KEYWORDS: Signal detection, Interference (communication), Signal processing, Phased arrays, Radio astronomy, Astronomy, Data modeling, Signal to noise ratio, Statistical analysis, Receivers
In this paper we develop two statistical rules for the purpose of detecting pulsars and transients using signals from phased array feeds installed on a radio telescope in place of a traditional horn receiver. We assume a known response of the antenna arrays and known coupling among array elements. We briefly summarize a set of pre-processing steps applied to raw array data prior to signal detection and then derive two detection statistics assuming two models for the unknown radio source astronomical signal: (1) the signal is deterministic and (2) the signal is a random process. The performance of both detectors is analyzed using both real and simulated data.
KEYWORDS: Field programmable gate arrays, Control systems, Signal processing, Software development, Computing systems, Radio astronomy, Radio telescopes, Observatories, Astronomy, Clocks
NRAO recently built the Digital Backend System (DIBAS) for the Shanghai Astronomical Observatory's (SHAO)
65 meter radio telescope. The machine was created from the design of the VErsatile GBT Astronomical Spec-
trometer (VEGAS) by adding pulsar search and timing modes to complement the VEGAS spectral line modes.
Together the pulsar and spectral line modes cover all anticipated science requirements for the 65 meter, except
VLBI. This paper introduces the radio telescope backend and explores the project management challenges of the
project. These include managing the high level of reuse of existing FPGA designs, an aggressive schedule for the
project, and the software design constraints imposed on the project.
The National Radio Astronomy Observatory Green Bank Telescope (GBT) is the world's largest fully steerable
telescope. The GBT has now been in routine operation for over two years, observing at frequencies up to 50 GHz. In
order to deliver the tracking accuracies required at 50 GHz, we solve simultaneously for gravitational and thermal
effects in the development of the static pointing and focus tracking models. A precision temperature sensor system then
generates additional real-time corrections to compensate for varying thermal gradients in the antenna. Collimation and
surface accuracy requirements are met by an active surface control system which combines initial corrections derived
from a finite element model of the antenna with additional terms derived from astronomical phase-retrieval holography
measurements. The GBT has a rich suite of instrumentation including receivers which cover almost the complete
frequency range from ~ 290 MHz to 50 GHz, and backends for spectroscopy, pulsar observing, broadband continuum,
very long baseline interferometry and planetary radar reception. A 64-pixel bolometer camera is under development by
a consortium including UPenn, NASA-GSFC, NIST, UCardiff and NRAO. Recent software developments include an
extremely flexible application which combines traditional interactive observing, scheduling-block based observing and
real-time monitoring and data display in a single, convenient interface. In this paper I will summarize the current
performance of the GBT, and review some recent science results. I will also describe how plans changed with time, and
review some of the lessons learned in the development of the telescope.
The NRAO Robert C. Byrd Green Bank Telescope is a new 100 meter radio telescope that is now in routine astronomical operation. This paper gives a status report on astronomical performance, operational capabilities, development plans, and early scientific highlights. The telescope is now in regular operation to frequencies of 50 GHz, which achieves the base design goal of the facility. Both the surface and pointing accuracies allow good performance at that frequency. The active surface of 2004 movable panels is in regular use at all frequencies above ~8 GHz and significantly improves both gain and diffraction beam shape at high and low elevation angles. Receivers exist at most primary observing frequencies ranging from ~290 MHz to 50 GHz. A 256k-channel correlation spectrometer with up to 16 independent inputs is in routine use for spectroscopy. Backends for pulsar observing, broadband continuum, Very Long Baseline Interferometry, and planetary radar reception also exist. An advanced development program for enhancing the performance of the GBT is also underway. This program includes the Precision Telescope Control Project that will extend operation to 115 GHz or 2.6 mm wavelength (see the paper by Prestage and Constantikes in this volume), a 26-40 GHz receiver, a 68-92 GHz receiver, a fast sampling continuum backend built by a Caltech/NRAO collaboration, and a 64-pixel bolometer camera developed by a consortium of UPenn, NASA-GSFC, NIST, UCardiff, and NRAO. Software development projects for enhanced user interfaces and data handling are underway, and plans to implement queue-based dynamic scheduling and remote observing are being developed. The status of these projects and their anticipated scientific impact will be discussed.
The NRAO Robert C. Byrd Green Bank Telescope (GBT) is a 100m diameter
advanced single dish radio telescope designed for a wide range of
astronomical projects with special emphasis on precision imaging.
Open-loop adjustments of the active surface, and real-time corrections
to pointing and focus on the basis of structural temperatures already allow observations at frequencies up to 50GHz. Our ultimate goal is to extend the observing frequency limit up to 115GHz; this will require a two dimensional tracking error better than 1.3", and an rms surface accuracy better than 210μm.
