Fermi Gamma-ray Space Telescope

LAT Modified Observing Strategy

From December 2013 to December 2014, Fermi was operated in a mixed observing mode. This mode alternated zenith pointing segments of the previous north-south rocking profile sky survey described in the LAT All-Sky Survey document with inertial pointing segments that bias the LAT exposure toward the galactic center. This modified observing strategy was recommended to the Fermi mission by a committee with the support of the Fermi Users Group in August 2013.

Coordinates for the inertial pointing segments were updated daily based on a right ascension of 261.4°, incorporating a 5° offset from the galactic center RA, and the galactic center declination of -29.01°. The inertial pointing target right ascension and declination were chosen to be the closest point to this position that lies on the Fermi orbit equator. The daily update in pointing coordinates followed the satellite's 53-day orbital precession period. Once each orbit, when the inertial pointing target coordinates were occulted, a zenith pointing segment was implemented for the equivalent north or south rocking profile survey. This maintained a relatively uniform all-sky coverage while simultaneously providing a boost in exposure at the Galactic center.

The committee report on alternate observing strategies is available in .pdf format here. The modified observing strategy was equivalent to the recommended option 4 discussed in the report.

Examples of spacecraft files, exposure maps and a comparison of all-sky survey and the adopted option 4 modified observing strategy are available here. Fermi was expected to operate using this modified observing strategy at least through December 2014.