Satellite Orbits PDF

Lecture 3 Satellite  Object placed into Earth orbit A satellite or artificial satellite is an object, typically a spacecraft, placed into orbit around Earth.  Earth orbits Altitude classifications – Low Earth orbit – Medium Ear...

Lecture 3 Satellite  Object placed into Earth orbit A satellite or artificial satellite is an object, typically a spacecraft, placed into orbit around Earth.  Earth orbits Altitude classifications – Low Earth orbit – Medium Earth orbit Semi-synchronous orbit – Geosynchronous orbit Geostationary orbit – High Earth orbit  Low Earth orbit The term low Earth orbit (LEO)(cyan) region is also used for the area of space below an altitude of 2,000 km (about 1/3 of Earth’s radius). – The red dotted line represents the orbit of the ISS. According to Kepler’s third law, this corresponds to an orbit period of 128 minutes. – Stellarium Web Online Star Map  Advantages A low Earth orbit requires the lowest amount of energy for satellite placement. It provides low communication latency. Satellites and space stations in LEO are more accessible for crew and servicing.  Applications Hubble Space Telescope Space station Communications satellite Earth observation satellite  Hubble Space Telescope The Hubble Space Telescope (HST) is one of the largest and most versatile space telescope. Hubble’s orbit outside the distortion of Earth’s atmosphere allows it to capture extremely high-resolution images with substantially lower background light than ground-based telescopes. https://www.youtube.com/watch?v=0V08M1NcdJQ  Maintained by astronauts Hubble is the only telescope designed to be maintained in space by astronauts. Five Space Shuttle missions have repaired, upgraded, and replaced systems on the telescope, including all five of the main instruments. – Hubble was launched in 1990, but its main mirror had a flaw. The optics were corrected by a servicing mission in 1993.  Space station A space station is a spacecraft which remains in orbit and hosts humans for extended periods of time. Most often space stations have been research stations, but they have also served military or commercial uses, such as hosting space tourists. All space stations to date have operated within LEO.  International Space Station The International Space Station (ISS) is a large space station assembled and maintained a collaboration of five space agencies and their contractors: NASA (United States), Roscosmos (Russia), ESA (Europe), JAXA (Japan), and CSA (Canada). The ISS is the largest space station ever built.  Scientific research Its primary purpose is to perform microgravity and space environment experiments. Research is conducted in a wide variety of fields, including astrobiology, astronomy, physical sciences, materials science, and human research including space medicine and the life sciences. This Photo by Unknown Author is licensed under CC BY-NC  Tourism 13 space tourists have visited the ISS. – Dennis Tito visited the ISS in 2001, becoming the world’s first space tourist. Tito paid a reported US$20 million for his trip.  Communications satellite A communications satellite 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. LEO satellites are less expensive to launch into orbit.  Relays & amplifies radio signals A communications satellite relays and amplifies radio telecommunication signals via a transponder. A satellite in LEO needs less powerful amplifiers for successful transmission.  Relay the signal around the curve of Earth The 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 Earth allowing communication between widely separated Rudolfs Davis Strazds, CC BY-SA 4.0, via Wikimedia Commons geographical points. Covers a small area that moves at high velocity A single satellite in LEO only covers a small area that moves as the satellite travels at high orbital velocity.  Satellite constellation A satellite constellation is a group of satellites working together as a system. Unlike a single satellite, a constellation can provide permanent global coverage. This Photo by Unknown Author is licensed under CC BY  Earth observation satellite An Earth observation satellite is a satellite used or designed for Earth observation from orbit, including spy satellites and similar ones intended for non- military uses such as environmental monitoring, meteorology, cartography and others.  See the surface of Earth clearly by being close Earth observation satellites use LEO as they are able to see the surface of Earth more clearly by being closer to it.  Polar orbit & global coverage This Photo by Unknown Author is licensed under CC BY-NC To get global coverage with a low orbit, a polar orbit is used. Earth will rotate around its polar axis between successive orbits. The ground track moves towards the west each orbit, allowing a different section of the globe to be scanned with each orbit. https://www.youtube.com/watch?v=y_jM_BxQGvE  Medium Earth orbit A medium Earth orbit (MEO) (yellow) has an altitude above a LEO. – The green dashed line is the orbit used for GPS satellites. The edge of MEO is the particular altitude of a geosynchronous orbit (black dashed line). All satellites in MEO have an orbital period ranging between 24 hours and about 2 hours.  