Alejandro Saez

An ALMA beamformer for ultra-high resolution VLBI and phased array science

By phasing all ALMA dishes together into a single effective aperture, ALMA can operate as both an exceptionally sensitive mm/sub-mm VLBI element and a beamformed array suitable for high frequency pulsar work. A detailed design for implementing a system to phase up the array has been completed and funding to build and integrate this capability into ALMA has been secured. Here we describe the basic elements of the system, outline the specifications, and review expected sensitivities. With this system in place, ALMA will become a key element in Global mm/sub-mm VLBI arrays that target a broad range of high sensitivity and high angular resolution science.

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.

Introducing hardware in the loop and model based simulation concepts in the ALMA observatory for software testing

The Atacama Large Millimeter /sub-millimeter Array (ALMA) has been working in operations phase regime since 2013. The transition to the operations phase has changed the priorities within the observatory, in which, most of the available time will be dedicated to science observations at the expense of technical time required for testing newer version of ALMA software. Therefore, a process to design and implement a new simulation environment, which must be comparable - or at least- be representative of the production environment was started in 2017. Concepts of model in the loop and hardware in the loop were explored. In this paper we review and present the experiences gained and lessons learned during the design and implementation of the new simulation environment.

Evaluation of controllers for tuning digitizers in the ALMA interferometer

In radio astronomy interferometers where the number of stations is large (in the ALMA case 66 antennas, where 8 digitizers are deployed in each antenna) tuning the digitizers parameters: thresholds and bias, is a process which needs to be repeated several times, therefore finding an algorithm that allows to speed up this process is a critical task. It is quite important to keep the digitizers properly adjusted in order to reach the maximal efficiency of the correlator, specially in a regime of coarse quantization (88% for 2 bits, 96% for 3 bits), and also is critical for avoiding signal artifacts which can degrade the collected data (DC bias or harmonics). This work presents a set of different approaches for automatically tuning the digitizers primary selected as: PID by using a proportional/integrative/derivative controller and defining a system to process a coupled MIMO system as an uncoupled SISO; Fuzzy Logic by making extensive advantage of the expert operator knowledge; and finally an hybrid scheme combining PID and Fuzzy Logic for a rapid and accurate tuning process. The aim of the present work is to evaluate the performance of each tuning method based on metrics like: required tuning time, stability and robustness under different extreme boundary conditions. In addition, we suggest the means for collecting the needed information considering an usual interferometer architecture. Furthermore, we provide an automated approach to find the best samplers clock timing profile. The aim of this work is to provide a guideline for implementing an algorithm which allows to tune a large set of digitizers under different conditions in a fast and precise automated process. The produced report will come in handy for integration into interferometer projects comprising a large number of individual stations (ALMA, SKA, VLA, CHIME, MeerKAT).

The evolution of the simulation environment in the ALMA Observatory

The Atacama Large Millimeter /submillimeter Array (ALMA) has entered into operation phase since 2013. This transition changed the priorities within the observatory, in which, most of the available time will be dedicated to science observations at the expense of technical time. Therefore, it was planned to design and implement a new simulation environment, which must be comparable - or at least- be representative of the production environment. Concepts of model in the loop and hardware in the loop were explored. In this paper we review experiences gained and lessons learnt during the design and implementation of the new simulation environment.

Implementing the concurrent operation of sub-arrays in the ALMA correlator

The ALMA correlator processes the digitized signals from 64 individual antennas to produce a grand total of 2016 correlated base-lines, with runtime selectable lags resolution and integration time. The on-line software system can process a maximum of 125M visibilities per second, producing an archiving data rate close to one sixteenth of the former (7.8M visibilities per second with a network transfer limit of 60 MB/sec). Mechanisms in the correlator hardware design make it possible to split the total number of antennas in the array into smaller subsets, or sub-arrays, such that they can share correlator resources while executing independent observations. The software part of the sub-system is responsible for configuring and scheduling correlator resources in such a way that observations among independent subarrays occur simultaneously while internally sharing correlator resources under a cooperative arrangement. Configuration of correlator modes through its CAN-bus interface and periodic geometric delay updates are the most relevant activities to schedule concurrently while observations happen at the same time among a number of sub-arrays. For that to work correctly, the software interface to sub-arrays schedules shared correlator resources sequentially before observations actually start on each sub-array. Start times for specific observations are optimized and reported back to the higher level observing software. After that initial sequential phase has taken place then simultaneous executions and recording of correlated data across different sub-arrays move forward concurrently, sharing the local network to broadcast results to other software sub-systems. The present paper presents an overview of the different hardware and software actors within the correlator sub-system that implement some degree of concurrency and synchronization needed for seamless and simultaneous operation of multiple sub-arrays, limitations stemming from the resource-sharing nature of the correlator, limitations intrinsic to the digital technology available in the correlator hardware, and milestones so far reached by this new ALMA feature.

