Information Exchange Items
The two lists below define the interface between a Spacecraft Calibration Team (SCT) and the USGS Lunar Calibration Team (LCT) for model comparison of lunar irradiance observations. The interface is intentionally bilateral, i.e. the information will be available to both teams. There are standard Exchange File Formats for all items.
The SCT will provide to the LCT:
- The system relative spectral response of each band. This is required only once unless the instrument is known to change.
- The Universal Time (UTC) of the lunar observation. For scanning sensors, this is the time that the geometric center of the Moon is scanned, regardless of illumination. E.g. for push-broom systems, this is the time when the detector line array crosses the center of the Moon.
- The location of the spacecraft at the above observation time, in (Earth-centered, inertial) J2000 Cartesian coordinates.
- The oversampling of the Moon, or alternatively the apparent angular size of the Moon in the downtrack direction, in milliradians (Moon_Y_Size).
- The apparent lunar irradiance measured in each band, without correction for distances or oversampling. Preferred units are microWatt m-2 nm-2. If the Moon is clipped, this is for the portion of the Moon observed. In order to track calibration histories, an identification of the radiometric calibration version used and the presumed pixel solid angle should be provided.
- If the observed Moon is clipped, then the
following two parameters are required for each focal
plane. This assumes that the edge of coverage is a
straight line across the lunar disk (a chord):
- The width of unobserved segment, as the angular distance from the center of the chord radially out to the lunar circumference expressed as a fraction of the lunar angular radius.
- The position angle of a line from the center of the Moon to the center of this chord, measured counterclockwise from equatorial celestial north.
The LCT provides to the SCT:
- The spectral response data for each band, converted to an effective wavelength for a nominal lunar reflectance.
- High precision geometry parameters pertaining to the locations of the Moon and Sun at the observation time, and all appropriate geometric correction factors.
- Computation of the lunar irradiance model for the time and geometry of the spacecraft observation, interpolated the effective wavelengths of the spacecraft instrument bands. This includes correction for oversampling based upon the Moon_Y_Size parameter. Interpolations to spacecraft band wavelengths between the ROLO observational bands are done along a representative lunar reflectance curve. Panchromatic bands utilize a weighted sum of ROLO bands.
- The discrepancy between the spacecraft measurement and the modeled lunar irradiance, expressed as a percentage: ((measured/model)-1.)*100%. This will include an identification of the lunar model version used.
- If spacecraft image was clipped, an appropriate fractional correction to predict the irradiance from the entire Moon. (Prior to availability of a reliable lunar radiance model, this will be a simple geometric factor)
SCT Irradiance Processing
Spacecraft teams should produce an irradiance by doing a full radiance calibration of a lunar image using their usual or "best" calibration coefficients, presumably those used to produce their standard radiance data products. However, commonly a better zero-level radiance can be found from a lunar image (the surrounding space level) than from the standard data product processing. The irradiance is found by summing all the radiance pixels attributed to the Moon and multiplying by the nominal solid angle of a pixel (solid angle defined by sampling separation, not the MTF size of a pixel).
Note: this process is described in: H.H. Kieffer, P. Jarecke and J. Pearlman, "Initial Lunar Calibration Observations by the EO-1 Hyperion Imaging Spectrometer", Proc. SPIE 4480, 247-258, (2002); however, the SCT should not do any of the oversampling or distance corrections described therein.
If the oversampling of the Moon in an observation is not known, then the SCT must determine the apparent length of the Moon in their image, in milliradians. This must be done as accurately as possible, as the value enters linearly into the irradiance calculation. This is the limb-to-limb extent in pixels, multiplied by the down-track angular line spacing for normal nadir imaging.
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