We report results from multi-epoch radio astrometry of the Cassini spacecraft with the Very Long Baseline Array (VLBA). These observations are part of a program to determine a series of accurate positions for the Saturn system barycenter in the inertial International Celestial Reference Frame (ICRF) and to use these position measurements to improve our knowledge of Saturn's orbit in the planetary ephemeris. Our VLBA observations cover the full duration of the orbital phase of the Cassini mission, from Saturn orbit insertion in 2004 to the end of mission in 2017. This period covers more than one-third of Saturn's orbital period, allowing us to obtain good orbit constraints for Saturn, particularly on the inclination and ascending node longitude. During the early years of Cassini's orbital mission our VLBA data dominated the determination of orbit orientation, while later in the mission range measurements become more significant. The orientation of Saturn's orbit is now known to approximately 0.25 milli-arcseconds (1.25 nrad), an order of magnitude improvement since the start of Cassini observations. Continuing improvements in the ICRF position accuracy for our phase reference sources, and possible improvements in the final orbit solutions for Cassini, may lead to a still better Saturn orbit over the coming years.

The Very Long Baseline Array (VLBA) is a ten antenna radio interferometer with baseline lengths up to 8000 km. It can provide astrometric measurements of spacecraft orbiting planets and other objects in our solar system with subnrad precision (5 nrad = 1 milli-arcsec). These measurements can be used to create a time series of positions for solar system objects in the inertial International Celestial Reference Frame, which in turn can be combined with other data to refine the planetary ephemeris. An accurate solar system ephemeris is critical for interplanetary spacecraft navigation, dynamical studies and tests of gravitational theories, the analysis of pulsar timing observations, predictions of transits, eclipses, and occultations, and other applications. We are using VLBA observations of the Juno spacecraft in orbit about Jupiter to provide accurate positions for the Jupiter system barycenter for the ephemeris development group at the Jet Propulsion Laboratory, using observing and data reduction techniques developed for similar observations of the Cassini spacecraft while it orbited Saturn from 2004 until 2017. The VLBA observations of Cassini helped to improve the accuracy of Saturn's orbit by nearly an order of magnitude, and we expect that our observations of Juno will produce a similar improvement in our knowledge of Jupiter's orbit. Astrometric positions are particularly useful in constraining the orientation (inclination and longitude of ascending node) of an orbit, while range measurements are most useful in constraining the semi-major axis and ellipticity. Juno's orbit around Jupiter has a longer period than initially planned due to a concern about the spacecraft main engine. The resulting extended mission duration will improve our constraints on Jupiter's orbit inclination beyond that originally expected. Our VLBA observations of Juno are scheduled during approximately every third or fourth perijove pass. During these times the Juno spacecraft is continuously tracked by the Deep Space Network and the most precise solutions for the orbit of Juno about Jupiter are available. A good spacecraft orbit solution is needed to transfer our spacecraft sky positions to planet system barycenter positions. VLBA astrometry of planetary spacecraft has previously been applied to Mars orbiting space-craft, and will also be used during the OSIRISREx mission to improve the accuracy of the orbit of the potentially hazardous asteroid Bennu.

The occultation of the radio galaxy 0141+268 by the asteroid (372) Palma on 2017 May 15 was observed using six antennas of the Very Long Baseline Array (VLBA). The shadow of Palma crossed the VLBA station at Brewster, Washington. Owing to the wavelength used, and the size and the distance of the asteroid, a diffraction pattern in the Fraunhofer regime was observed. The measurement retrieves both the amplitude and the phase of the diffracted electromagnetic wave. This is the first astronomical measurement of the phase shift caused by diffraction. The maximum phase shift is sensitive to the effective diameter of the asteroid. The bright spot at the shadow's center, the so called Arago–Poisson spot, is clearly detected in the amplitude time-series, and its strength is a good indicator of the closest angular distance between the center of the asteroid and the radio source. A sample of random shapes constructed using a Markov chain Monte Carlo algorithm suggests that the silhouette of Palma deviates from a perfect circle by 26 ± 13%. The best-fitting random shapes resemble each other, and we suggest their average approximates the shape of the silhouette at the time of the occultation. The effective diameter obtained for Palma, 192.1 ± 4.8 km, is in excellent agreement with recent estimates from thermal modeling of mid-infrared photometry. Finally, our computations show that because of the high positional accuracy, a single radio interferometric occultation measurement can reduce the long-term ephemeris uncertainty by an order of magnitude.