Porting the ALMA Correlator Data Processor from hard real-time to plain Linux

The ALMA correlator back-end consists of a cluster of 16 computing nodes and a master collector/packager node. The mission of the cluster is to process time domain lags into auto-correlations and complex visibilities, integrate them for some configurable amount of time and package them into a workable data product. Computers in the cluster are organized such that individual workloads per node are kept within achievable levels for different observing modes and antennas in the array. Over the course of an observation the master node transmits enough state information to each involved computing node to specify exactly how to process each set of lags received from the correlator. For that distributed mechanism to work, it is necessary to unequivocally identify each individual lag set arriving at each computing node. The original approach was based on a custom hardware interface to each node in the cluster plus a realtime version of the Linux Operating System. A modification recently introduced in the ALMA correlator consists of tagging each lag set with a time stamp before delivering them to the cluster. The time stamp identifies a precise 16- millisecond window during which that specific data set was streamed to the computing cluster. From the time stamp value a node is able to identify a centroid (in absolute time units), base-lines, and correlator mode during that hardware integration. That is, enough information to let the digital signal processing pipeline in each node to process time domain lags into frequency domain auto-correlations per antenna and visibilities per base-line. The scheme also means that a good degree of concurrency can be achieved in each node by having individual CPU cores process individual lag sets at the same time, thus rendering enough processing power to cope with a maximum 1 GiB/sec output from the correlator. The present paper describes how we time stamp lag sets within the correlator hardware, the implications to their on-line processing in software and the benefits that this extension has brought in terms of software maintainability and overall system simplifications.

Detecting anomalies in astronomical signals using machine learning algorithms embedded in an FPGA

Taking a large interferometer for radio astronomy, such as the ALMA1 telescope, where the amount of stations (50 in the case of ALMA’s main array, which can extend to 64 antennas) produces an enormous amount of data in a short period of time – visibilities can be produced every 16msec or total power information every 1msec (this means up to 2016 baselines). With the aforementioned into account it is becoming more difficult to detect problems in the signal produced by each antenna in a timely manner (one antenna produces 4 x 2GHz spectral windows x 2 polarizations, which means a 16 GHz bandwidth signal which is later digitized using 3-bits samplers). This work will present an approach based on machine learning algorithms for detecting problems in the already digitized signal produced by the active antennas (the set of antennas which is being used in an observation). The aim of this work is to detect unsuitable, or totally corrupted, signals. In addition, this development also provides an almost real time warning which finally helps stop and investigate the problem in order to avoid collecting useless information.

Low-cost Ku band interferometer for educational purposes

Latest discoveries in the field of astronomy have been associated to the development of extremely sophisticated instruments. With regards to radio-astronomy, instrumentation has evolved to higher processing data rates and a continuous performance improvement, in the analog and digital domain. Developing, maintaining, and using such kinds of instruments – especially in radio-astronomy – requires understanding complex processes which involve plenty of subtle details. The above has inspired the engineering and astronomical communities to design low-cost instruments, which can be easily replicated by the non-specialist or highly skilled personnel who possess a basic technical background. The final goal of this work is to provide the means to build an affordable tool for teaching radiometry sciences. In order to take a step further this way, a design of a basic interferometer (two elements) is here below introduced, intended to turn into a handy tool for learning the basic principles behind the interferometry technique and radiometry sciences. One of the pedagogical experiences using this tool will be the measurement of the sun’s angular diameter. Using these two Ku band receptors, we aim to capture the solar radiation in the 11-12GHz frequency range, the power variations at the earth spin, with a proper phase-lock of the receptors will generate a cross-correlation power oscillation where we can obtain an approximation of the angular sun’s diameter. Variables of interest in this calculation are the declination of the sun (which depends on the capture date and location) and the relation between maximal and minimal power within a fringe cycle.

Phasing ALMA with the 64-antenna correlator

The main characteristics of the ALMA 64-antenna correlator and its status are very briefly presented. It is recalled that the basic elements required to phase up the ALMA array, and thus enhance the ALMA science objectives, have been included in the correlator original design. We give an overview of the new phasing interface card which needs to be added to the 64-antenna correlator to implement the VLBI mode in ALMA. Finally, we briefly describe the test fixture which is planned to validate in the laboratory the new phasing card and the VLBI mode without disturbing ALMA current operation.