Measurement of the Ionospheric Scintillation Parameter $C_{k}L$ From SAR Images of Clutter

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.

Measurement of Ionospheric Scintillation Parameters From SAR Images Using Corner Reflectors

“…For example, an array of cheap, passive CRs distributed over ten or more kilometres might enable the variation of ionospheric irregularity statistics to be remotely inferred. An alternative approach to measuring the strength of turbulence using space-based SAR by measuring the change in clutter parameters when the ionosphere is disturbed is the subject of another paper [28].…”

Ionospheric Scintillation Observation Using Space‐Borne Synthetic Aperture Radar Data

“…We might conjecture that the RCS enhancement method may be less sensitive to low levels of scintillation, whereas the contrast method is responsive to irregularities that are too weak to produce an RCS enhancement and visual streaks in the image. This possibility was, in fact, conjectured by previous authors (Belcher et al, ), but one would need to consider a wider collection of data to establish whether this is the case.…”

“…Previous works by Belcher and Cannon (2014), Mannix et al (2017), and Belcher et al (2017) have demonstrated the capability of measuring S 4 and C k L using SAR signals. SAR data acquired under different ionospheric conditions may also have varying background returns due to changes in the terrain over time.…”

“…Previous authors (Belcher et al, ; Mannix et al, ) have estimated C k L from SAR measurements, using point targets and ground clutter statistics, respectively. In the absence of corner reflectors serving as point targets and accurate backscatter statistics due to inhomogeneous terrains, the knowledge of satellite position and viewing geometry were used to convert the S 4 measured by the contrast or RCS enhancement methods to C k L along with assumed values of anisotropy ratio of 50:1 and phase spectral index of p = 2.5.…”

“…Since space-borne microwave SARs operating at L-and P-bands are also affected by ionospheric irregularities, they can provide a useful source of scintillation diagnostics. Recently, a number of authors (Belcher & Cannon, 2014;Belcher et al, 2017;Mannix et al, 2017) have been leveraging space-borne SAR signals for this purpose.…”