  Gas Electron Multiplier (GEM) technology is being considered for the forward
muon upgrade of the CMS experiment in Phase 2 of the CERN LHC. Its first
implementation is planned for the GE1/1 system in the $1.5 < \mid\eta\mid <
2.2$ region of the muon endcap mainly to control muon level-1 trigger rates
after the second long LHC shutdown. A GE1/1 triple-GEM detector is read out by
3,072 radial strips with 455 $\mu$rad pitch arranged in eight $\eta$-sectors.
We assembled a full-size GE1/1 prototype of 1m length at Florida Tech and
tested it in 20-120 GeV hadron beams at Fermilab using Ar/CO$_{2}$ 70:30 and
the RD51 scalable readout system. Four small GEM detectors with 2-D readout and
an average measured azimuthal resolution of 36 $\mu$rad provided precise
reference tracks. Construction of this largest GEM detector built to-date is
described. Strip cluster parameters, detection efficiency, and spatial
resolution are studied with position and high voltage scans. The plateau
detection efficiency is [97.1 $\pm$ 0.2 (stat)]\%. The azimuthal resolution is
found to be [123.5 $\pm$ 1.6 (stat)] $\mu$rad when operating in the center of
the efficiency plateau and using full pulse height information. The resolution
can be slightly improved by $\sim$ 10 $\mu$rad when correcting for the bias due
to discrete readout strips. The CMS upgrade design calls for readout
electronics with binary hit output. When strip clusters are formed
correspondingly without charge-weighting and with fixed hit thresholds, a
position resolution of [136.8 $\pm$ 2.5 stat] $\mu$rad is measured, consistent
with the expected resolution of strip-pitch/$\sqrt{12}$ = 131.3 $\mu$rad. Other
$\eta$-sectors of the detector show similar response and performance.
