Minimize Terra Mission

Terra Mission (EOS/AM-1)

Terra (formerly known as EOS/AM-1) is a joint Earth observing mission within NASA's ESE (Earth Science Enterprise) program between the United States, Japan, and Canada. The US provided the spacecraft, the launch, and three instruments developed by NASA (CERES, MISR, MODIS). Japan provided ASTER and Canada MOPITT. The Terra spacecraft is considered the flagship of NASA's EOS (Earth Observing Satellite) program. In February 1999, the EOS/AM-1 satellite was renamed by NASA to “Terra”. 1) 2) 3) 4)

The objective of the mission is to obtain information about the physical and radiative properties of clouds (ASTER, CERES, MISR, MODIS); air-land and air-sea exchanges of energy, carbon, and water (ASTER, MISR, MODIS); measurements of trace gases (MOPITT); and volcanology (ASTER, MISR, MODIS). The science objectives are:

• To provide the first global and seasonal measurements of the Earth system, including such critical functions as biological productivity of the land and oceans, snow and ice, surface temperature, clouds, water vapor, and land cover;

• To improve the ability to detect human impacts on the Earth system and climate, identify the “fingerprint” of human activity on climate, and predict climate change by using the new global observations in climate models;

• To help develop technologies for disaster prediction, characterization, and risk reduction from wildfires, volcanoes, floods, and droughts

• To start long-term monitoring of global climate change and environmental change.

Complemented by aircraft and ground-based measurements, Terra data will enable scientists to distinguish between natural and human-induced changes.

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Figure 1: Illustration of the Terra spacecraft (image credit: NASA)

Spacecraft:

Terra consists of a spacecraft bus built by Lockheed Martin Missiles and Space (LMMS) in Valley Forge, PA. The spacecraft is constructed with a truss-like primary structure built of graphite-epoxy tubular members. This lightweight structure provides the strength and stiffness needed to support the spacecraft throughout its various mission phases. The zenith face of the spacecraft is populated with equipment modules (EMs) housing the various spacecraft bus components. The EMs are sized and partitioned to facilitate pre-launch integration and test of the spacecraft.

EPS (Electrical Power Subsystem): A large single-wing solar array (size of 9 m x 5 m = 45 m2), deployed on the sunlit side of the spacecraft, maximizes both its power generation capability and the cold-space FOV (Field of View) available to instrument and equipment module radiators. The average power of the satellite is 2.53 kW provided by a GaAs/Ge solar array (max of 7.5 kW @ 120 V at BOL). The solar array is based on on a prototype lightweight flexible blanket solar array technology developed by TRW (use of single-junction GaAs/Ge photovoltaics). A coilable mast is used for the deployment of the solar array. The Terra spacecraft represents the first orbiting application of a 120 VDC high voltage spacecraft electrical power system implemented by NASA. A PDU (Power Distribution Unit) has been designed to provide 120 DC (±4%) under any load conditions. This regulated voltage, in turn, is achieved via a sequential shunt unit (SSU) and the 2 BCDUs. A NiH2 (nickel hydrogen) battery is used (54 cells series connected) to provide power during eclipse phases of the orbit. 5) 6) 7)

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Figure 2: Coilable mast deployer for the Terra solar array (image credit: NASA)

GN&C (Guidance Navigation and Control) subsystem: Terra is a three-axis stabilized design with a single rotating solar array. The GN&C subsystem is made up of sensors, actuators, an ACE (Attitude Control Electronics) unit, and software. A three-channel IRU (Inertial Reference Unit) determines body rates in all control modes. Solid-state star trackers provide fine attitude updates, processed by a Kalman filter to maintain precise 3-axis inertial knowledge. A 3-axis magnetometer senses the Earth's geomagnetic field, primarily for magnetic unloading of reaction wheels, but also as a sensor to determine an attitude failure during a deep space calibration maneuver. 8)

The backup sensors include an ESA (Earth Sensor Assembly) for roll and pitch sensing, and coarse sun sensors for pitch and yaw sensing of the sun line relative to the solar array. A fine sun sensor is used in the event that one star tracker fails or during the backup stellar acquisition mode. In addition to these sensors, a gyro-compassing computation is performed for backup yaw attitude determination.

A reaction wheel assembly provides primary attitude control. During normal mode, a wheel speed controller is available to bias the wheel speeds at a range that avoids zero rpm crossings (stagnation point). Magnetic torquer rods regulate the wheel momentum to < 25% capacity in four-wheel mode and < 50% capacity in the three-wheel mode (backup mode). Thrusters are used for attitude control during all velocity change maneuvers and for backup attitude control and wheel momentum unloading.

GN&C is a fault-tolerant system that includes an FDIR (Fault Detection, Isolation and Recovery) capability unique to each of the different operational control modes. If an attitude fault is detected, FDIR transfers all control functions to the ACE unit configured to use all redundant hardware. Once in safe mode, FDIR is disabled.

Sensor component

Units

Manufacturer/model

Mission heritage

Solid State Star Tracker (SSST)

2

BATC / CT-601

MSX, XTE

Earth Sensor Assembly) (ESA)

2

Ithaco / conical scanning

UARS

Coarse Sun Sensor (CSS)

2

Adcole / 42060

UARS

Fine Sun Sensor (FSS)

1

Adcole / 42070

TOPEX

Three Axis Magnetometer (TAM)

2

NASA/GSFC

EUVE, UARS

Inertial Reference Unit (IRU)

2

Kearfott / SKIRU-DII

XTE

 

 

 

 

Actuator component

Units

Manufacturer/model

Heritage

Reaction Wheel Assembly (RWA)

4

Honeywell / EOS-AM

Similar to EUVE

Magnetic Torquer Rod (MTR)

3

Ithaco / TR500CFR

EUVE

Attitude Control Thruster

6 (x 2)

Olin Aerospace (Primex)

 

Delta-v thruster

2 (x 2)

Olin Aerospace (Primex)

 

Table 1: Overview of GN&C sensors and actuators

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Figure 3: Artist' view of the Terra spacecraft in orbit (image credit: NASA)

The design life of the Terra spacecraft is six years. The spacecraft bus is of size of 6.8 m (length) x 3.5 m (diameter) and has a total launch mass of 5,190 kg. The total payload mass is 1155 kg.

RF communications: The primary Terra telemetry data transmissions are via TDRS (Tracking & Data Relay Satellite) system. A steerable HGA (High Gain Antenna) and associated electronics are mounted on a deployed boom extending from the zenith side of the spacecraft. This location maximizes the amount of time available for TDRS communications via this antenna without obstruction by other pads of the spacecraft. Emergency communication is done via the nadir or zenith omni antenna. Command and engineering telemetry data are transmitted in S-band. The science data recorded onboard are transmitted via Ku-band at 150 Mbit/s. The nominal mode of operation is to acquire two 12 minute TDRSS contacts per orbit. During each TDRSS contact, both S-band and Ku-band transmission is being used.

The average data rate of the payload is 18.545 Mbit/s (109 Mbit/s peak); onboard recorders for data collection of one orbit. Mission operations are performed at GSFC. 9)

Broadcast of data: Besides Ku-band and S-band communication, Terra is also capable of downlinking science data via X-band. The X-band communication can be operated in three different modes, Direct Broadcast (DB), Direct Downlink (DDL) and Direct Playback (DP). DB and DDL is used to directly transmit real-time MODIS and ASTER science data respectively to users.

The DAS (Direct Access System) provides a backup option for direct transmission in X-band. DAS supports transmission of data to ground stations of qualified EOS users around the world. These users fall into three categories:

- EOS team participants and interdisciplinary scientists

- International meteorological and environmental agencies

- International partners who require data from their EOS instruments

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Figure 4: The Terra spacecraft in the cleanroom of LMMS at Valley Forge (image credit: LMMS)

 

Launch: The launch of the Terra spacecraft took place on Dec. 18, 1999 from VAFB, CA, on an Atlas-Centaur IIAS rocket.

