The
receiver
cannot
compensate
for
the
second
effect
because
the
signals
cancel
out
at
the
antenna,
not
inside
the
GPS
unit.
However,
as
the
reflected
signal
is
usually
much
weaker
than
the
direct
signal,
the
two
signals
will
not
cancel
out
completely.
The
reflected
signal
will
also
have
an
inverted
polarity
(left
hand
circular
rather
than
right
hand
circular),
further
reducing
the
signal
level,
particularly
if
the
antenna
has
good
polarization
selectivity.
Water
is
a
very
good
reflector;
so
all
marine
applications
require
special
attention
to
reflected
signals
arriving
at
the
antenna
from
the
underside,
i.e.
the
water
surface.
Also,
the
location
of
the
antenna
close
to
vertical
metal
surfaces
can
be
very
disruptive
since
metal
is
an
almost
perfect
reflector.
When
mounting
an
antenna
on
top
of
a
reflective
surface,
the
antenna
should
be
mounted
as
close
to
the
surface
as
possible.
Then,
the
reflective
surface
will
act
as
an
extension
of
the
antennas
ground
plane
and
not
as
a
source
of
multi-path.
1.5 Antennas
Even
the
best
receiver
cannot
bring
back
what
has
been
lost
at
the
antenna.
The
importance
of
the
attention
paid
to
this
part
of
a
GPS
system
cannot
be
stated
highly
enough.
1.5.1 Selecting the right Antenna
Several
different
antenna
designs
are
available
on
the
GPS
applications
market.
The
GPS
signal
is
right-hand
circular
polarized
(RHCP).
This
results
in
a
style
of
antenna
that
is
different
from
the
well-known
whip
antennas
used
for
linear
polarized
signals.
The
most
prominent
antenna
designs
for
GPS
are
the
patch
antenna
as
shown
in
Figure 6
.
Figure 6: Patch Antennas, EMTAC Technology Corp.
Another
style
is
the
quadrifilar
helix
antenna
shown
in
Figure 7
.
The
actual
geometric
size
of
both
antenna
designs
depends
on
the
dielectric
that
fills
the
space
between
the
active
parts
of
the
antenna.
If
the
antenna
is
only
loaded
with
air
it
will
be
comparatively
large,
high
dielectric
constant
ceramics
result
in
a
much
smaller
form
factor.
The
smaller
the
dimensions
of
the
antenna,
the
more
performance
critical
tight
manufacturing
tolerances
become.
Furthermore,
a
smaller
antenna
will
present
a
smaller
aperture
to
collect
the
signal
energy
from
sky
resulting
in
a
lower
overall
gain
of
the
antenna.
This
is
the
result
of
pure
physics
and
there
is
no
“magic”
to
get
around
this
problem.
Amplifying
the
signal
after
the
antenna
will
not
improve
the
signal
to
noise
ratio.
GPS
Modules
-
System
Integration
Manual
(SIM)
(incl.
Reference
Design)
GPS
Fundamentals
GPS.G4-MS4-05007-A1
Page 16
