Initial Analysis of GOES-13 Imager Solar Contamination During Eclipse

X. Wu; DraftedNov. 20, 2006; Revised Dec. 13, 2006

Introduction

The newly launched Geostationary Operational Environmental Satellite (GOES-13) is the first in a series (GOES-N/O/P) that is equipped with improved battery to enable normal operation throughout the semiannual eclipsethat occur around the solstices. During the Post Launch Tests (PLT), however, it was recognized that measurements should not be taken too close to the Sun. For the Imager, proximity of the filed of view (FOV) to the Sun is often associated with serious stray light contamination that compromises the value of the data for remote sensing applications.

Figure 1: Visible (top panels) and near infrared (3.9 μm, bottom panels) channel images from GOES-13 Imager around 0600 UTC (satellite midnight) on August 31, 2006. Horizontal lines are caused by noise in data reception not related to sensor performance.

The problem was first identified around 0600 UTC on August 31, 2006, when GOES-13 was stationed above the equator at 90ºW. The time coincides with the local midnight at the satellite nadir point.On this first day of the autumnal eclipse season for GOES, the Sun was at the top of the GOES-13 field of regard (FOR) before and after 0600 UTC, tangent to the Earth at the North Pole at 0600 UTC. (The Sun’s position around 0600 UTC would progressively move to the South Pole in the next 48 days.) Figure 1 shows that the visible channel detected strong signals where they are not expected, in the space views and Earth views that are fairly far away from the Sun. The infrared (IR) channels (only the 3.9 μm channel images were shown in Fig. 1) indicate contaminated data in those regions.

These contaminated data, while not a threat to the instrument safety and health, are useless nevertheless and represent a waste of valuable resources for observing other parts of the world. It was therefore decided to establish a “Keep Out Zone” (KOZ) such that the Imagers on GOES-N/O/P series will not acquire contaminated data once commissioned. This analysis summarizes the initial attempt to characterize the GOES-13 Imager KOZ. The objective is to answer the following questions:

  1. Which channels are affected? By how much?
  2. How to quantify the KOZ?
  3. What is the KOZ in terms of time and space?
  4. Does the KOZ depend on channel?
  5. Does the KOZ depend on the position of the Sun in the GOES FOR?

Sounder

Compared to the Imager, Sounder has longer dwell time for each pixel in normal operation, which caused part of the instrument temperature exceeding the safe limit on August 31, 2006 when exposed to direct sunlight. Sounder has since been excluded from KOZ experiment. Occasionally, Sounder data quality was compromised when the Sun is nearby, as suggested in Fig. 2. In general, it is expected that the KOZ derived from the Imager data analysis is also applicable to the Sounder, although that has not been verified and need to be carefully monitored.

Figure 2: Sounder data contamination at 0600 UTC on October 6, 2006, when the Sun came from behind the Earth.

Data

In collaboration with the GOES-13 PLT team, Space Science and EngineeringCenter at the University of Wisconsin (UW/SSEC), and Office of Satellite Data Processing and Distribution (OSDPD), three special sectors were designed for GOES-13 Imager during the KOZ experiment from September 12 to October 17, 2006. These sectors are full width in the East-West direction but approximately one third of the full disk in the North-South direction (from slightly above the North Pole to approximately 13ºN, between approximately 15ºN and 15ºS, and from approximately 13ºS to slightly below the South Pole).During the KOZ experiment, GOES-13 Imager was commanded to scan one of these sectors every 15 minutes from 0455 UTC to 0655 UTC. Two of these images, at 0555 UTC and 0655 UTC, were away from the Sun; the purpose was to confirm the absence of contamination in the “safe zone”. The rest of the images were following the position of the Sun on the day; the purpose was to monitor the evolution of the KOZ.

Figure 3 is an example of the solar contamination in one of the three sectors around 0510 UTC on Sept. 12, 2006. In the visible channel image (left), white indicates bright or contamination since no light is expected except at the left edge of the GOES FOR. In the IR image (right), black indicates hot, so the feature resembling a quarter of a ring to the left of the image is caused by contamination.

Figure 3: GOES-13 Imager Channel 1 (visible, left panel) and Channel 2 (3.9 μm, right panel) images at 0510 UTC onSept. 12, 2006, the first images collected for GOES-13 Imager KOZ study.

