PROGRESSES OF DEVELOPMENT OF CFOSAT SCATTEROMETER

National Space Science Center
Chinese Academy of Sciences

PROGRESSES OF DEVELOPMENT OF
CFOSAT SCATTEROMETER

Xiaolong Dong, Di Zhu
CAS Key Laboratory of Microwave Remote Sensing
National Space Science Center, CAS
PO Box 8701, Beijing, China
dongxiaolong@mirslab.cn, zhudi@mirslab.cn

IGARSS 2012, Munich, Germany Key Laboratory of Microwave Remote Sensing
July 22-27, 2012 Chinese Academy of Sciences
(MiRS, CAS)


Outline of the Presentation

• Introduction to the Mission
• Specifications of SCAT
• Description of SCAT system
• Simulation of SCAT system
performances
• Progresses
• Summary

(MiRS, CAS)


• Two payloads:
CFOSAT Mission – SWIM (Sea Wave Investigation and
Monitoring by satellite)
• A Ku-band real aperture radar for
• CFOSAT: Chinese French Oceanography measurement of directional ocean
wave spectra；
SATellie
– SCAT (SCATterometer)
• Launch plan: 2014
• A Ku-band rotating fan-beam radar
• Mission Objectives: scatterometer for measurement of
monitoring the wind and waves at the ocean ocean surface wind vector.
surface at the global scale in order to improve:
– The wind and wave forecast for marine
meteorology (including severe events)
– the ocean dynamics modeling and
prediction,
– our knowledge of climate variability
– fundamental knowledge on surface
processes linked to wind and waves

(MiRS, CAS)


Mission-measurement requirements
• Joint measurement of ocean surface wind vector
and sea-state parameters from radar
– Both wind vector and wave parameters can be measured using active micro-wave
remote sensing (heritage of altimeter, sactterometer and SAR missions, and airborne
radar measurements)
– Wind vector => optimal configuration at medium incidence angle (20-50°)
– Wave spectra => optimal configuration at low incidence angle (< 15°)

• CFOSAT mission with two payloads
– SWIM: wave scatterometer: multi-beam Ku-Band radar at low incidence
– SCAT: wind scatteromer: Fan beam Ku-Band radar at medium incidence

4
(MiRS, CAS) 4


Mission-Wind Vector Payload -SCAT
• A Ku-band rotating fan-beam radar
scatterometer (Ku-RFSCAT) for sea
surface wind vector retrieval by
measurement of the sea surface
backscattering coefficient. Rotating fan-beam antenna
• Adapted to the platform constraints
(small size);
• 2 fan beams (HH & VV) cover incident Nadir Point
angles from 26 degree to 46 degree
from nadir
• scanned with a rotation speed of around footprint
3.5 rpm.
• For each of the ground resolution cells,
more than four looking angles can be
obtained to retrieval wind vector
information.

July 22-27, 2012 5 Chinese Academy of Sciences
(MiRS, CAS)


Characteristics of CFOSAT SCAT
• Wide swath by rotating of beam;
– Decided by outer edge of incident angle of beam
• More number of azimuth look angles by overlap of beam;
– Decided by flying speed, rotating speed and beamwidth
• NRCS/sigma 0 dependent on antenna beam;
– Decided by local antenna gain along elevation
• Single antenna for all azimuth directions;
– No inter-beam balance required
– But azimuth fluctuation may exist due to rotating mechanism

(MiRS, CAS)


Azimuth look angle combinations
for surface resolution cells
Overlap between adjacent scanns
for large number of azimuth view for
each pixel. Dual polarization is used Surface resolution cell 1
to improve retrieval of outer and
center part of the swath

Track of nadir porints

Surface resolution cell 2

(MiRS, CAS)


Specifications for SCAT
• Objectives:
– Measurement of global surface sigma 0
– Retrieval of global ocean surface wind vector
• Data requirements
– Swath width: >1000km
– Surface resolution: 50km (standard); 25km (goal)
– Data quality (at 50km resolution)
– ° precision:
• 1.0dB for wind speed 4~6m/s
• 0.5dB for wind speed 6~24m/s
– Wind speed: 2m/s or 10% @ 4~25m/s
– Wind direction: 20deg @ 360deg for most part of the swath
• Life time: 3yrs

