COMS-1 (Communication, Ocean and Meteorological Satellite-1) / Chollian
In 1996, Korea established its long-term plan of the National Space Program which was revised in 2000 to accommodate the public and civilian demand for satellite utilization and to maintain the continuity of satellite services. The plan prospects the details of the future space activities of Korea until 2015 and serves as a basis for space development in Korea. In response to this space plan, the Korea Meteorological Administration (KMA) started to define and formulate the basic requirements for COMS-1, the first geostationary meteorological satellite mission of Korea. - Note: The nickname Collian means long distance view (literally “Thousand Li View”) in Korean.
COMS-1 is a geostationary meteorological satellite program of Korea with multifunctional applications in the fields of:
1) Experimental communications: a) in-orbit verification of developed communication technology, b) experiment of wide-band multimedia communication service
2) Ocean color monitoring: a) monitoring of marine environment and ecosystem, b) production of fishery information. The scope of the ocean color mission includes detecting, monitoring and predicting short-term biological phenomena such as HAB (Harmful Algal Bloom), studies on biogeochemical variables, monitoring health of the marine ecosystem, coastal zone and resource management and providing information for fishing communities.
3) Meteorological observations: a) continuous monitoring of the ground segment from GEO and extraction of meteorological products, b) early detection of severe weather phenomena, c) monitoring of long-term change of SST and clouds. The meteorological mission will complement the existing network of geostationary satellites, providing improved input data for numerical weather prediction models, and monitoring climate changes; the data, imagery and derived products will be freely available to both domestic and international community in real-time or near real-time basis through direct broadcasting or land lines.
The program definition started in 2002 and is sponsored by the Government of Korea with the following government institutions involved: MOST (Ministry of Science and Technology), KMA (Korea Meteorological Administration), MIC (Ministry of Information and Communication), MOMAF (Ministry of Maritime Affairs and Fisheries). The COMS program is part of a 15-year Korean space plan, a national program that began in the early 1990s, and which is being followed systematically ever since. A launch of a follow-up COMS-2 spacecraft is planned for 2014. 1) 2) 3) 4) 5) 6) 7) 8) 9)
On the implementation side, the following functions are split up among the following agencies:
• KARI (Korea Aerospace Research Institute), Daejeon, is responsible for the space segment (system development and integration), spacecraft operations and control, and ground station development. KARI is leading the project of COMS-1, building on the experience with KOMPSAT-1 and KOMPSAT-2.
• METRI/KORDI (Meteorological Research Institute/), Seoul, provide the user requirements of meteorological and ocean monitoring payloads
• ETRI (Electronic and Telecommunications Research Institute), Daejeon, provides the communications payload development, the satellite control system development, and ground station co-development
• The Korean space industry is the provider/manufacturer of system hardware and software components.
Table 1: Overall COMS-1 system requirements
Figure 1: Artist's rendition of the COMS-1 spacecraft in orbit (image credit: KARI, KMA)
In May 2005, EADS Astrium SAS received a contract award from KARI to design and manufacture the first Korean multifunctional geostationary satellite as well as the MI and GOCI payloads (prime contractor). In July 2005, EADS Astrium in turn awarded ITT Industries a contract to built the meteorological payload for COMS-1. The communications payload will be provided by KARI as a customer furnished equipment. The cooperative agreement between KARI and Astrium calls also for the training of over 40 Korean engineers by the Astrium design team.
The COMS-1 spacecraft structure is based on the Eurostar-3000 bus design. The satellite features a box-shaped structure, built around the two bi-propellant tanks. Imaging instruments and MODCS antennae are located on the Earth floor (Figure 2). A single-winged solar array with 10.6 m2 of GaAs cells is implemented on the south side, so as to keep the north wall in full view of cold space for the MI (Meteorological Imager) radiant cooler. The deployable Ka-band antenna reflectors are accommodated on the east and west walls. 10) 11) 12) 13)
The S/C is 3-axis stabilized. Attitude sensing in normal mode is based on a hybridized Earth sensor and a gyros concept; in addition, sun sensors are being used during 3-axis transfer operations. Reaction and momentum wheels (5) serve as actuators. Thrusters 2 x 7 (10 N) are being used for wheel desaturation, and for orbit control. The apogee firing boosts are provided by a 440 N liquid apogee engine.
Platform stability: Even with perfect optical instruments, the image quality is strongly dependent on the quality of the platform stabilization. Three strong requirements have been put on the quality of platform stabilization (Ref. 27):
1) Pointing accuracy (pitch and roll): this specification is essential to a priori knowledge to where the instrument line of sight is aiming at. This is important for Ka-band payload operations, for GOCI operation (due to further stitching of small images to construct the large imaging area) and for MI which can be commanded to frequently review some local areas.
2) Pointing kwoledge (pitch and roll): the pointing knowledge is mainly driven by the INR (Image Navigation and Registration) to start the landmark matching processing with a sufficient accuracy.
3) Pointing stability (pitch and roll): this specification is mainly driven by the GOCI instrument, requesting integrating times as long as 8 seconds, with a jitter less than 10 µrad.
