_Remote Sensing_**2010**, \(2\), 717-739; doi:10.3390/rs2030717  
_Review_  
**Near-Space Microwave Radar Remote Sensing: Potentials and Challenge Analysis**  
**[PERSON] \({}^{1,2,\star}\), [PERSON] \({}^{1}\) and [PERSON] \({}^{1}\)**  
\({}^{1}\) School of Communication and Information Engineering, University of Electronic Science and Technology of China, 611731 Chengdu, China; E-Mails: [EMAIL_ADDRESS] (J.C.);  
[EMAIL_ADDRESS] (Q.P.) \({}^{2}\) The Institute of Plateau Meteorology (IPM), China Meteorological Administration (CMA), China  
\({}^{\star}\) Author to whom correspondence should be addressed; E-Mail: [EMAIL_ADDRESS]  
_Received: 16 November 2009; in revised form: 11 January 2010 / Accepted: 26 January 2010 / Published: 9 March 2010_  
**Abstract:** Near-space, defined as the region between 20 km and 100 km, offers many new capabilities that are not accessible to low earth orbit (LEO) satellites and airplanes, because it is above storm and not constrained by either the orbital mechanics of satellites or the high fuel consumption of airplanes. By placing radar transmitter/receiver in near-space platforms, many functions that are currently performed with satellites or airplanes could be performed in a cheaper way. Inspired by these advantages, this paper introduces several near-space vehicle-based radar configurations, such as near-space passive bistatic radar and high-resolution wide-swath (HRWS) synthetic aperture radar (SAR). Their potential applications, technical challenges and possible solutions are investigated. It is shown that near-space is a satisfactory solution to some specific remote sensing applications. Firstly, near-space passive bistatic radar using opportunistic illuminators offers a solution to persistent regional remote sensing, which is particularly interest for protecting homeland security or monitoring regional environment. Secondly, near-space provides an optimal solution to relative HRWS SAR imaging. Moreover, as motion compensation is a common technical challenge for the described radars, an active transponder-based motion compensation is also described. **Keywords:** near-space; microwave radar; remote sensing; passive radar; synthetic aperture radar; bistatic radar; motion compensation Introduction  
Current spaceborne and airborne radars have been playing an important role in remote sensing applications [1, 2, 3]; however, even as good as they are envisioned and employed, it is impossible to provide a staring presence on a timescale of days, weeks, or months over a selected target or area of interest. Even if we can launch a satellite for a particular application, it would only be in view for a short time. As an example, most low earth orbit (LEO) satellites have a specific target in view for less than 10 minutes at a time and revisit the same site infrequently. Persistent coverage by multiple satellite constellation costs as much as billions of US dollars, which is prohibitively expensive [4]. In contrast, conventional airplanes cannot fly too high because there will not be sufficient oxygen for the engines to operate. Generally, satellites usually operate in the orbits above 200 km, and air-breathing airplanes routinely operate at lower than 18 km. We thus have two gaps. The first is a gap in capability of persistent remote sensing observation. The second gap is the absence of sensors in the altitude between that of airplanes and satellites. To overcome these disadvantages, we should seek some alternative platforms. Moreover, the requirements of stealth and robust survivability in military applications also call for new radar platforms other than satellites and airplanes [5]. Fortunately, those aims can be simultaneously obtained by near-space platforms at a fractional cost of the traditional platforms [6]. Near-space, defined as the region between 20 km and 100 km, is too high for conventional aircrafts and too low for LEO satellites. It offers many new capabilities that are critical to emerging remote sensing applications, but it is not accessible from satellites or airplanes.