ASTER is an advanced multispectral imager with high spatial, spectral, and radiometric performances for an EOS-AM1 polar orbiting platform which will be launched with four other instruments in June 1998 and covers a wide spectral region from visible to thermal infrared by 14 spectral bands. To meet a wide spectral coverage, optical sensing units of ASTER are separated into three subsystems, that is, visible and near infrared subsystem, a short wave infrared subsystem, and a thermal infrared subsystem, depending on the spectral region from a technical point of view. This ASTER instrument configuration with multi-telescopes leads to necessity of the ground processing on the generation of level-1 data products which are radiometrically calibrated and geometrically registered. The concept of level-1 processing algorithm on the ASTER Ground Data System is described.
ASTER/SWIR is an imager to cover the 1.6 to 2.5 micrometer wavelength region. The spatial resolution is 30 m and the spectral region is divided into 6 bands. The detectors are linear Pt- Si Schottky-barrier diode CCD arrays. In order to cool focal plane assembly down to 80 k with a single cryogenic cooler, all six bands CCD arrays are allocated in parallel on a focal plane, which causes band to band misregistration as a parallax effect of ground level variation. This paper deals with an approach to evaluate the parallax value from the images by means of band to band image position matching and reduce the effect to achieve band to band registration. The approach first finds corresponding points for a pair of images among the six spectral-band images, then interprets the data into an attitude of line of sight and geometric distortion by parallax. By using these data, image distortion can be corrected to eliminate parallax effect of all six bands by a resampling process named FFT-nearest neighbor selection. By applying this method on existing images taken by actual SWIR imager, misregistration for all six bands is reduced to about 1/8 of pixel spacing as root mean square value.
ASTER has three telescopes corresponding to VNIR, SWIR, and TIR. Unfortunately, there will be a band-to-band misregistration among the data of different telescopes, which is caused by pointing inaccuracy of the telescopes. Knowledge of the pointing has an error of several pixels in VNIR unit. So, the registration techniques are needed to get the registered image. In this study, these techniques are investigated. VNIR data is currently considered as reference. Consequently, registration between VNIR and SWIR and the one between VNIR and TIR is calculated and corrected. The primary candidate of the evaluation of the amount of misregistration is image correlation. It will work for VNIR and SWIR registration. But, another approach seems to be needed for the registration between VNIR and TIR, whose registration may or may not have good correlation.
ASTER instrument is a high spatial resolution imager on board the EOS-AM1 platform, which will be launched in mid 1998, and will provide the Earth's surface spectral data in the VNIR, SWIR, and TIR wavelength regions to the science community in the world. ASTER data will be captured by U.S. ground system via TDRSS, processed to level 0 data and then transferred to the ASTER ground data system in Japan for level 1 data products generation within an appropriate timeline. The system will produce approximately 780 scenes (about 80 GByte) per day to meet scientists requirements on data processing, data distribution, and so on. In this paper, key issues of the development of ASTER ground data processing system are pointed out and then a basic concept of its possible implementation is discussed to resolve them, considering COTS technology. Level 1 data product is ASTER standard product and the level 1 data product generation is an essential part of the ASTER data processing system. Discussion concentrates on this part. Finally, the development program of the system also is mentioned.
ASTER is a high resolution optical sensor for observing the Earth in a five-year mission on the EOS AM1 platform to be launched in 1998. ASTER consists of three radiometers. VNIR has three bands in the visible and near-infrared region, SWIR has six bands in the shortwave infrared region, and TIR has five bands in the thermal infrared region. The ASTER project is establishing a calibration plan including calibration requirements to the contractors. The major instrument characteristics specified are spectral characteristics, offset, nonlinearity of response, absolute responsivity, polarization effect and stray light effect. The ASTER pre- flight calibration of VNIR and SWIR adopts the working standard large integrating sphere of 1 m in diameter whose radiance levels are traceable to the primary standard fixed point blackbody. This is similar to the prelaunch calibration system of OPS of JERS-1 launched in 1992. The onboard calibration devices of VNIR and SWIR are halogen lamps and photodiode monitors used once in sixteen days. These calibrators are duplicated and used alternately to increase the reliability. The offsets of VNIR and SWIR are checked by looking at the dark side of the Earth. The TIR is unable to see the dark space. The temperature of the onboard blackbody of TIR remains at 270 K in the short term calibration for the offset calibration, and is varied from 270 K to 340 K in the long term calibration for the offset and gain calibration once in sixteen days. The TIR onboard blackbody is calibrated against a standard blackbody in a vacuum chamber before launch. The standard blackbody has a hood of 330 mm diameter and 600 mm length, the emissivity of more than 0.995 and the temperature range of 100 K to 400 K.
