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.