Tag Archives: satellite

Alternatives to GPS

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“GPS” is actually a brand name, and like “hoover” or “kleenex” has become a proprietary eponym, used to refer to a category of products despite just being one product in that category. There are other satellite navigation systems that offer the capabilities of GPS.

gnss-orbits

Operational

GLONASS

GLONASS (Globalnaya Navigatsionnaya Sputnikovaya Sistema) is the most developed of the GPS alternatives, and is operated by Russia. GLONASS uses twenty-nine satellites compared with the thirty-two satellites that make up the GPS constellation, and the GLONASS satellites are in a slightly lower orbit. Due to the orbital position of the satellites, GLONASS is slightly less accurate overall than GPS, but more accurate at high latitudes (both north and south) in locations such as Russia, because it is inclined at an angle of 65° to Earth’s equator as opposed to GPS’s 55°.

Many mobile phones already use a combination of GPS and GLONASS, and the availability of more satellites means that it is easier to get a line-of-sight satellite lock than when using GPS alone.

DORIS

DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite) is a French system that operates the other way around to other satellite navigation systems: each DORIS satellite receives signals from between fifty and sixty ground stations, rather than it being the satellites that are broadcasting the signals. There are no dedicated DORIS satellites, but DORIS receivers are fitted to a number of satellites such as TOPEX/Poseidon and Jason.

The accuracy of DORIS is lower than GPS as ground positioning is not its primary purpose; it is designed to measure the position the orbits of the satellites aboard which DORIS receivers are fitted and to measure the height of the oceans.

BeiDou/COMPASS

BeiDou was a test system developed by China that used four satellites and covered China, India, parts of Japan and other countries.

Beidou-coverage

The BeiDou system is being replaced to create BedDou-2 or COMPASS, and now uses sixteen satellites which cover the Asia-Pacific region, including Australia. By 2020 COMPASS should be able to offer global coverage, using thirty-five satellites, including five in geostationary orbit.

Development

Galileo

Galileo is a European satellite navigation system run by the EU and the European Space Agency; it plans to offer one-metre accuracy (compared with GPS’s fifteen-metre accuracy) and a search-and-rescue function for free, and will use a constellation of thirty satellites. Higher-precision (centimetre-resolution) data will be available to paying users. Five Galileo satellites have been successfully launched and placed, but the Soyuz rocket and Fregat “space-tug” used for the sixth satellite did not place it into the correct orbit.

Because Galileo satellites are placed in a higher orbit (23 200 km) than GPS satellites (20 200 km), there is a higher chance of being able to achieve a line-of-sight satellite lock, particularly in the “urban canyon” environment of modern high-rise cities. Galileo will also offer better coverage at high latitudes (e.g. northern Norway), where GPS coverage varies during the day due to the orbits of the GPS satellites.

It is highly likely that modern devices will use a combination of GPS, GLONASS and Galileo when the Galileo system becomes fully operational.

IRNSS

IRNSS (Indian Regional Navigation Satellite System) is an Indian satellite navigation system, the development of which is at least partially due to a desire to avoid reliance on foreign navigation systems. (During the India-Pakistan Kargil War in 1999, the US denied India access to GPS data.)

The system will consist of seven satellites in geostationary orbits, and will offer accuracy of below twenty metres within a radius of 3000 km of the centre of India, increasing to below ten metres over mainland India.

QZSS

QZSS (Quasi-Zenith Satellite System) is a Japanese system that is not a navigation system of its own, but rather augments the GPS system (it send signals compatible with standard GPS receivers). It will offer better coverage than GPS within Japan’s “urban canyons”, as the orbits of the four QZSS satellites will ensure that one satellite is always directly overhead of Japan. QZSS is specifically targeted at mobile applications, and will offer data transfer capabilities alongside positioning information.

