FY-3 (FengYun-3) 2nd Generation Polar Orbiting Meteorological Satellite Series
The FY-3 series of CMA/NSMC (China Meteorological Administration/National Satellite Meteorological Center) represents the second generation of Chinese polar-orbiting meteorological satellites (follow-on of FY-1 series). The FY-3 series represents a cooperative program between CMA and CNSA (China National Space Administration); it was initially approved in 1998 and entered full-scale development in 1999. Key aspects of the FY-3 satellite series include collecting atmospheric data for intermediate- and long-term weather forecasting and global climate research. 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14)
The overall objectives of the FY-3 series are:
• To provide global measurements of 3-D temperature and moisture soundings of the atmosphere, and to measure cloud and precipitation parameters in support of NWP (Numerical Weather Prediction).
• To provide global imagery of large-scale meteorological and/or hydrological events and biosphere environment anomalies
• To provide geophysical parameters in support of global change and climate monitoring.
• To provide global and local meteorological information for specialized meteorological users working in services of aviation, marine, etc.
• To collect and relay environmental data from the ground segment.
• The FY-3 operational phase will have two polar-orbiting satellites in service (one in the AM and one in the PM orbit, payload will be different for AM/PM satellites, time slots could be coordinated through WMO).
There are two development phases considered for the FY-3 series:
1) Experimental phase in the time period 2008-2010 with two spacecraft launches. These satellites have only limited sounding capabilities.
- FY-3A launch on May 27, 2008, LTDN (Local Time on Descending Node) = 10:00 hours.
- FY-3B launch on Nov. 4, 2010, LTDN = 14:00 hours.
2) Operational service phase beyond 2012. These satellites will have enhanced sounding and imaging capabilities.
- FY-3C launch on Sept. 23, 2013, LTDN =10:00 hours.
The CMA plans call for a constellation of two FY-3 spacecraft in orbit, one in the morning slot (AM) and one in the afternoon slot (PM).
Table 1: Overview of FengYun-3 spacecraft series of CMA/NSMC
In comparison with FY-1 spacecraft series, the principal improvements in the FY-3 series include:
1) Atmospheric sounding capacity
2) Microwave imaging capacity
3) Optical imaging with spatial resolution from 1 km to 250 m
4) Atmospheric composition detecting capacity
5) Radiation budget measuring capacity
6) Global data acquisition from within one day to within two to three hours.
The FY-3 series represents in fact a new chapter in the history of the Chinese meteorological satellites and satellite meteorology. The FY-3 series provides global air temperature, humidity profiles, and meteorological parameters such as cloud and surface radiation required in producing weather forecasts, especially in making medium numerical forecasting.
The FY-3 series satellites monitor large-scale meteorological disasters, weather-induced secondary natural hazards and environment changes, and provides geophysical parameters for scientific research in climate change and its variability, climate diagnosis, and predictions. The FY-3 series renders global and regional meteorological information for aviation, ocean navigation, agriculture, forestry, marine activities, hydrology, and many other economic sectors.
Note: The FY-3C spacecraft is equipped with all of the 11 payloads, but MWTS is upgraded to MWTS-II, MWHS to MWHS-II, and a new payload, GNOS (GNSS Occultation Sounder), is on board FY-3C. MWTS-II will increase the channels from 4 to 13, and MWHS-II will increase the channels from 5 to 15. GNOS will improve the measured temperature and moisture profiles in the upper atmosphere. 15)
Tentative Schedule for Future FY LEO Series:
For the FengYun LEO satellites, after FY-3A/B, the future FY-3 models (FY-3C/D/E/F) have been approved. CMA plans to develop certain observational capabilities for the follow-up models, for instance, the WindRAD for sea winds, the GAS for greenhouse gases absorption measurement. The atmospheric sounding shall be enhanced by replacing the current IRAS with HIRAS (Hyperspectral Infrared Atmospheric Sounder), and by the deployment of radio occultation sounder GNOS. Also, there is a plan to develop a rainfall measurement satellite: FY-3RM (2019), that shall carry Ku/Ka band radar, microwave sounding and imaging instruments.16)
Feasibility Study on FY-3 Use of Early Morning Orbit: FY-3 series serves at least for another 15 years with the additional four satellites. Sixteen (16) improved or new instruments will be configured on FY-3C/D/E/F. It is unrealistic for CMA to fly three orbits (AM, PM, and Early Morning) at the same time. Since FY-3C & 3D are being manufactured now, there is no chance to make them changed for the Early Morning orbit. FY-3E is possibly the only opportunity for CMA to fly early morning orbit before 2020.
Table 2: FY-3C/D/E/F payload configuration
The FY-3 series spacecraft are being designed and developed at SAST (Shanghai Academy of Spaceflight Technology). The spacecraft structure is a hexahedron of 4.4 m x 2.0 m x 2.0 m in stowed configuration and 4.4 m x 10 m x 3.8 m in the deployed state. The total spacecraft launch mass is estimated to be 2450 kg. The FY-3 features one solar panel mounted on one side of the satellite's main body (making the span length of the satellite 10 m in flight configuration). The attitude control of the satellite employs three-axis stabilization (bias momentum control) with a pointing precision of 50 m on the ground. The ADCS (Attitude Determination and Control Subsystem) employs a star sensor for attitude sensing.
