The 32m dish at Sodankylä

Nothing to report today, so instead we've just got a photograph of the EISCAT 32m VHF dish at Sodankylä. I took this photograph last night just before midnight. Still bathed in sunlight, of course.

The 32m EISCAT dish at Sodankylä Geophysical Observatory (Photo: D. McKay-Bukowski)

KAIRA EISCAT VHF intersection heights/elevations

In order for the KAIRA beam to intersect the vertical EISCAT VHF transmitter beam, the elevation must be set as follows.  (All data provided by I.I.Virtanen, U. Oulu).

Note that the azimuth to the EISCAT VHF is 313.95 degrees. The numbers for the above plots are as follows. Hgt is the height above the EISCAT VHF transmitter in kilometres and Elev is the require elevation of observation from KAIRA to have the beam centre at this height.

Hgt	Elev	Hgt	Elev	Hgt	Elev
0	-0.735	670	82.155	1340	85.69
10	6.101	680	82.259	1350	85.717
20	12.747	690	82.359	1360	85.743
30	19.044	700	82.457	1370	85.768
40	24.881	710	82.552	1380	85.793
50	30.199	720	82.644	1390	85.818
60	34.987	730	82.734	1400	85.843
70	39.261	740	82.821	1410	85.867
80	43.06	750	82.906	1420	85.891
90	46.431	760	82.989	1430	85.914
100	49.423	770	83.07	1440	85.937
110	52.082	780	83.148	1450	85.96
120	54.451	790	83.225	1460	85.982
130	56.568	800	83.3	1470	86.005
140	58.467	810	83.373	1480	86.027
150	60.176	820	83.444	1490	86.048
160	61.719	830	83.513	1500	86.069
170	63.117	840	83.581	1510	86.09
180	64.39	850	83.648	1520	86.111
190	65.551	860	83.712	1530	86.132
200	66.613	870	83.776	1540	86.152
210	67.589	880	83.837	1550	86.172
220	68.488	890	83.898	1560	86.192
230	69.318	900	83.957	1570	86.211
240	70.087	910	84.015	1580	86.23
250	70.801	920	84.072	1590	86.249
260	71.465	930	84.127	1600	86.268
270	72.084	940	84.181	1610	86.287
280	72.663	950	84.234	1620	86.305
290	73.205	960	84.286	1630	86.323
300	73.714	970	84.337	1640	86.341
310	74.192	980	84.387	1650	86.358
320	74.642	990	84.436	1660	86.376
330	75.067	1000	84.484	1670	86.393
340	75.468	1010	84.531	1680	86.41
350	75.847	1020	84.577	1690	86.427
360	76.207	1030	84.622	1700	86.443
370	76.548	1040	84.667	1710	86.46
380	76.872	1050	84.71	1720	86.476
390	77.18	1060	84.753	1730	86.492
400	77.473	1070	84.795	1740	86.508
410	77.752	1080	84.836	1750	86.523
420	78.019	1090	84.876	1760	86.539
430	78.274	1100	84.916	1770	86.554
440	78.518	1110	84.955	1780	86.569
450	78.751	1120	84.993	1790	86.584
460	78.974	1130	85.031	1800	86.599
470	79.188	1140	85.067	1810	86.614
480	79.394	1150	85.104	1820	86.628
490	79.591	1160	85.139	1830	86.643
500	79.781	1170	85.174	1840	86.657
510	79.963	1180	85.209	1850	86.671
520	80.139	1190	85.243	1860	86.685
530	80.308	1200	85.276	1870	86.698
540	80.471	1210	85.309	1880	86.712
550	80.628	1220	85.341	1890	86.725
560	80.78	1230	85.373	1900	86.739
570	80.927	1240	85.404	1910	86.752
580	81.068	1250	85.435	1920	86.765
590	81.205	1260	85.465	1930	86.778
600	81.337	1270	85.495	1940	86.79
610	81.466	1280	85.524	1950	86.803
620	81.59	1290	85.553	1960	86.815
630	81.71	1300	85.581	1970	86.828
640	81.826	1310	85.609	1980	86.84
650	81.939	1320	85.636	1990	86.852
660	82.049	1330	85.664	2000	86.864

We hope you read all these numbers and memorised them!

