LOFAR-UK observes the active sun

Over the last few days, our Sun has been very active. Apart from the dedicated solar instruments monitoring this activity, we've also been able to observe it with the LOFAR radio telescope at Chilbolton. This facility comprises two fields of small antennas, that link together to perform as a powerful VHF radio telescope. Although it is also possible to link the LOFAR system in the UK with others across Europe, on the 28th September 2011, it was operating independently.



At 16:10 UTC (= 5:10 pm local time), LOFAR Chilbolton was used to record an 'all-sky' image. Even before calibration and analysis, this image shows the bright objects in the radio sky. On the left-hand side of the image there are, from top to bottom, Cassiopeia-A (a supernova remnant), Cygnus-A (a radio-galaxy) and the broader region which is the centre of our own Galaxy. However, to the right of the image, the Sun can be seen with extremely strong intensity, just before it set below the western horizon.

The image shown was made using the UK608 LBA. It demonstrates what a LOFAR station is capable of, even when working alone and independent of the main International LOFAR network. The image is not calibrated nor cleaned in anyway. The observations were made by Menno Norden (ASTRON), and the report by Derek McKay-Bukowski (STFC).

The web release is also available from STFC's "RAL Space" website: http://www.stfc.ac.uk/RALSpace/Areas+of+expertise/Astronomy/LOFAR-UK/In+the+news/37611.aspx

Updated Science Image

Today I updated the figure (on the right panel) that shows the different science cases that can be addressed with KAIRA. I have added two more cases: Relative TEC tomography, and tropospheric echos.

Relative TEC tomography involves several different LOFAR stations simultaneously measuring the total electron content, which is basically the difference in ionospheric propagation delay. Because LOFAR can do this with radio stars, in theory it should be possible to perform these kinds of measurements on a huge number of directions simultaneously. This is a huge advantage compared to satellite tomography, which can only measure the total electron content between ground based stations and satellites. This large number of rays might make it possible to do extremely high resolution 3D ionospheric tomography with LOFAR. Currently the best location for doing this would be near the core of the array, where there are many stations located nearby.

The tropospheric echo science case was added after discussions with Ingemar Häggström of EISCAT, who told me that he often sees tropospheric echos in Kiruna, both from the VHF and UHF. With multistatic receivers, this would potentially allow tropospheric wind and scattering spectrum measurements.

HBA Rack installation

Last week Peter Gruppen ja Jan-Pieter de Reijer from ASTRON came by, and did the cabling work and receiver rack installation at the KAIRA site.

The HBA cables going into the digital receivers. 

Beamformer cables going into the switch. 

 Now if we could just get summators (which are in short supply right now) before snowfall, we could start measuring. 

LBA protype v 2.0

LBA platform version 2.0. Photo: Markku Postila

Here is another prototype of the raised LBA antenna platform designed by Markku Postila and Toivi Iinatti. This one allows arbitrary rotation of the antenna and has easily adjustable legs. The picture also shows the LBA antenna installed on top of the platform.

However, it seems that we will need to extend the ground plane with this type of a design. Recent antenna modeling results by Johan Borg of LTU suggests that this configuration has the effect of broadening of beam significantly at higher frequencies.

NEC model of the antenna with a raised ground sheet.

40 MHz beam pattern of an individual element. Slight broadening at 40 MHz, but nothing too serious.

80 MHz beam pattern of an individual element. The raised platform causes a broadening of the beam and a deeper minima at the zenith.

Optical fiber

Laying the last meter of fiber to the RF container. Photo: Markku Postila.

Yesterday there was good news, and some bad news. First of all, the fiber cabling work has now been completed. Whenever we want to, we can start using the internet over fiber cable! However, in terms of bad news, we didn't get any summators, and we are thus not able to start using the antenna yet. There seems to be some kind of a problem with the production of the summators, and nobody knows of a delivery date. If this is delayed a month more, it will be too cold and snowy to install them this winter, and we will have to wait for next summer before we can measure with the system.

TEC Measurements with LOFAR

TEC Measurements with LOFAR. Copyright: Mark Aartsen. 