Orbit: Sun-synchronous circular orbit, altitude = 705 km, inclination = 98.5º, period = 99 minutes (16 orbits per day, 233 orbit repeat cycles). The descending nodal crossing is at 10:30 AM.

Orbit determination is performed by TONS (TDRS Onboard Navigation System) which estimates Terra's position and velocity, drag coefficient, and master oscillator frequency bias. TONS is updated by Doppler measurements at the spacecraft's receivers and provides the attitude control software with a desired pointing ephemeris. Ground-based orbital elements are uplinked daily for backup navigation.

As of March 1, 2001, the Landsat-7, EO-1, SAC-C and Terra satellites are flying the so-called “morning constellation” or “morning train” (a loose formation demonstration of a single virtual platform). There is 1 minute separation between Landsat-7 and EO-1, a 15 minute separation between EO-1 and SAC-C, and a 1 minute separation between SAC-C and Terra. The objective is to compare coincident observations (imagery) from various instruments (synergistic effects). 10)

 


 

Mission status:

• The Terra spacecraft and its sensor complement (except the SWIR bands on ASTER) are operating nominally in 2014.

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Figure 5: Big Island of Hawaii captured by the MODIS instrument on Terra on January 26, 2014 (image credit: NASA Earth Observatory) 11)

Legend of Figure 5: The remarkably cloud-free view shows the range of ecological diversity present on the island. Many of the world’s climate zones can be found on Hawaii for two related reasons: rainfall and altitude. The Big Island is home to Mauna Kea, the tallest sea mountain in the world at 4,205 m and the tallest mountain on the planet—if you measure from seafloor to summit, a distance of more than 9,800 m.

Despite Mauna Kea’s height, it is Mauna Loa that dominates the island. With an altitude of about 4,169 m — the actual number varies depending on volcanic activity — Mauna Loa is the most massive mountain in the world. Temperatures dip low at the summit of these peaks, resulting in a tree-free polar tundra, pale brown in this image.

The mountains help shape rainfall patterns on Hawaii so that desert landscapes exist side-by-side with rainforests. In fact, average yearly rainfall ranges from 204 mm to 10,271 mm . Trade winds blow mostly from the east-northeast, and the sea-level breezes hit the mountains and get forced up, forming rainclouds. The east side of the island is lush and green with tropical rainforest. Much less moisture makes it to the lee side of the mountains. The northwestern shores of Hawaii are desert. Kona, on the western shore, receives plenty of rain because the trade winds curve back around the mountains and bring rain. Pale green areas on all sides of the island are agricultural land and grassland.

The other environmental force painting Hawaii’s canvas is volcanism. Mauna Loa and Mauna Kea are both volcanic, though only Mauna Loa has been active recently. However, in this department, Kilauea is the superlative: It is one of the world’s most active volcanoes. A small puff of steam rises from an erupting vent in this image. Black and dark brown lava flows extend from both Kilauea and Mauna Loa.

• January 2014: A swirling mass of Arctic air moved south into the continental United States in early January 2014. On January 3, the air mass began breaking off from the polar vortex, a semi-permanent low-pressure system with a center around Canada’s Baffin Island. The frigid air was pushed south into the Great Lakes region by the jet stream, bringing abnormally cold temperatures to many parts of Canada and the central and eastern United States.

- When the cold air passed over the relatively warm waters of Lake Michigan and Lake Superior, the contrast in temperatures created a visual spectacle. As cold, dry air moved over the lakes, it mixed with warmer, moister air rising off the lake surfaces, transforming the water vapor into fog—a phenomenon known as steam fog. 12)

The result: One of the coldest Arctic outbreaks in two decades has plunged into the USA, bringing bitterly cold temperatures to the Midwest, South and East. 13)

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Figure 6: Natural color image of MODIS on Terra captured on January 6, 2013 showing fog forming over the lakes and streaming southeast with the wind (image credit: NASA)

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Figure 7: A false color image of MODIS on Terra acquired on January 6, 2014which helps to illustrate the difference between snow (bright orange), water clouds (white), and mixed clouds (peach), image credit: NASA

• December 01, 2013: Offshore from Argentina, spring is in bloom. Massive patches of floating phytoplankton colored the ocean in November 2013. These microscopic, plant-like organisms are the primary producers of the ocean, harnessing sunlight to nourish themselves and to become food for everything from zooplankton to fish to whales. 14)

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Figure 8: The MODIS instrument on NASA's Terra satellite acquired this natural-color image on Nov. 26, 2013 (image credit: NASA)

Legend to Figure 8: The chalky blue swirls in the South Atlantic Ocean, as well as fainter streaks of yellow and green, are evidence of abundant growth of phytoplankton across hundreds of kilometers of the sea. These organisms contain pigments (such as chlorophyll) or minerals (calcium carbonate) that appear blue, green, white, or other colors depending on the species. The phytoplankton in this image are likely a blend of diatoms, dinoflagellates, and coccolithophores. Near the coast, the discoloration of the water could be phytoplankton or it might be sediment runoff from rivers.

These phytoplankton help fuel one of the world’s best fishing grounds, particularly for shortfin squid, hake, anchovies, whiting, and sardines. The area known as the Patagonian “shelf-break front,” is a crossroads of currents—Circumpolar, Brazil, and Malvinas—where nutrients are carried in from southern waters or churned up from the edge of the continental shelf.

• June 2013: The 2013 Senior Review evaluated 13 NASA satellite missions in extended operations: ACRIMSAT, Aqua, Aura, CALIPSO, CloudSat, EO-1, GRACE, Jason-1, OSTM, QuikSCAT, SORCE, Terra, and TRMM. The Senior Review was tasked with reviewing proposals submitted by each mission team for extended operations and funding for FY14-FY15, and FY16-FY17. Since CloudSat, GRACE, QuikSCAT and SORCE have shown evidence of aging issues, they received baseline funding for extension through 2015. 15)

- The Science Panel endorses the continuation of the Terra mission because it will extend the records for numerous data products used to monitor and understand changes in climate and the effects of those changes on land, ocean, and atmosphere over the next few years. The Terra mission has already accumulated 13 years of data from five instruments, each of which provides valuable data for scientific questions pertaining to the Earth and its changes, including 79 core products as well as support for monitoring and relief efforts for natural and man-made disasters. The continuation of the Terra mission would extend the baseline of these measurements and, for some instruments, provide continuity linking past and future missions.

- The products from Terra are invaluable to a large number of scientific investigations related to the Earth system and global change. From the perspective of the Science Panel, the data from MODIS, alone, justifies that the mission be continued.

• In June 2013, a wildfire broke out in Black Forest, a wooded suburb of Colorado Springs, CO, USA. The fire charred more than 5,700 hectare, destroying 509 homes and killing two people. The Black Forest fire was the most destructive in the state’s history. 16)

Figure 9 provides an image of the burn scar on June 21, 2013. Vegetation-covered land is red in the false-color image, which includes both visible and infrared light. Patches of unburned forest are bright red. Unburned grasslands are pink. The darkest gray and black areas are the most severely burned. Buildings, roads, and other developed areas appear light gray and white.

The most severe damage occurred north of Shoup Road, but the severity varied widely by neighborhood. Cathedral Pines, for instance, escaped largely unscathed. Many residents of that neighborhood put rocks around their homes, removed vegetation and dead trees from their yards, avoided using mulch, and followed other fire prevention strategies that helped keep flames back long enough for fighters to save homes

One key building that escaped the flames was Edith Wolford elementary school. Though it was in the middle of an area that was severely burned, the school survived intact partly because of the large, treeless parking lot surrounding it.

