Known Issues

12.17.09 - Known Issue on the Accuracy of the LANCE-MODIS Terra Geolocation Compared to the Standard Product

The Terra geolocation from the LANCE-MODIS system could have an average error of 250m and, in the worst case, 500m (1 pixel) when compared to the standard Collection 5 product as a result of differences in the correction applied to the instrument pointing direction. In the standard system, the attitude quaternion is used to determine the solar elevation angle correction applied to the instrument orientation. In the LANCE-MODIS system the attitude and ephemeris data entrained in the L0 data are used and no quaternion is available. In this case, a fixed attitude quaternion is determined by assuming that the satellite always has a nominal orientation. Although the difference in pointing is very small, it is large enough to introduce a noticeable error in geolocation. It is expected that this problem will be fixed early in 2010.

The impact of this error on the LANCE-MODIS L2 products is very small and, in fact, when the problem is fixed it is not expected that there will be noticeable changes in these products. These products remain validated without caveat. However, the Terra L2G products show visible error and user discretion is advised.

While there are, of course, differences between the Aqua LANCE-MODIS products and the standard products since the former uses the rate-buffered GBAD attitude and ephemeris data from EDOS, the status of these products is not affected by the issue noted above. All Aqua LANCE-MODIS products (including the L2G products) remain validated without caveat.