To track the degradation of the Imager visible channel on board NOAA’s Geostationary Operation Environmental Satellite (GOES), a research program has been developed using the stellar observations obtained for the purpose of instrument navigation. For monitoring the responsivity of the visible channel, we use observations of approximately fifty stars for each Imager. The degradation of the responsivity is estimated from a single time series based on 30-day averages of the normalized signals from all the stars. Referencing the 30-day averages to the first averaged period of operation, we are able to compute a relative calibration coefficient relative to the first period. Coupling this calibration coefficient with a GOES-MODIS intercalibration technique allows a direct comparison of the star-based relative GOES calibration to a MODIS-based absolute GOES calibration, thus translating the relative star-based calibration to an absolute star-based calibration. We conclude with a discussion of the accuracy of the intercalibrated GOES Imager visible channel radiance measurements.

A priori estimates of the vertical distribution of tropospheric water vapor are needed to solve the radiative transfer equation in the 15 μm CO2 band to derive tropospheric temperatures from radiances measured at orbiting satellites. This paper shows how errors in estimates of water vapor mixing ratio propagate into these solutions for temperature, and in simulation establishes the sensitivity of the rms errors in a collection of temperature soundings from the NOAA 2 satellite to errors in mixing ratio. The simulations predict that errors found in available estimates of mixing ratio degrade the solutions for temperature in the low troposphere so that they provide little new information over that already in the forecasts of the National Meteorological Center.

Monitoring the responsivities of the visible channel of the Imagers on operational GOES satellites is a continuing effort at NOAA. To estimate the rate of degradation of the responsivity, we have been analyzing the time series of star signals measured by the Imagers for attitude and orbit determination. In this report, we begin by showing our latest results of monitoring of the responsivities of the visible channels of GOES-8, GOES-9, GOES-10, GOES-11, GOES-12, and GOES-13. One complicating factor in the analysis has been the presence in the time series of an annual cycle that modulates the gradual long-term degradation whose rate we are trying to infer. We describe a method we are developing to reduce the influence of the annual cycle on the analysis. The method enables us to include in the analysis the star observations near local midnight, which had been excluded in the past to prevent loss of accuracy in the derived long-term degradation rate. With a fuller set of data, we can subdivide the data within each year into 48 bins and estimate the degradation separately in each bin, thereby reducing the influence of the annual cycle on the derived degradation rates. One indication that the method is valid is that the degradation rates estimated in all the bins are consistent.

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 when viewing stars, 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 of the Imagers on GOES-8, -9, -10 -11 and -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 populated 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 not only produced star signals more suitable for monitoring the changes in the Imager's outputs from views of constant-irradiance stellar sources, but also gave more information on the radiometric characteristics of the visible channels. We present specifics of this technique together with sample results. We discuss improvements in the quality of the time series that allow for more reliable inferences on the gradually changing responsivities of the visible channels. We describe further contributions of this method to monitoring of other performance characteristics of the visible channel of an Imager,

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.

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 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.

A method for accurately predicting simultaneous nadir overpasses (SNOs) among different sun-synchronous polar-orbiting meteorological satellites is presented for intersatellite radiometer calibration. At each SNO, the radiometers on the two satellites view the earth and its atmosphere at nadir within a few seconds of each other, providing an ideal scenario for the intercalibration of radiometers. The basic mechanism and frequency of occurrences of such events are analyzed. Prediction using the Simplified General Perturbations No. 4 (SGP4), an orbital perturbation model, is presented, and examples of SNOs among the NOAA-16, NOAA-17, Terra, and Aqua satellites are provided. Intersatellite calibration using this approach has the potential for achieving the calibration consistency and traceability required for long-term climate studies.

Solar impingement on the advanced very high resolution radiometers (AVHRRs) near the terminator can contaminate the onboard radiometric calibration system and degrade AVHRR data. The solar contamination causes disagreement between the sensor‐measured radiometric output of the onboard blackbody versus its bulk temperature measured by the platinum resistance thermometers. Despite the sun shield installed on the latest AVHRR unit, solar contamination can still contribute errors of more than 0.5 K in the 3.7 μm channel and 0.25 K in the long‐wave infrared channels. The timescale and spectral characteristics of the contamination are analyzed to find the possible causes. In addition, stray light in Earth scenes is found whenever the radiometric calibration is contaminated. These effects occur as the spacecraft moves out of the shadow of the Earth at spacecraft sunrise. The intensity of the effects varies by orbit and season and is related to the solar zenith and azimuth angles at the spacecraft.

We present the pre-launch infrared calibration of the Geostationary Operational Environmental Satellite (GOES) I-M Imager and Sounder. In addition to contractual performance verification, pre-launch calibration provides necessary information for on-orbit operations. These are system relative spectral response, non-linearity in radiometric response and verification of the accuracy of the on board calibration source. The JR channels are calibrated in a thermal vacuum chamber, under varying instrument operating conditions, with two external, temperature controlled blackbodies. A LN2 controlled target represents cold space, and a variable (200 K to 320 K) temperature target represents the Earth scene. We show methods and results for the following instrument performance parameters: system spectral response, noise, non-linearity and relative accuracy. As there is no absolute radiometric standard, the relative accuracy estimates are between the internal (used for on orbit operational calibration) and external calibration sources. Performance trends versus instrument operating condition and across serial number (SNO3-SNO7) are highlighted. We show residual calibration anomalies and describe probable causes.

A study of the solar contamination in the radiometric calibration for the NOAA -14, and -15 Advanced Very High Resolution Radiometers (AVHRR) is presented. The solar contamination results in a disagreement between the sensor-measured radiometric output of the onboard blackbody vs, its bulk temperature measured by the platinum resistance thermometers. The sunshield installed on the latest version of the instrument on NOAA-15 may have alleviated but not eliminated the problem. The anomaly can still contribute errors on the order of half a degree in the 3.7 mum infrared channel. Analysis of the time scale and the spectral characteristics of the radiometric anomaly suggest that the extraneous radiation is a combination of solar radiation and background radiation by an object other than the blackbody. Stray light in earth scenes is also found where the anomaly in the radiometric calibration occurs. The radiometric intensity and spectral characteristics of the stray light are analyzed in order to trace the source of the extraneous radiation. It is found that the effects occur as the spacecraft moves out of the shadow of the earth at a low sun elevation. The intensity of the effect appears to be related to the solar zenith and azimuth angles at the spacecraft.

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.

The imagers and sounders aboard NOAA's Geostationary Operational Environmental Satellites (GOES) provide quantitative data for weather forecasting and studies of the Earth's atmosphere and surface. This paper describes the post-launch radiometric testing of the imagers and sounders and present result from the instruments aboard the GOES-8, - 9, and -10 satellites. In these tests, we measure such quantities as nose and signal-to-noise ratios, radiometric responsivities and their variability, and detector 1/f noise. Performance anomalies specific to GOES, such as variation of scan-mirror reflectance with east-west scan angle, are characterized. The on-orbit results are compared with the performance specification, with pre-launch test result, and with results, and with results of post-launch tests of the imagers and sounders on other GOES satellites.