  In this paper we report the first airborne experiments of sparse microwave
imaging, conducted in September 2013 and May 2014, using our prototype sparse
microwave imaging radar system. This is the first reported imaging radar system
and airborne experiment that specially designed for sparse microwave imaging.
Sparse microwave imaging is a novel concept of radar imaging, it is mainly the
combination of traditional radar imaging technology and newly developed sparse
signal processing theory, achieving benefits in both improving the imaging
quality of current microwave imaging systems and designing optimized sparse
microwave imaging radar system to reduce system sampling rate towards the
sparse target scenes. During recent years, many researchers focus on related
topics of sparse microwave imaging, but rarely few paid attention to prototype
system design and experiment. We introduce our prototype sparse microwave
imaging radar system, including its system design, hardware considerations and
signal processing methods. Several design principles should be considered
during the system designing, including the sampling scheme, antenna, SNR,
waveform, resolution, etc. We select jittered sampling in azimuth and uniform
sampling in range to balance the system complexity and performance. The imaging
algorithm is accelerated $\ell_q$ regularization algorithm. To test the
prototype radar system and verify the effectiveness of sparse microwave imaging
framework, airborne experiments are carried out using our prototype system and
we achieve the first sparse microwave image successfully. We analyze the
imaging performance of prototype sparse microwave radar system with different
sparsities, sampling rates, SNRs and sampling schemes, using three-dimensional
phase transit diagram as the evaluation tool.
