Fine tuning FFT LPI

Going from initial first light to calibrated and cross validated ionospheric plasma parameter measurements is not an overnight step. However, the KAIRA team is making good progress. We have now managed to get fast Fourier transform lag-profile inversion working to some extent with real world signals contaminated with space debris and meteor head echos. We can now estimate ionospheric plasma incoherent scatter autocorrelation functions and spectra with a fairly nice accuracy and computational speed.

Here is an example of a 512 second integration combined from all of the 30 beams intersecting the Tromsø VHF beam at different altitudes. With this experiment that has a 128 microsecond baud length, we can reach altitudes up to 400 km.

(Click image to enlarge)

The plots are fairly similar with monostatic VHF spectra, although there are still lots of challenges with absolute calibration of electron density. These issues can hopefully be solved by comparison with monostatic Tromsø measurements, or with future plasma line measurements with KAIRA.

The completed mausolea

The cable mausoleum is used to store excess lengths of the cable from the array. There is one for each of the low-band and high-band arrays. Normally this cable is buried (in what is called a cable 'grave'). However, for KAIRA this needs to be above ground (hence the corresponding term 'mausoleum'). Today we have some photographs by Arttu Jutila of the two completed cable mausolea.

The completed LBA cable mausoleum. (Photo: A. Jutila)

The HBA cable mausoleum, in front of the RF-container. (Photo: A. Jutila)

Observing tips #2 — Keeping an observing log

In the old days, we all used to keep observing logs. It would have the date and time of all the things that we did, what we were looking at and any notes that we thought were important along the way. It was in a log book; yes, a physical book and the notes would all be hand-written.

Things have moved on, of course, and these days the telescope control, the data acquisition and the data processing are all done on computers. While some of us still keep log books, it is all too easy to forget to write down a command or two... usually the critical one that you need to remember later (because it had lots of arguments!).

As most of our systems use Linux (or some other UNIX-like environment), there is the "history" command, which can be used to see all the commands that have been put into the system. This is very useful. For example:

   % history

    1  ls

    2  ls - ltr
    3  cat obs_notes.txt
    4  history

But by default it does not include the time when the command was issued. However, there is a way around this. In bash (a common shell used on many Linux systems), the history command will check for the history format shell variable. So, you can set this before running your history command (in fact, it might be useful to put it in your startup script, such as .bashrc).

   % export HISTTIMEFORMAT="%Y%m%d-%H%M%S %% "

Then, when you run your history command, you get this:

   % history

    1014  20120827-114212 % swlevel 3
    1015  20120827-114218 % rspctl --regstat
    1016  20120827-114347 % poweruphba.sh 5
    1017  20120827-114410 % rspctl --rcu
    1018  20120827-114422 % cd

I prefer to use the compact format "YYYYMMDD-HHMMSS", because that way you can sort on this field. While this may seem superfluous for a simple example like this, what may happen is that there are commands coming in for all sorts of different terminals. So, what you can then do is to save them all, and then sort by time.

For example, in the first terminal:

   term1% history >> obs_notes.txt

And then, in the second terminal:

   term2% history >> obs_notes.txt

   term2% sort -k 2 obs_notes.txt

The sort command will then put them all into order for you. Note that you need to use ">>", not ">" in order to append to the obs_notes.txt file. Otherwise you simply overwrite the previous version.

Of course, you can play around with awk and grep and sed to get other fancy formatting, but you get the idea.

Hope it helps!

Lag-profile inversion

We have been extremely busy developing new analysis methods for efficient side-station inversion of the incoherent scatter measurements. Here is a preview of what we have accomplished so far. The follow plot combines lag-profile inversion results from 20 different beams simultaneously pointing at different altitudes along the EISCAT VHF beam. With this configuration, we can perform a bi-static measurement simultaneously along all the different altitudes. Measurements like this will be extremely useful in the future for determining e.g., ionospheric drift velocities. The plot shows standard E- and F-region incoherent scatter autocorrelation functions, along with what seems to be tropospheric propagation from the Tromsø antenna sidelobes.

