1.1.3 Principles of Measurement
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Figure 1.9 ERS-1 image June 21 1992, Mt. St. Augustine Volcano, Cook Inlet (Copyright ESA, 1992) |
The antenna beam of a side-looking radar is directed
perpendicular to the flight path and
illuminates a swath parallel to the platform
ground track. Due to the motion of the
satellite, each target element is illuminated by
the beam for a period of time
(integration time). As part of the on-ground
processing, the complex echo signals received
during this period are added coherently.
This process is equivalent to synthetically
forming a long antenna (called Synthetic Aperture).
Assuming a constant angular beam width along-track (azimuth) for the
entire swath, the achievable synthetic aperture
increases with the distance(slant range) between
radar and target.
The range resolution of a
pulsed radar system is limited fundamentally by
the bandwidth of the transmitted
pulse (slant range resolution = c/2B hence the
wider the bandwidth, the better the range resolution). A wide
bandwidth can be achieved by a short
duration pulse. However, the shorter the pulse,
the lower the transmitted energy (for a
fixed-peak power limitation) and the poorer
the signal-to-noise ratio, hence the radiometric
resolution. To preserve the radiometric
resolution, the technique adopted by ASAR is to
generate a long pulse with a linear frequency modulation (or
chirp). The length of the pulse is defined
to be consistent with the requirement for the
signal-to-noise ratio. The chirp bandwidth is
defined by the required range resolution.
After the received signal has been compressed,
the range resolution will be optimised without
loss of radiometric resolution.
The azimuth resolution of a real aperture radar system
is a function of the antenna length (the
larger the antenna, the better the azimuth
resolution). It can be shown that a spaceborne
real aperture radar, giving a useful azimuth
resolution for points on the Earth's
surface, will require an impractically large
antenna. Aperture synthesis, therefore,
offers a means of greatly improving the azimuth resolution for a
given antenna length.
The measurement principle of ASAR depends on the
use of coherent radiation, together with precise
knowledge of the transmit and receive point
of the radar pulse. For a given target, as the
platform moves, the distance from the radar to
the target (i.e., the slant range) changes
continuously, hence the phase of the reflected signal
changes according to a law given by the
geometry of observation. As this law is
deterministic, it is therefore possible to
correctly phase the return signals with
respect to each other so that the net effect is
equivalent to them all having been received
simultaneously by an antenna of length equal
to the path length over which the radar signals
were collected (i.e., the synthetic aperture).
In this way, the synthesised antenna can be
thought of as a number of independently
radiating elements (i.e., the real aperture),
whose separation is established by the Pulse Repetition Frequency
(PRF) and the platform velocity. The change of
phase with respect to time is the Doppler
angular frequency. The azimuth resolution is
determined by the Doppler bandwidth of the
received signal. For ASAR, the bandwidth of
target returns, in azimuth, is defined by the
Doppler bandwidth covered between the half-power
points of the one-way azimuth pattern. This
implies that pulses must be transmitted with a
repetition frequency greater than the
azimuth bandwidth in order to satisfy the
Nyquist sampling criterion.
There is an upper limit on the PRF imposed by the
geometry. If the PRF is so high that return
signals from two consecutively transmitted
pulses arrive simultaneously at the receiver,
there will be ambiguities in the response. This
will, therefore, define a set of unambiguous
intervals for a given geometry and PRF, which
corresponds to constraining the ground range extent of the
region illuminated within the elevation
beamwidth of the antenna footprint (i.e., the
swath width).
As a consequence of the ASAR antenna being used
for pulse transmission and echo reception, there
will be echoes that are not received due to
periods when the antenna is transmitting pulses
and hence not receiving echo returns. For a
given geometry and PRF, these
"blind" intervals will lie at constant
ground range positions.
The return from nadir (the ground point
vertically below the satellite) will be
significantly larger than the returns from the
required swath, because of its close range and
high reflectivity. To avoid this unwanted
signal saturating any other returns arriving at
the same time, the PRFs and swaths for ASAR are
chosen such that the nadir returns do not
occur in the imaging window.
The significant feature of the ASAR instrument is the active
phased array antenna, which allows independent
control of the phase and amplitude of the transmitted
radiators from different regions of the antenna
surface. It also provides independent
weighting of the received signal to each of
these regions. This offers great flexibility in
the generation and control of the radar beam,
giving the ASAR instrument the capability to
operate in a number of different modes. These
modes use two principal methods of taking
measurements; the ASAR instrument may operate as
a conventional stripmap SAR or as a ScanSAR.
