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ASTER User Advisory
Change in Status Alert - January 12, 2009

ASTER SWIR detectors are no longer functioning due to anomalously high SWIR detector temperatures. ASTER SWIR data acquired since April 2008 are not useable, and show saturation of values and severe striping. All attempts to bring the SWIR bands back to life have failed, and no further action is envisioned. VNIR and TIR data continue to show excellent quality, meeting all mission requirements and specifications.


ASTER User Advisory
Change in Status Alert - September 1, 2008

ASTER SWIR bands continue to be adversely impacted by an anomalously high SWIR detector temperature. Although the ASTER team continues to focus on this situation, no improvement is anticipated in the next few weeks. Users are advised that ASTER SWIR data acquired from late April 2008 to the present exhibit anomalous saturation of values and anomalous striping. Cloud cover assessment and TIR location accuracy have also been affected. Earlier archived data conform to mission specifications.

Additional advisories will continue to be provided.


ASTER User Advisory
Change in Status Alert - May 7, 2008

Users are advised that ASTER SWIR data acquired in late April and early May 2008 exhibit anomalous saturation of values and anomalous striping. Cloud cover assessment and TIR location accuracy have also been affected by the present situation.

Since January 2008, SWIR performance has been stable and data quality has been nominal. On April 23, 2008, the SWIR detector temperature rose precipitously, and SWIR Bands 5-9 saturated.

In an attempt to lower the SWIR detector temperature and improve data quality, the ASTER Team plans to commence another SWIR recycling procedure on May 7, 2008. If successful, stable SWIR performance and nominal data quality will be restored.

The data quality impacts referenced in the April 9, 2008 alert still apply for the periods specified.
Additional advisories will continue to be provided.


ASTER User Advisory
February 5, 2008

This advisory is written to users of ASTER SWIR data to alert them to the fact that some anomalous saturation of values has been observed in ASTER Bands 5 through 9 since May 2007. This problem is attributed to an increase in ASTER SWIR detector temperature believed to be caused by increased thermal resistance in the SWIR cryocooler. VNIR and TIR bands are unaffected by this problem.

The slow increase in SWIR detector temperature did not become a problem until early in 2007, and it did not really affect data quality until the detector temperature exceeded 83ēK. Figure 1 shows the trend in SWIR detector temperature for the past year. Note that the detector temperature first exceeded 83ēK on about May 1, 2007. Following that date, four attempts have been made to lower the detector temperature by recycling the cryocooler, including increasing the stroke length of the cryocooler piston. The first attempt in May succeeded in reducing the temperature to 82ēK, but the temperature soon began to increase again, exceeding 83ēK in late July. Second and third attempts to reduce the SWIR detector temperature essentially failed in October. However, a fourth attempt in mid-January, 2008 succeeded in reducing the SWIR detector temperature to 83ēK. Since that date the SWIR detector temperature has remained stable at 83ēK.

Figure 1. ASTER SWIR detector narrow temperature trend. Maximum (red), average (black), and minimum (green) lines are plotted.

As long as the detector temperature remains at 83ēK, little or no degradation of ASTER SWIR data is expected. However, users are advised that for ASTER SWIR data collected between late May 2007 and late January 2008, the SWIR detector temperature exceeded 83ēK, except for about six weeks in June and July. SWIR data acquired during these periods may exhibit anomalous saturation of values, particularly at high sun angles and for materials that are highly reflective in the SWIR bands.

Figure 2 shows an example of SWIR saturation in an extremely bright desert scene acquired over northern Africa in August 2007, when the detector temperature was at about 83.5ēK. Saturation is especially prevalent in Bands 5, 6, and 7. Saturated pixels with DN = 255 are displayed in black. All other colors are unsaturated pixels.

Figure 2. SWIR saturation example. August 24, 2007 Africa observation. Pixels with DN = 255 are colored black.

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