HY-2A (Haiyang-2A) / Ocean-2A
HY-2 is a second generation ocean observation/monitoring satellite series approved by CNSA (China National Space Administration) Beijing in Feb. 2007. The HY-2A mission represents a follow-up of the HY-1A and HY-1B missions.
The overall objective of HY-2 is the measurement of ocean dynamic and environmental parameters in the microwave region (i.e., all weather observations). The requirements call also for the collection of data on marine wind setup (wind vector), marine surface height, and SST (Sea Surface Temperature), along with aero-marine forecasts for the prevention and relief of disaster. 1) 2) 3)
Figure 1: Artist's rendition of the deployed HY-2A spacecraft (image credit: CAST)
The development of HY-2A, the first of the series of HY-2 series of satellites sponsored by the State Oceanic Administration (SOA), was approved by Chinese authorities on January, 2007. The spacecraft is three-axis stabilized using the CAST968 satellite platform. The spacecraft is being developed by the DFH (Dongfanghong Satellite Corporation Ltd.), a spin-off company of CASC (China Aerospace Science &Technology Corporation), Beijing, China.
The spacecraft has a pointing precision of < 0.1º in pitch, roll and yaw and a pointing knowledge of < 0.05º. Like its predecessors, HY-2A is a spacecraft with a mass of ~ 1500 kg. The HY-2A spacecraft will be operated by NSOAS (National Satellite Ocean Application Service) for a nominal lifetime of 3 years. 4)
A more complete description of the spacecraft will be provided when the information becomes available.
Figure 2: Photo of the HY-2A spacecraft during integration tests (image credit: AVISO, CNES)
Figure 3: Alternate view of the HY-2A spacecraft (image credit: CNES)
Orbit: Near sun-synchronous frozen orbit, altitude = 970 km, inclination of 99.3º, period = 104.45 minutes. Equator crossing time on descending node at 6:00 hours, repeat cycle: two phase (14 days and 168 days).
RF communications: The downlink of payload data is in X-band. Spacecraft monitoring and control is being done in S-band. Real-time transmission to X-band stations in Beijing, Hangzhou, SanYa and Mudanjiang.
Figure 4: Artist's view of the deployed HY-2A spacecraft (image credit: NSOAS, Ref. 9)
• The HY-2A spacecraft and its payload are operating nominally in 2013. The mission has been offering data to scientific users since the summer of 2012. Cross-overs of HY-2A along the period of time (2012-2013) with Jason-2 and CryoSat-2 are being investigated. 8)
• Sigma 0 distribution at the polar regions obtained by HY-2A SCAT in 2013. 9)
Figure 5: HY-2A-SCAT σο distribution at the Antarctic (left) and HY-2A SCAT σο distribution at the Arctic (image credit NSOAS)
• In a study performed in 2013, a comparison of wind field measurements from the HY-2A Scatterometer and the U.S. National Data Buoy Center (NDBC)’s moored buoys was performed. These comparisons were made in the Pacific Ocean and the Atlantic Ocean over a one month period of August 2012. The SCAT wind speed retrieval agreed well with the buoy measurements, with mean differences of -0.74 m/s and standard deviations of 1.52 m/s. The results (Figure 6) indicate that the SCAT-derived ocean surface wind speeds are as accurate as other scatterometers, such as QuikScat and ASCAT. 10)
However, the wind direction retrieval of SCAT still has some problems which need to be investigated in the future. Further analysis is underway to include more HY-2A data especially those in the higher wind domain, and validation results against ECMWF reanalysis and SAR winds.