Semi-synchronous orbit A MEO that is particularly significant is the semi- synchronous orbit. A satellite in the semi- synchronous orbit has an orbital period of 12 hours. This Photo by Unknown Author is licensed under CC BY-SA  Reliably predictable orbit The satellite passes over the same spots every day. The orbit is reliably predictable.  Global Positioning System The Global Positioning System (GPS) is a satellite-based radio navigation system.  Four satellites in view of the receiver It provides geolocation and time information to a GPS receiver anywhere on Earth where there is an unobstructed line of sight to four or more GPS satellites. This Photo by Unknown Author is licensed under CC BY-SA-NC  Three position coordinates & time Each GPS satellite carries an accurate record of its own position and time, and broadcasts that data continuously. Based on data received from multiple GPS satellites, an end user’s GPS receiver can calculate its own geolocation and time. However, at a minimum, four satellites must be in view of the receiver for it to compute four unknown quantities (three position coordinates and the deviation of its own clock from satellite time).  Satellite constellation The current GPS consists of 24 to 32 satellites in semi-synchronous orbit, so that the satellites pass over the same locations every day. The orbits are arranged so that at least six satellites are always within line of sight from everywhere on Earth’s surface. Paulsava, CC BY-SA 4.0, via Wikimedia Commons  Geosynchronous orbit A geosynchronous orbit (GSO) has an orbital period that matches Earth’s rotation on its axis (one day).  Geostationary orbit A special case of GSO is the geostationary orbit, which is a circular GSO in Earth’s equatorial plane. A satellite in a geostationary orbit remains in the same position in the sky to observers on the surface. Lookang many thanks to author of original simulation = Francisco Esquembre author of Easy Java Simulation = Francisco Esquembre, CC BY-SA 3.0, via Wikimedia Commons Antennas are pointed permanently at a position Communications satellites are often placed in a geostationary orbit so that Earth-based satellite antennas do not have to rotate to track them but can be pointed permanently at the position in the sky where the satellites are located. Vsatinet, CC BY-SA 4.0, via Wikimedia Commons  Record the entire hemisphere continuously A weather satellite is a type of Earth observation satellite that is primarily used to monitor the weather and climate of Earth. Geostationary weather satellites can record or transmit images of the entire hemisphere below continuously. https://www.youtube.com/watch?v=ECjEHbcydDA  High Earth orbit A high Earth orbit (HEO) is farther than that of the geosynchronous orbit. One of the main benefits of HEO is that it provides a nearly unobstructed view of Earth and deep space. This makes it an ideal location for astronomical observations and Earth monitoring.  Earth monitoring Vela was a group of satellites developed by the United States to detect nuclear detonations to monitor compliance with the 1963 Partial Test Ban Treaty by the Soviet Union. – Serendipitously, the Vela satellites were the first devices ever to detect cosmic gamma-ray bursts.  Astronomical observations Transiting Exoplanet Survey Satellite (TESS) is a space telescope designed to search for exoplanets. TESS had identified 7,203 candidate exoplanets, of which 482 had been confirmed. This Photo by Unknown Author is licensed under CC BY  Summary Satellite – Earth orbits Low Earth orbit Medium Earth orbit – Semi-synchronous orbit Geosynchronous orbit – Geostationary orbit High Earth orbit – Advantages – Applications  Lecture 3 – Part 2 Satellite (cont.) – Space debris – Satellite flare  Space debris Space debris are defunct human- made objects in space – principally in Earth orbit – which no longer serve a useful function. These include derelict spacecraft (nonfunctional spacecraft) and, particularly-numerous in-Earth orbit, fragmentation debris from the breakup of derelict rocket ESA/ID&Sense;/ONiRiXEL, CC BY-SA 3.0 IGO, via Wikimedia Commons bodies and spacecraft.  Accumulate in Earth orbit Space debris began to accumulate in Earth orbit with the launch of the first artificial satellite into orbit in 1957.  Risk to spacecraft Space debris represents a risk to spacecraft. Several spacecraft, both crewed and uncrewed, have been damaged or destroyed by space debris.  First major satellite collision The first major satellite collision occurred in 2009. Two communications satellites— the active Iridium 33 and the derelict Kosmos 2251— accidentally collided. Rlandmann, CC BY-SA 3.0, via Wikimedia Commons  Creating new debris Both satellites were destroyed, creating thousands of pieces of new smaller debris. https://www.youtube.com/watch?v=6Wf4H0e9rfU&t=10s  Kessler syndrome The Kessler syndrome was proposed by Donald J. Kessler in 1978. https://www.youtube.com/watch?v=LaKz8VDkDkI&t=30s  Density of objects in LEO is high enough The LEO environment is becoming congested with space debris because of the frequency of object launches. The Kessler syndrome is a theoretical scenario in which the density of objects in LEO is high enough that collisions between objects could cause a domino effect.  