  The National Science Foundation (NSF) Astronomy Division's Portfolio Review
of 2012 is no longer relevant to the Green Bank Telescope (GBT) of 2017 for two
principal reasons, one instrumental and the other astrophysical: 1) The GBT has
begun significant operations in the 3mm band, giving it unrivaled capabilities
for spectroscopy and continuum studies over 67-116 GHz. It is now an instrument
that is unique worldwide and is a critical complement to ALMA for the U.S.
scientific community. These capabilities had not been implemented at the time
of the review. 2) The detection of gravitational radiation by LIGO in 2015
places the GBT's work on pulsar observations of nano-Hz gravitational radiation
at the forefront of modern astrophysics.
  The Green Bank Telescope of 2017 is not the GBT that was reviewed by the
Eisenstein-Miller committee in 2012, a review that was specific to the NSF
Astronomy portfolio. The GBT serves a wide spectrum of science areas including
physics, chemistry, and planetary studies as well as astronomy. Besides its
well-documented intellectual merit, it thus has a significant broader impact.
The GBT is making significance advances in our understanding of gravitational
waves, the equation-of-state of nuclear matter, the mass of supermassive black
holes, the value of H0, and the physics of star-formation, all key science
goals for astronomy identified in a recent National Academy study "New Worlds,
New Horizons: A Midterm Assessment". In the era of ALMA and LIGO, other
countries have bolstered their mm-wave and cm-wave facilities; it is critical
that U.S. scientists have ready access to a large filled aperture to remain at
the forefront of research.
