FormoSat-2 / ROCSat-2 (Republic of China Satellite-2)
ROCSat-2 is an NSPO (National Space Program Office) of Taiwan Earth imaging satellite with the objective to collect high-resolution panchromatic (2 m) and multispectral (8 m) imagery for a great variety of applications such as in land use, agriculture and forestry, environmental monitoring, natural disaster evaluation, and in support of research interests, in particular with the ISUAL instrument. Daily image coverage of Taiwan and the surrounding region is required.
Background: A contract was signed in May 1999 between NSPO and DASA/DSS (Dornier Satelliten Systeme GmbH) of Germany to build a high-resolution optical imaging satellite. However, the German government refused to give DASA/DSS an export licence for the S/C (the People's Republic of China was protesting the deal). The stalemate was resolved in Dec. 1999 when NSPO signed a new contract with MMS (now Astrium SAS of France). Quick approval of the export of ROCSat-2 was provided by the French government. The ROCSat program is part of a long-term effort in Taiwan to develop an autonomous space capability.
Note1: A public naming competition took place in Taiwan in 2004 with regard to the ROCSat satellite program. At the end of this contest, the ROCSat program was given the new name of FormoSat in December 2004. Hence, ROCSat-2 became FormoSat-2.
Note 2: In 2005, the National Space Program Office was renamed to “National Space Organization”, keeping the same acronym, NSPO.
Figure 1: Artist's view of the FormoSat-2 spacecraft in orbit (image credit: NSPO) 1)
The spacecraft bus has been built by EADS Astrium SAS (prime contractor) of Vélizy, France, based on the Leostar 500 XO family. There were also contributions from Taiwanese industry (including satellite computers, S-band antennas, and sun sensors). The S/C structure consists basically of a hexagonal body of 1.6 m side length (diameter), 2.4 m in height.
The satellite is three-axis stabilized. The upper deck of the platform carries the payload (RSI and ISUAL) and also part of AOCS (Attitude and Orbit Control Subsystem), namely the star sensors, gyroscopes and an IRU (Inertial Reference Unit). The lower deck carries the four reaction wheels and the autonomous propulsion module. The pointing accuracy is < 0.7 km (0.12º); the position knowledge is < 70 m (0.02º).
The fixed solar array uses GaAs cells and consists of two deployable flaps. The entire S/C architecture is designed in such a way as to provide a low roll inertia, a key factor for satellite agility and instrument line-of-sight stability. The S/C provides a body-pointing capability of ±45º in roll and pitch (45º pitch in 60 s, 10º roll in 25 s, 30º roll in 60 s, respectively). The spacecraft wet mass is about 760 kg including 81 kg of propellant (N2H4) mass. The design life is five years or better. 2) 3) 4) 5) 6)
Figure 2: Illustration of FormoSat-2 (image credit: NSPO)
Figure 3: Illustration of FormoSat-2 spacecraft during prelaunch activiies (image credit: NSPO)
Figure 4: Schematic bus design of ROCSat-2 (image credit: NSPO)
Table 1: Overview of performance parameters of the ROCSat-2 spacecraft
Figure 5: Block diagram of the CDMU (Command and Data Management Unit), image credit: NSPO (Ref. 6)
Domestic components of FormoSat-2:
• OMU (Onboard Management Unit), developed by Acer Inc. OMU is the satellite's central control unit and the core of the secondary system for central commands and data processing. The objective of OMU is to control the satellite's operations, including the processing and execution of uploaded commands and the collection, processing, storage, and downloading the data of satellite's status. 7)
Figure 6: Illustration of OMU (image credit: NSPO)
• Onboard flight software module, developed by Tatung System Technologies Corp. The objective is to manage and control the satellite orbit, position, and operation. The flight software is responsible for the calculation of satellite position/orbit, operation of scientific instrument payload, and communication with ground antenna.
• Sun sensor, developed by Shihlin Electric & Engineering Corporation. The objective is to determine the spacecraft's relative position to the sun according to the direction of the source of sunbeam.
