## 3 HAP-borne Passive BiSAR Remote Sensing  
The HAP-borne passive BiSAR involves placing a passive receiver inside a HAP and utilizing opportunistic illuminators such as global navigation satellite systems (GNSS) and spaceborne or airborne radars. The use of GNSS as the transmitter has the advantages of entire planet coverage and simple transmitter-receiver synchronization [22], but the received GNSS signals are too weak [23, 24, 25, 26]. To overcome this disadvantage, this paper uses spaceborne or airborne radar as the passive BiSAR transmitter, in which the transmitter and receiver may have unequal velocities. Note that, although the passive receiver may be stationary, an aperture synthesis can still be obtained by the motion of the transmitter only. The HAP-borne passive receiver considered in this paper consists of two channels, as shown in Figure 1. One channel is fixed to collect the direct-path signals coming from the transmitter antenna sidelobes, which can be used as the reference signal for subsequent matched filtering or for BiSAR time and phase/frequency synchronization compensation [27]. The second channel is configured to gather the reflected signals with which remote sensing is attempted. Note that another configuration using digital beamforming on the receiver [28] is also implementable. In this case, the receiver antenna is split into multiple sub-apertures, and each sub-aperture signal is separately amplified, down-converted and digitized. The digital signals are then combined in a dedicated processor to form multiple sub-beams. When compared with spaceborne or airborne BiSARs, this passive BiSAR offers several promising superiorities. Besides possible persistent regional monitoring and low cost, another superiority is the robust survivability in military reconnaissance because detecting its passive receiver is a technical challenge. Moreover, rather than emitting signals, it relies on opportunistic transmitters; hence, it has no impacts on existing communication and navigation systems. This is an advantage for urban area and human settlement remote sensing. For a given imaging target, the target must be simultaneously within the line-of-sight (LOS) to both the transmitter and the receiver. That is to say, antenna direction synchronization must be ensured for the passive BiSAR system [29]. If airborne transmitter is used, the antenna direction synchronization can be implemented though a cooperative flying plan. However, if spaceborne transmitter is employed, it will be a challenge due to the fact that the transmitter and the receiver have a big difference in the flying velocities. Moreover, the satellite antenna direction is often uncontrollable for us. The pulse chasing technique proposed in [30] cannot be easily implemented for noncooperative passive BiSAR configurations, particularly when spaceborne transmitter is employed. For these reasons, we use a wide antenna beamwidth on receiver due to its advantage of high signal-to-noise ratio (SNR) in this system. This can be easily understood from the bistatic radar equation [31]. To implement successfully BiSAR remote sensing, both of the transmitter and receiver antennas should be steered to obtain an overlapping beam coverage on the ground and the exposure of a target is governed by this composite beam pattern. It is necessary to analyze the imaging time and imaging coverage. A space-time diagram-base method was investigated in [32] by employing a rectangular approximation. Consider a flat Earth model and parallel transmitter and receiver flying tracks without squint angles but with different velocities, the imaging time of the HAP-borne passive BiSAR is determined by  
\[T_{\text{image}}=\frac{D_{a,t}+D_{a,r}}{|v_{t}-v_{r}|} \tag{1}\]  
Figure 1: General geometry of HAP-borne passive BiSAR system. where \(v_{t}\) and \(v_{r}\) are the transmitter velocity and receiver velocity, \(D_{a,t}\) and \(D_{a,r}\) are the illuminated ground coverage in azimuth direction for the transmitter and receiver, respectively.