The FY-3 bus contains three major modules: a service module, a payload module, and a propulsion module. The spacecraft design life is 3 years.
Table 3: System design parameters of the FY-3 satellite series
Figure 1: Illustration of the FY-3 satellite (image credit: CMA/NSMC)
Launch: A launch of the first satellite in the series, FY-3A, took place on May 27, 2008. The spacecraft was launched on a LM-4C launch vehicle from the Taiyuan Launch Center. 17)
Orbit of FY-3A: Sun-synchronous near-circular orbit, average altitude of 836.4 km, inclination of 98.75º, period = 101.49 min, local solar time on descending node at 10:10 hours, 14.1735 orbits/day, tropical cycle of about 6 days.
Launch: The FY-3B spacecraft was launched on Nov. 4, 2010 (UTC). The spacecraft was launched on a LM-4C launch vehicle from the Taiyuan Launch Center.
Orbit of FY-3B: Sun-synchronous near-circular orbit, average altitude of 836.4 km, inclination of 98.75º, period = 101.49 min, local solar time on ascending node at 13:30 hours, 14.1735 orbits/day, tropical cycle of about 6 days.
Orbit of FY-3C: Sun-synchronous near-circular orbit, average altitude of 836 km, inclination of 98.75º, period = 101.49 min, LTDN (Local solar Time on Descending Node) at 10:00 hours, 14.1735 orbits/day, tropical cycle of about 6 days.
RF communications: The spacecraft communications links are S-band, L-band and X-band. Commands are via S-band only. Command and telemetry links are active in parallel. The S-band section of the communications subsystem provides primary telemetry and command (TT&C) service to and from FY-3A ground stations. The L-band and X-band section of the communication subsystem provide the science and engineering data downlink for the FY-3A common spacecraft.
Three modes of operation are provided:
1) DPT (Delayed Picture Transmission) - a direct playback mode. All the stored science and engineering data onboard (except the MERSI data) is transmitted in high data rate at 110 Mbit/s, to the NSMC national ground playback stations (Beijing, Guangzhou, Urumuqi, Jamusi, and Kiruna) whenever the satellite is passing over the acquisition range of these stations. The transmission band frequency will be within the range of 8025 - 8400 MHz, encoding: CONV (7, ¾).
2) MPT (Mission Picture Transmission). This mode provides the direct broadcast in X-band. The main function of this data format is for real-time broadcasting of the science and engineering data of the MERSI instrument with a data rate of 18.7 Mbit/s to any receiving station within view of the spacecraft. The broadcasting band frequency is in the range 7750-7850 MHz, QPSK modulation, encoding: CONV (7, ¾).
3) AHRPT (Advanced High Resolution Picture Transmission) -also referred to as HRPT. The AHRPT transmission band frequency is within the frequency range of 1698-1710 MHz at the data rate of 4.2 Mbit/s (real-time global broadcasting). The modulation is QPSK (Quadra‐Phase Shift Keying), encoding: CONV (7, ¾), real-time broadcasting. 20)
Table 4: Overview of FY-3 LEO constellation missions of CMA as of fall 2015 (Ref. 26)
To support global NWP (Numerical Weather Prediction) services within the coordination framework of CGMS (Coordination Group for Meteorological Satellites), Dr. Zheng made a commitment in WMO (World Meteorological Organization) EC-66 in 2014 21) that CMA will adjust its satellite plan to develop an early morning orbit mission. Hence, the FY-3E mission has now been changed as an early morning orbit satellite rather than the previously assigned morning orbit (AM).
Figure 2: Schematic illustration of the FY-3A spacecraft (image credit: CMA/NSMC)
• December 2016: Observations from MWHS-1 on board FY-3B and its more advanced successor, MWHS-2, on board FY-3C have been received at the UK Met Office since 2012 and 2014, respectively. Since then, both instruments have been the subject of several validation studies conducted internally at the Met Office and in collaboration with ECMWF (European Centre for Medium-Range Weather Forecasts). These studies concluded that observations from MWHS-1 and MWHS-2 183 GHz channels are, once appropriately bias corrected, of a quality matching well established operational instruments of similar sounding capability. 22)
- Several low and high resolution full system experiments showed the benefit of adding MWHS-1 and MWHS-2 observations to the global model. As a consequence, MWHS-2 has been successfully integrated into parallel suite 37 in November 2015, and has been assimilated in operations since March 2016, while MWHS-1 has been successfully integrated to the parallel suite 38 due to become operational on November 1, 2016.
- MWHS-2 operational monitoring showed the presence of a small residual bias in channels 11 and 12, and recurrent transient rises of temperature that are correlated to large bias changes affecting the channels 13 and 14. Nevertheless, forecast sensitivity to observations impacts analyses confirmed that MWHS-2 contributes to a significant level to the reduction of model forecast errors.
- Work is currently under way to implement the assimilation of MWHS-2 183 GHz channels over land. The addition of land observations is expected to further reduce forecast errors. — Finally, a bilateral Met Office-ECMWF evaluation of FY-3C MWRI data will lead the way to pre-operational testings in 2017.
