AN OVERVIEW OF THE 26-27 SEPTEMBER 2011 GEOMAGNETIC STORM AS MEASURED BY GPS RECEIVERS AND SUPERDARN HF RADARS Evan G. Thomas Advisors: Drs. J. B. H. Baker and J. M. Ruohoniemi Virginia Tech, Blacksburg, VA Many critical technologies relied upon by both commercial and military users around the world are directly impacted by events occurring in the Earth’s ionosphere. Strong geomagnetic storms are responsible for causing ionospheric clutter in over-the-horizon radar systems; scintillations and their associated errors in GPS signals; and induced electrical currents in power distribution networks. Here we present an overview of a large geomagnetic storm on 26-27 September 2011 which was caused by a coronal mass ejection from the sun. During this period, the Federal Aviation Administration (FAA) reported widespread outages in the Wide Area Augmentation System (WAAS), which is used to aid aircraft navigation and landing throughout North America. This research paper is a summary of the results published by Thomas et al.  and Zhang et al.  on the features observed in the Earth’s ionosphere during this large geomagnetic storm.