The first point is fulfilled by the heritage bus (E3000 platform), but the two last points have necessitated the implementation of a high precision Fiber Optic Gyro (FOG 120 HR developed by Astrium), furthermore the third point has been flown down to micro-vibration dampers under wheels, various AOCS tuning (solar array natural mode damping, optimised wheel zero crossing management), optimized manoeuvres (reaction wheel off loading, EW and NS maneuvers ) and a few operational constraints (stop solar array rotation during GOCI imaging period, ). See AOCS pointing requirements of Table 1.
The EPS (Electric Power Subsystem) makes use of GaAs solar cells and Li-ion batteries. A regulated power bus (50 V) distributes power to the various onboard applications through the power shunt regulator. During orbital eclipses, energy is provided by a 154 Ah Li-ion battery. The spacecraft launch mass is ~ 2500 kg, size (folded): 2.6 m x 1.8 m x 2.8 m.
Figure 2: Alternate view of the COMS-1 spacecraft (image credit: EADS-Astrium SAS)
The heart of the avionics architecture is implemented in hot redundant spacecraft computer units, based on 1750 standard processors with Ada object-oriented real-time software. A redundant MIL-STD-1553-B data bus serves as the main data path between the onboard units. Interface units are being used for the serial links, namely the actuator drive electronics with the bus units (including thermal control), the modular payload interface unit with the Ka-band communication payload, and the MI interface unit with the MI instrument.
RF communications: The onboard MODCS (Meteorological & Ocean Data Communication Subsystem) is required to collect all MI and GOCI data and to transmit it via CDAS to MODAC. The L-band and S-band frequencies are being used to transmit all MI and GOCI data. The S-band is also used to transmit all TT&C data. Use of HRIT/LRIT (High- and Low-Rate Information Transmission) services and CCSDS protocols. - In addition, there must be a spacecraft capability to relay the raw and processed MODAC data products to the end users. 14)
Figure 3: Block diagram of the spacecraft functional architecture (image credit: EADS Astrium SAS)
Figure 4: Photo of the COMS-1 spacecraft in its final integration stage, prior to shipment to Kourou (image credit: EADS Astrium)
Launch: The COMS-1 satellite was launched on June 26, 2010 (UTC) on Ariane-5 ECA from Kourou, French Guiana. Following launch, COMS-1 separated into a GTO (Geosynchronous Transfer Orbit). KARI signed a contract with Arianespace in Dec. 2006. 15) 16)
The primary payload on this flight was ArabSat-5A, a communication satellite of ArabSat, Saudi Arabia, with a launch mass of 4800 kg, a spacecraft power of 11 kW (EOL) and a design life of 15 years. ArabSat-5A was built by an industrial team of Astrium and Thales Alenia Space acting as co-prime contractors. Astrium, the team leader, supplied the Eurostar E3000 platform and assembled and tested the spacecraft.
Orbit of COMS-1: Geostationary orbit at the longitude of 128.2º E, altitude of ~ 35,786 km.
Figure 5: Alternate view of the COMS-1 spacecraft (image credit: KMA)
Status of Mission:
• October 2013: Mission operation characteristics. 17)
According to the user requirements of the meteorological observation mission, the observation modes of the MI (Meteorological Imager) is categorized into global, regional, and local mode and the MI observation areas have been defined as the five areas of Full Disk (FD), Extended Northern Hemisphere (ENH), Limited Southern Hemisphere (LSH), Asia and Pacific in Northern Hemisphere (APNH), and Local Area (LA). The five observation areas are shown in the Figure 6.
Figure 6: Typical meteorological image areas of COMS-1 (image credit: KARI)
In nominal operation, the user requirements call for:
- the global mode (FD) combined with the local mode (LA) which are executed every 3 hours
- the regional observation mode (ENH or LSH or APNH) combined with the local mode (LA) are evoked for 2hours and 30 minutes between two FD images which are separated by 3 hours.
- The combined global observation mode consists of the two images of “FD+LA” ; the combined regional observation mode is comprised of the four images of “ENH+LA+ENH+LA”.
In addition to Earth imaging in nominal operations, there are 3 special observation missions (calibration runs) for support of the meteorological observation mission.
- BBCal (Blackbody Calibration) usually performs every 30 minutes and meteorological images are taken between the BBCals. Hence the theoretical maximum numbers of FD, ENH, and LA images are 8, 80, and 88 per day, respectively.
- The 'albedo monitoring' is performed to have the Sun image once a day (at ~21:30 UTC ) for the check of the responsivity of the MI visible channel.
- The Moon observation and dark space observation are carried out monthly and quarterly, respectively, to check of the variation of the MI radiometric performance. — The Moon observation and the dark space observation are executed are also as albedo imaging modes. In general, whenever the albedo monitoring mode is used, the Earth imaging and the BBCal cannot be performed.
During the nominal operational period of the meteorological monitoring mission so far (over 2 years), which started in April 2011, an average daily operational imaging of 86.4% was achieved. For the ocean mission, the average daily monitoring support was 17% (Ref. 17).
• The COMS-1 spacecraft and its payload are operating nominally in 2013.
