CFOSAT (Chinese-French Oceanography Satellite)
CFOSAT is a joint mission of the Chinese (CNSA) and French (CNES) space agencies with the goal to monitor the ocean surface winds and waves and to provide information on related ocean and atmospheric science and applications. The primary objective of CFOSAT is to monitor on a global scale the wind and waves at the ocean surface in order to improve: 1) 2) 3) 4)
• 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.
An operational demonstration objective of CFOSAT is to provide observations over the ocean in near-real-time for assimilation in meteorological and wave forecast models. Wind and wave products must be available in operational centers within 3 hours after acquisition.
CFOSAT will also be used to complement other satellite missions for the estimation of land surface parameters (in particular soil moisture and soil roughness), and polar ice sheet characteristics.
Figure 1: Overview of organizational setup and responsibility allocations of CFOSAT contributions between CNSA and CNES (image credit: CNES) 5)
The project feasibility and preliminary design phases (A/B phases) were successfully carried out from 2006 until 2009. The project started the detailed design (Phase C) at the beginning of 2011 which will be followed by the implementation phase (Phase D).
Figure 2: Artist's rendition of the deployed CFOSAT spacecraft in orbit (image credit: CNES) 6)
CFOSAT is a spacecraft based on a CNSA funded platform provided by the DFH Satellite Corp., based on its CAST-2000 series satellite bus. The spacecraft has a launch mass of ~ 700 kg, the mission design life is 3 years. 7)
AOCS (Attitude and Orbit Control Subsystem): The spacecraft is three-axis stabilized.
• Three-axis pointing precision: 0.1º (3σ)
• Three-axis stabilization: 0.01º/s (3σ)
• Three-axis measurement precision: 0.03º (3σ).
EPS (Electrical Power Subsystem):
• Power generation: 1.5 kW (BOL), 1.3 kW (EOL)
• Battery capacity: More than 60 Ah.
Figure 3: Illustration of the CFOSAT spacecraft (image credit: CNES) 8)
RF communications: The TT&C communications are provided in S-band with an uplink rate of 2 kbit/s and a downlink data rate of 16.384 kbit/s. The payload data are provided in a separate downlink (more information is not available).
Launch: A launch of CFOSAT is planned for 2015 on a Long March 2C vehicle of China. The mission should be fully operational in 2015.
Orbit: Sun-synchronous near- circular orbit, altitude of 519 km, inclination = 97º, LTDN (Local Time on Descending Node) = 7:00 hours, the revisit time is 13 days.
Sensor complement: (SCAT, SWIM)
The objectives of CFOSAT will be implemented by joint measurements of the ocean surface wind vector and seastate parameters from radar payloads. Both OSVW (Ocean Surface Vector Wind) and OSWS (Ocean Surface Wave Spectra) can be measured using active microwave remote sensing technologies (with heritages from previous Chinese and French missions of radar altimeter, scatterometer and SAR missions, and airborne radar measurements).
Oceans cover 70% of the globe surface and play a fundamental role in climate, meteorology, environment, economy. Thus, monitoring, understanding and predicting the ocean surface processes is of major importance and even more critical for maritime countries. Particularly, surface wind and waves are key parameters affecting the marine meteorology, ocean dynamics, marine resources, pollution, economy (navigation, fisheries, oil industry, harbor activities, coastal tourism, etc.), coastal environment (sediment transport, pollution diffusion, etc.).
SCAT is a vector wind scatterometer designed and developed at MiRS/CSSAR/CAS (Microwave Remote Sensing Laboratory / Center for Space Science and Applied Research / Chinese Academy of Sciences), Beijing, China. SCAT will be the first RFSCAT (Rotating Fan-beam Scatterometer) flown on a spacecraft for global ocean vector wind observations. 9) 10) 11) 12)
SCAT is a Ku-band RFSCAT with HH and VV polarizations. The expected ocean wind vector retrieval performance is as follows (with 50 km OSVW resolution):
• Wind speed accuracy: 2 m/s or 10% (larger) within the 4~24 m/s wind speed range
• Wind direction accuracy: ±20º within the 360º wind direction range
• Ground geolocation accuracy OSVW resolution cells: < 5 km.
