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Sea Surface Temperature (SST) from Space
Past and Future…..
The evolution of Along Track Scanning Radiometer (ATSR) into the Sea and Land
Surface Temperature Radiometer (SLSTR)
Dr Chris Mutlow
Head of Earth Observation and Atmospheric Science
RAL Space
Science and Technology Facilities Council – Rutherford Appleton Laboratory
Sea Surface Temperature from Space - Appleton Space Conference 2013
Talk Outline
► Why are we interested in Sea Surface Temperature?
► Why observe it from space?
► Introduction to the Along Track Scanning Radiometers (ATSRs)
► Some of the Achievements of the ATSR Project
► The successor instrument - Sea and Land Surface Temperature
Radiometer (SLSTR)
► Summary
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Sea Surface Temperature from Space - Appleton Space Conference 2013
Why are we interested in SST
Why observe from space?
► The oceans cover over 70% of the Earth’s surface; likely to be important!
► Climate change is expected to have a strong signature in ocean
temperature
► Most of the energy driving the atmosphere comes from the Earth’s
surface - rather than directly from the Sun.
► The oceans transport heat from the tropics to the cold polar regions
► our maritime climate is the result of the Gulf Stream
► Associated with climate change there are expected to be other changes
in local SST patterns which will give early warnings
► Warmer tropical Atlantic waters = more Hurricanes
► Measurements from space are the only way to obtain global coverage.
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Sea Surface Temperature from Space - Appleton Space Conference 2013
How do we measure SST from space?
► By detecting the thermal
radiation emitted by the ocean
that reaches space.
► Filter-radiometers commonly
used working in either:
► Microwave – low accuracy,
spatial resolution but … can
penetrate clouds
► Infrared – delivers climate
accuracy but … can’t
penetrate clouds
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Sea Surface Temperature from Space - Appleton Space Conference 2013
How do we correct for the atmosphere?
► Multi-spectral method – correcting for atmospheric water vapour
► Makes use of varying strength of the absorption by water vapour in different infrared
wavelengths
►
►
►
Least absorption at 3.7μm
More at 11μm
Even more at 12μm
► Only technique used by the previous
generation of sensors (i.e. AVHRR)
►
►
Not robust enough for Climate
Monitoring purposes!
Doesn’t deal with aerosol well!
► Along track scanning – the real innovation in ATSR!
► Directly measuring the effect of the atmosphere by observing along different length paths to
the surface
►
Nadir (short) and a Slant (long) path
►
Need to maintain surface viewing angle to >55 degrees keep ocean “black”
► Works well for aerosols too!
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Sea Surface Temperature from Space - Appleton Space Conference 2013
Along Track Scanning Radiometer (ATSR)
► Designed in the early 1980’s by UK and Australian scientists – who were
then ahead of their time in recognising climate change as an important issue.
► Space-borne imaging radiometer
► Developed to measure sea surface temperature (SST) for Climate Change Detection
► ATSR was the first sensor specifically aimed at this task!
► Later capabilities added for land, cloud, and aerosol remote sensing
► Three ATSR instruments flown on ESA EO missions:
► ATSR(-1) on ERS-1 (1991-1996) – initial “experimental” instrument
► ATSR-2 on ERS-2 (1995-2003) – new “experimental” visible channels
► AATSR on ENVISAT (2002-2012)
► Series known collectively as (A)ATSR
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Sea Surface Temperature from Space - Appleton Space Conference 2013
ATSR-1, ATSR-2 and AATSR?
