The geometry of the earth - Physical Geodesy - page 174

be operational. See http://tycho.usno.navy.mil/gpscurr.html for the current
status.
With the nominal constellation, the space segment provides global cover-
age with four to eight simultaneously observable satellites above 15 ◦ elevation
angle at any time of day. If the elevation mask is reduced to 10 ◦ , occasionally
up to 10 satellites will be visible; and if the elevation mask is further reduced
to 5 ◦ , occasionally 12 satellites will be visible.
Satellites categories
Essentially, the GPS satellites provide a platform for radio transceivers,
atomic clocks, computers, and various ancillary equipment. The electronic
equipment of each satellite allows the user to measure a pseudorange to the
satellite, and each satellite broadcasts a message which allows the user to
determine the spatial position of the satellite for arbitrary instants. The aux-
iliary equipment of each satellite, among others, consists of solar panels for
power supply and a propulsion system for orbit and stability control.
There are several classes or types of GPS satellites. These are the Block I,
Block II, Block IIA, Block IIR, Block IIR-M, and the future Block IIF and
Block III satellites. An up-to-date description is dicult because new nota-
tions are introduced in a rather arbitrary way; an example is the recently
introduced notation Block IIR-M.
Eleven Block I satellites were launched in the period between 1978 to
1985. Today, none of them is in operation anymore.
The essential difference between Block I and Block II satellites is related
to U.S. national security. Block I satellite signals were fully available to
civilian users. Starting with Block II, satellite signals may be restricted for
civilian use. The Block II satellites are equipped with mutual communication
capability. Some of them carry retroreflectors and can be tracked by laser
ranging.
The Block IIR satellites (“R” denotes replenishment or replacement)
have a design life of 10 years. They are equipped with improved facilities for
communication and intersatellite tracking. Block IIR-M satellites incorpo-
rate two new military signals and a second civil signal. The first Block IIR-M
was launched on September 25, 2005.
Currently (April 2006), the first launch of a Block IIF satellite (“F”
denotes follow on) is scheduled for 2008 (instead of the previously projected
dates mid of 2006 and 2007). These satellites will broadcast a third civil
signal on L5 (see Sect 5.3.5).
Presently, the DOD undertakes studies for the next generation of GPS
satellites, called Block III satellites. Preliminary dates (likely to change) are
2011/12 for first launches and on-orbit tests (Civil GPS Service Interface