Table 1: Performance parameters of the SCAT instrument
The design of SCAT is based on simulations of the performance implementation, available technology heritages and the constraints of the platform. The key characteristics of the design are:
SCAT transmits a long LMF pulse and receives its signals with the de-ramp pulse compression, where the Tx pulse length and the Rx receiving window are 1.35 ms and 2.72 ms, respectively, which is a function of the observation geometry and the orbital altitude. To satisfy the downlink data rate limitation of 220 kbit/s, a digital I-Q receiver with an on-board pulse compression processing and resolution cell regrouping will be used, which results in a downlink σο surface resolution of about 10 km (azimuth) x 5 km (elevation).
SCAT features three operational modes:
• Nominal mode: dual polarization with rotation for OSVW measurement
• Test and calibration mode: raw waveform collection with lower PRF, including both rotating mode and fixed pointing mode
• Single polarization mode.
The SCAT instrument features the following functional subsystems:
• Antenna subsystem, including the antenna and feeding network, the scanning mechanism and the servo controller
• RF subsystem, including the RF switch matrix and the RF receiver
• Rx/Tx electronics subsystem, including the IF receiver, the frequency synthesizers and the Tx upconverter
• Power amplifier subsystem, including the TWT (Traveling Wave Tube) and the EPC (Electrical Power Conditioner)
• Digital subsystem, including the signal generator, the system controller, the signal processor and the data communication controller
• Secondary power supply subsystem, including the DC/DC power converter and the TC/TM module
• Waveguide and cable assembly.
To insure the liability and lifetime of the instrument, the Tx/Rx channels, except antenna and switch matrix, will be equipped with an identical primary/backup design.
Figure 4: Observation geometry (image credit: MiRS/CSSAR/CAS)
Figure 5: Functional block diagram of SCAT (image credit: MiRS/CSSAR/CAS, Ref. 11)
The onboard processing functions include:
• Source data rate reduction to ~ 220 kbit/s
• Reduction of the downlink data resolution to about 10 km (az) x 5 km (el) from the original resolution of 10 km (az) x (<1 km (el)
• Provision of the the “signal+noise” processing and the “noise-only” processing to satisfy the radiometric resolution for low wind speed cases.
The main functions of the SCAT onboard signal processing include the pulse compression, power estimation by accumulation, and elevation range bin regrouping.
Figure 6: Initial layout of the antenna and the rotating gyro (image credit: MiRS/CSSAR/CAS)
SWIM (Surface Waves Investigation and Monitoring instrument):
SWIM is a new CNES Ku-band radar instrument, manufactured by TAS (Thales Alenia Space), Toulouse, France; it is based on the technology of a spaceborne radar altimeter. SWIM is the first ever space radar concept that is mainly dedicated to the measurement of ocean waves directional spectra and surface wind velocities through multi-azimuth and multi-incidence observations. Orbiting on a 519 km sun-synchronous orbit, its multiple Ku-band (13,575 GHz) beams illuminating from nadir to 10º incidence and scanning the whole azimuth angles (0-360º) provide with a 180 km wide swath and a quasi global coverage of the planet between the latitudes of ±80º. 13) 14) 15) 16) 17) 18) 19) 20)
Such a wide range of observations, requiring high-range resolution (about 20 m on the ground), have led to design an instrument whose architecture and technology goes beyond what has been done on altimeter and scatterometer implementations. The global coverage and the reduction of telemetry budgets have required performing onboard range compression. The variety of signals at different incidences, the impact of the complex moving geometry of observation and the required real-time signal processing have led to propose onboard complete digital range compression on backscattered 320 MHz bandwidth signals. The design of the onboard compression and processing resulted from a trade-off between the instrument high level performances required, the needed correction for geometrical effects such as range migrations and performance of the acquisition and tracking loops.
The multi-azimuth multi-incidence observations requirements have led to design an ambitious antenna subsystem that rotates at 5.6 rpm while transmitting six high power RF signals towards tunable directions. This configuration (Figure 7) allows a 180 km wide swath and a quasi global coverage of the planet between the latitudes of ±80º.
Figure 7: SWIM geometry of observation (image credit: TAS, CNES)
The ground projection of several (6) nominal macro cycles of the SWIM instrument is illustrated in Figure 8.