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Figure 9: Aftermath of Colorado's most destructive wildfire observed by the ASTER instrument on the Terra satellite on June 21, 2013 (image credit: NASA)

• The Terra spacecraft and its sensor complement (except the SWIR bands on ASTER) are operating nominally in 2013. NASA extended the mission to 2015 (after the 2011 review). 17)

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Figure 10: MODIS image of a dust storm that blew out of Libya and across the Mediterranean Sea in late March 2013 (image credit: NASA)

Legend to Figure 10: MODIS on NASA's Terra satellite acquired this natural-color image of the dust storm on March 30, 2013. The dust plumes arose hundreds of kilometers inland, and dust stretched across the Mediterranean Sea toward southern Italy. - Southwest of the coastal city of Banghazi (Benghazi), an especially thick dust plume spanned roughly 100 km , and the plume was thick enough to completely hide the ocean surface below. 18)

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Figure 11: Air over Beijing China on January 14, 2013 as observed with the MODIS instrument (image credit: NASA, Jeff Schmaltz)

Legend to Figure 11: Residents of Beijing and many other cities in China were warned to stay inside in mid-January 2013 as the nation faced one of the worst periods of air quality in recent history. The Chinese government ordered factories to scale back emissions, while hospitals saw spikes of more than 20 to 30 % in patients complaining of respiratory issues, according to news reports. 19)

• The Terra spacecraft and its instruments are operating nominally in 2012 (> 12 years on orbit). - In June 2011, the NASA Earth Science Senior Review recommended an extension of the Terra mission as baseline up to 2013 and a further extension as baseline up to 2015.

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Figure 12: MODIS natural color image of the eastern half of the Black Sea observed on May 18, 2012 (image credit: NASA) 20)

Legend to Figure 12: Enriched by nutrients carried in by the Danube, Dnieper, Dniester, Don and other rivers, the waters of the Black Sea are fertile territory for the growth of phytoplankton. The bounty is a mixed blessing. The milky, light blue and turquoise-colored water in the middle of the sea is likely rich with blooming phytoplankton that trace the flow of water currents. Closer to the coast, the colors include more brown and green, perhaps a brew of sediment and organic matter washing out from rivers and streams, though it may also be a sign of phytoplankton. Puffs of spring clouds linger over parts of the coastline.

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Figure 13: Natural color image of MODIS acquired on January 23, 2012 showing a winter storn in the Pacific Northwest (image credit: NASA)

• The Terra spacecraft and its instruments are operating nominally in 2011. Terra is a huge success, and continuation of the data collection 11 year TERRA record from the five instruments: ASTER, CERES, MISR, MODIS, MOPITT, is critical to a wide array of earth system science.

According to the NASA Earth Science Senior Review 2011, the Terra platform is expected to remain fully functional through 2017 (battery, fuel, subsystems performance). The main failure to date is the SWIR bands on ASTER. But there continues to be significant use of the ASTER data from optical and TIR bands, and from the new global DEM. 21)

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Figure 14: Waves of dust dance off the African Coast - this MODIS natural color image was taken on Sept. 23,2011 (image credit: NASA)

Legend to Figure 14: The dust plumes sport a wave-like appearance—bands of thick dust alternating with bands of relatively clear air. Some waves extend westward while others curve toward the south in giant arcs. At the end of one curving wave of dust, a line of clouds extends southward over the sea. These ribbon-like patterns might result from atmospheric waves. - Sand seas sprawl over much of Mauritania, and the abundant sand provides plentiful material for dust storms. This dust storm hasn’t yet reached Cape Verde, which lies to the southwest, but the dust appears headed in that general direction.

• More than a decade after launch, the Terra spacecraft and its instruments are operating nominally in 2010 (design life of six years). The spacecraft remains in extraordinary good condition and with enough fuel to provide its service for another 6-7 years to come. 22) 23) 24)

All five instruments onboard the spacecraft continue to gather scientific data, although one of the three telescopes on ASTER is no longer working. ASTER stopped capturing useful SWIR imagery in 2008. The spacecraft is still working on its primary spacecraft components with one exception - the DASM (Direct Access System Module) which broadcasts MODIS data to 150 sites around the world, experienced a failure in 2008. The mission team switched the broadcast services to the redundant module.

The MISR instrument has been collecting global Earth data from NASA’s Terra satellite since February 2000. With its nine along-track view angles, four visible/near-infrared spectral bands, intrinsic spatial resolution of 275 m, and stable radiometric and geometric calibration, no instrument that combines MISR’s attributes has previously flown in space. The more than 10-year (and counting) MISR data record provides unprecedented opportunities for characterizing long-term trends in aerosol, cloud, and surface properties, and includes 3-D textural information conventionally thought to be accessible only to active sensors. Technology development is underway to extend future multiangle measurements to broader spectral range (ultraviolet to thermal infrared), wider spatial swaths (enabling more rapid global coverage), and accurate polarimetric imaging. 25)

• In the summer 2010, the project is reporting that many lessons have been learned from MODIS instrument operation, calibration, performance, algorithm refinements, and calibration coefficient LUT (Look Up Tables) updates. Listed in the following are some important factors that need to be considered to assure sensor performance and data quality: 26) 27)

- Comprehensive pre-launch calibration and characterization

- Dedicated calibration and validation effort throughout entire mission

- Close interactions among science and calibration teams and input from users

- Complete documentation on instrument operation concept, sensor calibration ATBD (Algorithm Theoretical Basis Document), algorithm and LUT update procedures, and sensor performance.

MODIS lessons have provided and will continue to provide valuable information for future missions and sensors, such as the VIIRS on the NPP and JPSS, ABI on GOES-R, OLI and TIRS on LDCM, and CLARREO. — Since launch, both Terra and Aqua MODIS have provided an unprecedented amount of high quality data and made significant contributions to the studies of short- and longterm changes in the Earth’s system.

Terra spacecraft deep space calibration: In early 2003, the Terra S/C performed two deep space calibration maneuvers. The objective of the maneuvers is to provide the science instruments with calibration opportunities using the cold background of deep space and also the stable lunar surface as calibration targets. These maneuvers help to identify and to quantify payload data inaccuracies, such as scan-dependent offsets, allowing for the correction and for more accurate data products. Additionally, the lunar calibration maneuver enables inter-calibration with other spacecraft (e.g. SeaWiFS/SeaStar, Aqua MODIS) observing the same illumination reference.

A 240º pitch maneuver is designed to protect the instrument deck from sun exposure and also to provide a steady-state slew during the lunar viewing. The 35 minutes eclipse period and the requirements for a nearly perfect moon placement and continuous communications coverage impose a strict timing constraint on the execution of the maneuvers. The GN&C has to perform beyond the experience and constraints of a heritage system design. - When Terra executed the maneuvers, FDIR protection as well as the S/C attitude and instrument performance met or exceeded all expectations.

• MOPITT operational history: First data were collected in March 2000 and then almost continuously from March 22, 2000 until May 7, 2001 at which point the instrument was shut down due to an anomaly. However, data collection in reduced mode (less height resolution) was resumed on August 23, 2001 and has continued since then. It has produced a complete dataset of CO over the globe period of 14 months from March 2000 to May 2001 (reduced resolution data set after Aug. 2001). It has provided one of the first global dynamic pictures of tropospheric pollution and its transport on both the regional and global scale. Continued coverage will enable the science team to examine more aspects of the large-scale transport within the lower atmosphere.

An appropriately chosen redundancy scheme has extended the life of the instrument beyond the mission requirements. The success of the instrument can be attributed to its long life mechanisms, which continue to operate at high speeds. With the LMC motors currently exceeding 2 billion rotations, and the choppers over 5 billion rotations, the successful mechanism design has been proven on orbit. MOPITT has made upwards of 60 million measurements, and an application has been made to NASA to extend the Terra mission from nominally 6 years to 10 years, based on the success of MOPITT and the other instruments on the spacecraft (see Ref. 60).

• The commissioning phase of Terra (checkout and verification) lasted until Feb. 23, 2000 when the spacecraft reached also its final orbit. After this the observatory began its observations phase collecting scientific data.

