Earth-emitted Irradiance at Triana

The Triana spacecraft will orbit the first Lagrangian point L-1, about a million miles from the Earth toward the Sun. The spacecraft will carry an active cavity radiometer NISTAR, which was made by NIST and Ball. This radiometer has 3 channels, which will measure the total irradiance from the Earth in the shortwave, near-IR and longwave ranges. The deconvolution theory for wide field-of-view radiometers can be used to compute efficiently the Earth-emitted iradiance as measured by NISTAR, so as to compute the mean Earth-emitted flux. It has been used to compute the bias of the irradiance near L-1 due to L-1 being over the Tropics and over the summer hemisphere. The method is extended to account for diurnal variations of Earth-emitted radiation and the effects of limb-darkening variations over the Earth. Because of the great distance from Earth, only a few terms are required to represent accurately the irradiance at Triana, making the method vastly superior to numerical integration of a detailed flux map. Results are shown for a nominal mission profile, beginning with the opening of the NISTAR 60 days after launch and through several cycles of its Lissajou orbit around L-1.