We have been using the Very Long Baseline Array (VLBA) to measure accurate sky positions of the Juno spacecraft since its insertion into orbit around Jupiter in July 2016. These positions are referenced to the inertial International Celestial Reference Frame. During Juno's perijove passes the relative positions of the spacecraft and the Jupiter system barycenter are extremely well determined from Doppler tracking by the Deep Space Network (DSN). Combining our VLBA measurements with DSN tracking gives us accurate Jupiter barycenter positions, which in turn can be used to improve our knowledge of Jupiter's orbit (particularly its orientation) as part of the JPL planetary ephemeris. A problem with the main engine has kept Juno in its current 53.5-day orbit instead of the originally planned 14-day orbit. This will extend the mission duration in Jupiter orbit to nearly 5 years, well over 1/4 of Jupiter's orbital period, which in turn will allow us to obtain stronger constraints on orbital inclination and ascending node longitude. DSN range measurements complement the astrometric measurements by providing the best constraints on orbit semi-major axis and eccentricity. We expect to reduce the uncertainty in the orientation of Jupiter's orbit with respect to the International Celestial Reference Frame from approximately 20 milliarcseconds (mas) to approximately 0.2 mas (1 nrad), based on the improvement in Saturn's orbit obtained from similar VLBA measurements of the Cassini spacecraft. Our initial VLBA observations of Juno, combined with the Ulysses flyby in 1992, have already improved the Jupiter ephemeris accuracy by about a factor of 4.

We demonstrate that extremely rapid and weak periodic and non-periodic signals can easily be detected by using the autocorrelation of intensity as a function of time. We use standard radio-astronomical observations that have artificial periodic and non-periodic signals generated by the electronics of terrestrial origin. The autocorrelation detects weak signals that have small amplitudes because it averages over long integration times. Another advantage is that it allows a direct visualization of the shape of the signals, while it is difficult to see the shape with a Fourier transform. Although Fourier transforms can also detect periodic signals, a novelty of this work is that we demonstrate another major advantage of the autocorrelation, that it can detect non-periodic signals while the Fourier transform cannot. Another major novelty of our work is that we use electric fields taken in a standard format with standard instrumentation at a radio observatory and therefore no specialized instrumentation is needed. Because the electric fields are sampled every 15.625 ns, they therefore allow detection of very rapid time variations. Notwithstanding the long integration times, the autocorrelation detects very rapid intensity variations as a function of time. The autocorrelation could also detect messages from Extraterrestrial Intelligence as non-periodic signals.

The Cassini spacecraft has been in orbit about Saturn since 2004. During this time, regular astrometric measurements of Cassini's sky position have been made with the Very Long Baseline Array (VLBA). These are high precision differential measurements that determine the position of Cassini with respect to angularly nearby extragalactic radio sources. Differential, narrow-angle astrometry reduces many error sources, particularly those associated with signal propagation effects in the ionosphere and troposphere. The background radio sources positions are tied to the inertial International Celestial Reference Frame (ICRF) by other international VLBI observations. Thus, we obtain a series of ICRF positions for Cassini, which can be combined with spacecraft orbit solutions from Deep Space Network Doppler tracking to get ICRF positions for the center of mass of the Saturn system. These positions have typical accuracies at the nano-radian level. For some epochs uncertainties in the background source positions are a major component of the total error, but these positions are being constantly improved as additional VLBI observations are incorporated into radio source catalogs. The planetary ephemeris group at the Jet Propulsion Laboratory uses our position measurements to fit improved orbital solutions for Saturn. As a result the orientation of the plane of Saturn's orbit is now known to approximately 0.25 milli-arcseconds (1.25 nrad), nearly an order of magnitude improvement over its pre-VLBA uncertainty. We will continue this observing program until the end of the Cassini mission in late 2017. By that time we will have covered about 1/3 of Saturn's orbital longitude range. Future improvements to this technique will include the use of higher spacecraft downlink frequencies (Ka band instead of X band) and higher ground array sensitivity to permit the use of weaker but angularly closer reference sources. In addition, the continuing international campaigns to enhance the accuracy of radio source catalogs will be extended to weaker sources, improving their ties to the ICRF.

