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    Live demo of the PED web interface. No username or password required. Contains a step by step tour showcasing functionality.

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History | Hardware | Software



        
        
        
        
        
        
        

History of PED

The PED project was initially conceived by members of the Karoo Array Telescope (KAT) computing team in late 2005. At this stage the team had little or no experience with building interferometric telescopes and PED was seen as having a valuable role to play in various risk mitigation tasks.

The primary objectives for PED are as follows:

  • Develop understanding of radio interferometry.
  • Provide a real world testbed for KAT operating and control software.
  • Use a "hardware-in-the-loop" approach for some aspects of the simulation work being carried out by KAT.
  • Provide an instrument for use in KAT outreach and educational programs.

It was also interesting to us that the team building the Allen Telescope Array (ATA) had followed a similar approach and built a small prototype array (Rapid Prototype Array) in order to test and evaluate various aspects of their proposed design for the ATA.

By March 2006 approval had been granted for the construction of PED at a vacant area on the grounds of the South African Astronomical Observatory (SAAO). A small out building that had once formed part of a solar transit telescope was dedicate for use as the PED control room. It was discovered that the building had not been entered for around 35 years. Some cleanup work was necessary.

During the second quarter of 2006 design was completed on the antenna controller and the receiver chain was constructed and exercised. Receiver first light (detection of a GPS satellite) happened in July 2006.

Unfortunately many procurement delays plagued us, and coupled with a lack of manpower due to other projects, meant that construction of the first antenna only occured during March 2007. However, much of the site preparation had been done by this stage so construction was relatively quick. A work party of interested individuals was convened and the first dish was completed in a matter of hours.

Another delay followed in which the focus of the KAT team shifted to delivery of our first antenna prototype located at the Hartebeeshoek Radio Observatory. Once this system had been successfully delivered in Dec 2007 some time became available for work on PED to continue. A project manager was appointed and a couple of people made time available to PED.

After completing the hardware phase and performing a number of single dish experiments our first interferometric observations were done. On February 15 2008 we produced our first fringe of the sun, proving the basic operation of the facility.

PED Hardware

The hardware used in PED is almost entirely off the shelf in order to save time and cost.

Antennas and Mounts

Currently we have 6 x 2.5m and 1 x 3.4m antennas installed on the site. These are all Orbitron amateur television mesh antennas optimised for reception at C-Band.

These antennas were chosen primarily for their low cost (around $250) and their suitability for use at 1.4 GHz. As shipped the antennas come with a basic fixed polar mount. The larger of the antennas has been left as a fixed mount with a Zenith pointing.

The smaller antennas are equipped with an horizon to horizon driven mount from Jaeger. This has a ball and screw jack driven declination adjustment with a geared horizon drive. Coverage is fairly limited with around 90 degrees horizontal and 30 degrees declination.

 

Antenna Layout

The design of the antenna layout was "outsourced" to students from the University of Cape Town as part of the NASSP program. The students used an antenna layout package produced by SKA South Africa known as Antconfig. Roger Deane designed the layout that we determined to be optimal for our work.

This layout is shown below (the open circles are snapshot UV points, the closed circles are physical antenna placings):

The baselines vary in length from a minimum of 5m to a maximum of around 40m.

Antenna Controller

The H-H drives we purchased did not have controllers and we could not find any off the shelf controllers that met our requirements. Each axis is driven by a PWM 36v signal and has a simple optical chopper providing pulsed feedback on the movement of the axis. Although of fairly poor resolution (around 10 pulses per degree) this was deemed sufficient for our tracking purposes.

Since part of the PED mandate was to test software we are using on our full scale telescopes the decision was made that the controller should be ethernet enabled so as to fit in with software that was under development at the time. To this end we constructed a controller consisting of a Microchip ENC28J60 Ethernet MAC and PHY, an Atmel AVR Mega microcontroller and a power driver.

A simple UDP communications protocol was implemented that allowed the software to query the mount status and position and accept a desired velocity command for either axis in order to drive.

Feed and LNA

As our interest was primarily the observation of strong continuum sources as well as galactic HI it made sense to optimise our receiver chain for 1.42 GHz. We briefly investigated manufacturing a feed in-house but came to the conclusion that we could not outperform the SETI Hydrogen Line feeds which are for sale commercially from Radio Astronomy Supplies.

 

The plots below show the result of the feed and dish analysis as carried out by the project. Shown is the far field pattern for the combined feed and dish system. It was determined that the optional choke supplied by RAS was not required. (The beam pattern cut is from -180 to 180 degrees)

 

As we had an HI optimised feed it made sense to use a similarly optimised LNA. We decided on the SLN series from SSB-Electronic which has a 0.3 dB noisefigure, 30 dB of gain and a 40 MHz 3dB bandwidth. At a price of around $300 these represented great value at the time.

Receiver

Post amplication the RF signal is transported via WBC-400 RF cable to the PED control room. Our digital receiver setup consists of 4 USRP digital receiver boards from Ettus Research. These flexible boards have two receive and two transmit channels per board and output I and Q for each receive channel via USB to a host computer.

These boards are closely linked to the Gnu Radio project which provides a framework for Software Defined Radio (SDR) use. This means that we have easy access to a complete digital receiver and extensive background processing suite.

