Very Long Baseline Interferometry Correlator Research Articles

Dynamic dual-target VLBI correlator technology

The dynamic dual-target very long baseline interferometry (VLBI) processing technology applied to the CVNScorr software processor is introduced. Chang’e 5 (CE-5) has achieved China’s first sample return mission from an extraterrestrial body. The characteristics of VLBI are that there are several target probes and the telemetry tracking and command (TT&C) technical requirements are high in CE-5. In the rendezvous and docking process, a real-time in-beam VLBI technology is required to perform the high-precision orbit measurement of two dynamic targets, the orbiter and the ascender. In addition, the VLBI correlator (CVNScorr) should tackle two-phase centers from two probes simultaneously. In the special power flight phases such as rendezvous and docking, near-moon braking, powered descending, and lunar takeoff, the VLBI high-precision delay model cannot be derived owing to the low accuracy of the predicted orbit. If the conventional data processing method is adopted, the VLBI fringe cannot be obtained. Therefore, the VLBI correlator should have the function of real-time dual-target fringe search and delay model reconstruction. CVNscorr adopts the principle of FX-type software correlation and has the function of dual-target correlation module, multibeacon fringe search, and delay model reconstruction. By taking advantage of the characteristics of TT&C signals from different probes, multiple beacons fringes are automatically searched and the delay values of the two targets are obtained simultaneously to reconstruct the high-precision delay models. CVNScorr uses MPI, GPU, and other technologies to realize parallel computing, and it is run on a CPU+GPU parallel cluster platform. In the CE-5 mission, the real-time processing capability is up to four stations of the Chinese VLBI Network and the postprocessing capability is up to six stations. The real-time data rate reaches 128 Mbps per station, the VLBI delay accuracy reaches 0.4 ns, and the real-time performance is better than 25 s.

GPU-Based Parallel Implementation of VLBI Correlator for Deep Space Exploration System

Very Long Baseline Interferometry (VLBI) solution can yield accurate information of angular position, and has been successfully used in the field of deep space exploration, such as astrophysics, imaging, detector positioning, and so on. The increase in VLBI data volume puts higher demands on efficient processing. Essentially, the main step of VLBI is the correlation processing, through which the angular position can be calculated. Since the VLBI correlation processing is both computation-intensive and data-intensive, the CPU cluster is usually employed in practical application to perform complex distributed computation. In this paper, we propose a parallel implementation of VLBI correlator based on graphics processing unit (GPU) to realize a more efficient and economical angular position calculation of deep space target. On the basis of massively GPU parallel computing, the coalesced access strategy and the parallel pipeline strategy are introduced to further accelerate the VLBI correlator. Experimental results show that the optimized GPU-based VLBI method can meet the real-time processing requirements of the received data stream. Compared with the sequential method, the proposed approach can reach a 224.1 × calculation speedup, and a 36.8 × application speedup. Compared with the multi-CPUs method, it can achieve 28.6 × calculation speedup and 4.7 × application speedup.

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

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

Prospects for Wideband VLBI Correlation in the Cloud

This paper proposes a cloud architecture for the correlation of wide bandwidth Very Long Baseline Interferometry (VLBI) data. Cloud correlation facilitates processing of entire experiments in parallel using flexibly allocated and practically unlimited compute resources. This approach offers a potential improvement over dedicated correlation clusters, which are constrained by a fixed number of installed processor nodes and playback units. Additionally, cloud storage offers an alternative to maintaining a fleet of hard disk drives that might be utilized intermittently. Here, we describe benchmarks of VLBI correlation using the DiFX-2.5.2 software on the Google Cloud Platform to assess cloud-based correlation performance. In our analysis, the number of virtual central processing units per virtual machine was varied to determine the optimum configuration of cloud resources. The number of stations was varied to determine the scaling of correlation time with VLBI arrays of different sizes. Data transfer rates from Google cloud storage to the virtual machines performing the correlation were also measured. Based on the results, we present an example cloud correlation configuration. Current cloud service and equipment pricing data is used to compile cost estimates allowing an approximate economic comparison between cloud and cluster processing. We note that the economic comparisons are based on cost figures which are a moving target, and are highly dependent on factors such as the utilization of cluster and media, which are a challenge to estimate. Our model suggests that shifting to the cloud is an alternative path for high data rate, low duty cycle wideband VLBI correlation that should continue to be explored. In the production phase of VLBI correlation, the cloud has the potential to significantly reduce data processing times and allow the processing of more science experiments in a given year for the petabyte-scale data sets increasingly common in both astronomy and geodesy VLBI applications.

The correlation process in Very Long Baseline Interferometry

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

Using an atmospheric turbulence model for the stochastic model of geodetic VLBI data analysis

Space-geodetic techniques at radio wavelength, such as global navigation satellite systems and very long baseline interferometry (VLBI), suffer from refractivity of the Earth’s atmosphere. These highly dynamic processes, particularly refractivity variations in the neutral atmosphere, contribute considerably to the error budget of these space-geodetic techniques. Here, microscale fluctuations in refractivity lead to elevation-dependent uncertainties and induce physical correlations between the observations. However, up to now such correlations are not considered routinely in the stochastic model of space-geodetic observations, which leads to very optimistic standard deviations of the derived target parameters, such as Earth orientation parameters and station positions. In this study, the standard stochastic model of VLBI observations, which only includes, almost exclusively, the uncertainties from the VLBI correlation process, is now augmented by a variance–covariance matrix derived from an atmospheric turbulence model. Thus, atmospheric refractivity fluctuations in space and time can be quantified. One of the main objectives is to realize a suitable stochastic model of VLBI observations in an operational way. In order to validate the new approach, the turbulence model is applied to several VLBI observation campaigns consisting of different network geometries leading the path for the next-generation VLBI campaigns. It is shown that the stochastic model of VLBI observations can be improved by using high-frequency atmospheric variations and, thus, refining the stochastic model leads to far more realistic standard deviations of the target parameters. The baseline length repeatabilities as a general measure of accuracy of baseline length determinations improve for the turbulence-based solution. Further, this method is well suited for routine VLBI data analysis with limited computational costs.

