The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part Bl. Beijing 2008
The lower limit of near-space is not only determined from
operational considerations, being above controlled airspace, but
meteorological one as well. The 20km altitude is above the
troposphere, the atmosphere region where most weather occurs.
There are no clouds, thunderstorms, or precipitation in near
space. In fact, there is a region in near-space where average
winds are less than 20 knots, with peak winds being less than
45 knots for 95 percent of the time.
4.2 Robust Survivability
Near-space ffee-floaters are inherently survivable. They have
extremely small RCS (radar cross section) making them
relatively invulnerable to most traditional tracking and locating
methods. Estimates of their RCS are as small as that of a bird
(Tomme, 2005), and as a result currently documented radars are
unable to find them. At this altitude the acquisition and tracking
will be technical challenges even without considering what sort
of weapon could reach them since few weapons are designed to
engage a target with very low RCS. Even if the acquisition and
location problems are overcome, near-space assets are still
difficult to be destroyed. The way they are manufactured has a
lot to do with them relative invulnerability. Near-space ffee-
floaters can be manufactured in two basic types: super-pressure
and zero-pressure. Super-pressure ones are inflated and sealed,
much like a child’s toy helium balloon. Zero-pressure ones
have venting system that ensures the pressure inside the balloon
is same as the surrounding atmosphere. The second kind is less
vulnerable to puncture. Imaging an inflated, lightweight plastic
garment bag floating on the wind; even if there are many small
holes in such a bag, it still can float in the air for a long time.
4.3 Bistatic Observation
Bistatic observation can provide many specific advantages, like
the exploitation of additional bistatic information (Kuang and
Jin, 2007), reduced vulnerability in military systems(Wang and
Cai, 2007b), and improved detection capability of slowly
moving targets (Li, et al., 2007). Objects detection in
heterogeneous environments, e.g., homeland security, will
further take advantage of reduced retro-reflector effects
(Fernandez, et al., 2006). The segmentation and classification
of natural surface and volume scatterers are alleviated by
comparing the spatial statistics of mono- and bistatic scattering
coefficients. Bistatic observations may also increase the RCS of
manmade objects and/or the sensitivity to specific scattering
centres of objects composites. Furthermore, bistatic observation
in a forward scattering geometry have also great potential for
systematic vegetation monitoring. Homeland monitoring will
take advantage of the specular coherent reflection, which
enables more sensitive object estimates over a wider dynamic
range with lower saturation.
4.4 Low Cost
When cost is the concern, near-space has no peer. Their
inherent simplicity, recoverability, relative lack of requirement
for complex infrastructure, and lack of space-hardening
requirements all contribute to this strong advantage for assets.
Requiring only helium for lift, near-space platforms do not
require expensive space launch to reach altitude. If the payloads
they carried have malfunction, they can be brought back down
and repaired. When they become obsolete, they can be easily
replaced. Not being exposed to the high levels of radiation
common to the space environment, payloads flown in near
space require no costly space-hardening manufacture.
Additionally, operating in near-space obviously eliminates a
great deal of expense involved in space sensor construction.
The infrastructure cost savings involved with near-space are
huge. Near-space platforms require extremely minimal launch
infrastructure. Only a simple tie-down and an empty fielded are
required, but a space-launch complex or even a hard-surface
runway must be built for satellites and airplanes.
5. CHALLENGES
5.1 Synchronization Techniques
The near-space remote sensing discussed in this paper is a
bistatic configuration, it is subject to the problems and special
requirements that are either not encountered or encountered in
less serious form for current monostatic SAR systems. The
biggest challenge lies in the synchronization between the near
space receiver and the GNSS satellites: phase synchronization,
the near-space receiver and the GNSS satellites must be
coherent over extremely long periods of time; spatial
synchronization, the near-space receiving antenna and the
GNSS transmitting antennas must simultaneously illuminate the
same spot on the ground.
There is no cancellation of low-frequency phase noise as in a
monostatic radar, where the same oscillator is used for
modulation and demodulation. We can express the synchroniz
ation errors as
A T
A = ¡Mf.-fX' (12)
o
where f and f r denote the GNSS transmit carrier frequency and near
space receive demodulation frequency, respectively, AT is the
integrated time and should be greater than one aperture time. A
typical requirement for the maximum tolerable ISLR
(integrated sidelobe ratio) is -20dB. Unfortunately, the phase
synchronization errors are usually random and too complex to
apply autofocus algorithms.
As shown in Fig. 4 for four point targets, it is evident that
oscillator phase noise may not only defocus the radar image,
but also introduce significant positioning errors along the scene
extension, so some synchronization technique or compensation
algorithms must be applied. One possible solution is the direct-
path signal based synchronization technique (Wang, et al.,
2008). However notice that, although the feasibility of general
bistatic radar concept was already demonstrated by
experimental investigations, the synchronization including time,
spatial and phase is still the primary impediment to current
bistatic radar development in general, not only to the near-space
passive bistatic radar discussed in this paper.
