Friday 17 August 2012

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.

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