A GOES-15 (GOES-West) Star Tracker failed on 23 April 2015 at 2032 UTC. This leaves just one working Star Tracker; consequently image navigation has degraded. The image above shows three successive visible images centered near Crater Lake, OR, on 3 May 2015. The navigation shifts over time. An animation of 3.9 µm imagery, below, also from May 3rd (available here as an mp4), shows image navigation shifts throughout the day. A two-day animation of visible imagery centered on the Washington coast near Hoquiam (bottom, available here as an mp4) also shows the navigation anomalies that can be as much as 8 km in the infrared. Users who require precise animation in their GOES-15 imagery should be alert to this issue. NOAA/NESDIS, NASA and factory engineers are investigating possible fixes; GOES-15 status updates will appear here.
The longwave infrared (10.7 µm) and shortwave infrared (3.9 µm) channels on GOES-13 have been shown in the past to have poor co-registration, meaning that the sensors are not viewing the same pixel at the same time. This error can lead to false signals in (for example) the IR Brightness Temperature Difference product that has historically been used to detect fog and low stratus. The error can propagate to other products as well, such as GOES-R IFR Probabilities (a data fusion product used to detect fog). The error is most obvious along north-south shorelines.
The figure below (Courtesy Tony Schreiner, SSEC/CIMSS) shows differences averaged over 6 pixels near the eastern shore of northern Lake Superior. The orange line (labeled ‘Original’) represents differences arising from the operational algorithm used before November 2014. Note that at 0700 UTC for this date and (clear) location (0600 UTC Surface Map) the brightness temperature difference nevertheless showed a negative value because the 10.7 µm pixel was over the cold lake and the 3.9 µm pixel was over warmer land. Shoreline on the western side of the lake would have a positive value: there, the 10.7 µm pixel would be over warm land and the 3.9 µm pixel (co-registered too far to the east) would be over cold water. This positive signal is consistent with fog detection.
The red line in the figure above also shows a shift at 1700 UTC, and that shift is apparent in data as well, as shown in the brightness temperature difference product, below, north of Lake Superior. The apparent shift occurs in the 1700 UTC image.
Between the 1515 and 1545 UTC imagery on 9 February 2015, a software change was implemented by NESDIS (link), and that now operational software is represented by the green line (labeled ‘Resample’) in the figure above. Rather than a step function change, a smoothly varying change is applied to the co-registration over the course of the day. This has reduced the obvious changes in brightness temperature difference fields that occurred between 0645 and 0700 and between 1645 and 1700 UTC. Consider the two animations below (Courtesy Jim Nelson, SSEC/CIMSS). In both, the former operational technique (the red line in the figure above) is on the left and the current operational technique (the green line in the figure above) is on the right. The operational change has certainly eliminated the jump that was occurring at 0700 and 1700 UTC.
Note that even the green line in the figure up top shows errors approaching 1 C at times during the day (and that may change over the course of the year). It is therefore still possible to find cases in which the brightness temperature difference field from GOES erroneously indicates fog or low stratus. The toggle below shows data from 10 February, after the operational change. A fog/stratus signal is indicated by the GOES Brightness Temperature Difference product along the eastern shore of Lake Michigan; however, there is no signature of fog/stratus on the VIIRS Day Night Band and brightness temperature difference (11.35µm – 3.74µm) imagery from Suomi NPP. As always, a positive indication of a phenomena in data should always be verified with other data types.
[Added, Friday the 13th]: The co-registration error between the longwave and shortwave infrared bands on the GOES-13 Imager is larger than on any of the other Imagers from GOES-8 through GOES-15. For more information, see here and here.
Previous posts on this blog (and elsewhere) have detailed the co-registration misalignment that exists between the 3.9 µm and 10.7 µm channels on the GOES-13 Imager. Because of this diurnally-varying co-registration error, a 3.9 µm pixel may be offset to the right or left of a 10.7 µm pixel; if this occurs near a pronounced temperature gradient (such as along a lakeshore), a false brightness temperature difference signal can ensue.
Consider, for example, the toggle above from 26 September 2014. A strong brightness temperature difference exists at 1825 UTC along the shorelines of Lakes Michigan, Huron and Erie; it is gone five minutes later, at 1830 UTC. There is no discernible change in the visible image over the same 5-minute interval (Link).
NESDIS operations alters the GVAR signal just before 1830 UTC (when the 3.9 µm imagery is shifted one pixel to the West) and at 0630 UTC (when the 3.9 µm imagery is shifted one pixel to the East) to mitigate the effects of the diurnally-varying co-registrations differences between the 3.9 µm and 10.7 µm channels. The imagery above shows the visible and two infrared (10.7 µm and 3.9 µm) channels at 1825 and 1830 UTC (GOES-13 was in Rapid Scan Operations mode at this time). The 3.9 µm imagery shows a one-pixel westward shift that is especially evident if you look at the unchanging navigation along the eastern shore of Lake Michigan. (1825 UTC imagery: Visible, 3.9µm and 10.7µm; 1830 UTC imagery: Visible, 3.9µm and 10.7µm) A similar link between 1815 and 1830 UTC on 25 September shows the same shift in the shortwave IR. A toggle between 0615 and 0630 UTC on 29 September shows the eastward shift in the 3.9 µm imagery that occurs then.
NOAA/NESDIS continues to monitor this co-registration issue.
GOES-13 is currently in Autumn Eclipse Season, when the Earth-Satellite-Sun geometry means that solar energy can reach the satellite sensors directly. NOAA NESDIS has software to mitigate the effects of Stray Light in the Sensor Processing System (SPS) that transforms the raw GOES Imager data to navigated and calibrated (GVAR) data. However, earlier this month, the SPS at Wallops inadvertently omitted the Stray Light Correction. The animation above, from 16-27 August, shows how Stray Light intruded into the 3.9 µm imagery on the GOES-13 Imager; on 25 August the Stray Light Correction was turned back on, and the final two images show no major Stray Light effects over the satellite view (Stray Light is still recorded in outer space). The animation above is for 5:15 UTC, when Stray Light affected the eastern part of the full disk scan. At 4:45 UTC, Stray Light affected the western part of the disk, and at 05:00 UTC, the central part of the disk.
Click here for more about the Stray Light Correction.