HJ-1 (Huan Jing-1: Environmental Protection & Disaster Monitoring Constellation)
The HJ-1 minisatellite constellation is a national program by the National Committee for Disaster Reduction and State Environmental Protection Administration (NDRCC/SEPA) of China, to construct a network of Earth observing satellites. The overall objective is to establish an operational Earth observing system for disaster monitoring and mitigation using remote sensing technology and to improve the efficiency of disaster mitigation and relief.
Note: HJ is the Chinese abbreviation for "Huan Jing" - meaning "environment".
The main application fields for China are environmental monitoring and prediction, solid waste monitoring, disaster monitoring and prediction (flood, drought, typhoon and wind damage, sand storm, earthquake, landcreep, frost and grassland fires, coal fires, crop pest monitoring, ocean disaster monitoring).
The HJ-1 program was ratified in 2003. In May 2007, the China national Space Administration became the member of the International Charter 'Space and Major Disasters', a joint initiative that works to provide emergency response satellite data free of charge to those affected by disasters anywhere in the world. Each member of the International Charter 'Space and Major Disasters' has demonstrated its commitment to use its space assets when it is most needed, when disasters of natural and human origin strike the world's communities, or wreak havoc on the environment.
The HJ-1 constellation is to be realized in two stages to achieve eventually a daily revisit:
• The first stage is comprised of three minisatellites, referred to as the 2+1 constellation
• The second stage will consist of a total of 8 spacecraft (4 with optical payloads and 4 with SAR payloads). The launch of all satellites is to be accomplished by 2012 through international cooperation.
The first stage implementation is referred to as "Environment-1": The HJ-1 constellation includes three small satellites (2+1 constellation). The spacecraft of the constellation are referred to as HJ-1A, HJ-1B, and HJ-1C. 1) 2) 3) 4) 5) 6) 7) 8) 9) 10)
• HJ-1A is an optical satellite with a CCD camera and and an infrared camera
• HJ-1B is also a optical satellite with a CCD camera and a hyperspectral camera on board
• HJ-1C is a spacecraft with a S-band SAR payload.
Background: More than half the world's population lives in the Asian and Pacific area in approximately one-fifth of the earth's land. As natural disasters increase in both intensity and severity around the world, the Asian and Pacific region continues to suffer a disproportionate number of hazard events and related losses of lives, infrastructure, stability and economic progress. The Asian and Pacific region accounts for 42% of the world's natural disasters, and a disproportionate 65% of disaster victims. In 2008, almost a quarter of a million people died as a result of natural disasters in the region, representing a staggering 97% of fatalities worldwide. Since 1990, one in every twenty persons in the region was affected by disasters in any given year. Natural disasters pose a significant challenge to development in the region. 11)
HJ-1A and HJ-1B spacecraft (optical payload)
The minisatellites are based on the CAST-968B bus of DFHSat (DFH Satellite Co Ltd.) of CAST (China Academy of Space Technology), Beijing and by the China Aerospace Science and Technology Corporation. The spacecraft are 3-axis stabilized.
Each spacecraft (HJ-1A, HJ-1B) has a launch mass of 470 kg. The size is 1.2 m x 1.1 m x 1.03 m. The spacecraft design life is 3 years.
Unfortunately, there is no more spacecraft description available.
Figure 1: Artist's rendition of the HJ-1 deployed optical spacecraft (image credit: CAST)
Launch: A Long March-2C (CZ-2C) launch of two spacecraft, HJ-1A and HJ-1B, took place on Sept. 5, 2008 (UTC). The launch site was TSLC (Taiyuan Satellite Launch Center), located in the northwest part of the Shanxi Province, China.
Orbit: Sun-synchronous circular orbit, altitude = 650 km, inclination = 97.95º, LTDN (Local Time of Descending Node) equator crossing at 10:45 hours.
HJ-1A and HJ-1B are in a coplanar orbit with a phasing of 180º.
