John C. Webber

Precision Geodesy Using the Mark-III Very-Long-Baseline Interferometer System

Very-long-baseline interferometry (VLBI) has been used to make precise measurements of the vector separation between widely separated antennas. The system for acquiring and processing VLBI data known as Mark-III is described. Tests of the system show it to have millimeter-level accuracy on short baselines; measurements of baselines longer than a few hundred kilometers suggest that accuracy is limited by the uncertainty in the calibration of tropospheric path delay to the level of a few centimeters. VLBI experiments conducted between 1976 and 1983 have demonstrated the stability of the North American plate by showing that there is no change in the distance between easternl-California and Massachusetts at the level of a few millimeters per year or greater. Experiments made from 1980 to 1984 indicate that the distance from Massachusetts to Sweden is increasing by 1.7 ± 1 cm/year where the quoted standard deviation includes the estimated effects of systematic atic errors

Very-Long-Baseline Radio Interferometry: The Mark III System for Geodesy, Astrometry, and Aperture Synthesis

The Mark III very-long-baseline interferometry (VLBI) system allows recording and later processing of up to 112 megabits per second from each radio telescope of an interferometer array. For astrometric and geodetic measurements, signals from two radio-frequency bands (2.2 to 2.3 and 8.2 to 8.6 gigahertz) are sampled and recorded simultaneously at all antenna sites. From these dual-band recordings the relative group delays of signals arriving at each pair of sites can be corrected for the contributions due to the ionosphere. For many radio sources for which the signals are sufficiently intense, these group delays can be determined with uncertainties under 50 picoseconds. Relative positions of widely separated antennas and celestial coordinates of radio sources have been determined from such measurements with 1 standard deviation uncertainties of about 5 centimeters and 3 milliseconds of arc, respectively. Sample results are given for the lengths of baselines between three antennas in the United States and three in Europe as well as for the arc lengths between the positions of six extragalactic radio sources. There is no significant evidence of change in any of these quantities. For mapping the brightness distribution of such compact radio sources, signals of a given polarization, or of pairs of orthogonal polarizations, can be recorded in up to 28 contiguous bands each nearly 2 megahertz wide. The ability to record large bandwidths and to link together many large radio telescopes allows detection and study of compact sources with flux densities under 1 millijansky.

The ALMA correlator

Aims. The Atacama Large Millimeter Array (ALMA) is an international astronomy facility to be used for detecting and imaging all types of astronomical sources at millimeter and submillimeter wavelengths at a 5000-m elevation site in the Atacama Desert of Chile. Our main aims are: describe the correlator sub-system which is that part of the ALMA system that combines the signal from up to 64 remote individual radio antennas and forms them into a single instrument; emphasize the high spectral resolution and the configuration flexibility available with the ALMA correlator. Methods. The main digital signal processing features and a block diagram of the correlator being constructed for the ALMA radio astronomy observatory are presented. Tables of observing modes and spectral resolutions offered by the correlator system are given together with some examples of multi-resolution spectral modes. Results. The correlator is delivered by quadrants and the first quadrant is being tested while most of the other printed circuit cards required by the system have been produced. In its final version the ALMA correlator will process the outputs of up to 64 antennas using an instantaneous bandwidth of 8 GHz in each of two polarizations per antenna. In the frequency division mode, unrivalled spectral flexibility together with very high resolution (3.8 kHz) and up to 8192 spectral points are achieved. In the time division mode high time resolution is available with minimum data dump rates of 16 ms for all cross-products.

The ALMA 64-antenna correlator: Main technical features and science modes

We present the main features of the Atacama Large Millimeter/submm Array (ALMA) 64-antenna correlator sub-system and describe some original parts of the design. All correlator parts have been constructed, 3 quadrants are delivered to the 5000-m high site and 2 have been commissioned for ALMA Early Science. The basic observing modes are described and the huge flexibility embedded in the design is underlined.

