Meet the OMIS IOT
Hi! We are the OMIS IOT members, Jacques Claas (KNMI, NL), Leo van Lent (DS, NL), Mirna van Hoek (KNMI, NL) and Ayman Mekhail (NGES, USA). We are responsible for the health and safety of the instrument. One of us is on call 24/7 (every third week the same person). Lucky for us OMI and IAM don't have many problems. We monitor the engineering parameters looking for unexpected trends. When there is a problem, we investigate, analyse and try to diagnose the problem. We try to find and implement solutions, if there are any.
From the point-of-view of operations, there are three important phases in the lifetime of OMI:
- The testing phase
- The Launch and Early Orbit operations (LEO) phase, and
- The Nominal Operations phase.
The Nominal Operations phase
We are now in the Nominal Operations phase. Measurements are performed according to the Nominal baseline. For certain special events, for example in case of a volcanic eruption or during a (validation) measurement campaign, off-nominal operations with special instrument configurations may be required.
Engineering parameter trending
We monitor engineering parameters, like temperatures and mechanism parameters, for unexpected trends. Temperature increases and duty cycle decreases are expected trends and are also monitored. An automated job runs every day on the OMISIPS. It produces logfiles with warnings about possible trends and a postscript file, since a picture says more than a 1000 words. Since launch in 2004 we spotted (only) two troublesome trends. The first is the so-called Folding Mirror Mechanism anomaly that occured in February 2006, the second is the so-called row-anomaly (see section on "OMIS problems" below).
Ozone hole season
Each year during the ozone hole season we also monitor the intensity levels of the CCD detectors. Less ozone means less absorption, more reflected light into the instrument, and possible ADC saturated pixels. In order to prevent saturation we change the measurement types during the ozone hole season. See the Ozone hole monitoring page for further details.
OMIS problemsOMIS only experienced a few problems: Folding Mirror Mechanism Anomaly and row anomaly.
Folding Mirror Mechanism Anomaly
The Folding Mirror Mechanism (FMM) behaviour changed in the last week of February 2006 and on the 28th of February 2006 the FMM did not return to it's nominal position. What followed where many discussions of what could be the problem, and many tests of the mechanism to confirm our suspicions. This greatly improved our understanding of the FMM, but the FMM moved smoothly during all of our tests and none of our suspicions were confirmed. However, since we suspected the bouncing of the FMM against the end-stop to be the cause of the anomaly that occured on February 28th 2006, we decided to change the way the FMM is operated. This entailed updating all the calibration measurements that use the FMM and testing of all the updated measurement code. After 2 years all the calibration measurements have been updated.
In 2007 OMI started to experience the so-called row anomaly. Two rows seemed to be (partially) blocked. This row anomaly was follwed by another row anomaly in 2008. Like the first row anomaly a few rows seemed to be (partially) blocked. But these rows also suffered from reflected sunlight during part of the orbit and earthshine from another scene. This second row anomaly is changing through time (see below for more details). We had many discussions and looked at many pictures of the instrument, but have not been able to find a satisfying explanation for all the row anomalies. Corrections (if possible) and a flagging algorithm are being developped and for an update of the situation, please see the "Important Information for OMI Data Users" below.
Important Information for OMI Data Users
OMI has suffered from a so-called “row anomaly”. It affects particular viewing directions, corresponding to rows on the CCD detector, hence the name “row anomaly”. Other viewing directions are of optimal quality. Please read this information carefully prior to using OMI data.
The first appearance of the row anomaly is on June 25th, 2007. At that time, it appeared only in rows 53 and 54 (zero-based). From May 11th, 2008, cross-track scenes 37-41 (0-based) are affected as well. In January 2009 the range of rows extended to 27 - 44, while the original rows 53 and 54 remain affected. Since then the behavior of the row anomaly has remained dynamic. Which rows are affected and to what extent varies with time. Please visit the “Overview” section of our detailed technical information page for the most up to date information and details. A graphical overview of the affected rows is available on the background page.
Recommendations To Users
At the moment no corrections have been implemented in the operational level 1B and level 2 data. Please use the flagging provided in the level 1B and level 2 products to filter out affected data. All other OMI data, meaning unflagged scenes and earlier observations, is of optimal quality. Level-3 products are being produced after filtering for the flagged scenes.
Please visit the README and Data Product Quality Information for the OMI data product of your interest at the NASA DISC pages for OMI data.
Row Anomaly Corrections
The OMI team is aware of the anomaly and is currently investigating whether corrections for the effects can be implemented in the data-processing software. Please visit this information page regularly for updates on the status of corrections implemented and visit our detailed technical information page.
Row Anomaly Flagging
The Level 1B data are flagged for the row anomaly via the XTrackQualityFlags field. Most level 2 products have this information available now, either by copying the XTrackQualityFlags field into the level 2 product or via other newly defined flags in the level 2 product. Please check the README file of the L2 products for more information. Please visit our detailed technical information page for details on the current flagging status of the Level 1B and Level 2 products.