Introduction - Understanding GPS: Principles and Applications - page 4

Positioning Service (PPS). The SPS is designated for the civil community, whereas
the PPS is intended for U.S. authorized military and select government agency users.
Access to the GPS PPS is controlled through cryptography. Initial operating capabil-
ity (IOC) for GPS was attained in December 1993, when a combination of 24 proto-
type and production satellites was available and position determination/timing
services complied with the associated specified predictable accuracies. GPS reached
full operational capability (FOC) in early 1995, when the entire 24 production satel-
lite constellation was in place and extensive testing of the ground control segment
and its interactions with the constellation was completed. Descriptions of the SPS
and PPS services are presented in the following sections.
1.3.1 PPS
The PPS is specified to provide a predictable accuracy of at least 22m (2 drms, 95%)
in the horizontal plane and 27.7m (95%) in the vertical plane. The distance root
mean square (drms) is a common measure used in navigation. Twice the drms value,
or 2 drms, is the radius of a circle that contains at least 95% of all possible fixes that
can be obtained with a system (in this case, the PPS) at any one place. The PPS pro-
vides a UTC time transfer accuracy within 200 ns (95%) referenced to the time kept
at the U.S. Naval Observatory (USNO) and is denoted as UTC (USNO) [7, 8].
Velocity measurement accuracy is specified as 0.2 m/s (95%) [4]. PPS measured per-
formance is addressed in Section 7.7.
As stated earlier, the PPS is primarily intended for military and select govern-
ment agency users. Civilian use is permitted, but only with special U.S. DOD
approval. Access to the aforementioned PPS position accuracies is controlled
through two cryptographic features denoted as antispoofing (AS) and selective
availability (SA). AS is a mechanism intended to defeat deception jamming through
encryption of the military signals. Deception jamming is a technique in which an
adversary would replicate one or more of the satellite ranging codes, navigation data
signal(s), and carrier frequency Doppler effects with the intent of deceiving a victim
receiver. SA had intentionally degraded SPS user accuracy by dithering the satellite's
clock, thereby corrupting TOA measurement accuracy. Furthermore, SA could have
introduced errors into the broadcast navigation data parameters [9]. SA was discon-
tinued on May 1, 2000, and per current U.S. government policy is to remain off.
When it was activated, PPS users removed SA effects through cryptography [4].
1.3.2 SPS
The SPS is available to all users worldwide free of direct charges. There are no
restrictions on SPS usage. This service is specified to provide accuracies of better
than 13m (95%) in the horizontal plane and 22m (95%) in the vertical plane (global
average; signal-in-space errors only). UTC (USNO) time dissemination accuracy is
specified to be better than 40 ns (95%) [10]. SPS measured performance is typically
much better than specification (see Section 7.7).
At the time of this writing, the SPS was the predominant satellite navigation ser-
vice in use by millions throughout the world.