- Applications: GOCI (Geostationary Ocean Color Imager) observations can be employed to investigate dynamic variations in the coastal water properties, in particular, in an environment affected by semi-diurnal tides such as the west coast of the Korean Peninsula. GOCI was effectively employed to the monitoring of coastal water turbidity variations based on the tidal cycle. These hourly variations in coastal water properties can be basic dataset to develop an algorithm for catching the ocean current movement, i.e. velocity and direction which are crucial information for seawater circulations, fisheries, shipping control, military purpose, etc. in the coastal area. 18) 19) 20)
- GOCI data can be used to generate surface PAR (Photosynthetically Available Radiation) imagery over the East Asia Seas and Northwest Pacific at a 0.5 km resolution and a daily temporal scale for studies of aquatic photosynthesis. An algorithm has been developed to estimate daily PAR from GOCI data and tested on actual GOCI imagery. 21)
- Another excellent application of GOCI in terms of the short-term variability is surveillance of waste disposal activity at sea. GOCI could even clearly trace the disposal activity of a sewage sludge disposal ship near real-time for several hours due to its great temporal resolution. Through that hourly-based traceable variability, the ship’s cruising speed could also be estimated. 22)
Figure 7: Tracing marine sand gathering with GOCI (image credit: KOSC)
Legend to Figure 7: The left 4 figures indicate the total suspended sediments (TSS) concentrations derived from GOCI. The figures are from multiple time periods from July to August 2012 which were observed in various ocean conditions around the southern sea of Korea about 50 km away from the southern coast. KOSC could verify that those positions correspond to the marine sand collection spots operated by the Korea Water Resources Corporation (K-water). - Highly variable sea surface turbidity was detected by GOCI around the deposits, which is considered to be caused by the suspended sand coming off the bottom of the water when the marine sand was gathered. 23)
• In late March 2013, KOSC (Korea Ocean Satellite Center) announced the release of GDPS (GOCI Data Processing System) Beta Version 1.2 with enhanced functional capacity to process GOCI (Geostationary Ocean Color Imager) data via its website.24)
Users can now output the data concerning to red tides, current vectors, fishing ground information as well as to vegetation index that were not included in the previous versions of GDPS.
• Fall 2012: The COMS-1 mission of meteorological observation, ocean monitoring, and telecommunication service is currently under nominal operation service since April, 2011. The MI (Meteorological Imager) and GOCI (Geostationary Ocean Color Imager) perform the Earth observation mission of meteorological observation and ocean monitoring, respectively. 25)
• On June 26, 2012, the COMS-1 spacecraft and its payload marked two years on orbit.
• The COMS-1 spacecraft and its payload are operating nominally in 2012.26)
- COMS-1 produces full disk imagery every 3 hours and extended Northern Hemisphere imagery every 15 minutes. In particular, COMS has been focusing on the Korean Peninsula 8 times an hour to expect early detect of abrupt high-impact weather events such as typhoons and summertime heavy rains which frequency increase by a factor of 4 compared with the past.
- The CMDPS (COMS-1 Meteorological Data Processing System) provides 16 baseline products including information on Atmospheric motion vector, Asian dust, SST (Sea Surface Temperature) and LST (Land Surface Temperature) over the East Asian region. These products will help improve the performance of NWP models for weather analysis and forecast. In the long term, they will be used in the analysis and prediction of climate change around the Asian region.
- Starting on April 1, 2011, COMS-1 data are provided free-of -charge to the countries in the Asia Pacific region; KMA actively deploys the satellite receiving system support project for the countries in Southeast Asia to improve the utilization of COMS-1 data. In particular, KMA/NMSC (Korea Meteorological Administration/National Meteorological Satellite Center) has been providing a training program for about one month for the forecasters and satellite imaging analysis experts in the Asia Pacific region including the Philippines, Vietnam, Mongolia and Papua New Guinea every year since 2007.
• In the summer of 2011, first 3D-HDTV broadcasting and broadband VSAT communication trial services were conducted with the Ka-band payload (Ref. 53).
• After one full year in orbit in the late summer of 2011, the COMS satellite is behaving beautifully and meets all requirements whether in terms of image quality (radiometrically and geometrically), or in terms of availability, and fuel consumption. 27)
Although driven by conflicting mission requirements, the COMS is the first geostationary 3-axis stabilized satellite for Earth observation, with pointing performances showing large margins. - The versatility of the missions and the adaptability of the platform have pushed to affectionately nickname the satellite as a “Swiss knife” satellite, and it is found that this nickname is well deserved (Ref. 27).
• In July 2011, the COMS project handover meeting took place when Astrium officially passed control of the COMS satellite to the customer, namely KARI (Korea Aerospace Research Institute), which assumed full responsibility for COMS operations. The handover followed in-orbit acceptance of the satellite, which was successfully completed on March 17, 2011.28) 29)
This final formal phase of the delivery gave the Astrium team and the customer the chance to carry out a joint check of the satellite configuration and the relevant documentation, and to ensure that no outstanding issues remained to be addressed.
• KOSC (Korea Ocean Satellite Center) has been disseminating and distributing data from GOCI (Geostationary Ocean Color Imager) since April 20, 2011. Currently, GOCI RGB and Level-1B full resolution images are being produced, and available for three time periods per day (11h, 12h, 13h local time) and will be extended to the full service of 8 times per day in due course.
