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The ESE program is a highly ambitious series of satellite flights extending well into the first decade of the 21st century. A chart showing these many missions is shown, with the proviso that launch dates can slip and missions may be scrubbed (usually from funding cutbacks). The first EOS launch in the cavalcade to follow is Terra, which is described on this page in its conceptual mode (its actual launch and first results will follow on pages 16-9 and 16-10). Special attention is given to one of its prime sensors, MODIS, with a simulated image promising what to expect. Landsat-7 is recalled again to show how it fits into the grand scheme.

EOS Platforms/Sensors and Mission Profiles

Addressing the scientific questions posed on the preceding pages requires that the ESE program utilizes a variety of instruments–each addressing a different but related issue–to work together. We mount some combination of suitable instruments on a spacecraft that launches into a carefully-designed, Earth orbit. The instruments dictate the spacecraft’s size and its design, as well as the size of available launch vehicles, etc. The Earth Observing Satellite (EOS) series meet these specifications (get more details on this and the rest of the EOS program at the EOS Web Site).

The EOS program involves multi-year missions, with twenty-one different orbit-based components, similar in concept (if not in detail) to EOS AM-1. By designing many such spacecraft, based on the guidelines outlined above, we formulate a mission profile. The mission profile, over time, provides the necessary data. We present the updated EOS Mission Profile here (keep in mind that future launch dates occasionally slip):

Chart presenting the ESE/EOS program in terms of missions and actual or projected launch times through the period 997-2003.

Although more tenuous (particularly susceptible to budgetary changes), the satellites planned for launch between 2004 and 2010 are specified in this chart:

The EOS program proposed for 2004 through 2010.

Some of these missions are described in more detail at the EOS Missions Web Site. Also, the above mission profiles may be more sharply displayed on this original site (the download at 72 dpi accounts for the fuzziness in the profile charts on this page) if you wish to see some of the fine print.

To allow for flexibility in the missions, NASA and cooperating agencies haven’t assigned developed or proposed instruments to specific flights in some instances. In several places the ESE may fly an instrument as a "flight of opportunity," or "FOO." While the above profile extends only to 2003, additional flights of instruments and flights may continue beyond 2006 to provide a full 15-year-long data set, as required by scientific drivers to the program, as described earlier.

The flagship mission, EOS-AM-1, has been renamed Terra. Its main thrust is to significantly improve our understanding of how the atmosphere works, especially by integrating inputs from the oceans, biosphere, and land surface. It thus is primarily a climate-oriented mission but with capabilities for studying other aspects of the Earth System including land surfaces that have now long been the purview of Landsat. After more than a year's delay, it was successfully launched on December 18, 1999 and is fully operational (discussed in detail on pages 16-9 and 16-10). Some first results from the sensors described below are presented on the next page. More information about its design, purpose, and instrumentation, along with more sample imagery, can be found on the Terra Home Page.

Designed to have an orbital lifetime of at least six years, Terra (EOS AM-1) provides data on clouds, aerosols, and radiative balance. It allows us to characterize the terrestrial surface and provide data on the global carbon cycle.

Because Terra is complex, it has neither the proper instrument complement nor the longevity to handle many of the other issues addressed by ESE and related programs. How, then, do we acquire all the needed data? The answer lies in the mission profile which clearly demonstrates that a fleet of simultaneously operating satellites with different sensors is the approach required to gather interactive data.

An example of a state-of-the-art instrument built for the first EOS flight (Terra), is the Moderate Resolution Imaging Spectroradiometer, or MODIS. MODIS measures biological and physical processes on a global basis, every one to two days. The instrument provides long-term observations, from which we enhance our understanding of global dynamics and processes occurring on the Earth’s surface and in the lower atmosphere. The MODIS instrument employs a conventional imaging radiometer, consisting of a cross-track scan mirror and collecting optics, and a set of linear detector arrays with spectral interference filters located in four focal planes. The optical arrangement provides imagery in 36 discrete bands (hyperspectral mode), selected for diagnostic significance in Earth science. The spectral bands have spatial resolutions of 250 m, 500 m, or 1 km at nadir (the point directly below the instrument). The instrument's data rates range from 6.2 Mbps (average) to 10.6 Mbps (day) to 3.2 Mbps (night). It is temperature-controlled by radiative cooling. The instrument’s mass is 225 kg, it has a 100% duty cycle, and requires 175 W (average) and 225 W (peak) power. In its stowed (launch) configuration, its dimensions are about 95 x 158 x 134 cm.

MODIS-N cutaway diagram showing the main components of this sensor.

Because MODIS is the pivotal instrument on EOS, it is appropriate to list in detail its specifications.

MODIS technical specifications table.

