Abstract
Space-based synthetic aperture radar (SAR) can be affected by the ionosphere, particularly at L-band and below. A technique is described that exploits the reduction in SAR image contrast to measure the strength of ionospheric turbulence parameter ${C}_{k}{L}$ . The theory describing the effect of the ionosphere on the SAR point spread function (PSF) and the consequent effect on clutter is reviewed and extended. This theory can then be used to determine ${C}_{k}{L}$ from both corner reflectors (CRs) and K-distributed SAR clutter. Measuring the K-distribution order parameter allows ${C}_{k}{L}$ values much lower than those that defocus the image to be determined. The results of an experiment in which a CR on Ascension Island was repeatedly imaged by PALSAR-2 in the spotlight mode during the scintillation season are described. The value of ${C}_{k}{L}$ obtained by measuring the clutter was compared with that obtained from a nearby CR. The correlation between the two was good using a median value of the spectral index $p$ . This correlation was improved by using the measured value of $p$ derived from the CR PSF. The technique works for any homogeneous K-distributed SAR clutter and is thus applicable to extra-terrestrial bodies as well as PALSAR-2 images of Ascension Island.
Highlights
Previous work [8] has shown that the primary effect of phase scintillation is to induce an along-track point spread function (PSF) that is dependent on the scintillation parameters
This PSF is from a 47-dBm2 corner reflectors (CRs) on Ascension Island imaged with PALSAR-2 at 1-m resolution [9]: the left and right halves of the PSF have been averaged to produce a single-sided plot
It is possible to measure the effect of phase scintillation by measuring the K-distribution order parameter
Summary
T HE ionosphere has a number of effects on a spacebased synthetic aperture radar (SAR) operating at L-band and lower frequencies, such as Faraday rotation [1], [2], image geometric distortion [3], phase shift and defocusing [4], amplitude modulation [5], [6], and loss of contrast [7]. For SAR clutter that is K-distributed, this loss of contrast is more appropriately described by a rise in the K-distribution order parameter, which is a more accurate measure. By measuring the rise in the order parameter, Ck L can be determined This forms the basis for the technique described and tested in this paper. It is not possible to measure the rise in the order parameter using only one SAR image. Measuring the order parameter of two SAR images of the same place, one with ionospheric effects, and one without, allows the rise in the order parameter due to the ionosphere to be determined.
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