AQUA, ENVISAT, and AURA - Remote Sensing Application - Completely Remote Sensing, GPS, and GPS Tutorial

The afternoon segment (PM) of the EOS flagships, Aqua, was successfully launched by a Delta II rocket from Vandenberg Air Force base at 2:55 AM PDT on May 4, 2002. Here is an artist's painting of the spacecraft:

The Aqua spacecraft from an artist's perspective
Aqua's initial orbit of 705 km (438 miles) leads to an average equatorial crossing time (going north) of 1:30 PM. Its mission is to complement Terra observations by providing data later in the day; however, the new sensors on Aqua are designed to obtain information of interest primarily to meteorologists and oceanographers. A better understanding of the planet's water cycle is the goal, as these sensors will lead to improved data on global precipitation and evaporation, radiative balances, humidity and temperature profiles through the atmosphere, soil moisture, etc. The overall objective is to integrate the water studies into a better general understanding of the world's environments. The three host participants in Aqua are the United States, Japan, and Brazil, although other nations are close associates.

Aqua carries six state-of-the-art instruments in a near-polar low-Earth orbit. The six instruments are the Atmospheric Infrared Sounder (AIRS), managed by JPL, the Advanced Microwave Sounding Unit (AMSU-A), the Humidity Sounder for Brazil (HSB), the Advanced Microwave Scanning Radiometer for EOS (AMSR-E), the Moderate-Resolution Imaging Spectroradiometer (MODIS), and Clouds and the Earth's Radiant Energy System (CERES). Each has unique characteristics and capabilities, and all six serve together to form a powerful package for Earth observations. Information about each of these sensor systems is rather involved and will not be summarized here, but the reader is encouraged to consult the section on Instruments at the Aqua Home Page.

Several months were involved in completing the instrument test phase, after which Aqua went operational. (There were a few minor glitches during this period.) The first imagery released by the Aqua team is this AMSR-E pair that give improved sea surface and brightness temperatures on June 2-4 (three days data integrated):

The first ocean water temperatures, both sea surface and surface brightness, obtained by the AMSR-E on Aqua.

AMSR-E has been used also to locate and measure rainfall on the continents. This set of data maps shows measurements made simultaneously within the same hour on June 5, 2002 by AMSR-E from space and by the U.S. Weather Bureau's ground-based NEXRAD Doppler radar and by TRMM (see page 14-5) three hours later, with corresponding NEXRAD readings. The space and ground measurements were 0.31 and 0.29 mm/hr respectively.

AMSR-E and TRMM determinations of rainfall distribution on the night of June 5, 2002, both compared with NEXRAD scans from the ground.

A very important task in applying microwaves to the Earth's surface is the detection of soil moisture and estimation of its amount. Here is a map of the generalized variations in soil moisture on a global basis as determined by AMSR in June 2001.

Global variations in soil moisture, as mapped by Aqua's AMSR-E.

The second sensor to "go public" with data is CERES. We have described this sensor on the previous page. Recall that CERES measures reflected thermal radiation from the Sun and heat from the Earth's surface warmed by the Sun. Here is another pair of images centered on North America and acquired on June 22, 2002:

CERES images from the Aqua spacecraft, showing reflected and emitted radiation over a broad region centered on the United States.

Then, on July 6, images from the AIRS (Atmospheric Infrared Sounder) group were released. The sensors making up this instrument are designed to measure temperatures along vertical profiles so as to derive a three-dimensional map of the atmosphere. The first image is a Visible view of Tropical Cyclone Ramasun in Asia:

Visible bands image of Cyclone Ramasun.

Released simultaneously were images from the AIRS group of sensors for a region from Italy east to Turkey and south to the North African Coast. This is the Visible image obtained from AIRS itself:

AIRS group Visible image of the Mediterranean area of Europe/Asia.

Beneath this visible view as a geographic reference in succession are:

1) An AIRS infrared image at 11 µm, which measures surface and cloud temperatures:

AIRS infrared image

2) An AMSU (Advanced Microwave Sounding Unit) temperature plot, sensed in the microwave at 31.4 GHz:

AMSU temperatures

3) An HSB (Humidity Sounder, developed and operated by Brazil) map, made from a sensor operating at 150 GHz, designed to obtain mid-troposphere temperatures, and sensistive to moisture, precipitation, and ice crystals; its dry land temperatures are close to those of AIRS:

HSB mid-troposphere temperatures.

