Dr. O'Brien's primary areas of research are GPS and Global Navigation Satellite System (GNSS) receivers, navigation systems, adaptive antennas, antenna electronics, and array signal processing.  He is currently serving on the Science Team for CYGNSS, a NASA Earth Venture-2 Program.  CYGNSS is a constellation of satellites which use space-based GPS reflectometry for remote sensing.  Dr. O'Brien is Co-I of a NASA Instrument Incubator Program (IIP) developing Next-Generation GNSS Bistatic Radar Instrument for spaceborne GNSS-R applications for which he is responsible for GNSS-R receiver instrument signal processing algorithm design, implementation and testing.  Previously, Dr. O'Brien developed cubesat mission simulation tools for CubeRRT, a NASA InVEST program that a developed a 6-40 GHz radio frequency interference (RFI) technology demonstration satellite.  Dr. O’Brien is also involved with all aspects of antenna design and characterization, and he has worked on projects developing novel GPS reflectometry antenna for small sats and commercial cubesats.   He took part in a project developing a real-time wave sensing radar for ship motion predictions and was responsible for coordinating ship motion systems and radar signal processing.  He served as PI of a project dealing with hardware characterization of advanced digital receiver exciter modules for large-scale defense radar systems.  He has also done work in a variety of areas, including development of foldable dish antenna for GPS and communication systems, non-destructive material evaluation to extract complex material parameters from aircraft coatings, RF emitter geolocation from airborne platforms, characterizing rotorcraft-mounted antenna arrays, and calibration approaches for the use of adaptive antenna arrays in precise GPS applications.