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

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

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

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

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

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

Azimuth look angle combinations for surface resolution cells

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 10% @ 4~25m/s Wind direction: 20deg @ 360deg for most part of the swath Life time: 3yrs

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

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

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

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°)

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

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

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

System Diagram

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

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

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

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

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

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

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

Internal Calibration Loop

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

Simulation model

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

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

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

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

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

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

Retrieval performance of parallel wind field U 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

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

FOM varying with wind speed

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

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

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

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

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

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 0.89-1.79 0.4 44.1 1.43-3.07 1.1 44.7 0.66-2.07 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 10.4 16 0.40-0.49 2.1 0.44-0.54 2.2 27.6 11.3 20 0.40-0.48 2.9 24.8 0.43-0.50 30.1 0.40-0.47 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

Assessment by FOM

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

RFC test and SCAT integrated test

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.

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.

Thanks for your attentions!