(MiRS, CAS)


Specifications of SCAT

Ku-RFSCAT Parameters
Antenna Spinning rate : 3.5 rpm
Polarization: VV, HH
PRF/channel: 75 Hz/channel
Pulse peak power (Pt): 120 W
Pulse bandwidth (B): 0.5 MHz
Pulse duration (τp): 1.3 ms
Swath width: 1000 km
Receive gate length(Tg): 2.82 ms
Receive gate delay: 3.74 ms
Inclination: 97.5 deg

(MiRS, CAS)


Description of SCAT system
• System overview
• Choice of system type
• Operation mode
• System configuration
• Key parameters

(MiRS, CAS)


System overview
• Ku-band rotating fan-beam scatterometer
– Platform dimension
– Technology heritage
– Available GMFs
• Long LMF pulse with de-ramp pulse compression
– TX: 1.35ms
– RX: 2.72 ms
• Digital I-Q receiver with on-board pulse compression
processing and resolution cell regrouping
• TX/RX channel except antenna and switch matrix
identical primary/backup design to ensure liability

(MiRS, CAS)


Choice of system type
-Why rotating fan beam?

• Why rotating beam? • Why rotating fan beam?
– Overlap of surface coverage with – Lower rotating speed to ensure
SWIM is requirement, nadir gap life time of rotating mechanism;
should be avoided. – Multiple incident angles for
– Deployment of multiple fan-beam better wind direction retrieval;
antenna is not allowed due to – Large incident angle ranges
platform capability. (20~46°) for investigation of
– Large swath at a relatively low ocean surface scattering
orbit (~500km) requires scanning. characteristics, by compensating
with SWIM (0~10°)

(MiRS, CAS)


Other constraints
• Antenna dimension: <1.2m
• Available Pulsed Ku-TWTA: <140W
• Available TWTA PRF: >150Hz
• Data rate: <220kpbs
• Rotating speed and mechanism lifetime

(MiRS, CAS)


Operation mode
• Normal mode: dual polarization with rotation;
• Test/cal mode:
– raw waveform with lower PRF;
– Including both rotating mode and fixed pointing
mode;
• Single polarization mode

(MiRS, CAS)


System configuration
• Antenna subsystem • Power amplifier subsystem
– Antenna and feeding network; – TWT and EPC
– Scanning mechanism; • Digital subsystem
– Servo controller; – Signal generator;
• RF subsystem – System controller;
– Switch matrix; – Signal processor;
– RF receiver; – Communication controller;
• RX/TX electronics subsystem • Secondary power supply subsystem
– IF receiver; – DC-DC power converter;
– Frequency synthesizers; – TC/TM module
– TX up-converter • WG & cable assembly

(MiRS, CAS)


System Diagram
(MiRS, CAS)


System configuration

• Interface with structure
subsystem
– Antenna and part of the
servo mechanism installed
outside the satellite；
– Other equipments
installed inside the
satelltie

(MiRS, CAS)

Basic radar parameters
Parameter Specifications

Frequency 13.256GHz

Signal bandwidth 0.5MHz

Internal calibration precision Better than 0.15dB
Receiver NF ≤2.0dB
Insertion loss of TX channel ≤1.5dB

Insertion loss of RX channel ≤3.0dB

Transmitting power (peak) 120W
Pulse width 1.35ms
PRF 2×75=150Hz

EUMETSAT/ESA Scatterometer Science Conference 2011
April 11-13, Darmstadt, Germany


Optimization of radar parameters

• Optimization:
trade-off between SNR, measurement
samples of each look and number of looks.
maximization of wind vector retrieval
performance
– Surface resolution
– Signal bandwidth
– Rotating speed
(MiRS, CAS)

Resolution in azimuth direction
& azimuth beam-width
• Fan beamlower gainantenna as long as
possible
• Decided by antenna beamwidth
• Limited by satellite dimension: ≤1.2m
• Beamwidth ~1.1 deg resolution in azimuth
direction: 10.5~14.5km

