## 3 Near-Space Passive Radar Remote Sensing
### Radar Configurations  
To achieve persistent regional remote sensing, we proposed the concept of near-space platform-based passive radar imaging. This concept involves placing a passive receiver inside a near-space platform and utilizing opportunistic illuminators such as frequency modulation (FM) radio [34], digital television [35], digital broadcasters [36] or spaceborne imaging radars [37]. A promising opportunistic illuminator is global navigation satellite systems (GNSS). Taking global position systems (GPS) as an example, GPS broadcasts a civilian-use signal at 1.57542 GHz, referred to as \"L1\". Additionally, a second carrier signal for military use is also broadcasted at 1.2276 GHz, referred to as \"L2\", but civilian reception of this signal requires complicated processing algorithms. Both \"L1\" and \"L2\" signals are constantly scattered off earth surface, and are known to contain valuable and varied information on the targets or environment [38, 39, 40, 41, 42]. The use of GNSS signals has many advantages such as entire planet coverage, simple transmitter-receiver synchronization, and precise knowledge of the transmitter position information. Although the near-space receiver may be stationary, an aperture synthesis can still be achieved with the motion of the GPS satellite only [43, 44]. This configuration provides many specific advantages, like the exploitation of additional information contained in the bistatic reflectivity of targets [45, 46], reduced vulnerability in military systems [47], forward-looking SAR imaging [48], and improved detection capability of slowly moving targets [49]. More importantly, this passive bistatic SAR has a particular value for persistent region monitoring. Rather than emitting signals, it relies on opportunistic illuminators to monitor the area of interest without influencing normal communication and broadcast systems. One attractive configuration is the two-channel receiver configuration, as shown in Figure 3(a). One channel is fixed to collect the direct-path signals, which can be used as the reference signal for matched filtering. The second channel is configured to gather the scattered signals with which the detection of targets is being attempted. Typically, the signals scattered from a target are much weaker than the direct-path signals. Unless the transmit antenna attenuates sufficiently in the direction of the illuminator, the reflected channel will contain more energy from the direct-path signals than from the scattered signals. The use of array antennas is being considered to overcome this problem. The configuration using two or more near-space receivers located far apart is also feasible. Each using its independent antenna, as shown Figure 3b. In this configuration, each near-space receiver performs its own matched filtering using the received signal, the results of which can then be combined in some manner to provide a single target positioning and imaging. In fact, the configuration using only a single-channel near-space receiver is also feasible. In this case, the received signals would contain energy from both the direct-path signals and the scattered signals. Once they are separated, successful matched filtering can then be performed. Another promising configuration is digital beamforming on receive [50]. As shown in Figure 4, the satellite antenna footprint exceeds by far the size of the near-space receiver footprint. To prevent the simultaneous data collection area from being restricted by the small receiver footprint, the receiver antenna is split into multiple subapertures. This idea for spaceborne bistatic SAR has been proposed by [PERSON] and [PERSON] [22]. In this way, each subaperture signal is separately amplified, down-frequency converted and digitized. The digital signals are then combined in a dedicated processor to form multipleantenna-beams with arbitrary shapes. Digital beamforming on receive also allows for a selective interference suppression.