2.1 Products and Algorithms Introduction
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| Figure 2.1 |
The specifications
for the standard Advanced Synthetic
Aperture Radar (ASAR) data products are
shown in table 2.1 below. Many of the
product types and definitions are similar to
those used for the European Remote Sensing
Satellite (ERS). For instance, Precision, Ellipsoid Geocoded, and Single-Look Complex images
have the same nominal resolution and pixel spacing as their ERS
equivalents. (For a discussion on the
differences between resolution and pixel spacing
see )
ASAR
Measurement Modes
The Advanced Synthetic Aperture Radar is a
high-resolution imaging radar that can be
operated in 5 distinct Measurement Modes:
Within each mode, several different image swaths may be used. IM, AP, and WS
modes are designated as High Rate (HR) modes, and
have a downlink rate of 100 Mbps. GM and WV are Low
Rate (LR) modes and have a data generation rate
of 0.9 Mbps.
In addition, ASAR supports 2 Auxiliary Modes (Test
Mode and Module Stepping Mode 2.5.1.2.3.3. ) and one
Calibration Mode (External Characterisation
Mode 2.5.1.2.2.4. ), which are used for testing, calibration
and instrument monitoring.
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Table 2.1 Standard specifications for ASAR data products
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For additional product summary tables refer to
"Organisation of Products. 2.2. "
Product Generation
Product generation may fall into two
major classes. These are:
-
Systematic - Medium resolution
and browse products are generated automatically
for all received data in strip-line format.
-
On Request - Products are not
generated unless specifically requested by a
user, such as Precision, Ellipsoid
Geocoded and Single-Look Complex images.
In addition, the systematic products may be
subdivided into two categories based on the
production time:
-
Near Real Time - There are two
types of Near Real Time (NRT) products, those to
be produced within 3 hours and those to be
produced within one day.
-
Off-line - Some products will
be processed off-line. The availability time
depends on the complexity of the
processing and the availability of auxiliary
data. Delivery time ranges from two business
days to four weeks. Off-line products benefit
from the more precise orbit information which
becomes available later.
Stripline Products
Stripline products are generated along a complete
segment or an orbit. Geometric and radiometric
continuity are ensured along the complete
segment or orbit. Scenes can be ordered from within
a stripline, then extracted and distributed via the
PDS User Services. Scene sized is based on
granules, an area consisting of the along-track
distance by the full swath width. The framing
constraints are that the scene must contain an
integer number of granules, beginning and ending
anywhere within the stripline and must be a full
swath width. Child products may be sized to be
any multiple of a granule. Stripline processing of
browse, medium- and low-resolution data will produce
products for up to 10 minutes data acquisition in
Image, Alternating Polarisation and Wide Swath
Modes, and up to a full orbit for Global
Monitoring Mode.
In order to achieve stripline processing, the full
data segment is divided across multiple PF-ASAR
processors, as shown in figure2.2 below. The output of each
processor is called a slice, and the slices are
concatenated together to form the final stripline
product. Each slice is divided into an integer
number of granules, which are used to facilitate
child product extraction.
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| Figure 2.2 System design for stripline processing |
For a detailed description of strip-line
processing see the section entitled "ASAR Stripline Product
Processing" 2.6.1.2.5. .
Floating Scene Concept on
Archived Products
There is no reason why a user's region of
interest should be limited to that which matches the
archived product, either in terms of size or
geographical position. For these reasons, production
has been disassociated from the dissemination and,
according to the floating scene concept, a
multiple of the minimum scene size only can be
disseminated if necessary. The size of a product can
be anything from the whole data downlink segment
to the minimum scene size, without any constraint on
the beginning of the product. Regardless of the
size, the extracted product will have a size
equal to a multiple of the minimum scene size.
For ASAR, the minimum product size is a single
granule, which is a pre-defined, along-track
distance by the full swath width.
This flexibility is obtained by the Archiving
Facility (ARF), which allows a product to be to
selectively extracted from an existing archived
product without any processing. This corresponds to
the child product concept, which is discussed below.
(Also see "Floating scene concept for
NRT products" discussed below.)
For products processed systematically, the user
selects the position of one or more scenes along the
acquired data segment. The scene starts at
granule boundary, as shown in figure2.3 below.
 full size
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| Figure 2.3 The selection of the scene starts at granule boundary |
For products generated on request, such as
Alternating Polarisation (AP) and Image Mode (IM)
precision products, the user can select a scene
starting from any time along the acquisition data
segment (no framing constraint), as shown in figure2.4 below. These products have
fixed sizes.
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| Figure 2.4 The selection of a scene can start from any time along the acquisition data segment |
Child Product Concept
A child is the result of the extraction of a set of
data from a parent product (Level 0, Level 1B, Level
2). Generation of child products can be
formulated in terms of time, Data Set (DS), or
Instrument Source Packet (ISP) selection, in terms
of either segments or scenes; that is:
- time window
- geographic zone (search using the LADS with the
help of the PDS inventory)
- data subset
For generation of scenes, the only extraction
criteria available are times. When performing child
extraction, it is possible to output either a
segment corresponding to the time window, or a set
of scenes containing the time window, as depicted in
figure2.5 below.
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| Figure 2.5 Child product extraction |
When a child extraction is required against
consecutive slices archived at the ARF, the child
product generation can apply to several parent
products. These parent products are identified
through their physical names and their Stripline
Continuity Indicators (SCI).
Again, the result of such a child product extraction
could be formulated in terms of either segment or scenes.
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| Figure 2.6 Child product extraction from consecutive slices |
For a further discussion of these products, as they
relate to ASAR, refer to "Child Products" 2.1.1. .
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| Figure 2.7 Product generation and extraction |
Floating scene concept for NRT products
The ARF will be able to generate and distribute
scenes associated to the concatenation or the
extraction of the different consecutive slices
it has received. For NRT mode distribution, the ARF
will transmit as quickly as possible each scene to
the DF, not waiting for the reception of the set
of complete slices before building and transmitting
the associated scenes.
Each scene will be transmitted immediately as it
becomes complete.
The NRT scene generation is illustrated below:
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| Figure 2.8 Scene generation in NRT |
General Product Layout
The data production format for one
product will consist of:
All data sets are filled with DSRs, and each DSR
starts with a time entry. The time entry permits
correlation between all data sets at record
level and permits subset extraction as will be
defined later. The GADS is an exception to this rule.
For a further discussion on each of these components,
refer to the section entitled "Definitions and Conventions." 2.3.
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EO Data Access
