Navigation Satellites

Means to help navigation have always been sought by humans. It is only normal, considering that one of the inherent traits of our species is exploration of the unknown. In the beginning, people relied on the stars to tell them the way. However, with the dawn of the satellites era, a new way to support navigation became the norm: satellite navigation.

Navigation satellites can tell position anywhere with an accuracy of five to ten meters, regardless of weather condition. Instruments on board can even indicate the position of a stationary object within a few centimeters. ¹ Navigation satellites are placed in highly stable Medium Earth Orbits (MEOs), at high altitudes (1,200 to 14,000 miles above the Earth) in order to allow precise positioning. MEOs are stable orbits with exact orbit predictions. At this altitude satellites move slower and can be observed longer, allowing that at least four at a time be visible from any place on Earth, at all times. ² All one needs is a receiver that can pick up their signal.

Transit, also known as Navsat, was the first navigation satellite to be launched by the United States in 1959-1960, and used for the navigation of the Polaris submarines. The research done by APL scientists George Weiffenbach and William Guier on Sputnik’s orbit revealed that if the exact position of a satellite is known and predictable, the Doppler shift could be used to locate a receiver back on Earth.³ Transit functioned until 1996 when the Global Positioning System (GPS) took over. Because of the many applications of the GPS, it has been decided to extend its capabilities in order to serve the civil community.

GPS satellites are placed on a semi-synchronies orbit, and are set up to into six orbital planes, with four to five satellites per orbital plane, and therefore multiple satellites are in view from one location at the same time. This set-up gives GPS worldwide coverage. The orbits are inclined by 55° with respect to the equator and have orbital periods of approximately 11 hours, 58 minutes. Every time we get an exact location on a GPS receiver, the result is the work of four satellites: the first one gives a roundabout area, the second and third satellite refines the information, and the fourth gives the exact coordinates. The receiver measures the time the signal needed to travel from the satellite, and the measured distance tells the requested location is on the circle where two spheres with the satellites at center meet. The radii of these spheres are the distance between the satellite and receiver.⁴

The Soviet Union also launched a positioning system in 1982. GLONASS became fully operational in 1993, and was made of 21 satellites in three orbital planes. This system is still operational at the moment, in a degrade mode, with only eight satellites.⁵ The European Space Agency plans to launch a navigation satellite system similar to GPS, called the Galileo positioning system, planed to become operable in 2014. China is working on a similar system to become available by 2020.

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Notes:

[1] European Space Agency (ESA). About Satellite Navigation. February 2, 2005. http http://www.esa.int/esaNA/GGGYC650NDC_index_2.html (Accessed September 26, 2011), para. 2.

[2] Ibid, para. 14

[3] Gavaghan, Helen. Something New Under the Sun: Satellites and the Beginning of the Space Age. Copernicus (Springer-Verlag), 1998.

[4] Graham, Rebecca. The Edge of Space. The Universe. 10 February 2009, History Channel.

[5] Space and Tech. GLONASS – Summary. http://www.spaceandtech.com/spacedata/constellations/glonass_consum.shtml (Accessed September 26, 2011), para. 2.

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References

European Space Agency (ESA). About Satellite Navigation. February 2, 2005. http://www.esa.int/esaNA/GGGYC650NDC_index_2.html (Accessed September 26, 2011).

Gavaghan, Helen. Something New Under the Sun: Satellites and the Beginning of the Space Age. Copernicus (Springer-Verlag), 1998.

Graham, Rebecca. The Edge of Space. The Universe. 10 February 2009, History Channel.

Space and Tech. GLONASS – Summary. http://www.spaceandtech.com/spacedata/constellations/glonass_consum.shtml (Accessed September 26, 2011).