2.2 Organisation of Products The data from all the ENVISAT instruments
is classified according to a hierarchical data product
scheme. Details of this scheme are as follows:
Raw Data is data as it is received
from the satellite (serial data stream, not
demultiplexed). Raw data is recorded from the X
and Ka band demodulator output interfaces and stored
on High-Density Data Tapes (HDDTs). The Raw Data is
not considered a product, and its structure is
not defined in this document.
Level 0 is reformatted,
time-ordered satellite data (no overlap), in
computer-compatible format. The Level 0 product is
the lowest level product in the ENVISAT PDS. Table 2.7 in the section
"Level 0 Products" shows a full list of
ASAR Level 0 products.
Level 1B products are geolocated
engineering foundation products in which data has
been converted to engineering units, auxiliary
data has been separated from measurements, and
selected calibrations have been applied to the data.
These products are the foundation from which
higher level products are derived.
The Level 0 product is transformed into a Level 1B
Product by application of algorithms and calibration
data to form a baseline engineering product. The
Level 1B product may be based on the whole of the
source Level 0 product, or only a selected set of
instrument packets from within the Level 0
product, based on a time range or other selection criteria.
Level 1B products may be produced in
the following ways:
Unconsolidated: generated in
Near Real Time (NRT); an intermediate step in
near real time processing, used for
instrument performance monitoring. It is
produced in NRT using available NRT auxiliary
data (i.e., may not contain the most precise
orbit vectors or calibration information).
Fully Consolidated: generated
off-line, time ordered, no overlap, no data gaps
(except where the instrument has been
turned off), fully validated; the basis for any
further off-line processing. It is produced
using the most precise auxiliary information
available. It is compiled to respect the defined
product boundaries (e.g., ascending node to
ascending node at the equator for full orbit products).
Partially Consolidated: a
product that has been consolidated in all ways
except that it does not use the most
precise auxiliary data possible to perform
processing. Since several different levels of
accuracy exist for auxiliary data (e.g., 5
possible orbit state vector sources), varying
levels of consolidation may exist for a product.
Level 1B products make up the majority of ASAR
products, and are outlined in the section "Level 1B Products" 2.6. .
Table 2.39 in that section provides
a summary of these products.
Level 2 consists of geolocated
Near Real Time: generated in
near real time from unconsolidated Level 1B products
Off-line: generated from
consolidated Level 1B products
In general, the Level 1B product is transformed into
one or more Level 2 products through higher-level
processing to convert engineering units into
geophysical quantities and to form a more directly
interpretable and useful measurement data set. The
Level 1B products used are created at the time
of Level 2 processing.
The general rule for product structure is that NRT
and off-line versions of Level 2 products share the
same structure, the only difference being that
the off-line product is of better quality due to the
fact that more accurate (fully consolidated) data is
used during production than for the NRT product;
and that the start and stop times of the data may be
different to reflect consolidation of the data to
standardised portions of the orbit. Level 2 products
may be processed systematically or upon user request.
At present, there is only one ASAR Level 2 product,
which is the "Wave Mode Ocean Wave
Spectra" product (ASA_WVW_2P). This is
discussed more fully in the section "Level 2 Products and Algorithms." 2.7.
Browse products are decimated images which can be
ordered from the ENVISAT inventory. These products
are very small to support electronic
transmission while querying the catalogues. They may
be further reduced in size through the use of
standardised data compression algorithms.
Browse products will be generated systematically for
ASAR (except wave mode) and will contain image lines
derived from a data segment (up to a full orbit
The user will view the browse product to assist in
ordering the required products by displaying scenes
starting anywhere in the browse product. A
comprehensive description of the Browse products is
provided in the section entitled "Browse Products." 188.8.131.52.3.
Figure2.9 below shows the generalised
|Figure 2.9 The Generalised Product Tree
As discussed in the Introduction 2.1. , ASAR products can be
classified according to the mode in which the SAR
instrument is operating. These are:
Mode - these products are
high-resolution, narrow swath products based on
data acquired at one of seven subswaths.
product is a multi-look, ground
range, digital image (in
Polarisation Received (HH)
Vertical Polarisation Received
suitable for most applications. It
is intended for multi-temporal
analysis and for deriving backscatter
corrections and relative calibration are
applied to compensate for well
understood sources of system
variability. Absolute calibration
parameters are provided in the
The Ellipsoid Geocoded
Image product is similar to a
Precision Image, but with rectification
to a map projection. It is
geocoded using a map projection
selectable by the user, such as UTM, or
calibration parameters are provided
in the product annotations.