• The FY-3A spacecraft was retired on January 5, 2015, ending the global image coverage service. FY-3A provided a substantial contribution to ocean and ice monitoring, climate monitoring - and a significant contribution to atmospheric chemistry and space weather. 23)
- FY-3A mission instruments: MWRI failed soon after launch; IRAS failed in October 2008 (inactive); SBUS failed in December 2008 (inactive); ERM-1 failed in May 2008 (inactive); MWTS-1 failed in December 2012 (inactive).
• The FY-3B spacecraft is operating in 2016 providing operational meteorology. 24)
- FY-3B mission instruments: ERM-1 failed in August 2011 (inactive).
• The FY-3C spacecraft and its instruments are operating in 2016. 25)
- FY-3C mission instruments: MWTS-2 failed on Feb. 2, 2015. Operational service suspended on 31 May 2015 for anomaly investigation. Recovered from 30 July onwards.
Figure 3: FY-3 instrument status as of November 2015 according to Ref. 26)
Figure 4: In-orbit FengYun Satellites(6/7) (6 operational, 1 retired) as of fall 2015 (image credit: CMA) 26)
• Feb. 2015: A new methodology is developed to detect the cloud structures at different vertical levels using the dual oxygen absorption bands located near 60 GHz and 118 GHz, respectively. Observations from MWTS (Microwave Temperature Sounder) and MWHS (Microwave Humidity Sounder) on board the recently launched Chinese FengYun-3C satellite are used to prove the concept. It is shown that a paired oxygen MWTS and MWHS sounding channel with the same peak weighting function altitude allows for detecting the vertically integrated cloud water path above that level. A cloud emission and scattering index (CESI) is defined using dual oxygen band measurements to indicate the amounts of cloud liquid and ice water paths. The CESI distributions from three paired channels reveal unique three-dimensional structures of clouds and precipitation within Super Typhoon Neoguri that occurred in July 2014. 27)
• Status of FY-3 satellite series in January 2015: 28)
- FY-3A (launch on May 27, 2008): Only reduced operations are possible in 2015. MWRI failed soon after launch; IRAS failed in October 2008; SBUS failed in December 2008; ERM failed in May 2010; MWTS failed in December 2012.
- FY-3B (launch on Nov. 4, 2010): The FY-3B spacecraft and its payload are operating nominally in 2015.
- FY-3C (launch on Sept. 23, 2013): The FY-3C spacecraft and its payload are operating nominally in 2015.
• October 2014: FY-3C/MERSI has some remarkable improvements compared to the previous MERSIs including better SRF (Spectral Response Function) consistency of different detectors within one band, increasing the capability of lunar observation by space view (SV) and the improvement of radiometric response stability of solar bands. During the In-orbit verification (IOV) commissioning phase, early results that indicate the MERSI representative performance were derived, including the signal noise ratio (SNR), dynamic range, MTF, B2B registration, calibration bias and instrument stability. The SNRs at the solar bands (Bands 1–4 and 6-20) was largely beyond the specifications except for two NIR bands. 29)
• The FY-3A and FY-3B spacecraft and their payloads are operating nominally in 2013 (Ref. 32).
The FY-3 project conducted cross-calibrations between the MWTS/FY-3 channels with those of the AMSU-A and AMSU-B channels of the NOAA fifth-generation satellites (NOAA-15, -16, -17, -18 and -19) using the RM (Ray-Matching) method over the South Pole and North Pole study area in 2011. 32)
The results show that the in-orbit calibrations of AMSU-A on the fifth-generation NOAA satellites are identical with averaging errors less than 0.45 K, except the channel 8, in which the averaging error is up to -1.53 K. No obvious impact of solar illumination on AMSU-A/NOAA channels was found. - The in-orbit calibrations of MWTS/FY-3 channels are basically consistent with those of the AMSU-A/NOAA-19 channels, and a small influence of solar illumination on MWTS/FY-3B channel 4 was observed.
Large in-orbit calibration discrepancies were found between the MWHS/FY-3 channels and the AMSU-B/NOAA-16 channels, especially in MWHS/FY-3A channel 5. Strong influences of solar illumination on MWHS/FY-3 channels 3, 4 and 5 were observed.
• The FY-3A and FY-3B spacecraft and their payloads are operating nominally in 2012. 33)
- On 16 December 2011, sounding data from FY-3B began to be received and test dissemination started on EUMETCast. The new products will be made available to EUMETSAT Member States on January 24, 2012. This is in addition to the data already received and disseminated from FY-3A since the end of 2010. 34)
The addition of FY-3B data to EUMETCast follows discussions between EUMETSAT and CMA. During a bilateral meeting between the two organizations in May 2011 in Geneva, CMA agreed to add FY-3B data to the FY-3A data it already shares with EUMETSAT and its Member States.
