Category The Universe, Exploring the Universe, Solar System, The Moon, Space, Space Travel

Satellites can help scientists learn a great deal more about the planet than instruments on aircraft and ships can. They use Earth-resources satellites to monitor every part of the world in order to find out information about the planet’s condition. Satellites can detect things such as the amount of water in a field of crops, which will give early warning of a harvest failure. They can also detect large areas of deforestation, showing changes over large periods of time.

ERS (Earth Resources Satellite) are the first two remote sensing satellites launched by ESA (European Space Agency). Their primary mission was to monitor Earth’s oceans, ice caps, and coastal regions.

The satellites provided systematic, repetitive global measurements of wind speed and direction, wave height, surface temperature, surface altitude, cloud cover, and atmospheric water vapor level. Data from ERS-1 were shared with NASA under a reciprocal agreement for Seasat and Nimbus 7 data. ERS-2 carries the same suite of instruments as ERS-1 with the addition of the Global Ozone Measuring Equipment (GOME) which measures ozone distribution in the outer atmosphere. Having performed well for nine years – more than three times its planned lifetime – the ERS-1 mission was ended on March 10, 2000, by a failure in the onboard attitude control system.

The length of its operation enabled scientists to track several El Nino episodes through combined observations of surface currents, topography, temperatures, and winds. The measurements of sea surface temperatures, critical to the understanding of climate change by the ERS-1 Along-Track Scanning Radiometer were the most accurate ever made from space. All these important measurements are being continued by ERS-

Meteorology satellites, which orbit in geostationary and polar orbits, can keep a constant watch over the weather systems at work around the planet. They record data, such as cloud formation and movement, pressures, wind speeds and humidities, and send them to Earth, where scientists can use them to predict weather in preparation for weather forecasts. Satellites are also used to detect hurricanes — fierce tropical storms with wind speeds of over 130km/h (80mph). These storms can strike with very little warning, but satellites can detect them before they hit land, warning people of danger in time for them to take cover.

Weather satellites carry instruments called radiometers (not cameras) that scan the Earth to form images. These instruments usually have some sort of small telescope or antenna, a scanning mechanism, and one or more detectors that detect either visible, infrared, or microwave radiation for the purpose of monitoring weather systems around the world.

The measurements these instruments make are in the form of electrical voltages, which are digitized and then transmitted to receiving stations on the ground. The data are then relayed to various weather forecast centers around the world, and are made available over the internet in the form of images. Because weather changes quickly, the time from satellite measurement to image availability can be less than a minute.

Most of the satellites and instruments they carry are designed to operate for 3 to 7 years, although many of them last much longer than that. Weather satellites are put into one of two kinds of orbits around the Earth, each of which has advantages (and disadvantages) for weather monitoring. The first is a “geostationary” orbit, with the satellite at a very high altitude (about 22,500 miles) and orbiting over the equator at the same rate that the Earth turns. This allows the satellite to view the same geographic area continuously, and is used to provide most of the satellite imagery you see on TV or the internet.

For instance, GOES-East and GOES-West provide coverage of much of the Western Hemisphere, from the western coast of Africa to the West Pacific, and the Arctic to the Antarctic. The European Space Agency’s Meteosat satellite provides coverage of Europe and Africa. The disadvantages of a geostationary orbit are (1) its very high altitude, which requires elaborate telescopes and precise scanning mechanisisms in order to image the Earth at high resolution (currently, 1 km at best); and (2) only a portion of the Earth can be viewed.

The other orbit type is called near-polar, sun-synchronous (or just “polar”), where the satellite is put into a relatively low altitude orbit (around 500 miles) that carries the satellite near the North Pole and the South Pole approximately every 100 minutes. Unlike the geostationary orbit, the polar orbit allows complete Earth coverage as the Earth turns beneath it.

These orbits are “sun-synchronous”, allowing the satellite to measure the same location on the Earth twice each day at the same local time. Of course, the diadvantage of this orbit is that the satellite can image a particular location only every 12 hours, rather than continuously as in the case of the geostationary satellite. To offset this disadvantage, two satellites put into orbits at different sun-synchronous times have allowed up to 6 hourly monitoring.

But because of the lower altitude (500 miles rather than 22,000 miles), the instruments the polar-orbiting satellite carries to image the Earth do not have to be as elaborate in order to achieve the same ground resolution. Also, the lower orbit allows microwave radiometers to be used, which must have relatively large antennas in order to achieve ground resolutions fine enough to be useful. The advantage of microwave radiometers is their ability to measure through clouds to sense precipitation, temperature in different layers of the atmosphere, and surface characteristics like ocean surface winds.