Averaged lag-profile matrix (incoherent scatter autocorrelation functions for different ranges) measured with KAIRA.  Ranges given as round-trip time, i.e., half of the time that light travels from VHF to KAIRA given in range, assuming speed of light in vacuum. 

The LBA cable mausoleum

As you will have seen over the last month or so, the excess cable lengths from the low-band antenna (LBA) array are zig-zagged (not looped!) back and forth in a second mausoleum. Well, this part of the project has now been finished off... and here are some photographs from Arttu Jutila of the work.

All cable work complete and the temporary roof removed ready for the finishing work. (Photo: A. Jutila)

Another view. The frames are converted HBA tile frames, of course. (Photo: A. Jutila)

The next step is to add the purlins. (Photo: A. Jutila)

And then the roof itself is added. (Photo: A. Jutila)

Observing tips #1 — Introduction

There are three main observing modes for LOFAR stations:

• Network mode — where the station is controlled from Dwingeloo as part of the overall LOFAR network.
• Single-station mode —  when the station is controlled from Dwingeloo, but not as part of the network.
• Stand-alone mode — when the station is controlled completely locally.

For KAIRA, this will mostly be in stand-alone mode. As a result we will be doing a lot of observing and will gain a lot of experience with using the LOFAR systems directly.

We've noticed that as we've been doing our work we've picked up some useful tips that would apply to any of  the other LOFAR international stations. These are mostly command-line tricks that can be used on the local control unit, but also for some of the other data processing aspects of the system. Hopefully these will be of use to other LOFAR users and even other scientists running their own experiments, even on other equipment. And, for our general readers, it will give a small glimpse of some of the strange commands that we use to interact with our experiments.

We shall be occasionally interspersing these amongst the photographs and results that we post here. Watch out for them, or search for them all with the observing tips label.

EISCAT_3D Science Case

The second revision of the EISCAT_3D Science Case has been publicly announced via the EISCAT_3D website. This science case document was prepared as a part of the EISCAT_3D Preparatory Phase. It is updated regularly and is an excellent summary of the scientific goals. Obviously, apart from EISCAT_3D itself, much of this will impact on the work being done by KAIRA — both as a pathfinder for the EISCAT_3D project and as a stand-alone experiment.

Source: EISCAT_3D

Ref: http://www.eiscat3d.se/drupal/content/second-version-eiscat3d-science-case-available

An odd numbering scheme

We've had a lot of feedback on our recent successes — thanks to everyone who commented or sent e-mails! However, there was one particular question which came up which probably wants some explanation. Paraphrased, it was:

How did you come up with your antenna numbering scheme?

Yes, I agree that it certainly looks a bit odd, so here's a recap of the reasons and a bit more explanation.

For the High-Band Antenna array, the logic is as follows:

• The project has 48 'tiles' and we devised the 24+12+12 layout to suit our primary science objectives and environmental concerns.
• However, we wanted to still keep the option of converting to a full-size LOFAR station one day. That has 96 tiles in a circular pattern, numbered by row, but with a "hole" in the centre (Tile ID = "HCC"). Sets of LOFAR layouts are shown here: http://kaira.sgo.fi/2011/02/hba-layout-part-1-lofar.html
• So, we superimposed our layout on the standard full-size layout and used the numbering system that resulted. The one tile that would not be in the full size layout was originally referred to by Tile ID = "HXX", which later became "H96".
• There is a diagram (which should help) here: http://kaira.sgo.fi/2011/06/hba-configuration.html

For the Low-Band Array, the layout (LINK) works like this:

• A standard LOFAR remote station has 48 aerials in a scatter pattern.
• We started with this layout for antennas "L00" to "L45".
• These start "L00" in the centre, then increment anti-clockwise in rings.
• However, there are two "out-rigger" antennas ("L46" and "L47") in the LOFAR setup which are 80-90 metres away. These could not be used at KAIRA owing to terrain issues.
• So we made them "fit" somewhere in our array circle, but ran a massive simulation to find the spots where they would give the best beam performance. That's why these last two seem a bit oddly placed.

