Weather radar

Yesterday we described our walk up the ski slopes of Luosto on that hot summer's evening. But what was at the top?

Well, it turns out there is a large weather radar (= säätutka) there. This is operated by our colleagues at the Finnish Meteorological Institute (FMI).

The "blue stone". (Photo: D. McKay-Bukowski)

The FMI weather radar site. (Photo: D. McKay-Bukowski)

Lassi ignores the warning signs. (Photo: D. McKay-Bukowski)

It's a really lovely place. A bit like a lighthouse. A pity it is remotely operated, as I'd so love to work in a place like that.

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!

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.

EISCAT UHF

A couple of weeks ago, the KAIRA team was at the EISCAT radar site at Ramfjordmoen, near Tromsø, to carry out some observations. We planned to control KAIRA remotely and do the observations with EISCAT locally. It didn't quite work out that way, as an unexpected power failure required a "quick" return to the site for a reset, but otherwise all went okay.

The weather we had in Tromsø was lovely and we took some photographs to share. Today we are featuring the 32m UHF radar.

EISCAT transmitter hall, control building and UHF 32m dish. (Photo: D. McKay-Bukowski)

The 32m UHF dish, seen from the feeder-bridge of the VHF antenna. (Photo: D. McKay-Bukowski)

Passive Radar

Passive radar is a form of radar that utilizes radio transmissions of opportunity as illuminators for targets. Examples of such transmissions are commercial FM radio, GPS signals, TV transmissions, and military radars.

A little while ago, we had several plots showing DAB stations in Norway seen via anomalous propagation at KAIRA. This is a very primitive form of passive radar. A more advanced solution is to measure the transmitted waveform using one receiver, and use this measured waveform when deconvolving the echoes. Today we have some results from FM radio based passive radar operating at the MIT Haystack Observatory using two dipole antennas and a USRP N200. The plots were made while commissioning a new 16-channel USRP based system called MIDAS-Mini, which will eventually replace older ISIS Echotek digital receivers used for this same purpose.

The concept of passive radar is not that new. For example, the Manastach Ridge Radar or the Haystack Observatory ISIS system can be used to observe specular meteor trails, ionospheric E-region irregularities and E-region drift velocities -- but only using commercial FM radio transmitters and relatively cheap hardware. To read more on the topic, refer to eg., Frank Lind's Ph.D. thesis.

In the video above, you can see airplanes with time variable Doppler shifts flying around the Boston and Rhode Island. This was recorded using a USRP N200 with one yagi antenna pointing towards the transmitter and another antenna pointing towards the opposite direction.

ISR Radar School 2014

I has been announced that the 2014 Incoherent Scatter Radar School will be held at the Arecibo Radio Observatory in Puerto Rico. This will be a joint event between the European and United States radar communities.

Arecibo Radio Observatory (Soruce: PD-USGOV-DOC-NOAA/Wikipedia)

The Arecibo Radio Telescope is a most distinctive facility, owing to its unusual design and huge size. It is build into the landscape, using the natural shape of the terrain to assist in the forming of the main reflector. This is fixed, and provides a limited view of the sky overhead. However a moveable gregorian secondary focus allows some degree of flexibility. That, and the earth's natural rotation, allow it to observe a narrow band of the sky between Declinations -1 and +38 degrees.

It is also extremely large. The 305-metre diameter means a total collecting area of 73,000 square metres... this holds the record for the largest physical size of any radio receiving system ever since its completion in 1963. And although it will be exceeded in the near future by other telescopes that are under construction (e.g. when finished, the LOFAR HBA will be 76,000 m² and FAST will be 196,000 m²) and those that are planned (such as SKA and  KARST), the Arecibo Radio Telescope will remain an important and powerful scientific instrument for many years to come.

Links:

• https://en.wikipedia.org/wiki/Arecibo_Observatory
• http://www.naic.edu/

2013 ISR school (MIT, Haystack)

This week, there is the 2013 Incoherent Scatter Radar school being held at MIT Haystack Observatory. Sodankylä Geophysical Observatory will have a lecturer there and we can expect reports on the EISCAT_3D weblog during the week.