A satellite in geostationary orbit takes the same time to orbit the Earth as the Earth does to spin, therefore always remaining over the same point on the planet. This orbit is mainly used for communications satellites. Low-Earth orbits, often used by spy satellites, can be lower than 250km (155 miles) above the planet. Polar-orbit satellites orbit at around 800km (590 miles), while highly-elliptical-orbit satellites have very low altitudes when they are closest to Earth, but pass far beyond the planet when they are at their most distant.

A geosynchronous orbit is a high Earth orbit that allows satellites to match Earth’s rotation. Located at 22,236 miles (35,786 kilometers) above Earth’s equator, this position is a valuable spot for monitoring weather, communications and surveillance. “Because the satellite orbits at the same speed that the Earth is turning, the satellite seems to stay in place over a single longitude, though it may drift north to south,” NASA wrote on its Earth Observatory website.

Satellites are designed to orbit Earth in one of three basic orbits defined by their distance from the planet: low Earth orbit, medium Earth orbit or high Earth orbit. The higher a satellite is above Earth (or any other world for that matter), the slower it moves. This is because of the effect of Earth’s gravity; it pulls more strongly at satellites that are closer to its center than satellites that are farther away. 

So a satellite at low Earth orbit — such as the International Space Station, at roughly 250 miles (400 km) — will move over the surface, seeing different regions at different times of day. Those at medium Earth orbit (between about 2,000 and 35,780 km, or 1,242 and 22,232 miles) move more slowly, allowing for more detailed studies of a region. At geosynchronous orbit, however, the orbital period of the satellite matches the orbit of the Earth (roughly 24 hours), and the satellite appears virtually still over one spot; it stays at the same longitude, but its orbit may be tilted, or inclined, a few degrees north or south.

A great many of the satellites sent into space by the USA and Russia are used for military activities. These range from eaves-dropping on important telephone calls to detecting the x-rays and electromagnetic pulses given off by nuclear explosions. Early military satellites were used to take close-up pictures of enemy territory but had to return home to have their film developed. Modern satellites use digital technology to take photographs, so they never run out of film. Amazingly, they can photograph things as small as the headlines on a newspaper.

The military space program is a significant but largely unseen aspect of space operations. Nearly a dozen countries have some kind of military space program, but the U.S. program dwarfs the efforts of all these other countries combined.

Military space operations are divided into five main areas: reconnaissance and surveillance, signals intelligence, communications, navigation, and meteorology. Only the United State and Russia operate spacecraft in all five areas. Several other countries have long used communications satellites for military purposes. In the 1990s, several countries in addition to Russia and the United State began developing reconnaissance satellites.

Reconnaissance and surveillance involve the observation of Earth for various purposes. Dedicated reconnaissance satellites, like the United States’ Improved CRYSTAL and the Russian Terilen, take photographs of targets on the ground and relay them to receiving stations in nearly real time. These satellites, however, cannot take continuous images like a television camera. Instead, they take a black-and-white photograph of a target every few seconds. Because they are in low orbits and are constantly moving, they can photograph a target for only a little over a minute before they move out of range. The best American satellites, which are similar in appearance to the Hubble Space Telescope, can see objects about the size of a softball from hundreds of miles up but they cannot read license plates. The Russians also occasionally use a system that takes photographs on film and then returns the film to Earth for processing. This provides them with higher-quality photos. The United States abandoned this technology in the 1980s after developing superior electronic imaging technology.

Other surveillance satellites, such as the American DSP and Space-Based Infrared System (SBIRS, pronounced “sibirs”) and the Russian Oko (or “eye”), are equipped with infrared telescopes and scan the ground for the heat produced by a missile’s exhaust. They can be used to warn of missile attack and can predict the targets of missiles fired hundreds or thousands of miles away. There are also satellites that look at the ground in different wavelengths to peer through camouflage, try to determine what objects are made of, and analyze smokestack emissions.

Signals intelligence satellites can operate either in low Earth orbit or in extremely high, geosynchronous orbit, where they appear to stay in one spot in the sky. These satellites listen for communications from cellular telephones, walkie-talkies, microwave transmissions, radios, and radar. They relay this information to the ground, where it is processed for various purposes. Contrary to popular myth, these satellites do not collect every conversation around the world. There is far more information being transmitted every day over the Internet than can be collected by evens the best spy agency.