Joint KAIRA/EISCAT observations

Continuing on with an overview of Poppy’s observations.



When not working on pulsar observations, Poppy has been running a riometry experiment that is also run at the LOFAR station in Chilbolton (UK).

The most exciting part of this involved simultaneous observations of the ionosphere with KAIRA, EISCAT at Tromsø, and Chilbolton. As KAIRA and EISCAT are co-located, they can be used to observe the same section of the ionosphere. This is done by observing zenith at both sites (using the VHF at EISCAT and using the LBA at KAIRA), then by pointing the UHF over KAIRA, and by pointing some of KAIRA’s beamlets over EISCAT. This allows us to simultaneously observe the ionosphere above both EISCAT and KAIRA using two different instruments, verifying KAIRA’s riometry observations.

A diagram of the simultaneous observations of the ionosphere
using EISCAT and KAIRA. (Credit:  P. Martin)

Derek sciencing [ed: this was Poppy's caption, not mine!] (Photo: P. Martin)

 

Text/images: P. Martin

EISCAT VHF

Yesterday we showed some photographs of the EISCAT 32m UHF system. Today, we are featuring the 120x40m VHF system. This antenna is actually split into four "panels", which are 30x40m each and can be controlled in elevation independently.

The EISCAT VHF system with the nearest panel tipped
forward. Note the person on the access-way
for scale. (Photo: D. McKay-Bukowski)

The EISCAT VHF system with all panels aligned.
The feed is off-axis, so in this configuration the radar
beam is actually straight up. (Photo: D. McKay-Bukowski)

Looking along the feeder-bridge. The crossed-
dipoles are used for both transmission and
reception. (Photo: D. McKay-Bukowski)

The observing project was being run by the University of Southampton, UK, which has time on both the EISCAT UHF and VHF systems as well as KAIRA.

KAIRA plasma parameter fit

Ilkka Virtanen and Niklas Siipola have been analysing some of the KAIRA data that was recorded during the previous Finnish EISCAT campaign. The following plots show bi-static incoherent scatter plasma parameter profile plots obtained using KAIRA. The results are in good agreement with the Tromsø measurements, and also show for the first time that a simultaneous bi-static plasma parameter profile can be obtained using a phased array incoherent scatter radar receiver. This is still work in progress, but this is yet another step in the direction of KAIRA performing routine incoherent scatter plasma parameter measurements.

The PI of the experiment was Anita Aikio and the purpose of the experiment was to 1) obtain the first tri-static EISCAT VHF measurements, and 2) to leverage the multi beam capability of KAIRA to obtain a profile of wind velocity vectors (due to the lack of a third station, only two components of the velocity vector can be obtained).

Plasma parameters obtained using KAIRA.

Plasma parameters obtained using EISCAT VHF.

Tracking Starts Friday 20:00 UT

24 hours till the first ping of asteroid 2012 DA14! EISCAT Tromsø is now calibrated and secured to track the flyby of DA14.

We will post updates both in the Kaira blog as well as in Twitter during the DA14 flyby from the EISCAT control room (see photo).

KAIRA VHF-altitude angles

One of the primary experiments for KAIRA is to observe incoherent scatter from the EISCAT VHF transmitter near Tromsø, Norway. In order to do this, the digital beam of KAIRA needs to be correctly pointed to intersect the correct volume of the ionosphere. To assist experimenters in planning this, the following graph can be used to estimate the centre elevation for a given altitude. Click on the graph to see the enlargement.

KAIRA pointing elevation angles to intersect at specific altitudes
above the VHF transmitter near Tromsø. (Graph: J. Vierinen)

Please note that depending on the observing frequency, the beamsize can actually be quite large, spanning many degrees.