Figure 6: Sea surface wind retrieval from HY-2A/SCAT (Aug 8, 2012), image credit: State Key Laboratory of Remote Sensing Science
• Sept. 2012: The HY-2A project has demonstrated that the mission fulfills the requirements of high precision altimetry. In particular, it allows continuing the observation of the MSL (Mean Sea Level) variations at the same accuracy as ERS-1. The accuracy of SSH (Sea Surface Height) derived from GDR products of HY-2A, is about 8.5 cm. 11) 12)
Figure 7: HY-2A SLA (Sea Level Anomaly) with MOE (Medium Orbit Ephemeris) and orbit error correction (m), image credit: CLS, CNES, NSOAS
• On March 2, 2012, control of the HY-2A spacecraft was handed over to SOA (State Oceanic Administration) by DFH of CASC. HY-2A has passed the commissioning phase and was declared “operational”. HY-2 will work in collaboration with HY-1B, the Chinese oceanic surveying satellite with a launch of HY-1B on April 11, 2007.13) 14)
• The HY-2A spacecraft is on its nominal mission orbit since October 1, 2011. Precise orbits are computed at CNES on the basis of 7 days arcs since the beginning of the mission (repeat cycle is 14 days).
• On August 31, 2011, the DORIS and GPS instruments were switched on and the first data obtained by DORIS show good results on the orbit determination.15)
Sensor complement: (MWRI, RA, SCAT, DORIS, LRA)
MWRI (Microwave Radiometer Imager):
MWRI is a multichannel nadir-viewing radiometer with the objectives to observe ocean surfaces and coastal zones for biological resources, pollution monitoring and prevention, and monitoring of estuaries, bays and navigation routes.
Table 1: User requirements of the MWRI measurement parameters
Table 2: Parameter specification of the MWRI instrument 16)
RA (Radar Altimeter):
RA/HY-2A was designed and developed at NMRSL (National Microwave Remote Sensing Laboratory) of CSSAR (Center for Space Science and Applied Research), Beijing. RA is an active instrument, a dual-frequency (Ku-band and C-band) radar altimeter. The Ku-band portion of the instrument comes with a DDS (Direct Digital Synthesizer), creating LF-chirps (Low Frequency) with bandwidths of 320, 80 and 20 MHz. The bandwidth of the C-band is fixed at 160 MHz. A MCT (Model Compatible Tracker), in which both MLE (Maximum likelihood Estimate) algorithm and OCOG (Off Center of Gravity) algorithm cooperating in parallel, is used in RA/HY-2A.
The user requirements call for:
- Sea surface precision: < 8 cm
- SWH (Significant Wave Height) range: 0.5-20 m; and an SWH precision of < 10 % or 0.5 m (whatever is greater).
• Antenna: The antenna of RA/SCAT is a paraboloidal reflector antenna. The two frequencies will share the reflector with a dual-frequency feed. The antenna gain is greater than 42.5dB (Ku-band) and more than 32.5dB (C-band). The antenna looks to the nadir, and the pointing accuracy is 0.1º.
• T/R switch and internal calibration loop. This unit includes the T/R switch and the internal calibration loop.
• Frequency synthesizer: The frequency synthesizer provides all the frequency signal for the system, including the local oscillator frequency, the reference frequency and the clock signals.
• Transmitter: The transmitter consists of a DDS signal generator, the up-converter and the solid-state RF power amplifier.
• Receiver: The receiver consists of the LNA (Low Noise Amplifier), the down-converter, the deramp-mixer and the I-Q detector.
• DSP control and processing unit: This unit consists of a AD converter, the DSP (Digital Signal Processing) unit and the system control, and management unit. The DSP unit provides the FFT operation for the frequency-to-time transform for the full deramp-receiver and the digital tracking operation to obtain the average sea surface height and significant wave height. The command and data transfer between RA and the HY-1B platform is also being done by this unit.
• Secondary power supply unit. Like other similar spaceborne radar altimeters, RA is equipped with a three-band microwave radiometer for atmospheric and ionospheric delay correction. The frequencies of these radiometers are 18.7, 23.8 and 37 GHz, respectively; in which the 18.7 and 37 GHz channels are atmosphere window frequencies and the 23.8 GHz channel is for vapor delay correction.
