Geostationary orbit
British science fiction author Arthur C. Clarke popularised and expanded the concept in a 1945 paper entitled Extra-Terrestrial Relays – Can Rocket Stations Give Worldwide Radio Coverage?, published in Wireless World magazine.
[11] Although these projects had difficulties with signal strength and tracking, issues that could be solved using geostationary orbits, the concept was seen as impractical, so Hughes often withheld funds and support.
Although its inclined orbit still required moving antennas, it was able to relay TV transmissions, and allowed for US President John F. Kennedy in Washington D.C., to phone Nigerian prime minister Abubakar Tafawa Balewa aboard the USNS Kingsport docked in Lagos on August 23, 1963.
[19][20][21] Geostationary communication satellites are useful because they are visible from a large area of the earth's surface, extending 81° away in latitude and 77° in longitude.
This means that Earth-based observers can erect small, cheap and stationary antennas that are always directed at the desired satellite.
[23]: 537 However, latency becomes significant as it takes about 240 ms for a signal to pass from a ground based transmitter on the equator to the satellite and back again.
[25] Geostationary satellites are directly overhead at the equator and appear lower in the sky to an observer nearer the poles.
As the observer's latitude increases, communication becomes more difficult due to factors such as atmospheric refraction, Earth's thermal emission, line-of-sight obstructions, and signal reflections from the ground or nearby structures.
[22] Because of this, some Russian communication satellites have used elliptical Molniya and Tundra orbits, which have excellent visibility at high latitudes.
[36] Geostationary satellite imagery has been used for tracking volcanic ash,[37] measuring cloud top temperatures and water vapour, oceanography,[38] measuring land temperature and vegetation coverage,[39][40] facilitating cyclone path prediction,[34] and providing real time cloud coverage and other tracking data.
[41] Some information has been incorporated into meteorological prediction models, but due to their wide field of view, full-time monitoring and lower resolution, geostationary weather satellite images are primarily used for short-term and real-time forecasting.
[42][40] Geostationary satellites can be used to augment GNSS systems by relaying clock, ephemeris and ionospheric error corrections (calculated from ground stations of a known position) and providing an additional reference signal.
[48] Additionally, launching from close to the equator allows the speed of the Earth's rotation to give the satellite a boost.
[65] A second effect to be taken into account is the longitudinal drift, caused by the asymmetry of the Earth – the equator is slightly elliptical (equatorial eccentricity).
[64] The correction of this effect requires station-keeping maneuvers with a maximal delta-v of about 2 m/s per year, depending on the desired longitude.
[65] Solar wind and radiation pressure also exert small forces on satellites: over time, these cause them to slowly drift away from their prescribed orbits.
[67] In the absence of servicing missions from the Earth or a renewable propulsion method, the consumption of thruster propellant for station-keeping places a limitation on the lifetime of the satellite.
Hall-effect thrusters, which are currently in use, have the potential to prolong the service life of a satellite by providing high-efficiency electric propulsion.
From Newton's second law of motion, the centripetal force Fc is given by: As Fc = Fg, so that Replacing v with the equation for the speed of an object moving around a circle produces: where T is the orbital period (i.e. one sidereal day), and is equal to 86164.09054 s.[69] This gives an equation for r:[70] The product GME is known with much greater precision than either factor alone; it is known as the geocentric gravitational constant μ = 398600.4418±0.0008 km3 s−2.
[72] The gravitational constant GM (μ) for Mars has the value of 42830 km3 s−2, its equatorial radius is 3389.50 km and the known rotational period (T) of the planet is 1.02595676 Earth days (88642.66 s).