With the GOCI In-Orbit Test Results for 8 months, KORDI (Korea Ocean Research and Development Institute) confirms that GOCI has great advantage for real-time ocean environment observation and monitoring, comparing with conventional LEO OC (Ocean Color) satellites. The enhanced temporal resolution, hourly observation in daytime, shows better effectiveness than expected. 30)
• GOCI radiometric calibration: The radiometric radiometric calibration has been performed through an in-orbit solar calibration using the onboard calibration device, Solar Diffuser (SD), on a daily basis. The sun image, which is a known reference source, will be observed utilizing the SD. Spectral irradiation from the sun is not really constant, but the yearly variations are known. If the characteristic of the SD is known, gain parameters of each pixel can be calculated using this sun image. Then, the radiance of an observed image will be computed by employing the gain parameters. Stability of the SD characteristics between ground and orbit is ensured by design and by adequate protections against contamination. However, small aging over the mission life remains possible due to frequent exposure to the sun, up to 30 min per day. The SD aging can then be monitored and compensated using the DAMD (Diffuser Aging Monitoring Device), which is much less frequently exposed to the sun. The GOCI radiometric gain parameters have been observed through in-orbit calibration activity over the COMS IOT duration. 33)
Figure 8: GOCI image of April 11, 2011 (image credit: Yu-Hwan Ahn of KORDI)
Legend to Figure 8: The Level-1B composite of eastern China, South Korea and Japan was captured by the GOCI instrument on board the geostationary COMS satellite and processed by KOSC (Korea Ocean Satellite Center) of KORDI. A turbid plume of resuspended sediment originating from the discharge of the Yangtze River is clearly visible on the left hand side of the image. Resuspended sediments are prevalent in winter and spring when strong seasonal winds cause mixing of the water column.
• In December 2010, a newly developed COMS Flight Dynamics System (FDS) has been successfully validated during a six months of IOT (In-Orbit Test) operations (Ref. 37).
• In October 2010, the COMS spacecraft is still in the IOT (In-Orbit Test) phase, exhibiting very promising early stage IOT data including the images from the two on-board optical sensors, MI (Meteorological Imager) and GOCI (Geostationary Ocean Color Imager). 34)
• On Sept. 9, 2010, ETRI (Electronics and Telecommunications Research Institute) of Korea announced that it had completed the in-orbit test of the communication payload onboard the COMS-1. 35)
Ka-band IOT: The in orbit testing of the Ka-band payload was conducted during a few days dedicated to these activities. The payload was thoroughly tested from the ETR ground station with the support of the KARI SOC (Spacecraft Operation Center). Some satellite off pointing was necessary to analyze the antenna coverage, emitted power, EIRP, AFR, G/T, etc. for beams covering the territories out of South Korea. All performances were found within the specifications, and no antenna pointing adjustment was found necessary.
Figure 9: Schematics for Ka-band in orbit testing (image credit: EADS Astrium, KARI)
Figure 10: GOCI L1A mosaic image (image credit: KORDI) 36)
• On July 4, 2010, COMS-1 reached its GEO location after three apogee engine firings and two station-acquisition maneuvers.37)
• The satellite successfully reached orbit and separated from the second stage rocket about 32 minutes after takeoff. Full operational service of the spacecraft is expected in December 2010 after the end of the commissioning phase.38)
Sensor/experiment complement: (GOCI, MI, COPS)
GOCI (Geostationary Ocean Color Imager):
GOCI is the world’s 1st geostationary ocean color imager with the objective to provide multispectral (8-band) VIS/NIR data. The radiometer is designed to be operated in a 2D staring-frame capture mode. The instrument provides an important new capability for imagining the coastal zone where the phenomena varying on shorter space and time scales demand a simultaneous increase in spatial and temporal resolution. GOCI will provide multiple views of many locations within the fixed region during a single day (i.e. 8 images during the daytime and 2 images during the nighttime). The data from GOCI will therefore address various research areas in coastal, oceanographic and atmospheric sciences.
The overall observation objectives of GOCI include the following capabilities:
• Detecting, monitoring and predicting of short-term biophysical phenomena
• Support for studies on bio-geochemical variables and cycle
• Detecting, monitoring and predicting noxious or toxic blooms of algae of notable extension
• Monitoring of the health state of marine ecosystems
• Permitting an assessment of the geological and biological response to physical dynamics
• Support of coastal zone and resource management
• Provision of improved information on marine fisheries to the fisherman communities.
The aim of the GOCI observations include: monitoring of the marine environment in the vicinity of the Korean peninsula to permit analysis of the short-term and long-term change of the marine ecosystem. The coverage area has a size of 2500 km x 2500 km. The GSD (Ground Sampling Distance) at the center of the target region (defined at 130º E - 36º N) is 500 m x 500 m. Such a resolution is equivalent to a GSD of 360 m in nadir direction on the equator. 39) 40) 41) 42) 43) 44) 45) 46) 47)
Figure 11: Schematic view of the GOCI optical system (image credit: KARI)
Table 2: Major requirements of GOCI
Figure 12: Spectral response of GOCI (image credit: KARI, Astrium, Ref. 39)
Table 3: Spectral bands of the GOCI instrument
GOCI has been jointly designed and developed by EADS Astrium and KARI. The GOCI instrument concept is based on a dioptric camera design of 150 mm pupil diameter. A step-and-stare imaging technique is used with a CMOS/APS detector array of 1,415 x 1,432 pixels. The detector is passively cooled and regulated to a working temperature of 10ºC. The GOCI matrix is a custom CMOS imaging sensor featuring rectangular pixel size to compensate for the Earth projection. The detector comes from the COBRA family, developed and qualified by Astrium in cooperation with ISAE/CIMI for circuit design and with E2V for back-end manufacturing.