ASTER will be calibrated in the laboratory by reference to sources traceable to NRLM and NIST standards and through the use of transfer radiometers. Partial aperture on-board calibration systems will be used in the solar-reflective range and an on-board blackbody source will be used in the infrared. An important independent source of calibration data will be provided through the in-flight radiometric calibration of ASTER by reference to well- characterized scenes. The latter is the subject of this paper. Methods that make use of ground reflectance and radiance measurements made simultaneously with atmospheric measurements at selected sites and used as input to radiative transfer codes are described. The results of error analyses are presented indicating that, depending on the method used, the predicted uncertainties fall between +/- 2.8% and +/- 4.9%, for the solar-reflective range. In the thermal infrared, the goal is an uncertainty of less than 1 K. A method that provides in-flight cross calibrations with other sensors also is described.
ASTER data has two kinds of misregistrations, (1) inter-telescope misregistration among VNIR, SWIR, and TIR, and (2) intra-telescope misregistration of SWIR. These misregistrations are caused by pointing inaccuracy and different look direction for each band of SWIR. To make registered images, these misregistrations should be corrected. Before the launch of ASTER, we need to establish the correction algorithms for these misregistrations. To evaluate the algorithms, we made the ASTER misregistered simulation data. This data includes the two misregistrations mentioned above. We used NASA/JPL's AVIRIS data for VNIR and SWIR bands, and NASA/JPL's TIMS data for TIR bands. We also used a digital elevation model (DEM) of 15 m grid interval to simulate the misregistration. The misregistrations of the data were corrected by the developers of algorithms, and we evaluated each correction algorithm. As the first step, we have made only the misregistered simulation data. However our goal is to evaluate the entire algorithms of ASTER level-1 processing, including misregistration corrections. So, we will make the ASTER simulation data, which includes other factors, in the future.
ASTER is an advanced high spatial resolution stereo multispectral optical imager selected for flight on the NASA EOS-AM1 spacecraft in 1998. It is being provided by the Ministry of International Trade and Industry of Japan. The EOS-AM1 spacecraft is being developed under the management of Goddard Space Flight Center. ASTER consists of three telescope subsystems: VNIR, SWIR, and TIR and has a broad spectral coverage from the visible to the thermal infrared. The telescopes have ground resolutions (IFOVs) of 15, 30, and 90 meters, respectively. Because of these high spatial resolutions and the need for close registration of the three telescopes and their internal bands, jitter and stability are important factors in achieving satisfactory geometric and radiometric performances in the instrument's science data products. Integrated finite element model/optical analyses of ASTER jitter and stability were performed as part of an overall assessment of the EOS-AM1 instrument/spacecraft system. Significant disturbance sources and mitigation approaches were identified. The modeling techniques and results are described.
The moderate resolution imaging spectroradiometer is a space-based imaging spectroradiometer designed to observe small changes in Earth system processes over long periods of time. The first of several MODIS instruments is scheduled to fly on the Earth Observation System (EOS)-AM spacecraft in 1998. The engineering model for the MODIS is well into build and will be in system test later this year. This paper provides an overview of the MODIS instrument and highlights many of the technical achievements during the engineering model development. Results of subsystem testing of critical assemblies is presented indicating a high probability of success in critical performance areas at the system level.
The MODIS airborne simulator (MAS), a scanning spectrometer built by Daedalus Enterprises for NASA Goddard Space Flight Center and Ames Research Center, is used for measuring reflected solar and emitted thermal radiation in 50 narrowband channels between 0.55 and 14.3 micrometers . The instrument provides multispectral images of outgoing radiation for purposes of developing and validating algorithms for the remote sensing of cloud, aerosol, water vapor, and surface properties from space. Nineteen of the channels on MAS have corresponding spectral channels on the moderate resolution imaging spectroradiometer (MODIS), an instrument being developed for the Earth Observing system (EOS) to be launched in the late 1990s. Flown aboard NASA's ER-2 aircraft, the MAS has a 2.5 mrad instantaneous field of view and scans perpendicular to the aircraft flight track with an angle of +/- 43 degree(s) about nadir. From a nominal ER-2 altitude of 20 km, images have a spatial resolution of 50 m at nadir and a 37 km swath width. We report on the status of the instrument, discuss recent design changes, and provide comparisons with MODIS. We also summarize MAS calibration work, especially efforts to calibrate those channels with strong water vapor absorption.