• On June 2, 2011, the FY-3B spacecraft started its operational service after finishing a six-month commissioning period (the spacecraft was launched on Nov. 4, 2010). The operational service was handed over to CMA on May 27, 2011. The FY-3B will join the FY-3A to create a comprehensive weather satellite system. 35)
The MWRI instrument of FY-3B is providing continuous and stable data sets since launch. Compared with the MWRI instrument on FY-3A, the MWRI instrument on FY-3B features a higher stability and a much lower nonlinearity. 36)
• In Jan. 2011, the FY-3B spacecraft is on-orbit (launch Nov. 4, 2010, in the afternoon orbit). All 11 instruments of the payload have been switched on. The spacecraft and its payload are currently in their commissioning phase conducted by the NSMC (National Satellite Meteorological Center) of CMA. 37) 38)
• The FY-3A spacecraft and its payload are operating nominally in 2011. 39)
• On Nov. 7, 2010, a first image of the VIRR instrument was obtained from the FY-3B spacecraft. 40)
• The FY-3A spacecraft and its payload are operating nominally in the fall of 2010. 41)
• After launch (May 27, 2008) the spacecraft and its payload were in the commissioning phase. However, in the following months, FY-3A has served the Beijing 2008 Olympic Games and the flood season in 2008 at the same time.
Figure 5: Typhoon Fung-wong monitored by the MERSI instrument of FY-3A on 27 July, 2008 (image credit: NSMC)
Figure 6: Total ozone in DU (Dobson Units) monitored from TOU on Nov. 1, 2008 (image credit: NSMC)
Sensor complement: (VIRR, MERSI, MWRI, IRAS, MWTS, MWHS, SBUS, TOU, ERM, SEM, SIM, GNOS)
Initially, it was planned that the two experimental spacecraft, FY-3A and FY-3B, would carry only a limited sensor complement, consisting of VIRR, MERSI and MWRI instruments. The follow-on spacecraft would fly the full sensor complement. However, it turns out that all FY-3 spacecraft are already equipped with the full sensor complement.
Table 5: FY-3A major remote sensing instruments
Table 6: Overview of instruments and some instrumentation parameters 44)
VIRR (Visible and Infrared Radiometer):
The objective is to obtain observations for the following applications: diurnal cloud charts, Earth surface temperature and sea surface temperature, land feature, cloud feature, vapor content in lower layer, and brine color. The instrument features 10 channels in the spectral range of 0.43 - 12.5 µm. The spatial resolution at nadir is 1.1 km on a swath of 2800 km (FOV=±55.4º). VIRR is a scanning whiskbroom radiometer by design.
Table 7: Performance characteristics of VIRR
The retrieval of atmospheric water vapor is intended to further understand the role played by the energy and water cycle to determine the Earth's weather and climate. 45)
MERSI (Medium Resolution Spectral Imager):
The instrument is of MVISR (Multichannel Visible and IR Scanning Radiometer) heritage (flown on the FY-1 series) and is being built by SITP (Shanghai Institute of Technical Physics). The objective is to obtain imagery for the following applications: diurnal cloud charts, Earth's surface temperature and sea surface temperature, brine color, land feature, cloud feature, aerosol and atmospheric vapor. The instrument features 20 detecting channels in the spectral range of 0.44 - 12.5 µm. The instrument FOV is ±55.4º providing a swath of 2800 km. Due to its wide swath, MERSI has the capability to observe the entire Earth twice daily. 46)
Table 8: Performance characteristics of MERSI instrument
The SMA (Scan Mirror Assembly) uses a 45º rotating scan mirror, and a three-mirror system (K-mirror) to offset image rotation from 45º rotating scan mirror (Figure 7).
The optical system consists of an aspherical telescope and four refractive objective assemblies which work in the VIS, VIS/NIR, SWIR and LWIR spectral regions to cover a total spectral range of 0.4 to 12.5 µm. The 20 cm aperture telescope is a coaxial two-mirror system, and there is an intermediate image behind the telescope.
The multi-sensor scanning technique is adopted. Each 1-km band has a 10-element linear detector array and each of the 250 m bands has a 40-element array, respectively.
A high-performance passive radiative cooler provides cooling to 100 K for the one LWIR spectral band on one HgCdTe FPA (Focal Plane Assembly). The cooler provides also cooling to 150 K for the two SWIR spectral bands on another HgCdTe FPA at the same time.
A novel photodiode-silicon readout technique for the visible and near infrared provides an unsurpassed quantum efficiency and low-noise readout with exceptional dynamic range. Analog programmable gain and offset and the FPA clock and bias electronics are located near the FPAs in two dedicated electronics modules, the SAM (Space-viewing Analog Module) and the FAM (Forward-viewing Analog Module). A third module, the MEM (Main Electronics Module) provides power, control systems, command and telemetry, and calibration electronics. The system also includes an on-board calibrator.
Figure 7: Schematic layout of the optical subsystem (image credit: CMA/NSMC)
MWRI (Microwave Radiometer Imager):
The objective is to provide all-weather observation/derivation of parameters like: precipitation, cloud feature, vegetation, soil humidity, sea ice, etc. The instrument, a total power radiometer, provides conical beam scanning on a swath of 1400 km; it features 10 channels with five frequencies in the range 10.65-89 GHz. The nadir spatial resolution range varies from 15-85 km depending on frequency.
