Similar to some of the first Metsats, several U.S. missions in the past 18 years have been flown to primarily perform atmospheric/climate research. Of particular importance in these studies is daily and seasonal knowledge of the amount of sunlight - total solar irradiance - reaching the Earth. Satellites above the main atmospheric column are especially adept at measuring this. Over the years, the satellites have led to these two general plots:
On October 5, 1984, the Earth Radiation Budget Satellite (ERBS) went into a non-synchronous orbit that permitted sampling at various times of day. The ERBS consisted of two instruments: a scanner and a non-scanner, designed to measure diurnal variations of incoming solar radiation and its differential absorption by the atmosphere. The latter had five broadband channels, one looking sunward, and the others at Earth, over wavelength intervals within the 0.2-5.0 µm range. The scanner's three channels undertook similar functions. The ERBS also carried the Stratospheric Aerosol and Gas Experiment II (SAGE-II), a limb sounder, which used four bands (0.385, 0.45, 0.60, and 1.0 µm) to measure aerosol and Rayleigh (fine-particle) scattering, whenever the satellite is in position in its orbit to view sunrise and sunset. The next images show ERBS-determined, long-wavelength and short-wavelength, radiant-flux densities during January, 1985.
Here we show aerosol concentrations over a period of six weeks, during summer 1991, as measured by SAGE-II. Note: these instruments flew on other satellites, including two in the NOAA series.
The Upper Atmosphere Research Satellite (UARS) deployed from a Shuttle (STS-48) on September 12, 1991, with ten instruments including the Microwave Limb Sounder (MLS), the High Resolution Doppler Imager (HRDI), and the Improved Stratospheric and Mesospheric Sounder (ISAMS). In the next images, on the top is a plot of the distribution around the South Pole of chlorine monoxide (ClO) and nitrogen dioxide (NO2), in terms of predictive models and actual results from the MLS and Cryogenic Limb Array Etalon Spectrometer (CLAES). On the bottom is a CLAES plot of methane (CH4) distribution through the atmosphere along a specific orbit.
Among the many gases studied by UARS is ozone (O3), of which much attention is being paid to the so-called Ozone Hole at each Pole because the depletion of ozone can have serious consequences to health (loss of ozone increases the risk of damage to life by solar UV rays). This set of maps shows variations in ozone in the Arctic region as detected by UARS' MLS instrument in wintertime during two dates three years apart; the distribution is at different elevations in the upper atmosphere as indicated by temperatures (degrees Kelvin):
UARS is a good example of the use of multiple sensors to study the properties of a single target medium, in this case the atmosphere. As such, it is a precursor model of the use of multisensors to monitor the Earth's diverse environments, as is being done now in the EOS Terra and Aqua programs (Section 16). A worthwhile review of UARS is found at its Home Page.
Between 1980 and 1991, three solar irradiance satellites, the ACRIM (Advanced Cavity Radiometer Irradiance Monitor) series, were placed in operation. Here is a typical plot of measured solar irradiation:
The Advanced Earth Observation Satellite (ADEOS) is a Japanese spacecraft, launched on August 17, 1996 and renamed by them as MIDORI, to conduct land, sea, and atmospheric studies using ten instruments supplied by NASA, NOAA, NASDA (Japan) and CNES (France). Sadly, this versatile satellite failed on September 12, 1996 but not before sending back some excellent, proof-of-concept data.
It is now becoming common to piggy-back sensors from several nations on a given satellite. Among ADEOS sensors are a NASA Scatterometer (NSCAT), discussed on page 14-12. Another NASA instrument is TOMS (Total Ozone Mapping Spectrometer), developed at Goddard Space Flight Center. Besides ADEOS, TOMS has flown earlier on Nimbus 7, Meteor-3 (Russian), and since ADEOS on Earth Probe (see below). Here is one of the few TOMS maps from ADEOS.
Next, we show examples of ADEOS sounder plots using instruments supplied by two Japanese agencies: the Interferometer for Monitoring of Greenhouse gases (IMG), which looks at absorption bands for CO2, CH4, H2O, O3, and NO2 in a 0.715-2.0 µm channel, CO in a 2.0-2.5 µm channel, and CH4 in a 2.32-3.05 µm channel; and the Infrared Limb Atmospheric Spectrometer (ILAS), which examines ozone distribution.
Another ADEOS instrument, developed by the French, is POLDER (Polarization and Directionality of the Earth's Reflectance). It includes 8 channels in the Visible and Near IR and can take images from different directions. It has a polarimeter that allows polarized light properties to be assessed. The image set below shows part of the Mediterranean in 4 successive views on the left and polarized light components (dominated by blue from scattering) on the right.
The early loss of ADEOS meant that one satellite monitoring ozone was no longer able to supply this critical data set, first collected in 1978 by the TOMS instrument on Nimbus-7. Fortunately, continuity was preserved by the launch in 1996 and continuing operation of Earth Probe (EP) which included a TOMS. This is a typical result for the South Polar region:
The next image was taken over a short period around July 20, 2000:
The previous week, the EP TOMS produced a neat image of dust storms raging in North Africa and the Near East. Here, TOMS was measuring aerosols associated with the dust storm:
A recent earth radiation budget satellite is SORCE (Solar Radiation and Climate Experiment, launched on January 25, 2003 and managed jointly by NASA Goddard and the University of Colorado. Here is the spacecraft:
This is a typical data set for its main instrument, TIM (Total Irradiance Monitor):
SORCE has proved invaluable for monitoring both the occurrence and the effect of solar flares on incoming radiation:
The instruments on the several irradiance monitoring satellites are carefully calibrated to correspond to "absolute" values. Nevertheless, each has returned slightly different levels of irradiant energy, as indicated by this diagram (note the small range of values on the ordinate):