Over the next five years the Naval Research Laboratory (NRL) will fly a series of ultraviolet satellite instrument packages to measure vertical profiles of atmospheric airglow emission. The objective of this program is to test new techniques for optical remote sensing of the mesosphere, thermosphere, and ionosphere using limb scanning spectrographs. Emphasis will be placed on day- and night-remote sensing of the F-region through measurement of profiles of airglow emission from the O+ ion. Other objectives include remote sensing of vertical profiles of neutral density, minor species and temperature. These observations will be used to study the composition, photochemistry, thermodynamics, and couplings between atmospheric regions. A phased approach will be used which provides for: (1) comprehensive multi-parameter measurements; (2) high spectral resolution studies; and (3) long-term operational observations from DoD weather satellites. The first of these payloads is the multi- sensor experiment called the remote atmospheric & ionospheric detection (RAIDS). RAIDS, a collaboration between NRL and The Aerospace Corporation, contains two spectrographs, three scanning grating spectrometers, and three photometers. Space flight for RAIDS will be provided by the Air Force Space Test Program (STP). The phase 2 component is the high resolution airglow/aurora spectroscopy (HIRAAS) experiment, a collaboration between NRL and the Naval Postgraduate School. HIRAAS will fly aboard the STP ARGOS Satellite in early 1996. The third phase of this program involves flight of a series of five limb scanning instruments called the special sensor ultraviolet limb imager (SSULI) aboard Defense Meteorological Satellite Program weather satellites in the last quarter of this decade. The long- term observations from these satellite experiments will provide a comprehensive database of mesospheric, thermospheric, and ionospheric density profiles from which to search for the effects of global change.
The tropical atmosphere plays a major role in the climate dynamics, through the influence of water vapor and clouds on the radiative budget of the Earth, and the release of latent heat in rainy systems. Present operational satellites do not allow reliable estimations of atmospheric water parameters, because they suffer from inappropriate instrumentation (geosynchronous) or from insufficient sampling (polar orbiters). TRMM is the only scheduled mission devoted to the tropics, but will still provide an insufficient sampling. Small satellites may alleviate these problems by placing several key passive instruments into appropriate orbits. TROPIQUES, studied by CNES, will carry two key instruments, one for radiative budget and one microwave imager. The orbit will have a low inclination (10 to 15 degree(s)), and the swath 2000 km, allowing repetitive cover of the equatorial band. The altitude will be at least 1000 km, due to microwave imager geometry constraints. The resolution will be relatively coarse, 40 km for the radiative budget instrument, 15 to 100 km for the microwave imager. TROPIQUES will be used in synergy with the polar platforms and geosynchronous satellites of the years 2000. It will provide a useful complementary system for the global study of our climate.
There is a growing interest on small satellites for Earth observation and communication services. Nevertheless, these low cost systems have shown a certain number of limitations, they may be overcome through the use of available technology and on-board data handling capabilities. One of the most important limitations is the attitude and orbit determination and control system (AODCS). PoSAT-1, launched in September 1993, has a passive ADCS system, with magnetometers for attitude determination (sun sensors and earth horizon sensors are implemented only for redundancy), a 4.5 m gravity gradient boom and magnetorquers for attitude control. The satellite also carries a star imaging system (SIS) and a GPS receiver for more accurate and standalone attitude and position determination. Both experimental subsystems are working according to specifications. The authors present their current work on an integrated model for AODCS, to be implemented on PoSAT-1, which is based on the SIS the GPS and the parallel nature of on-board transputer processing, and discuss it as a powerful and efficient navigation unit for small and low earth orbiting satellites.