In the final stage of the constellation, 4 satellites will be distributed in the same orbital plane at phase angles of 90º.
RF communications of HJ-1A: The TT&C communication is in S-band while the payload data are transmitted in X-band at a data rate of 120 Mbit/s.
RF communications of HJ-1B: The TT&C communication is in S-band while the payload data are transmitted in X-band at a data rate of 60 Mbit/s.
Three ground receiving stations for HJ-1 satellites are located in Miyun (Beijing), Kashi and Hainan, covering the entire land area of China and its surrounding countries and regions. CRESDA (China Center for Resource Satellite Data and Application) is responsible for the processing and analysis of the imagery.
Figure 2: Schematic view of the HJ-1A or HJ-1B spacecraft in launch configuration (image credit: CAST)
Mission status of HJ-1A and HJ-1B:
• According to WMO OSCAR, the satellites HJ-1A and HJ-1B are operational in 2017. 12)
• Both spacecraft, HJ-1A and HJ-1B, are in nominal operation in 2015, six years after commissioning. 13)
Figure 3: Sample image of the IRMSS instrument on HJ-1B with a resolution of 150 m (image credit: CRESDA) 14)
• Jan. 2015: The radiometric stability of satellite sensors is crucial for generating highly consistent remote sensing measurements and products. We have presented a radiometric responsivity tracking method designed especially for optical sensors without on-board calibration systems. Using a temporally stable desert site with high reflectance, the sensor responsivity was simulated using the Second Simulation of the Satellite Signal in the Solar Spectrum (6S) radiative transfer model (RTM) with information from validated MODIS atmospheric data. Next, radiometric responsivity drifting was identified using a linear regression of the time series BRDF (Bidirectional Reflectance Distribution Function) normalized coefficients. 15)
The proposed method was applied by CRESDA (China Center for Resources Satellite Data and Application) to the Chinese HJ-1A/-1B CCD sensors, which have been on-orbit operations for more than 5 years without continuous assessment of their radiometric performance. Results from the Dunhuang desert site between 2008 and 2013 indicated that the CCD sensors degraded at various rates, with the most significant degradation occurring in the blue bands, ranging from 2.8% to 4.2% yr-1. The red bands were more stable, with a degradation rate of 0.7-3.1% yr-1. A cross-sensor comparison revealed the least degradation for the HJ-1A CCD1 (blue: 2.8%; green: 2.8%; red: 0.7%; and NIR: 0.9% yr-1), whereas the degradation of HJ-1B CCD1 was most pronounced (blue: 3.5%; green: 4.1%; red: 2.3%; and NIR: 3.4% yr-1).
The tracking results are valuable for earth observation applications when time series HJ-1A/-1B CCD data are used which calls for effective correction of sensor degradation, and when data from different sensors are integrated due to inconsistency of varied radiometric responsivity.
• May 2014: The HJ-1 A/B satellites have now worked more than 5 years, and their payloads are operating well, while the application of HJ-1C is very limited into qualitative analysis due to the severe damaged antenna. 16)
• Both spacecraft are in nominal operation in 2012. 17)
• Both spacecraft are in nominal operation in 2010. The spacecraft play an important role in disaster monitoring next to their role in nominal Earth observations. 18)
China, a member of the International Charter "Space and Major Disasters", provided imagery of the HJ-1A and HJ-1B spacecraft to the relief authorities in Haiti after the devastating Haiti earthquake in January 2010.
Sensor complement: (WVC, HSI, IRMSS)
The sensor complement of the HJ-1A and HJ-1B spacecraft consists of three kinds of optical devices. The optical devices are: Wide View CCD Cameras (WVC), Infrared Multispectral Scanner (IRMSS), and the Hyperspectral Imager(HSI).21) 22)
The configuration of instruments onboard HJ-1A and HJ-1B satellites is as follows:
• HJ-1A satellite includes two CCD optical cameras (WVC) and one hyperspectral imager (HSI)
• HJ-1B satellite includes two CCD optical cameras (WVC) and one infrared multispectral scanner (IRMSS).
