Drunken forests and cable trays

As we discussed in the last article, cables for LOFAR stations are typically buried. This is not possible at KAIRA. The reason for this is the problems caused by the freezing conditions. The fact that the soil will soak with water and then freeze means that trapped cables can be stretched, crimped or even broken under the strain of the ice. Because of the volume of snow (and hence melt-water) and the depth of the ground freezing, this is a serious problem.

KAIRA is the only deployment of LOFAR technology in a boreal subarctic climate. This sort of environment is subject to frost heaving, ice-lensing, palsas and hydrolaccoliths. These are the sorts of phenomena that lead to a arboreal effect which can be seen in the region. This is where ground distortion can cause trees to lean at a seemingly random angles because of soil displacement around the root base. Trees affected by such ground upheaval are said to be drunk and the large areas where this occurs are referred to as 'juopunut metsä' (= drunken forest).

Although one or two seasons might be okay, the lifetime of the KAIRA project is a minimum of 15 years and over these sorts of timescales the effects can be significant - especially in the case of an abnormal precipitation or freezing event.

We anticipate that the site will be affected by this and so distortions of the power poles, reindeer fence and other such installations is to be expected. This is partly the reason for the raised platforms for all the KAIRA antennas, as it will allow us to adapt the framesets to ground distortion, or to get under the array, jack up the affected locations and re-level the soil.

However, also as a result of the ground movement, the conventional burying of the signal cables is not a option. Instead we will route our cables on timber trays above ground, just under the antenna tiles themselves. Because of gaps in the array, there will be the need to dip down temporarily underground, but this needs to be done through flexible ducting, to ensure that ground movement will not lock into the cables and affect them — that is, the cables must have free movement in these tunnels. Of course, it might be conceivable to route all the cables in free-air ducts underground, but this is prohibitive in our case because of time, cost and access issues. So, cable trays it is, and these are now under production.

Mikko Tilja and Pertti Nissinen during the preparation of a feeder cable tray.

The first trays themselves are a 'V' shape, made from two long planks nailed together. These are stock-piled for later deployment under the array.

Some completed 'V' trays.

The installation of the cable trays is a pretty major task and has taken up the bulk of the time between the completion of the antenna tile installation and the delivery of the electronics and cabling.

Photos: D. McKay-Bukowski

Dealing with the cables

Every High-Band Array (HBA) tile antenna has two cables that connect it to the main KAIRA processing electronics. These electronics are located in the so-called RF-container to the north of the array. The two cables from each antenna carry the RF (= radio frequency) signal back to the RF-container. There is one cable for each polarisation. In addition to that, one cable carries a low-baud-rate command signal from the RF-container out to the antenna and the other carries DC power to energise the circuits in the antenna tiles themselves.

Coils of LOFAR signal cables ready for testing prior to the field
deployment. Photo: D. McKay-Bukowski, STFC/SEPnet/LOFAR-UK

For most LOFAR stations, the cables are buried underground. This provides security against wildlife, but it also provides thermal stability. When a cable is heated up, its properties change. Most notably, the copper expands and thus the cable actually gets a little longer. This means that it takes the signal longer to travel through the cable. If the cables are not of equal length, or the heating of them is not uniform, then it means that there are errors introduced into the time of arrival of the signals at the central electronics in the RF-container. This means a degradation in performance of the instrument.

So, to mitigate these effects, the cables are buried about half a metre underground to keep their temperature more stable. There are two ways of doing this.

One is to level the field and then dig trenches for the cables. These are lined with fine sand to prevent rocks from pressing against the cables themselves, which are then laid in. A second layer of sand is then put down and the trenches backfilled and re-levelled.

Signal cables in trenches on the LOFAR-UK station (UK608).
Photo: D. McKay-Bukowski, STFC/SepNET/LOFAR-UK

The alternative is to dig down to the base level, put all the cables in place and then cover the entire region with sand and soil to bring the surface level back up to the required height, thus leaving the cables buried.