RF Chain Overview

A schematic view of the overall receive chain for a single antenna is shown below:

Computing Hardware

There are 4 rackmounted IBM x3250 server machines that serve as the computing platform for PED.
  • Control: This machine is responsible for running the Telescope Operating System (more in the software section below)
  • 3 x Data Capture: Each USRP board (two antennas) has a dedicated capture machine that is responsible for dumping the raw data from the USRP board to disk and handling processing thereof.
A 24 port switch and a DSL router make up the remainder of the computing hardware.

Time Standard

A key requirement for an interferometric array is the coherent sampling of the signal from each antenna. This means that we need a stable reference clock that is distributed to each receiver board.

In its default configuration a USRP board runs from a local 64 MHz XO. With slight modifications these boards can accept an external reference. To generate this we start with a 64 MHz VCXO and a reference 10 MHz signal provided by a Trimble Thunderbolt GPS disciplined rubidium oscillator. Using a Reflock II board the 64 MHz is then phase referenced to the 10 MHz signal giving a very clean and stable 64 MHz clock which is then distributed to the USRP boards.

PED Software

A wide range of software is used by PED. Part of the design goals for PED were to use widely deployed open source software wherever possible. This allowed us to bring solutions from other areas into the radio astronomy space without huge cost.

A fair portion of the operating software is an extension of the work carried out by the CONRAD collaboration under the CTOS (CONRAD Telescope Operating System) project. In particular the CONRAD-SW-0011 document may be of interest.

An overview of the software is shown below (each major component is described in the sections that follow):

Monitoring and Control

Control and monitoring of the PED system is performed via SNMP set and get requests. This standard protocol is ideally suited to use in this environment as it has a low overhead, is simple to implement, has a good open source implementation available, and can easily be debugged "on the wire".

Each component that requires control or provides monitoring information runs an SNMP agent that communicates with a host SNMP daemon via an internal protocol known as AgentX. This allows for any number of agents to run on a single host with a single SNMP daemon handling network traffic.

Control commands are issued via a number of SNMP set instructions that configure the parameters for a particular command. The command is then triggered to execute either at a specified time or in an ASAP fashion. Due to this time trigger, real time control of the facility is not required as control can be performed a priori.

Each physical machine in the facility makes a range of monitoring information available via SNMP such as CPU temperature, fan speeds, etc... We chiefly use the IPMI tool to extract system parameters in place of the less reliable lm-sensors package.

Interaction with the system is done via the HELIOS Python libraries which abstract the low level SNMP commands and provide an object based interface for use by the various user interfaces. An LDAP directory is used to store system layout and configuration information which is essential for providing the SNMP abstraction.

Data Capture

As described in the hardware section above, we use USRP boards as digital receivers. As these boards are the target hardware platform for use with the GNU Radio software defined radio project, a large software stack is readily available.

Currently PED uses gnuradio 3.1 to handle the capture of the I and Q complex values for the two channels on each board. GNU Radio uses a streams and filters based approach to signal processing and includes a large number of components for manipulation of the input data.

The data capture setup consists of three data capture machines running Debian 4.0, with a single USRP board connected to each. One of the machines is designated the host and the other two slaves. At capture startup the master machine emits a clock reset pulse to the slaves in order to sychronise the sample counters.

The processing software has three components. The first writes the raw I/Q values for each channel, along with a common samplecounter to disk. The second produces the real time cross correlation between channel A and channel B, and the third produces a total power value for each channel.

The cross correlation and total power value are averaged and written to a UDP socket once every second. These values are used by the online interface for diagnostic purposes.

Antenna Control

There are six instances of the antenna control stack running at the PED facility. This allows each antenna to be individually controlled. As discussed earlier, control is via SNMP. In this case an object containing information about the target to track is constructed and sent to the Antenna Control Software Module via SNMP.

This ACSM is an in house software library that provides a wide range of antenna control and tracking functionality. This generic component communicates with a PED dish driver instance that converts standard coordinate requests into mount specific control (in this case a velocity request for each axis).

The ACSM is currently being used in PED, at the XDM at HartRAO, and will be used for KAT-7 and MeerKAT (for more information on these projects see www.ska.ac.za

The dish driver sends velocity requests to the mount which contains an embedded drive controller (also developed in house) that enacts the requested velocity for each axis.

Offline Correlator

For PED we only correlate the antenna pairs of a single USRP board in realtime. This means that we only get 3 out of the possible 15 baselines displayed to the user. This is fine for diagnostic purposes but the final PED data product needs to contain all the baselines.

The offline correlator is stand alone GNU Radio based software written in Python. For each antenna combination the software is run and produces a single baseline. The first step is two align the samples from each Antenna. This is done by matching the sample counter from Antenna A with that of Antenna B. Once the streams are aligned the same time domain correlator used in the realtime correlation is used and the cross correlation is written to disk.

The data from the slave machines is copied to the master machine prior to correlation.

User Interfaces

There are two user interfaces that are provided by PED. The first is the web interface which is used to control and monitor the facility remotely. For access to the live interface or to check out the demo see the links at the top of this page.

The web interface has been developed in house using Adobe Flex.

The second interface is an IPython interactive shell that exposes the low level functionality provided by the main HELIOS python library. This is primarily a debug and diagnostic interface but does provide more direct control of the facility.