1.4 Considerations for GPS Performance
GPS
works
with
weak
signals.
The
signal
strength
on
earth
is
approximately
15dB
below
the
thermal
noise
floor.
In
order
to
design
a
reliable
GPS
system,
the
following
parameters
have
to
be
considered
carefully
during
the
design
phase
as
they
may
significantly
degrade
the
GPS
performance.
1.
Antenna
limitations
•
Poor
gain
of
the
GPS
antenna
•
Poor
directivity
(radiation
pattern)
of
the
GPS
antenna
•
Improper
orientation
of
the
antenna
to
the
sky
•
Poor
matching
between
antenna
and
cable
impedance
•
Poor
noise
performance
of
the
receivers
input
stage
or
the
antenna
amplifier
2.
Electrical
Environment
•
Jamming
from
external
signals
•
Jamming
from
signals
generated
by
the
receiver
itself
3.
GPS
related
effects
•
Signal
path
obstruction
by
buildings,
foliage,
covers,
snow,
etc.
Multi-path
effects
•
Satellite
constellation
•
an nna
related
issues
from
the
above
list
and
geometry
The
te
will
be
further
discussed
in
Section 1.5
.
Jamming
and
interference
The
Dilution
of
Precision
(DOP)
is
a
unit
less
value
that
indicates
when
the
satellite
geometry
provides
the
most
bination
of
horizontal
Note
A
point
calculated
with
a
PDOP
of
30.0
can
deviate
by
more
than
150
m
from
its
true
location.
issues
will
be
extensively
discussed
in
Section 1.6
.
1.4.1 Dilution of Precision (DOP)
accurate
results.
It
is
the
mathematical
representation
of
the
quality
of
the
navigation
solution,
based
on
the
geometry
of
the
satellites
used
in
the
calculation.
The
number
of
visible
satellites
and
their
relative
positions
in
the
sky
mainly
control
DOP.
Satellites
spread
over
the
sky
give
better
results
(lower
DOP).
The
most
commonly
used
DOP
is
position
dilution
of
precision
(PDOP),
which
is
the
com
dilution
of
precision
(HDOP)
and
vertical
dilution
of
precision
(VDOP).
A
PDOP
value
of
1
indicates
an
optimum
satellite
constellation
and
high-quality
data.
The
quality
of
the
data
decreases
as
the
PDOP
value
increases.
PDOP
values
in
excess
of
8
are
considered
poor.
GPS
Modules
-
System
Integration
Manual
(SIM)
(incl.
Reference
Design)
GPS
Fundamentals
GPS.G4-MS4-05007-A1
Page 14