The planetary ephemeris is a fundamental tool of astronomy that is essential for dynamical studies of the solar system, pulsar timing, tests of general relativity, occultation and eclipse predictions, and interplanetary spacecraft navigation. Since Jupiter and Saturn dominate the dynamics of our solar system, improved knowledge of their orbits will result in a global improvement in the accuracy of the ephemeris. The Cassini spacecraft has been orbiting Saturn for over a decade, a third of Saturn's orbital period. This has provided an unprecedented opportunity to improve all components of Saturn's orbit by combining periodic very long baseline inferterometry (VLBI) measurements of Cassini's sky position with respect to background radio sources, which in turn can be tied to the inertial International Celestial Reference Frame (ICRF). The orbit of Cassini about the center of mass of Saturn is determined from Doppler tracking by the Deep Space Network. Combining these observations, we obtain the barycenter position of the Saturn system in an inertial frame at multiple epochs, with typical uncertainties of 0.3 milli-arcseconds in right ascension and 0.4 milli-arcseconds in declination. These results are then provided to JPL's ephemeris group for inclusion in future ephemeris solutions. At most epochs the largest component of the error budget is uncertainty in the ICRF position of the phase reference radio source used. These source positions are being continuously improved through additional VLBI observations. These VLBA observations have improved our knowledge of Saturn's orbit by nearly an order of magnitude. This technique will be expanded to include astrometric observations of the Juno spacecraft as soon as it enters Jupiter orbit in mid-2016. Although the orbital phase of the Juno mission is expected to last only a bit over one year, it will still allow a significant improvement in Jupiter's orbit. Previous missions to Jupiter have been single-epoch flybys with the exception of Galileo, for which the accuracy of VLBI position measurements was severely limited by failure of the high gain antenna. The Juno mission is scheduled to end in February 2018, five months after the scheduled end of the Cassini mission. At the ends of their missions the Juno and Cassini spacecraft will be destroyed in the atmospheres of Jupiter and Saturn to eliminate the possibility of a future crash onto any of the liquid-containing moons of these planets that may be habitats of life.

This paper presents astrometric observations of Mars that are reduced from Very Long Baseline Array (VLBA) measurements of Mars-orbiting satellites. These observations provide angular positions for Mars in the International Celestial Reference Frame (ICRF). Nine observing epochs were used: eight from 2008 and one from 2013. For each epoch, observed R.A. and decl. are provided with associated uncertainties. The post-fit rms residuals of these measurements against JPL's DE430 ephemeris are 0.13 mas and 0.18 mas for R. A. and decl., respectively, with average uncertainty of 0.24 mas in R. A. and 0.32 mas in decl. The results are generally in good agreement with single-baseline Very Long Baseline Interferometry and range measurements of Mars-orbiting satellites. The VLBA measurements of Mars are used to determine the orientation of the dynamical system of Earth and Mars relative to the ICRF with uncertainty of 0.23 mas.

We present a demonstration of near real-time spacecraft astrometry with the VLBA. We detect the X-band downlink signal from Mars Reconnaissance Orbiter and Odyssey with the VLBA and transmit the data over the internet for correlation at the VLBA correlator in near real-time. Quasars near Mars in the plane of the sky are used as position references. In the demonstration we were able to obtain initial position measurements within about 15 minutes of the start of the observation. The measured positions differ from the projected ephemerides by a few milliarcseconds, and the repeatability of the measurement is better than 0.3 mas as determined from measurements from multiple scans. We demonstrate that robust and repeatable offsets are obtained even when removing half of the antennas. These observations demonstrate the feasibility of astrometry with the VLBA with a low latency and submilliarcsecond repeatability.