 


 

Sensor complement: (ASTER, CERES (2 units), MISR, MODIS, MOPITT)

Measurement Region

Measurement

Instruments used

Atmosphere

Cloud properties
Radiative energy flux
Tropospheric chemistry
Aerosol properties
Atmospheric temperature
Atmospheric humidity

MODIS, MISR, ASTER
CERES, MODIS, MISR
MOPITT
MISR, MODIS
MODIS
MODIS

Land surface

Land cover and land use change
Vegetation dynamics
Surface temperature
Fire occurrence
Volcanic effects

MODIS, MISR, ASTER
MODIS, MISR, ASTER
MODIS, ASTER
MODIS, ASTER
MODIS, MISR, ASTER

Ocean

Surface temperature
Phytoplankton and dissolved organic matter

MODIS
MODIS, MISR

Cryosphere

Land ice change
Sea ice
Snow cover

ASTER
MODIS, ASTER
MODIS, ASTER

Table 2: Overview of major physical process measurements of the Terra instruments

 

ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer):

ASTER is a Japanese instrument sponsored by METI (Ministry of Economy, Trade and Industry) and a cooperative project with NASA. The ASTER team leaders are Hiroji Tsu of ERSDAC (Japan) and Anne B. Kahle of JPL. ASTER management is provided by JAROS (Japan Resources Observation System Organization). ASTER was built by NEC, MELCO, Fujitsu, and Hitachi. A Joint US/Japan Science Team is responsible for instrument design, calibration, and validation. Previous instrument name: ITIR (Intermediate Thermal Infrared Radiometer).

Objective: Provision of high-resolution and multispectral imagery of the Earth's surface and clouds for a better understanding of the physical processes that affect climate change. Applications: studies of the surface energy balance (surface brightness temperature), plant evaporation, vegetation and soil characteristics, hydrologic cycle, volcanic processes, etc. 28) 29) 30) 31) 32) 33)

The ASTER instrument consists of three separate instrument subsystems; each subsystem operates in a different spectral region, has its own telescope(s), and is built by a different Japanese company. The subsystems are in the VNIR (Visible Near Infrared), SWIR (Shortwave Infrared) and TIR (Thermal Infrared) spectral regions. The VNIR and SWIR subsystems employ pushbroom imaging while the TIR subsystem performes whiskbroom imaging. ASTER is pointable in the cross-track direction such that any point on the globe may be observed at least once within 16 days in all 14 bands and once every 5 days in the VNIR bands. The absolute temperature accuracy is 3K in the 200-240 K range, 2K in the 240-270 K range, and 2 k in the 340-370 K range for TIR bands.

Total instrument mass=421 kg; power=463 W average, 646 W peak; data rate = 8.3 Mbit/s average and 89.2 Mbit/s peak; thermal control by 80 K Stirling cycle coolers, heaters, cold plate/capillary pumped loop, and radiators; pointing accuracy: for control = 1 km on ground (all axes), knowledge= 342 m on ground (per axis), stability=2 pixels for 60 seconds. shown in this figure.

Parameter

Band No

VNIR

Band No

SWIR

Band No

TIR

Spectral bands in µm

1

0.52 - 0.60

4

1.600 - 1.700

10

8.125 - 8.475

2

0.63 - 0.69

5

2.145 - 2.185

11

8.475 - 8.825

3N

0.76 - 0.86

6

2.185 - 2.225

12

8.925 - 9.275

3B

0.76 - 0.86

7

2.235 - 2.285

13

10.25 - 10.95

Stereoscopic viewing
capability along-track

8

2.295 - 2.365

14

10.95 - 11.65

9

2.360 - 2.430

 

 

Ground resolution

15 m

30 m

90 m

IFOV (nadir)

21.5 µrad

42.6 µrad

128 µrad

Data rate

62 Mbit/s

23 Mbit/s

4.2 Mbit/s

Cross-track pointing

±24º (±318 km)

±8.55º (116 km)

±8.55º (116 km)

Swath width

60 km

60 km

60 km

Detector type

Si (CCD of 5000 elements,
4000 are used)

PtSi-Si Schottky barrier linear array, cooled to 80 K (Stirling cooler)

HgCdTe
cooled to 80 K
(Stirling cooler)

Data quantization

8 bit

8 bit

12 bit

Radiometric accuracy

4%

4%

 

Table 3: ASTER instrument parameters of the three subsystems

The cooling capacity of the SWIR cryocooler is a nominal value of 1.2 W at 70 K; the measured power consumption is 43.5 W, which satisfies the requirement that it be less than 55 W. The cooling capacity of the TIR cryocooler is a nominal value of 1.2 W at 70 K; the measured power consumption is 50 W, which satisfies the requirement that it be less than 55 W. 34)

The VNIR subsystem, built by NEC Corporation, is a reflecting-refracting improved Schmidt design. VNIR features two telescopes, one nadir-looking with a three-spectral-band detector, and the other backward-looking with a single-band detector. The backward-looking telescope provides a second view of the target area in band 3B for stereo observations. Cross-track pointing is accomplished by rotating the entire telescope assembly. Band separation is through a combination of dichroic elements and interference filters that allow all three bands to view the same ground area simultaneously. Calibration of the nadir-pointing detectors is performed with two halogen lamps.

 

ASTER

TM on Landsat 4/5

Wavelength Region

Band No.

Spectral Range (µm)

Band No.

Spectral Range (µm)

VNIR

 

 

1

0.45 - 0.52

1
2
3

0.52 - 0.60
0.63 - 0.69
0.76 - 0.86

2
3
4

0.52 - 0.60
0.63 - 0.69
0.76 - 0.90

SWIR

4

5
6
7
8
9

1.60 - 1.70

2.145 - 2.185
2.185 - 2.225
2.235 - 2.285
2.295 - 2.365
2.360 - 2.430

5

1.55 - 1.75

7

2.08 - 2.35

TIR

10
11
12
13
14

8.125 - 8.475
8.475 - 8.825
8.925 - 9.275
10.25 - 10.95
10.95 - 11.65

6

10.4 - 12.5

Table 4: Spectral range comparison of ASTER and TM (on Landsat)

The SWIR subsystem, built by MELCO (Mitsubishi Electric Company), uses a nadir-pointing aspheric refracting telescope. Cross-track pointing is accomplished by a pointing mirror. The size of the detector/filter combination requires a wide spacing of the detectors, causing in turn a parallax error of about 0.5 pixels per 900 m of elevation. This error is correctable if elevation data (DEM) are available. Two halogen lamps are used for calibration. The maximum data rate is 23 Mbit/s. 35)

The TIR subsystem employs a Newtonian catadioptric system with aspheric primary mirror and lenses for aberration correction. The telescope of the TIR subsystem is fixed to the platform, pointing and scanning is done with a single mirror. The line of sight can be pointed anywhere in the range ± 8.54º in the cross-track direction of nadir, allowing coverage of any point on Earth over the platform's 16 day repeat cycle. Each channel uses 10 mercury cadmium telluride (HgCdTe) detectors in a staggered array with optical bandpass filters over each detector element to define the spectral response. Each detector has its own pre- and post-amplifier for a total of 50. The detectors are being operated at 80 K using a mechanical split-cycle Stirling cooler. - In scanning mode, the mirror oscillates at about 7 Hz with data collection occurring over half the cycle. The scanning mirror is capable of rotating 180º from the nadir position to view an internal full-aperture reference surface, which can be heated to 340 K. 36)

Overview of some ASTER instrument characteristics:

• The Visible Near InfraRed (VNIR) telescope subsystem features a backward viewing band (next to a nadir viewing band) for high-resolution along-track stereoscopic observation (two-line VNIR imager)

• Provision of multispectral thermal infrared data of high spatial resolution (8 to 12 µm window region, globally)

• ASTER provides the highest spatial resolution surface spectral reflectance, temperature, and emissivity data within the Terra instrument suite

• The instrument provides the capability to schedule on-demand data acquisition requests

• The VNIR and SWIR subsystems employ pushbroom imaging while the TIR subsystem performes whiskbroom imaging

• ASTER provides band-to-band registration of the 14 spectral bands, not only within each subsystem, but also among the three subsystems. Accuracies of 0.2 pixels within each subsystem and 0.3 pixels among different subsystems are achieved.