VHF upgrade at SGO — Part 3

And finally, the new receiver is installed on the 32 m antenna.

The new prime-focus, VHF feed. (Photo: D. McKay-Bukowski)

VHF upgrade at SGO — Part 2

Today work started on replacing the sub-reflector with a VHF prime-focus feed.

The crane (right) prepares to suspend the old sub-reflector, while the mobile platform allows the workers to get in close. (Image: D. McKay-Bukowski)

VHF upgrade for the EISCAT antenna at SGO

Due to the encroachment of radio-frequency interference (RFI) it was decided to convert the two EISCAT remote stations to operate at VHF frequencies. This is great news for KAIRA as it gives us some long baselines at the common frequency. Of course we've mentioned Sodankylä before; this is where the host institute is located.

At SGO, there is a 32m antenna (which used to have a UHF receiver). This is now being converted to a VHF system. The new receiver and feed can be seen in the lower left of the photograph.

The final image shows a close-up shot of the new system. It will replace the sub-reflector addionally converting the dish to a primary feed. Here, it is still on the trailer as it was brought over from Kiruna, Sweden.

The conversion is due to take place on 23rd October 2012. (Images: D. McKay-Bukowski)

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.

Construction plans — Part 5

With the cabling run out, the next stage is to install the internal electronics within the RF-container. Within it there are four, full-height 482.6mm (19") rack frames. The sub-rack assemblies go into the first three of these and the fourth is for the control computers, clocks and networking. Once the sub-rack assemblies are in place, they can be cabled up with the white signal cables that come from the back of the patch panels.

In addition to this, site clean-up and storage will be done. This means 'dressing' the site, securing all items, packing up equipment and recycling waste. The site is then cleared and we are ready for winter.

Over winter 2011-2012, the first tests between KAIRA and the VHF transmitter at Tromsø will be carried out. And then, in the following year, the LBA will be deployed and the data connection made back to the supercomputer in Groningen.

But that's another story.

So ends our mini-series on what is planned for the summer installation... and the action starts NEXT WEEK!

Construction plans — Part 4

There are two massive tasks planned for July. This is the month with the best weather, 24-hour-a-day sunshine and the warmest temperatures (sometimes even exceeding 10 celsius). During this month the HBA tiles will be deployed and then they will be cabled up. This is also the month when the largest part of the workforce will be on the site.

The HBAs are deployed in a semi-sparse pattern. Although we have a final design in mind, this will not be confirmed for certain until the end-of-winter test evaluation has been completed. In any case, the pattern will be in small groups of antennas, oriented towards the VHF Tromsø transmitter, and set so that they are compatible with an upgrade to a full international LOFAR station HBA field.

Although there are only 48 HBA antennas, the plan is one day to expand this to the full 96 tiles (+1 dummy tile). The KAIRA project still has a full LBA (96 aerials) and RF-container, but there is a smaller HBA installation in the first instance.

Additionally, the HBA will be built on a raised platform. This is for a number of reasons. Firstly, it assists in snow control. Drift snow can be a problem, so by raising the antenna field, it allows the wind to blow snow and thus help clear the array naturally. Additionally, it decouples the tiles from the ground, thus giving us a lot more flexibility with the drilling of anchors for the framesets. Finally, it lets the cabling be done above the ground which, apart from protecting the cables from the frost, makes installation and repair access so much easier.

The frames are 1.5 metres high. As the tiles themselves are 0.5 metres high, this puts the top surface of the HBA approximately 2 metres above the ground.

In order to deploy the HBA, the framesets need to be assembled from their component frames. They are then lifted into place, as per the precision surveyed marker pegs from the array layout survey. They are anchoured to the ground and each other to form the secure platform for the HBA tiles.

Once each frameset is in place, the HBA tiles themselves can be deployed. The tiles are delivered by lorry to the storage area on the site. They are then moved by all-terrain forklift to the unfolding area, where the packaging material and securing harnesses are removed, the tile is unfolded and fitted. The tile is then lifted by crane onto the frameset, where it is anchored into place and the cover is applied.

