Global Positioning System Reference
In-Depth Information
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the first IIF (F for follow on) satellites. These satellites will transmit a third civil
frequency, called L5.
The U.S. government's current policy is to make GPS available in two services.
The precise positioning service (PPS) is available to the military and other authorized
users. The standard positioning service (SPS) is available to anyone. See SPS (2001)
for a detailed documentation of this service. Without going into detail, let it suffice to
say that PPS users have access to the encrypted P(Y)-codes on the L1 and L2 carriers,
while SPS users can only observe the public C/A-code on L1. The encryption of the
P-codes began January 31, 1994. SPS positioning capability was degraded by SA
measures, which entailed an intentional dither of the satellite clocks and falsification
of the navigation message. In keeping with the policy, SA was implemented on March
25, 1990, on all Block II satellites. The level of degradation was reduced in September
1990 during the Gulf conflict, but was reactivated to its full level on July 1, 1991, until
it was discontinued on May 1, 2000, upon direction of the U.S. president.
Over time, both satellite and receiver technologies have improved significantly.
Whereas older receivers could observe the P(Y)-code more accurately than the C/A-
codes, this distinction has all but disappeared with modern receiver technology. Dual-
frequency P(Y)-code users do have the advantage of being able to correct the effect of
the ionosphere on the signals. However, this simple classification of PPS and SPS by
no means characterizes how GPS is used today. Researchers have devised various, of-
ten patented procedures that make it possible to observe or utilize the encrypted P(Y)-
codes effectively, and in doing so, make dual-frequency observations available, at
least to high-end receivers. In certain surveying applications where the primary quan-
tity of interest is the vector between nearby stations, intentional degradation of SA
could be overcome by differencing the observations between stations and satellites.
However, in many applications, positioning with GPS works much better without SA.
The six orbital planes of GPS are evenly spaced in right ascension and are inclined
by 55° with respect to the equator. Because of the flattening of the earth, the nodal
regression is about
[74
Lin
0.0
——
Lon
PgE
[74
0 . 04187° per day; an annual orbital adjustment keeps the orbits
close to their nominal location. Each orbital plane contains four satellites; however,
to optimize global satellite visibility, the satellites are not evenly spaced within the
orbital plane. The orbits are nominally circular, with a semimajor axis of about 26,660
km. Using Kepler's third law (3.42), one obtains an orbital period of slightly less than
12 hours. The satellites will complete two orbital revolutions in one sidereal day. This
means the satellites will rise about 4 minutes earlier each day. Because the orbital
period is an exact multiple of the period of the earth's rotation, the satellite trajectory
on the earth (i.e., the trace of the geocentric satellite vector on the earth's surface)
repeats itself daily.
Because of their high altitude, the GPS satellites can be viewed simultaneously
over large portions of the earth. Usually the satellites are observed only above a
certain vertical angle, called the mask angle. Typical values for the mask angle are 10-
15°. At low elevation angles the tropospheric effects on the signal can be especially
severe and difficult to model accurately. Let ε denote the mask angle, and let
denote
the geocentric angle of visibility for a spherical earth; then one can find the relation
( ε
α
=
0°,
α =
152°), ( ε
=
5°,
α =
142°), ( ε
=
10°,
α =
132°). The viewing angle
from the satellite to the limb of the earth is about 27°.
 
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