Observations of planets by the Imagers on the present GOES satellites and by the Advanced Baseline Imagers (ABI's) on the future GOES-R satellites have the potential to augment the thermal IR (TIR) calibration performed with the onboard blackbody, the Internal Calibration Target (ICT). The gain and offset of the TIR channels are determined by measuring their signals when they view the ICT, but the ICT has a limited temperature range and is observed only at one pair of azimuth and elevation angles. ICT measurements alone are not able to detect on-orbit shifts in spectral response functions (SRF's) or variations in the reflectivity of the E/W scan mirror as a function of scan angle. Changes in these parameters over the life of the mission can produce bias errors and uncertainties in long-term trending.

Observations of a planet with a TIR spectrum that is very different from the blackbody spectrum of the ICT have the potential to detect anomalies in the SRF's of the TIR channels. A sequence of observations of a planet as it moves from west to east through the field of regard of the Imager or the ABI can be used to quantify the angular dependence of a scan mirror's reflectivity. Observations of a planet can also be used to verify the co-registration among TIR and reflective channels (which is especially useful for those channels that lie in atmospheric absorption bands and thus cannot produce clear images of landmarks on the Earth), and to cross-calibrate among instruments on different satellites by co-observation.

During the time interval from May 1 to May 19, 2011, the two inner planets, Mercury and Venus, had elongations (angular separations from the Sun) greater than 20 deg., separations from the moon greater than 3.5 deg., and declinations from -10.5 deg. to +10.5 deg. that caused them to pass through the Imager's field-of-regard (FOR). Observations were made with three Imagers: GOES-11 at the GOES-West location, GOES-12 at the GOES-South America location and GOES-13 at the GOES-East location: all with nominal latitudes of 0 deg. and with nominal longitudes of 135 deg. W, 60 deg. W, and 75 deg.W, respectively. The observation times were restricted to occur when a planet was at least 1 deg. above the Earth's limb (> 9.7 deg. from nadir), and at times normally reserved for star sensing, intervals 30 minutes apart, to avoid disrupting NOAA's operational schedule of Earth observations. Mercury was targeted on two consecutive small frames, one minute apart. Venus, which was separated from Mercury by slightly more than 1 deg. during many observations, was also detected on several occasions.

When viewed from geostationary orbit, Mercury exhibits phases, having an angular diameter that varies from about 25 microradians in its gibbous phase to 50 microradians in its crescent phase. Its sunlit surface, the only portion that emits significant amounts of TIR radiation, is smaller than the ABI's 56x56 microradian instantaneous-fields-of-view (IFOV's), as well as the Imager's 112x112 microradian and 224x224 microradian TIR IFOV's, but it is much hotter than the ICT and the Earth scenes. Since the sunlit surface can also be detected in the ABI's solar reflective channels (Ch 1-6), it is an excellent target for measuring the co-registration among all of the ABI's channels. The effective temperatures of Mercury and Venus, i.e. the temperatures of a full aperture blackbody that would produce an equivalent signal in each of the Imager's channels, were computed using NOAA's GOES VARiable format (GVAR) algorithm.

Since Mercury's spectrum is more heavily weighted toward short wavelengths, its effective temperatures in the Imager's TIR channels were observed to increase rapidly with decreasing wavelength and to lie within their dynamic ranges in most cases. The effective temperatures were >300K in the Imager's Channel 2 (WL = 3.9 microns) and < 200K in Channels 4, 5, and 6 (WL's > 10 microns). Mercury's temperature in Channel 6 on GOES 13, (WL = 13.3 microns, IFOV = 224x244 microradians) was about 150K, too low for it to be a good calibration source. Mercury's effective temperatures trended upward as its phase transitioned from crescent to gibbous.

Since the ABI has smaller IFOV's than the Imager, the effective temperature of Mercury will be greater in any of the ABI's TIR channels than it is in the Imager's TIR channel at comparable wavelengths. Observation of Mercury promises to be a sensitive method for detecting anomalies in the SRF's of ABI's TIR channels. A 0.1% decrease in the SRF's wavelength is estimated to produce an increase of about 0.1K in Mercury's effective temperature, comparable to the noise-equivalent temperature difference (NEDT) in most of the ABI's TIR channels.

The wavelength-dependent thermal profile of Venus (which has an atmosphere) is more complex, but it also differs greatly from those of Earth and the ICT, making it a useful calibration target for the Imager and the ABI. It appears likely that its effective temperature will usually exceed the nominal maximum temperatures in all of the ABI's TIR channels except for the 400K maximum temperature in the 3.9 micron channel, but it might not exceed their saturation temperatures.

Mercury and possibly Venus also appear to be good targets for the other proposed uses: measurement of variations in the reflectivity of the E/W scan mirror, co-registration among channels, especially those in the atmospheric absorption bands, and cross-calibration with instruments on other platforms. Whenever they enter the ABI's FOR and satisfy solar and lunar separation constraints, these planets can be observed with minimal modifications to the ABI's observational schedule.