Figure 7: Illustration of the sun sensor (image credit: NSPO)
• S-band antennas, designed and manufactured by Victory Industrial Corp. These antennas are responsible for receiving commands and transmitting microwave signals. They allow the uploading and downloading of the ground commands and satellite information.
Figure 8: View of the S-band antennas (image credit: NSPO)
• Flight harness, designed and manufactured by Aerospace Industrial Development Corp. The harness wirings are used for connecting satellite components and payload and for transmitting power, commands, and signals.
• Vertical installation dolly, designed and manufactured by Taiwan Aerospace Corp. It is the main loading equipment for the satellite integration and testing procedures. It has anti-shock and anti-tilting functions and can prevent people from approaching too close to the dolly.
• Satellite primary structure, designed, analyzed and tested by NSPO and manufactured by Aerospace Industrial Development Corp. and Taiwan Aerospace Corp. The satellite primary structure carries the different satellite components and payloads.
Figure 9: Alternate view of the FormoSat-2 spacecraft (image credit: EADS Astrium)
Launch: A launch of ROCSat-2 took place on May 20, 2004 (UTC) on a Taurus-XL vehicle of OSC (Orbital Sciences Corporation) from VAFB, CA (maiden flight of Taurus-XL configuration which offers greater lift capability compared to previous versions of the Taurus rocket).
Orbit: Sun-synchronous circular orbit, mean altitude = 888 km, inclination = 99.1º, period of 102.9 minutes, the LTDN (Local Time of Descending Node) is 9:26 hours (14 orbits/day). The agility of the spacecraft provides a daily revisit capability for event/disaster monitoring.
Note: Following the early-orbit checkout, the initial satellite orbit has been raised from 728 km to 891 km altitude in the period May 23 to June 2, 2004. A total of 32 burns were performed by the propulsion module with 4 burns for inclination change, to enter into the mission orbit with an altitude of 891 km and an inclination of 99.14º.
Figure 10: Schematic for mission operations during various orbital phases (image credit: NSPO)
• 2014: The FormoSat-2 mission is operating nominally. On May 21, 2014, FormoSat-2 will have been on-orbit for 10 years, with continuous monitoring for Taiwan and the world (the design life of 5 years has been doubled). 8)
- The remaining on-board propellant is 27 kg, which is sufficient for orbit maintenance of another 10 years.
- FormoSat-2 is currently in a good state of health, as investigations of the trending data of each subsystem verified (including RSI, C&DH, TT&C, Power, Thermal, Propulsion, and AOCS). The spacecraft experienced malfunctions of one gyro and one reaction wheel.
• Fall 2013: The FormoSat-2 satellite has been operated on orbit for more than 9 years (design life of 5 years). The RSI instrument is operated in the daytime (sunlit period), while the ISUAL is active during the nighttime (eclipse period) of the orbit. 9)
To meet both mission objectives and to accomplish conflict-free planning and scheduling, the mission operations have been more complicated than traditional dedicated Earth remote sensing satellites. For instance, the usage of the satellite’s limited onboard resources, especially the SSR (Solid State Recorder) and the power status, needs to be fully cross-checked before a command load generation for either mission objective of payload.
- Malfunction of the IRU (Inertial Reference Unit) and noise of gyro A: The IRU contains 4 gyros, 2 processing units and 2 power supplies; its configuration is illustrated in Figure 11. Gyro input axes are arranged in a skewed redundant configuration such that any of three of four gyro axes provides a full set of three-axis data, i.e., the gyros works in a 3 among 4 cold redundancies for the inertial attitude estimation purpose. Two processing units and two power supplies (PSs) are also redundant, i.e., only one for each is used in operation. In summary, the IRU features four gyros in a tetrahedron configuration, with a full redundancy for power supply and electronics.
The IRU anomaly was first found on February 7, 2006, the time was not yet the completion of 2 years on orbit operation. The configuration of using PS2/CPU2 and gyro A/B/C has been operated smoothly for about seven months. Unfortunately, the 2nd IRU malfunction happened in September 2006. This anomaly led to the 2nd IRU reconfiguration with gyro A off. From that time, the IRU was configured to use PS2/CPU2 and gyro B/C/D. Thanks to the full redundant considerations and cross-strap design, the IRU could be used without degrading its functions and performances even undergone these two malfunctions.