► Channels to capture atmospheric state; water vapour and aerosol
► Other sensors rely on external for their operation;
► All ATSRs have infrared channels at 1.6, 3.7, 10.8 and 12µm
► used for SST determination and cloud identification
► ATSR-2 and AATSR have extra visible channels at 0.55, 0.67 and
0.87µm
► used for land, aerosol and cloud applications
► Climate-quality levels of “performance” required the following
technical developments over previous sensors:
► Along track scanning
► Low noise detectors → active cooling using Sterling cycle cooler
► On-board calibration → ultra-stable, and accurate blackbodies
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Sea Surface Temperature from Space - Appleton Space Conference 2013
Gulf Stream
The Along Track Scanning Radiometer (ATSR) Mission
•
•
•
Primary objective to measure Sea Surface Temperature (SST) with an accuracy of 0.3K
(1-sigma limit)
Secondary objective thermal and visible data for land studies (e.g. temperature,
vegetation):
Provision of a long-term dataset for global climate change studies
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Sea Surface Temperature from Space - Appleton Space Conference 2013
ATSR viewing geometry – ground track
Forward view is the
2 x shortest path to surface
i.e. more absorption
Nadir view is the
shortest path to
surface
500 km wide Swath
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Sea Surface Temperature from Space - Appleton Space Conference 2013
ATSR Optical Layout
FPA
Field Stop
Scan Mirror
M2
Opal
UV Filter
Baffle
Sun
VISCAL
Off-Axis Paraboloid
(f = 540mm)
M1
Black-Body
Along-Track
View
Nadir View
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Sea Surface Temperature from Space - Appleton Space Conference 2013
The AATSR instrument
Forward view
aperture
Nadir
view
aperture
VISCAL
This way
to Earth
Blackbody
calibration
targets
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(A)ATSR Data compared to in situ
Good agreement of independent satellite with in situ datasets
regarding global sea surface temperature changes over time
Gives high confidence
in global SST changes
Satellite only, not tuned
Two in-situ only datasets
ESA SST CCI
European Space Agency
Sea Surface Temperature
Climate Change Initiative
ATSRs capture SST variability cleanly : even better with greater SLSTR coverage
ENSO as seen in ATSRs
behemoth.nerc-essc.ac.uk/ncWMS/godiva2.html
Also see animation in exhibition
space of CCI data
ESA SST CCI
European Space Agency
Sea Surface Temperature
Climate Change Initiative
Stability of SST across 3 ATSR Sensors
 ATSR SSTs are low bias and highly stable in time against
other data – independent not “tuned” against other data
 This gives ATSRs and future SLSTR a unique role for SST
climate data records
Spread of difference
Average difference
ATSR-1
ATSR-2
AATSR
Satellite vs. global drifting buoy SSTs
ESA SST CCI
European Space Agency
Temperature
CourtesySea
of Surface
Prof Chris
Merchant,
Climate Change Initiative
Reading
University
vs. Argo floats
ATSRs are used as reference for other
meteorological sensors
 This means we can aim for AVHRR sampling …
 … with accuracy, stability and independence derived
from ATSRs
Dual-view reference SST coverage
Tied AVHRR SST coverage
ESA SST CCI
European Space Agency
Sea Surface Temperature
Climate Change Initiative
Sea Surface Temperature from Space - Appleton Space Conference 2013
Example Cloud Products
Cloud fraction 1996-2001
Cloud top temperature 1996-2001
ATSR cloud products
Cloud optical depth 1996-2001
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Sea Surface Temperature from Space - Appleton Space Conference 2013
Comparison with ISCCP data
ATSR-2 May 1999 Optical depth
ISCCP Optical depth May 1999
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Sea Surface Temperature from Space - Appleton Space Conference 2013
ATSR Aerosol Measurements – now in ESA CCI
Thomas et al, ACP, 2010
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Land Surface Temperature (LST)
Day - LST
340K
Night - LST
220K
Courtesy University of Leicester
ESA SST CCI
European Space Agency
Sea Surface Temperature
Climate Change Initiative
