military users, NATO military users, and other selected military and civilian users such as the

Australian Defense Forces and the U.S. Defense Mapping Agency.  The PPS is specified to

provide 16 metres Spherical Error Probable (SEP) (3 D, 50%) positioning accuracy and 100

nanosecond (one sigma) Universal Coordinated Time (UTC) time transfer accuracy to authorized

users.  This is approximately equal to 37 metres (3 D, 95%) and 197 nanoseconds (95%) under

typical system operating conditions.  PPS receivers can achieve 0.2 metres per second 3 D

velocity accuracy, but this is somewhat dependent on receiver design.

Access to the PPS is controlled by two features using cryptographic techniques, Selective

Availability (SA) and Anti Spoofing (A S).  SA is used to reduce GPS position, velocity, and time

accuracy to the unauthorized users.  SA operates by introducing pseudorandom errors into the

satellite signals.  The A S feature is activated on all satellites to negate potential spoofing of the

ranging signals.  The technique encrypts the P code into the Y code.  Users should note the C/A

code is not protected against spoofing.

Encryption keys and techniques are provided to PPS users which allow them to remove the

effects of SA and A S and thereby attain the maximum accuracy of GPS.  PPS receivers that have

not been loaded with a valid cryptographic key will have the performance of an SPS receiver.

PPS receivers can use either the P(Y) code or C/A code or both.  Maximum GPS accuracy is

obtained using the P(Y) code on both L1 and L2.  P(Y) code capable receivers commonly use the

C/A code to initially acquire GPS satellites.

1.3.2  Standard Positioning Service

The SPS is a less accurate positioning and timing service which is available to all GPS users.  In

peacetime, the level of SA is controlled to provide 100 metre (95%) horizontal accuracy which is

approximately equal to 156 metres 3D (95%).  SPS receivers can achieve approximately 337

nanosecond (95%) UTC time transfer accuracy.   System accuracy degradations can be increased

if it is necessary to do so, for example, to deny accuracy to a potential enemy in time of crisis or

war.  Only the President of the United States, acting through the U.S. National Command

Authority, has the authority to change the level of SA to other than peacetime levels. 

The SPS is primarily intended for civilian purposes, although it has potential peacetime military

use.  Refer to "Technical Characteristics of the Navstar GPS" for additional details regarding SPS

performance characteristics.

1.4  GPS THEORY OF OPERATION

The ranging codes broadcast by the satellites enable a GPS receiver to measure the transit time

of the signals and thereby determine the range between a satellite and the user.  The navigation

message provides data to calculate the position of each satellite at the time of signal transmission. 

From this information, the user position coordinates and the user clock offset are calculated using

simultaneous equations.  Four satellites are normally required to be

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