In this paper, we study the feasibility of a method for vicarious calibration of the GOES Imager visible channel using the Moon. The measured Moon irradiance from 26 unclipped moon imagers exhausted all the potential Moon appearances between July 1998 and December 2005, together with the seven scheduled Moon observation data obtained after November 2005, were compared with the USGS lunar model results to estimate the degradation rate of the GOES-10 Imager visible channel. A total of nine methods of determining the space count and identifying lunar pixels were employed in this study to measure the Moon irradiance. Our results show that the selected mean and the masking Moon appears the best method. Eight of the nine resulting degradation rates range from 4.5%/year to 5.0%/year during the nearly nine years of data, which are consistent with most other degradation rates obtained for GOES-10 based on different references. In particular, the degradation rate from the Moon-based calibration (4.5%/year) agrees very well with the MODIS-based calibration (4.4%/year) over the same period, confirming the capability of relative and absolute calibration based on the Moon. Finally, our estimate of lunar calibration precision as applied to GOES-10 is 3.5%.
The Imagers carried by NOAA's Geostationary Operational Environmental Satellites observe the Earth and its atmosphere in four channels in the thermal infrared and one in the visible part of the spectrum. Because of angle-dependent anomalous absorption in the scan mirror's SiOx coatings, the throughput of the Imagers in the infrared depends on the east-west angle of observation. If not accounted for, this effect would introduce artificial east-west gradients into the Imagers' observations of scene brightness temperature. Therefore, NOAA includes a model of the radiative processes at the scan mirror in its operational calibration. The input to the model includes a-priori values of the emissivity of the scan mirror vs east-west scan angle. The values NOAA uses operationally are estimated during post-launch testing from observations of frames of space that span the entire east-west extent of the Imagers' field of regard above and below the Earth's disk. However, we also have an alternative source of emissivity-vs-angle values - laboratory measurements made on witness samples of the scan mirrors at MIT Lincoln Laboratory. Are those emissivity values as effective as (or more effective than) the ones NOAA estimates in post-launch testing? To answer this question, we compared the results of in-orbit calibrations with the two sets of emissivity vs scan angle. Although the results depend slightly on channel and observation conditions, the values from the post-launch measurements in space are usually the better choice.
Stars are regularly observed in the visible channels of the GOES Imagers for real-time navigation operations. However, we have been also using star observations off-line to deduce the rate of degradation of the responsivity of the visible channels. We estimate degradation rates from the time series of the intensities of the Imagers' output signals, available in the GOES Orbit and Attitude Tracking System (OATS). We begin by showing our latest results in monitoring the responsivities of the visible channels on GOES-8, GOES-10 and GOES-12. Unfortunately, the OATS computes the intensities of the star signals with approximations suitable for navigation, not for estimating accurate signal strengths, and thus we had to develop objective criteria for screening out unsuitable data. With several layers of screening, our most recent trending method yields smoother time series of star signals, but the time series are supported by a smaller pool of stars. With the goal of simplifying the task of data selection and to retrieve stars that have been rejected in the screening, we tested a technique that accessed the raw star measurements before they were processed by the OATS. We developed formulations that produced star signals in a manner more suitable for monitoring the conditions of the visible channels. We present specifics of this process together with sample results. We discuss improvements in the quality of the time series that allow for more reliable inferences on the characteristics of the visible channels.
The National Environmental Satellite, Data, and Information Service (NESDIS) of the National Oceanic and Atmospheric Administration (NOAA) recently implemented enhancements to its operational calibration processing to mitigate the effects of two performance anomalies affecting the Imagers and Sounders aboard the current Geostationary Operational Environmental Satellites (GOES). This paper describes the anomalies, our algorithm enhancements to mitigate their effects, and results. In the first anomaly, the values of the computed calibration slopes in the infrared channels of the Imagers exhibit erroneous spikes during the six hours surrounding satellite midnight, causing observations of scene temperatures to be too low. We believe the spikes are the result of radiation from the solar-heated scan-cavity that reaches the detectors during the Imagers' calibration cycles. In November 2003, NOAA/NESDIS implemented a statistical algorithm that provides more realistic slopes around midnight. The second anomaly is "banding" in frames of observations by the Sounders' infrared channels. This also occurs during the six hours centered on satellite midnight. We believe the source of this anomaly is rapid changes in the temperatures of Sounder fore-optics components. They cause large and rapid changes in calibration offsets, which are not accounted for properly by the Sounder calibration updates, which only occur once every two minutes. In October 2004, NOAA/NESDIS implemented a remedial algorithm that removes the banding by interpolating the offsets to 1.1s intervals.
