Imaging a moving target using forward-look synthetic aperture sonar

Abstract

Mid-frequency active, surface ship sonar data from a recent shallow-water sea trial have been processed using synthetic aperture sonar (SAS) algorithms. The target of opportunity closed head-on directly toward the surface ship at a relative speed of 13 knots through an acoustic environment that was characterized by a downward refracting sound speed profile in 100-m-depth water over a sand/mud bottom during calm seas. This situation represents an endfire (90-degree squint angle) scenario for which the resolution of any line array (synthetic or real) is considerably degraded relative to the broadside case. The test was not designed for SAS application so the aperture is significantly undersampled. The processing approach utilizes signal adaptive, ping-to-ping correlation autofocusing to correct for the effects of platform and target motion, and for ocean instabilities. The processing is tuned to the specific environment by setting the cross-correlation window size based on the two-way group arrival structure of the channel. The aliasing that results from the undersampled aperture is reduced to acceptable levels through the appropriate use of cross-range matched field processing. Images formed by processing the data from a forward-look synthetic aperture measuring 2200 wavelengths with a mean target distance of 9.5 km are presented.

The post-maximum behaviour of the changing-look Seyfert galaxy NGC 1566

“…This result can be seen independently from spectra obtained by SALT in 2012 and 2018 (Ochmann et al 2020). A similar effect has been detected in other CL AGNs as they change their states (see e.g., Shappee et al 2014;Edelson et al 2000), and it is probably a common property of CL AGNs.…”

“…The dramatic changes of factors 1.5 orders of magnitude in the X-ray flux along with correlated variations at UV/Opt wavelengths is a typical property of CL AGNs. However, whilst a strong correlation of the X-ray and UV/Opt is not common among AGNs (see e.g., Edelson et al 2000;Buisson et al 2017), there are exceptions for some other CL AGNs, viz. NGC 2617, NGC 4151 and NGC 5548 (see e.g., Shappee et al 2014;Oknyansky et al 2017;Edelson et al 2017;McHardy et al 2014).…”

Changing looks of the nucleus of the Seyfert galaxy NGC 1566 compared with other changing‐look AGNs

“…The dramatic changes of factors few tens in the X‐ray flux (see Paper I) along with correlated variations at UV and optical wavelengths is a typical property of CL AGNs. However, such a strong correlation of the X‐ray and UV/Opt is not common among AGNs (see e.g., Buisson et al 2017; G. S. Edelson et al 2000), there are exceptions for some other CL AGNs, viz. NGC 2617, NGC 4151, NGC 3516, and NGC 5548 (see e.g., R. Edelson et al 2017; McHardy et al 2014; Oknyansky et al 2021, 2017; Shappee et al 2014).…”

Changing looks of the nucleus of the Seyfert galaxy NGC 1566 compared with other changing-look AGNs

“…The dramatic changes of factors few tens in the X-ray flux (see Paper I) along with correlated variations at UV and optical wavelengths is a typical property of CL AGNs. However, such a strong correlation of the X-ray and UV/Opt is not common among AGNs (see e.g., Buisson, Lohfink, Alston, & Fabian, 2017;G. S. Edelson, Gedney, Abbot, Dyer, & Rolt, 2000), there are exceptions for some other CL AGNs, viz.…”