| Sentinel-1 | |
The Sentinel-1 mission is the European Radar Observatory for
the Copernicus joint initiative of the European Commission (EC) and the
European Space Agency (ESA). The mission is composed of a constellation of two
satellites, Sentinel-1A and Sentinel-1B, sharing the same orbital plane with a
180° orbital phasing difference. The mission provides an independent
operational capability for continuous radar mapping of the Earth with enhanced
revisit frequency, coverage, timeliness and reliability for operational
services and applications requiring long time series.
Sentinel-1 carries a single C-band synthetic aperture radar
instrument operating at a centre frequency of 5.405 GHz. It includes an active
phased array antenna providing fast scanning in elevation and azimuth. The
C-SAR instrument supports operation in dual polarisation (HH+HV, VV+VH)
implemented through one transmit chain (switchable to H or V) and two parallel
receive chains for H and V polarisation.
A single Sentinel-1 satellite is able to map the entire
world once every 12 days. The two-satellite constellation offers a six day
exact repeat cycle. The constellation will have a repeat frequency
(ascending/descending) of 3 days at the equator, less than 1 day at the Arctic
and is expected to provide coverage over Europe, Canada and main shipping
routes in 1-3 days, regardless of weather conditions.
Stripmap (SM) imaging mode is provided for continuity with
ERS and ENVISAT missions. Stripmap provides coverage with a 5 m by 5 m
resolution over a narrow swath width of 80 km.
The ground swath is illuminated by a continuous sequence of
pulses while the antenna beam is pointing to a fixed azimuth angle and an
approximately fixed off-nadir angle. SM images have continuous along track
image quality at an approximately constant incidence angle. The incidence angle
is the angle between the incident SAR beam and the axis perpendicular to the
local geodetic ground surface. One of six imaging swaths can be selected by
changing the beam incidence angle and the elevation beamwidth.
The Interferometric Wide (IW) swath mode is the main
acquisition mode over land and satisfies the majority of service requirements.
It acquires data with a 250 km swath at 5 m by 20 m spatial resolution (single
look). IW mode captures three sub-swaths using Terrain Observation with
Progressive Scans SAR (TOPSAR). With the TOPSAR technique, in addition to
steering the beam in range as in ScanSAR, the beam is also electronically
steered from backward to forward in the azimuth direction for each burst,
avoiding scalloping and resulting in homogeneous image quality throughout the
swath [R11].
TOPSAR mode is intended to replace the conventional ScanSAR
mode, achieving the same coverage and resolution as ScanSAR, but with a nearly
uniform SNR (Signal-to-Noise Ratio) and DTAR (Distributed Target Ambiguity
Ratio).
Azimuth resolution is reduced compared to SM due to the
shorter target illumination time of the burst. Using the sweeping azimuth
pattern, each target is seen under the same antenna pattern, independently from
its azimuth position in the burst image. By shrinking the azimuth antenna
pattern, as seen by a target on the ground, scalloping effects on the image can
be reduced. Bursts are synchronised from pass to pass to ensure the alignment
of interferometric pairs.
IW SLC products contain one image per sub-swath and one per
polarisation channel, for a total of three (single polarisation) or six (dual
polarisation) images in an IW product.
Each sub-swath image consists of a series of bursts, where
each burst has been processed as a separate SLC image. The individually focused
complex burst images are included, in azimuth-time order, into a single
sub-swath image with black-fill demarcation in between, similar to ENVISAT ASAR
Wide ScanSAR SLC products.
Due to the one natural azimuth look inherent in the data,
the imaged ground area of adjacent bursts will only marginally overlap in
azimuth by just enough to provide contiguous coverage of the ground. The images
for all bursts in all sub-swaths are resampled to a common pixel spacing grid
in range and azimuth while preserving the phase information.
The Extra Wide (EW) swath imaging mode is intended for
maritime, ice and polar zone operational services where wide coverage and short
revisit times are demanded. The EW mode works similarly to the IW mode
employing a TOPSAR technique using five sub-swaths instead of three, resulting
in a lower resolution (20 m by 40 m).
EW SLC products contain one image per sub-swath and one per
polarisation channel, for a total of five (single polarisation) or 10 (dual
polarisation) images in an EW product.
Like IW, EW mode can also be used for interferometry since it shares the
same characteristics for burst synchronisation, baseline and Doppler stability.
SENTINEL-1 Wave mode in conjunction with global ocean wave
models, can help determine the direction, wavelength and heights of waves on
the open oceans.
SENTINEL-1 Wave mode is similar to ERS and ENVISAT wave mode
imaging but with improved resolution, larger vignettes and a new 'leap frog'
acquisition pattern. WV acquisitions consist of several vignettes exclusively
in either VV or HH polarisation, with each vignette processed as a separate
image. WV mode products can contain any number of vignettes, potentially
amounting to an entire data-take. Each vignette will be contained in an
independent image within the product.
Wave mode acquires data in 20 km by 20 km vignettes, at 5 m
by 5 m spatial resolution, every 100 km along the orbit, acquired alternately
on two different incidence angles. Vignettes on the same incidence angle are
separated by 200 km. Swaths alternate incidence angles between near range and
far range (23° and 36.5° respectively).