This procedure is repeated until the total deployment is completed.

Once the HBA tiles are in place, the cables can start to be connected. There are a pair of cables for each tile. They run back to the RF-container where they are connected to a patch panel. Each cable is a coaxial-type and will carry the RF signal in one direction and power and commands in the other direction.

Because it is important to control the cable lengths for the LOFAR radio telescope, each cable is made to a specific length. And to ease the manufacturing process, these lengths are standardised to a small number of set lengths. Because of the positions of the HBA tiles, there can be several tens of metres of excess cable which needs to be carefully zigzagged back and forth before the remainder of the cable goes into the RF-container.

Typically, this is done underground; cables on all LOFAR stations are buried. However, for KAIRA, the cables must be kept above the surface due to the agressive ground freezing. Thus rather than excess cable length being stored in a 'cable grave', it is instead stored in a raised box — the so-called 'cable mausoleum'.

Radio bands

Throughout this web log, there are lots of references to the radio bands: UHF, VHF, VLF, etc. For example, KAIRA will receive VHF frequencies, whereas some of the EISCAT radars operate at VHF, where others work at UHF frequencies. This chart is to put them all in perspective with respect to each other.

Abbreviation	Frequency Range	Wavelength Range	Name
ELF	3 to 30 Hz	10,000 to 100,000 km	Extremely low frequency
SLF	30 to 300 Hz	1000 to 10,000 km	Super low frequency
ULF	300 to 3000 Hz	100 to 1000 km	Ultra low frequency
VLF	3 to 30 kHz	10 to 100 km	Very low frequency
LF	30 to 300 kHz	1 to 10 km	Low frequency
MF	300 to 3000 kHz	100 to 1000 m	Medium frequency
HF	3 to 30 MHz	10 to 100 m	High frequency
VHF	30 to 300 MHz	1 to 10 m	Very high frequency
UHF	300 to 3000 MHz	10 to 100 cm	Ultra high frequency
SHF	3 to 30 GHz	1 to 10 cm	Super high frequency
EHF	30 to 300 GHz	1 to 10 mm	Extremely high frequency

The two arrays of KAIRA will operate in the high and low ends of the VHF band. The KAIRA HBA will work from 120-240MHz and the KAIRA LBA will operate from 30-80 MHz. As explained earlier, there is a gap between the two, as this is where FM-radio broadcasts occur, and it is impossible to conduct delicate scientific experiments in this region of the spectrum due to such broadcasts.

Ref: http://en.wikipedia.org/wiki/ITU_Radio_Bands

High-power large aperture radars

High-power large aperture radars were first envisioned by Bill Gordon (1958) as instruments that can measure the incoherent scatter from free electrons in the Earth's ionosphere. He also proposed that such a radar could be used to observe the Sun and various planetary targets. The first experimental measurements of ionospheric incoherent scatter was soon thereafter reported by Bowles (1958), and many of the other goals were also soon realized when the Jicamarca Radio Observatory and the Arecibo Ionospheric Observatory were built. Both of these radars are still the largest in the world, and have contributed much to our knowledge of Earth's atmosphere and space. The Jicamarca radar located in Peru has a square shaped phased array antenna field with dimensions of 300*300 m. The Arecibo Ionospheric Observatory in Puerto Rico has a spherical dish with a diameter of 305 meters.


