New Technology

The very weak incoherent scatter echoes returned from the ionosphere have similar power to the instrumental and sky-background noise which is encountered at the frequencies used by EISCAT.

This requires the use of sensitive receivers, which themselves add very little extra noise to the system. EISCAT has successfully developed low noise, wideband, GaAsFET amplifiers able to operate close to the powerful transmitters. At the receiver sites, these amplifiers are cooled to a few degrees above absolute zero, bringing the overall system noise down to a level rivalling those of the best radio astronomy telescopes.

The demand for UHF frequencies has increased substantially in the last few years and the EISCAT UHF band is now bracketed by many commercial transmitters. A major revision of the EISCAT receivers improved their large-signal handling capability and linearity to the point where operation can continue, at full sensitivity, essentially undisturbed by any potential transmissions in adjacent bands.

Developments are also being undertaken to allow precise monitoring of the transmitters using dedicated micro-computers. These machines protect the delicate components of the high power amplifiers by ensuring that any faults are detected before damage can occur and the power safely diverted away from the transmitter itself.

In addition to such hardware projects, the EISCAT staff continue to develop and refine the software which allows the whole system to be used effectively. Many of the advances made at EISCAT have been made possible by the extremely flexible real-time, radar control software but nevertheless it requires continuous development.

Substantial advances have been made using new pulse coding schemes, including complementary codes - which allery low altitude, high resolution studies by eliminating the effects of ground echoes (called 'clutter') - and alternating codes, which use the available transmitter power very efficiently.

Work is in progress to refine the techniques used to extract physical parameters from the recorded data and these currently include initiatives in which large data- sets are considered together to produce improved estimates of the basic, and derived, parameters. As the available computer resources increase, so these techniques will also be applied to analyse the data in real-time, increasing the range of monitoring and control facilities available to the experimenters.