Figure 8: Ground swath geometry created by several nominal macro cycles (image credit: TAS, CNES)
The RF (Radio Frequency) radiating performances have to be warranted for each of the 6 beams on the 360º rotation under flight conditions. In particular, the antenna subsystem shall ensure the pointing stability of each beam under rotation throughout the 3 years of nominal mission life. The antenna design shall also minimize the impact of mechanical and thermal environments on its critical components (active equipments).
SWIM antenna assembly:
The antenna design, operating in transmit and receive modes, features an offset reflector geometry combined with the RFA (Rotating Feed Assembly), including 6 horns and a switch matrix, allowing to switch the RF signal between each horn. The RF rotary joint, which is part of the complex RMA (Rotary Mechanism Assembly), ensures the interface between the fixed RF part and the rotating one. A stepper motor allows the precise rotation of the RFA. The rotating RFA is electrically fed and controlled using a collector, the RMA is aligned by a set of bearings.
A calibration probe is implemented close to the reflector to be able to calibrate each of the 6 beams in flight conditions (Figure 9). The SWIM antenna is electrically fed and commanded by a dedicated APCE (Antenna Power and Command Equipment).
The multibeam SWIM antenna is comprised of the following elements:
• A composite mechanical structure supporting the reflector and the rotating feed
• A rotating feed including:
- RMA (Rotary Mechanism Assembly) composed of a RF rotary joint, a motor, a collector, two bearings, and a HARM (Hold And Release Mechanism) system
- RFA (Rotary Feed Assembly) including a switch matrix in ferrite technology with associated drivers and 6 horns with associated mechanical plate
• A passive calibration system allowing to calibrate in flight conditions the 6 antenna beams
• An antenna thermal control device.
Figure 9: Illustration of the SWIM antenna (left) and the antenna calibration probe (right), image credit: TAS, CNES
RFA (Rotary Feed Assembly): The RFA is composed of 6 horns, a switch matrix and a mechanical interface structure. The RFA is rotating at 6 rpm (revolutions per minute) in the instrument nominal operating mode. To obtain the optimal performance for each beam, three different designs have been optimized for the horns. The location of the horns on the rotating plate has been optimized to comply with the beam pointing requirement as well as the center of gravity, inertia, and the interfaces with the switch matrix.
The switch matrix allows to switch the RF signal between the 6 beams - which operate in transmit and receive modes. The matrix includes 5 ferrite switches with electronic driver devices. The ferrite switches are RF interconnected by WR62 waveguide junctions. The switch locations and the routing of the waveguide junctions have been enhanced to minimize the RF losses and to be compliant with the rotating plate mechanical interfaces.
Figure 10: Illustration of the RFA showing the horn and the switch matrix sides (image credit: TAS, CNES)
RMA (Rotary Mechanism Assembly). The RMA is developed in the frame of the CNES/TAS SWIM instrument. It is a high lifetime mechanism enabling the holding, the release and the rotation of 6 RF beams integrated on an antenna structure at 5,6 rpm continuously during 3 years (qualification needs: 13,6 millions of turns). This mechanism is composed of a rotary for the rotation (with a RF joint and a slip-ring for digital commands in particular), and by 3 HRMs for the holding and the release. 21)
The RMA supports the following functions:
• Stowage of the RFA during the launch sequence
• Rotate the RFA with accuracy and stability
• Provide the RFA angular positioning
• Transfer the feed and command signals from the APCE to the switch matrix
• Transfer the RF signal between instrument RF unit and RFA
• Transfer the electrical grounding to RFA.
The RMA is composed of the following elements:
• A stepper motor
• A collector for electrical signal transmission
• The RF rotary joint
• Two bearings
• A set of 3 HARMs (Hold and Release Mechanisms)
• A set of mechanical interfaces.
Figure 11: Illustration of the RMA device (image credit: TAS, CNES)
Mechanical and thermal design of RFA and RMA: The mechanical and thermal designs have been optimized to limit the constraints on the RFA and RMA devices.
SWIM antenna RF performance: One of the most critical RF performance parameters to optimize was the beam aperture evolution minimization (aperture evolution lower than 10%) as presented in Figure 12 for the 10º beam example. For this particular beam, strongly depointed from nadir, the unsymmetrical side lobe effects have been minimized (Figure 13).
The worst case RF performance analysis has met compliance with the RF antenna requirements.