Communications satellites are used for many different tasks, including television broadcasts and telephone calls. A telephone call made from England to the USA would be sent to the nearest Earth station, which would use its giant antenna to beam the call into space in the form of radio waves. The satellite would receive these radio waves and beam them back down to an antenna on the other side of the planet.

A communications satellite is an artificial satellite that relays and amplifies radio telecommunications signals via a transponder; it creates a communication channel between a source transmitter and a receiver at different locations on Earth. Communications satellites are used for television, telephone, radio, internet, and military applications. There are about 2,000 communications satellites in Earth’s orbit, used by both private and government organizations. Many are in geostationary orbit 22,236 miles (35,785 km) above the equator, so that the satellite appears stationary at the same point in the sky, so the satellite dish antennas of ground stations can be aimed permanently at that spot and do not have to move to track it.

The high frequency radio waves used for telecommunications links travel by line of sight and so are obstructed by the curve of the Earth. The purpose of communications satellites is to relay the signal around the curve of the Earth allowing communication between widely separated geographical points. Communications satellites use a wide range of radio and microwave frequencies. To avoid signal interference, international organizations have regulations for which frequency ranges or “bands” certain organizations are allowed to use. This allocation of bands minimizes the risk of signal interference.

Launched by NASA in 1962, Relay 1 was one of several satellites placed in orbit in the decade after Sputnik to test the possibilities of communications from space. Relay 1 received telephone and television signals from ground stations and then transmitted them to other locations on the Earth’s surface. The satellite relayed signals between North America and Europe and between North and South America, and it also monitored the effects of radiation on its electronics. In conjunction with the Syncom 3 communications satellite, Relay 1 transmitted television coverage of the 1964 Olympics in Japan.

This prototype of Relay 1 is covered with solar cells. The antenna on top is for receiving and transmitting communications signals; those at its base are for telemetry, tracking, and control. In orbit, Relay used spin-stabilization to orient the antennas to communicate with Earth.

Satellites must be launched into orbit with enough speed to prevent Earth’s gravity from pulling them back down to the ground. Imagine throwing a ball horizontally. Gravity pulls the ball back to Earth very quickly. If the ball could be thrown hard enough, however, then it would have enough force to keep on travelling horizontally forever. It would be in orbit. A satellite at an altitude of 200km (120 miles) must be travelling at 7.8km/s (4.8mp/s) to prevent it being pulled back down to Earth.

An artificial satellite is a marvel of technology and engineering. The only thing comparable to the feat in technological terms is the scientific know-how that goes into placing, and keeping, one in orbit around the Earth. Just consider what scientists need to understand in order to make this happen: first, there’s gravity, then a comprehensive knowledge of physics, and of course the nature of orbits themselves. So really, the question of How Satellites Stay in Orbit is a multidisciplinary one that involves a great of technical and academic knowledge.

First, to understand how a satellite orbits the Earth, it is important to understand what orbit entails. Johann Kepler was the first to accurately describe the mathematical shape of the orbits of planets. Whereas the orbits of planets about the Sun and the Moon about the Earth were thought to be perfectly circular, Kepler stumbled onto the concept of elliptical orbits. In order for an object to stay in orbit around the Earth, it must have enough speed to retrace its path. This is as true of a natural satellite as it is of an artificial one. From Kepler’s discovery, scientists were also able to infer that the closer a satellite is to an object, the stronger the force of attraction, hence it must travel faster in order to maintain orbit.

Next comes an understanding of gravity itself. All objects possess a gravitational field, but it is only in the case of particularly large objects (i.e. planets) that this force is felt. In Earth’s case, the gravitational pull is calculated to 9.8 m/s2. However, that is a specific case at the surface of the planet. When calculating objects in orbit about the Earth, the formula v=(GM/R)1/2 applies, where v is velocity of the satellite, G is the gravitational constant, M is the mass of the planet, and R is the distance from the center of the Earth. Relying on this formula, we are able to see that the velocity required for orbit is equal to the square root of the distance from the object to the center of the Earth times the acceleration due to gravity at that distance. So if we wanted to put a satellite in a circular orbit at 500 km above the surface (what scientists would call a Low Earth Orbit LEO), it would need a speed of ((6.67 x 10-11 * 6.0 x 1024)/(6900000))1/2 or 7615.77 m/s. The greater the altitude, the less velocity is needed to maintain the orbit.

So really, a satellites ability to maintain its orbit comes down to a balance between two factors: its velocity (or the speed at which it would travel in a straight line), and the gravitational pull between the satellite and the planet it orbits. The higher the orbit, the less velocity is required. The nearer the orbit, the faster it must move to ensure that it does not fall back to Earth.