Observing Modes of the Fermi Gamma-ray Space Telescope

The Fermi Gamma-ray Space Telescope (formerly GLAST) is a low-earth orbiting mission supporting two science instruments, the Gamma-Ray Burst Monitor (GBM), a near-all-sky detector in the 8 keV to 30 MeV range, and the Large Area Telescope (LAT), a wide field pair-conversion detector sensitive in the 20 MeV to 300 GeV range. The dominant observing mode of Fermi is a sky survey in which the observatory will be rocked about the orbital plane to provide almost uniform coverage of the entire sky every three hours. This results in continuous monitoring of all sources on timescales greater than 3 hours, guaranteeing gamma-ray data for a wide range of studies. Fermi is also capable of performing pointed observations which may be useful for studies of very bright, rapidly variable sources that benefit from more densely sampled observations, such as gamma-ray bursts and their afterglows. However, pointed observations will have a detrimental effect on coverage of the rest of the sky, which may hurt multiwavelength campaigns and uniformity of time monitoring studies. We describe these observational modes of Fermi and how they are affected by multiple operational constraints such as data generation rate and downlink, sun avoidance, and Earth avoidance. We show how the pointing profile of the observatory affects the LAT sky exposure, and discuss the relative merits of different observing strategies for various science objectives.