Figure 11: IRU configuration (gyro 1/2/3/4 equals A/B/C/D), image credit: NSPO
- Malfunction of the RW (Reaction Wheel) interface (Figure 12): The design concept of the reaction wheels in FormoSat-2 is to use a cluster of four wheels in hot redundancy, in order to take advantage of the degree of freedom to avoid wheel rate zero-crossing. The RWA (Reaction Wheel Assembly) consists of one RW and a single channel WDE (Wheel Drive Electronics).
The RW malfunction occurred in November 2009, while FormoSat-2 has been operated for five and a half years, exceeding design life around 6 months. The project managed to find a solution for this problem.
Figure 12: Schematic view of the 4 RW interfaces (image credit: NSPO)
- For more than 9 years of fully engaged operations, the FormoSat-2 spacecraft has encountered many anomalies, including 43 AROs (Automatic Reconfiguration Orders) and lots of error events. Most of the anomalies were transient incidents and the satellite could be switched back to normal situation shortly without interrupting the routine operations. The only permanent failures were the IRU malfunction and gyro-A noise and interface malfunction between RW3 and TIF (Telecommand Interface) board. However, since the failure of IRU/gyro was replaced with spares, the handicapped three-wheel configuration with the degraded maneuver capability did cause significant impacts on the mission operations.
Except for the two permanent malfunctions and three-wheel induced operations impacts, the FormoSat-2 spacecraft behaved quite well. It is expected to be operated on orbit for the next decade under such an experienced mission operations team (Ref. 9).
• The FormoSat-2 spacecraft and its payload are operating nominally in the summer of 2012 - in its 9th year on orbit - providing imagery for the domestic needs of Taiwan as well as for the international community. Astrium GEO-Information Services is the sole distributor of the Formosat-2 satellite’s imagery outside of Taiwan.
• On March 11, 2011, an earthquake of magnitude 9.0 struck Japan, and the resulting 10 m high tsunami swept the country’s northeast coast within 15 minutes. The United Nations Platform for Space-based Information for Disaster Management and Emergency Response started the International Charter on Space and Major Disasters to coordinate the satellites of different countries in acquiring images of disaster affected areas. 11) 12)
Figure 13: Quick look images of three strips of Sendai, Fukushima, and Ichihara on March 13, 2011 (image credit: NSPO)
FormoSat-2 responded rapidly to Japan's needs of information about the disaster-stricken regions. Once the first image of the affected area after the disaster was acquired and downloaded at noon on March 12, it was processed within 2 hours and published on GEMDAS (Global Environment Monitoring and Disaster Assessment System).
The role that GEMDAS played in the Japanese disaster can be attributed to three factors. The daily-revisit orbit and high agility of FormoSat-2, the automatic image processing system of the satellite (F2 AIPS), and the web-based geospatial information system used to make the images public all contributed to the effective disaster response (Ref. 11).
• The FormoSat-2 spacecraft and its payload are operating nominally in 2011. Although the mission is beyond its design life of 5 years, the health of the spacecraft is such that NSPO expects continued observations until the follow-on mission is launched.
Note: The FormoSat-2 mission is to be followed by the FormoSat-5 mission of NSPO with a launch scheduled for 2014. The PDR (Preliminary Design Review) of FormoSat-5 was completed on Dec. 16, 2010. 13)
Figure 14: FormoSat-2 satellite image showing in natural colors the Chuquicamata Copper Mine, Chile (image credit: NSPO, Astrium) 14)
• The spacecraft and its payload are operating nominally in the fall of 2010. The satellite successfully complements existing high spatial resolution imaging satellites such as SPOT-5, IKONOS, and QuickBird, among others, with its unique capability of daily revisits worldwide. FormoSat-2 has now been operated beyond its mission lifetime of five years and is in its extended mission phase. 15) 16) 17)
- Besides providing imagery for the domestic needs of Taiwan, the spacecraft is frequently being used to deliver high-resolution imagery for event monitoring applications. For the recent large disasters in the world, like southern Asia tsunami, Sichuan earthquake, typhoon Morakot over Taiwan, and Chile earthquake, FormoSat-2 took the first images almost every time, and provided the intensive monitoring images to domestic and international organizations for aftermath relief.
