1. 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

2. Outline of the Presentation
Introduction to the Mission
Specifications of SCAT
Description of SCAT system
Simulation of SCAT system performances
Progresses
Summary

3. CFOSAT Mission Two payloads:
SWIM (Sea Wave Investigation and Monitoring by satellite)
A Ku-band real aperture radar for measurement of directional ocean wave spectra；
SCAT (SCATterometer)
A Ku-band rotating fan-beam radar scatterometer for measurement of ocean surface wind vector.
CFOSAT: Chinese French Oceanography SATellie
Launch plan: 2014
Mission Objectives:
monitoring the wind and waves at the ocean 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

4. 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

5. 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.
Adapted to the platform constraints (small size);
2 fan beams (HH & VV) cover incident angles from 26 degree to 46 degree from nadir
scanned with a rotation speed of around 3.5 rpm.
For each of the ground resolution cells, more than four looking angles can be obtained to retrieval wind vector information.
Rotating fan-beam antenna
Nadir Point
Flight direction
Swath
footprint

6. 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

7. Azimuth look angle combinations for surface resolution cells

8. 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)
s° precision:
1.0dB for wind speed 4~6m/s
0.5dB for wind speed 6~24m/s
Wind speed: 2m/s or 4~25m/s
Wind direction: 360deg for most part of the swath
Life time: 3yrs

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

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

11. 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

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

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

14. 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

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

16. System Diagram

17. System configuration Interface with structure subsystem
Antenna and part of the servo mechanism installed outside the satellite；
Other equipments installed inside the satelltie

18. Basic radar parameters
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

19. Optimization of radar parameters
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

20. 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

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

22. 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

23. 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

24. Internal Calibration Loop

25. Simulations of system performances
Simulation model
Simulation of sigma 0 precision
Simulation of wind vector retrieval performance

26. Simulation model

27. Simulation of s° precision
Modeling
Radar equation
SNR
s° precision

28. Statistics: number of looks (left) number of independent samples (right)

29. SNR distribution
U=4m/s U=8m/s
U=16m/s U=24m/s

30. Kp distribution (25km)
U=4m/s U=8m/s
U=16m/s U=24m/s

31. Simulation of wind retrieval
Only s°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

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

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

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

35. FOM varying with wind speed

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

37. U=8m/s U=4m/s

38. U=12m/s U=16m/s

39. U=20m/s U=24m/s

40. Wind vector retrieval performance
Input wind speed
RMS of wind speed (m/s)
RMS of wind direction (o) 4 6 8 10 12 14 16 18 20 22 24
0.7
0.8
1.0
1.2
1.4
1.6
1.9
2.2
2.5
2.8
35.5
22.3
16.6
15.7
16.0
16.3
16.9
17.8
19.0
20.0
20.7

41. Retrieval performance
U(m/s)
Near nadir
(-100~+100)
Far range within footprint
(>400km)
Near range within footprint
(100~400km) 0
(dB) U
(m/s)
phi
() 4
0.4
44.1
1.1
44.7
31.8 8
0.9
24.2
1.3
26.9
0.6
11.0 12
1.2
22.7
1.8
26.6
10.4 16
2.1
2.2
27.6
11.3 20
2.9
24.8
30.1
12.8 24
3.8
26.3
3.6
32.0
2.4
14.1

42. Assessment by FOM

43. Progresses of CFOSAT/SCAT
PDR of SCAT
Detailed design review
Delivery of electrical models (except antenna subsystem) and satellite electrical performance test
System specifications, interface compatibility confirmed
Delivery of mechanical and thermal models
Satellite mechanical test
Satellite thermal test
RF compatibility test
Onboard full operation mode test

44. RFC test and SCAT integrated test

45. 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.

46. 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.

47. Thanks for your attentions!