Improved spectral observations of equatorial spread F echoes at Jicamarca using aperiodic transmitter coding

Abstract

We present preliminary radar results obtained with a new method of spectrum estimation of moderately overspread equatorial spread F (ESF) echoes over Jicamarca. This new method is similar to the classical pulse-to-pulse methods but with nonuniform pulse spacings, i.e., using aperiodic transmitter pulses, allowing us to avoid range and frequency aliasing, at the cost of extra clutter at some ranges. We also present different ways of dealing with the extra clutter, depending on the spectral characteristics of the clutter. We have identified two kinds of clutter: ground clutter (due to mountains and transmitter pulses) and ionospheric clutter (due to ESF and equatorial electrojet echoes). Preliminary radar results are very encouraging, and ways of improving them are proposed based on Monte Carlo simulations. Particularly, we propose alternating the phases of the uncoded aperiodic pulses using random binary phase codes to flatten the spectrum of the ionospheric clutter. The theoretical implications of the newly estimated spectra, particularly from topside echoes, are presented and discussed in a companion paper.

Introduction

Plasma irregularities in equatorial spread F (ESF) are observed by multiple ground- and space-based scientific instruments. They are also known to occur frequently enough and with enough intensity to interfere severely with communications and navigation equipment used by nonscientists. Various devices, i.e., in situ probes, radars, ionosondes, airglow imagers, scintillation receivers, etc., are sensitive to the different parts of the spectrum of the irregularities. A good review of these almost 70-years of multi-instrument and theoretical studies is given by Hysell (2000) and references therein.

In this paper, we present the preliminary radar results obtained with an “aperiodic” technique recently implemented at the Jicamarca Radio Observatory (JRO) that allows us to measure the ESF 3-m irregularities with a wider frequency window than before. In the past, relatively long interpulse periods (IPPs) were used in order to avoid range aliasing. Typical values were 1500 km or more, limiting unambiguous velocity measurements to be $\pm 150\mathrm{m}{\mathrm{s}}^{-1}$ or less. As we show below, with the aperiodic technique we are now able to estimate ESF spectra with a velocity window between $\sim \pm 650\mathrm{m}{\mathrm{s}}^{-1}$.

The aperiodic technique was presented to the radio science community by Uppala and Sahr (1994) to be used in studies of auroral electrojet echoes in particular and moderately overspread targets in general. Some ESF echoes belong to this category, particularly those from the topside (e.g., Woodman and La Hoz, 1976; Hysell, 2000).

The aperiodic technique is based on transmitting pulses at nonuniform intervals, sampling the range of interest at a suitable rate while undersampling the clutter ranges. The clutter signals result from a different pulse or pulses than the desired signal. If the pulse rate is constant, then the clutter will always be the same set of ranges, but if the pulse rate is varied, the cluttering ranges vary. If one can find a sequence of pulses covering the correlation time of the clutter, so that each clutter range affects only one signal pulse, then the clutter spectrum is flat, and the signal spectrum is derivable from the irregularly spaced pulse sequence.

In the past, the frequency aliasing problem in ESF observations was avoided at Jicamarca and with CUPRI (Hysell et al., 1994) using the double-pulse technique (Sahr et al., 1989). Although high-mean Doppler velocities were measured, the technique did not allow a good estimation of the spectral shape or even the spectral width. The double-pulse technique, as well as the more general multi-pulse technique (Farley, 1969) and the alternating code technique (Lehtinen and Haggstrom, 1987), also depend on turning the aliased signals into random clutter, but they are suitable for severely overspread targets.

Recent improvements to the data-acquisition system and radar controller at JRO allowed us to implement the aperiodic technique. The paper is organized as follows: first we present details of the experimental setup and the radar results. Then, we present and discuss ways of further improving the aperiodic spectral estimation technique based on Monte Carlo simulations. Finally, we summarize our results and main conclusions. Comparisons of the new topside ESF spectral shapes with theoretically derived spectra are presented and discuss in a companion paper (Hysell and Chau, this issue).