I hope that clears things up!

Shipping lane

The Three-Corner-Cairn -- Treriksröset (in Swedish), Treriksrøysa (in Norwegian), Kolmen valtakunnan rajapyykki (in Finnish) -- is the point at which the borders of Sweden, Norway and Finland meet. It is a pleasant walk from near where KAIRA itself is, across by Pikku Malla and along the northern shores of Kilpisajärvi to the Norwegian border.

However, there is an alternative route during the summer. One can take a boat across Kilpisajärvi and then follow the trail up past Kuohkamijärvi to Koltajärvi, which is where the Three-Corner-Cairn is located. This photograph was taken by one of the ASTRON engineers during a brief time off during the HBA commissioning in July.

The return trip across Kilpisjärvi. (Photo: H. Meulman)

Although there are a few small dingheys, this is the only boat of any reasonable size on the entire lake. However on the tourist map by the side of the lake, there is a dashed line showing the path that it takes and, in the English subtitle, it is labelled "Shipping Lane"!

First radar light

First ISR spectrum measurement with KAIRA with a 60 second integration time with a beam pointed towards the F-region peak.   

Last Friday was a big occasion for KAIRA. We performed our first bi-static VHF measurement together with EISCAT, which turned out to be a success. We are proud to present the first ISR spectrum measured with the system. In our first measurement we transmitted a simple long pulse of length 1.28 ms. We then measured the weak ionospheric plasma scatter from a signal transmitted in Tromsø and produced a spectrum on 31 beams pointed along different common volumes along the VHF beam.  The resulting spectrum clearly shows the typical ion-line shoulders that one would expect in this type of a measurement.

 

A 64 second integrated lagged product on beam 20 showing the approximate received power at different arrival times. 

  Next week we will perform our next measurement with the aim of obtaining better range and lag resolution. Stay tuned for more results. We have already seen plenty of space debris and meteor head echos with the system too.

Tetrastatic VHF

The EISCAT facility has a number of facilities scattered across Fenno-Scandinavia. These include two so-called "remote stations", located at Kiruna (Sweden) and Sodankylä (Finland), where there are 32m antennas which currently operate in the UHF band. However, due to encroaching radio-frequency interference the decision (announced this morning on the EISCAT weblog) has been made to convert these two dishes to VHF to be able to receive scattered signals from the Tromsø VHF transmitter. To quote from their post:

The EISCAT Council, on recommendation from the EISCAT Science Oversight Committee (SOC), decided that the remote UHF receivers located at Kiruna, Sweden, and Sodankylä, Finland, should be converted to the current VHF frequency of 224 MHz. The background to this is that the UHF frequency of 930 MHz lies within the radio band, which is nowadays used for GSM mobile communication, and the frequency protection, which was in place at the remote sites, has ended a while ago. In Sodankylä, e.g., we have noticed in autumn 2011, that new transmitters had come on-line, which makes incoherent scatter observations impossible due to interference. Thus the EISCAT Scientific Association has lost its unique capability of tristatic ISR measurements.

In order to preserve this ability, a plan has been made to convert the UHF 32-m parabolic dishes to 224 MHz in order to receive echoes of the Tromsø VHF radar signal when the radar points to zenith. Unfortunately, the VHF is not allowed to point further south than zenith, but it is estimated, that even at zenith, the remote sites will see the VHF signal. Astonishingly, even though the remote antennae are designed for the higher frequency, when considering all advantages and disadvantages of the conversion, they should perform at the VHF frequency just as well as at the UHF frequency.

What is not mentioned in the announcement is that KAIRA also operates in these frequencies. This means that not only is there the ability to carry out tri-static measurement with EISCAT at VHF freqencies, but with KAIRA there is a fourth station providing the ability for tetrastatic measurements. This is a fantastic oppotunity for us and makes some of the science possibilities even more interesting.