The Precision Telescope Control System project has two main components. One aspect is the continued deployment of appropriate metrology systems, including temperature sensors, inclinometers, laser rangefinders and other devices. An improved control system architecture will harness this measurement capability with the
existing servo systems, to deliver the precision operation required.
The second aspect is the execution of a series of experiments to identify, understand and correct the residual pointing and surface accuracy errors. These can have multiple causes, many of which depend on variable environmental conditions. A particularly novel approach is to solve simultaneously for gravitational, thermal and wind effects in the development of the telescope pointing and focus tracking models. Our precision temperature sensor system has already allowed us to compensate for thermal gradients in the antenna, which were previously responsible for the largest "non-repeatable" pointing and focus tracking errors. We are currently targetting the effects of wind as the next, currently uncompensated, source of error.
The National Radio Astronomy Observatory Green Bank Telescope (GBT), the world's largest fully steerable telescope, is now undergoing commissioning and early scientific operation. The GBT has many innovative design features that advance imaging quality, sensitivity, and versatility. These include an unblocked aperture, an active surface, and a six-degree of freedom Gregorian subreflector. The GBT has an advanced laser rangefinder metrology system, which will measure the position of the active surface panels, and also guide the precision pointing of the telescope. Early commissioning results have confirmed the performance of the telescope, and exciting scientific discoveries are already being made. This paper describes the various features of the telescope in more detail, and presents the latest results.
KEYWORDS: Control systems, Telescopes, Java, Heterodyning, Observatories, Telecommunications, Control systems design, Human-machine interfaces, Data integration, Calibration
The James Clerk Maxwell Telescope is escaping from its dependence on VAX/VMS legacy software by incrementally upgrading its control system using platform-independent tools. The first stage of the upgrade includes a GUI, written in Java and modeled on project planning software, for defining complex observing recipes. Another GUI serves as an editor for observation definition files describing the details of an individual observation. The recipe designer also serves as an observation subsystem controller, executing observing scripts containing combinations of low- level and high-level commands, using the parameters from the observation definition file. The next stage of the upgrade includes a Java-based queue manager. The DRAMA system developed at the Anglo Australian Observatory provides a convenient bridge between the existing VAX/VMS-based instrument tasks and the new controller and queue manager.
As part of the upgrades to our observatory control software, we have developed a replacement for the astrometric kernel and the servoing system at the James Clerk Maxwell Telescope. The new PTCS astrometric kernel operates within the DRAMA tasking system. An EPICS servo controls the antenna drives. The THI, a DRAMA task, translates demands from the kernel and sends them to the servo, and then feeds back to the kernel the current drive encoder positions and the expected time of arrival at source position. The TEL DRAMA task at the top level interfaces between the existing observation control software and the telescope-independent PTCS task. This new system is replacing components of an existing observatory a Vax platform and sent demands via GPIB to a servo on a VME platform. The new PTCS and the TKI software run on a Unix machine and communicate via the DRAMA message system to the upper layer observatory system and via Channel Access to the EPICS servo running VxWorks on the VME platform. In this paper we will review the main features of the new telescope control system, and report on progress with implementation.
The portable telescope control system (PTCS) project is a collaboration between the Anglo-Australian Observatory and the Joint Astronomy Centre, Hawaii. The project aims to develop telescope control software in a form which is portable between a wide range of computer systems, and which can easily be adapted to different telescopes. PTCS uses the DRAMA software environment which provides a high degree of operating system independence. The PTCS design is based on tested algorithms used in existing successful telescope control systems. The initial version of PTCS is now being interfaced to an EPICS based drive servo system for use with the James Clerk Maxwell sub-millimeter telescope on Mauna Kea.
KEYWORDS: Telescopes, Space telescopes, Servomechanisms, Raster graphics, Seaborgium, Stereolithography, Control systems, Telescope design, Control systems design, Astronomical telescopes
Many telescope control systems now make use of the so-called 'virtual telescope' concept -- a software abstraction of the real telescope which masks imperfections in the hardware from higher levels of the software. In general, this approach allows for elegant and rigorous control of telescope pointing and tracking. When slewing, however, while the virtual telescope arrives on source immediately, the real telescope only catches up after some time. This is especially a problem when performing raster-scanned observations: since the demand position and velocity have discontinuities at the end of each row, a naive implementation of the standard virtual telescope system results in missing the demand positions at the start of each row. In the existing JCMT telescope control system (TCS), this problem is solved by having the TCS calculate a route in (az,el) space for the real telescope to follow which results in it arriving at the correct position, moving with the correct velocity, at a predictable time in the future. In this paper we describe a generalized implementation of this technique, which has the added advantage that the 'astrometric kernel' and 'telescope servo' layers are cleanly separated, allowing telescope-specific hardware to be combined with a generic astrometric kernel. Since the solution requires only minimal changes to the standard virtual telescope design, this approach may be of interest to other telescopes which are currently using, or are planning to use that design.
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