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G P S ( t h e Global Positioning System) represents the state of the art
in
precise,
continuous, world wide satellite navigation.
T h e constellation of satellites will ultimately consist of 2 4 operational satellites
(21
plus
3
operating spares). The
US
Department o f Defense
will
declare
t h e GPS constellation fully operational w h e n
2 1
operational (Block satellites
are
functioning in their assigned orbits. This is expected t o occur
in
Users are cautioned t h a t the system is under development, and signal availabili-
t y a n d accuracy are subject t o change
without warning a t the discretion o f the
Therefore, until t h e system is declared Operational, any use o f t h e system
is a t the user's o w n risk.
GPS will provide two levels of service
-
a Standard Positioning Service
a n d a Precise Positioning Service
(PPS).
SPS is a service w h i c h
be
available t o all
GPS
users o n a con-
tinuous, worldwide basis with no direct charge. SPS will be provided on the
GPS
frequency w h i c h contains
a
course acquisition code and a
data message. SPS is planned to provide the capability t o obtain horizontal
position accuracy w i t h i n meters
(95
percent probability) and
300
meters
99.99
percent probability. The GPS frequency also contains
a
preci-
sion code, that is not p a r t o f t h e
SPS.
PPS is
a
highly accuracy military positioning service that
will not be
publicly
ava i la b I e.
The SPS accuracy is a result o f a degradation of the basic GPS accuracy,
caused b y a cryptographic technique called selective availability Without
t h e typical horizontal accuracy is f o u n d in the range 1 5 t o 30 meters
Unlike Decca and Loran-C navigators, the geographical accuracy and the posi-
tion repeatability are identical for
navigators.
There is a t present n o information in the GPS signals telling the civilian user
whether S / A is used or not. However, if your b o a t is moored, and the navigator
s h o w s a steady Course Over Ground and ( l o w ) Speed Over Ground, y o u m a y
expect t h a t
S / A is in use.
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