Radio Astronomy Data Model for Single-Dish Multiple-Feed Telescopes, and Robledo Archive Architecture
All the effort that the astrophysical community has put into the development of the Virtual Observatory (VO) has surpassed the non-return point: the VO is a reality today, and an initiative that will self-sustain, and to which all archival projects must adhere. We have started the design of the scientific archive for the DSS-63 70-m antenna at NASA’s DSN station in Robledo de Chavela (Madrid). Here we show how we can use all VO proposed data models to build a VO-compliant single-dish, multiple-feed, radio astronomical archive data model (RADAMS) suitable for the archival needs of the antenna. We also propose an exhaustive list of Universal Content Descriptors (UCDs) and FITS keywords for all relevant metadata. We will further refine this data model with the experience that we will gain from that implementation.
To appear in the accompanying CD-ROM to Highlights of Spanish Astrophysics IV, 2007, eds. F. Figueras, J.M. Girart, M. Hernanz, C. Jordi (Springer)
The AMIGA project (Analysing the interstellar Medium of Isolated GAlaxies) was born in 2003, and intends to provide a statistical characterisation of a strictly selected sample of isolated galaxies composed by more than 1000 objects, by means of multi-wavelength data, and with a particular emphasis on radio data at cm, mm, and sub-mm wavelengths. All these data are being periodically released via the web page of the project111<http://www.iaa.csic.es/AMIGA.html>, which will soon provide a Virtual Observatory-compliant interface.
AMIGA+ is the natural extension to AMIGA, with three different goals: exploitation of the AMIGA catalog selecting the best candidates for a detailed study of isolated galaxies, scientific extension to the millimeter and submillimeter range, and participation in the development of systems allowing the access and display of large radio astronomical databases, both single-dish and interferometric.
1.1 Radio Astronomy in the VO
The VO can be defined as a set of protocols and data models that allow for easy discoverability of interoperable data-sets, which share an unified description by means of a common data model.
The protocols define how data are archived, searched for, and accessed, while data models describe the set of entities needed for information storage in a particular field, and specify both the data being stored, and the relationships between them. By establishing generic relationships, a data model able to store information for many different instruments can be defined, but in order to set the parameters for each antenna each telescope and instrument has to be separately studied.
Within the VO, the common interchange format is the VOTABLE, a FITS Hanisch:1999fk () replacement based upon XML Quin:2006fk (), while the communication protocols (such as the Simple Spectral Access Protocol, SIAP Dolensky:2006fk ()) are based upon common web-services technologies (SOAP Mitra:2003uq (), XML-RPC Winer:1999kx ()).
These common standards have been possible by the joint work of an international standardisation body, the International Virtual Observatory Alliance (IVOA) Hanisch:2003vn (). IVOA is a federation of national and supranational VO groups, and steers and sanctions the development of the different parts of the VO infrastructure, thanks to its different Working Groups. In particular, the Data Access Layer and Data Modelling groups are the ones trying to standardise access protocols and data models within the VO, such as the already mentioned SSAP, or the Space and Time Coordinates (STC) data models, and many others.
However, there is only one radio-related data model proposal within the IVOA, Lamb and Power’s IVOA Data model for raw radio telescope data LamPow0310IVOA (), but it is prior to many IVOA data modelling developments, and it constitutes a proposal for radio interferometry. Some of our efforts have been inspired by this proposal.
In a search for VO-compliant radio archives we only found the ATCA (Australian Telescope Online Archive) Data Model 2006astro.ph..1354 (). Again, even when it provides VO interfaces, it makes use of very few standard data models, and relies in custom implementations instead.
As no suitable data model existed for single-dish radio astronomy, and in order to develop a VO-compliant archive for single-dish antennas, while contributing to the development of the radio VO, a complete radio data model had to be defined.
2 RADAMS: Radio Astronomy DAta Model for Single-dish radio telescopes
Our goal is to develop a data model suitable for storage of single-dish spectroscopic data, in order to develop the scientific archive for the DSS-63 antenna, as a result of our collaboration with the LAEFF-INTA .
The DSS-63 is a very sensitive 70-m antenna, part of NASA’s Deep Space Network (DSN), located in the Madrid Deep Space Communications Centre (MDSCC) at Robledo de Chavela, and its main use is the monitoring and remote command of NASA’s missions in the Solar System. However, when not performing DSN-related tasks, the DSS-63 can make use of its 22MHz K-band receiver to perform spectroscopic observations with a 2 to 16 MHz bandwidth digital (384-samples) spectrometer. Observing time is allocated by LAEFF-INTA by agreement with NASA, with up to 260h reserved for host-country astronomers.
2.1 Data Model Overview
The data model that we have developed is specific to single-dish radio telescopes, and can hold data from multiple instruments for a single antenna, as long as they are spectrometers.
In order to build the RADAMS, we have reviewed the different data models offered by the IVOA, selected the most suitable for our needs, and then decided how to better group those data models into a single one.
Three are the main references for our data model:
The main components of the RADAMS, together with the documents inspiring each section, are shown in Fig. 1. In gray we have marked the components that needed further specification, because they have not been developed by current IVOA standards, or whose definition had to be changed in order to accommodate RADAMS’ specifications. The SDM is only used in the data import/export sequence.
In particular, Packaging and Policy had to be fully specified, and for our packaging and data delivery needs we have developed the VOPack, a system for fully characterising and delivering VO-compatible datasets. As for the Policy class, a role based mechanism will be provided, that allows for different data access levels for different roles, instead of basing permissions upon users.
More than 40 classes and subclasses are specified by the RADAMS. For each class, the RADAMS specifies an Attribute name, the corresponding FITS Keywork (both for importing and exporting data), the associated UCD (Universal Content Descriptor: gives semantic information about the attribute, and allows for attribute matching between databases with different attribute names), and finally a full Description of the attribute, explaining the meaning and possible values of each attribute.
2.2 Archive Infrastructure
The different archive components have been distributed in different layers in order to minimize layer and component communication, which allows for easier development of the archive infrastructure. The different layers and components are shown in Fig. 2.
From left to right, the different layers contain software packages, running from the Instrument (the Control System) to the Interfaces for users or software packages performing queries to the archive.
The telescope’s Control System writes data, and after being processed by a semi-automated workflow, the Archive Backend will store both the raw data and the workflow-generated metadata. The Archive Services layer will allow several components to perform queries on the stored data and metadata, and provides the infrastructure for the Interfaces layer, where user- or machine-operated clients will be able to use standard VO protocols to query the archive.
3 Conclusions and Future Work
The IVOA modelling efforts have built a solid foundation upon which we have built the RADAMS. However, we needed to perform a careful selection and synthesis of the provided data models in order to provide a unified data model for the DSS-63.
In particular, the RADAMS provides definitions for the Provenance, Policy and Packaging classes, with the addition of the VOPack, a VO-specific XML-based packaging and delivery format.
We plan to release the RADAMS as an IVOA Note for the IVOA Radio interest group, and present the VOPack to the IVOA Data Access Layer Working Group for evaluation.
At the same time, we will implement the RADAMS as the foundation for the scientific archive of the DSS-63 antenna, and we will extend its capabilities in order to be able to use the RADAMS with additional antennas.
This work has been partially supported by DGI Grant AYA 2005-07516-C02-01 and the Junta de Andalucía (Spain). JDSV acknowledges support at the IAA/CSIC by an I3P Postgrade Grant, funded by the European Social Fund. The IAA-LAEFF collaboration is partially funded by the Spanish Virtual Observatory Network, funded by the PNAYA, AYA2005-24102-E.
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