Domino effect In the domino effect, each collision generates space debris that increases the likelihood of further collisions. One implication is that the distribution of debris in orbit could render space activities and the use of satellites in particular low Earth orbits difficult for many generations. https://www.youtube.com/watch?v=gjE2Kffq8Fk&t=2s  International Telecommunication Union The International Telecommunication Union (ITU) is a specialized agency of the United Nations (UN) responsible for many matters related to information and communication technologies.  Requirement on designers of a new satellite Designers of a new vehicle or satellite are frequently required by the ITU to demonstrate that it can be safely disposed of at the end of its life, for example by use of a controlled atmospheric reentry system or a boost into a graveyard orbit. – A graveyard orbit is an orbit that lies away from common operational orbits.  Summary Space debris (domain multi- disciplinarity) – Satellites (technology) – LOE (Science) – Applications (economics) – ITU requirement (legal)  Satellite flare Satellite flare is a satellite pass visible to the naked eye as a brief, bright “flare”. It is caused by the reflection toward Earth below of sunlight incident on satellite surfaces. Andreas Möller, CC BY-SA 3.0 DE, via Wikimedia Commons  Light pollution Streaks from satellite flare are a form of light pollution that can negatively affect ground-based astronomy. The Iridium constellation was one of the first anthropogenic sources of near-space light pollution to draw criticism. Larger satellite constellations, like Starlink, have received increased criticism.  Iridium satellite constellation Iridium Communications owns and operates the Iridium satellite constellation. Satellites are placed in LEO. The constellation consists of 66 active satellites in orbit, required for global coverage, and additional spare satellites to serve in case of failure.  Name Early calculations showed that 77 satellites would be needed, hence the name Iridium, after the metal with atomic number 77. It turned out that just 66 were required. Pumbaa (original work by Greg Robson), CC BY-SA 2.0 UK, via Wikimedia Commons  Reflective antennas The first-generation satellites were deployed in 1997–2002. Due to the shape of their reflective antennas, the satellites focused sunlight on a small area of Earth surface in an incidental manner. Cliff, CC BY 2.0, via Wikimedia Commons  Iridium flares Brocken Inaglory, CC BY-SA 3.0, via Wikimedia Commons This resulted in a phenomenon called Iridium flares, whereby the satellite momentarily appeared as one of the brightest objects in the night sky and could be seen even during daylight. This flashing caused some annoyance to astronomers, as the flares occasionally disturbed observations. https://www.youtube.com/watch?v=uymzXNdXwmo  New generation does not produce flares From 2017 to 2019 the satellites were replaced with a new generation that does not produce flares, with the first generation completely deorbited by 2019. This Photo by Unknown Author is licensed under CC BY-SA-NC  Summary Iridium flares (domain multi- disciplinarity) – Iridium Communications (economics) – LOE (science) – Satellite constellation (technology) – Satellite flare (environment)  Starlink Starlink is a satellite internet constellation operated by Starlink Services, LLC, a wholly owned subsidiary of SpaceX.  Satellite internet constellation It consists of over 6,000 mass- produced small satellites in LEO. Nearly 12,000 satellites are planned to be deployed, with a possible later extension to 34,400. This Photo by Unknown Author is licensed under CC BY  Small satellites This Photo by Unknown Author is licensed under CC BY Satellites can be built small to reduce the large economic cost of launch vehicles and the costs associated with construction. Miniature satellites, especially in large numbers, may be more useful than fewer, larger ones for radio relay. https://www.youtube.com/watch?v=veMts1Khido  De-orbit & avoid collisions Concerns have been raised about how the satellites will contribute to an already congested orbital environment. SpaceX has attempted to mitigate the concerns. The satellites are equipped with thrusters allowing them to de- orbit at the end of their lives. They are also designed to autonomously and smoothly avoid collisions based on uplinked tracking data.  Effect on astronomy Astronomers have raised concerns about the effect the constellation may have on ground-based astronomy. While astronomers can schedule observations to avoid pointing where satellites currently orbit, it is “getting more difficult” as more satellites come online. Egon Filter, CC BY 4.0, via Wikipedia  Reducing brightness This Photo by Unknown Author is licensed under CC BY SpaceX has attempted to mitigate astronomy concerns by implementing several upgrades to Starlink satellites aimed at reducing their brightness during operation. This Photo by Unknown Author is licensed under CC BY-ND  Summary Starlink (domain multi- disciplinarity) – SpaceX (economics) – LEO (science) – Satellite constellation (technology) – Satellite flare (environment)