Spectral band selection is ensured by a 9-position filter wheel: 8 positions correspond to the 8 wavebands, and the ninth one is used for detector dark current measurement by providing a full occultation view. 48)
Figure 13: Illustration of the detector array, shown with temporary window (image credit: Astrium)
The instrument images a portion of the specified image frame (also called slot) at a time, for each of the spectral bands. A pointing mirror, located close to the entrance of the instrument then provides a bi-dimensional elliptical scanning on the Earth. Thus, by successively pointing the line-of-sight in 16 pointing directions, the detector array is moved in the field of view so as to cover the complete image area. A 4-position mechanism is positioned at the entrance of the instrument and serves as a shutter and as a radiometric calibration mechanism: one of the positions allows to image periodically the sun through a Lambertian diffuser, while another one offers direct sun imaging through a pinpoint so as to assess diffuser ageing, if any.
A slot acquisition takes about 100 seconds for the 8 bands and dark signal acquisition. The time required to acquire a full image, comprising the 16 slots in all 8 wavebands, is < 30 min, including image integration and readout time and filter wheel motion. The acquired data are directly transferred to the MODCS for immediate downlink in L-band to the ground segment.
The principle of the pointing mechanism is an assembly of two rotating actuators mounted together with a cant angle of about 1º, the top actuator carrying also the Pointing Mirror (PM) with the same cant angle. When rotating the lower actuator the LOS (Line of Sight) is moved on a circle and by rotating the second actuator, a second circle is drawn from the first one. It is thus possible to reach any LOS position inside the target area by choosing appropriate angle position on each circle. The mechanism pointing law provides the relation between rotation of both actuators and the LOS with a very high stability. This high accuracy pointing assembly used to select slots centers is able to position the instrument LOS anywhere within a 4º cone, with a pointing accuracy better than 0.03º (500 µrad). Position knowledge is better than 10 µrad (order of pixel size) thanks to the use of optical encoders.
Figure 14: Photo of the GOCI pointing mechanism without mirror (image credit: Astrium)
The GOCI instrument has a mass of 83.3 kg and a power consumption of 106 W (max). The size of GOCI is 1270 mm (W) x 790 mm (D) x 898 mm (H). The telescope employs a SiC (Silicon Carbide) structure provided by Boostec Industries, France. A PIP (Payload Interface Plate) is part of the main unit (Figure 16). It supports a highly stable full SiC telescope, mechanisms and proximity electronics. The PIP is larger than the instrument to carry the satellite Infra-Red Earth Sensor (IRES).
The main unit includes an optical module, a two-dimensional FPA (Focal Plane Array) and an FEE (Front End Electronics). The optical module of GOCI consists of a pointing mirror, TMA (Three Mirror Anastigmat) mirrors, a folding mirror, and a filter wheel. The FEE is attached near the FPA in order to amplify the detector signal with low noise prior digitization. In Figure 16, the shutter wheel is located in front of pointing mirror carrying four elements: shutter which will protect optical cavity during non-imaging period, open part for the ocean observation, SD (Solar Diffuser) and DAMD (Diffuser Aging Monitoring Device) for solar calibration.
A QVD (Quasi Volumetric Diffuser) has been chosen for the SD and the DAMD among several candidates because it is known to be insensitive to the radiation environment.
Figure 13 shows the FPA for GOCI which is a custom designed CMOS image sensor featuring rectangular pixel size to compensate for the Earth projection over Korea, and electron-optical characteristics matched to the specified instrument operations. The CMOS FPA having 1432 x 1415 pixels is passively cooled and regulated around 10ºC. It is split into two modules which are electrically independent. The CMOS detector is advantageous due to its low susceptibility to blooming. This is also important when observing the relatively dark ocean surface near bright clouds.
Figure 15: Illustration of the GOCI instrument (image credit: EADS Astrium SAS)
Figure 16: Alternate view of the GOCI instrument (image credit: EADS Astrium SAS)
Calibration is achieved by sunlight at night (every night in orbit) through a full pupil solar diffuser, made of fused silica insensitive to radiation aging. A second diffuser of smaller size is used to verify the main diffuser stability. The full pupil solar diffuser and diffuser monitoring device are both carried by the shutter wheel which also provides an open or closed position in front of the optical aperture.
The radiometric calibration of the GOCI will be performed for all pixels of detector matrix at ground segment by using the on-ground and in-orbit calibration data. The radiometric response of the GOCI has been characterized through on-ground calibration. The radiometric gain parameters will also be estimated through in-orbit calibration. The change of radiometric response will be corrected periodically through in-orbit calibration using the on-board calibration devices.
