Journal of Atmospheric and Solar-Terrestrial Physics
Improved spectral observations of equatorial spread F echoes at Jicamarca using aperiodic transmitter coding
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 or less. As we show below, with the aperiodic technique we are now able to estimate ESF spectra with a velocity window between .
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).
Section snippets
Experimental setup
The observations presented in this paper were made with an experimental setup similar to the one used extensively to study ESF irregularities with the JULIA (Jicamarca unattended long-term investigations of the ionosphere and atmosphere) system (Hysell and Burcham, 1998, Hysell and Burcham, 2002). Briefly, the north and south quarters of the main Jicamarca antenna were used for transmission, while the east and west quarters were used for reception independently, forming an east–west
Radar observations
In Fig. 1, we show an example of the type of spectrograms we have obtained during this experiment after 20 s integration. In order to identify the clutter better, we are showing the signal-to-noise-ratio (SNR) for each radial velocity bin of the east (west) antenna in Fig. 1a(b) along with the SNR profiles in Fig. 1c (east: red, west: Green). A single noise level is calculated for each range gate using the algorithm of Hildebrand and Sekhon (1974). The magnitude (phase) of the spectral
Simulated results
Our preliminary observations using the aperiodic technique are very satisfactory. However, we think there is still room for improvement at either the spectrum estimation stage or with the estimation of the spectral parameters of the desired signal.
In this section, we present a simple scheme to improve the spectrum estimation of the desired signal when ionospheric correlated clutter (i.e., with very wide spectral widths) is present. We test the feasibility of our proposed scheme with Monte Carlo
Concluding remarks
We have shown that the aperiodic technique works for the study of moderately overspread ESF echoes. In the companion paper (Hysell and Chau, this issue), we show the theoretical implications of the newly characterized spectrum. Although our preliminary results are very encouraging, under some circumstances, the desired spectrum is contaminated by ground clutter and/or ionospheric clutter.
The ground clutter is due to mountain echoes and/or the transmitting pulse. The spectral characteristics and
Acknowledgements
We thank John Sahr for suggesting that we look into phase coding in order to deal with clutter signals with long correlation times. DLH was supported by NSF Grant ATM-0225686 to Cornell University. The Jicamarca Radio Observatory is operated by the Instituto Geofísico del Perú, with support from the NSF Cooperative Agreement ATM-9911209 through Cornell University.
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