Satellites
Your Global Positioning System receiver (GPSr) must utilize information transmitted
from one or more of the following GPS satellite constellations to accurately and precisely
calculate your current position. Which systems are used and the number of satellite signals
available will impact performance.Satellite Constellations
GPS
The Global Positioning System is a space-based satellite navigation system that provides
location and time information in all weather conditions, anywhere on or near the Earth where
there is an unobstructed line of sight to four or more GPS satellites. The original concept for a DefenseNavigation Satellite System (DNSS) in September 1973 was later named Navstar
(NAVigation System using Timing And Ranging), then Navstar-GPS to identify the constellation of Navstar satellites, and eventually shortened simply to GPS. Achieving initial operational
capability (IOC) in December 1993, Full Operational Capability (FOC) was declared by Air
Force Space Command (AFSPC) in April 1995. The highest quality signals were reserved for
military use, and the signal available for civilian use was intentionally degraded. Selective
Availability was discontinued on May 2, 2000, allowing users to receive a non-degraded
signal globally. GPS is maintained by the United States government and is freely accessible
to anyone with a GPS receiver.
The GLObal NAvigation Satellite System is a space-based satellite navigation system operated
by the Russian Aerospace Defence Forces. It both complements and provides an alternative
to the United States' Global Positioning System (GPS) and is the only alternative navigational
system in operation with global coverage and comparable precision. GLONASS initially achieved
Full Operational Capability (FOC) in December, 1995. Financial difficulties crippled the Russian
space program, leaving them with only 6 operational satellites by 2001. In 2010, GLONASS had
achieved 100% coverage of Russia's territory, and by October 2011, the full orbital constellation
of 24 satellites was restored, enabling full global coverage.
GBAS
The Ground Based Augmentation System (also referred to as Local Area Augmentation System
in U.S.) is intended primarily to support precision aviation approach operations. GBAS information is transmitted using VHF signals the Oregon 6xx is not equipped to receive.
SBAS
A Satellite Based Augmentation System supports wide-area or regional augmentation through
the use of additional satellite-broadcast messages.
The Wide Area Augmentation System is a satellite based augmentation system (SBAS) jointly
developed by the United States Department of Transportation (DOT) and the Federal Aviation
Administration (FAA). WAAS was commissioned for use on July 10, 2003.
The European Geostationary Navigation Overlay Service is a satellite based augmentation system (SBAS) developed by the European Space Agency, the European Commission, and EUROCONTROL. EGNOS operations were initiated in July 2005. An EGNOS Data Access Service was made
available in July 2012.
MSAS
The Multi-functional Satellite Augmentation System is a Japanese satellite based augmentation
system (SBAS) commissioned on September 27, 2007.
GAGAN
The GPS Aided Geo Augmented Navigation is a regional satellite based augmentation system
(SBAS) implemented by the government of India. GAGAN is scheduled to be fully operational
in early 2014.
Satellite Identification
Have you ever considered what the numbers assigned to each GPS satellite on your Garmin GPSr represent, or how they are determined?
PRN
Global positioning systems use a form of spread-spectrum or code-division multiple access (CDMA) communications where each signal source is modulated by a PRN (pseudo-random noise) code.
The PRN code (along with other encoded sub codes) dithers the L1 carrier
(or the coarse-acquisition carrier) of the signal. All receivers listen on the same L1 frequency,
but they sort out the satellite signals by demodulating them according to their PRN encoding.
NMEA
Garmin GPS receivers employing the NMEA 0183 specification assign each GPS satellite signal
a unique NMEA identification number based on the satellite PRN number (GPS) or slot number
(GLONASS):