East Siberian Center for the Earth's Ionosphere Research (SDC ESCEIR)

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Incoherent scatter ionosphere diagnostics subdivision
SDC technical and engineer instruments support subdivision

Incoherent Scatter Radar

Technical characteristic Antenna system Receivers Modernization project

The Irkutsk incoherent scatter radar is one of the world's nine active radars which give the opportunity to investigate ionosphere by the incoherent scatter method. The geographical locations of the radars can be seen from the IS Radar Map. Initially, in the 60s, under USA funding a USA latitudince chain of four radars was created: Sonderstrom (Greenland), Millstone Hill (USA), Arecibo (Puerto-Rico), and Jacamarca (Peru). The goal of this chain was to study for latidunce distribution of ionospheric parameters. Later, during active polar ionospheric investigations, a three-position instrument was constructed in Scandinavia funded by the EC. In the 90s the EC constructed also the Svalbard radar on Spitsbergen. The USSR in the 70s have been constructed the only special-purpose IS radar in Kharkov. In the 80s a radar in Kioto (Japan) was constructed which was used as a radiolocator.

The ISTP was very lucky in receivins under the conversion program in 1993 the radiolocation station "Dnepr", which is situated 120 km to the north-west from Irkutsk. The opportunity to use the station for IS method measurements is determine by its main characteristics:

Working frequencies	154 - 162 Mhz
Impulse power	2.5 - 3.2 Mw
Impulse length	70 - 820 Mcs
Impulse repetition frequency	24.4 Hz
Antenna gain coefficient	38 dB
System noise temperature after modernization	440îK
Antenna	Sectoral horn
Exposure system	Two antennas
Beam main lobe size	0.50(N-S), 100(E-W)
Scanning sector due to sounding frequency changing	(N-S)

Antenna system

To get an idea what station "Dnepr" antenna system looks like, let us consider some of the radar views which are given by the photographs General view and Side view. Antennae by itself is two-sided sectoral horn. Antennae sizes are 244 m length, 12 m width and 20 m height. Summar area is about 3000 m ².

Let us consider in some detail the radar design and principles of operation. The radar antenna system (Fig. 1) consists of two waveguide-slot antennae (WSA) 4 connected in diameter by sectoral horn 1 with partition 2. Two WSA can be powered from two ends by exciting horns 3. Using special positions of the slots and delaying system 5, we can change the antenna beam (DD) from the zenith to 30 degrees in both directions along WSA (Fig. 2), thus changing the frequency from 154 MHz to 162 MHz.

Fig.1 Antenna system structure

Fig.2 Radar beam

The antenna beam width is 0.5 degree in the longtitudial direction. When both WSA are phased, the beam width in the longitudial direction is 10 degrees, and when the phase difference is p, then there are two beams 20 degrees wide each, separated by 20 degrees. The antenna axis is rotated by 7 degrees from the meridian (Fig. 2), and all antenna system is titled by 10 degrees to the west. Antenna system of this kind can radiate and receive only one linear polarization of the signal. Moreover, there is a polarization filter in the antenna, which consists of metal bands 6, thus preventing the signal with the orthogonal polarization from penetrating.

Each of the two waveguide-slot radar antennas can be excited from two sides, and so the system can operate with four receivers, each with a peak power of 1.1-1.4 MW. Furthermore , on each side there is an additional "reserve" receiver, which can be used instead of any of the main receivers. So, Receivers Hall combines 6 sets of one-type receivers. The modulators and charging lines are placed on low technological level under the receivers. The modulator working cycle is about 40 ms, and so the impulse repetition frequency is 24.4 Hz. A maximum receiver impulse length is 960 ms.

For preventing the antenna structure from firing, the operation of the receivers in opposite direction is separated in time, and so there are two intervals of radiation in a cycle of a length ~1ms, in each of these we are forming beams directed to the north or south from a normal. The beam is tilted to the side opposite to the powered end of the antenna.

We have the opportunity to power simultaneously two WSA from the other sides. If on one of the WSA the working frequency is smaller that the normal frequency154 MHz by some frequency df (that is, if DD of the beam is titled to the powered end), and the other WSA has the frequency 154 MHz + df, then two beams with a difference frequency 2*df in space will be simultaneously formed, which can be chosen equal to the plasma frequency at some height in the ionosphere. It is assumed that as a result of the interaction of these two waves we may have a resonance oscillation in plasma which may cause to make artificial irregularities. Methodological details of these experiment are now discussed

Receivers

Due to the special properties of the IS radar operation, the original station "Dnepr" receiver set did not meet our requirements for technical characteristics. During the 1998 modernization, this set was completely changed for special equipment. We installed new good amplifiers; to heterodyne the signal we installed SW receivers R-160P. However, these efforts did not completely resolved problem of carrying out good experiments, because the technical characteristics of these receivers can provide to carry measurements only in the oxygen line of the IS spectrum.

However, the increase of solar activity in the years 1999-2000 compels us to seek an opportunity to measure the hydrogen line of the IS spectrum and to change over to measurements of the IS signal correlation function for complex impulses. These two problems compel us to modernize essentially our recording equipment and software for primary and secondary data processing. The development of communication facilities has also some influence on the recordins complex composition and properties which gave rise to a new modernization project now in progress.

Modernization project

The project includes:

• Modernization of the set of receivers with the ultimate goal to carry out correlation measurements with complex impulse and to register hydrogen line of IS spectrum.
• Development of the system for forming complex sounding impulses.
• Development of the system for control and governing the receiver power and sounding signal phases.
• Development of a modern registration complex on the base of fast analogue digital equipment and ADSP signal processors.
• Connecting the local radar net to the Internet and real-time information transfer .
• Development of the real time experimental data processing software with real-time transfer to the database.

The project will be implemented during the years 2001-2002.