Introduction to GNSS - GNSS Remote Sensing: Theory, Methods and Applications - page 7

Fig. 1.1 Global Navigation Satellite Systems (GNSS)

and the Department of Transportation (DOT). The optimum system should have
global coverage, continuous/all weather operation, the ability to serve high-dynamic
platforms, and high accuracy. The first satellite navigation system with a con-
stellation of five satellites, Transit, was first successfully tested in 1960, which
was used by the United States Navy. The Transit could provide a navigational fix
approximately once per hour (Guier and Weiffenbach 1997 ). In 1967, the U.S. Navy
developed the Timation satellite that proved the ability to place accurate clocks
in space. In the 1970s, the ground-based Omega Navigation System with phase
comparison of signal transmission from pairs of stations became the first worldwide
radio navigation system. In 1973, the Defense Navigation Satellite System (DNSS)
was created, which was named Navstar later. With the individual satellites being
associated with the name Navstar, a more fully encompassing name was used to
identify the constellation of Navstar satellites, Navstar-GPS, shortly named GPS
(Rip and Hasik 2002 ). In 1983, President Ronald Reagan issued a directive making
GPS freely available for civilian use. Until the 24th satellite was launched in 1994,
GPS has become operational fully.
The system originally broadcast two signals, a C/A code available to civilian
uses and a more precise encrypted code reserved for military uses. In 2000 the
Selective Availability (SA) - the feature where the signal available for civilian use
could be intentionally degraded - was discontinued, and every user on the globe
was allowed to receive a non (intentionally) degraded signal. Between 1997 and
2009, 20 satellites of blocks IIR and IIR-M were launched, and constitute now
a large portion of the current GPS constellation (20 out of 31 healthy satellites,
as for January 2011). The block IIR-M satellites made available to civilian users
a civil encoded signal in the L2 band. An additional safety-of-life civilian-use
signal (in the L5 band) will be provided with the block IIF generation of GPS
satellites, which is being implemented. The first block IIF satellite was launched on
28 May, 2010, and set operative on 27 August, 2010. All GPS satellites broadcast by
employing the Code Division Multiple Access (CDMA) technique as channel access
method. Nowadays the official GPS space segment is composed of 24 satellites
(plus three operational spares) distributed over six orbital planes, separated by 60
right ascension of the ascending node, 55 -inclined, and with an orbital radius of
about 26,600 km. Each satellite orbits the Earth exactly twice each sidereal day,
repeating the same ground track once a day. Therefore, the same constellation