The Wave mode at VV polarisation is the default mode for
acquiring data over open ocean. WV mode is acquired at the same bit rate as SM
however, due to the small vignettes, single polarisation and sensing at 100 km
intervals, the data volume is much lower.
SENTINEL data products are made available systematically and
free of charge to all data users including the general public, scientific and
commercial users. Radar data will be delivered within an hour of reception for
Near Real-Time (NRT) emergency response, within three hours for NRT priority
areas and within 24 hours for systematically archived data.
All data products are distributed in the SENTINEL Standard
Archive Format for Europe (SAFE) format.
Each mode can potentially produce products at SAR Level-0,
Level-1 SLC, Level-1 GRD, and Level-2 OCN.
Data products are available in single polarisation (VV or
HH) for Wave mode and dual polarisation (VV+VH or HH+HV) and single polarisation
(HH or VV) for SM, IW and EW modes.
The SAR Level-0 products consist of the sequence of Flexible
Dynamic Block Adaptive Quantization (FDBAQ) compressed unfocused SAR raw data.
For the data to be usable, it will need to be decompressed and processed using
focusing software.
Level-0 data includes noise, internal calibration and echo
source packets as well as orbit and attitude information.
Level-1 data is the generally available products intended
for most data users. Level-1 products are produced as Single Look Complex (SLC)
and Ground Range Detected (GRD).
Level-1 Single Look Complex (SLC) products consist of
focused SAR data geo-referenced using orbit and attitude data from the
satellite and provided in zero-Doppler slant-range geometry. The products
include a single look in each dimension using the full TX signal bandwidth and
consist of complex samples preserving the phase information.
The products include a single look in each dimension using
the full available signal bandwidth and complex samples (real and imaginary)
preserving the phase information. The products have been geo-referenced using
the orbit and attitude data from the satellite and have been corrected for
azimuth bi-static delay, elevation antenna pattern and range spreading loss.
Stripmap SLCs contain one image for its single swath per
polarisation band. IW, having three swaths, has three images in single
polarisation and six images for dual polarisation. EW, having five swaths, has
five images for single polarisation and ten images for dual polarisation.
For IW and EW, each sub-swath consists of a series of
bursts. Each burst has been processed as a separate SLC image. The individually
focused complex burst images are included, in azimuth-time order, into a single
sub-swath image, with black-fill demarcation in between, similar to the ENVISAT
ASAR Wide ScanSAR SLC products.
For IW, a focused burst has a duration of ~2.75 seconds and
a burst overlap of approximately ~0.4 seconds. For EW, a focused burst has a
duration of ~3.19 seconds with an overlap of ~0.1 seconds. The overlap slightly
increases in range within a sub-swath. Unlike ASAR WSS which contains a large
overlap between beams, for SENTINEL-1 TOPSAR products, the imaged ground area
of adjacent bursts only marginally overlap in azimuth just enough to provide
contiguous coverage of the ground. This is due to the one natural azimuth look
inherent in the data.
Images for all bursts in all sub-swaths of an IW SLC product
are re-sampled to a common pixel spacing grid in range and azimuth. Burst
synchronisation is ensured for both IW and EW products.
The Swath Timing data set record in SLC products contains
information about the bursts including dimensions, timing and location that can
be used to merge the bursts and swaths together.
Level-1 Ground Range Detected (GRD) products consist of
focused SAR data that has been detected, multi-looked and projected to ground
range using an Earth ellipsoid model.
The ellipsoid projection of the GRD products is corrected
using the terrain height specified in the product general annotation. The
terrain height used varies in azimuth but is constant in range.
Ground range coordinates are the slant range coordinates
projected onto the ellipsoid of the Earth. Pixel values represent detected
magnitude. Phase information is lost. The resulting product has approximately
square resolution pixels and square pixel spacing with reduced speckle at a
cost of reduced geometric resolution.
In addition to the corrections applied to Level-1 SLC
products, GRD products have thermal noise removed to improve the quality of the
detected image.
For the IW and EW GRD products, multi-looking is performed
on each burst individually. All bursts in all sub-swaths are then seamlessly
merged to form a single, contiguous, ground range, detected image per
polarisation channel
GRD products can be in one of three resolutions:
·
Full Resolution (FR)
·
High Resolution (HR)
·
Medium Resolution (MR).
The resolution is dependent upon the amount of multi-looking
performed. Level-1 GRD products come in MR and HR for IW and EW modes, MR for
WV mode and MR, HR and FR for SM mode.
Level-2 Ocean (OCN) products include components for Ocean
Swell spectra (OSW) providing continuity with ERS and
The OSW is a two-dimensional ocean surface swell spectrum
and also includes an estimate of the wind speed and direction per swell
spectrum. The OSW is generated from Stripmap and Wave modes only. For Stripmap
mode, there are multiple spectra derived from internally generated Level-1 SLC
image. For Wave mode, there is one spectrum per vignette.
The OWI is a ground range gridded estimate of the surface
wind speed and direction at 10 m above the surface derived from internally
generated Level-1 GRD images of SM, IW or EW modes.
The RVL is a ground range gridded difference between the
measured Level-2 Doppler grid and the Level-1 calculated geometrical Doppler.