(MiRS, CAS)

Design of rotating speed
• Trade-off between independent
sigma 0measuremrent samples for
single look and number of looks
• Optimization of 3.5rpm

EUMETSAT/ESA Scatterometer Science Conference 2011
April 11-13, Darmstadt, Germany

Resolution in elevation direction
& signal bandwidth

• Low SNR due to low antenna
gain
• Bandwidth 0.5MHz
resolution:380~650m
• On-board non-coherent re-
grouping to improve sigma 0
precision
resolution of 5km

(MiRS, CAS)


Onboard processing
• Reduce data rate to ~220kbps
• Downlink data resolution: ~10km(az) × 5km(el)
– (original resolution: 10km(az) × (<1km(el))
• Signal+noise processing & noise-only processing

(MiRS, CAS)


Internal Calibration Loop
P1
K1
C1 LPF1
P31
P21 Circulator TWTA
P22 P32

K6

C2 K2
D K3 K5
A
Ns LPF2
K4 P31

P32

Compositions:
•C1，C2：directional couplers
Ports： •K1，K2，K3，K4：ferrite switches
•P1： BJ-140 to TWTA •K5，K6：mechanical switches
•P2： BJ-140 to antennas •LPF1/2：EMC filters
•P3： BJ-140 to RF receiver •D: power monitoring detector
•Ns：internal noise source

(MiRS, CAS)


Simulations of system performances

• Simulation model
• Simulation of precision
• Simulation of wind vector retrieval
performance

(MiRS, CAS)


Simulation model

(MiRS, CAS)


Simulation of ° precision
• Modeling 2
G2 
Pr Pt dA SNR SNR 
– Radar equation
3 4
L 4 A
R Pr
where: N
– SNR P 120W 50.8dBm t 1 2
G2 
Pt dA
– ° precision
3
2.263cm 16.5dB kBT 4 L A
R4
L 3.5dB (insturment loss)

2 2

P true 1 1 1 1
Kp 
1 
1 
P true Neff SNR true Nnoise SNR true

Kp(dB) 10log 1 Kp

(MiRS, CAS)


• Statistics:
number of looks (left)
number of independent samples (right)

(MiRS, CAS)


SNR distribution

U=4m/s U=8m/s

IGARSS 2012, Munich, Germany U=16m/s U=24m/s
Key Laboratory of Microwave Remote Sensing
(MiRS, CAS)


Kp distribution (25km)

U=4m/s U=8m/s

IGARSS 2012, Munich, Germany U=16m/s U=24m/s
Key Laboratory of Microwave Remote Sensing
(MiRS, CAS)


Simulation of wind retrieval

• Only °data with precision better than 1.0dB will be used for
wind retrieval;
• Standard MLE method and NSCAT GMF are used for simulation;
• Median filter algorithm for wind direction ambiguity removal

• 2 kinds of wind field simulated
– Spatially correlated parallel wind field and circular wind field
– Random wind field

(MiRS, CAS)


Parallel and circular wind field
(U~[2,24])

(MiRS, CAS)


Retrieval performance of parallel wind
field

U Qe(m/s)-U QRMS(m/s)-U Qe (o)-phi QRMS (o)-phi
4 0.43 0.65 51.5 65.9
6 0.45 0.66 15.9 22.3
8 0.55 0.76 10.9 16.5
10 0.68 0.93 10.1 15.4
12 0.83 1.13 10.2 15.6
14 0.99 1.33 10.3 15.7
16 1.17 1.57 10.8 16.3
18 1.38 1.83 11.3 17.0
20 1.60 2.12 12.1 18.1
22 1.86 2.43 12.9 19.1
24 2.13 2.75 13.4 19.8

(MiRS, CAS)

Retrieval performance of circular wind
field

U Qe(m/s)-U QRMS(m/s)-U Qe (o)-phi QRMS (o)-phi
4 0.44 0.68 39.1 51.9
6 0.46 0.68 15.6 22.3
8 0.55 0.77 10.9 16.6
10 0.69 0.95 10.2 15.7
12 0.86 1.17 10.4 16.0
14 1.02 1.38 10.6 16.3
16 1.22 1.62 11.1 16.9
18 1.44 1.90 11.8 17.8
20 1.68 2.20 12.7 19.0
22 1.93 2.51 13.5 20.0
24 2.19 2.83 14.0 20.7