- Example of spacecraft agility: When the Sichuan earthquake occurred on May 12, 2008, FormoSat-2 was able to take imagery over the disaster area during its first pass; this was followed immediately by a +30º roll maneuver to dump the data to the NSPO ground station nearly 3000 km away in real-time. The agility of the spacecraft is in particular ideal for disaster monitoring support by shortening the data transfer time significantly.
- In the first five years of operation, ISUAL (Imager of Sprites and Upper Atmospheric Lightning) recorded more than 12,000 TLE events. Among them, ELVES (Emissions of Light and Very Low Frequency Perturbations from Electromagnetic Pulse Sources) are found to be the most abundant type (~80%) of TLEs (transient luminous events), whereas sprites and halos only combine to account for ~20%. The distribution of sprites and halos closely resemble that of the cloud-to-ground lightning. However, nearly 60% of ELVEs occurred over the ocean, a feature indicating that the high peak current lightning is more abundant over the ocean (Ref. 15).
• After 5 years of operations, the spacecraft is still in a generally very good state-of-health condition and is expected to provide its services for several years (3 to 5) to come. 18)
• In Feb. 2009, NSPO issued a RFP (Request for Proposal) for the FormoSat-5 Remote Sensing Instrument (RSI) to provide continuity of observation data. FormoSat-5 is planned for launch in 2013. 19)
• The spacecraft and its payload are operating nominally as of 2009. So far, the FormoSat-2 imagery has also been used in many applications on a global scale, such as disaster monitoring, land-use (change) detection and environmental monitoring.
• When the Sichuan Province of southwest China was hit by an earthquake of magnitude 7.9 on May 12, 2008, FormoSat-2 was among the first providing its imagery of the stricken region to the Chinese Academy of Sciences. 20)
• In the timeframe 2007/8, the high-resolution imagery of FormoSat-2 was being used by the government of Brazil to plan field interventions to stop illegal deforestation in progress in the Amazon basin. 21)
• In 2006, the IRU power supply failed and the behavior of gyro-A turned out to be rather noisy. Due to these events, the project was forced to use the redundant devices. Thanks to the full redundant design of the spacecraft operations could be continued (Ref. 15).
• In May 2006, NSPO implemented a F2T (FormoSat-2 Terminal) in Svalbard, Norway, to extend the data acquisition capability of the mission.
SOH (State-of-Health) trending during two years of mission operations (May 2004-May 2006) on five major subsystems, including AOCS and EPS (Electric Propulsion Subsystem), and one major payload have been conducted. All parameters were well within specifications. The C&DH and the TT&C do not exhibit any SOH problems so far. 22)
• In April 2005, NSPO implemented a terminal in Kiruna, Sweden, to serve as an additional receiving station.
• The high-resolution FormoSat-2 imagery was used extensively to monitor the devastation on the coasts of south-east Asia caused by the Tsunami that struck the Indian Ocean region on Dec. 26, 2004 (the epicenter of the undersea earthquake was off the coast of Sumatra, Indonesia - killing more than 225,000 people in 11 countries; in retrospect the earthquake was referred to as the “Great Sumatra-Andaman earthquake”). The imagery of FormoSat-2 proved extremely helpful to disaster relief activities and area reform tasks.
• Mission operations started in June 2004 (the checkout and performance verification for satellite bus and RSI were started on May 21 and completed through June 2004; all performance requirements of FormoSat-2 had been successfully verified in orbit). Besides providing imagery for the domestic needs of Taiwan, the S/C is frequently being used to deliver high-resolution imagery for event monitoring (coverage of the Tsunami in Asia on Dec. 26, 2004 and thereafter, coverage of Hurricane Katrina in Aug. 2005, coverage of Typhoons on the Pacific, coverage of earthquake regions, etc.). The spacecraft is operating nominally as of 2006. 23) 24) 25) 26)
Figure 15: Panchromatic image of Kao Hsiung, Taiwan observed in July 2004 (image credit: SPOT Image)
• NSPO has contracted to SPOT Image S. A. for the international distribution of FormoSat-2 images since June 2004.
