Radiometer for Coastal Altimetry

Abstract

Water vapour in the troposphere extends the satellite-to-surface round-trip time of satellite altimeter radars and is a significant factor in the overall altimetry error budget. Due to the spatial and temporal variability of water vapour, many satellite altimeters embark Microwave Radiometers (MWR) to concurrently measure and perform the so called Wet Tropospheric Correction (WTC) associated with water vapour.
ESA is currently having two parallel studies to define a new European radiometer instrument design that tackles the more challenging geophysical situations encountered in the coastal zones and inland water regions where strong gradients of atmospheric water vapour exist. This new radiometer is aiming at higher spatial resolution in order to improve coastal altimetry and also to cover other applications such as topography over sea ice, ice sheets and possibly also inland hydrology applications. This is achieved by embarking a set of high frequency channels to enable smaller antenna beam-widths and thereby more accurate WTC correction and altimetry in addition including the classical MWR channels for ensuring observation continuity. Such an instrument would be used by future operational altimetry missions.

The latest finding from the ESA studies will be presented including design aspects at system and equipment level, important trade-off aspects aimed at design optimisation and performance predictions.

Full text

1 R.Midthassel, 2 B.Picard,
3 M.Labriola, 4 S.Varchetta
1 ESA ESTEC Noordwijk, 2 CLS Toulouse,
3 Airbus Defence and Space Madrid,
4 Thales Alenia Space Rome
Radiometer for Coastal
Altimetry

ESA UNCLASSIFIED -For Official Use Author | ESRIN | 18/10/2016 | Slide 2
Overview presentation
•Coastal altimetry radiometer
introduction.
•ESA activities related to coastal
altimetry radiometers.
•Results so far.

ESA UNCLASSIFIED -For Official Use Author | ESRIN | 18/10/2016 | Slide 3
Current Altimeter Radiometers
•Provides Wet Tropospheric Correction (WTC).
•Accounts for the delay of radar signal caused by
Water Vapour to give actual sea surface height.
•Water Vapour has high spatial and temporal
variability.
•Sentinel 3 radiometer MWR:
•Uses “low frequency” channels.
•Frequencies 23.8 and 36.5GHz.
Sentinel-3A –launched 2016
Sentinel-3B –launched 2018

ESA UNCLASSIFIED -For Official Use Author | ESRIN | 18/10/2016 | Slide 4
Current Altimeter Radiometers
•The current radiometer resolution limits
performance close to coast.
•Land contamination occurs when 30-
40km from coast.
•This is an area with strong water
vapour gradients.
Altimeter footprints/resolution:
LRM Jason’s -> ~ 10km (Ku-band)
LRM Altika, -> ~ 6km (Ka-band)
S 3, Jason CS -> ~300m SAR along track resolution
Microwave Radiometers footprints:
Jason’s ~ 35 km (18.7 GHz, 23.8 GHz, 34 GHz)
Envisat ~ 30 km (23.8 GHz, 36.5 GHz)
Sentinel-3 ~ 20 km (23.8 GHz, 36.5 GHz)
AltiKa ~ 10 km (23.8 GHz, 37 GHz)

ESA UNCLASSIFIED -For Official Use Author | ESRIN | 18/10/2016 | Slide 5
Coastal Altimeter Radiometer
•A Coastal Radiometer augments a traditional
radiometer with an additional set of high
frequency channels.
•This allows a smaller radiometer footprint.
•Can resolve water vapour closer to coast.
•Also supports other applications:
•High resolution SSH.
•Inland hydrology applications.
•Access to additional atmospheric properties.

ESA UNCLASSIFIED -For Official Use Author | ESRIN | 18/10/2016 | Slide 6
ESA activities; Sentinel 6 / Jason CS
•First mission with coastal radiometer.
•Three additional channels:
•90GHz, 130GHz and 166GHz.
•Experimental, non redundant.
AMR-C
Sentinel 6a launch 2020 Sentinel 6b launch 2025
High frequency channels
Radiometer mages from article “The Advanced Microwave Radiometer –
Climate Quality (AMR-C) Instrument For Sentinel-6”, 28th International
Symposium On Space Terahertz Technology, March 2017;
Low frequency channels

ESA UNCLASSIFIED -For Official Use Author | ESRIN | 18/10/2016 | Slide 7
ESA activities; Coastal altimetry studies
•“ACCRA” coastal radiometer study
•JCR Systems, CNRS, CNRM, CLS, RAL, Uni. of Southampton, SMT consultancies.
•12 months ESA paper study, ended October 2016.
•“ACCRA” radiometer antenna study
•JCR Systems, TK instruments, Uni. of Cardiff, SMT consultancies.
•16 months ESA paper study. Finished beginning 2018.
•“ICARO” and “CoastRad” coastal radiometer studies
•ICARO = Airbus and CLS. CoastRad = TAS-I and CLS.
•More detailed follow up to ACCRA.
•18 months on-going parallel ESA paper studies. Both finish end 2018.
•Copernicus Polar Ice and Snow Topographic Mission Phase A/B1 studies
•OHB and Airbus primes. Airbus and TAS-I radiometer responsible.