That flat cap. I've seen that before somewhere... (Photo: Th.Ulich)

Polar mesospheric summer echoes

This week, we've been supporting EISCAT observations of polar mesospheric summer echoes (PMSE). These are unusually strong VHF radar reflections ("echoes") that occur at around 80-85 km altitude (hence "mesospheric"). This is the altitude of what is called the "D-Region" of the ionosphere, and it is a similar altitude to that where noctilucent clouds are found. They are noted at high latitudes ("polar") from June to August in the north (thus "summer").

PMSE are not well understood and remain an area of active research. The campaign that is currently running is making use of multiple facilities in Norway, Sweden and Finland, and KAIRA is playing its role, observing the radar echoes from the EISCAT VHF using a multibeam experiment. In total 60 HBA beamlets (20 pointing directions x 3 subbands each) are being recorded, at raw signal levels using our local pipeline programme (KLP).

We hope that the additional spatial information provided by KAIRA will assist the scientists who are researching this phenomenon. Hopefully, we'll be able to report some successful results soon.

International EISCAT Radar School 2012

From the SGO website...

In the near future the EISCAT Scientific Association will face a major instrumental upgrade with the ESFRI Roadmap project "EISCAT_3D: A European Three-Dimensional Imaging Radar for Atmospheric and Geospace Research."

EISCAT, in co-operation with the Sodankylä Geophysical Observatory, will run a training course for new users of the EISCAT radars, from 27th August to 1st September, 2012 (Monday to Saturday). The training course will be held at Sodankylä, where one of the EISCAT UHF remote sites is located.

The course will cover all essential aspects of the current EISCAT systems, including the science programme. An overview of the existing hardware and software will be provided and future plans will be discussed, with a strong emphasis on EISCAT_3D. The course will have a very strong emphasis on practicals, i.e. work in groups of participants on real data. There will be a dedicated radar experiment for every group, and the groups will then analyse their data and present their results under the guidance of an experienced team of instructors.

The deadline for registration is 12th July 2012.

For more details, see:  http://www.sgo.fi/Events/RS2012/

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.

EISCAT Svalbard Radar — Part 2

The ESR transmitter is modular in design, using 16 small klystrons based on TV transmitter technology. The transmitter has a central operating frequency of 500 MHz. At the time of writing this article, the two dishes cannot be used exactly simultaneously. However it is possible to alternate between the dishes on a timescale of approximately 5 seconds. Two receiver systems are available which (for instance) allow ion line and plasma line data to be recorded simultaneously.

The transmitter hall of the ESR. (Photo: Derek McKay-Bukowski)

The EISCAT UHF transmitter

If you read the previous web log post about the EISCAT site at Sodankylä, you may be wondering how a dish fitted with a UHF receiver can receive signals from a VHF transmitter at Tromsø. Well, that's because there is actually a second transmitter at the Tromsø site.

As we mentioned in the first post about the EISCAT facilities near Tromsø (Norway), there is a radar there which operates at 931 MHz. Like the receiver dishes in Kiruna and Sodankylä, the UHF transmitter system in Tromsø is also a 32-metre diameter dish, although of a slightly modified design. This is because of the additional waveguides required by the transmitter, which take the high-power signals from the klystrons in the so-called transmitter hall, out to the dish and then through a couple of rotating wave-guide joints to get it to the focal position for beaming into the upper-atmosphere.

Together with the passive receivers at Kiruna in Sweden and Sodankylä in Finland, EISCAT’s Tromsø UHF radar transmitter forms this tri-static system. The UHF system has been operating since 1981, with several major upgrades in the intervening period.

EISCAT-Sodankylä

If you have looked at the map showing the positions of LOFAR, EISCAT and KAIRA, it will be apparent that there is actually three EISCAT sites on the Fenno-Scandinavian mainland. Apart from the transmitter site at Tromsø and the receiver site at Kiruna (home of the EISCAT_3D demonstrator array) there is also an EISCAT site in Finland.

This is actually located at Sodankylä Geophysical Observatory, the institute behind the KAIRA project. Like the Kiruna site, the EISCAT system in Sodankylä is a receiving station (there is no transmitter). The two dishes are copies of each other, built at the same time and deployed to the two sites. They are fully-steerable, 32m-diameter, prime-focus paraboloids. Both are fitted with UHF receivers (although on occasion other receiving systems have been fitted for specific experiements). The photograph shows well the backing structure and counterweights of the Sodankylä dish.