Today the ASTRON daily image featured a very interesting image in terms of geophysics. It featured a total electron content measurement produced by listening to a strong radio source using four different stations. This is interesting because if one can produce such measurements using many different radio sources, it would be possible to use this for three dimensional ionospheric tomography. 

LBA Prototype platform

The LBA platform prototype, built by Markku Postila and Toivi Iinatti. 

We are now already thinking about the LBA build next year. This will be challenging because we cannot level the field, as it will probably cause erosion and water quality problems with the nearby Siilasjärvi lake, were all the water drains to. We are now focusing on a design which involves similar raised platforms, which hopefully will allow easy position adjustments, when permafrost moves our antennas around. Another reason for the platform is that we will need to use them in order to level out the antennas, if we cannot level out the ground. In anticipation of this, Johan Borg of LTU is currently investigating the effect of such a platform on the beam pattern of an individual antenna.

Summators and signal processing installation

Wireless internet mast installed on the RF-container. The Barracks have also been moved next to the RF container. 

This week ASTRON will visit the KAIRA site and install the summators inside the HBA tiles. After this, they will install the A/D converters and beamformers inside the container. After this, we they will hopefully test all of the antennas, and we can hopefully start calibrating the antenna. With Ilkka now in the Netherlands, this is a prime opportunity for us to learn how to use the signal processing hardware.

Lego radio antennas — Part 1

Putting the KAIRA site together has been sometimes described as being akin to assembling flat-pack furniture or building Lego. Ah... if only it was that simple! Despite the failings of this analogy, it is a fun one. And it can certainly be a great way to demonstrate or explain concepts. So, in this article (the first of a little special series), we'll take a look a some examples of miniature radio antennas built from that famous Danish toy: Lego!

And, in order to be topical, our first example comes from the SKA (Square Kilometre Array) community. Although the SKA will have aperture arrays like KAIRA, it will also have sets of conventional radio dishes as well.

Currently, there are a variety of prototypes for antennas (both dishes and phased arrays), which are being considered for the two main site candidates of the final SKA project. At this stage, it has not been decided where the final telescope system will be located, but the list has been shortened to two strong candidates: Southern Africa and Australasia.

Our Lego model today is from the second of the two: the anzSKA project.

Here are the pieces...


And the build sequence...


And the result!


More models of other systems are to follow on this web log (including some stunning ones of KAIRA and LOFAR). We'll also be using them to show how they can be used to explain radio astronomy and engineering concepts to children. So stay in touch for our future instalments!

Path to SKA-low Workshop

This week, there is the "Path to SKA-low Workshop". SKA is the Square Kilometre Array (see our two previous articles: LINK1, LINK2), and this important meeting focuses on the aperture arrays that will be used in this up-coming radio telescope.

As part of the SKA's Aperture Array Verification Programme (AAVP), there will be reports on the latest science and system results from low-frequency pathfinders (LOFAR, MWA, KAIRA, etc.), discussions on the major system design challenges, and progress reports on the crucial technologies and techniques. Not only will KAIRA be contributing its experience to the discussions, but we hope to learn some important lessons to assist with the EISCAT_3D project.

The workshop is being hosted by the International Centre for Radio Astronomy Research (ICRAR), Perth, Western Australia from the 6th to 9th September, 2011.

Link: http://www.icrar.org/news/ska-low

EISCAT International Workshop

The 15th EISCAT International Workshop will be held in Qingdao, China, 5-9 September 2011, hosted by the China Research Institute of Radiowave Propagation (CRIRP). A ceremony for the 30th anniversary of the EISCAT measurements will highlight the Workshop.

There will be a presentation about KAIRA on Friday.

Looking forward to an interesting week.

EISCAT_3D Executive Meeting

The EISCAT_3D Executive Board held its third physical meeting at the Sodankylä Geophysical Observatory (SGO) on Wednesday and Thursday of this week (31.8.-1.9.2011).

A full report is available on the EISCAT_3D web log.

LOFAR Data School

This week, there has been the LOFAR data school, held in the United Kingdom. A report is available on the LOFAR-UK web log (along with some photographs).

Official website: http://www.jb.man.ac.uk/meetings/LOFAR_UK/