The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part Bl. Beijing 2008 
target-to-receiver distance R [km] 
Figure 1: Power budget analysis of SNR 
3.3 Noise Equivalent Sigma Zero 
A quantity directly related to radar image performance is the 
noise equivalent sigma zero (NESZ). The NESZ is the mean 
RCS necessary to produce a SNR image of unity. The NESZ can 
be interpreted as the smallest target cross section which is 
detectable by the SAR system against thermal noise. Setting 
SNR image = 1 > Eq- (8) gives 
NESZ = 
\kR)FKT {] 
n 0 4^ 
(9) 
where a c and G sl are the RCS of the clutter per unit area and 
the sidelobe gain of the antenna, respectively. From radar 
equation we get the clutter-to-target power ratio (CTPR) as 
CTPR = - = 2n- — • ■ R 2 • log 
P cr n G. 
f \ 
1 + ^ 
R. 
(11) 
To estimate the clutter power, we suppose that G sl =-10dB, 
a c = -20dB , and the other parameters are same as the last 
sections. Figure 3 gives the clutter-to-target power ratio for 
different values of R r and A r ( A r = A 2 G r / 4n ). The results 
show that the clutter contains almost as much as power as the 
target returns. This situation can be improved by reducing the 
magnitude of the antenna sidelobe gain or the application of 
space-time adaptive signal processing algorithms, e.g., 
fractional Fourier transform. Moreover, the CTPR can be 
further improved owing to subsequent range compression and 
azimuth compression. 
4. POTENTIALS 
In this section, we addressed the potential analysis of near 
space passive remote sensing for homeland security 
applications, while compared with spacebome and airborne 
remote sensing. 
Assuming again the same parameters as the last section, the 
calculated NESZ is illustrated in Fig. 2. This results clearly 
show that a comparable RCS requirement to current radar 
systems is possible. 
Figure 2: Power budget analysis of NESZ 
3.4 Cluster Power Estimate 
Another consideration is clutter, which can be assumed to enter 
the system via the antenna sidelobes only. Take GPS as an 
example, the transmitted signal is a spread spectrum system 
with a chip rate of 1.023M Hz, the clutter power at the receiver 
antenna can be represented by 
P c =n 0 -^-log 
1 + ^ 
^G si 
An 
4.1 Persistent Coverage 
Due to the unavoidable consequences of orbital mechanics, a 
satellite at other than GEO altitudes cannot remain within view 
of an area indefinitely. Generally speaking, air-breathing 
aerodynamically lifted platforms cannot routinely operate much 
above 18.3km. Similarly satellites usually operate in the orbits 
above 200km, otherwise tenuous atmospheric drag will 
significantly reduce their lifetimes. As a result, physical 
limitations due to orbital mechanics and fuel consumption 
prevent a persistent coverage for current radars. Fortunately this 
can be achieved through the use of near-space free-floaters 
flying in the region where the prevailing winds are relatively 
mild because it is above storms and jet steam. Being defined as 
the region between 20km and 100km, near-space offers a 
number of benefits, but the most promising is persistence. 
target-to-receiver distance R [km] 
(10) 
Figure 3: Calculated clutter-to-target power ratio.