In fact, at the time these were built, there were two other identical dishes built, which were deployed as part of the European VLBI Network in Italy (one at Medicina and one at Noto).

Apart from some of its own unique research projects, the 32m dish at Sodankylä acts as a one of the receivers for bi- or tri-static ionospheric radar observations.

Photo credit: Th. Ulich

Space debris

Space debris is an ever growing environmental problem occurring in space. After over 50 years of space rocket launches, our near space is littered with approximately 10^6 objects larger than 1 cm in diameter, weighing a total of 5500 tons (Nasa ODQN Jan 2010). As the atmosphere is very thin in near-Earth space, these objects do not come down very quickly, causing danger to Earth orbiting spacecraft for tens or hundreds of years to come. Once these objects are in space, there is also a probability that these objects will eventually collide with each other and create even more debris. As the probability of in flight collisions increases as a function of debris in orbit, a catastrophic collisional cascade process that renders regions of near-Earth space unusable is a real risk. This scenario is often referred to as the Kessler syndrome. A video of the evolution of space objects is shown below.

A hypervelocity collision.

EISCAT has been active in space debris measurements for over 10 years, producing hundreds of hours of space debris measurements, covering two major break ups: the Chinese anti-satellite collision and the collision of the Iridium and Cosmos satellites.

Most of the measurements are so called beam-park measurements where the antenna is positioned at a fixed pointing. During a 24-hour period, while the Earth rotates around its axis, a representative statistical sample of debris is measured, containing information on orbital elements of the debris. The time of day provides information on the longitude of the ascending node, while the Doppler shift gives information on the inclination of the object. An example measurement produced after the collision of the Iridium and Cosmos satellites is shown in the figure below.

EISCAT UHF beam park measurement of the Iridium-Cosmos collision
debris clouds. Each point represents a detection of an object passing
the radar beam, the color represents radial Doppler shift. full sized version.

One of the potential uses of the planned EISCAT3D system is to track space objects and space debris. One of the advantages of a phased array system is the capability of observing a large volume of space simultaneously and making interferometric direction angle measurements. This will allow accurate trajectory measurements that can be used for collision avoidance with operational spacecraft, such as the International Space Station or Envisat.

For more information on EISCAT space debris activities can be obtained here.

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!

EISCAT_3D

Mentioned on several occasions is the EISCAT_3D project.

The EISCAT Scientific Association operates three incoherent scatter radars in Tromsø (Norway) and on Svalbard. The UHF radar, which operates at 930 MHz, is the only tristatic incoherent scatter radar in the world. The transmitter is located in Tromsø and additional receiver sites are in Kiruna (Sweden) and Sodankylä (Finland).

However, there are ambitious plans afoot to build the next generation incoherent scatter radar, which will provide comprehensive 3D monitoring of the atmosphere and ionosphere above Northern Fenno-Scandinavia. This new radar system — called EISCAT_3D — will consist of multiple phased arrays, using the latest digital signal processing to achieve ten times higher temporal and spatial resolution than the present scientific radars. It will have pplications in a wide range of European research areas including Earth environment monitoring and technology solutions supporting sustainable development, well beyond atmospheric and space sciences.

EISCAT_3D will be a volumetric radar capable of imaging an extended spatial area with simultaneous full-vector drift velocities, having continuous operation modes, short baseline interferometry capability for imaging sub-beamwidth scales, real-time data access for applications and extensive data archiving facilities.

The design of the antenna arrays will be modular at different scales allowing for mass-production of the components. Some arrays will be very large, in the scale of 32,000 individual antenna elements. The receiver arrays will be located at 50-150 km distance from the transmitters, with some smaller arrays close by to support continuous interferometric observations. The total system will comprise approximately 100,000 elements. Construction is due to start in 2015.

KAIRA is an integral part of the EISCAT_3D development process. It will test whether or not LOFAR production items are suitable to the EISCAT_3D project, possibly leading to a mass deployment of LOFAR hardware in the far north. However, it is not the only possibility, and other array designs are being considered. One of these prototypes under evaluation is the Kiruna Demonstrator Array; there will be a web log post about this soon!

EISCAT_3D website: http://www.eiscat3d.se/