Table 9: Channel characteristics of the MWRI instrument
The MWRI instrument has a mass of 175 kg and a power consumption of 125 W. It is comprised of an offset parabolic reflector with the dimensions of 977.4 mm x 897.0 mm which are illuminated by 4 separate feed horns. The reciewers at 18.7 and 23.8 GHz share one feed horn. The reflector and feed horn are mounted on a drum which contains the radiometers, digital data subsystem, mechanical scanning subsystem, and power subsystem. To realize an end-to-end calibration, the project developed two quasi-optical reflectors with diameters of 860 mm and 1300 mm, respectively, which are mounted on the opposite direction of hot load and the top of satellite platform. These reflectors can reflect the radiation from the hot load and cold space to the main reflector (Ref. 36).
Figure 8: Illustration of the MWRI instrument (image credit: CMA/NSMC)
IRAS (Infrared Atmospheric Sounder):
The objective is to measure atmospheric temperature, the humidity profile, and ozone. IRAS is a sounder with 26 channels in the spectral range 0.69 - 15 µm measuring profiles in the troposphere. The sub-satellite resolution is 17 km, the FOV (Field of View) is ±49.5º (symmetrical about nadir) with 56 measurements in cross-track.
Table 10: Spectral characteristics of the IRAS instrument
Figure 9: Illustration of the IRAS instrument (image credit: CMA/NSMC)
MWTS (Microwave Temperature Sounder):
The objective is to provide an all-weather detection capability for atmospheric profiles. The instrument features four channels. The ground resolution at nadir is 50-70 km, depending on the channel. The FOV is ±48.3º with 13 measurements in cross-track per scan line.
Table 11: MWTS parameter specification
Table 12: MWTS instrument performance parameters
Figure 10: Illustration of the MWTS instrument (image credit: CMA/NSMC)
MWHS (Microwave Humidity Sounder):
MWHS is designed and developed at CAS/CSSAR (Chinese Academy of Sciences/Center for Space Science and Applied Research), Beijing. MWHS is a dual-frequency, 5-channel millimeter-wave radiometer (similar to AMSU-B). The objective of MWHS is to provide meteorological sounding for the measurement of the global atmospheric water vapor profiles. The instrument is 1st version of the microwave humidity sounder to be deployed on FY-3 satellite, which is China's 2nd generation polar-orbiting meteorological satellite. 47)
Table 13: Performance characteristics of MWHS
Figure 11: Block diagram of the MWHS instrument (image credit: CAS/CSSAR)
MWHS consists of three units: antenna and receiver unit, power supply unit and electronic unit (Figure 11). One motor drives two separated reflectors for the 150 GHz and the 183.31 GHz channels, respectively. The cross-track scanning is carried out by the rotation of the reflectors.
MWHS is a total power type microwave radiometer based on a heterodyne receiver. The water vapor vertical profile is retrieved from the measured brightness temperatures of 3 different frequency channels around the water vapor absorption line at 183.31 GHz. The antenna reflector is scanning in the cross-track direction. During each scan period, when the Earth surface is out of sight, a two-point calibration are performed to calibrate the receiver gain and noise. The calibration is implemented by observing an onboard hot target and the background emission of the cold sky. The separated 3-channel IF signals are detected, integrated, and digitized by the electronic unit. The detected data are finally transferred to the satellite through a MIL-STD- 1553B data bus. The electronic unit also controls the scanning mechanism and measures physical temperature of the on-board hot target for calibration.
Figure 12: Illustration of the MWHS instrument (image credit: CMA/NSMC)
Table 14: MWHS channel characteristics
Figure 13: Illustration of the scanning scheme and of the imaging geometry (image credit: CAS/CSSAR)
The main beams of the antenna scan over the observing swath (±53.35º from nadir) in the cross-track direction at a constant periodicity of 1.71s. The time for observing the internal calibration hot target and cold sky background is 0.1s, respectively. The residual time of the period are used for acceleration and deceleration of the motor. To satisfy both the integration time for the radiometric sensitivity requirement and the scan to scan interval, it is necessary to slow down the motor rotation during the Earth observation view and to speed it up during the period when observing the calibration targets (Figure 13).
Figure 14: MWHS footprint size from nadir to the edge of the swath (image credit: CAS/CSSAR)
MWHS-II (Microwave Humidity Sounder-II):
The MWHS-II instrument (2nd generation) includes many improvements when compared to the MWHS instrument onboard the FY-3A/B satellites, launched in 2008 and 2010, respectively. 48)
The MWHS-II instrument will be flown for the first time on the FY-3C mission (replacing MWHS). The rest of the sensor complement for FY-3C will be identical to the one flown on the FY-3A/3B missions.
MWHS-II is a four-frequency, fifteen-channel millimeter wave radiometer, a total power type radiometer based on a heterodyne receiver and performs cross-track scanning. When compared to MWHS, MWHS-II includes a 89 GHz (vertical polarization) in the atmospheric transparent window and oxygen absorbing lines around 150 GHz (vertical polarization), respectively. Furthermore, the mainly sounding channels are working at 118.75 GHz for 8 horizontal polarization channels and 183.31 GHz for 5 horizontal polarization channels.
