5.2 NISTAR

5.2.1 Scientific Goals and Objectives

The Triana NISTAR measures the whole-Earth radiation in three absolute (self-calibrated) broad band channels:

1. A visible to far infrared (0.2 to 100 µm) channel to measure total radiant power in the UV, visible, and infrared wavelengths
2. A solar (0.2 to 4 µm) channel to measure reflected solar radiance in the UV, visible, and near infrared wavelengths
3. A near infrared (0.7 to 4 µm) channel to measure reflected IR solar radiance
4. A non-absolutely calibrated photodiode (0.3 to 1 µm) channel for calibration reference for the spectroradiometer

The earliest precedent would be Suomi¹s black and white flat-plate radiometers on the first Tiros satellite in 1963. Like NISTAR, Suomi¹s radiometers were designed to study the radiation balance of Earth, had a field of view encompassing the whole disk of the planet (from LEO), and co-flew with an imager.

Since Suomi, wide-field-of-view (WFOV) radiometers continued flying in LEO, but they were paid less and less attention by the radiation community. The tendency in radiation balance research has been toward a more statistical mechanics point of view, in which every pixel on Earth is accounted for separately. This has many advantages, including the production of monthly-average maps and the separation of cloud from non- cloud effects. But another reason for disinterest in WFOV data was that the orbits were just too low for them to be of much use. They could not reveal meaningful patterns, except for something as big and long-lasting as El Nino, because the spatial resolution was too low; and they could not be added up to give the integrated global picture. The big fields of view were a jigsaw puzzle impossible to fit together perfectly and many assumptions about diurnal cycle were necessary to fill the unsampled times of day. The NISTAR returns to the simple thermodynamic view of the Earth that Suomi was pursuing, but from a much better orbit that does not require merging data from successive orbits or making assumptions regarding the diurnal cycle.

The NISTAR also begins the process of looking at the Earth as a planet, rather than as a collection of pixels. In spite of Earth¹s complexity when seen from a worm's-eye LEO view, this complexity must average out over time and space to produce a planet satisfying some relatively simple laws. Some of these laws are not known yet, for example those relating global cloudiness to global warming. We are as unlikely to discover those laws from a worm's-eye view as to discover the perfect gas laws from tracking individual gas molecules. NISTAR cannot entirely solve this problem, but it is a first step down a worthwhile path which has, for a time, been somewhat abandoned.

Technologically, NISTAR is the avant-garde member of the Triana instrument suite. Like Suomi's experiment in its time, the NISTAR pushes the limits of absolute radiometry. Indeed, radiometry experts initially said that it couldn't be done. With any practical collection aperture at L­1, they said, there were too few photons to cause measurable changes in detector temperature. The Sun was easy, but the Earth, with a radiance less than one hundred thousandths that of the Sun, was very hard. The NISTAR is the answer to this challenge. It achieves a remarkable 0.1% accuracy without cryo-cooling. And it defines a technological path forward that will someday lead to spatially resolved absolute radiometry of Earth from L­1, affording the advantages both of the pixelated view and the integrated global view simultaneously.

The Triana NISTAR radiances will be used in several ways:

1. to make integrative measures of global change (longwave channel);
2. to estimate Earth albedo;
3. to attempt to interpret the near-infrared to visible albedo ratio;
4. to test EOS-CERES algorithms in an integral sense;
5. to provide an extensive integral test of how well radiative transfer in the Earth- atmosphere system is understood.

These usages are discussed in corresponding sub-sections below.