Reaction wheels and pointing satellites

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The Kepler telescope, a satellite telescope in orbit around the Sun* designed to look for exoplanets, has come to the end of its original mission. The end of the mission was caused by the failure of ‘s reaction wheels, which are used to point the telescope. Reaction wheels are electric motors connected to metals discs, usually with masses between . As the speed of the motors are altered the conservation of angular momentum imparts a force on the spacecraft, causing it to rotate around its centre of mass. lro-reaction-wheels

The aboard the Lunar Reconnaissance Orbiter.

Reaction wheels are usually employed in groups of three, the x-, y- and z-axes, enabling the telescope to be pointed accurately in . Kepler was fitted with , with all , and each acting as a spare for the other three. After wheels (#2) failed in July 2012 the spacecraft was still able to operate normally, but reaction wheel #4 began malfunctioning in January 2013, and whilst the wheel returned to working order initially, it failed completely on, leaving the spacecraft now unable to move and point properly. kepler-reaction-wheels

Reaction wheels aboard Kepler.

Reaction wheels are often confused with momentum wheels, but the two are very different. Momentum wheels are much heavier, and spin at much higher rates, and their role is not to point or steer the spacecraft but rather to use the gyroscopic effect to keep it in a fixed position when subjected to perturbing forces such as solar wind or radiation pressure. Satellites in orbit close around Earth can also use a device called a magnetorquer to control their position. By altering the flow of current through a set of coils (again usually three, with the spacecraft pushes against Earth’s magnetic field and the reaction force against this push causes the satellite to rotate.

* Technically an Earth-trailing heliocentric orbit.

Salt flats and giant space mirrors

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A salt flat is formed when a pool of salt water evaporates, depositing salt as it does. This layer of salt builds up over time and seasonal flooding causes a very flat surface to form.

Salar de Uyuni in Bolivia, the largest salt flat in the world.

When covered in water, salt flats become the largest mirrors in the world.

Salt flats are commonly used to calibrate observation satellites, as they provide very large and flat areas (Salar de Uyuni has an area of more than 10 000 square kilometres and varies in height by less than one metre). The surface of salt flats are highly reflective and because they occur in desert areas there is usually very little cloud cover and very clear air.

Orbits

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Thanks to my Dad for the inspiration for this post.

Earth is orbited by thousands of artificial satellites (and one natural one). These satellites orbit in a number of different ways according to their purpose.

Low Earth Orbit (LEO)

Examples: International Space Station, Hubble Space Telescope, Iridium communication satellites.

Objects in LEO orbit at between 200 and 2000 kilometres from Earth. The low distance from Earth means that it is relatively easy (in terms of energy/fuel) to get into LEO and this orbit is therefore popular. This popularity is boosted by the fact that devices on Earth’s surface do not need to be high-powered to transmit a signal to the satellite. Objects in LEO orbit the Earth about once every 90 minutes and therefore do not remain fixed over a given location for any length of time, requiring that a constellation of satellites be present to provide continuous coverage of/to any one area. For example, the Iridium satellite phone constellation is composed of sixty-six active satellites orbiting at about 780 kilometres above Earth.

Current position of: ISS and Hubble Space Telescope.

Medium Earth Orbit (MEO)

Examples: Global Positioning System (GPS), GLONASS and Galilieo positioning systems.

Satellites in MEO orbit between 2000 and 35 000 kilometres. At this orbit fewer satellites are required to cover the globe: GPS uses thirty-one satellites orbiting at 2o 200 km; and the Russian equivalent, Glonass, uses twenty-four satellites orbiting at 19 100 km. At this distance devices that communicate with satellites have to be high-powered to reach them, but in the case of navigation satellites, which are transmit only,* this is not a problem.

Current position of: GPS Navstar satellites and GLONASS Cosmos satellites.

Geosynchronous Orbit

Examples: Satellite television broadcasting, weather and reconnaissance satellites.