• On 4 July 2004, ISUAL successfully observed the first images of sprites, sprite halo, and elves.
• It took 3 days to check the performance and function of all subsystems of satellite bus, and 11 days to raise the satellite from 723 km parking orbit to 888 km mission orbit. Then the primary payload, a remote sensing instrument (RSI), was turned on and the first image was taken on 4 June 2004 (Ref. 26).
Sensor complement: (RSI, ISUAL)
Generally, the RSI is operated during the daytime (requiring sun illumination) while the ISUAL instrument is active during the nighttime phase of the orbit. Without any payload activities during an orbit, the spacecraft is sun-pointing by default to maximize its energy. The spacecraft attitude during the eclipse phase is geocentric (nadir pointing) for RSI thermal regulation and for communication coverage purposes. The mode transition from sunlit to eclipse orbit takes place automatically. For RSI observations, the attitude pointing is geocentric by default.
RSI (Remote Sensing Instrument):
RSI is a pushbroom-type imager built by EADS Astrium SAS, France. RSI is made up of the camera and IPU (Instrument Processing Unit). The camera itself consists of the optical subassembly, the secondary structure, and the FPA (Focal Plane Assembly). The Korsch telescope (mirrors & structure) design and the focal plane structure are being made of SiC (Silicon Carbide). The video electronics, sequencer, DC/DC converter, and compression cards comprise the IPU. 27) 28) 29) 30)
Table 2: Performance characteristics of RSI
RSI utilizes the agility of the satellite bus for stereo imaging over a specific region, continuous imaging over a slender region, and mosaic imaging over a large region. The imaging capability is 8 minutes per orbit, and the imaging areas during one cycle can be one 3000 km x 24 km continuous strip, two 100 km x 24 km stereo pairs, four 100 km x 24 km strips, or eight scenes.
Figure 16: Schematic structure of RSI (image credit: NSPO)
In-flight radiometric calibration: Although the RSI instrument was calibrated on-ground prior to launch with an uncertainty of 5%, the in-flight calibration has to be done regularly with the pre-flight calibration data as reference. The in-flight calibration includes two major parts: absolute and relative radiometric calibrations. Use of test sites in the ground segment. NSPO and CNES are using the following test sites: La Crau test site (43.56ºN, 4.86ºE) and desert sites Libya 1 (24.42ºN, 13.35ºE), Libya 2 (25.05ºN, 20.48ºE), Libya 3 (23.15ºN, 23.1ºE) and Mauritania 2 (20.85ºN, 8.78ºW), respectively. These sites are selected because of their spatial uniformity and temporal stability of the surface reflectance. 31) 32)
ISUAL (Imager of Sprites and Upper Atmospheric Lightning):
ISUAL is a joint international research program of NSPO, UCB (University of California at Berkeley), National Cheng Kung University of Taiwan, and Tohoku University, Japan. The objective is to observe the natural upward lightning discharge phenomena toward the ionosphere on top of the troposphere, referred to as TLEs (Transient Luminous Events). The requirements call for: 33) 34) 35)
• To determine the location and timing of luminous phenomena above thunder clouds to investigate their spatial, temporal and spectral properties
• To obtain a global survey of upper atmospheric optical flash transients (sprites, elves, blue jets, gigantic jets, etc.).
The instrument consists of four elements: a) intensified sprite imager, b) a six-channel spectrophotometer, c) a two-channel array photometer, and d) an electronics package. The imager is a staring-type frame CCD camera, taking 180 frames/s with a resolution of 512 x 80 pixels in a FOV of 20º x 3.15º. In Figure 18, the large disk of the ISUAL instrument contains six color filters, which can be rotated to select wavelength for measuring the emission spectrum of red sprites. ISUAL is operated in three modes:
• The sprite continuous mode to take images with a sample rate of 100 Hz
• The sprite burst mode with a sample rate of 1000 Hz
• The auroral mode at a constant rate of 1 sample/s.
