The United States metsat program traces its inception to the launch of Vanguard 2 in February of 1959, just 12 months after our first ever successful satellite, Explorer 1. The spacecraft failed because of a wobbly attitude. An improper orbit on the next attempt, with Explorer 6, led to a few low resolution pictures of Earth. We achieved success with Explorer 7, orbited on October 13, 1959, which carried a radiometer designed by Verner Suomi, America's pioneer in satellite meteorology, and his colleagues. Using black and white hemispheres, this instrument measured solar and terrestrial radiative energy (determining reflective and absorbed components) to estimate radiation balance.

The first experimental meteorology series began with TIROS-1, launched on April 1, 1960. TIROS stands for Television and Infrared Observational Satellite. The main instrument was a vidicon, which is a modified television camera that scanned through 500 lines, each containing 500 pixels. We show here one of the nearly 23,000 images returned from TIROS-1:

14-8: Why is there a blocky disjointedness associated with this image? ANSWER

TIROS-2 included the Medium Resolution Infrared Radiometer (MRIR). The TIROS-8 vidicon had an 800-line resolution and was the first to use the Automatic Picture Transmission (APT) technique. The TIROS series (10 in all) was non-polar following orbital inclinations between 48° and 58° . The first eight TIROS maintained orientation by spinning at 12 revolutions/minute, which limited the time during which the camera pointed at Earth. TIROS-9 followed a "cartwheel" spin pattern (spin axis perpendicular to the orbital plane), which facilitated coverage to allow strips of images that users could mosaick to provide global composites:

The next fleet of metsats, the Nimbus series (total of seven), was dedicated largely to research and development experiments. We name the instruments used in this series in this table

Nimbus-1, launched on August 28, 1964, was the first that we put in a sun-synchronous orbit (now the norm). It used a three-axis stabilization technique (based on flywheels) that kept it pointed constantly at Earth. It carried several instruments including the Advanced Vidicon Camera System (AVCS, an APT) and the High Resolution Infrared Radiometer (HRIR ), which operated in the 3.6-4.2 µm interval. An example of a Nimbus 1 HRIR image, taken at night over western Europe, appears next, on the top (note the distortion that enlarges Germany and Sweden relative to southern countries-the Italians may be aggrieved by the shrinking of their "boot"!). On the bottom is a visible Image Dissector Camera System (IDCS) image of the southeast U.S., as seen by Nimbus 3:

To compare the information obtained in the visible (left) and the thermal infrared, look at this image pair of the Gulf of Mexico acquired simultaneously by Nimbus 4.

14-9: The thermal image shows extended groups of swirly white patterns. Are these clouds or thermal patterns in the Gulf of Mexico seawater? Cite a logical clue to your conclusion. ANSWER

In the next image we get a feel for what we can see at the 1.1 km (0.7 mi) resolution, provided by the IDCS visible channel on Nimbus 3. Under optimal viewing conditions, parts of Wyoming and Utah appear here (this scene was the first space imagery ever worked on by the writer in 1970).

The basins generally are highly reflective, and the mountains tend to stand apart because of dark tones associated with evergreen vegetation. Compare this image with the Landsat 1 MSS image of the Wind River Mountains (upper right in the Nimbus view) in Section 2 .

14-10: This Nimbus image was the first ever worked on by the writer (NMS) when I transferred from the planetary to the remote sensing programs at NASA Goddard Space Flight Center. Then, the Landsat image that you may have checked out by clicking was the first I examined (in the field ultimately) after that satellite sent its first pictures in July of 1973. These are my "holy grail". Just for fun, why don't you fit the Landsat image into the Nimbus image - and check the answer. ANSWER

Nimbus 3 (April 14, 1969) also was the first to use atmospheric sounders extensively, with its Infrared Interferometer Spectrometer (IRIS ), operating between 6 µm and 25 µm and Satellite Infrared Spectrometer (SIRS ), sensing in the 15 µm region. Nimbus 4 (April 8, 1970) carried the Backscatter Ultraviolet (BUV ) radiometer, becoming the first metsat to measure atmospheric ozone.

Two instruments on Nimbus 5 (December 11, 1972) are of special significance. The Surface Composition Mapping Radiometer (SCMR) uses two thermal bands, 8.4-9.5 µm and 10.2-11.4 µm, to produce color-coded temperature maps, such as this image of Florida and Cuba and surrounding waters, made from the 8.8 µm channel:

Ratios of radiant temperatures measured by the two bands provide a qualitative estimate of SiO₂ content of rocks and soils. This process uses the concept of "restrahlen", a German term that refers to decreased emissivity (a parameter mentioned in Section 9) because of resonance vibrations associated with silicon-oxygen bonds in silica tetrahedra. An emissivity trough shifts towards longer wavelengths as silica content increases.

14-11: How might this silica shift be used practically? ANSWER

The Electronically Scanning Microwave Radiometer (ESMR ) on Nimbus-5 operated at a 19.35 GHz frequency (1.55 cm wavelength) to sense brightness temperatures of the surface and atmosphere. This instrument was capable of sensing surface ice temperatures, especially in the polar regions, as shown in this time series of maps that plot the percentage of ice cover around Antarctica on a monthly basis in 1974. The ESMR was also the first to use microwave absorption to estimate precipitation (rain rates) by quantifying increases in optical depth, which correlate to higher brightness temperatures. The ESMR on Nimbus 6 (June 12, 1975) was set at 37 GHz (0.81 cm).

The Scanning Multichannel Microwave Radiometer (SMMR), flown on Nimbus 7 (October 24, 1978), and again on Seasat, consists of a 5-channel (0.81, 1.36, 1.66, 2.80, 4.54 cm), dual-polarization instrument that provides data of value to many applications: ocean circulation, low altitude winds, water vapor, cloud liquid water content, sea ice type, extent, and concentration, snow cover and moisture, and rainfall rates. Data on tropical rainfall for the months of 1986 over the Indian Ocean and South Asia is one such product:

The SMMR monitored ice in the Antarctic shelf during 1985, as displayed for four months here:

The first Total Ozone Mapping Spectrometer (TOMS) on Nimbus 7, which is still operating, measures UV reflectivities at 0.312, 0.317, 0.331, and 0.340 µm. It calculates ozone quantitiesfrom the ratio of the returns in the 0.312/0.331 µm wavelengths. As an example of a global ozone map, here is the plot of data obtained on May 14,1992.

(Note: A Dobson unit is the response from a compressed column of ozone equivalent to a 0.01 mm thick, measured over a fixed area centered on Labrador in eastern Canada.)

We can also use the TOMS to monitor SO₂ in the atmosphere. After some major volcanic eruptions, it tracked extensive clouds of SO₂-enriched ash and gases injected into the upper atmosphere daily across much of the world, until they dissipated below detection levels. Here is the status on June 20, 1991 of the cloud produced by the Mt. Pinatubo eruption in the Philippines.

The first Solar Backscatter UltraViolet (SBUV ) senser was on Nimbus 7 (also on NOAA-9 and 11), sharing some of the components of the TOMS. The SBUV had 12 channels in the UV region. We discuss the Coastal Zone Color Scanner (CZCS), which flew exclusively on Nimbus 7 in some detail later in this Section

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