Three-dimensional atmospheric maps developed from multisensor observations such as the above three data sets have now been released. Here is the western Mediterranean (France on left and North Africa on right) with the bottom shown as a landscape image, and atmospheric layers at 8 and 12 km shown in colors with red the warmest and purples coolest.

Generalized temperature ranges in two layers above the Mediterranean Sea (view looks east towards the top).

AIRS also measures the total water vapor content of the atmosphere from its base to the outer limits of H2O occurrence. This can be done globally for an extended period of time, as shown in this map which uses daily averages summed up for January 2004 to arrive at estimates of rainfall potential expressed in millimeters. The tropical zones can thus produce more than a half meter of precipitation over that period.

Total water vapor content in the relevant part of the atmosphere, integrated for the month of January, 2004 using AIRS measurements.

AMSU measures surface emissivities in the microwave region as well as atmospheric emissivities. These in turn are used to calculate temperatures. An excellent illustration of end products from this sensor is given by these observations of the eastern half of the U.S. before and after passage of Hurricane Isidore in late September of 2002 (these dates are determined by the 16-day orbital repeat cycle of Aqua). The left image was obtained on September 12; the right on the 28th, one day after the hurricane made landfall.

AMSU images on Sept. 12 and 28, 2002 of the eastern U.S.

Increasing temperatures range from the colder blues to the warmest reds. In the right image, the blues around the Mississippi River drainage basin result mainly from surface soils that have absorbed water from the hurricane (it appears further north as a blue area) such that the cooling effect of the rainfall into these soils has lowered emissivity. In the image below, differences in temperature determined by subtracting the second from the first AMSU image show a new map in which the hurricane cooling effects stand out in blue and red pinpoints an area of warmer temperatures in the air evident in the Carolinas:

AMSU temperature difference map.

The first MODIS images were released near the end of June, 2002. An example of its large area coverage is this natural color MODIS image, taken on June 24, that shows the U.S. West Coast from northern Oregon to the Mexican border. The red spots are active wildfires in the Klamath Mountains of SW Oregon, and elsewhere in that state. Most of the clouds off northern California are actually a thick fog bank.

MODIS natural color image of Oregon and California (Nevada inland) taken on June 24th, 2002 during a time of several major (100000 acres plus) wildfires (in bright red).

MODIS on both Terra and Aqua took measurements of CO2 throughout 2002. From knowledge of the Carbon Cycle (page 16-4) relations between CO2 and the amount fixed in vegetation (land and ocean), a measure of productivity could be made at different times of the year. The first pair of plots below show the distribution of global productivity in June and December of that year. The next map indicates the net productivity for the year.

Primary Productivity (as a function of kg/ worldwide at the end of June and December 2002, based on MODIS data
Net Primary Productivity for the year 2002.

MODIS has also proved its value at detecting plankton and thus complements SeaWiFS and other ocean-dedicated satellites. This image shows chlorophyll distribution in the Arabian Sea between Pakistan and Oman. The higher than normal concentrations of planktonic life are due to a period of significant rainfall in the region.

Chlorophyll variations in the Arabian Sea.

Other satellites in the EOS program are now operational. ICESat (Ice, Clouds, and land Experiment Satellite; see page 14-14), whose sole instrument is GLAS (Geoscience Laser Altimeter System) and SORCE (Solar Radiation and Climate Experiment) were both successfully launched in 2003 but large quantities of data are still forthcoming.

Aura, discussed below, will concentrate on atmospheric chemistry and climate conditions, looking with its instruments also at the dynamics involved in climate change.


We previously described the ESA (European Space Agency) environmentally-oriented program with its Envisat as the lead satellite, near the second page of the Overview.Check this for details, and note that Envisat was also launched in May of 2002. ESA claims their satellite is the biggest of the EOS family and has the most sensors (10). Count these in this artist's drawing - you should find 10. These are described in the Envisat component of ESA's Web site.

ESA's Envisat; the two prime sensors are MERIS and ASAR.

