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Tltc Basics
of
GPS
GPS. or Global Positioning
Systern.
is
a satellite navigation system
designed to
provide
worldwide positioning
and tirne
infbrmation
with
all
weather
capability.
quick
response
time and accuracy tct within
several
yards.
GPS
is
maintained by the
United States Department
of
Defense 2rl hours
a
day.
GPS
will
be a system ol-24 satellites orbiting
at an altitudc
very
high
above the earth. Because of the
great
altitude. the satcllitcs' signals
are
not interl-ered with
by the terrain or
geographv.
and the high
fiequency of
the signals
(l-57-5
MHz) results in
practicallv
no
interence from
shipboard cngines or other
electrcnic
svstems.
GPS uses the
principle
of ranging
to
givc
vr)u
vr)ur
position.
This
is
done by
measuring
the distance from
your position
to the
position
of
at least three satellites. The
distance to
a
satellite is
calculated bv
measuring how long it
takes radio signals
to arrivc froln the krrown
position
of each of the respective satellites
in their orbits.
The
satellites and the
Star
Pilot
6
must
be
precisely
tinted to
generate
the
exact
same radio signal
at the exact
same time in order
to
mcasure
the tirne and distancc
calculations
with
a high degree of
accuracy.
This is
accomplished by the use of atomic
clocks on all the satellitcs.
and by comparing
what part
of
the
radio
signal code is rece rvcd
at
the
exact same time fiom
all the satellites.
To
complete
its
calculations filr
your position.
the
Star
Pilot
6 looks
up ir-r its data base
the
known position
o1'each of the satellites
at that
exact time. Because thc
GPS satellites
are
in
such a high orbit.
their
positions
at any
-eiven
tinre are
quite predictable.
Now.
all the Star
Pilot
6
needs
to do is calculate
all this for three or firur satcllitcs
every f-ew seconds.
and
you
have continuous position
updatingl
GPS satellites are
constantly monitored
by Department
of
Def'ense
ground
stations. Satellite
clocks and exact
pclsitionin_t
infbrmation
is
computed and any
variations
are noted
or
corrected as
neccssary.
and
transmitted to the appropriate satellites.
WARNING: As of April 1992,
there are 16 workin-e satellites
in
orbit.
Full implementation
of
2l satcllites
(and
three
sparesl
is
currently
pro.yccted
fbr 1993.
Users are
cautioned that the
systcm is
still in an experimental
operational status
and the system
may be
turned
off or
satellites
moved
at any time. without warning.
DO
NOT
place
yourself
or your vcssel
in a
situation
where
GPS
is your
sole
means of navigation.
Check with
the Coast Guard
tor currcnt
operational
slalu\
and messages.
WARNIN(i: Selective Availability
(S/A)
has been implemenred
by
the
U.S.
DOD
to
restrict
the accuracy of
GPS
positions.
As of April
1992.
the accuracy of
the GPS system is
such that 95%
of the time
a
fix will
be
within
100 rneters
of the actual
geographic
position.
2l