16-17: In the right column are two abbreviations: SNR and NE deltaT. What do these stand for and what do they measure? ANSWER

The instrument will provide long-term observations from which to derive an enhanced understanding of global dynamics and processes occurring on the surface of the Earth and in the lower atmosphere. The MODIS instrument employs a conventional imaging radiometer concept, consisting of a cross-track scan mirror and collecting optics, and a set of linear detector arrays with spectral interference filters located in four focal planes. The optical arrangement will provide imagery in 36 discrete bands (quasi-hyperspectral mode) selected for diagnostic significance in Earth science. The instrument's data rates range from 6.2 Mbps (average) to 10.6 Mbps (day) to 3.2 Mbps (night). It is temperature-controlled by radiative cooling. In its stowed (launch) configuration, its dimensions are about 95 x 158 x 134 cm.

16-18: Looking over the channels listed in the "specs", and their intended uses, how would you categorize MODIS as a satellite class? ANSWER

To ensure that measurements provided by the instrument are accurate, it uses a complex series of on-board calibrators. As the instrument’s scan mirror makes a revolution, these calibrators provide information on a per-scan basis that places the measurements in a trustworthy framework.

Here is an example of a characteristic 3-band color composite made with the MODIS Airborne Simulator (MAS) sensor using selected MODIS bands, showing a scene in Central Brazil taken in August of 1995.

Color composite MAS sensor 3-band image of Central Brazil, August 1995.

MODIS has its own Home Page that gives updated information about that sensor.

In the case of the first EOS flight, earlier known as AM-1 because of its designated morning equator-crossing time, four other instruments join MODIS, two of them coming from some of the program's International Partners:

  • CERES - Clouds and the Earth's Radiant Energy System
  • MOPITT - Measurements of Pollution in the Troposphere (Canada)
  • MISR - Multi-angle Imaging Spectroradiometer
  • ASTER - Advanced Spaceborne Thermal Emission and Reflection Radiometer (Japan)

These instruments are on the Terra spacecraft, as shown here (the order in the list conforms to the instruments in the diagram from left to right, plus MODIS on the far right):

Artistís drawing of Terra (EOS-AM) in space, turned so that the instruments are visible.

Of special interest in this battery of instruments is ASTER, shown here.

Artistís drawing of Terra (EOS-AM) in space, turned so that the instruments are visible.

A combination of wide-bandwidth coverage, high-resolution mapping, and stereo DEM is unique to ASTER. It operates in tandem with MODIS. ASTER contains a radiometer in the VNIR, which images in three bands (between 0.52 and 0.86 µm (15-m spatial resolution). A second radiometer uses six bands in the SWIR region between 1.6 and 2.43 µm (30-m resolution). The TIR (thermal emission) radiometer has five bands between 8.3 and 11.3 µm (90-m resolution). There is also a separate single band at 0.76-0.86 µm, inclined backward at 27.6Á from nadir that provide along-track imagery, capable of stereoscopic viewing with one of the VNIR bands to permit images in the DEM (Digital Elevation Model) mode. The VNIR bands measure snow, water, vegetation, and degree of oxidation of the land surface. The SWIR bands are optimized for mineral identification, especially hydrated minerals in clay-rich soils. The TIR bands identify major rock types and also take land-surface temperatures.

16-19: What groups of scientists and technologists are most likely to use ASTER? ANSWER

Each instrument on Terra has a mathematical (computer-based) model that allows scientists and engineers to determine its response to operating conditions. In addition, because each data-collecting measurement goes through an algorithm (a set of mathematical and operational instructions), a detailed analysis of all algorithms is available in an Algorithm Theoretical Basis Document (ATBD). The ATBD presents the scientific and physical basis for the measurement, and documents how to translate that basis into computer code that converts the physical measurement into data products. The data products give the geophysical parameters required to answer the questions that scientists and government planners ask.

Landsat-7 launched on April 15, 1999. It operates both independently and in synergistic support of EOS. We now consider it an integral part of the ESE program. It mounts only one sensor, the Enhanced Thematic Mapper (ETM+) that has identical spectral wavelength coverage as earlier TMs (with the same 30-m resolution, except that the thermal band improved to 60 m). It also has (the +) a panchromatic (0.5-0.9 µm) band with 15-m resolution. The special feature of Landsat-7 is its launch and orbital parameters, which allow the space vehicle to trail (follow) the EOS-AM platform (within its swath) by 15-30 minutes each day. Thus, we can integrate observations from both systems, and we can use atmospheric data from EOS sensors to correct for the effects of water vapor and temperature on Landsat images.

The first image from Landsat-7 is included on page I-21 of the Introduction. Here is another: a cropped full scene of eastern Washington state, with Spokane in the upper right (northeast) corner, mountain ranges near the top, the Columbia River, fields still mostly barren in April, and some large dark gray areas along flowing or inactive streams which comprise part of the "Scablands" formed when a great Pleistocene lake burst sending cascades of water that strongly eroded the landscape.

An example of a Landsat-7 ETM image, showing the Channeled Scablands of central Washington state.

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