Geoscience Reference
In-Depth Information
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Aperture Synthesis Radar Imaging
for Upper Atmospheric Research
D. L. Hysell and J. L. Chau
Earth and Atmospheric Sciences, Cornell University, Ithaca, New York
Jicamarca Radio Observatory, Lima
U.S.A., Peru
1. Introduction
Radars used for upper-atmospheric applications can be engineered to measure the Doppler
spectra of their targets adequately for most intents and purposes, the spectral resolution
being limited only by the observing time and the constraints of stationarity. Likewise,
they can measure the range to their targets adequately for most intents and purposes,
range resolution being limited by system bandwidth, the power budget, and the constraints
of stationarity. Problems arise for “overspread” targets, where range and frequency
aliasing cannot simultaneously be avoided using pulse-to-pulse methodologies, and more
complicated pulse-to-lag or aperiodic pulsing methods are required (see for example (Farley,
1972; Huuskonen et al., 1996; Lehtinen, 1986; Sulzer, 1986; Uppala, 1993)). Important examples
of this situation include incoherent scatter experiments (Farley, 1969), observations of meteor
head echoes (Chau & Woodman, 2004), and observations of plasma density irregularities
present in certain rapid flows, as are found in the equatorial ionosphere during so-called
“equatorial spread
F
” (Woodman, 2009; Woodman & La Hoz, 1976).
Where capabilities are most limited is in bearing determination and the associated problems
of imaging in the directions transverse to the radar beam. Electronic beam steering using
phased-array radars offers a means of radar imaging (e.g. Semeter et al. (2009)), but the
number of pointing positions that can be used is limited by the incoherent integration time
required for each position. If the power budget permits, transmission can be done using a
broad beam, and beam forming can be done “after the fact”, such that all pointing positions are
examined simultaneously (e.g. Kudeki & Woodman (1990)). Even so, the angular resolution
will be limited by the size of the antenna array unless the diffraction limit is removed through
numerical deconvolution. The half-power beamwidth of large-aperture radars used for upper
atmospheric research is usually of the order of one degree. At ionospheric altitudes, this
translates to a transverse resolution of a few to a few tens of kilometers, which may be larger
than the scales at which primary plasma waves are excited. The resolution of medium-sized
and small research radars with their relatively smaller antenna arrays is relatively poorer
still. In applications involving coherent scatter from plasma density irregularities, targets
of interest may exhibit backscatter intensities spanning 30 dB or more of dynamic range. For
such targets, the 3 dB beamwidth of the antenna is essentially irrelevant, and even targets in
the sidelobes of the antenna radiation pattern can contribute to the power assigned to a given
