Csillag kolléga hozzászólásához még azt tenném hozzá, hogy a PPS _nem_csak_ az SA nem létét jelenti. Hanem elveiben is egy kicsit más...

a részletekre lasd ezt:

Precise Positioning Service (PPS)
The highest level of military dynamic positioning accuracy provided by GPS, using the dual-frequency
P-code. PPS is the most accurate positioning, velocity, and timing information
continuously available, worldwide, from the basic GPS. This service is limited to authorized U.S.
and allied Federal Governments; authorized foreign and military users; and eligible civil users.
PPS information is usually (but not always) encrypted to prevent use by unauthorized users. The
encryption process is known as Anti-Spoofing. GPS receivers that can use the P code provide a
predictable positioning accuracy of at least 22 meters (2 drms) horizontally and 27.7 meters (2
sigma) vertically. They can also provide timing/time interval accuracy within 90 nanoseconds (95
percent probability). This improved accuracy is provided in two ways. First, P-code users are not
subjected to Selective Availability. Second, access to the L2 channel allows the user to correct
for atmospheric propagation errors. Equipment that uses the P code is available for civilian use;
however, The capability is useless when Anti-Spoofing is active. Access to GPS receivers that
can decrypt the Y code is tightly controlled and is subject to National Security considerations. If
you are a civilian user, you will need a really good excuse to get one.

A lényeg: egy titkositott u.n. "P" kód (nem mindig titkos, de ki tudja mikor nem az)
azon túl két frekis vevő.
(a mezeiek pld. GARMIN -bocs NaviGate! egy frekisek)

üdv

rugo