Several images from Envisat appear elsewhere in the Tutorial. Two excellent MERIS (for Medium Resolution Imaging Spectrometer) images are found in the Overview. Here is another MERIS wide swath image showing France, the Alps, and a cloud bank over Germany:

MERIS image of France and neighboring regions.
An example of a scene that has strong environmental information is this MERIS image of the West Coast of Africa, including part of Senegal. The Kaffrine River delta is densely vegetated with brackish water coastal vegetation. Dakar is at the end of the westernmost peninsula. Note the sediment from the river.
MERIS image of Senegal, with the westernmost tip of Africa.

Data taken by AATRS (Advanced Along Track Scanning Radiometer) are used primarily to determine sea surface temperatures. But several bands can be processed in images that resemble false color composites. These tend to look "funny" because of their extended elongation (along the orbital track) but they have the advantage of being a continuous "picture" taken at one time. Here is one that begins in Kenya and ends in the Sinai Peninsula across the Gulf of Suez; to fit better on the page it has been turned 90° so that north is to the right.

AATRS image of East Africa.

We have seen several ASAR (Advanced SAR) images in Section 8, page 8-7. Here is another, showing the Elbe River south of Berlin, in color because several band modes were used.

ASAR image of the Elbe River.

Many of the ASAR images are in black and white. Here is one showing Patagonia in Argentina, with the Andes to the left (west):

ASAR image of the Patagonia plateau and the Andes, in Venezuela.

As an example of the environmental utility of Envisat, we look again at the oil spill in the eastern Atlantic of Galicia in northwest Spain, which occurred in November of 2002 when the tanker Prestige sank with most of its cargo of 25 million barrels of oil. About 1.5 million barrels did escape, some reaching coastal beaches, as seen here in this ASAR image.

ASAR (radar) image of the oil spill off the Spanish coast in late 2002.

As with several other satellites that mount both visible-NIR sensors and radar, these images can be combined. The three panels below show parts of Corsica and Sardinia in the Tyrranean Sea and the west coast of Italy just north of Rome. The left panel is an ASAR radar image; the right panel was made from MERIS images. The bottom panel is a registered combination of both - red areas over the water are rain clouds; red on land is NOT vegetation, but in the band combinations used this color coincides with vegetation-poor land areas.

One of Envisat's tasks is to monitor various chemicals (including pollutants) in the global atmosphere. Its Scanning Imaging Absorption Spectrometer for Atmospheric Cartography is capable of determining the amount and distributon of the noxious gas NO2, released largely in auto emissions. Here is a map of the distribution of this gas over the eastern U.S. in June, 2003; yellows and browns are higher concentrations.

Nitrous oxide in the air above the eastern U.S.

Aura will extend this compositional capability for air analysis and will dovetail results with Envisat.


The natural chemistry of the Earth's atmosphere is dominated by three elemental gases: O2, N2, and Argon; much of the CO2 in air is introduced by natural processes such as transpiration. Some sulphur compound gases also are brought into the atmosphere by volcanic processes. Ozone, O3, is likewise in large part present naturally. But as we have seen earlier in this Section, man's activities have altered natural balances of the above chemicals and added still others, mostly in burning fuels. A general diagram illustrating chemical transfer in the atmosphere is as follows:

Diagram of the general chemistry of Earth's atmosphere; credit Barbara Summely.

The EOS program has included a spacecraft designed to primarily monitor the Earth's atmosphere. The third flagship in NASA's Earth Observatory spacecraft, AURA, was successfully launched in the early morning of July 15 2004 from the Vandenberg Air Force Base facility near Lompoc, California. Being a night launch, the visual effect of its takeoff, after 6 delays, was a "glorious" sight for the many scientists, engineers, and technicians who had patiently waited until all systems were finally set at "GO".

The night launch of Aura

Aura, like the other EOS spacecraft, has its own website.

Aura was inserted along an orbital path and altitude that allows its to follow just 8 minutes behind Aqua, so that here is an example of formation flying, discussed on the next page. With two assemblages of different instruments, the Aqua-Aura pair are acquiring complementary data sets.