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Figure 15: Illustration of the VNIR and SWIR subsystems of ASTER (image credit: JPL)

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Figure 16: Illustration of the TIR subsystem of ASTER (image credit: JPL)

 

CERES (Clouds and the Earth's Radiant Energy System):

The CERES instrument of NASA/LaRC was built by Northrop Grumman (formerly TRW Space and Technology Group) of Redondo Beach, CA (PI: Bruce Wielicki). Objective: Long-term measurement of the Earth's radiation budget and atmospheric radiation from the top of the atmosphere to the surface; provision of an accurate and self-consistent cloud and radiation database (input to WCRP international programs like TOGA, WOCE, and GEWEX). Retrieval of cloud parameters in terms of measured areal coverage, altitude, liquid water content, and shortwave and longwave optical depths. Specific science objectives are: 37) 38) 39) 40)

• For climate change analysis, provide a continuation of the ERBE record of radiative fluxes at the top of the atmosphere (TOA), analyzed using the same algorithms that produced the ERBE data.

• Double the accuracy of estimates of radiative fluxes at TOA and the Earth's surface.

• Provide the first long-term global estimates of the radiative fluxes within the Earth's atmosphere.

• Provide cloud property estimates that are consistent with the radiative fluxes from surface to TOA.

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Figure 17: View of one CERES radiometer and location of instruments on the Terra spacecraft (image credit: NASA/LaRC)

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Figure 18: Observation geometry of the CERES instruments on Terra (image credit: NASA/LaRC)

The CERES instrument assembly (of ERBE heritage) consists of a pair of broadband scanning radiometers (two identical instruments), referred to as FM-1 (Flight Module-1) and FM-2; one instrument operates in the cross-track mode for complete spatial coverage from limb to limb; the other one operates in a rotating scan plane (biaxial scanning) mode to provide angular sampling. The cross-track radiometer measurements are a continuation of the ERBS mission. The biaxially scanning radiometer provides angular flux information to improve model accuracy. A single cross-track CERES instrument is flown on TRMM (Tropical Rainfall Measuring Mission), while the dual-scanner instrument is flown on Terra (EOS/AM-1) and Aqua (EOS/PM-1).

The CERES instrument consists of three major subassemblies: 1) Cassegrain telescope, 2) baffle for stray light, and 3) detector assembly, consisting of an active and compensating element. Radiation enters the unit through the baffle, passes through the telescope and is imaged onto the IR detector. Uncooled infrared detection is employed.

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Figure 19: Schematic view of the CERES instrument (image credit: NASA/LaRC)

Instrument parameters (2 identical scanners): total mass of 100 kg , power = 103 W (average, 2 instruments), data rate = 20 kbit/s, duty cycle = 100%, thermal control by heaters and radiators, pointing knowledge = 180 arcsec. The design life is six years. CERES measures longwave (LW) and shortwave (SW) infrared radiation using thermistor bolometers to determine the Earth's radiation budget. There are three spectral channels in each radiometer:

- VNIR+SWIR: 0.3 - 5.0 µm (also referred to as SW channel); measurement of reflected sunlight to an accuracy of 1%.

- Atmospheric window: 8.0 - 12.0 µm (also referred to as LW channel); measurement of Earth-emitted radiation, this includes coverage of water vapor

- Total channel radiance in the spectral range of 0.35 - 125 µm;. reflected or emitted infrared radiation of the Earth-atmosphere system, measurement accuracy of 0.3%.

Limb-to-limb scanning with a nadir IFOV (Instantaneous Field of View) of 14 mrad, FOV = ±78º cross-track, 360º azimuth. Spatial resolution = 10-20 km at nadir. Each channel consists of a precision thermistor-bolometer detector located in a Cassegrain telescope.

Instrument calibration: CERES is a very precisely calibrated radiometer. The instrument is measuring emitted and reflected radiative energy from the surface of the Earth and the atmosphere. A variety of independent methods used to verify calibration: 41)

• Internal calibration sources (blackbody, lamps)

• MAM (Mirror Attenuator Mosaic) solar diffuser plate. MAM is used to define in-orbit shifts or drifts in the sensor responses. The shortwave and total sensors are calibrated using the solar radiances reflected from the MAM's. Each MAM consists of baffle-solar diffuser plate systems, which guide incoming solar radiances into the instrument FOV of the shortwave and total sensor units.

• 3-channel deep convective cloud test

- Use night-time 8-12 µm window to predict longwave radiation (LW): cloud < 205K

- Total - SW = LW vs Window predicted LW in daytime for same clouds <205K temperatures

• 3-channel day/night tropical ocean test

• Instrument calibration:

- Rotate scan plane to align scanning instruments TRMM, Terra during orbital crossings (Haeffelin: reached 0.1% LW, window, 0.5% SW 95% configuration in 6 weeks of orbital crossings of Terra and TRMM)

- FM-1 and FM-2 instruments on Terra at nadir

Instrument heritage

Earth Radiation Budget Experiment (ERBE)

Prime contractor

Northrop Grumman (formerly TRW)

NASA center responsible

LaRC (Langley Research Center)

Three channels in each radiometer

Total radiance (0.3 to 100 µm); Shortwave (0.3 to 5 µm); Window (8 to 12 µm)

Swath

Limb to limb

Spatial resolution

20 km at nadir

Instrument mass, duty cycle

50 kg/scanner, 100%

Instrument power

47 W (average) per scanner, 104 W (peak: biaxial mode) both scanners

Data rate

10 kbit/scanner

Thermal control

Use of heaters and radiators

Thermal operating range

38±0.1ºC (detectors)

FOV (Field of View)

±78º cross-track, 360º azimuth

IFOV

14 mrad

Instrument pointing requirements (3σ)
Control
Knowledge
Stability


720 arcsec
180 arcsec
79 arcsec/6.6 sec

Instrument size

60 cm x 60 cm x 57.6 cm/unit

Table 5: CERES instrument parameters

The international CERES Science Team includes scientists from NASA, NOAA, US universities, France (CNRS), and Belgium (RMIB).

Data: A key element in the success of CERES, beyond the development of an instrument, is the development of data analysis and interpretation techniques for producing radiation and cloud products that meet the scientific objectives of the project.

 

MISR (Multi-angle Imaging SpectroRadiometer):

The MISR instrument was designed and developed by NASA/JPL (PI: D. J. Diner). Objective: provision of multiple-angle continuous sunlight coverage of the Earth with high spatial resolution (multidirectional observations of each scene within a time scale of minutes). MISR uses nine CCD pushbroom cameras to observe the Earth at nine discrete viewing angles: one at nadir, plus eight other symmetrical views at 26.1º, 45.6º, 60.0º, and 70.5º forward and aft of nadir. Images at each angle are obtained in four spectral bands centered at 0.446, 0.558, 0.672, and 0.866 µm. Each of the 36 instrument data channels (i.e. four spectral bands for each of the nine cameras) is individually commandable to provide ground sampling of 275 m, 550 m, or 1100 m. The swath is 360 km; multi-angle coverage (repeat cycle) of the entire Earth in nine days at the equator, and in two days at higher latitudes. By design, MISR is an along-track nine-line camera system, offering multidirectional observations of each ground (or target) scene. 42) 43) 44) 45)

Camera

View angle

Boresight angle

Swath offset angle

Effective focal length

Df

70.3º forward

57.88º

-2.62º

123.67 mm

Cf

60.2º forward

51.30º

-2.22º

95.34 mm

Bf

45.7º forward

40.10º

-1.71º

73.03 mm

Af

26.2º forward

23.34º

-1.06º

58.90 mm

An

0.1º nadir

-0.04º

0.04º

58.94 mm

Aa

26.2º aftward

-23.35º

1.09º

59.03 mm

Ba

45.7º aftward

-40.06º

1.76º

73.00 mm

Ca

60.2º aftward

-51.31º

2.24º

95.33 mm

Da

70.6º aftward

-58.03º

2.69º

123.66 mm

Table 6: MISR as-built camera pointing specifications

Application: MISR provides global maps of planetary and surface albedo (brightness temperature), and aerosols and vegetation properties. Monitoring of global and regional trends in radiatively important optical properties (eg., opacity, single scattering albedo, and scattering phase function) of natural and anthropogenic aerosols.