A geosynchronous orbit is one that is in sync with the rotation of Earth: it takes geosynchronous satellites exactly one day to complete one orbit. If a geosynchronous satellite is placed in the same plane as the equator then that satellite is said to be geostationary – relative to an observer on Earth it remains in the same place in the sky all the time. This means that the antennae used to communicate with geostationary satellites can remain in a fixed position – they do not have to move to track the satellite as it moves across the sky.

The idea of a geostationary satellite was popularised by the science fiction author Arthur C. Clarke in an article for Wireless World magazine entitled Extra-Terrestrial Relays – Can Rocket Stations Give World-wide Radio Coverage? so geostationary orbits are sometimes known as Clarke Orbits.

Calculating the height of a geostationary orbit is relatively simple. Any object moving in a circle requires a centripetal force towards the centre of that circle to keep it moving on a curved path. By setting this centripetal force equal to the force of Earth’s gravity pulling on the satellite and solving to find the radius of the circle you find that geostationary satellites orbit at 35 768 kilometres above Earth.†

Not all geosynchronous orbits are geostationary. A geosynchronous polar orbit, in which the satellite passes over both poles causes the satellite to be above the same location at the same time every day, which is useful in making day-to-day comparisons. When using this system more than one satellite is required because each satellite will spend a large amount of time facing the dark (i.e. non sun-facing) side of the Earth.

Polar Orbit

Examples: Imaging/reconnaissance satellites

A polar orbit is one in which a satellite passes over both of Earth’s geographic poles as it orbits; on each orbit it will therefore be above a strip of land west (as the earth rotates east-to-west) of the piece it previously orbited. Polar orbits like this are useful because a low number of satellites can image the entire Earth. NASA’s MODIS system is composed of two satellites (Terra and Aqua) that image the entire Earth once every one or two days.

An image showing one day of swaths from the TRMM satellite.

Current position of: Terra, Aqua and TRMM satellites.

Elliptical Orbits

Examples: Specialised communication satellites, Sirius satellite radio

All of the previous orbits are circular, each satellite remains at a constant distance from Earth’s surface. Satellites in elliptical orbits change their distance from Earth, speeding up as they approach closer to Earth and slowing down as they move away. Satellites in elliptical orbits spend long periods of time over one area of Earth, providing coverage of that particular area with a small number of satellites.

A communication satellite in a geostationary orbit is further away from northern latitudes than the equator and therefore requires more power to be able to reach these locations. Russian Molniya communication satellites (after which the Molniya orbit is named) orbit at an angle of 63° to the equator at a distance between 500 and 40 000 km and take half a day to complete one orbit, enabling them to loiter above 55°N for six hours a day, which means that only three satellites are required for all-day coverage at high latitudes (as they can broadcast to 55°N from below this latitude). The Sirius Satellite Radio system, which broadcasts to North America, uses three satellites in Tundra orbits which are similar to Molniya orbits in their inclination and eccentricity, but with an orbital period of one day, making them geosynchronous. In contrast, the XM satellite radio system uses two satellites placed in geostationary orbits at longitudes of 85°W and 115°W (corresponding approximately to the west and east coasts of the USA).

Graveyard Orbit

Graveyard orbits are where satellites go to die. At the end of their useful life, if there isn’t enough energy to push a satellite into Earth’s atmosphere where it will burn up (or land in the “spacecraft cemetery” in the Southern Pacific Ocean off the coast of New Zealand) then satellites are pushed into an orbit above their normal location so that they do not collide with active satellites in their original orbit.

An image of space debris from the European Space Agency.

You can use WolframAlpha to find out which satellites are currently above your location.

* The US Department of Defense, which runs the GPS, does transmit updates to the satellites, from the Master Control Station at Schriever Air Force Base and from one of four monitoring stations on the islands of Hawaii and Kwajalein in the Pacific Ocean, the Ascension Islands in the South Atlantic Ocean and Diego Garcia in the central Indian Ocean.

† For the interested, the radius of a geostationary orbit from the centre of a planet with mass M that rotates once every T seconds is the third root of GMT2/4π2 where G is the universal constant of gravitation.