Table 3: Parameter specification of the RA instrument
Figure 8: Block diagram of the dual-frequency RA instrument (image credit: NMRSL/CSSAR)
Table 4: System parameters of the RA instrument
Design of the operation mode: The operation mode of RA/HY-2A includes 6 sub modes, which are noise bias measurement, inter calibration measurement, acquisition measurement, tracking measurement, bus communication, and data transmission (Ref. 19).
1) Noise bias measurement: This sub mode measures the noise and bias of radar altimeter system, by which to determine the threshold of acquisition and tracking, and the correction value of echo signal.
2) Internal calibration measurement: RA/HY-2A samples the internal calibration signal every 30 s, and then put the sampled data into scientific data packet.
3) Acquisition measurement: Acquisition is the previous step of tracking measurement, by which to enable echo signal to fall in the tracking window.
4) Tracking measurement: This measurement is the core of the altimeter. RA/HY-2A adopts the model compatible tracker to perform tracking measurement.
5) Bus communication: RA/HY-2A can obtain satellite current information (such as time tag, attitude data, and etc.) through the 1553B bus. The working parameters coming from the ground station are also transferred to RA/HY-2A by the 1553B bus.
6) Data transmission: RA/HY-2A puts the scientific data into packets, and then transfers the packets to the satellite's large capacity memory. The packets are then transmitted to the ground receiving station when the satellite passes over it.
Implementation of the MCT (Model Compatible Tracker): A new tracker, MCT, is adopted in RA/HY-2A. In MCT, two tracking algorithms, MLE and OCOG, cooperate in parallel. MLE is suitable for ocean with better accuracy, but it is easy to lose lock when the target is land, ice or coast zone. While OCOG is robust on account of its model free, but its accuracy is worse than that of MLE. Therefore, not only the good accuracy but also the robustness can be realized by MCT. So MCT is very suitable for the ocean-land compatible radar altimeter (Figure 9).
Figure 9: Block diagram of the MCT (image credit: NMRSL/CSSAR)
One of the goals of MCT is to measure both ocean and land without loss of tracking, so the MCT should have the ability to distinguish the different echoes from diverse surfaces and choose the different tracking strategies automatically. MCT is comprised of OCOG tracker, MLE tracker, and fusion unit.
Three bandwidths for chirp signal are adopted in MCT, and they are 320 MHz, 80 MHz and 20 MHz. The 320 MHz signal is arranged for ocean measurement, the 80 MHz is for the coastal zone, and the 20 MHz is for land or ice coverage. The echo signal from the coast or land is more dramatic than that from the ocean, so the OCOG discriminator can determine which bandwidth is the candidate at anyone time. The OCOG tracker also sends the selected bandwidth to the MLE discriminator.
The fusion unit receives all the results from the two trackers. MCT selects the MLE tracker’s output as the current working parameters when MLE locks the target. Otherwise, when MLE loses locking, the OCOG tracker’s output is chosen (Ref. 19).
KU-RFSCAT (Ku-band Rotational Fan-beam Scatterometer) -referred to as SCAT:
KU-RFSCAT is a Ku-band instrument designed and developed at NMRSL/CSSAR, Beijing. KU-RFSCAT is a pencil-beam conically scanning radar scatterometer. It uses a full-deramp pulse compression receiver with LFM (Linear Frequency Modulation) capability, which increases the number of the independent measurement samples to improve the measurement precision. A transmit signal generator signal generator creates a LF-chirp pulse with a bandwidth of 1~3 MHz in transmit mode; a separate receive mode reference signal generator creates a LF-chirp pulse with a bandwidth of 1~5 MHz as the de-ramp receiving reference. The separate design of the transmit signal generator and the reference signal generator allows the full range of onboard internal calibrations. 20) 21) 22)
The user requirements call for:
- Wind speed range: 2-24 m/s within an accuracy of 2 m/s or 10% of maximum value
- Wind direction accuracy: ±20º.
Instrument: The KU-RFSCAT instrument is comprised of the following elements:
• Antenna and scanning servo unit: The antenna of KU-RFSCAT is a paraboloidal reflector antenna with a feed very slightly off-set. It features two off-nadir pointing beams for the two orthogonal polarizations.
