Monday 6 October 2014

EISCAT Demonstrator Array

Kiruna Demonstrator Array. Photo: Craig Heinselman.
During the FP6 EISCAT_3D Design Study, which was completed in 2009, a small radar receiver array was built at the Kiruna EISCAT site to test reception of radar signals from the Tromsø VHF transmitter using a phased-array receiver. The receiver was used later on for science applications, namely for measurements of so-called Polar Mesospheric Summer Echoes (PMSE). The photo above was taken by EISCAT Director Craig Heinselman using his quad-copter drone, which he demonstrated to his colleagues a day later.

One of three instrument cabinets of the Demonstrator Array: this is clearly an experimental setup for prototyping work. The final EISCAT_3D receiver will look much different, probably incorporating many parts into single custom-made components. Photo: Thomas Ulich.
During the FP7 EISCAT_3D Preparatory Phase, the array was modified by SGO scientists in collaboration with Technical University of Luleå and National Instruments to demonstrate reception of VHF signals using a different approach. This time the array was converted to using software-defined radio (SDR) receivers. The results of these tests are documented in the reports of the projects work packages 7 and 11 (see Deliverables and Milestones).

One of the aerials of the Demonstrator Array with the EISCAT 32-m receiver in the background. Photo: Thomas Ulich.
The aerials are derived from TV antennae, since the VHF radar operates at very similar frequencies around 224 MHz. Also EISCAT_3D is expected to operate in this part of the radio spectrum, around 233 MHz, although negotiations about frequency allocations in the Nordic countries are on-going.

With the Demonstrator Array in the background, the photo shows a small transmitter antenna, which sends a calibration signal to the array. Photo: Thomas Ulich.
Calibration of arrays is essential. The direction in which a phased array looks depends on the time delay of the signals coming in from all of its aerials. Therefore one needs to know the absolute delays introduced by the components of the array, including their manufacturing inaccuracies, before applying delays related to the viewing direction.

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