Although the visible channel of the Imagers carried by NOAA's operational Geostationary Operational Environmental Satellites (GOES) has no onboard calibration device, the decrease in the responsivity of this channel over time must be known if we are to make the data in this channel useful for detecting trends in the signals from the Earth. Therefore, some external method is required to provide this information. In this paper, we examine an external technique for monitoring responsivity changes based on empirical distribution functions (EDFs) of observations of the Earth's full disk. A time series of instrument outputs (in digital counts) at fixed levels at the tops of the EDFs is produced. A nonlinear least squares technique is then employed to adjust the time series for solar and seasonal effects and to fit it with an exponential, whose argument provides the rate of degradation of the responsivity. This technique assumes that the probabilistic structure of the signal from the earth does not change over time. The resulting time series and estimated responsivity degradation rates for the visible channels of GOES-8 and -10 Imagers will be presented. These results are similar to those obtained earlier with a star-based technique, thus increasing our confidence in the results of both techniques. The EDF technique and the star-based technique are synergistic, as they use very different approaches and data sets. Also, the star based technique works at the low end of the Imager's output signal range, whereas the EDF technique works at the high end.
Monitoring the responsivities of the visible channels of the operational Geostationary Operational Environmental Satellites (GOES) is an on-going effort at NOAA. Various techniques are being used. In this paper we describe the technique based on the analysis of star signals that are used in the GOES Orbit and Attitude Tracking System (OATS) for satellite attitude and orbit determination. Time series of OATS star observations give information on the degradation of the detectors of a visible channel. Investigations of star data from the past three years have led to several modifications of the method we initially used to calculate the exponential degradation coefficient of a star-signal time series. First we observed that different patterns of detector output versus time result when star images drift across the detector array along different trajectories. We found that certain trajectories should be rejected in the data analysis. We found also that some detector-dependent weighting coefficients used in the OATS analysis tend to scatter the star signals measured by different detectors. We present a set of modifications to our star monitoring algorithms for resolving such problems. Other simple enhancements on the algorithms will also be described. With these modifications, the time series of the star signals show less scatter. This allows for more confidence in the estimated degradation rates and a more realistic statistical analysis on the extent of uncertainty in those rates. The resulting time series and estimated degradation rates for the visible channels of GOES-8 and GOES-10 Imagers will be presented.
This paper discusses the operational in-orbit GOES-8 and GOES-10 imager scan-mirror emissivity trends, as well as their diurnal cycles. The imagers (and sounders) aboard both GOES-8 and GOES-10 experience a variation in scan-mirror emissivity along the east-west scan direction. The most obvious manifestation of this phenomenon is a difference in output between the east and west sides when the insthiments view space, but it is also present in observations of the Earth. The phenomenon is accounted for in the calibration process with an algorithm that makes use ofcoefficients incorporating the variation ofthe scanmirror emissivity with east-west scan angle. The coefficients are derived from measurements of space above the north pole and below the south pole made during GOES station-keeping maneuvers, which are performed a few times a year. Over time, these measurements allow us to compile a trend ofthe east-westemissivityvariation. Operational full-disk images are used to diagnose the diurnal behavior of the residual (after correction) east-west output differences. A comparison between the scan-mirror emissivity of GOES-8 and that of GOES-10 is made to search for patterns related to specific satellites. This paper also reviews how the east-west scan-mirror emissivity coefficients are derived and evaluates the effect ofuncertainties in the band-averaged emissivity measurements on the GOES calibration. An effective scan-mirror temperature is proposed to minimize the residual east-west output differences.
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