NEAR-SPACE PASSIVE REMOTE SENSING FOR HOMELAND 
SECURITY: POTENTIAL AND CHALLENGES 
Wen-Qin Wang a ’ b ' * 
a Lab 140, School of Communication and Information Engineering 
University of Electronic Science and Technology of China, Chengdu, 610054, P. R. China 
b Beijing Key Lab of Spatial Information Integration and 3S Application, Peking University, Beijing 100871, China 
‘ wqwang@uestc.edu.cn 
Commission I, ThS-1 
KEY WORDS: Near-space, homeland security, remote sensing, passive radar, monitoring, microwave imaging, disaster. 
ABSTRACT: 
Inspired by recent technical advances in near-space defined as the space region between 20km and 100km, which is above the storms 
and jet stream, and not constrained by the orbital mechanics of the satellite platforms or the high fuel consumptions of the airborne 
platforms, they can stay at a specific site almost indefinitely to provide a persistent coverage, this paper proposed the system concept 
of near-space remote sensing for homeland security applications. To the author’s knowledge, I am the first author that proposes the 
concept of near-space remote sensing. This concept involves a passive radar receiver placed inside a near-space platform is in 
conjunction with the illuminator of opportunistic signals, so as to provide persistent mapping and monitoring of homeland. This 
paper deals with conceptual analysis, as opposed technological implementation. It is shown that the novel use of near-space 
platforms can provide the solutions that were thought to be out of reach for remote sensing scientists. The techniques and concepts 
discussed may be regarded as the Phase I work towards promising near-space remote sensing for homeland security applications. 
1. INTRODUCTION 
To protect civilian population, mass transmit, civil aviation, 
and critical infrastructure from terrorist attacks employing 
explosive devices, vast improvements in our current capability 
of protecting homeland security are required, so that 
efficiently neutralize these threats without significantly 
impacting our normal day-to-day activities. It appears that 
microwave remote sensing may play an important role in 
homeland security (Baker and Griffiths, 2005). The current 
spacebome radar has playing an important role in remote 
sensing applications (Trouve et al., 2007); however, even as 
good as they are, they cannot provide a staring presence on a 
timescale of days, weeks, or months over a selected target or 
area of interest. Costing as much as billions of U. S dollars, or 
at least millions each with multiple satellites required to 
provide persistent coverage makes it prohibitively expensive. 
In contrast, conventional airplane cannot fly very high because 
there is insufficient oxygen to allow the engines to operate. 
We thus have two identified gaps; one is the gap in capability 
and the other is the gap in the altitude between satellite and 
airplane. Fortunately, these two gaps can be simultaneously 
filled through the use of near-space platforms. Near-space 
defined as the region between 20km and 100km was a cultural 
blind spot - too high up for conventional aircrafts, but too low 
for LEO satellites. Inspired by recent advances in near-space 
technology, this paper presents the system concept of near 
space remote sensing for homeland security applications. To 
the author’s knowledge, I am the first author that proposes the 
concept of nears-pace remote sensing (Wang, 2007a). This 
concept involves a passive radar receiver placed inside a near 
space platform is in conjunction with the illuminator of 
opportunistic signals, so as to provide persistent homeland 
mapping and monitoring. More importantly, rather than 
emitting signals, this imaging system relies on the illuminators 
of opportunistic signals. This is particularly attractive, because 
it is desirable for such sensor to serve also other purposes like 
disaster monitoring, traffic monitoring and weather prediction. 
The remaining sections of this paper are organized as follows. 
In Section 2, the system concept of near-space remote sensing 
is presented, followed by the possibility analysis in Section 3. 
Next, the potentials and challenges are investigated in Section 
4 and Section 5, respectively. Finally, Section 6 concludes the 
whole paper. 
2. SYSTEM CONCEPT 
As an example, we take the global navigation satellite systems 
(GNSS) signals as the opportunistic signals. The use of GNSS 
has many advantages such as entire planet coverage, simple 
transmitter -receiver synchronization, and precise knowledge of 
the transmitter spatial information. Some applications have 
surfaced, but the application of this technique to homeland 
security was mostly ignored or overlooked. 
The passive receiver consists two channels. One channel is 
fixed to collect the signal arriving at the receiver travelling 
directly from the GNSS transmitters. This signal can be used as 
the reference signal for matched filtering, and referred to as 
x d (t). The second channel is configured to gather the scattered 
signal, and referred to as x r (r). This signal is sampled in a 
delay window that can be predicted using the knowledge of the 
near-space receiver, along with that of the GNSS transmitter 
and surface elevation. For homeland security applications, the 
GNSS signal reflection or scattering by an object is based on its 
surface reflectivity, which is the ratio of the reflected power to 
direct power. This configuration is of great interest because it 
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