Posted for "Charles R. Patton" <charles.r.patton@ieee.org>:

 Assuming it is some sort of standard GPS antenna puck, the ground plane is
already in the antenna.  The typical construction is a thick piece of ceramic,
an inch or so square, coated on the top and bottom  with fired on conductors.
 The size and dielectric constant make the puck resonant at the GPS receive
frequency, but this tuning is very susceptible to the proximity and size of
the ground plane the puck is mounted on.  So any standard GPS antenna also has
a metal plane that he puck is mounted on as part of the package.  As long as
you mount this assembly as is, additional ground planes (such as a glare
shield, plane body, etc.) or not is far enough away as to not bother this
antenna.  A important aspect of these type of antennas is the omnidirectional
 characteristics with 360 degree azimuthal capability extending almost to the
horizon.
 Charles Patton
 LNC2 360JM