KAIRA viewed from Pikku-Malla

Today we have another great photograph of the site from Arttu Jutila. This was taken a couple of weeks back from the summit of Pikku-Malla. The LBA array had just been completed at that stage.

The KAIRA site, as seen from the summit of Pikku Malla. (Photo: A. Jutila)

Dawn over the Kitinen River

Just a nice photograph today. This photograph was taken at dawn on 2nd August 2012 from the middle of the Kitinen River adjacent to the Sodankylä Geophysical Observatory. Enjoy!

Kitinen River. (Photo: D. McKay-Bukowski)

Cable work in the RF-container

We now have installed all the patch cables in the RF-container. These are the 6-metre cables that go from the back of the patch-panel under the false floor and into the signal processing racks.

Under-floor cabling. (Photo. D. McKay-Bukowski)

Close-up of the back of the patch panel. (Photo: D. McKay-Bukowski)

Complete antenna layout

It has been pointed out there has not been a complete antenna layout posted up anywhere. So, we'll fix that today! Here is an overview of both the HBA array and LBA array for KAIRA. This diagram is to scale, including the spacing between the two arrays. The directions to north and the EISCAT VHF transmitter near Tromsø. Also shown are the antenna ID numbers.

Click to enlarge

Another view of the KAIRA site

Today, we've another great photograph from Arttu Jutila. This time it is a view of the KAIRA site, including the recently completed Low-Band Antenna (LBA) array. The water in the foreground is part of Siilasjärvi.

The KAIRA site. (Photo: A. Jutila)

HBA all-sky uv-coverage and point-spread function

The following plots were made as part of the calibration. These show the scatter of the instantaneous uv-coverage of the HBA, when selecting a pseudo-random cell pattern. In order to get good all-sky performance and to suppress and grating lobes, different cells in each tile are chosen. The result is the instantaneous uv-coverage as seen from zenith.

From this we can derive a point-spread function. This is the response of the phased array to an isolated point source.

These plots were made with various packages and calibration files. The final displays were made with a modified version of the dftAcc.py programme by Griffin Foster (U. Oxford).

Looking across the LBA

Today we have another photograph of the newly completed low-band antenna array. This photograph is a very wide view, it is probably best to enlarge it and pan across it to see all the details. The view is looking northwest across the two antenna fields. The mountains in the background are Gihcibákti (Iso-Malla) and Goallarrássa... still with a few patches of snow on them.

The newly completed LBA array, with the HBA tiles in the background. Click on the image to see a larger version. (Photo: A. Jutila)

Beacon satellites revisited

Beacon satellite signal power measured using a stationary beam pointed towards the point of closes approach. Both the full beamformed signal power and the power of all the individual tiles is plotted. As expected, the power recorded on all individual tiles is nearly the same -- also showing the sidelobe. The full beamformed beam is more narrow but contains some irregualrities that we will hopefully get rid of after performing a new calibration. 

A lot has happened after the first beacon satellite measurement, which seemingly was successful. However, when more satellites were measured, the signal didn't peak at the time when the satellite was transiting the beam. Even the beams of the individual tiles indicated that analog beamforming was not pointed in the correct direction. After several days of work, we have tracked the problem down to a small error in the antenna coordinate files, which caused the azimuths to be rotated 90 degrees clockwise. After fixing this minor problem, everything seems to be working more or less as expected and we will soon be ready for the first EISCAT measurement.

In other news: after replacing the quad gigabit ethernet card on the measurement PC, we can now receive all 244 beamlets simultaneously without dropping samples anymore. This amounts to about 400 MB/s! With the previous card, which from outside appearance looks nearly the same, we could only get 124 beamlets.

The next steps will be to redo the calibration with the corrected antenna coordinates and perform some incoherent scatter radar experiments with the Tromsø VHF system.