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Figure 20: A camera of the MISR instrument with support electronics (image credit: NASA/JPL)

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Figure 21: Cut-away view of the MISR instrument (image credit: NASA/JPL)

MISR images are acquired in two observing modes: global and local. The global mode provides continuous planet-wide observations, with most channels operating at moderate resolution; some selected channels operate at the highest resolution for cloud screening and classification, image navigation, and stereo-photogrammetry. The local mode provides data at the highest resolution in all spectral bands and all cameras for selected 300 km x 300 km regions. In addition to data products providing radiometrically calibrated and geo-rectified images, global mode data will be used to generate two standard (level 2) science products: TOA (Top-of-Atmosphere)/Cloud Product and the Aerosol/Surface Product.

MISR on-orbit radiometric calibration is performed bi-monthly, using deployable white spectralon panels to reflect diffuse sunlight into the cameras, and a set of photodiodes to measure the reflected radiance. Additionally, vicarious calibrations using field and AirMISR data are done on six-month intervals. Geometric calibration of the cameras is done using ground control points.

Parameter

Description

Mission life

6 years

Global coverage time

Every 9 days, with repeat coverage between 2-9 days depending on latitude

Cross-track swath width

360 km common overlap of all 9 cameras, FOV = ±60º along-track and ±15º cross-track.

Nine CCD cameras

Named An, Af, Aa, Bf, Ba, Cf, Ca, Df, and Da where fore, nadir, and aft viewing cameras have names ending with letters f, n, a respectively and four camera designs are named A, B, C, D with increasing viewing angle respectively

View angles at Earth surface

0º, 26.1º, 45.6º, 60.0º, and 70.5º

Spectral coverage

Four bands centered at 0.446, 0.558, 0.672, and 0.866 µm (blue, green, red, and NIR)

Spatial resolution

275 m, 550 m, or 1.1 km, selectable in-flight

Detectors

CCDs, each camera with 4 independent line arrays (one per filter),1504 active pixels per line

Radiometric accuracy

3% at maximum signal

Detector temperature

-5 ±0.1º C (cooled by thermo-electric cooler) of focal plane

Structure temperature

5º C

Instrument mass, power

148 kg, 131 W peak and 83 W average

Instrument size

0.9 m x 0.9 m x 1.3 m

Data rate

3.3 Mbit/s average, 9.0 Mbit/s peak

Table 7: MISR instrument specification

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Figure 22: Illustration of the MISR observing concept from Terra (image credit: NASA/JPL)

 

MODIS (Moderate-Resolution Imaging Spectroradiometer):

MODIS is a NASA/GSFC instrument; prime contractor is Raytheon SBRS, Goleta, CA, formerly Hughes SBRS (Science team leader: V. Salomonson); MODIS algorithm development by an international team of scientists from USA, UK, Australia, and France; there are four discipline groups: Atmosphere, Land, Oceans, and Calibration. 46) 47) 48) 49)

The instrument is flown on the Terra and Aqua satellites (prime instrument). Objective: to measure biological and physical processes on a global basis on time scales of 1 to 2 days. Specific science goals are:

• To determine surface temperature at 1 km resolution, day and night, with an absolute accuracy of 0.2 K for ocean and 1 K for land

• To obtain ocean color (ocean-leaving spectral radiance) from 415 to 653 nm

• To determine chlorophyll fluorescence within 50% at surface water concentrations of 0.5 mg per cubic meter of chlorophyll a

• To obtain chlorophyll a concentrations within 35%

• To obtain information on vegetation and land surface properties, land cover type, vegetation indices, and snow cover and snow reflectance

• To obtain cloud cover with 500 m resolution by day and 1000 m resolution at night

• To obtain cloud properties and aerosol properties

• To determine information on biomass burning

• To obtain global distribution of atmospheric stability and total precipitable water.

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Figure 23: Artist's rendition of the MODIS instrument showing the 360º scan mirror (image credit: Hughes SBRS, NASA)

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Figure 24: Schematic view of the MODIS instrument (image credit: Raytheon SBRS, NASA)

Parameter

Value

Parameter

Value

Instrument type

Opto-mechanical design (whiskbroom scanner)

Data rate

10.6 Mbit/s (peak daytime), 6.1 Mbit/s (orbital average)

Scan rate

20.3 rpm

Data quantization

12 bit

Telescope

17.8 cm diameter off-axis, afocal (collimated) with intermediate field stop

Spatial resolution

250 m (bands 1-2)
500 m (bands 3-7)
1000 m (bands 8-36)

Size

1.0 m x 1.6 m x 1.0 m

Swath width, FOV

2330 km, 110º (1354 pixels in cross-track)

Mass

229 kg

Swath length/scan

10 km (10 pixels in parallel along track)

Power

162.5 W

Design life

6 years

Table 8: Some specification parameters of the MODIS instrument

MODIS is an optomechanical imaging spectroradiometer (whiskbroom type), consisting of a cross-track scan mirror (continuously rotating double-sided scan mirror assembly) and collecting optics, and a set of linear detector arrays with spectral interference filters located in four focal planes. To accommodate frequent infrared calibration (every 1.47 s), a 360º rotating paddle-mirror is centered within a scan cavity to provide the optical subsystem with sequential views of the five calibrators and the Earth.

The optical arrangement provides imagery in 36 discrete bands between 0.4 and 14.5 µm (21 bands within 0.4-3.0 µm range, 15 bands within 3-14.5 µm range). The spectral bands provide a spatial resolution of 250 m, 500 m, and at 1 km at nadir. MODIS heritage: AVHRR (POES), HIRS (POES), TM (Landsat), CZCS (Nimbus-7). In fact, the MODIS instrument is considered to be a next-generation AVHRR instrument, having 36 bands (AVHRR/3 has 6) and a spatial resolution of 250 m (AVHRR has 1 km).

A high-performance passive radiative cooler provides cooling to 83 K for the infrared bands on two HgCdTe FPAs (Focal Plane Assemblies). A new photodiode-silicon readout technology for the VNIR range provides unsurpassed quantum efficiency and low-noise readout with a very good dynamic range.

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Figure 25: Functional architecture of the MODIS instrument (image credit: Raytheon SBRS)

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Figure 26: Major elements of the MODIS instrument (image credit: NASA)

MODIS polarization sensitivity < 2% for the visible range out to 2.2 µm; the performance goal for SNR (Signal-to-Noise Ratio) and NEΔT (Noise-Equivalent Temperature Difference) values is 30-40% better than the required values in Table 9.; absolute irradiance accuracy of 5% for <3 µm and 1% for >3 µm; absolute temperature accuracy of 0.2 K for oceans and 1 K for land; daylight reflection and day/night emission spectral imaging; swath width of 2330 km at 110º FOV; scan rate = 20.3 rpm across track; instrument mass = 250 kg; duty cycle = 100%; power = 225 W (average); data rate = 6.2 Mbit/s (average), 10.6 Mbit/s (peak daytime), 3.2 Mbit/s (night); quantization = 12 bit. Instrument IFOV (spatial resolution) = 250 m (bands 1-2), =500 m (bands 3-7), = 1000 m (bands 8-36).

The observations are made at three spatial resolutions (nadir): 0.25 km for bands 1-2 with 40 detectors per band, 0.5 km for bands 3-7 with 20 detectors per band, and 1 km for bands 8-36 with 10 detectors per band. All the detectors, aligned in the along-track direction, are distributed on four focal plane assemblies (FPAs) according to their wavelengths: visible (VIS), near infrared (NIR), short- and mid-wave infrared (SMIR), and long-wave infrared (LWIR).

Primary Use

Band No.