Figure 17: Full pupil solar diffuser (external side), image credit: Astrium)
Figure 18: The target area of the GOCI instrument (image credit: EADS Astrium SAS)
One of the major advantages of ocean observation with the GOCI is that continuous monitoring is possible with images provided every hour, which maximizes the chance of clear observation of the whole field even in the cloudy season. No sun glint occurs thanks to the angular position of the field of view during daytime, while it discards many observations in low orbit.
Figure 19: Photo of the GOCI flight model during integration (MLI removed), image credit: EADS Astrium SAS 49)
Instrument calibration: KARI performed in-orbit optical performance tests with GOCI L1B imagery. Generally, the performance indicators are GSD (Ground Sample Distance), MTF (Modulation Transfer Function), and SNR (Signal to Noise Ratio) that verify the optical performance of payload. 50)
Each band coverage of GOCI and those estimated SNR value are listed in Table 4. The SNR result is averaged from 4 SNR results which were obtained of the acquired UTC-03 L1B imagery on Jan. 31, Mar. 10, Apr. 5, and May 8, 2011. All results cannot satisfy the GOCI requirements. However, those compared requirements set the high value because the SNR target condition is homogeneous area when testing the SNR value. In addition, the target image in this presentation is L1B data without atmospheric correction so the noise includes the atmospheric effects. Nevertheless, the relative estimated SNR results will be useful to compare the image quality and optical performance of other ocean color sensor in the near future.
Table 4: Estimated SNR comparison according to bands
To cover the predefined target area of 2500 km x 2500 km, a step-and-stare method has been selected. The array size of CMOS FPA chosen for GOCI is not large enough to observe the target area with a GSD of 500 m x 500 m by the staring method. As can be seen in Figure 20, the GOCI takes 16 slots (4/EW x 4/NS) step by step through a scan of pointing mirror. The positions of the pointing mirror for each slot and slot acquisition sequence are commanded by ground. At each slot imaging, the filter wheel is sequentially rotated to achieve 8 spectral images and 2 dark images which are used for offset correction. Each slot image includes a set of 10 spectral images of 1432 x 1415 pixels observed at a certain pointing mirror position. The rotation angle of the pointing mirror is controlled to have an overlap region between adjacent slots enough to avoid any gap in the observation area.
Figure 20: Step & staring imaging method for the GOCI (image credit: EADS Astrium SAS)
Each slot image is sequentially transmitted to the ground after completing each slot imaging. It takes less than 30 minutes to take and transmit a whole image, 16 slot images, from the target area.
MI (Meteorological Imager):
The instrument is an off-the-shelf model of ITT-A/CD [ITT Industries Inc. Aerospace/Communications Division (Fort Wayne, IN, USA)]. Note: The MI is also referred to as CAGI (Commercial Advanced Geo-Imager) as well as “Meteo Imager”. The instrument is a visible and infrared imaging radiometer that measures energy from Earth's surface and atmosphere. The objective of MI observations is to provide a continuous monitoring capability for the near-realtime generation of high-resolution meteorological products - to be used in weather forecasting of severe local storms, floods, yellow sand transport in the atmosphere, as well as for the extraction of data on long-term change analysis of sea surface temperature and cloud coverage.
Figure 21: Illustration of the MI radiometer (image credit: ITT, Ref.8)
The MI radiometer consists of three distinct modules:
• The sensor module attached to the Earth facing side of the spacecraft. The module has zoned, proportional temperature controllers that depend upon built-in heaters and space-viewing radiators.
• The electronics module attached to a heat sink in the spacecraft. Operating temperature limits are - 5 to + 40ºC. A wire harness in the spacecraft connects the sensor and electronics modules.
• The power module installed in the bus, which provides the required voltages to the other modules.
MI is a multispectral two-axis scanning radiometer, and is capable of providing imagery and radiometric information of the Earth's surface and cloud cover over 5 channels. MI scans a scene as large as the full Earth disk every 30 minutes, or regional areas such as 1,000 x 1,000 km (1.6º x 1.6º) in less than 1 minute. MI interleaves frequent, small area scans with a full Earth disk scan to deliver regular global data and evolving events. The addressable FOV is 23º x 21º centered on the 17.4º diameter Earth disk. For a full disk scan, east-to-west scan lines have a length which is sufficient to cover the disk and nearby space. A view to cold space provides a zero-radiance radiometric reference at the end of each scan line, followed by a north-south step. Small area scans may range in size from a pixel to a full disk; 1.6º x 1.6º is an example of what can be achieved.
Table 5: Major MI instrument observation requirements
Table 6: Spectral bands of the MI instrument
The GIFOV (Geometric Instantaneous Field of View) is required to be ≤ 28 µrad (i.e., 1 km pixel size) for the VIS band and ≤ 112 µrad (i.e., 4 km pixel size) for all IR bands. The calibration accuracy for infrared channels is better than 1 K, traceable to NIST (National Institute of Standards and Technology) via thermometry standards. The accuracy in the visible channel is better than 5%, and is traceable to NIST via luminous source standards. MI uses a built-in blackbody with precision thermometry to maintain infrared calibration. Infrared calibration is performed upon command from the ground or automatically. There is also a visible channel stability monitor, accessible by command once a day using the scan system that directs sunlight through all optics to the visible channel detectors. Built-in electrical signals monitor all other properties of the signal path. An auto-zero function of the infrared channel electronics is used to maintain a stable digital number for zero radiance.