(MiRS, CAS)


FOM varying with wind speed

(MiRS, CAS)


Random wind field
• Parallel wind field simulated
• Wind speed range: 4~24m/s
• Wind direction search interval: 10deg
• 25km WVC resolution

(MiRS, CAS)


U=4m/s
U=8m/s

(MiRS, CAS)


U=16m/s
U=12m/s

(MiRS, CAS)


U=24m/s
U=20m/s

(MiRS, CAS)


Wind vector retrieval performance

RMS of wind speed RMS of wind direction
Input wind speed (m/s) (o)

4 0.7 35.5
6 0.7 22.3
8 0.8 16.6
10 1.0 15.7
12 1.2 16.0
14 1.4 16.3
16 1.6 16.9
18 1.9 17.8
20 2.2 19.0
22 2.5 20.0
24 2.8 20.7
(MiRS, CAS)


Retrieval performance

Near nadir Far range within footprint Near range within footprint
(-100~+100) (>400km) (100~400km)
U(m/s)
0 U phi 0 U phi 0 U phi
(dB) (m/s) ( ) (dB) (m/s) ( ) (dB) (m/s) ( )

4 0.89-1.79 0.4 44.1 1.43-3.07 1.1 44.7 0.66-2.07 0.4 31.8
8 0.46-0.66 0.9 24.2 0.51-1.01 1.3 26.9 0.44-0.74 0.6 11.0
12 0.41-0.53 1.2 22.7 0.45-0.63 1.8 26.6 0.41-0.54 0.9 10.4
16 0.40-0.49 2.1 22.7 0.44-0.54 2.2 27.6 0.40-0.49 1.3 11.3
20 0.40-0.48 2.9 24.8 0.43-0.50 2.9 30.1 0.40-0.47 1.8 12.8
24 0.39-0.47 3.8 26.3 0.43-0.48 3.6 32.0 0.39-0.46 2.4 14.1

(MiRS, CAS)


Assessment by FOM

(MiRS, CAS)


Progresses of CFOSAT/SCAT
• 2010.04 PDR of SCAT
• 2010.12 Detailed design review
• 2011.07 Delivery of electrical models (except antenna
subsystem) and satellite electrical performance test
– System specifications, interface compatibility confirmed
• 2011.11 Delivery of mechanical and thermal models
• 2011.12 Satellite mechanical test
• 2012.02 Satellite thermal test
• 2012.05 RF compatibility test
• 2012.07 Onboard full operation mode test

(MiRS, CAS)


RFC test and SCAT integrated test

160
定标
回波
140

120

Amplitude(dB)
100

80

60

40
-750 -562.5 -375 -187.5 0 187.5 375 562.5 750
Frequency (KHz)

(MiRS, CAS)


Summary
• Design and performance of CFOSAT SCAT is presented:
– When U<4m/s, SCAT/CFOSAT cannot provide useful wind
retrieval due to its low SNR;
– For U=4~8m/s, SCAT/CFOSAT can provide wind retrieval
similar to QSCAT only within swath of 800km;
– For U>8m/s, SCAT/CFOSAT can provide better wind
retrieval with its designed swath of 1000km, compared
with QSCAT;
– For U>16m/s, the advantage of SCAT/CFOSAT become
obvious, due to its more number of looks.
• Development of SCAT on time for the scheduled launch
in 2014.

(MiRS, CAS)


Further to do…
• New quality control for sigma-0 measurement
with more number of looks and lower SNR;
• Development of retrieval making use of increased
number of looks;
• Evaluation of rain effect, compared with pencil
beam system like QSCAT;
• Calibration for rotating fan beam system:
– In-orbit antenna pattern calibration;
– In-orbit possible azimuth-dependent antenna gain
variation due to rotary joint.
(MiRS, CAS)


Thanks for your attentions!

(MiRS, CAS)

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