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SAGE II Mission Instrument
The platform for SAGE II is the Earth Radiation Budget Satellite (ERBS).
Nominal orbit parameters for ERBS are:
- Launch Date: October 5, 1984
- Planned Duration: 2 years
- Actual Duration: powered off on August 26, 2005
- Orbit: non-sun synchronous, circular at 650 km
- Inclination: 57 degrees
- Nodal Period: 96.8 minutes
The SAGE II instrument was a seven-channel Sun photometer that used a
Cassegrainian-configured telescope, holographic grating and seven silicon photodiodes,
some with interference filters, to define the seven spectral channel bandpasses.
Solar radiation is reflected off a pitch mirror into the telescope, forming an image of the sun
at the focal plane. The instrument's instantaneous field-of-view,
defined by an aperture in the focal plane, is a 0.5-by-2.5 arc-minute slit that produces
a vertical resolution at the tangent point on the Earth's horizon of about 0.5 kilometers.
Radiation passing through the aperture is transferred to the spectrometer section of the
instrument containing the holographic grating and seven separate detector systems.
The holographic grating disperses the incoming radiation into the various spectral regions
centered at the 1020, 940, 600, 525, 453, 448, and 385 nanometer wavelengths. Slits on the
Rowland circle of the grating define the spectral bandpass of the seven spectral channels.
The spectrometer system is inside the azimuth gimbal to allow the instrument to be pointed
at the Sun without image rotation. The azimuth gimbal can be rotated over 370 degrees so
that measurements can be made at any azimuth angle.
The operation of the instrument during each sunrise and sunset measurement is totally automatic.
Prior to each sunrise or sunset encounter, the instrument is rotated in azimuth to its predicted
solar acquisition position. When the Sun's intensity reaches a level of one percent of maximum in
the Sun sensor, the instrument adjusts its azimuth position to lock onto the radiometric center
of the Sun to within +/-45 arc-seconds and then begins acquisition of the Sun by rotating its
pitch mirror in a predetermined direction depending on whether it is a sunrise or a sunset.
When the Sun is acquired, the pitch mirror rotates back and forth across the Sun at a rate of
about 15 arc-minutes per second. The radiometric channel data are sampled at a rate of 64 samples
per second per channel, digitized to 12-bit resolution, and recorded for later transmission back to Earth.
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