HALOE data with potential problems
Two artifacts have been uncovered in the HALOE data set that need to be brought to attention of the users of the data. These artifacts potentially affect only about 2.5% of the data but nevertheless they need to be noted. A brief description is provided here and detailed descriptions are available via the links.
Trip angle: Under normal circumstances, during sunrise events, the HALOE instrument acquires and begins tracking the solar image in the troposphere; the instrument continues to track the sun as it appeares to rise though the atmosphere until the image is above a tangent point altitude of 150km or more at which time HALOE switches to the solar scan mode. Due to a commanding error, on some events the instrument switches to performing solar scans before the 150km point is reached; this leads to potential data corruptions. A few sunset events had a similar problem. In both cases, the data corruptions can lead to poor data quality. A full description with a list of potentially bad events is available. Full Description
Constant lockdown angle: To promote stability in the retrievals some signal processing as a function of altitude was utilized. In the upper portions of the retrievals where the signal to noise ratio is small, the signals were smoothed; this smoothing (channel dependent) was gradually relaxed once sufficient signal levels were reached. In addition, the lockdown angle, the angle measured from the top of the sun to where the FOV was positioned on the solar image, was also processed to help reduce retrieval jitter. Above where there was significant signal to noise ratio, the lockdown angle was set artificially to a constant when used in signal simulations. Once the signals increased, the lockdown angle was abruptly switched to a smoothed value and then gradually switched to the actual lockdown angle. For small lockdown angles such as those used in the very early part of the mission, this abrupt switch from a constant lockdown angle induced artifacts in the retrieved temperature, water vapor and ozone. For a full description of this problem including a list of events with small lockdown angles that potentially exhibit artificial features in some profiles, see the Full Description.