Geodetic VLBI correlation in software

Correlation algorithms for geodetic very long baseline interferometry (VLBI) can now be effectively implemented on parallel computers of modest size. We have undertaken a detailed comparison of the output from a trusted geodetic correlator, one that has supported global geodetic VLBI observations for many years, with the output of a software correlator implemented on a small parallel computing cluster. We show that the correlator outputs agree closely, within expected error bounds, after accounting for the differences in the adopted geometric delay models, and therefore that use of the software correlator is feasible for geodetic VLBI processing, as a first step toward routine geodetic data processing. Recent developments in software correlation for geodesy are discussed, including the possibility of real-time processing options.

DiFX: A Software Correlator for Very Long Baseline Interferometry Using Multiprocessor Computing Environments

ABSTRACTWe describe the development of an FX‐style correlator for very long baseline interferometry (VLBI), implemented in software and intended to run in multiprocessor computing environments, such as large clusters of commodity machines (Beowulf clusters) or computers specifically designed for high‐performance computing, such as multiprocessor shared‐memory machines. We outline the scientific and practical benefits for VLBI correlation, these chiefly being due to the inherent flexibility of software and the fact that the highly parallel and scalable nature of the correlation task is well suited to a multiprocessor computing environment. We suggest scientific applications where such an approach to VLBI correlation is most suited and will give the best returns. We report detailed results from the Distributed FX (DiFX) software correlator running on the Swinburne supercomputer (a Beowulf cluster of ∼300 commodity processors), including measures of the performance of the system. For example, to correlate all Stokes products for a 10 antenna array with an aggregate bandwidth of 64 MHz per station, and using typical time and frequency resolution, currently requires an order of 100 desktop‐class compute nodes. Due to the effect of Moore’s law on commodity computing performance, the total number and cost of compute nodes required to meet a given correlation task continues to decrease rapidly with time. We show detailed comparisons between DiFX and two existing hardware‐based correlators: the Australian Long Baseline Array S2 correlator and the NRAO Very Long Baseline Array correlator. In both cases, excellent agreement was found between the correlators. Finally, we describe plans for the future operation of DiFX on the Swinburne supercomputer for both astrophysical and geodetic science.

The S2 very long baseline interferometry tape recorder

The S2 is a new generation very long baseline interferometry (VLBI) tape recorder designed at the Institute for Space and Terrestrial Science (ISTS). The unique features of the S2 are its use of consumer-type videocassette transports to achieve low-cost, high data rate, high-density recordings; modularity in system design and extensive internal system self-monitor and diagnosis; and standardized record and playback interfaces for VLBI systems compatibility. The S2 sustains a maximum data rate of 128 megabits per second for up to 8.5 h, and has a maximum storage capacity of approximately 4/spl middot/10/sup 12/ bits; multiple S2's may be used for increased data rates of 512 megabits per second and beyond. This paper reviews the role of the VLBI tape recorder in the signal-processing chain of a complete VLBI system, a system in which it may be argued that the VLBI recorder has historically been the weakest link, then describes the S2 and its key features which serve to increase the reliability of the recorder in the VLBI data acquisition process at the radio telescope and the general means by which S2 recorded data may be reproduced at the VLBI correlator.

VLBI in Australia ? A Review

After two decades of Australian VLBI (very long baseline interferometry), high-resolution radio astronomy continues to be an active and fruitful research field. The status of Australian VLBI programs in astrophysics, astrometry and geodesy is reviewed and likely future developments are outlined. In addition to research programs with the Australian VLBI network, a number of successful collaborative projects are underway with overseas VLBI observatories. The inception of the Asia-Pacific Telescope will provide an important formal basis for fostering and extending international VLBI experiments in the Australian hemisphere. The APT will also serve a vital function in coordinating ground-based observations when the Soviet and Japanese VLBI space missions, Radioastron and VSOP, are launched in the middle of this decade. However, continued viable Australian participation in VLBI into the nineties will require new wide-bandwidth recording systems and an Australian VLBI correlator.

Multiagency VLBI Correlator Facility opens

A new $1.2 million facility to process geodetic Very Long Baseline Interferometry (VLBI) observational data was dedicated August 25, 1986, at the U.S. Naval Observatory headquarters, Washington, D.C., by representatives of the National Oceanic and Atmospheric Administration (NOAA), the Naval Research Laboratory (NRL), the U.S. Naval Observatory (USNO), and the National Aeronautics and Space Administration (NASA). The new facility will provide timely correlation of the data tapes produced by the POLARIS (Polar (Motion) Analysis by Radio Interferometric Surveying) earth orientation monitoring network. The new facility will also be used to process VLBI data collected by the NASA Crustal Dynamics Project (CDP) and by NRL and USNO programs in astrometry and basic research.