The DMSP Block 5D3 Special Sensors will provide the first comprehensive operational space-based investigation of the physical and chemical processes in the Earth's upper atmosphere (above 80km). The upper atmosphere is the region that contains the mesophere, thermosphere, and ionosphere. This region is poorly understood due to the difficulty in carrying out in situ measurements, its inherent complexity, and the need to develop a comprehensive global picture of this environment. While the basic physics that controls this region is understood, it does represent a difficult region to model since the atmospheric temperature and temperature gradients reach their largest values, composition changes from molecular to predominantly atomic, complex chemical and electrodynamic processes become the major determinants of composition, and where the combination of these effects prevent an adequate global description of the upper atmospheric "weather".

The study of the upper atmosphere has witnessed a major change in the last decade as ultraviolet technology has made the transition from spectroscopy to remote sensing. Traditionally the concern of optical aeronomers (scientists who study the structure, composition, and dynamics of the upper atmosphere using optical means) had been the identification of the excitation and emission mechanisms of spectral features. With the advent of an adequate physical description of the phenomena it has become possible to move beyond the simple identification of features to their interpretation in terms of geophysical parameters.

The FUV is ideally suited to determining thermospheric and ionospheric environmental parameters. It possesses optical signatures of all the major thermospheric species O, N₂, and O₂ (O₂ is seen in absorption on the limb) and the dominant F-region ion, O⁺ (on the nightside). Since we understand the processes which produce this radiation, we know that we need not telemeter down the entire FUV spectrum but can identify a few wavelength bands or "colors" that provide all the information required for an unambiguous determination of these environmental parameters.

The tables below summarize our anticipated performance as a function of geophysical region. The values listed for maximum intensities are the largest signals that we generally expect to see. The minimum detectable intensities (in Rayleighs) are our measurement goals for a signal with 10% counting statistics in a superpixel. The minimum detectable signal for 10% counting statistics is determined based upon co-adding SSUSI imaging mode spatial pixels into a larger spatial area super pixel during ground processing. The super pixel spatial resolution at nadir is 200 km by 200 km in the day- and night-side regions, and 30 km by 400 km in the auroral region.

In the presence of all UV maximum intensities, the maximum input count rate capability of the UV detector (200kHz) will not be exceeded. In fact, we expect to see a peak input counting rate of about 130kHz. The intensity at any single UV wavelength could exceed the maximum value as long as the total input rate across all UV wavelengths was below the detector maximum count rate. The maximum intensity at any visible wavelength cannot be exceeded, or the corresponding photometer detector output will saturate.

Airglow Intensity Requirements