Global Positioning System Reference
In-Depth Information
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conditions. The U.S. government operates GPS. It was designed as a dual-use system,
with the primary purpose of meeting the military's needs for positioning and timing.
Over the past decade, the number of civilian applications has increased significantly,
with no end in sight. Because GPS is so well known by now, not just by the experts but
by general citizens as well, there is no need to dwell on which innovative application
will be next or even attempt to list its numerous current uses.
The buildup of the satellite constellation began with the series Block I satellites.
These were concept validation satellites that did not have selective availability (SA)
or antispoofing (AS) capability. They were launched into three 63° inclined orbital
planes. Their positions within the planes were such that optimal observing geometry
was achieved over certain military proving grounds in the continental United States.
Eleven Block I satellites were launched between 1978 and 1985 (with one launch
failure). The average lifetime was 8-9 years. They were designed to provide 3-4 days
of positioning service without contact with the ground control center. The launch of
the second generation of GPS satellites, called Block II, began in February 1989. In
addition to radiation-hardened electronics, these operational satellites had full SA/AS
capability and carried a navigation data message that was valid for fourteen days.
Additional modifications resulted in the satellite called Block IIA. These satellites
can provide about six weeks of positioning service without contact from the control
segment. Twenty-eight Block II/IIA satellites were launched between 1989 and 1997
into six planes, 55° inclined. The first third-generation GPS satellite, called Block IIR
(R for replenishment), was successfully launched in 1997. These satellites have the
capability to determine their orbits autonomously through UHF cross-link ranging
and to generate their own navigation message by onboard processing. They are able
to measure ranges between themselves and transmit observations to other satellites as
well as to ground control. Currently, GPS is undergoing a major modernization. Most
important, GPS satellites will transmit more signals that allow a better delineation of
military and civilian uses, and thus increase the performance of GPS even more. Table
3.3 shows the expected progression of the modernization. We anticipate the launch
of the IIR-M (M for modified) satellites soon. These satellites will transmit new civil
codes on L2 and new military codes on L1 and L2. Given the continued progress in
microelectronics, it will then be possible to manufacture inexpensive, compact dual-
frequency receivers for civilian uses. Around 2005/2006, we expect the launch of
[73
Lin
—
0.7
——
No
PgE
[73
TABLE 3.3
New GPS Signals
IIR
IIR-M
IIF
Signal
1978-2003
2003 (expected)
2005 (expected)
L1 C/A
X
X
X
L1 P(Y)
X
X
X
L1M
X
X
L2C
X
X
L2 P(Y)
X
X
X
L2 M
X
X
L5C
X