Patch panel

In the RF-container, there is the patch panel which has the field cable connected in on the outside and on the inside, there are the patch cables going into the receiver units in the equipment racks. Now that the LBA cabling is finished, we've completed this outside patch work. However, still more cabling remains to be done (sorry!). These photographs show the current situation:

The patch panel, as seen from the outside. (Photo: D. McKay-Bukowski)

The inside of the patch panel, showing the F-type connectors. There is one pair per antenna. The orange connectors are the X-polarisation and the black are for the Y-polarisation. (Photo: D. McKay-Bukowski)

A look at the patch panel. The lower section has the completed HBA patching. There is then a single pair left as a spare and, starting with #47, there is the first of the LBA patch cables. These will be filled in over the next few days. (Photo: D. McKay-Bukowski)

The completed LBA array

Today's photograph is one of the low-band antenna array. All the LBA's themselves have been erected, but the cable-mausoluem is still being worked on. Additionally, this was before the guy lines on the LBAs were adjusted, so not all the posts are perfectly vertical yet. Still, it is very gratifying to see this progress on the site. (Click on the photograph to see a larger version.)

The completed aerials of the KAIRA LBA array. (Photo: A. Jutila)

Daily bread

For the most part, the KAIRA work teams live at the Kilpisjärvi Biology Station, which is run by the University of Helsinki. It is located a few kilometres away from the KAIRA site, making it very convenient. The Bio-Station has its own kitchens and every day, the staff there prepare fresh meals for everyone working there... both the biologists and us physicists. The food is always excellent. In fact, I have had some of the best food I've ever tasted here. Even the bread is hand made and baked each day. If you a lucky, you will catch it just as it is coming out of the oven. MMmmm...

Hand-baked bread at the Bio-Station. Luxury.

Triple-A: Aperture Arrays at ASTRON

There's a new weblog specifically for aperture arrays in radio astronomy, written by Ilse van Bemmel.

Triple-A: Aperture Arrays at ASTRON

Although KAIRA has a much bigger scope that just astronomy, this new feature might well be of interest to many of our readers. We asked Ilse for an introduction, and this is what she sent us:

When you think about radio astronomy, most people imagine the fantastic big dishes that collect the radio emission from the farthest regions of the Universe. However, to see the faintest possible structures, a very large number of dishes is needed, and this rapidly becomes very expensive. A new technology to make very cheap and flexible radio telescopes is aperture arrays. These are in essence large collections of fairly simple and cheap receivers, which are combined with a computer to mimic a single large dish.

This technology was developed for radio astronomy at ASTRON over the last decades, and is now at a level at which it can be used to build radio telescopes. The first one of this kind is the LOFAR (www.lofar.org) telescope, currently the world's largest radio observatory. However, LOFAR is still small compared to the ultimate goal: the Square Kilometre Array (SKA, www.skatelescope.org).

For low frequencies there is no doubt that aperture arrays are the best technology for the SKA. But for the 'classical' radio astronomy frequencies around 1GHz, aperture arrays have to compete with the well-known and well-understood dishes. Aperture arrays are cheaper, can look in multiple directions at once, can observe over enormous frequency ranges, and can be triggered to look at a particular source pretty much instantaneous. However, they are the new kid on the block, and processing the data into scientifically sensible images is still a challenge.

In this blog I describe my efforts at understanding how aperture arrays work from an astronomical perspective, and my attempts to engage fellow astronomers to embrace the technology (pun towards EMBRACE fully intended). Follow me through the circle which engages engineers ('do you have an exact number for it?'), discussions with fellow astronomers ('within an order of magnitude is fine...'), and the management ('there is currently no budget for that').

It's a great web log and is recommended to all who are interested in the progress being made in radio astronomical phased arrays.

Link:  http://aperturearrays.blogspot.nl/

Building the final aerial

I know it has been a week, but we've finally caught up with the photographs of the LBA aerial build. Here's the last one, as the team put together aerial L38. The picture shows the tightening of the F-type connectors onto the low-noise amplifier (LNA) unit, which is at the top of each aerial post.

