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G P S ( t h e Global Positioning System) represents the state of the art
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 (DOD) will declare
t h e GPS constellation fully operational w h e n
are functioning
in their assigned orbits. This is expected t o occur
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 n d a Precise Positioning Service
SPS is a positioning service w h i c h will
tinuous, worldwide basis with no direct charge. SPS will be provided on the
GPS
frequency w h i c h contains a course acquisition ( C / A ) code and a naviga.
position accuracy w i t h i n 100 meters 2drms
meters 99.99 percent probability. The GPS
code, that is not p a r t o f t h e SPS.
sion
PPS is a highly accuracy military positioning service that
ava i la b I e.
The
SPS accuracy is a result o f a degradation of the basic
caused b y a cryptographic technique called selective availability (S/A). Without
Unlike Decca and Loran-C navigators, the geographical accuracy and the posi-
tion repeatability are identical for 6 P S 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.
2 1
operational (Block
a Standard Positioning Service (SPS)
-
(PPS).
be
available t o all
(95
percent probability) and 300
frequency also contains
3
precise,
in
satellites
in 1993.
GPS
users o n a con-
a
preci-
will not
be
publicly
GPS
accuracy,
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