(SLC) image data is intended
for SAR image quality assessment,
interferometric or wind/wave
applications. A small number of
corrections and interpolations are
performed on the data in order to
allow freedom in the derivation of
higher-level products. Absolute
calibration parameters are
provided in the product annotations.
Image product will be available
at 150 m resolution, which is
specifically aimed at sea ice
and oceanography applications. Absolute
calibration parameters are provided
in the product annotations.
Alternating Polarisation (AP)
Mode - these products are similar to
Image Mode products but include a second image
acquired using a second polarisation
combination. The raw data is acquired in bursts
of alternating polarisation.
polarisation combinations are:
- the co-polarisation submode (one image
HH and one image VV)
- the cross-H polarisation submode (one
image HH and one image HV)
- the cross-V polarisation submode (one
image VV and one image VH)
Products contain two images corresponding to
one of the three polarisation combination
submodes, at reduced spatial resolution in
comparison to Image Mode.
Wide Swath (WS)
Mode - these products provide greater
swath width at reduced spatial resolution
through the use of the ScanSAR technique.
The standard product available for Wide
Swath Mode will be a 150 m resolution image
with the full 405 km swath width.
Wave (WV) Mode -
these products are similar to those in Image
Mode but are much smaller, covering an
area of approximately 5 km by 5 km. Wave mode
products may also contain secondary information
like cross spectra.
Wave Mode products are based on small high-resolution, complex images of
ocean scenes also called imagettes. These are
processed to derive spectra of the ocean
backscatter and consequently the
wavelength and direction of ocean waves.
An SLC Imagette and Imagette Cross
Spectrum is the basic Level 1B product from
the ASAR Wave Mode. A minimum number of
corrections and interpolations are performed
on the SLC data in order to allow freedom in
the derivation of higher-level
products. Absolute calibration parameters
are provided in the product annotations.
ASAR Ocean Wave Mode Spectra are available as
a Level 2 product. A
Level 2 product is an ocean wave
wavelength and direction measurement.
Global Monitoring (GM)
Mode - is similar to WS mode, but has a
lower data rate since the data is
subsampled on board the satellite.
The Global-Monitoring Mode product is a 1
km resolution image with a 405 km swath
width, suitable for dissemination by
electronic links in near real time.
The nominal characteristics of the various ASAR
measurement modes are summarised in the following table:
Table 2.4 Nominal characteristics of ASAR measurement modes
ASAR Image Mode (IM)
|VV or HH polarisation images from
any of 7 selectable swaths. Swath
width between approximately 56 km
(swath 7) and 100 km (swath
1) across-track. Spatial resolution
of approximately 30 m (for precision product).
Polarisation Mode (AP)
|Two co-registered images per
acquisition, from any of 7
selectable swaths. HH/VV HH/HV or
VV/VH polarisation pairs possible.
Spatial resolution of approximately
30 m (for precision product)
ASAR Wide Swath Mode (WS)
|400 km by 400 km wide swath image.
Spatial resolution of approximately
150 m by 150 m for nominal product.
VV or HH polarisation.
ASAR Global Monitoring Mode (GM)
|Spatial resolution of approximately
1000 m in azimuth by 1000 m in range
for nominal product. Up to a full
orbit of coverage, HH or
ASAR Wave Mode (WV)
|A small imagette (dimensions range
between 10 km by 5 km, to 5 km by 5
km) is acquired at regular intervals
of 100 km along-track. The
imagette can be positioned anywhere
in an Image Mode swath. Up to two
positions in a single swath or in
different swaths may be
specified, with acquisitions
alternating between one and the
other (successive imagettes will
hence have a separation of
200 km between acquisitions at a
given position). HH or VV
polarisation may be chosen.
Imagettes are converted to wave
spectra for ocean monitoring.
Table 2.5 below gives a summary of the
ASAR products, categorised by product level, and Table 2.5 below gives a product
summary categorised by mode. As for all ENVISAT
instruments, the interface for browsing and ordering
these products is via the PDS User Service Facility (USF).
Table 2.5 Summary of ASAR data products
To view the summary table that provides the standard
specifications for ASAR data products, see "Products and Algorithms Introduction." 2.1.
The Image Mode (IM), Alternating Polarisation mode
(AP), and the Wide Swath modes (WS) are all high
data rate products. Figure2.10 below gives the generalised
product tree for high data rate products.
|Figure 2.10 ASAR high data rate product tree
The Global Monitoring (GM) and Wave Modes (WV) are
low data rate products. The generalised trees for
these products is given below in Figure2.11 .
|Figure 2.11 ASAR low data rate product tree