ESA UNCLASSIFIED -For Official Use Author | ESRIN | 18/10/2016 | Slide 8
Coastal radiometer studies
e2e simulation & retrieval approaches
“Observed” TBs “Virtual” instrument
(Semi) Empirical
Neural Network
(NN) Retrieval
Model WTC
Instrument Performance
DTROPO = WTCRADIOMETER -WTCMODEL [cm]
Measured TBs
(AMSU-A / MHS)
“Virtual RM”
AROME
Est.
TB
1D VAR Minimisation
Most
Suited
Profile(s)
WTC
1DVAR
WTC
NN
Model TBs Instrument
Model
Radiometer WTC

ESA UNCLASSIFIED -For Official Use Author | ESRIN | 18/10/2016 | Slide 9
Coastal radiometer studies
Example simulation results
Frontal system SW France, Iberian peninsula
AROME analysis at 0.01°.
Wtc cm
distance km

ESA UNCLASSIFIED -For Official Use Author | ESRIN | 18/10/2016 | Slide 10
Coastal radiometer studies
Example simulation results
-GLOBAL: whole dataset
-DRY: WTC <= 10 cm
- WET: WTC >= 30 cm
- CLEAR (sky): CLWC <= 0.2 g/cm2
-CLOUDY: CLWC > 50 g/cm2
Sensitivity of the difference channels for
different atmospheric conditions
[wtc impact per TB; cm/K].

ESA UNCLASSIFIED -For Official Use Author | ESRIN | 18/10/2016 | Slide 11
Coastal radiometer studies
Selection of frequencies
Initial starting point. Recommended configuration.
Also
18.7

ESA UNCLASSIFIED -For Official Use Author | ESRIN | 18/10/2016 | Slide 12
Coastal radiometer –potential solution / status
TAS-I; CoastRad
•Two baselines solutions identified after trading off more than 60 combinations of Antenna, receivers and
calibration subsystems.
•Down selection has been based on a mix of parameters:
•Performance
•Complexity
•Technology readiness
•The selected baseline solutions are:
•Baseline 1, Single aperture antenna for reduction of overall instrument envelope.
•Baseline 2, Double aperture antenna to harmonize technology constraints.
Antenna Channel Centre Freq Typology Calibration Architecture
Single Aperture
CH1 18,7 Heterodyne External Total Power
CH-2 23,8 Direct detection, External Total Power
CH-3 36.5 Direct detection, External Total Power
CH-4 50,3 Direct detection External Total Power
CH-5 53,6 Direct detection External Total Power
CH-6 89 Direct detection External Total Power
CH-7 165 Heterodyne External Total Power
CH-8 172.3 Heterodyne External Total Power
Antenna Channel Centre Freq Typology Aperture Calibration Architecture
Dual
Aperture
CH-2 23,8 Direct detection,
1st Aperture
Internal Dicke
CH-3 36.5 Direct detection, Internal Dicke
CH-4 50,3 Direct detection Internal Dicke
CH-5 53,6 Direct detection Internal Dicke
CH-6 89 Direct detection
2nd Aperture
External Total Power
CH-7 165 Heterodyne External Total Power
CH-8 172.3 Heterodyne External Total Power

ESA UNCLASSIFIED -For Official Use Author | ESRIN | 18/10/2016 | Slide 13
Coastal radiometer –potential solution / status
Airbus; Icaro
•Two baselines solutions also identified after
extensive trade offs.
•Both baseline solutions are single aperture but
differs in calibration approach.
•Baseline solutions now undergoing more
detailed design.

ESA UNCLASSIFIED -For Official Use Author | ESRIN | 18/10/2016 | Slide 14
Coastal radiometer studies
Retrieval performance coastal
Using a E2E simulator (1D-VAR) to assess
the improvement of a coastal radiometer:
•AMR-like classical 3-channels low frequency
•Coastal-like 8 channels
•WTC reference = 1-km AROME
OCEAN LAND
Performance (Mediterranean):
AMR-like: ocean= 1.9 / land= 3.7 cm
Coastal: ocean=1.5 /land=2.0 cm

ESA UNCLASSIFIED -For Official Use Author | ESRIN | 18/10/2016 | Slide 15
Coastal radiometer studies
Retrieval performance coastal
•With traditional channels: •Including high frequency channels:
DTROPO = WTCRADIOMETER –WTCMODEL
σ=1.7 σ=1.4

ESA UNCLASSIFIED -For Official Use Author | ESRIN | 18/10/2016 | Slide 16
Coastal radiometer studies
Retrieval performance coastal
•With traditional channels: •Including high frequency channels:
DTROPO = WTCRADIOMETER –WTCMODEL
σ=1.7 σ=1.4
σ=1.4

ESA UNCLASSIFIED -For Official Use Author | ESRIN | 18/10/2016 | Slide 17
Coastal radiometer studies
Retrieval performance ocean
•1D-Var retrieved WTC normalized
RMS error difference between the
assimilation of LF+HF and of LF
artificial measurements.
•Statistics are averaged in 4°
latitude x 4°longitude boxes,
over a one-month period in June
2015.
•(Hermozo et al 2018, accepted)

ESA UNCLASSIFIED -For Official Use Author | ESRIN | 18/10/2016 | Slide 18
Coastal altimetry radiometer work –way ahead
•Work shows promising performances for a new coastal altimetry radiometer.
•Work on-going to consolidate final solutions and performances.
•Sensitivity analysis for key performance aspects.
•Consolidation of concepts including final performance assessment.
•ICARO and CoastRad studies concludes soon -by end of this year.
•Findings can feed into Copernicus Polar Ice activity.

ESA UNCLASSIFIED -For Official Use Author | ESRIN | 18/10/2016 | Slide 19
Contact details
Silvio Varchetta, Thales Alenia Space Italia, CoastRad project
silvio.varchetta@thalesaleniaspace.com
Massimo Labriola, Airbus Spain, ICARO project
massimo.labriola@airbus.com
Bruno Picard, CLS, end to end simulation for both projects
bpicard@cls.fr
Rolv Midthassel, ESA
rolv.midthassel@esa.int

ESA UNCLASSIFIED -For Official Use Author | ESRIN | 18/10/2016 | Slide 20