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Sea Surface Temperature from Space - Appleton Space Conference 2013
ATSR Fires and Gas Flares
Indonesia
n
Fires
After Casadio et al; ALGO3 persistent hot spot sites (1991–2009) RSE 2012
Amazon
Deforestation
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Evolution from ATSR-1  SLSTR
1991-2000 ATSR-1
Next Generation
2015 - SLSTR
1995-2008 ATSR-2
2002-2012 AATSR
ERS-1
ERS-2
(A)ATSR Heritage Series
ENVISAT
Sentinel-3
Sea Surface Temperature from Space - Appleton Space Conference 2013
Sea and Land Surface Temperature Radiometer
(SLSTR) for Copernicus
► Part of the payload Copernicus Sentinel 3 Mission
► Equivalent baseline performance to AATSR
► Recognition of LST (land) as being important in addition to SST (sea)
► Differences from AATSR
► Backwards oblique view
►
►
►
►
►
►
►
double scanner
Flip mirror
Wider swath (improved re-visit)
Increased dual view coverage
Extra SWIR (cloud) channels
Improved fire channels
Visible channels at 0.5 km resolution
► Launch late 2014/Early 2015
See Coppo et al, J. Mod. Opt, 2010, Donlon et al, RSE, 2012
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S3 SLSTR: Basic Geometry
SLSTR Design Features
• To enable a wider swath SLSTR uses two scan
systems (nadir and oblique) and optical paths
• A flip mirror (new) is used to select which optical
path is directed to the detectors
• The nadir swath has a westerly offset to
completely overlap the OLCI swath
• The oblique view 55°inclination
maintains a longer atmospheric path
length compared to nadir
• better atmospheric correction
• Both scan chains view the same
blackbody and VISCAL targets
S3 SLSTR: Optical path overview
The scanning system uses two flat rotating mirrors and one flip
mirror - switching between one view and the other
Multi-element detector arrays
sample 2 earth sample lines
per scan rotation
The scan mirrors rotate on a
300 msec time basis, ~half
period of AATSR (~35μs
integration time on detectors)
The in flight Blackbody
calibration period is 600 msec.
Each Blackbody calibration
source is viewed
independently by each
scanner once every second
scan cycle.
Alignment
Optics
Earthshine
Plate
International
Standards
ATS
R
Point source
+ collimator
ATSR-2
Instrument
Electronics
Blackbody
Source
Platform
Simulator
SLSTR Calibration Facility at RAL
→ Facility commissioning completed 19th March 2012
→ Structural Test Model (STM) Integrated & MLI fitted 28rd March 2012
→ Tests with STM in completed last year!
SLSTR Model Testing scheduled to start Q1 2014
AATSR
Copernicus
SLSTR
The achievements and the future of ATSR?
► The ATSR Programme has delivered the high-quality SST data it was
designed to provide!
► (A)ATSR is now regarded as the “gold-standard” for SST determination
► ATSR SST data is now routinely used to calibrate other sensors as demonstrated by
GODAE GHRSST Pilot Project
► When AATSR failed the Met Office forecast noticed the loss of skill
► ATSR has matured
► experimental to operational in the journey from ATSR-1 to AATSR, and then SLSTR
► Significant new technology was developed to enable the programme
► Much of this technology has been successfully spun-out to commercial products
► Important new science applications on clouds, aerosols, and land remote
sensing have been developed.
► Sentinel 3 will carry the SLSTR to continue and extend the ATSR mission
until beyond 2025
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Sea Surface Temperature from Space - Appleton Space Conference 2013
Acknowledgements
► The many colleagues who worked on the ATSR Programme over the years!
► Chris Merchant, University of Reading.
► Craig Donlon and the ESA S3 Teams at ESTEC and ESRIN,
► Anne O’Carroll and EUMETSAT
► Roger Saunders and the UK Meteorological Office
► David Llewellyn-Jones, John Remedios and Gary Corlett at the University of Leicester
► The SLSTR Teams at Thales (Cannes), Selex Galileo (Florence) and Jena Optronik (Jena)
► Dave Smith, Caroline Poulsen, Gareth Thomas and the rest of the (A)ATSR and SLSTR
Team at RAL
► Funding: NERC, STFC, DECC and ESA
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