Abstract
Abstract. Both ionospheric and weather radar communities have already adopted the method of transmitting radar pulses in an aperiodic manner when measuring moderately overspread targets. Among the users of the ionospheric radars, this method is called Aperiodic Transmitter Coding (ATC), whereas the weather radar users have adopted the term Simultaneous Multiple Pulse-Repetition Frequency (SMPRF). When probing the ionosphere at the carrier frequencies of the EISCAT Incoherent Scatter Radar facilities, the range extent of the detectable target is typically of the order of one thousand kilometers – about seven milliseconds – whereas the characteristic correlation time of the scattered signal varies from a few milliseconds in the D-region to only tens of microseconds in the F-region. If one is interested in estimating the scattering autocorrelation function (ACF) at time lags shorter than the F-region correlation time, the D-region must be considered as a moderately overspread target, whereas the F-region is a severely overspread one. Given the technical restrictions of the radar hardware, a combination of ATC and phase-coded long pulses is advantageous for this kind of target. We evaluate such an experiment under infinitely low signal-to-noise ratio (SNR) conditions using lag profile inversion. In addition, a qualitative evaluation under high-SNR conditions is performed by analysing simulated data. The results show that an acceptable estimation accuracy and a very good lag resolution in the D-region can be achieved with a pulse length long enough for simultaneous E- and F-region measurements with a reasonable lag extent. The new experiment design is tested with the EISCAT Tromsø VHF (224 MHz) radar. An example of a full D/E/F-region ACF from the test run is shown at the end of the paper.
Highlights
The simplest design of a radar experiment is to transmit short pulses at equal inter-pulse periods (IPP) between the pulses, i.e. to use a uniform pulse-repetition frequency (PRF)
While the Pulse Aperiodic Transmitter Coding (PPATC) experiment described in this study can provide a continuous range coverage up to 1000 km, it can be increased up to tens of thousands of kilometers, e.g. using longer simple difference covers, arithmetic modulus type aperiodic transmission sequences (Uppala and Sahr, 1994), or with a ramped IPP scheme (Vierinen et al, 2009)
The utilisation of aperiodic transmitter coding for incoherent scatter radar experiments using long phase-coded pulses was demonstrated
Summary
The simplest design of a radar experiment is to transmit short pulses at equal inter-pulse periods (IPP) between the pulses, i.e. to use a uniform pulse-repetition frequency (PRF). To cover the whole range extent of the target, the IPP must be larger than or equal to the roundtrip time of the signal to and from the furthermost target range. A radar target is called overspread if its scattering spectrum is wider than the largest possible uniform PRF. Overspread radar targets, whose scattering spectrum is much wider than the largest possible PRF, can be sufficiently sampled by only measuring lag values clearly shorter than the signal roundtrip time to and from the target. They can be efficiently measured using long, possibly phase-coded pulses. Different techniques for measuring the severely overspread targets based on these ideas have been presented by several authors (e.g. Farley, 1969, 1972; Sulzer, 1986a,b; Lehtinen and Haggstrom, 1987; Sulzer, 1993)
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