• T/R frontend unit: This unit includes a high-isolation (> 130 dB) T/R switch matrix and an internal calibration loop. A very small portion of the output microwave RF power from the transmitter is being coupled to the internal calibration loop for output power and receiver gain floating calibration.
• Receiver RF-IF unit, including the low noise amplifier (LNA), the down-converting mixer, the IF filter and amplifier and the I-Q detector unit
• Transmitting signal unit: It generates the linear FM chirp signal for the transmitting mode.
• Reference signal unit: It generates the linear FM chirp signal as the reference for the deramp receiver. This signal uses the same FM slope as the transmitted chirp signal, but its duration is longer than the transmitted pulse to cover the range extending within the footprint of the antenna beam.
• RF power amplifier unit: It is a TWTA (Travelling Wave Tube Amplifier) to generate the required 100 W transmitting power.
• Frequency synthesizer unit
• DSP, system control and management unit: DSP includes the FFT (Fast Fourier Transform) operation for full-deramp receiving and the noncoherent resolution cell average.
• Secondary power supply unit.
Table 5: Parameter specification of the KU-RFSCAT instrument
Figure 10: Observation geometry of the KU-RFSCAT instrument (image credit: NMRSL/CSSAR)
Figure 11: Block diagram of the KU-RFSCAT instrument (image credit: NMRSL/CSSAR)
Figure 12: Illustration of the KU-RFSCAT structure (image credit: NMRSL/CSSAR)
Instrument calibration: KU-RFSCAT employs an internal calibration loop. The fluctuations of the receiver gain and the transmitter output power can be calibrated by a internal calibration procedure when the antenna is rotated to the cross-track directions. 23)
The ratio method is employed in the internal calibration to eliminate the negative impacts caused by parameter fluctuation from receivers and transmitters during in-orbit operation. Internal calibration is carried out by coupling portion of the TWTA output-power to the receiver to form a closed loop. Figure 13 illustrates the timing sequences for observation and internal calibration during each rotation. Internal calibration is performance twice with each rotation cycle with an interval of 1.8s, at the edge of the swath.
Figure 13: Measurement and internal calibration sequence during each rotating cycle (image credit: NMRSL/CSSAR)
An important problem in SCAT calibration is that only one pulse can be received by the transponder in a calibration routine, which is inadequate to acquire high accuracy calibration. To solve the problem without disturbing the operations of SCAT, an innovative scheme is employed: SIMO (Single-Input-Multiple-Output). After the transponder receives a pulse, a sequence of short (~100us) chirps is transmitted. Each chirp has the same chirp rate (bandwidth divided by the time duration) as the transmitter chirp of the SCAT, and each one is modulated by a different carrier so that they can be resolved in the range window of SCAT. - In the full de-ramp theory, once the two chirps have the same chirp rate, the signal after the mixer is a sinusoid. As a result, there are many echoes of one single transponder in the SCAT range window. Therefore more information of the SCAT antenna pattern can be exploited and a more accurate calibration is possible (Ref. 18).
DORIS (Determination Orbite Radiopositionnement Integres Satellite):
HY-2A is the first mission which will serve the French-Chinese cooperation in the space oceanography domain. The cooperation will consist in the provision of the satellite data by NSOAS (National Satellite Ocean Application Service) and for CNES in the provision of the POD (Precise Orbit Determination) based on data supplied by a DORIS instrument, a GPS and the LRA (Laser Ranging Assembly) measurements.. This precise orbit will be used by NSOAS to create altimetry products that will then be integrated by CNES to realise multi-mission products distributed by AVISO (Archivage Validation Interprétation des Données des Satellites Océanographiques). DORIS data will be made available through the IDS (International DORIS Service).
LRA (Laser Ranging Assembly):
Figure 14: The LRA configuration for HY-2 (image credit: NASA) 26)
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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.