Forward-Scatter Radar facility

Physics of the Middle Atmosphere and Meteors

 
 

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Leonids 1999

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Introduction

The forward scatter (FS) radar method has experienced a resurgence of interest in recent years by the advent of digital technology which offers the possibility of a full on-line analysis of the echo from a meteor. There are considerable potential advantages to be gained from the FS configuration, where transmitter and receiver can be a  1000 km or more apart, and the scattering is at oblique incidence. For a given meteor the signal usually decays much more slowly at oblique incidence than at the normal incidence of backscatter.  Even at frequencies where interference is not a problem, the backscatter (BS) systems are unable to detect echoes from heights much above 100 km (the "echo ceiling"). This is, in effect, because at high altitudes the echo decays much faster than the train is formed. The slower decay of forward scatter offers the possibility of using HF and VHF radars to examine the distribution at greater heights than hitherto.  The persistence of the signal also facilitates the discrimination between factors contributing to the ionisation loss; of particular interest are chemical effects, which become apparent only in the longer enduring echoes. There are also technical advantages. For sufficiently long baselines, gating of aircraft is unnecessary, the aircraft being over the horizon from transmitter or receiver, or both. This makes the use of continuous wave (CW) attractive, for which one may use narrow bandwidths, thus increasing  sensitivity and minimising interference with other radio users.

 

 

the System

The FS meteor radar has the transmitting section at Budrio (44.6N), near Bologna, and the receiving parts at Lecce (40.3N) in Southern Italy and at Modra (48.3N) near Bratislava (Slovakia). The system utilises a continuous wave (CW) transmitting frequency at 42.7MHz with a fixed modulating tone at 1 kHz and about 1kw mean power. The FS transmitting and receiving antennas at the three separated places are horizontally polarised with an elevation angle of 15 along the Bologna-Lecce (700 km, 53 south-east from Bologna) and the Bologna-Modra (650 km, 35 north-east from Bologna) directions (Fig.1).

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Fig.2) the FS meteor radar system block diagram.

Click on diagram for a larger image

 

These tones are then sent through the combiner and the DRIVER, to the final stage of the transmitter (TX) and the system of YAGI antennas. The receiving system (RX) at Lecce and Modra transfers the two tones to the MIXER to reconvert the signals at the frequency of 10 and 11 kHz, which are sent then to the data acquisition system (ACQ.SYS.).  A simple calculation of the peak received power from a typical FS meteor echo suggests that most meteors with an electron line density q> 5x1012 el/m will be detected by the system (3).

Fig.3) the FS meteor radar transmitter at Budrio (Italy)

Fig.4) the FS meteor radar receiver at Lecce (Italy)

Fig.5) The Astronomical Observatory at Modra (Slovakia)

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