Filling the entire area area at the Swedish LOFAR station
(SE607). Photo: Leif Helldner, Onsala Space Observatory.

However, for KAIRA this is simply not possible. And the reason for it is curiously related to trees that seem to have drunk far to much alcohol!

Puzzled? Then wait for the next article, which will explain all!

Reindeer fence

They say that every LOFAR station has its own special wildlife problems. And KAIRA will probably also fit that pattern too. One issue (among many) for us will be reindeer. As a result, a reindeer fence will be erected around the perimeter of the high-band and low-band arrays.

Putting the fence out has been a mixed experience. In parts, the ground has been clear, soft and easy to work on. As can be seen from the photograph left, a long stretch on the western edge went out without any problems.

However, there are other sections where the work is not so easy. In part, along the eastern edge, the ground is extremely rocky and driving in the posts has been next to impossible. In other sections to the north, the ground is sodden and the fence needs to span a small meltwater stream. Everywhere in this area is marshy and the mosquitoes and midges swarm about in clouds.

To the south, the ground itself is okay, but the vegetation is extremely thick. In the photograph on the right, Marko Kauranen comes back from a shift with a tundra-cutting machine. This device (a bit like a strimmer, but with a large metal saw blade, rather than the nylon lines) is used to slice through the dense ground cover to allow us to place the posts more easily and to then get through with the machine to stretch out the fencing mesh.

Putting up the reindeer fence will be a fairly long task, as there is so much of it. It is our new 'background' task for days when the weather is too poor for working on the antennas or other parts of the site.

Photos: D. McKay-Bukowski

The first sunset

The season of Midnight Suns starts in Kilpisjärvi on about the 20^th May and lasts until the 23^rd July. Although the sun technically will set today, it only just dips below the horizon and the land remains in a strong twilight. In fact, it will be quite a while before we get some serious darkness.

Crow's nest

One of the KAIRA team is Stuart Keenan. Stuart has been involved in many LOFAR installations in the past and he is helping out for us as well. In addition to his LOFAR know-how, he is also an expert on wildlife and is always on the lookout for new and interesting species. In the spare moments during breaks and while waiting for deliveries or preparation work, Stuart doesn't waste any time and is on the lookout for different wildlife in the KAIRA area.

As it turns out, there is actually a lot of wildlife about, from lemmings to sea eagles, or ravens to reindeer. And it is very useful to have an expert to hand to explain what something is — animals and plants alike.

Summer grasses

It is the end of summer and the most of the plants have gone to seed, ready for the winter which is not far away now. This view of the unfolding area was taken from ground level, looking past some Arctic grasses on the field edge.


Have a nice weekend!

Marking out the LBA

We've said along the way that the limited building time (just a few months over the summer) would limit the amount of work that we could do. Additionally, the fact that the site needs a lot of extra work in terms of ground preparation and framesets and other 'KAIRA-specials', puts even more pressure on what we will do this year.

As a result, the Low-Band Array (LBA) will be built in 2012.

However, even though that is some way off yet, we still need to start doing the preparation work for it now (in the same way that we were planning the HBA around this time last year). Especially as it impacts on some of the work being done as part of the HBA build. Things such as the cable routing, fencing and site access all require a knowledge of the LBA plans.

As a result, we've started marking out the field now.

The LBA field is a swampy, tundra region just to the north west of the HBA array. It is considerably lower and collects a lot of meltwater from the surrounding hills. As a result, the ground is sodden and difficult to negotiate. We will need to carry out drainage works to ensure that the field can be used.

Over the last couple of days, we've marked out the field and done some of the basic planning. Soon, we'll be putting out the reindeer fence, so that will certainly mark the field off.

Markku Postila and Stuart Keenan measure out the LBA field.

Although there are only a few markers in place at the moment, already the scale of it is impressive. We also have the advantage that we can get a good overview of the LBA field from the HBA mound, so we'll be able to take lots of interesting photographs as the work progresses.

More to follow, of course!