Analysis

KOZ Identification

The KOZ was identified through comparison with a reference image. For the visible channel,since no reflected sunlight is expected at the time of observation, all signals above noise level are due to contamination. For the infrared channels, KOZ causes large and consistent difference between the contaminated and reference images that cannot be explained by change in scene temperature between the observing times, as shown in Fig. 4. This analysis was applied to all images. It was found that images taken around the satellite midnight but are sufficiently far away from the Sun (in the sector from approximately 13ºS to slightly below the South Pole, scanned at 0555 UTC and 0655 UTC) are not subject to the stray light contamination.

Figure 4: Brightness temperature difference between 0510 UTC (contaminated) and 0525 UTC (not contaminated) on Sept. 12, 2006 for the four IR channels of GOES-13 Imager (the Channel 2 image in the upper left panel is that of Tb instead of δTb). The impact is severe on Channel 2 (3.9 μm), up to 75°K, and progressively smaller for longer wavelength but still traceable even for Channel 6 (13.3 μm). (T. Schmit)

KOZ Features

As can be seen from previous figures, the solar contamination is not uniformly decreasing away from the Sun. Rather, there seems to be a “ring feature” when the Sun is to the west, and this feature seems similar for both channels. Other features also exist, for example the “rays from the center” feature when the Sun is to the east. Examination of all contaminated images collected during the KOZ experiment suggests that these features are persistent from day to day. Some of these features are collected in Fig. 5. It seems that the shape of contamination, especially for Channel 2, is similar to that of Imager’s Secondary Mirror, with some variations at different time and day. It also seems that the contaminated area in the visible channel, though lacking many fine structures, is otherwise similar to that in the IR channels.

Figure 5: Selected Channel 2 images to show features in the contaminated data. Upper right: 0625 UTC on September 12, 2006. Upper left: 0510 UTC on October 12, 2006. Lower right: 0610 UTC on October 12, 2006. Lower left: 0525 UTC on October 17, 2006.

KOZ Quantification

If the Imager records solar radiance when it looks away from (though close to) the Sun, it is contamination. When the Imager looks sufficiently further away from the Sun, the contamination disappears. The KOZ can therefore be quantified by the distance, in terms of the angle between the Sun and the pixel when viewed from the satellite, that separate the contaminated and un-contaminated pixels.

For an image, the mean sensor response can be computed and plotted as a function of the angular distance. Figure 6 is such a plot for the 0510 UTC image on September 12, 2006, by GOES-13 Imager visible channel. There is a general trend of decreasing contamination intensity away from the center of the Sun, with a secondary peak around 4° that corresponding to the ring feature in Fig. 3, followed by a sharp decline after 5°. The mean signal is never reduced to zero, most likely because of the few bright lines in Fig. 2 (some invisible in the small image) that were caused by ground receiving.

Figure 6: Mean sensor response (raw count – space count) for GOES-13 Imager visible channel at 0510 UTC on Sept. 12, 2006, plotted as a function of angular distance between the Sun and a pixel with respect to the Imager. 0° means the Imager is looking toward the center of the Sun.

It might be reasonable to state that the KOZ is 5° or 6° based on Fig. 6, however it would be premature to characterize the KOZ that way for all images. Figure 7 shows that the KOZ can be quite different, depending on the position of the Sun. Similar plot should be analyzed for other UTC times, especially when the Sun is at the other side of the Earth, to gain a more comprehensive view. Current results suggest that the KOZ is normally around 5° but can be up to 10° when the Sun is near the pole.

Figure 7: As Fig. 6, but for all 0510 UTC images during the investigation. Color is used to indicate the date.

Conclusions:

  1. Stray light from the Sun during eclipse can affect all channels of the Imager. The impact is large (several hundred of counts) for Channel 1 & 2, smaller for IR channels with longer wavelength, but still noticeable even for Channel 6.
  2. Images obtained around the satellite midnight but sufficiently far away from the Sun are not affected.
  3. A Keep Out Zone (KOZ) is recommended such that the Imager should not scan closer to the Sun than the KOZ to avoid solar contamination.
  4. The KOZ does not seem to depend on channel.
  5. The KOZ seems to be smaller (~5°) when the Sun is near the equator and larger (up to 10°) when the Sun is near the poles.
  6. Recommend to repeat the data collection and analysis during spring equinox
  7. Confirm the effect of yaw-flip (or the lack thereof)
  8. Monitor the KOZ before/after the Sun approaches the poles