An ALMA beamformer for VLBI and phased array science

Phasing all of the 12m ALMA dishes together will enable the array to function as a single telescope with an effective aperture of ∼ 85m diameter. In conjunction with other (sub)mm wavelength facilities, a phased ALMA will serve as the high sensitivity anchor for (sub)mm VLBI arrays capable of resolving super massive black holes on Schwarzschild radius scales. Current (sub)mm VLBI arrays have already detected time variable Schwarzschild radius scale structure in Sgr A∗, the presumed ∼ 4×106 Msun black hole at the center of the Milky Way [1,2]. Harnessing the full collecting area of ALMA will transform short wavelength VLBI arrays by doubling angular resolutions and improving sensitivity by an order of magnitude. At 1.3mm and 0.8mm wavelength, VLBI arrays including a phased ALMA will be able to time-resolve changing structures at the event horizon of Sgr A∗, search for periodic signatures of orbiting hot-spots in the innermost accretion flow, and study the jet launching region of the M87 jet with Schwarzschild radius resolution. A phased ALMA will also be a sensitive pulsar/transient observatory with the ability to search for shallow spectrum pulsars towards the Galactic Center and study known high frequency magnetars with sub-ns time resolution. This presentation will focus on the technical considerations of constructing and integrating a phased-array processor into the ALMA system. A detailed plan and design that conforms to all ALMA requirements and the construction schedule will be described. This design will enable initial VLBI and phased array science projects to be carried out with ALMA within 3 years.

Wideband correlators for radio astronomy

A description of modern wideband digital correlators being developed at the National Radio Astronomy Observatory (NRAO) for radio astronomy is presented. The NRAO is developing several modern wideband instruments for spectroscopic analysis in radio astronomy. One instrument now in the system test phase is a spectrometer intended for use on the 100 meter Green Bank Telescope (GBT) being constructed by the NRAO. This spectrometer has a maximum bandwidth of 6.4 GHz comprised of 8 separate 800-MHz-wide segments and can develop 16,384 points of spectral resolution in its wideband mode. Factor of 2 trade-offs between bandwidth and resolution can be made. In an alternate narrow bandwidth mode, the GBT spectrometer can develop spectra with 262,144 point resolution over a maximum bandwidth of 1600 MHz comprised of 32 separate 50-MHz-wide segments, again with bandwidth/resolution trade-offs possible. Additional spectrometers being built or planned using hardware developed for the GBT system include a spectrometer for the NRAO 12 Meter Telescope in Tucson and a test correlator for the proposed Millimeter Array (MMA). A design for the final MMA correlator is also being studied at the NRAO. The MMA is projected to be an aperture synthesis array consisting of about 40 radio telescopes (the exact array size is currently uncertain). The correlator proposed for the MMA will be able to analyze the auto- and cross-spectra of the array output with a bandwidth of up to 16 GHz per antenna.

Wideband digital filter using FPGAs

A digital filter being developed by the National Radio Astronomy Observatory (NRAO) for the Atacama Large Millimeter Array (ALMA) is presented here. The filter is designed using field programmable gate array (FPGA) integrated circuits and has an equivalent clock rate of 4 GHz. The ALMA radio astronomy array being developed by the NRAO in cooperation with several European scientific agencies will consist of up to 64 twelve-meter diameter antennas to be used for observing astronomical sources at millimeter and submillimeter wavelengths. This instrument is to observe with bandwidths up to 16 GHz per antenna in eight unbroken RF bands of 2 GHz each. Bandwidth-resolution trade-offs are done in ALMA by providing each of the eight 4-Gsample/s digitizers per antenna with a programmable Finite Impulse Response (FIR) digital filter. The filter can be selected to perform 1/1, 1/2, 1/4, . . ., 1/64 band filtering and sample decimation. In the 1/2 band mode, the filter has the equivalent of 128 tap weight multiplications. Tap weights increase by factors of two to the limit of 2048 tap weights when the filter is programmed to be a 1/32 band low-pass or high-pass filter. The filter is designed to process the output of a four-bit digitizer. The filter utilizes RAM look-up tables for tap weight multiplications. The output of the tap weight multiplications drive an adder tree which has 10-bit precision in the early stages and 7-bit precision in latter stages. Output quantizing of the adder tree output is done in look-up table RAM to 2- bit, 4-level precision. The filter is implemented on a 280-mm, 6-U, Euro card using FPGA ICs running with a system clock rate of 125 MHz.