Assembling the last aerial. (Photo: D. McKay-Bukowski)

Aerial to node

Going the other way, the 40m cables are run from the aerial back to the node and then from there to the mausoleum. This is because the new metal armours are two large to easily get through the thin antenna ducts.

Fitting the draw clip to the cable pair. (Photo: D. McKay-Bukowski)

Drawing the cables through to the node. (Photo: D. McKay-Bukowski)

By now the cables have nearly reached the mausoleum (left far distance). Note the metal armour near the left. This was fed on from the leading end and will eventually end up at the base of the post. (Photo: D. McKay-Bukowski)

Cables, cables, cables...

Today we have some more photographs from the cable work.

Protecting the cable ends. Because the fitting process takes several days, the exposed cable ends are covered to protect them from the elements. The pegs keep them up off the ground and stop water from running into them. (Photo: D. McKay-Bukowski)

Although one end is out across the site, the other end goes into the mausoleum. (Photo: D. McKay-Bukowski)

Just another photo of cables. (Photo: D. McKay-Bukowski)

LOFAR station UK608

It is always interesting to see new LOFAR stations start to appear on the satellite photographs and Internet mapping sites. And one of the more recent additions is the LOFAR station in the United Kingdom. This is a particularly good site to look at, as there is a 25m dish nearby for comparison. It certainly gives a good idea of the scale of the LOFAR site!

This particular resolution photograph was probably taken in late 2011. We can tell this based on the materials around the LOFAR field itself and the state of the field. There are also cars parked near the gate to the main observatory compound, which would date from the time of the preparation of painting works for the 25m dish.

View Larger Map Get Directions View Bird's Eye

The UK608 station was officially opened by Dame Jocelyn Bell Burnell on 20 September 2010. Its LOFAR designation is UK608. It is operated by the LOFAR-UK consortium, which is the largest astronomy collaboration in the United Kingdom, involving scientists from 24 institutes. LOFAR-UK also have a web log (http://blog.lofar-uk.org/) for their project.

References:

• Chilbolton Observatory on Bing Maps
• LOFAR-UK home page

Adapting the posts

As we saw yesterday, we have fitted additional armour to some of the cables. The only small setback is that the added diameter means that we need to adapt the base of the LBA aerial posts. Fortunately, this is easily done and there are not many to do.

An adapted cable post. (Photo: D. McKay-Bukowski)

Incidentally, it may seem a bit odd that we are going into such painstakingly fine detail in this weblog.Surely we just want the glorious photographs of the finished product and the amazing data that it will produce. Well, this is certainly true. But this weblog also serves other purposes as well. It acts as a complete record of the work that we've done on the site. It also explains to our readers at ASTRON and EISCAT what modifications we needed to make to the standard LOFAR system to adapt it to the local Arctic conditions. And it is also a step-by-step guide for our colleagues on projects like POLFAR and I-LOFAR, helping them to understand the steps involved, how they are sequenced and what little challenges we meet along the way.

So, yeah, sure... cutting a slightly larger slot in the LBA post may seem a bit trivial, but there is a really good reason for writing about it and posting it for public view.

Cable armour

Because we were cutting existing cables, it means that half of them will be without metal armour. The armour is a short sleeve of metal that goes around the cable. In the final installation, it is located at the bottom of the LBA aerial posts, where the cable comes out the base of the post and goes into the ground. Obviously, the cable is briefly exposed at this point, and the armour prevents animals from chewing on the cable or any other form of damage from being exposed. For the extra armour, we cut short lengths of metal hose and fitted it where necessary.

A collection of prepared metal armour. (Photo: D. McKay-Bukowski)

Fitting the armour is easy. The choice of diameter is such that it easily slips over the female F-type connector on the cable. It is then slid all the way to the other end, near the male connector, which is where it is required. (Photo: D. McKay-Bukowski)