## 3 Near-Space Passive Radar Remote Sensing
### Potential Applications  
The near-space passive radar provides many promising potentials due to the superiorities of persistent region coverage, robust survivability, bistatic observation and low cost. [PERSON] _et al._[55] have presented a status report on the applications of SAR for detecting human settlement, estimating population, assessing the impact of human activities on physical environment, mapping urban land use patterns and change, and interpreting socioeconomic condition. Certainly, all of these applications are also possible for near-space passive radar. Moreover, near-space passive radar can provide the following additional potentials:  
One promising potential application is homeland security. To protect civilian population, mass transit, civil aviation and critical infrastructure from terrorist attacks employing explosive devices, significant improvement in our capability to detect and mitigate explosive threats are required, so that we can efficiently combat these threats without impacting normal day-to-day human activities and commerce activities. To reach this aim, radar has long been used in a variety of military and civilian applications and has been an essential component of current defensive systems. Presently, many countries have civil aviation radar networks. Such radar networks are specifically designed to ensure early alert against potential hostile targets. However, the range and nature of potential threat targets are becoming even more diverse. The tragic event on 11 September 2001 is just one example. Others might include missile attack and unmanned aerial vehicles (UAV). This means that the source of attack can be in a much wider variety of new and different forms, and cannot be well dealt with current radar systems. Fortunately, it appears that passive radar using opportunistic illuminators can provide a potential solution to these problems. More importantly, synthetic aperture (_i.e._, SAR) can be obtained for the near-space passive radar. In this way, it can not only detect moving targets but also monitor regional scene persistently.  
Figure 5: Calculated NESZ with a function of target-to-receiver distance.  
Another advantage is that, rather than emitting signals, this passive imaging system relies on opportunistic transmitters to image potential targets or scene. This allows the receiver to operate without emitting signals. This is particularly attractive for homeland security applications, because it is desirable for such sensor to also serve other purposes like traffic monitoring and weather prediction. In addition, the fact that they require no dedicated transmitters also means significant cost savings.  
Another promising potential application is persistent region environment monitoring. A good example is disaster monitoring. In recent years, the frequency of natural disasters has shown rapid increase [56]. Examples of this trend are related to floods, earthquakes, tsunamis, hurricanes, and forest fires [57]. The tsunami that killed thousands of people in the coastal areas of India, Indonesia, Thailand and Sri Lanka has brought the awareness that we cannot no longer take natural disasters as something inevitable or unavoidable and remain as helpless observers. Methods and strategies along with effective devices have to be developed to predict and fight natural disasters. To reach this aim, immediately after that tsunami, natural disaster monitoring has received much recognition [58], but there is still lack of feasible and practicable solution. Near-space passive radar remote sensing does indeed seem to be a promising solution. Moreover, since natural disasters are relatively infrequent events, it is desirable for such sensor to serve also as other purpose like weather and ocean monitoring (floods, sea-swells, cyclones) as well as surveillance, reconnaissance, and traffic monitoring [59, 60].