Bandwidth
(µm)

Spectral Radiance
NESR
(W m-2 µm-1 sr-1)

Required SNR
(Required NEΔT
in K)

Spatial
Resolution at nadir

Land/Cloud
Boundaries

1
2

0.620 - 0.670
0.841 - 0.876

21.8
24.7

128
201

250 m

Land/Cloud
Properties

3
4
5
6
7

0.459 - 0.479
0.545 - 0.565
1.230 - 1.250
1.628 - 1.652
2.105 - 2.155

35.3
29.0
5.4
7.3
1.0

243
228
74
275
110

500 m

Ocean Color/
Phytoplankton/
Biogeochemistry

8
9
10
11
12
13
14
15
16

0.405 - 0.420
0.438 - 0.448
0.483 - 0.493
0.526 - 0.536
0.546 - 0.556
0.662 - 0.672
0.673 - 0.683
0.743 - 0.753
0.862 - 0.877

44.9
41.9
32.1
27.9
21.0
9.5
8.7
10.2
6.2

880
838
802
754
750
910
1087
586
516

1000 m

Atmospheric
Water Vapor

17
18
19

0.890 - 0.920
0.931 - 0.941
0.915 - 0.965

10.0
3.6
15.0

167
57
250

Surface/Cloud
Temperature

20
21
22
23

3.660 - 3.840
3.929 - 3.989
3.929 - 3.989
4.020 - 4.080

0.45
2.38
0.67
0.79

(0.05)
(2.00)
(0.07)
(0.07)

Atmospheric
Temperature

24
25

4.433 - 4.598
4.482 - 4.549

0.17
0.59

(0.25)
(0.25)

Cirrus Clouds

26

1.360 - 1.390

6.00

150

Water Vapor

27
28
29

6.535 - 6.895
7.175 - 7.475
8.400 - 8.700

1.16
2.18
9.58

(0.25)
(0.25)
(0.25)

Ozone

30

9.580 - 9.880

3.69

(0.25)

Surface/Cloud
Temperature

31
32

10.780 - 11.280
11.770 - 12.270

9.55
8.94

(0.05)
(0.05)

Cloud Top
Altitude

33
34
35
36

13.185 - 13.485
13.485 - 13.785
13.785 - 14.085
14.085 - 14.385

4.52
3.76
3.11
2.08

(0.25)
(0.25)
(0.25)
(0.35)

Table 9: MODIS spectral performance parameters

MODIS onboard calibration employs various techniques for comprehensive verification of spectral, radiometric and spatial measurements. They include: 50) 51) 52) 53)

• Spectroradiometric Calibration Assembly (SRCA)

- Spectral calibration of reflective channel channel bandpasses

- Verification of spectral band registration

- DC restoration on every scan using a direct view of space

- Lunar calibration via the space-view port as well as periodic rotations of the S/C to enable full scans across the moon through the active scan aperture

• Blackbody (BB) calibration of thermal bands on every scan (a v-groove blackbody)

• Solar Diffuser (SD) reference

• Solar Diffuser Stability Monitor (SDSM)

The spectral mode of the SRCA device consists of a light source, a grating monochromator, and a beam collimator. The light source is a SIS (Spectral Integration Sphere) with lamps distributed inside. By combining the use of the spectral filters mounted on the filter wheel assembly and the grating monochromator, the SRCA is capable of performing spectral characterizations of the RSB (Reflective Solar Bands) ranging from 0.41 to 2.2 µm. Its spectral calibration is referenced to the ground equipment (SpMA) with high accuracy.

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Figure 27: Schematic view of the SRCA device (image credit: NASA/GSFC)

The SD/SDSM system is used for the RSB calibration and BB for the TEB (Thermal Emissive Bands) calibration. The SRCA is primarily used for the sensor's spectral (RSB only) and spatial (TEB and RSB) characterization. The RSB calibration is reflectance based using a sensor’s view of diffusely reflected sunlight from a solar diffuser (SD) plate with a known bi-directional reflectance and distribution function (BRDF). Because of the solar exposure onto the SD plate, its reflectance properties slowly degrade on-orbit.

The Blackbody is located in front of and slightly above the Scan Mirror, which views the BB with every revolution. The BB assembly provides a full-aperture radiometric calibration source of the MWIR and LWIR bands to within 1 percent absolute accuracy. It provides known radiance levels and is also used in the DC restore operation (a space-view signal level provides the second level for all bands in the two-point calibration). In normal operation the BB is kept at the instrument’s ambient temperature (nominally 273 K), though it is possible to heat and control the BB to 315K. Twelve sensors below the assembly's surface monitor its temperature. Each sensor is calibrated to National Institute of Standards & Technology (NIST) traceable standards, and can determine the temperature of the assembly to within ± 0.1 K.

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Figure 28: View of the BB assembly (image credit: NASA/GSFC)

To maintain the calibration and data quality, a solar diffuser stability monitor (SDSM) is used in tandem with the SD to track its degradation or BRDF changes. The SDSM system has a small integration sphere (SIS) with a single input aperture and nine filtered detectors. Each filter has a narrow spectral bandpass so that the change in reflectance is effectively monitored at nine discrete wavelengths between 0.4 µm and 1.0 µm. A three-position fold mirror enables the detectors to view sequentially a dark scene, direct sunlight, and illumination from the SD (Solar Diffuser). The direct sunlight is attenuated via a two-percent transmitting screen to keep the radiance within the dynamic range of the SDSM’s detector/amplifier combination.

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Figure 29: The MODIS SD device (image credit: NASA/GSFC)

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Figure 30: The SDSM device (image credit: NASA/GSFC)

MODIS product overview: MODIS provides global coverage every 1 to 2 days. It provides specific global survey data, which includes the following (some standard data products):

• Cloud mask: at 250 m and 1 km resolution by day and at night

• Aerosol concentration and optical properties: at 5 km resolution over oceans and 10 km over land during the day

• Cloud properties: characterized by optical thickness, effective particle radius, cloud droplet phase, cloud-top altitude, cloud-top temperature

• Vegetation and land-surface cover, conditions, and productivity, defined as:

- Vegetation indices corrected for atmospheric effects, soil, polarization, and directional effects

- Surface reflectance

- Land-cover type with identification and detection of change

- Net primary productivity, leaf-area index, and intercepted photosynthetically active radiation

• Snow and sea-ice cover and reflectance

• Surface temperature with 1 km resolution, day and night, with absolute accuracy goals of 0.3-0.5ºC for oceans and 1ºC for land surfaces.

• Ocean color: defined as ocean-leaving spectral radiance within 5% from 415-653 nm, based on adequate atmospheric correction from NIR sensor channels

• Concentration of chlorophyll-a within 35% from 0.05 to 50 mg/m3 for case 1 waters

• Chlorophyll fluorescence within 50% at surface water concentrations of 0.5 mg/m3 of chlorophyll-a.

 

MOPITT (Measurement of Pollution in the Troposphere):

MOPITT is a Canadian sensor supported by CSA, built by COM DEV, Cambridge, Ontario (PI: J. R. Drummond, University of Toronto). The MOPITT instrument design is of MAPS (Measurements of Air Pollution from Space) heritage, flown on STS-2 (November 12.-14, 1981), then on STS-13 (October 5 -13, 1984), and then twice in 1994 (STS-59, STS-68). MOPITT is the first satellite sensor to use gas correlation spectroscopy (A technique to increase the sensitivity of the instrument to the gas of interest by separating out the regions of the spectrum where the gas has absorption lines and integrating the signal from just those regions. The specific wavelengths are located using a sample of the gas as a reference for the spectrum). By using correlation cells of differing pressures, some height resolution can be obtained. Thus MOPITT has multiple channels to provide height resolution, it also carries multiple channels to afford some redundancy. Definitions of acronyms in Table 10: LMC (Length Modulator Cell), PMC (Pressure Modulator Cell). 54) 55) 56) 57) 58) 59) 60) 61)

The CO profile measurements are made using upwelling thermal radiance in the 4.6 µm fundamental band. The troposphere is resolved into about four layers with approximately 3 km vertical resolution, 22 km horizontal resolution and 10% accuracy. Pressure Modulated Cells (PMCs) are used to view the upper layers whilst Length Modulated Cells (LMCs) are used for the lower troposphere measurements. By varying the cell pressures the modulators can be biased to view the different layers.

