  Since by the International Telecommunications Union (ITU) regulatory the
radio spectrum available to spaceborne synthetic aperture radar (SAR) is
restricted to certain limited frequency intervals, there are many different
spaceborne SAR systems sharing common frequency bands. Due to this reason, it
is reported that two spaceborne SARs at orbit cross positions can potentially
cause severe mutual interference. Specifically, the transmitting signal of a
SAR, typically linear frequency modulated (LFM), can be directly received by
the side or back lobes of another SAR's antenna, causing radiometric artefacts
in the focused image. This paper tries to model and characterize the artefacts,
and study efficient methods for mitigating them. To this end, we formulate an
analytical model for describing the artefact, which reveals that the mutual
interference can introduce a two-dimensional LFM radiometric artefact in image
domain with a limited spatial extent. We show that the artefact is low-rank
based on range-azimuth decoupling analysis and two-dimensional high-order
Taylor expansion. Based on the low rank model, we show that two methods, i.e.,
principle component analysis and its robust variant, can be adopted to
efficiently mitigate the artefact via processing in image domain. The former
method has the advantage of fast processing speed, for example, a sub-swath of
Sentinel-1 interferometric wide swath image can be processed within 70 seconds
via block-wise operation, whereas the latter provides improved accuracy for
sparse point-like scatterers. Experiment results demonstrate that the
radiometric artefacts caused by mutual interference in Sentinel-1 level-1
images can be efficiently mitigated via the proposed methods.
