Active cavity radiometers (ACRs) have been used for years to measure solar radiation and Earth radiation from low Earth orbit. NISTAR is a low-noise, near room-temperature ACR that will provide unprecedented sensitivity in a space qualified instrument. The key features that enable low noise and drift are: high-sensitivity positive temperature coefficient (PTC) thermistors for low-noise high-gain temperature control, digital servo loops employing a drop-floor algorithm for the most efficient use of measurement time, and AC-bridge based measurement electronics that provide sub-ppm resolution, all coupled to a low-mass, low-conductance receiver cavity design. A prototype radiometer was recently reported (Rice et al., 1999) at NIST and used to demonstrate the feasibility of measuring a microwatt signal with a signal-to-noise ratio of 100:1 or better with an ambient temperature instrument.
Orbiting the L-1 point aboard Triana, the ~0.5° full Earth disk will be within the 1° full-angle field of view of NISTAR. NISTAR consists of four channels, three electrical substitution radiometers and a silicon photodiode detector, that will make continuous measurements of the sunlit Earth disk. The three radiometers will be used to make simultaneous measurements in three bands. Band-A is unfiltered and therefore measures the total radiant flux coming from the Earth both emitted and reflected across all wavelengths. The radiometer is spectrally flat with very broadband response that ranges from below 200 nm to beyond 100 µm. Band-B is the solar channel and employs a water-free quartz filter that transmits from 200 nm to 4 µm. By cutting off radiation above 4 µm the thermal emission from the Earth is blocked. Band-C is a near-infrared channel defined by a filter that transmits from 720 nm to 4 µm. This spectral range represents approximately half of the reflected energy from the Sun, separating the UV- VIS from the near and shortwave IR solar reflected radiation. The filters are mounted in a 12-position wheel with redundant filters for Bands B and C thereby allowing monitoring of filter degradation during the lifetime of the mission.
A silicon photodiode detector based radiometer channel has been included in the instrument in order to obtain a faster time series (<1 second) than what can be obtained by the radiometers. This channel provides for a number of measurements such as tracking the stability of the filters, verifying the co-alignment of NISTAR with EPIC, and providing a continuous broadband measurement of the solar reflected radiation from the Earth with high temporal resolution. The spectral response of the photodiode channel is 190 nm to 1100 nm.