README file for OMI Field-of-View predictions. Edward A. Celarier edward.a.celarier@nasa.gov edward.celarier@gmail.com Telephone: 240.620.3748 (Cell, text messages, and WhatsApp) Abstract I have generated a set of daily orbital coverage maps for the OMI instrument, covering the period of the DISCOVER-AQ mission in Colorado (15 July to 15 August). These images are available from the Aura Validation Data Center at Goddard. Introduction The Aura satellite is in a polar, sun-synchronous orbit, with an ascending node local equator crossing time of 13:45. Aura carries four instruments, of which two (the Ozone Monitoring Instrument, OMI, and the Microwave Limb Sounder, MLS) are continuously taking data. The other two are the Thermal Emission Spectrometer (TES), which is currently running part-time only, and HRDLS, which failed soon after launch. As OMI is of particular interest to the studies being undertaken as part of DISCOVER-AQ and FRAPPE, I have calculated the predicted trajectories of OMI's 60 fields of view for the orbits that occur during the field campaign's duration. This can be used to anticipate when ground- and aircraft-based measurements may be made collocated in time and space with an OMI-based measurement. OMI Measurements OMI is a pushbroom-type instrument that makes 60 simultaneous measurements in fields of view (FOV) that are arranged along the cross-track direction. The fields of view near the nadir (positions 29 and 30) are approximately 26 km (cross-track) by 13 km (along-track). However, the FOV size in the cross-track direction grows rapidly toward the port-most and starboard-most FOVs, i.e. scan positions 0 and 59. The larger the FOV, the less representative its measurements of trace gases and aerosols may be of the value at a particular ground position. The measurement simply takes in a lot of ground area that is as much as hundreds of km from any site of interest. Furthermore, a large FOV that is identified with a particular site (say, Boulder) may extend over the urban-to-rural plateau in the South Platte River Valley, but may also extend over the Rocky Mountains. Some of the retrieved trace gas column amounts, including ozone and nitrogen dioxide, depend upon the terrain altitude at a representative point for each field of view. That point is generally the field-of-view centroid point. In the case of Boulder and similar locations, the "representative point" may have a very different altitude from the ground point of interest. The Aura instruments, including OMI, have now been flying for ten years, and have degraded in some ways from their original design specifications. Since November 2007 (and perhaps earlier), OMI has developed a problem that is usually called the "Row Anomaly." ("Row," in this context, means cross-track position: A particular FOV.) It appears that a piece of multi-layer insulation (MLI) debonded from the spacecraft's body, and insinuated itself in front of OMI's entrance optics. This causes a partial optical occlusion. The set of FOVs affected has changed from time to time since this first appeared. Although measurements made at affected FOVs sometimes appear to be reasonable, they should not, generally speaking, be trusted. Because of these considerations, the orbital coverage maps distinguish between (a) good-quality FOVs [bright color], (b) near-edge, large FOVs [greyed color], and (c) FOVs that are affected by the row anomaly [blacked-out]. Prediction Maps The OMI FOV prediction maps show a particular geographical region, along with the OMI orbital swaths that come over that region on a particular day. In a small region like the area covered by the DISCOVER-AQ/FRAPPE/GEO-TASO campaigns, only 1-3 Aura/OMI orbits are relevant for a single day. The prediction map shows up to three orbits that are color-coded as red, blue, and yellow. Boundaries between the FOVs are drawn, and the region between the boundaries are colored in one of three shades: Bright, indicating the most reliable OMI data, Greyed, indicating very large FOVs (and hence less reliable correlative measurements), and black, indicating FOVs affected by the row anomaly, where the OMI data should be disregarded. Transverse to the FOV boundaries, isochrons (curves of constant satellite measurement time) are drawn. These are labeled with the orbit number, the time in Coordinated Universal Time (something like GMT), and selected FOV numbers. Although the prediction map is complicated in appearance, information may readily be read from it to allow the planning of ground- and aircraft-based measurements that will be very well collocated with OMI's measurements. Feedback Please direct comments, questions, error-reports, and brickbats to me using the contact information at the head of this document.