All the instruments are mounted on a common platform and boresighted in the same direction. The imager is equipped with a six-filter wheel. ISUAL monitoring occurs during each nightside pass. The source data are stored and compressed in a separate 128 MByte memory. The data are downlinked in S-band. The total data volume is about 1 Gbit/day.
Figure 17: Functional block diagram of the ISUAL instrument (image credit: UCB/SSL)
The selection of filters are:
1) 623-750 nm - N2 1st positive band filter for observing sprites, removing the lightning induced at 777.4 nm line and minimizing the contribution of the airglow in the 760 nm O2 band
2) 762 nm - O2 (0,0) atmospheric band for observing airglows and auroras
3) 427.8 nm - energetic electron induced emissions of sprites and auroras
4) 630 nm - aurora and airglow emissions
5) 557.7 nm - aurora and airglow emissions
6) blank filter with IR blocking by the camera lens.
The photometers are fitted with the following filters:
1) 150-280 nm - bandpass ultraviolet filter to look at N2 LBH bands
2) 250-390 nm - bandpass ultraviolet filter; for sprites emission in this range with attenuating lightning contamination
3) 337.0 nm - N2 2nd positive narrow band filter to act as a particle energy spectrometer
4) 427.8 nm - narrow band filter to function as an energetic electron detector in sprites and aurora
5) 623-750 nm - bandpass filter to detect N2 1st positive emission from sprites
6) 777.4 nm - narrow band filter to detect lightning and act as a possible TLE event trigger.
The two array photometers are identical in every aspect, except one fitted with a blue band filter and another equipped with a red band filter. The wavelength selections for the two array photometers are 360-450 nm and 525-880 nm, respectively.
Figure 18: Illustration of the ISUAL instrument with sprite imager (left) and spectrophotometer at right (image credit: NSPO)
The imager is designed to capture five images in quick sequence. The imager operates continuously and five data frames are captured when the photometer signals the presence of a flash event in the field of view. This method of operation obtains high temporal resolution framing of the image.
Figure 19: Schematic view of the imager with filter wheel (image credit: UCB/SSL)
Figure 20: Imager optics of sensor layout (image credit: UCB/SSL)
Figure 21: Spectrophotometer sensor head (image credit: UCB/SSL)
Figure 22: TLEs generated by the upward discharge from thundercloud to the ionosphere (image credit: V. P. Pasko, 2007)
Status of ISUAL
• In 2014, FORMOSAT-2’s scientific payload ISUAL is still operational (Ref. 8).
• During the period July 4, 2004 to May 31, 2013, ISUAL recorded 31,369 TLEs (Transient Luminous Events); of this total sum, there were the following classes of TLEs according to their appearances: 73.65% elves, 6.53% red sprites, 6.00% halos, 13.51% blue jets, and 0.30% gigantic jets. 36)
Due to the orbital characteristics of FORMOSAT-2, unobserved gaps exist between each two orbits. Also, the ISUAL has its detection threshold. An estimated global occurrence rate is 24 TLEs per minute, but this number is still much less than the possible potential rate of 195.
Table 3: Number of TLEs observed by ISUAL from 4 July 2004 to 31 May 2013
Table 4: Accumulated observation orbits and hours of ISUAL from 4 July 2004 to 31 May 2013
Figure 23: Global distribution of 31,369 TLEs observed by ISUAL from 4 July 2004 to 31 May 2013 (image credit: NSPO, Ref. 36)
Figure 24: The first Sprite image captured by ISUAL on July 4, 2004 over the Philippines (image credit: NSPO, Ref. 24)
The basic elements of the ROCSat-2 ground segment are the SOCC (Spacecraft Operations and Control Center) and the XAS (X-band Acquisition System) located in Hsinchu, Taiwan. SOCC in turn consists of MOC (Mission Operations Center), MCC (Mission Control Center), SCC (Science Control Center), FDF (Flight Dynamics Facility), and GCN (Ground Communications Network). FormoSat-2 X-band imagery reception is also made available to third parties (international partners) with their own ground stations through cooperative agreements. 37) 38)
Figure 25: Major elements of the FormoSat-2 system architecture (image credit: NSPO, Ref. 5)
Figure 26: Architecture of the imaging processing system (image credit: NSPO)
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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.