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The South Atlantic Anomaly

Satellites launched into low Earth orbits with moderate inclinations will pass through the South Atlantic Anomaly (SAA). For Swift SAA passage occurs on approximately seven consecutive orbits per day. During passage through the SAA the high particle flux will affect Swift's detectors. The orbital inclination (20.6 degrees) reduces the time that Swift will spend in the SAA but does not eliminate the problem. Therefore, the Swift instruments take several measures to protect themselves during SAA passage.

The BAT high voltage remains enabled during an SAA passage, but event collection and GRB triggering are disabled.

Simulations indicate that the XRT CCDs will not suffer damage from soft protons reflecting off the grazing incidence optics. The hard protons encountered in the SAA will slowly degrade the spectral resolution of the XRT instrument, an effect that is mitigated by features of the instrument's design. In order to avoid saturating the XRT telemetry with particle events during SAA passage the XRT's data collection is turned off during each SAA passage. However, if a GRB occurs during an SAA passage the XRT will still attempt to provide a fine position for the afterglow.

In order to protect the UVOT detectors the UVOT cathode voltage is set to zero and the MCP23 voltage is set to 70% of its nominal value during SAA passage. UVOT begins observing again after Swift has exited the SAA.

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Eleonora Troja 2013-09-03