Thursday, 6 March 2014

Recent developments in pulse-compression - Upgrading EISCAT UHF

Pulse-compression techniques are regularly used in high-power ionospheric incoherent scatter radars. For example, the EISCAT Tromsø has binary coding possibility. As I have understood pulse-compression, it is typically due to two factors:

  1. High-power amplifier technology supports transmitting longer pulses with lower peak power instead of shorter pulses with very high peak power.
  2. The wanted range resolution is of the order of microseconds or less while range extent of the target is typically milliseconds. Thus if we send microsecond pulse in millisecond intervals, the transmitter is nonoperational for a very large portion of time.
Because of these reasons, we use pulse-compression techniques in high-power radars, i.e. Barker codes, chirped signals etc.

SGO team has concentrated on designing a number of different coding and analysis techniques. The first significant breakthroughs were done already in 1980s with alternating coding development. During recent years, we have been working on perfect pulse-compression coding techniques. These codes were first reported in

[1] M. Lehtinen, B. Damtie, P. Piiroinen and M. Orispää, Perfect and almost perfect pulse compression codes for range spread targets, Inverse Problems and Imaging 3 (2009) 465-486.

A succession to this paper was reported in

[2] L. Roininen and M. S. Lehtinen, Perfect pulse-compression coding via ARMA algorithms and unimodular transfer functions, Inverse Problems and Imaging (2013) 649-661.

The paper [1] included a crucial discussion on comparison of measurements developed by Petteri Piiroinen from University of Helsinki. This mathematical formalism gives us a solid background to compare different kinds of measurements and hence also coding techniques. Paper [2] shows the relation of the coding problem to the classical study of unimodular polynomials with constrained coefficients.

As [1] and [2] need amplitude and binary phase coding, we have not yet run any real measurements with these codes in high-power radars. Of course at some point we will do this also! However, in 2012, we ran a series of polyphase coded experiments with the Millstone Hill ISR in Massachusetts, USA. These were reported in Radio Science

[3]  I. I. Virtanen, F. Lind, L. Roininen, P. Erickson, W. Rideout, M. Orispää, J. Vierinen, and M. Lehtinen,  Polyphase-coded incoherent scatter measurements at Millstone Hill, Radio Science, 48 (2013).

To our knowledge, this was the first reported polyphase-coded ISR experiment.  Of course we want to do similar things with EISCAT system. Hence during the Finnish November EISCAT campaign, we discussed a possibility for doing quadriphase coded experiments by minor modifications of the current EISCAT UHF. This option is now available and we are going to schedule these experiments for the May campaign. Provided that everything will go ok, we will report these experiments in the web log and hope to publish them in peer-reviewed journals!

Naturally we are also continuing the development of the mathematical background for pulse-compression code comparison. This week, we are going to submit a paper on pulse-compression of continuous codes to Inverse Problems and Imaging. The reference is:

[4] L. Roininen, M. S. Lehtinen, P. Piiroinen and I. I. Virtanen, Perfect pulse compression coding via unimodular Fourier multipliers, manuscript in progress.

Hence, we are working on both the mathematical background of the pulse-compression techniques and upgrading the existing hardware to more advanced coding possibilities. And of course... The EISCAT_3D needs a whole lot more!

EISCAT UHF Klystrons!

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