## 1 Introduction  
Satellite and airplane represent two well established platforms in radar and remote sensing arena for years [1,2]. However, in recent years a new platform has received much attention, which operatesat an altitude between \(20\) km and \(100\) km above the Earth's surface and is called high-altitude platform [3, 4, 5, 6] or near-space vehicle [7]. Traditionally very few sensors can operate in the altitude between \(20\) km and \(100\) km because the atmosphere is too thin to support flying for airplanes and too thick to sustain orbit for satellites. In this region, there are no ionospheric scintillations that may significantly degrade microwave communication and navigation performance. Moreover, not constrained by orbital mechanics like satellites and high fuel consumption like airplanes, HAPs can move at a high speed. More importantly, HAPs can provide a large footprint and a long mission duration that are commonly associated with satellites and fast responsiveness that is commonly associated with unmanned aerial vehicles. These advantages are particularly useful for some specific remote sensing applications that require a fast revisiting frequency [8]. This also explains why HAPs have received so much attention in recent years [9].  
By placing a radar transmitter or receiver inside a HAP, many functions that are currently performed by satellites or airplanes could be performed in a more efficient way [10]. In [11], we investigated the application of HAP passive radar for regional remote sensing. In this paper, we present azimuth-variant signal processing for HAP passive bistatic synthetic aperture radar (BiSAR) with spaceborne/airborne transmitter for high-resolution remote sensing. In recent years, BiSAR has received much recognition [12, 13, 14]. There is also a resurgence of interest in passive radar [15], partly because bistatic radar requires no expensive transmitter. The BiSARs discussed in literature are mainly azimuth-invariant configurations, for example the \"Tandem\" configuration where the transmitter and receiver moving one after another with the same trajectory and the \"translational invariant\" configuration where the transmitter and receiver moving along parallel trajectories with the same velocity. Several representative azimuth-variant configurations are the SARs with forward-looking receivers [16, 17] or different experimented configurations with the moving transmitter (receiver) and fixed receiver (transmitter) [18]. Several signal processing algorithms have also been proposed for these configurations [19, 20]. The HAP-borne BiSAR described in this paper is an azimuth-variant configuration; when compared with the azimuth-invariant configurations, it has much more complex characteristics in signal models, spatial resolution, and image formation algorithm.  
In this paper, we present the azimuth-variant signal processing in HAP-borne passive BiSAR with spaceborne or airborne transmitter for high-resolution remote sensing. First, the system model and azimuth-variant characteristics are introduced. Then, an extended nonlinear chirp scaling (NCS) imaging algorithm is derived to address the azimuth-variant Doppler signal characteristics. The remaining sections are organized as follows. In Section 2, the background of HAP and the motivation of this paper are introduced. In Section 3, the system configuration and geometry model are introduced. Next, the azimuth-variant Doppler chirp rate and two-dimensional (2-D) spectrum model are analyzed in Section 4. Finally, an imaging algorithm using equivalent velocity and NCS is derived in Section 5. This paper is concluded in Section 6.