Designers of future remote sensing spacecraft, including platforms for Mission to Planet Earth and small satellites, will be driven to provide spacecraft designs that maximize data return and minimize hardware and operating costs. The attitude determination subsystems of these spacecraft must likewise provide maximum capability and versatility at an affordable price. Hughes Danbury Optical Systems (HDOS) has developed the Model HD-1003 Miniature Star Tracker which combines high accuracy, high reliability and growth margin for `all-stellar' capability in a compact, radiation tolerant design that meets these future spacecraft needs and whose cost is competitive with horizon sensors and digital fine sum sensors. Begun in 1991, our HD-1003 development program has now entered the hardware qualification phase. This paper acquaints spacecraft designers with the design and performance capabilities of the HD- 1003 tracker. We highlight the tracker's unique features which include: (1) Very small size (165 cu. in.). (2) Low weight (7 lbs). (3) Multi-star tracking (6 stars simultaneously). (4) Eighteen arc-sec (3-sigma) accuracy. (5) Growth margin for `all-stellar' attitude reference.
This paper describes the latest results of the performance in space of the experimental star imaging system for attitude determination based on commercially available imaging technology. The device was integrated on PoSAT-1, the first Portuguese satellite, which was launched into sunsynchronous LEO on September 26, 1993. The main features of the system are described, its data processing philosophy, attitude errors, and sensitivity analysis. The most important conclusion thus far is that low cost technology can provide attitude knowledge to within 0.05 degrees, a figure of utmost interest for small and low cost remote sensing spacecrafts.
Stanford University's Department of Aeronautics and Astronautics has commenced full scale development of a new microsatellite initiative. Known as the satellite quick research testbed (SQUIRT) program, the project's goal is to produce student engineered satellites capable of servicing state-of-the-art research payloads on a yearly basis. This program is specifically designed to meet the education and research goals of the department's Satellite Systems Development Laboratory. SQUIRT vehicles are envisioned to consist of a 25 pound, 9 inch tall, 16 inch diameter hexagonal structure with complete processor, communications, power, thermal, and attitude subsystems. These spacecraft cater to low power, volume, and mass research experiments and student developed educational packages. Mission lifetimes of up to one year are considered. Through student participation, voluntary mentoring from the academic and industrial communities, and the extensive use of off-the-shelf components, the cash outlay target for SQUIRT class vehicles is $50,000. This paper discusses the educational and research issues surrounding the development of Stanford's spacecraft design curriculum and the formulation of the SQUIRT program. A technical review of the first SQUIRT satellite, named SAPPHIRE, and an outline of the conceptual plans for other missions is also presented. Additionally, initiatives concerning partner academic institutions and public domain design information are featured.
The Stanford Satellite Systems Development Laboratory (SSDL) is currently investigating advanced satellite control technologies aimed at improved spacecraft system operations. One technique involves a unique real-time payload control strategy for satellites operating within a crosslinked constellation. This strategy effectively merges payload scheduling, traditionally considered a high level planning problem, with low level actuator control. The developed method utilizes attractive artificial potential fields of varying strength to exert `planning' forces on the payload. As the payload nears a particular goal, it is critically damped to that specific position. This approach has been simulated for the simple repositioning of a mechanically actuated space telescope. Applied to constellation operations, this strategy provides the basis for simple, efficient, and robust satellite cooperation in dynamic environments. This paper describes the strategy under consideration and discusses preliminary plans for its flight testing through the use of a small constellation of satellite quick research testbed (SQUIRT) micro satellites.
There is concern that the exploitation of Earth observation data in Europe is not progressing sufficiently quickly towards operational status. Although Earth observation data have been available for approximately 20 years, financial and programmatic responsibility has not been transferred from the national and international space agencies to users, except in very limited cases. As this is one long term goal, it is important to assess what is required by European programs to achieve operational status. This must be considered within the broader international environment where an increasing number of small satellite missions are being proposed. Although small satellites can be considered as a response to shrinking budgets, they are also more consistent with some user requirements and have the potential to play an important role in the development of operational capabilities.