Eight channels near 118.75GHz are being applied for the first time to improve the spatial resolution and investigate the temperature retrieval capabilities of MWHS-II. For antennas of the same size, the view pixels of 118.75 GHz channels are approximately half of them for 50-60 GHz channels. Since the channels near 118.75 GHz include more information about cloud and water vapor compared to the 50-60 GHz channels, combining the 183.31 GHz channels and window channels of 89 GHz and 150 GHz, MWHS-II can retrieve atmospheric temperature profiles, humidity profiles, and water vapor information simultaneously (Figure 15).
Figure 15: Block diagram of reflectors and polarizations for MWHS-II (image credit: CAS/CSSAR)
Table 15: Channel characteristics of the MWHS-II receivers
MWHS-II consists of three units: antenna and receiver unit, power supply unit and electronic unit. The antenna and receiver unit collect emissions from the atmosphere. The received signal is focused to the horn-feed and the first element of the high frequency front end, and then down-converted by a double side band mixer to IF; the IF signal is down-converted to LF by a detector, and then integrated. The electronic unit digitizes the LF signal, controls the scanning mechanism and measures physical temperature of the on-board hot target for calibration, and communicates with satellite through a MIL-STD-1553B data bus.
Table 16: Performance characteristics of MWHS-II
Figure 13 shows the scanning mode and observing geometry of channels operated at 183 GHz and 150 GHz of MWHS-II. For the 118 GHz and 89 GHz channels, the angle resolution is 2º. One motor drives two separated reflectors for 15channels, which realizes vertical and horizontal polarization with a polar separated grid, respectively. The scanning period is 2.667 s. The main beams of the antenna scan over the observing swath is ±53.35º (from nadir) in the cross-track direction at a constant periodicity of 1.71 s. During each period, a two-point calibration is performed to calibrate the receiver gain and noise. Figure 16 shows the time distribution in different scanning angles of the calibration period.
Figure 16: Time distribution in different scanning angles of the calibration period (image credit: CAS/CSSAR)
SBUS (Solar Backscattering UV Sounder):
The objective is the measurement of the vertical ozone distribution in the atmosphere. The instrument provides two operational modes: the atmospheric mode and the sun mode.
• The vertical ozone (O3) distribution (profile) is measured in the atmospheric mode by selecting among 12 channels in the spectral range of 250~340 nm.
• The sun mode is used in continuous scanning to measure the solar irradiance spectrum in incremental steps of 0.21 nm, the spectral range is 160~400 nm. The dynamic range of the instrument is 106 with stray ray ≤10-6. The FOV is 11.3º x 11.3º. The discrete solar mode involves the measurement of the solar irradiances in 12 channels.
Table 17: Spectral parameters of the SBUS in atmospheric mode
TOU (Total Ozone Unit):
The objective of TOU is to measure the quantity of ozone in the atmosphere by selecting among 6 channels from 300-360 nm. The dynamic range is 104 with a stray ray of < 10-3. The ground resolution at nadir is 50 km; the FOV is ±56º using 31 measurement spots in cross-track per scan line. The relative calibration accuracy is 1% of radiation intensity and irradiance; spectrum 0.03 nm. 49)
Table 18: Spectral parameters of TOU
The UV TOU instrument is composed of the optical assembly and the electronics assembly. The optical assembly is a grating spectrometer with responsible for the selection of measuring channels, the measurement of their backscatter ultraviolet radiation and solar irradiance signals and onboard calibration. The electronics assembly is responsible for the power supply, operation control, data acquisition and communication with satellite.
Table 19: Key parameters of the TOU instrument
TOU has three work modes, i.e., scanning mode, radiometric calibration mode and wavelength monitoring mode. The scanning mode is a main operational mode, to collect the scientific data at six wavelengths. The radiometric calibration mode is operated once per orbit and the wavelength monitoring mode is operated about once per two days when the satellite moves in the shadow area of the orbit.
Scanning unit: TOU's scanning unit uses a 45º scanning mirror in object space driven by a stepper motor. It scans perpendicular to the orbital plane in cross-track at ±54º from nadir in steps of 3.6º for a total of 31 samples.
Optical subsystem: The optical subsystem consists of foreoptics unit, monochromatic unit, wavelength selector and focusing unit, etc. The foreoptics unit is used to match TOU's field of view with the f-number of the monochromator, and to depolarize the entrance light. The monochromatic unit features a single Ebert-Fastie monochromator with a fixed grating and an array of exit slits, which cooperates with the wavelength selector to achieve the selection of the detected wavelengths and the measurement of their signals and dark currents alternately. The focusing unit collects the ultraviolet radiation from the six exit slits on PMT photocathodes and reduces the non-uniform influence of PMT photocathode sensitivity.
Electronics subsystem: It consists mainly of the following components: electrometer circuit, the phase-locked steady velocity circuit, the stepper motor driver circuits, the high-voltage power supply, the wavelength monitor power supply, the signal acquisition-control interface circuit, the secondary power supplies, and the CPU- RTU.