The MOPITT instrument contains four optical chains initiated by four scan mechanisms, which are split into eight independent channels. Each channel uses a technique known as correlation spectroscopy to perform the science measurements. This uses a sample of gas in the optical path. By performing synchronous demodulation of the detected infrared signal, the system functions as a complex filter, providing very good spectral resolution and good sensitivity by incorporating several molecular lines simultaneously.

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Figure 31: Isometric optical system layout of the MOPITT instrument (image credit: University of Toronto)

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Figure 32: Schematic illustration of the MOPITT instrument (image credit: University of Toronto)

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Figure 33: Schematic view of the correlation radiometry concept (image credit: NCAR, University of Toronto)

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Figure 34: Photograph showing the finished PMC for MOPITT (image credit: Oxford Physics)

Channel No

Cell type

Cell Pressure (kPa)

Center Wavelength (cm-1)

Spectral band constituent

1

LMC1

20

2166 (52)

CO thermal

2

LMC1

20

4285 (40)

CO solar

3

PMC1

7.5

2166 (52)

CO thermal

4

LMC2

80

4430 (140)

CH4 solar

5

LMC3

80

2166 (52)

CO thermal

6

LMC3

80

4285 (40)

CO solar

7

PMC2

3.75

2166 (52)

CO thermal

8

LMC4

80

4430 (140)

CH4 solar

Table 10: Channel definition of MOPITT

The instrument measures emitted and reflected infrared radiance in the atmospheric column. Analysis of these data permit retrieval of tropospheric CO profiles and total column CH4. Objective: study of how these gases interact with the surface, ocean, and biomass systems (distribution, transport, sources and sinks). Measurements are performed on the principle of correlation spectroscopy utilizing both pressure-modulated and length-modulated gas cells, with detectors at 2.3, 2.4, and 4.7 µm. Vertical profile of CO (carbon monoxide) and total column of CH4 (methane) are to be measured; CO concentration in 4 km layers with an accuracy of 10%; CH4 column abundance accuracy is 1%.

Swath width = 616 km, spatial resolution = 22 x 22 km; instrument mass = 182 kg; power = 243 W; duty cycle = 100%; data rate = 25 kbit/s; thermal control by an 80 K Stirling cycle cooler, capillary-pumped cold plate and passive radiation; thermal operating range = 25º C (instrument) and 100 K (detectors).

MOPITT is designed as a scanning instrument. IFOV = 1.8º x 1.8º (22 km x 22 km at nadir). The instrument scan line consists of 29 pixels, each at 1.8º increments. The maximum scan angle is 26.1º off-axis which is equivalent to a swath width of 640 km. - MOPITT data products include gridded retrievals of CH4 with a horizontal resolution of 22 km and a precision of 1%. Gridded CO soundings are retrieved with 10% accuracy in three vertical layers between 0 and 15 km. Three-dimensional maps to model global tropospheric chemistry.

The instrument is self-calibrating in orbit and performs a zero measurement every 120 seconds and a reference measurement every 660 seconds. The instrument operation is practically autonomous, requiring very little commanding to keep it within the mission profile at all times. 62)

Terra_Auto0

Figure 35: View of the MOPITT instrument (image credit: COM DEV)

MOPITT operations: MOPITT has suffered two anomalies since launch. On May 7, 2001 one of the two Stirling cycle coolers, which are used to keep the detectors at about 80 K, failed. The cooler fault compromised half of the instrument. After the fault, only channels 5, 6, 7, and 8 are delivering useful data. On Aug. 4, 2001 chopper 3 failed. Fortunately, it stopped in the completely open state, which permits to continue to use the data by adjusting the data processing algorithm accordingly.

 


 

EOS (Earth Observing System)

EOS is the centerpiece of NASA's Earth Science Enterprise (ESE). It consists of a science component and a data system supporting a coordinated series of polar-orbiting and low inclination satellites for long-term global observations of the land surface, biosphere, solid Earth, atmosphere, and oceans. 63) 64) 65) 66)

Background: The EOS program is a NASA initiative of the US Global Change Research Program (USGCRP). Planning for EOS began in the early 1980s, and an AO (Announcement of Opportunity) for the selection of instruments and science teams was issued in 1988. Early in 1990 NASA announced the selection of 30 instruments to be developed for EOS. Major budget constraints imposed by the US Congress forced the EOS program into a restructuring process in the time frame of 1991-92. In addition a rescoping of the EOS program occurred in 1992 leading to just half of the 1990 budget allocation (the HIRIS sensor was eliminated). The instruments adopted as part of the restructured/rescoped EOS program were chosen to address the key scientific issues associated with global climate change. This action reduced the required instruments to 17 that needed to fly by the year 2002 (six were deferred and seven instruments were deselected from the original 30). Furthermore, a shift occurred in the conceptual design of the EOS satellite platforms from “large observatories” to intermediate and smaller spacecraft that may be launched by smaller and existing launch vehicles. The EOS program experienced a further rebaselining process in 1994, due to a budget reduction of about 9%. This resulted in the cancellation of the combined EOS Radar and Laser Altimeter Mission (but rephasing the latter as two separate missions), deferring the development of some sensors and spreading the launch of missions by increasing the basic re-flight periods of missions from 5 to 6 years, and flying some EOS instruments on missions of partner space agencies (NASDA, RKA, CNES, ESA) in a framework of international cooperation. The EOS program includes instruments provided by international partners (ASTER, MOPITT, HSB, OMI) as well as an instrument developed by a joint US/UK partnership (HIRDLS).

The overall goal of the EOS program is to determine the extent, causes, and regional consequences of global climate change. The following science and policy priorities are defined for EOS observations (established by the EOS investigators working group and in coordination with the national and international Earth science community):

• Water and Energy Cycles: Cloud formation, dissipation, and radiative properties which influence the response of the atmosphere to greenhouse forcing, large-scale hydrology, evaporation

• Oceans: Exchange of energy, water, and chemicals between the ocean and atmosphere, and between the upper layers of the ocean and the deep ocean (including sea ice and formation of bottom water)

• Chemistry of the Troposphere and Lower Stratosphere: Links to the hydrologic cycle and ecosystems, transformations of greenhouse gases in the atmosphere, and interactions including climate change

• Land-Surface Hydrology and Ecosystem Processes: Improved estimates of runoff over the land surface and into the oceans. Sources and sinks of greenhouse gases. Exchange of moisture and energy between the land surface and the atmosphere. Changes in land cover

• Glaciers and Polar Ice Sheets: Predictions of sea level and global water balance

• Chemistry of the Middle and Upper Stratosphere: Chemical reactions, solar-atmosphere relations, and sources and sinks of radiatively important gases

• Solid Earth: Volcanoes and their role in climate change.

The original EOS mission elements (AM S/C series, PM S/C series, Chemistry S/C series) was redefined again in 1999. The EOS program space segment elements are now: Landsat-7, QuikSCAT, Terra, ACRIMSat, Aqua, Aura and ICESat.

Parameter

Terra (EOS/AM-1) S/C

Aqua (EOS/PM-1 S/C)

Downlink center frequency

8212.5 MHz

8160 MHz

EIRP

14 W

27.2 W

Bandwidth

26 MHz

15 MHz

Data modulation

OQPSK

SQPSK

Data format

NRZ-L

NRZ-L

I/Q power ratio (nominal)

1:1

1:1

Operational duty cycle

100%

100%

Antenna coverage from nadir

±64º

±64º

Antenna polarization

RHCP

RHCP

Data rate

13 Mbit/s

15 Mbit/s

Data protocol standard

CCSDS

CCSDS

Instrument data provided

MODIS

MODIS, AIRS, AMSU-A, CERES, HSB, AMSR-E

Table 11: Specification of Direct Broadcast (DB) service of Terra and Aqua satellites

EOS policy includes providing Direct Broadcast (DB) service to the user community; this applies to real-time MODIS data from the Terra spacecraft, as well as to the entire real-time data stream of the Aqua satellite. These data may be received by anyone with the appropriate receiving station, without charge. The broadcast data are transmitted in X-band. A 3 m antenna dish (minimum) should be sufficient for X-band data reception.


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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.