Since the early days, many more high-power large aperture radars have been built in various places around the world: These include the Millstone Hill, Svalbard, Tromsø UHF, Tromsø VHF, Kharkiv, Irkutsk, MU, Sondrestrom, PROUST, Poker Flat, and Resolute Bay radars. There are also various large radars of comparable size around the world used for space surveillance purposes. The most recently built Poker Flat and Resolute Bay radars are digital phased array radars, which allow fast beam steering and allow 3D imaging of the ionosphere. The KAIRA receiver array will also be a phased array system, which will function as a bistatic receiver for the Tromsø VHF radar. Some of these radars are shown in the following figure

High-power large aperture radar systems of the world. Photograph credits: Arecibo (NAIC), Jicamarca (JRO), Tromsø (EISCAT Scientific Association), Svalbard (Tony van Eyken), Millstone Hill (MIT Haystack), Kharkiv (Institute of Ionosphere, Kharkiv), Poker Flat and Resolute Bay (Craig Heinsleman).

As the name already suggests, high-power large aperture radars are radars with large antenna aperture and transmission power. As the beam width of an antenna is typically inversely related with the collecting area, these radars also have fairly narrow beams (typically 1 degree). They also typically transmit fairly long coded pulses in order to increase the average transmitted power. In some bi-static planetary radar applications the transmission can be continuous.

While the primary purpose of most high-power large aperture radars is the study of ionospheric plasma, they can also be used for a large variety of other uses, including meteor, space debris, planetary , and lower atmospheric studies.

How do KAIRA and LOFAR work? — Part 8 : Steering a phased array

Last time, we saw that a couple of detectors can be used to collect signals and that because they will add in phase from some directions and not others, there is a certain directionality to the system.

In principle, you can ‘steer’ this system to look in different directions by tipping it, but that’s not particularly efficient. The real advantage is that without moving the antennas you can change the directionality of the overall system by changing the lengths of the cables.

Let’s consider the off-zenith case from the last part.

If we now add a bit of extra cable, these off-zenith signals now add in-phase again.

In fact, the zenith signals are the ones now out of phase when they are combined electrically. So, by adding some cable length, you can control the direction in which the array is sensitive. That is, you can steer its ‘looking direction’ around the sky without actually moving the antennas themselves.

Some phased arrays indeed use cables to adjust their pointing direction. The VHF radar in Tromsø is one such system. It is mechanically steered in the vertical direction and horizontally pointed with a phased array. By manually changing the cables, the horizontal pointing direction of the array can be altered by 15 degrees.

A view inside the feeder bridge of the VHF system
at Tromsø. (Photo courtesy Mike Rietveld)

Because this needs to be done by hand, it is not patch the cables that quickly, so these sorts of directional changes are not done too often.

Although still useful (rotating a 120×40m antenna in azimuth is tricky!) there is another technique which has recently become affordable and which makes arrays like KAIRA, LOFAR and the SKA practical.

The EISCAT VHF transmitter

As we have seen, the site at Tromsø run by the EISCAT Scientific Association has several instruments. The most significant of these to KAIRA is the VHF transmitter, because its frequency (224 MHz) is within the high-band of the KAIRA system. This frequency is also very close to the proposed frequency of the EISCAT_3D project.

The Tromsø VHF system is a scientific radar. Using the incoherent scatter principle, it transmits radio power into the upper atmosphere and detects the faint radio echoes. From these, it can readily determine the electron density, ratio of the electron temperature to ion temperature, ratio of the ion temperature to ion mass, and the line-of-sight ion velocity.

The Tromsø VHF antenna is huge: 120×40 metres in size. It comprises 4 panels, each 30×40 metres, which can be steered in elevation independently of each other. The system cannot turn in azimuth, but it is possible to reconfigure the phase of the transmitter array to provide a small amount of horizontal directional control.

The transmission power is provided by a klystron. The transmission frequency is 224 MHz (well inside the range of a LOFAR HBA antenna) and the peak transmission power is 1.6 MW. The feed system is a line of 128 crossed dipoles at the focal line of the parabolic-cylindrical antenna panels. This line can be seen to the right of the panels in the first photograph. To get an idea of the scale, that line is actually a corridor and engineers can walk along the inside of it. The second photograph shows a close-up of one of the individual crossed dipoles.

EISACT Tromsø

Two of the EISCAT radar transmitters are located in Ramfjordmoen, close to the city of Tromsø in Norway. The people working here are employed by The University of Tromsø.

