The Global Positioning System, usually called GPS,
is the only fully-functional satellite navigation system. A
constellation of more than two dozen GPS satellites broadcasts precise
timing signals by radio to GPS receivers, allowing them to accurately
determine their location (longitude, latitude, and altitude) in any
weather, day or night, anywhere on Earth.
States Department of Defense developed the system, officially named
NAVSTAR GPS (Navigation Signal Timing and Ranging GPS), and the
satellite constellation is managed by the 50th Space Wing at Schriever
Air Force Base. Although the cost of maintaining the system is
approximately US$400 million per year, including the replacement of
aging satellites, GPS is available for free use in civilian
applications as a public good.
has become a vital global utility, indispensable for modern navigation
on land, sea, and air around the world, as well as an important tool
for map-making, and land surveying. GPS also provides an extremely
precise time reference, required for telecommunications and some
scientific research, including the study of earthquakes.
late 2005, the first in a series of next-generation GPS satellites was
added to the constellation, offering several new capabilities,
including a second civilian GPS signal called L2C for enhanced accuracy
and reliability. In the coming years, additional next-generation
satellites will increase coverage of L2C and add a third and fourth
civilian signal to the system, as well as advanced military
Wide-Area Augmentation System (WAAS), available since August 2000,
increases the accuracy of GPS signals to within 2 meters (6 ft) for
compatible receivers. GPS accuracy can be improved further, to about 1
cm (half an inch) over short distances, using techniques such as
Differential GPS (DGPS).
is used by people around the world as a navigation aid in cars,
airplanes, and ships. The system can also be used by computer
controlled harvesters, mine trucks and other vehicles. Hand-held GPS
receivers can be used by mountain climbers and hikers. Glider pilots
use the logged signal to verify their arrival at turnpoints in
competitions. Low cost GPS receivers are often combined in a bundle
with a PDA, car computer, or vehicle tracking system.
More costly and precise receivers are used by land surveyors to locate
boundaries, structures, and survey markers, and for road construction.
(Russian & Global'naya Navigatsionnaya Sputnikovaya Sistema. GLObal
NAvigation Satellite System) is a radio satellite navigation system,
the Russian counterpart to the United States' GPS system and the
European Union's embryonic Galileo positioning system. It is operated
for the Russian government by the Russian Space Forces.
peak efficiency the system offered a standard (coarse-acquisition or
C/A) positioning and timing service giving horizontal positioning
accuracy within 57-70 meters, vertical positioning within 70 meters,
velocity vector measuring within 15 cms and timing within 1 µs, all
based on measurements from four satellite signals simultaneously. A
more accurate signal (precision or P(Y)) was available for Russian
GPS, the complete nominal GLONASS constellation consists of 24
satellites, 21 operating and three on-orbit 'spares' placed in three
orbital planes. Each plane contains eight satellites identified by
"slot" number, which defines the corresponding orbital plane and the
location within the plane: 1-8, 9-16, 17-24. The three orbital planes
are separated by 120°, and the satellites equally spaced within the
same orbital plane, 45° apart. The GLONASS orbits are roughly circular,
with an inclination of about 64.8° and a semi-major axis of 25,440 km.
The planes themselves have 15° argument of latitude displacement.
constellation orbits the Earth at an altitude of 19,100 km (slightly
lower than that of the GPS satellites). Each satellite completes an
orbit in approximately 11 hours, 15 minutes. The spacing of the
satellites in orbits is arranged so that a minimum of 5 satellites are
in view at any given time.
satellite transmits two types of signal: standard precision (SP) and
high precision (HP). SP signal L1 have a frequency division multiple
access in L-band: L1= 1602 MHz + n0.5625 MHz, where n is frequency
channel number (n=0,1,2...).
satellites have been launched from Baikonur Cosmodrome. The first three
test satellites were placed in orbit in October 1982 with the first
operational satellites entering service in December 1983. The system
was intended to be operational in 1991, it was announced to be
operational on September 24, 1993 but the constellation was not
completed until December 1995.
characteristic of the GLONASS constellation is that the satellite
orbits repeat after 8 days. As each orbit plane contains 8 satellites,
there is a non-identical repeat (i.e., another satellite will occupy
the same place in the sky) after one sidereal day. This differs from
the GPS identical repeat period of one sidereal day.
to the economic situation in Russia there were only eight satellites in
operation in April 2002 rendering it almost useless as a navigation aid.
the economic situation in Russia has improved, 17 satellites were in
operation by December 2005. Additionally, an advanced GLONASS
satellite, the GLONASS-M, with an operational lifetime of 7 years, has
been developed. A 3-satellite block of this new version was launched on
26 December 2004. A further improved GLONASS-K satellite, with a
reduced weight and an increased operational lifetime of 10-12 years, is
due to enter service in 2008. Reducing its weight by 50% will allow to
significantly lower launch costs. Following a joint venture deal with
the Indian Government, which will launch two GLONASS-M satellites on
its GSLV rockets, it is proposed to have the system fully operational
again by 2008 with 18 satellites, providing full coverage of Russia
territory, and by 2010 with all 24 satellites. During the December 2005
summit between Indian Prime Minister Manmohan Singh and Russian
President Vladimir Putin, it was agreed that India would share the
development costs of the GLONASS-K series and launch them from India.