The miniature sensor technology integration (MSTI) program sponsored by the United States Department of Defense (DoD) exploits advances in sensor and small satellite bus technology for theater and national missile defense. MSTI-1 and MSTI-2 were used to demonstrate the capability of the common bus and to build up the integration and management infrastructure to allow for `faster, better, cheaper' missions. MSTI-3 is the newest of the MSTI series and the first to fully exploit the developed infrastructure. Given the foundation laid down by MSTI-1 and MSTI-2, MSTI-3's mission is totally science-driven and demonstrates the quality of science possible from a small satellite in low earth orbit. The MSTI-3 satellite will achieve bus and payload performance historically attributable only to much larger satellites -- while maintaining the cost and schedule advantages inherent in small systems. The MSTI program illustrates the paradigm shift that is beginning to occur and has the mantra: `faster, better, cheaper.' The disciples of smallsat technology have adopted this mantra as a goal -- whereas the MSTI program is demonstrating its reality. The new paradigm illustrated by MSTI-3 bases its foundation on a development philosophy coined the `Three Golden Truths of Small Satellites.' First, bus and payload performance do not need to be sacrificed by a smallsat. Second, big science can be done with a smallsat. And third, a quick timeline minimizes budget exposure and increases the likelihood of a hardware program as opposed to a paper study. These themes are elaborated using MSTI-3 as an example of the tremendous potential small satellites have for making space science more affordable and accessible to a large science community.
The U.S. Department of Defense (DoD) and the National Aeronautics and Space Administration (NASA) started a cooperative program in 1992 to flight qualify recently developed lightweight technologies in a radiation stressed environment. The spacecraft, referred to as Clementine, was designed, built, and launched in less than a two year period. The spacecraft was launched into a high inclination orbit from Vandenburg Air Force Base in California on a Titan IIG launch vehicle in January 1994. The spacecraft was injected into a 420 by 3000 km orbit around the Moon and remained there for over two months. Unfortunately, after successfully completing the Lunar phase of the mission, a software malfunction prevented the accomplishment of the near-Earth asteroid (NEA) phase. Some of the technologies incorporated in the Clementine spacecraft include: a 370 gram, 7 watt star tracker camera; a 500 gram, 6 watt, UV/Vis camera; a 1600 gram, 30 watt Indium Antimonide focal plane array NIR camera; a 1650 gram, 30 watt, Mercury Cadmium Telluride LWIR camera; a LIDAR camera which consists of a Nd:YAG diode pumped laser for ranging and an intensified photocathode charge-coupled detector for imaging. The scientific results of the mission will be first analyzed by a NASA selected team, and then will be available to the entire community.
As a payload of KITSAT-2, also known as KO-25, the CCD earth imaging system is equipped with a single chip color CCD camera system as well as a monochrome camera system. The black and white narrow angle camera has a mean ground pixel resolution of about 200 m and the color wide angle camera has a meteorological mean ground pixel resolution of 2 km. Brief test results of the narrow angle camera are presented. The paper also presents image processing activities, including image correction and the vegetation index comparison of narrow images with readily available satellite data such as AVHRR. General information on the color imager is also given.
EOS-AM1 is the first element of NASA's Earth Observing System (EOS). The primary goal of EOS, which serves as the centerpiece of Mission to Planet Earth (MTPE), is to provide satellite observations to determine the extent, causes, and regional consequences of global climate change. The EOS series of spacecraft will provide continuous, well calibrated data sets over a period of fifteen years. The EOS-AM1 instrument complement is tailored to the characterization of terrestrial and oceanic surfaces; clouds, radiation, and aerosols; and the earth's radiative balance. In addition, vertical profiles of important tropospheric greenhouse gases, the contribution of volcanoes to climate, and ocean primary productivity will be measured. The payload consists of five advanced facility and principal investigator (PI) instruments: advanced spaceborne thermal emission and reflection radiometer (ASTER), clouds and earth's radiant energy system (CERES), multi-angle imaging spectroradiometer (MISR), moderate resolution imaging spectroradiometer (MODIS), and measurements of pollution in the troposphere (MOPITT). These instruments are being provided by the Ministry of International Trade and Industry of Japan, Langley Research Center, Jet Propulsion Laboratory, Goddard Space Flight Center, and the Canadian Space Agency, respectively. The project is currently in its C/D phase and is on-track in its development for a June 1998 launch. The EOS-AM project is managed by Goddard Space Flight Center.
Arianespace introduced a launch capability for small payloads in the late 1980s that has resulted in the successful launch of 20 small satellites. This launch capability, known as the Ariane Structure for auxiliary payloads (ASAP) has been used on five Ariane IV launches since 1990. ASAP consists of a mounting ring located below the primary payload on which as many as six small satellites can be mounted. Each satellite is limited to a maximum weight of 50 kg. ASAP has provided invaluable assistance to the small satellite industry through the introduction of a readily available, low cost means of putting small scientific and commercial payloads into orbit.