The EISCAT UHF (Ultra High Frequency) radar operates at 931 MHz and is driven by two klystrons with a peak transmission power of 2 MW. The antenna is a 32-metre diameter dish, which weighs about 100 tonnes. It is fully steerable, and can scan at speeds of 80 degrees per minute in both the azimuth and elevation axes. It can accelerate to full speed within 2 seconds.

The other radar is the VHF system, shown in the second photograph. This is a 120×40 metre antenna with a 224 MHz transmitter. This radar is of particular significance to the KAIRA project, so we'll write about it in full tomorrow.

In addition to the radars, there is a Heating Facility, which is used for modification experiments. It applies high-power transmissions of high-frequency electro-magnetic waves to the ionosphere to study its plasma parameters. The name Heating stems from the fact that these high power electromagnetic waves, which are transmitted into the ionosphere with high-gain antennas, heat the electrons and thus modify the plasma state. To create plasma turbulence, the transmitted frequencies have to be close to the plasma resonances, which are 4 to 8 MHz.

There is also a Dynasonde (a digital HF sounder) covering a frequency range of approximately 1-20 MHz. Six dipoles are used as spaced receiving antennas. Each half of each dipole is made from an aluminium tube, 11 metres long and 15 centimetres in diameter. These tubes are suspended about 2 metres above the ground. Even so, they do break sometimes through weight of snow and metal fatigue as they vibrate in the wind.

More information about these systems can be found at the EISCAT website: http://www.eiscat.se/about

But there will be more about the VHF system tomorrow!

Mound Mesh Model

As reported in our piece on the site survey, we need to accurately model the region where we intend to put the KAIRA antennas. Unlike most LOFAR stations, KAIRA is located in pretty rugged terrain. This means that finding a suitable space is not as easy as it first appears. Especially when we are trying to avoid cutting down trees and keeping our impact on the Arctic wilderness to the barest minimum.

The site we have selected fits these requirements pretty well. The ground is mostly material from civil works, allowing us to re-use an existing development. However, the space is not easy to work in, especially when the main antenna array has an area of over 1200 m²., which needs to be levelled to an accuracy of ±3 cm. As a result, we need to consider well the exact location of the array and the amount of levelling required.

The plot shown here is from part of that analysis work. From the survey results, a topographic mesh is fitted to the data. That is then interpolated to derive a working model of the surface. From this, the detailed planning and layout can be completed.


The map is oriented with North at the top and the direction to the EISCAT VHF transmitter marked.

LOFAR

LOFAR (LOw Frequency ARray) is a pan-European radio telescope designed to observe the radio universe at VHF frequencies. It is designed, built and operated by ASTRON, the Netherlands Foundation for Radio Astronomy. The LOFAR system is split across multiple sites — known as stations — each of which typically has two arrays of antennas.

The antennas in each of these two arrays are different in design. One design is a vertical 'aerial' which stands about 1.8 m tall, optimised for radio frequencies in the range 30 to 80 MHz. The other design is a large 'tile', with a square area of 5 × 5 m. It is for the higher frequency range of 120 to 240 MHz.

Did you note the gap from 80 to 120 MHz? LOFAR deliberately avoids these frequencies, as this is where commercial and public FM-radio stations broadcast. There is no point trying to listen for faint cosmic radio signals at the same frequency as a high-powered neighbouring radio station.

In addition to a large cluster of LOFAR stations in the Netherlands, there are additional participants in the main LOFAR project: Germany, France, Sweden and the United Kingdom. This map is a closer view than the one posted earlier, and shows the locations of the LOFAR stations (green = complete, yellow = under construction).


KAIRA makes use of the same antenna technology that is used in LOFAR, making it compatible with the LOFAR system. This opens up some exciting prospects of linking the two projects. We'll be looking at these antenna systems soon.

In addition, there have just been some important achievements by the LOFAR project. More on that in the next post!