Figure 12: Beam of 10º: illustration of beam evolution along the rotation axis (image credit: TAS, CNES)
Figure 13: Radiating pattern for beam 10º (plate position = 0º), image credit: TAS, CNES
SWIM instrument description and operations:
SWIM is a RAR (Real Aperture Radar) to provide 1D range sampled echoes, from which an adequate processing leads to the retrieval of radar cross-section profiles with respect to incidence and wave spectrum characteristics. Observations in 2D require a rotating system in order to cover the azimuth direction. Useful profiles require to span the beam incidences up to 10º.
The instrument consists in a rotating antenna intended to transmit signal towards six distinct directions. The antenna consists of a rotary feed placed in front of a reflector in an offset configuration. This rotary feed is composed of 6 horns mounted on a circular plate rotating at ~6 rpm in order to reach the six distinct directions. Such a configuration provides with a filled 180 km swath at an altitude of 500 km.
The functioning of the instrument relies on the definition of six distinct periods of time, called cycles, each related to one given beam. A cycle is constituted of three distinct subcycles, a first one where pulses are only transmitted but no backscattered echoes are received yet, a second one where transmitted and received signals alternate and finally a third one where the last transmitted signals are received. The architecture of a cycle is illustrated in Figure 14 (Ref. 18).
Figure 14: Architecture of a cycle on a given beam (image credit: TAS, CNES)
Legend to Figure 14:
- RA = Rank of Ambiguity
- NIMP = Number of useful transmitted pulses per cycle
- PRF = Pulse Repetition Frequency.
The SWIM functional block diagram is shown in Figure 15. The instrument is comprised of 6 distinct units. The SIU (SWIM Interface Unit) communicates on the one hand with the platform (power and TM/TC), and on the other hand with the heart of the radar, providing with power to the DPU (Digital Processing Unit), HB (Hyperfrequency Box) and HPA (High Power Amplifier); it is also taking care of the TM/TC dialog with the DPU.
HB (Hyperfrequency Box): HB gathers all the analog functions of a radar (bandwidth expansion, frequency translations, local oscillator generation, low noise amplification, base band conversion, bandwidth filtering). It is a new design compared to the current nadir altimeters architecture. Indeed, it considers an integrated architecture:
- at the transmission level, the direct translation from an analog baseband signal with 160 MHz bandwidth on I & Q to a 320 MHz bandwidth RF signal carried at 13,575 GHz using a direct modulator
- at the reception level, the direct demodulation of the received 320 MHz bandwidth signal in Ku band to base band using a direct demodulator.
It leads to a symmetric frequency plan between transmission and reception paths and a significant decrease in mass and power budgets.
DPU (Digital Processing Unit): The DPU gathers all the digital (hardware and software) processing of the whole instrument (digital chirp generator, digital compression, range migration correction, radar echoes processing, instrument interface with the platform computer). The swath durations increase significantly when going at high incidences, then providing with a large amount of samples that have to be processed onboard with large FFT’s. Figure 16 illustrates the digital compression stage. Processing has to be performed at high rate (up to 7 kHz) on 32768 complex samples. It requires the use of two ASICs.
Table 2 gives the main specification of the six 0º to 10º incidence beams, and the corresponding signal to noise ratio (effective SNR after integration). The beam aperture is 2º x 2º, the scan is over an azimuth of 360º at a rate of 5.7 rpm.
The data from obtained observations will provide the SWH (Significant Wave Height) for the off-nadir beams (i.e., 6-10º), and the ocean wave directional spectra; the nadir beam will provide the SWH and the surface wind speed, similarly to altimeter missions. The wave spectra will be analyzed in terms of the wavelength, direction and energy of different partitions of the wave spectra. In addition, the backscattering coefficient profiles will be provided with a continuous sampling from 0º to 10º.
The ground segment is composed of:
• One Mission Center in Beijing
• One Control Center in Xi'an
• An Instrument Mission Center in Toulouse
• A Waves & Wind Mission Center in Brest
• Ground stations at various locations. CNES will provide a polar stations network based on two Earth stations located in Inuvik (Canada) and Kiruna (Sweden). The system has therefore the capability to download the science data on every orbit.
Figure 17: Overview of the CFOSAT ground segment (image credit: CNES)
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.