Table 7: Performance parameter overview of the MI instrument
The mass of the MI radiometer is 139 kg, the power consumption is < 195 W, data quantization = 10 bit.
Figure 22: Scanning principle of the MI radiometer (image credit: EADS Astrium SAS)
COPS (Ka-band Communication Payload System):
COPS is an onboard demonstration payload for the performance verification and space qualification of advanced communication technologies in the field of wideband multimedia services for the government and the public domain. COPS is being jointly developed by SI (Satrec Initiative) and ETRI (Electronics and Telecommunications Research Institute). 51) 52) 53)
The objectives of COPS call for 3 multibeam functions within the coverage regions of South Korea, North Korea, and the Dongbei region (northeast China). The goal of the Ka-band is to provide communication services in support of natural disasters such as its prediction, prevention, recovery service in the government communication network. In addition, provision of a beam switching function for high-speed multimedia services including the internet via satellite in the public communications network for all coverage regions. COPS provides a bent-pipe type function for the communication services of natural disaster coverage (prediction, prevention, and recovery) within the government communication network of South Korea. The requirements of COPS call for a link availability 99.7% during the service period at BER 10-6 quality.
The COPS system consists of a switching transponder subsystem, supporting the transponder channel in redundancy, and a multibeam deployable antenna subsystem. The receive and transmit antenna employes a linear polarization scheme using vertical polarization for the territory of South Korea, horizontal polarization for the territory of North Korea, and vertical polarization for the region of China covered.
The Ka-band payload provides 100 MHz wide four channels for fixed satellite service. Three channels are assigned for on-board switching for multi-beam connection, and one channel is assigned for bent pipe connection. The uplink is frequency band is 30 GHz and the downlink frequency band is 20 GHz. The multi-beam switching is performed at 3.4 GHz band. Channel allocation and frequency plan of Ka-band payload are shown in Figure 23.
Figure 23: Ka-band payload frequency channels (image credit: ETRI)
As shown in Figure 24, the service coverage for the Ka-band payload system is two regions, named by beam 1, 2 and 3. Beam 1 was assigned to South Korea for national disaster service network and satellite multimedia service network, while beam 2 and 3 will be assigned to the North Korea and the north-east of China, respectively.
Figure 24: Beam coverage of the Ka-band payload (image credit: ETRI)
The Ka-band payload consists of multi beam antenna and on-board switching transponder subsystem, which includes all the necessary microwave hardware in order to receive, switch, amplify and transmit microwave signals within the defined coverage area. The Ka-band payload system was designed to be capable of the communication service function among the individual beams.
The transponder layout is designed according to the RF signal flow as shown in Figure 25. The received signal from the ground goes to the Downconverter through the LNA (Low Noise Amplifier). The down converted signal is controlled by the RF switch matrix and goes to the Upconverter. The up converted signal is amplified through the channel amplifier and TWT (Traveling Wave Tube).
Figure 25: Transponder design layout (image credit: SI)
Table 8: Major requirements of COPS
Table 9: COPS system characteristics
Table 10: Transponder and antenna subsystem performance requirements
Table 11: Specifications of CTES (Communication Test Earth Station) requirements
Figure 26: Isometric views of the transponder panel (image credit: SI)
In the overall architecture of the system, the ground segment features the following elements:
• CDAS (Command and Data Acquisition System) for the services of command transmission, all data reception, tracking and ranging, and data product relay
• MODAC (Meteorological/Ocean Data Application Center) for the dissemination, processing, and archiving functions of the payload data. Distribution of data products. MODAC generates calibrated image data as well as derived products and uplinks them in S-band back to the spacecraft via the MODCS function for regional distribution to the user community. This techniique complies with the international HRIT/LRIT (High Rate and Low Rate Image Transmission) distribution standard of CGMS (Coordination Group of Meteorological Satellites). The processed meteorological and ocean data products are also being distributed to domestic end users through existing ground networks.
• CTES (Communication Test Earth Station) for the Ka-band communication services
• CSMC (Communication System Monitoring Center) for the monitoring of the COPS system and for network control.
Figure 27: Overview of the COMS system architecture
Figure 28: View of the overall COMS-1 system (image credit: KMA, KARI)
Figure 29: The COMS-1 system architecture (image credit: ETRI)
SGCS (Satellite Ground Control System) consists of five subsystems: TTC, ROS, MPS, FDS, and CSS as shown in Figure 30. The FDS (Flight Dynamics Subsystem) operations include spacecraft orbit determination, orbit prediction, event prediction, fuel accounting, station-keeping maneuver planning, and station-relocation maneuver planning. All of the orbit dynamics functions in FDS consider COMS specific twice a day wheel off-loading operations affecting the COMS orbit. FDS also provides COMS operation related functions such as oscillator updating parameter calculation, sensor interference management, and Earth acquisition parameter calculation after emergency sun reacquisition.
The generic flight dynamics operations support includes spacecraft orbit determination, orbit prediction and ephemeris prediction, event prediction, fuel accounting and life prediction, station-keeping maneuver planning, and station-relocation maneuver planning. In general, FDS is a computer-based system, which is comprised of flight dynamics software and computer hardware. FDS in the COMS SGCS include only the functions required for the geostationary orbit spacecraft operations.