Software correlation, where a correlation algorithm written in a high-level language such as C++ is run on commodity computer hardware, has become increasingly attractive for small- to medium-sized and/or bandwidth-constrained radio interferometers. In particular, many long-baseline arrays (which typically have fewer than 20 elements and are restricted in observing bandwidth by costly recording hardware and media) have utilized software correlators for rapid, cost-effective, correlator upgrades to allow compatibility with new, wider-bandwidth, recording systems and to improve correlator flexibility. The DiFX correlator, made publicly available in 2007, has been a popular choice in such upgrades and is now used for production correlation by a number of observatories and research groups worldwide. Here, we describe the evolution in the capabilities of the DiFX correlator over the past three years, including a number of new capabilities, substantial performance improvements, and a large amount of supporting infrastructure to ease use of the code. New capabilities include the ability to correlate a large number of phase centers in a single correlation pass, the extraction of phase-calibration tones, correlation of disparate but overlapping sub-bands, the production of rapidly sampled filter-bank and kurtosis data at minimal cost, and many more. The latest version of the code is at least 15% faster than the original (and in certain situations, many times this value). Finally, we also present detailed test results validating the correctness of the new code.

A digital backend based on the ROACH board has been developed jointly by the National Radio Astronomy Observatory and MIT Haystack Observatory. The RDBE will have both Polyphase Filterbank and Digital Downconverter personalities. The initial configuration outputs sixteen 32-MHz channels, comprised of half the channels from the PFB processing of the two IF inputs, for use in the VLBI2010 geodetic system and in the VLBA sensitivity upgrade project. The output rate is 2x109 bits/second (1x10(exp 9) bits/sec = 1 Gbps) over a 10 GigE connection to the Mark 5C with the data written in Mark 5B format on disk.

In this paper, storage clouds are online services for leasing disk storage. A new modeling tool, formulated from empirical data spanning many years, lets organizations rationally evaluate the benefit of using storage clouds versus purchasing hard disk drives. The business of selling infrastructure as a service through the Internet is growing. This technology trend, also known as cloud computing, lets individuals and organizations outsource their IT requirements to remote data centers, paying for only what they use.Several online services currently lease storage infrastructure. These storage clouds let anyone with a credit card purchase storage capacity online, paying a monthly fee for the storage they use. With the significant growth of society's storage requirements, and the availability of pay-per-use online storage services, when should a consumer consider using storage clouds? The paper focuses on the problem of resolving this buy-or-lease storage decision.

The VLBA was a major participant in the original VSOP mission. NRAO hopes to play a similar role in the VSOP-2 mission, if commensurate support can be obtained for VLBA operations in such a collaboration. While the VLBA's original data system is not compatible with the planned VSOP-2 specifications, the current VLBA Sensitivity Upgrade project will produce a new system that is well-matched to VSOP-2. This upgrade involves replacement of the entire data path downstream from the IFs, and includes a digital sub-band processor, a wideband recording system, and a software correlator. The project's goal is to achieve sustained 4-Gbps operation by 2011, with wideband operation available much earlier for the most scientifically compelling observations. These goals appear to be well matched to the VSOP-2 timeline. This paper presents an overview of the new systems under development, and compares the capabilities of each to the requirements for VSOP-2. Further topics include adaptation of the entire system, and the correlator in particular, to Space VLBI operations, and upgrade aspects that should minimize the data-format incompatibilities that were a substantial difficulty in the first VSOP mission.

This triennium began with an action to re-create the Terms of Reference for the Working Group Global VLBI (WG-GV). These had been lost over the years since the Group was established in 1990. Fortunately, the personal archive of one long-term member yielded a copy of the original memorandum by R. D. Ekers, which was found to coincide quite well with current practice and areas of interest. New Terms of Reference, based on modern conditions, were drafted and accepted by both IAU and URSI.