Interlaced Limb Scanning for Tomography

The Naval Research Laboratory has developed and flown far-ultraviolet limb scanners for characterizing thermospheric composition, densities, and temperature and ionospheric electron density. Each sensor collects measurements in the orbital plane, and the numerous intersecting lines-of-sight oversample the region and provide the capability to perform tomographic retrievals. However, these sensors were not originally designed for tomography so their operating mode is not optimized for 2-D retrievals. Under normal operations each instrument collects 90-sample limb scans over altitudes 75--750~km at a 90-second cadence, which provides good vertical resolution (down to 6 km) but limited horizontal resolution (5.6 degrees in latitude). For example, the 5.6-degree resolution limits the accuracy with which sharp gradients - --such as those associated with the Appleton anomalies - --can be retrieved. By utilizing an interlaced scan pattern, the same 90 lines-of-sight are sampled in a different order to improve horizonal resolution by a factor of 2--3 while retaining the nearly the same vertical resolution over each 90-sec period. Scan strategies, signal-to-noise concerns, spatial resolution, and retrieval accuracy considerations are presented.