Leica Geosystems GPS Basics - Manual (Page 18)

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GPS Basics -1.0.0en How GPS works
4 5. Selective Availability (S/A)
Selective Availability is a process
applied by the U.S. Department of
Defense to the GPS signal. This is
intended to deny civilian and hostile
foreign powers the full accuracy of GPS
by subjecting the satellite clocks to a
process known as ‘dithering’ which
alters their time slightly. Additionally, the
ephemeris (or path that the satellite will
follow) is broadcast as being slightly
different from what it is in reality. The end
result is a degradation in position
accuracy.
It is worth noting that S/A affects civilian
users using a single GPS receiver to
obtain an autonomous position. Users
of differential systems are not signifi-
cantly affected by S/A.
Currently, it is planned that S/A will be
switched off by 2006 at the latest.
6. Anti-Spoofing (A-S)
Anti-Spoofing is similar to S/A in that it’s
intention is to deny civilian and hostile
powers access to the P-code part of the
GPS signal and hence force use of the
C/A code which has S/A applied to it.
Anti-Spoofing encrypts the P-code into a
signal called the Y-code. Only users with
military GPS receivers (the US and it’s
allies) can de-crypt the Y-code.
3.1.4 Why are military receivers more
accurate ?
Military receivers are more accurate
because they do not use the C/A code to
calculate the time taken for the signal to
reach the receiver. They use the P-code.
The P-code is modulated onto the carrier
wave at 10.23 Hz. The C/A code is
modulated onto the carrier wave at 1.023
Hz. Ranges can be calculated far more
accurately using the P-code as this code
is occurring 10 times as often as the C/A
code per second.
The P-code is often subjected to Anti
Spoofing (A/S) as described in the last
section. This means that only the
military, equipped with special GPS
receivers can read this encryted P-code
(also known as the Y-code).
For these reasons, users of military GPS
receivers usually get a position with an
accuracy of around 5m whereas, civilian
users of comparable GPS receivers will
only get between about 15-100m
position accuracy.
A military handheld GPS receiver
(courtesy Rockwell)