The two CCD cameras work together to form a 700 km wide swath. Each CCD camera has three visible bands (430-520 nm, 520-600 nm, and 630-690 nm) and a near-infrared band (760-900 nm). The spatial resolution of the CCD camera is 30 m.
Figure 4: Overview of instrument allocations to the spacecraft HJ-1A and HJ-1B
WVC (WIDE View CCD Cameras):
Table 1: Parameters of the WVC multispectral CCD cameras
Figure 5: Illustration of the WVC instrument with dual camera configuration (image credit: CAS/CSSAR)
HIS (Hyperspectral Imager):
The HSI is composed of an interferometer, Fourier mirror, a calibration system, swing mirror, a detector array and some other components.
Table 2: Parameters of the HSI instrument
The HIS instrument is carried by the HJ-1A satellite with a ground swath width of 50 km, the resolution of the ground pixels is 100 m, includes 110-128 spectral bands, has ± 30 ° side observing ability and calibration function on satellite.
Figure 6: HSI system structure diagram (image credit: Beijing Normal University)
Figure 7: Photo of the HSI instrument (image credit: CAST)
Figure 8: Flow chart of the spectrum reconstruction after data reception (image credit: Beijing Normal University)
IRMSS (Infrared Multispectral Scanner):
The instrument provides four bands in the spectral regions of: NIR (Near Infrared), SWIR (Short Wave Infrared) and TIR (Thermal Infrared).
The optomechanical instrument is of IRMSS heritage flown on the CBERS-1 (ZY-1) satellite.
Table 3: Parameters of the IRMSS instrument
The IRMSS instrument is flown on the HJ-1B satellite; it features a swath width of 720 km, the spatial resolution is 150 m or 300 m (TIR).
Figure 9: Schematic view of the IRMSS instrument (image credit: CAST)
HJ-1C (Huan Jing-1C / Environment-1C) spacecraft with SAR payload
The platform for the SAR spacecraft is based on the CAST-2000 bus with necessary modifications and upgrades.The satellite predicted operational life is three years. This is the first civil SAR (Synthetic Aperture Radar) system of China. 25) 26) 27)
Table 4: Overview of spacecraft parameter requirements
Figure 10: Artist's rendition of the HJ-1C spacecraft (image credit: CAST)
Figure 11: The HJ-1C SAR satellite during system assembly (image credit: CAST)
RF communications: X-band payload data transmission with QPSK modulation. The downlink data rate is of 2 x 160 Mbit/s. Use of the CCSDS standard to format the compressed raw data and its auxiliary data. An on board data storage capacity of 40 Gbit is provided.
Launch: The HJ-1C spacecraft was launched on Nov. 18, 2012 (UTC) on a Long March 2C vehicle (CZ-2C). The launch site was TSLC (Taiyuan Satellite Launch Center), located in the northwest part of the Shanxi Province, China. 28)
Orbit: Sun-synchronous circular dawn-dusk orbit, altitude = 502 km, inclination = 97.3º, period = 94 minutes, LTDN (Local Time on Descending Node) at 6:00 hours, repeat cycle = 31 days.
The secondary payloads on this flight were:
• NewTech-1 (New Technology Demonstration Satellite-1) of CASC
• HummerSat-1 (a minisatellite of 160 kg) of CAST (Chinese Academy of Space Technology)
• HummerSat-1A (a microsatellite of 30 kg) of CAST
• According to WMO OSCAR, the HJ-1C spacecraft is operational as of 2017 (Ref. 12).
• Only limited functionality is availaböle due to a damaged antenna of the SAR-S instrument.
Sensor complement: (SAR-S)
The S-band SAR instrument has been developed with the assistance of NPO Mashinostroyenia & Vega Corp., Russia. A technical description of SAR instrument is not available.
Table 5: Parameters of the SAR instrument
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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).