked into co-rotation with the black hole ergosphere, and launch Poynting flux dominated outflows. The inner portion of the accretion disk is not only the source of the magnetic fields threading the black hole, but also launches a disk-wind via the Blandford-Payne (BP) mechanism (4), which serves to collimate the jet. This BZ/BP combination forms a spine/sheath morphology in which a high velocity and narrow central jet from the black hole is surrounded by a slower outflow originating from the inner disk (6,24). In the case of M87, our line of sight is sufficiently off axis that the dominant contribution to the 1.3mm VLBI emission is from the slower moving sheath, anchored within the accretion disk (21). Thus, the critical size scale associated with the jet footprint is the Innermost Stable Circular Orbit (ISCO) of the black hole, within which matter quickly p

A continuing thrust in the space geodetic community is to deploy instruments using different techniques at common sites. While the close proximity (of order 100 meters) of the instruments to each other affords improved inter-comparison tests, it also increases the potential for interference between instruments. Of present concern to VLBI are DORIS beacons and the aircraft surveillance radars used in conjunction with satellite laser ranging (SLR). Initial numerical studies(1) were conducted to obtain rough estimates of the degree to which the VLBI SNR is degraded for various levels of DORIS and SLR radar interference. Numerical studies are only as good as the models upon which they are based, however, and there is sufficient uncertainty regarding their accuracy that field and laboratory validation is warranted. In this contribution, we present a measurement methodology designed to resolve the major uncertainties in the models. We also summarize the experimental results to date.

The next generation geodetic VLBI instrument is being developed with a goal of 1 mm position uncertainty in twenty-four hours. The broadband signal chain, which is essential for obtaining the required delay accuracy from a network of relatively small antennas, has been implemented on the 12-meter antenna at the Goddard Space Flight Center, Maryland, USA, and on the 18-meter Westford antenna at Haystack Observatory, Massachusetts, USA. Data have been obtained in four 512 MHz bands spanning the range 3.2 to 9.9 GHz using commercially available broadband feeds, LNAs, digital back ends, and recorders. The first geodetic-style observing session has been completed. While demonstrating that the broadband hardware functions as expected, the six-hour session has illuminated areas of the scheduling, correlation, and post-correlation process that require improvement. 1. The VLBI2010 Broadband Observing System The potential of the broadband delay and some of the expected challenges in processing the data were presented in the report on the Proof-of-Concept (PofC) system in the Proceedings for the previous IVS General Meeting [1]. Since that time a fundamental element of the VLBI2010 concept, a fast-slewing 12-meter antenna [Figure 1], has been installed adjacent to the 5-meter antenna at the Goddard Geophysical and Astronomical Observatory (GGAO) on the grounds of the Goddard Space Flight Center, and the PofC instrumentation has been replaced with production versions of the broadband signal chain. The principal replaced components are the feed, digital back end, and recorder, each of which are described in the following paragraphs. A significant improvement to the VLBI2010 system is the incorporation of the quadruple-ridged flared horn (QRFH) feed in place of the Lindgren feed that was used for the PofC demonstration. The QRFH feed was developed at Caltech [2] and provides the two desirable features for geodetic VLBI, beamwidth and phase center that are largely independent of frequency over the 2-14 GHz range. Equally important, this is achieved using only one low noise amplifier (LNA) per polarization. As with all proposed feeds, the QRFH output is dual linear polarizations. Different versions of the QRFH feed are used for the two antennas due to the different f/D ratios. The digital back end, designated RDBE-H (hereafter referred to as RDBE), is a completely new design developed by NRAO-Socorro and MIT Haystack Observatory [3]. Features new to the IVS 2012 General Meeting Proceedings 13 Arthur Niell et al.: First Broadband Results with a VLBI2010 System Figure 1. The MIT 12-m antenna installed at Goddard Space Flight Center. This is the first antenna fully configured for VLBI2010 operation. RDBE compared to the DBE1 used previously are selectable channel output, improved threshold setting for quantization, adjustable attenuators for setting power levels on input, and control and synchronous detection of an external noise diode for measurement of system temperature. The output of the RDBE is via 10 Gigabit Ethernet in Mark 5B format for recording on the Mark 5C. Conversion of the RF signal to IF is accomplished by the UpDown Converter, a component used in the PofC that did not require any improvement. Another step in the move to VLBI2010 is the use of the DiFX software correlator [4]. This has required the addition of the capability of converting the native output into Mark IV format, which has been accomplished by the creation of a program difx2mark4, which parallels difx2fits for conversion to the astronomical data format. A basic assumption in the broadband concept is that the data from the four bands and both linear polarizations will be fit coherently for the delay observable and for the differential ionosphere dispersion. This, and the use of all available phasecal tones in a channel, have required significant modifications and additions to the estimation program fourfit. The broadband concept is that data from four bands spanning approximately 2.2 GHz to up to 14 GHz will provide sufficient phase accuracy to estimate the VLBI delay and differential ionosphere with no ambiguity. Thus a total of four parallel hardware paths are required. The signal chain that has been designed to implement this concept is shown in Figure 2.