The detector is a dual-alkaline cathode PMT device. The electrometer circuit amplifies the output of the PMT, and provides the A/D conversion. To achieve rapid signals response and the required dynamic range, the electrometer circuit includes three amplifiers in parallel with different gains. The high voltage power supply provides an operating voltage for the PMT. The phase-locked steady velocity circuit drives the brushless DC motor turn steadily. The stepper motor driver circuits drive the scanning motor and radiometric calibration motor to achieve a scene scan and the selection of the diffusers, respectively.
The wavelength monitoring power supply provides an ignition voltage for the mercury lamp. The signal acquisition-control interface circuit achieves signals and dark current collection, A/D conversion, data pre-procession and TOU operation control. The CPU-RTU realizes the collection and packaging of the scientific data and the engineering parameters; the communications with the satellite is provided via a MIL-STD-1553B bus.
TOU on the FY-3 series spacecraft represents the first instrument for global total ozone monitoring in China. TOU provides a daily global map of total ozone using the self-developed inversion method. The performance of TOU is at the same level as that of similar instruments. In addition, the total ozone product derived from TOU agrees with that of other in-orbit instruments, indicating that both instrument development and the method of retrieval in China have come to an initial success.
ERM (Earth Radiation-budget Measurement):
The overall objective is to measure accurately the incident sun radiation, and the reflected short-wave and long-wave radiation from the Earth-atmosphere system for the study of the Earth-atmosphere radiation budget. ERM consists of two units to measure the sun radiation and the Earth-atmosphere system respectively.
• Sun Irradiance Monitor. The instrument consists of three absolute cavity radiometers in the spectral range of 0.2 - 50 µm. The radiative flux can be measured in the range of 100-2000 W/m2; the measurement sensitivity is 0.2 W/m2 with a calibration precision of 0.5%.
• Earth-atmosphere Radiation Sounder. The instrument is capable to measure the Earth-atmosphere reflected radiation in two channels/modes: 1) a wide-field non-scanning mode and 2) a narrow-field scanning mode.
Table 20: Performance of wide-field non-scanning channel
Table 21: Performance parameters of narrow field scanning channel
SEM (Space Environment Monitor):
The instrument measures space environment parameters ensuring normal operation of the spacecraft. It consists of a high-energy ion detector, a high-energy electronic detector, three radiation dosage meters, two surface potential detectors and a single-particle event detector.
SIM (Solar Irradiation Monitor):
The objective is to provide solar irradiance monitoring. The instrument takes measurements of the sun in the spectral range of 0.2~50 µm. The SIM sensitivity is 0.2 W m-2.
GNOS (GNSS Occultation Sounder)
The GNOS mission is a GNSS (Global Navigation Satellite System) radio occultation mission of China for remote sensing of Earth's neutral atmosphere and the ionosphere. GNOS will use both the GPS (Global Positioning System ) and the BeiDou navigation satellite systems on the China FY-3 series satellites. The first FY3-C spacecraft, with GNOS onboard, was launched on 23 September 2013. 50) 51)
The GNOS instrument was developed by CSSAR (Center for Space Science and Applied Research) of CAS (Chinese Academy of Sciences), Beijing, China. The GNOS instrument consists of three antennas, the PA (Positioning Antenna), the ROA (Rising Occultation Antenna), and the SOA (Setting Occultation Antenna) in physical structure or five antennas, the PA, the RIOA (Rising Ionosphere Occultation Antenna), the SIOA (Setting Ionosphere Occultation Antenna), the RAOA (Rising Atmosphere Occultation Antenna), and SAOA (Setting Atmosphere Occultation Antenna) in electrical structure. There are three RFUs (RF Units) and one GNSS EU (Electronics Unit). Each antenna is connected to its RFU with sharp cavity filters, which are placed close to the antennas to protect the GNOS from the complex RF environment on board FY3-C. Each RFU is connected to the EU.
Figure 17: The GNOS instrument configuration (image credit: CSSAR)
The PA is a wide beam with hemispherical coverage, low-gain antenna pointing into the zenith direction. The GNOS instrument is capable of tracking up to six BeiDou satellites and more than eight GPS satellites through this antenna. These measurements are used for realtime navigation, positioning as well as for precise orbit determination through post-processing on the ground.
The ROA ((including RIOA and RAOA) and anti-velocity viewing antennas SOA (including SIOA and SAOA) are used for rising and setting occultation tracking. The GNOS has the capability of tracking up to four BeiDou and six GPS occultations simultaneously. The atmosphere occultation antennas (including RAOA and SAOA) have a pattern that is wide in azimuth and narrow in elevation. A gain of approximate 10 dBi is reached over the coverage range between ±35º in azimuth and from ±7.5º in elevation.
The EU of GNOS is based on a FPGA (Field-Programmable Gate Array)+DSP (Digital Signal Processor) framework. After filtering and down-conversion in the RFU, the signals are digitally down converted with ADC (Analog to Digital Converter), then sampled at a high rate and transmitted to the channel processor of the EU, where the GNOS accomplishes navigation, positioning and occulting GNSS satellite prediction and selection, signal acquisition and tracking, and data handling. An USO (Ultra-Stable Oscillator) is used as a reference oscillator with very stable frequency (1 s Allan deviation of 10-12) in order to retrieve atmospheric measurements with high accuracy. It also allows using the zero-difference method to invert the excess phase measurements.