ASAR Stripmap Modes (Image, Wave)
When operating as a stripmap SAR, the phased
array antenna gives ASAR the flexibility to
select an imaging swath by changing the beam
incidence angle and the elevation beamwidth. In
addition, the appropriate PRF required to ensure
acceptable ambiguity performance and to
suppress unwanted nadir returns is selected.
In the Image Mode, ASAR operates in one of seven
predetermined swaths with either vertically or
horizontally polarised radiation; the same
polarisation is used for transmit and receive
(i.e., HH or VV). See figure1.10 .
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Figure 1.10 Image Mode |
The Wave Mode uses the same swaths
and polarisations as Image Mode. However, a
continuous strip of data is not required.
Instead, small areas of the ocean are imaged at
regular intervals along the swath. This
intermittent operation provides a low data rate,
such that the data can be stored on board the
satellite, rather than being downlinked
immediately to the ground station. See figure1.11 .
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Figure 1.11 Wave Mode |
ASAR ScanSAR Modes (Wide Swath,
Global Monitoring, and Alternating Polarisation)
While operating as a stripmap SAR,
ASAR is limited to a narrow swath which is
imposed by the ambiguity limitation. This
constraint can be overcome by utilising the
ScanSAR principle, which achieves swath widening
by the use of an antenna beam which is
electronically steerable in elevation.
Radar images can then be synthesised by scanning
the incidence angle and sequentially
synthesising images for the different beam
positions. The area imaged from each particular
beam is said to form a sub-swath. The principle
of the ScanSAR is to share the radar
operation time between two or more separate
sub-swaths in such a way as to obtain full image
coverage of each.
The system transmits pulses to, and receives
echoes from, a sub-swath for a period long
enough to synthesise a radar image of the area
within the beam footprint at the required
resolution. It then switches beams to illuminate
a different sub-swath and continues in this
manner until the full-wide swath is covered, at
which point it returns to the original sub-swath
and the scanning cycle is repeated.
The imaging operation is, therefore, split into a
series of bursts of pulses, each burst providing
returns from one of the sub-swaths. Each
burst will be processed to provide an image of a
section of the corresponding sub-swath. The
imaging operations must therefore be such
that it cycles around the full set of sub-swaths
sufficiently rapidly for the imaged sections in
any one sub-swath to be adjoining or overlapping.
ASAR operates according to the ScanSAR principle,
as described above, in two measurement modes:
the Wide Swath Mode and Global Monitoring
Mode. These use five predetermined overlapping
antenna beams which cover the wide swath.
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Figure 1.12 Wide Swath Mode |
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Figure 1.13 Global Monitoring Mode |
An additional ASAR measurement mode, called
Alternating Polarisation Mode, has also been
defined which employs a modified ScanSAR
technique. Instead of scanning between different
elevation sub-swaths, the Alternating
Polarisation Mode (co-polar) scans between two
polarisations, HH and VV, within a single swath
(which is preselected, as for Image and Wave
Modes). In addition, there are two cross-polar
modes, where the transmit pulses are all H or
all V polarisation, with the receive chain
operating alternatively in H and V, as in the
CO-polar mode.
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Figure 1.14 Alternating Polarisation Mode |
The ASAR Instrument's Capabilities
Table 1.1 summarises the ASAR capabilities.
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Table 1.1 Summary of ASAR capabilities
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Instrument Parameters |
Image Mode |
Alternating Polarisation Mode |
Wide Swath Mode |
Global Monitoring Mode |
Wave Mode |
Swath width |
up to 100 km |
up to 100 km |
> 400 km |
> 400 km |
5 km vignette |
Operation time |
up to 30 min per orbit
rest of orbit |
Data Rate |
up to 100 Mbit/s 0.9 Mbit/s |
Power |
1365 W |
1395 W |
1200 W |
713 W |
647 W |
The use of the ASAR generic processor for near
real-time (NRT) and off-line processing in the
processing and archiving centres (PACs) and
national stations offering ESA services, is a
simplification for processing and product
validation. This allows full product
compatibility between the different processing centres.
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