Figure 30: Satellite Ground Control System (SGCS), image credit: ETRI
FDS is made up of six major functions and three supporting functions. Six major functions are orbit determination, orbit prediction and ephemeris prediction, event prediction, stationkeeping maneuver, station-relocation maneuver, and fuel estimation. Three supporting functions are system management, database management, and thruster modeling. Figure 31 shows the functional structure of the FDS and its interfaces with TTC, ROS, and MPS.
Figure 31: FDS functional block diagram and interfaces (image credit: ETRI)
Figure 32: Functional allocations of the COMS-1 meteorological processing system (image credit: KMA)
COMS INR (Image Navigation and Registration):
COMS INR, unlike other INR systems that have been used in previously flown geostationary remote sensing satellites, employs a new design approach. The key concept of COMS INR system can be succinctly summarized as the following two aspects (Ref. 34):
1) It is based on “a posteriori” compensation of the geometric error of the image location or the effects of any perturbations on the image geometry (the ‘image motion compensation’) to be performed on-ground.
2) The error compensation is mainly performed by a simple linear interpolation, called the ‘linear error correction’, by estimating (short term) and applying the ‘correction angle’ “CorrAngles(t).”
The adopted approach of pre-processing the GOCI and MI imagery consists in observing perturbations within the images themselves, after acquisition, with respect to contribution models and then correcting those images, by ground processing, utilizing landmarks (from observation within image), attitude measurements (from instrument telemetry or from platform telemetry at a higher rate if needed) and ranging data (orbit data). The image correction that is performed is more on observations than on models prediction, since it uses all available information, including those extracted from the images themselves.
Obviously, this approach depends on landmarks and the availability of landmarks, both in terms of quantity and quality is expected to play a critical part in its function and performance. It is also observed that it was aimed at obtaining the low dependency of INR performances on predictive models and low coupling between on-board and ground. Figure 33 shows the functional chain architecture of the COMS INR system.
Figure 33: Functional chain architecture of the COMS INR system (image credit: KARI, EADS Astrium)
The COMS INR system architecture and the resulting INR ground processing is composed of the following four mains modules, as shown in the Figure 34.
• The Image Observation Data Extraction Module
• The Navigation and Registration Filter Module
• The Image Geometric Correction Module
• The Image Quality Control Module
Figure 34: COMS INR ground processing architecture (KARI, EADS Astrium)
These modules provide functions that are sequenced in order to perform the whole INR processing and to obtain Level 1B data from Level 1A images. At the end of this processing, geometric accuracy is estimated through calculation of residual errors and statistics on landmarks database.
There are many more details of the COMS INR pre-processing architecture beyond the scope of this description (see Ref. 34).
In summary it can be stated, that this new retrospective INR concept employed in COMS, when its final performance is successfully demonstrated at the end of the commissioning phase, could be applied to very diverse observation missions from the geostationary orbit, because of this model independency. The preliminary results have confirmed the fundamental assumptions, and even evidence, in some aspects, that significant margins with respect to the specification allocations could be obtained.
CMDPS (COMS Meteorological Data Processing System):
With the development of the COMS system, NMSC/KMA (National Meteorology Satellite Center/Korea Meteorological Administration) has been focusing on utilizing the COMS observed data and the foreign satellite data as well. For this purposes, NMSC/KMA has been developing the CMDPS within the frame of COMS program. The major functions of the CMDPS are extraction of meteorological products from the calibrated and geo-located level 1B data and development of calibration algorithm for monitoring and upgrading. Sixteen (16) baseline products have been developed as shown in the flow of CMDPS in Figure. 35.
The raw data of MI and GOCI payloads transmitted are being acquired and preprocessed at MSC (Meteorological Satellite Center) and KOSC (Korea Ocean Satellite Center), respectively. The data centers also produce Level 2 products such as meteorological and ocean product from the preprocessed payload image data for further processing. The SOC (Satellite Operation Center) has also backup functions of raw data reception, preprocessing, and LRIT/HRIT dissemination service (Ref. 58).
Figure 36: GDPS (GOCI Data Processing System), image credit: KIOST (Korea Institute of Ocean Science & Technology) 59)
For transmission services to users the preprocessed MI image data and meteorological products are formatted in LRIT/ HRIT and GOCI image data from KOSC are also contained in the LRIT files (Figure 37).
The COMS LRIT/HRIT files are generated and disseminated by MSC. The contents and dissemination schedule of LRIT/HRIT will be controlled in accordance with the KMA data policy.
In the COMS ground system, the LHGS (LRIT/HRIT Generation Subsystem) performs the LRIT/HRIT formatting and transmission from the received data stream via external systems according to the CGMS (Coordination Group for Meteorological Satellites) global specification and the COMS LRIT/HRIT mission specification.
Figure 37: COMS LRIT/HRIT generation/dissemination flow (image credit: KARI)
Figure 38: Processed data distribution coverage (image credit: KMA) 60)
Legend to Figure 39: Clockwise from the left top to the right, are the illustrative examples presented for the individual products: fishing ground information (FGI), the under water visibility (VIS), the primary productivity (PP), and the water current vector (WCV). These products can be produced by GDPS that has been newly upgraded to version 1.2.
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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.