GNOS is a multi-frequency receiver with BeiDou/GPS compatibility, B1/B2 closed-loop (CL) tracking, GPS L2 codeless-mode operating for P code, GPS L2C closed-loop tracking and GPS L1 C/A closed-loop and open-loop (OL) tracking capabilities. The BeiDou and GPS-compatible instrument increases the number of transmitting sources and promises significant enhancements in throughput of the measurements. A multi-frequency operating instrument is needed for ionosphere parameter retrieval and ionospheric correction in pre-processing of atmospheric parameters. The receiver measures the following observable parameters for each tracked GPS and BeiDou satellite:
- L1 C/A-code phase
- L1 carrier phase
- L1 signal amplitude
- L2 P-code phase
- L2C code phase (if present)
- L2 carrier phase
- L2 signal amplitude
- B1I code phase
- B1 carrier phase
- B1 signal amplitude
- B2I code phase
- B2 carrier phase
- B2 signal amplitude.
In the lower part of the troposphere where highly dynamic signal conditions are frequently encountered due to the strong atmospheric modulation, the GPS L1 signal is tracked in open loop in parallel with the closed loop tracking. In open-loop tracking, the signal is down-converted using a numerically controlled oscillator, which generates a frequency given by an onboard Doppler model pre-calculated in GNOS without a feedback from received signal. Particularly, for the rising occultation, an a priori range model of the atmospheric delay is also calculated on board the GNOS. The baseband signal is then sampled at a rate of 100 kHz. Furthermore, a sample rate of 100 Hz of open-loop tracking is proven to be sufficient to capture the signal modulated by the atmosphere dynamics and uncertainties of the Doppler model.
The design specifications of GNOS are summarized in Table 22; it can be seen that some parameters of the FY-3 GNOS are comparable to those of COSMIC or MetOp/GRAS.
The mass for the whole GNOS instrument is around 14 kg. The power consumption in full operation (BeiDou and GPS, navigation and occultation) is about 40 W. The average GNOS data rate is 86 kbit/s, with peaks of up to 170 kbit /1. The characteristics of GNOS are displayed in Table 23.
Table 22: GNOS instrument parameters
Table 23: GNOS instrument characteristics
The GNOS project performed measurements in closed-loop (CL) and open-loop (OL) modes, similar to COSMIC and MetOp. The results show that the GNOS instrument provides more than 500 GPS occultations plus about 200 BeiDou occultations per day. The performance of the GNOS instrument in laboratory tests was found to agree with requirements of the GNOS instrument. In mountain-based experiments, the refractivity profiles of GNOS from GPS and BeiDou were compared with those of nearby radiosonde data within 1 hour. The comparison showed that the refractivity profiles obtained by GNOS were consistent with those of the radiosonde. The rms difference between the GNOS and radiosonde was <3 %.
FY-3 ground segment:
NSMC / CMA (National Satellite Meteorological Center / China Meteorological Administration) is responsible for receiving, processing the data of Chinese and foreign meteorological satellites, and distributing the data and information products to users for application. Other responsibilities include establishing the ground segment of the Chinese meteorological satellite observation system, conducting applied research in satellite meteorology, making plans and programs for developing Chinese meteorological satellite system based on the national requirements.
The FY-3 ground segment is comprised with 5 receiving stations. One of the stations will be in high latitude place (Svalbard, Norway). The FY-3 data products will be transmitted via DVB-S (Digital Video Broadcast-Satellite).
Figure 18: FY-3A receiving network (image credit: CMA/NSMC)
Table 24: Geographic location of ground stations
The FY-3A ground segment includes the data processing center, operation and control center, ground receiving center, and data archiving center. The technical systems are the Data Acquisition System (DAS), Computer and Network System (CNS), Operation Control System (OCS), Data Pre-Processing System (DPPS), Products Generation System (PGS), Quality Control System (QCS), Utilization Demonstration System (UDS), Archive and Service System (ARSS), Monitoring and Analysis System (MAS), and Simulation and Technical Supporting System (STSS).
Figure 19: FY-3A ground segment framework (image credit: NSMC)
Data Sharing and Service (Ref. 15):
Fengyun satellite data is provided to both domestic and international users via the following ways.
1) Direct Broadcast Service. Users with appropriate receiving equipment can directly receive data transmission of each operational Fengyun satellite.
2) DVB-S Dissemination System. The CMACast system uses the DVB technology to disseminate realtime products to subscribers.
3) Internet. The Fengyun Satellite Data Service Network (Website: http://satellite.cma.gov.cn) is one of the ways to download realtime or historical products.
4) FTP Service. For users demanding for large bulk data in realtime or near-realtime, the system initiatively pushes the data to user-specified FTP servers.
5) Manual Service. If large volume data is requested and has been approved by CMA (China Meteorological Administration), manual service is also available.
Figure 20: Satellite application facilities in China (image credit: CMA/NSMC)
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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 (firstname.lastname@example.org).