Its primary mission is to determine the "health" of the world's air envelope. This includes an inventory of global three-dimensional distribution of various chemicals (most as pollutants) in the atmosphere, as well as more detailed measurements of ozone, and evidence of short- and long-term climate change. We show here the $736 million dollar spacecraft (as big as a small bus and weighing over 3 tons) :

The Aura spacecraft.

Below is the spacecraft "innards" in the assembly facility before its outer cover was emplaced.

The instruments on Aura.

Here is a chart indicating the chemicals that can be detected and quantified

Instruments on the Aura spacecraft and their capabilities for monitoring the atmospheric chemicals indicated in the chart.

The tasks of its four principal sensors are:

The joint U.K./U.S. High Resolution Dynamics Limb Sounder (HIRDLS) instrument will measure trace gases, temperature and aerosols in the upper troposphere, stratosphere, and mesosphere.

The Microwave Limb Sounder (MLS) will measure important ozone-destroying chemical species in the upper troposphere and stratosphere and measure trace gases in the presence of ice clouds and volcanic aerosols. MLS was built by the Netherlands and Finland in conjunction with NASA.

Ozone Monitoring Instrument (OMI) is Aura's primary sensor for tracking global ozone change and will continue the record of space-based ozone monitoring that began in 1970. NASA's Jet Propulsion Laboratory built OMI.

Tropospheric Emission Spectrometer (TES) will measure tropospheric ozone directly and other gases important to tropospheric pollution. JPL also made TES.

All four images are capable of making ozone measurements.

As with most satellites of this size and complexity, it took almost 90 days to purge the system of unwanted gases, check out instruments, adjust orbit, and do other appropriate housekeeping.

One major disappointment soon was evident. The Sun Shield on HIRDLS had upon opening become clogged with some debris in this instrument shaken loose during launch. The Shield had deployed only 20% of its full open condition. This severely compromises data acquisition because of the limited field of view. Hence, images from this sensor have not been released.

Far better luck befell the other three instruments. We look first at some MLS images; read their captions for brief descriptions.

Chemistry and temperature data obtained by the MLS over a short time frame.
Oxygen-3, ClO, and HCl concentrations in the atmosphere over a three months span as determined by the MLS.
Vertical profile along an orbital path from which the two gases indicated were measured with the MLS.
Water vapor volume mixing ratio as a function of height and date from a parcel of atmosphere of large size; MLS data,

Here are two ozone measurements made by the OMI:

OMI ozone measurement.

Another example of the OMI data is this map of the a Pacific island group in a small nation called Vanuatu. Here the Ambrym volcano is exuding S02 gases through vents filled with lake water in large enough amounts to be dangerous to livestock.

Plot of sulphur dioxide gases released from the Ambrym volcano.

NO2 concentrations in the eastern United States measured by OMI on January 29, 2005 are plotted in this map:

Nitric oxide concentrations over the eastern U.S.

A solar flare on January 17, 2005 apparently had a distinct effect on O3 and OH in the Antarctic atmosphere which the OMI was able to detect and map; read the description in the diagram for details.

Solar flare-induced changes in trivalent oxygen and hydroxide in the Antarctic ozone hole.

The TES has measured CO levels worldwide at two different altitudes (in atmospheric pressure terms):

Two maps showing CO levels simultaneously at two different altitudes

The TES obtained vertical variation data for ozone and water along an orbital path from northern Canada to the Yucatan in Mexico.

Aqua AIRS image of the orbital path followed shortly thereafter by AURA
Vertical profile data for ozone and water along the flight path shown in the previous image.

The next is a short-term aerosol map made at a global scale.

Global aerosol map made from Aura data.

The three Earth Observing satellites are all functioning well as of mid-2005. Imagery covering new types of data will be added from time to time.

As the EOS program moves on, some of the companion satellites shown in the diagram on Page 16-7 will be launched. One of these, JASON-1, was placed in orbit on Dec. 7, 2001. A joint endeavor between NASA JPL and the French Space Agency, it will provide improved wind speed measurements over the oceans and will yield data giving more detailed ocean surface topography. Here is one of the first products made public in 2002.

Sea Surface state as determined from JASON data.

When these EOS satellites are all up and flying, they will represent one type of Satellite Formation Flying configuration. Another is the NPOESS program which will be developed in the first decade of th 21st Century. Page 16-11 discusses these innovations.