Radar
The term RADAR was coined in 1941 as an acronym for Radio Detection and Ranging.
Radar is a system that uses electromagnetic waves to identify the range, altitude, direction, or speed of both moving and fixed objects such as aircraft, ships, motor vehicles, weather formations, and terrain.
A transmitter emits radio waves, which are reflected by the target and detected by a receiver, typically in the same location as the transmitter. This enables a radar to detect objects at ranges where other emissions, such as sound or visible light, would be too weak to detect. Radar is used in many contexts, including_
Ø meteorological detection of precipitation,
Ø air traffic control, police detection of speeding traffic,
Ø and by the military.
Satellite
In the context of spaceflight, a satellite is any object which has been placed into orbit by human endeavor. They are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon.
A satellite is an object that orbits around something else. For example, the moon orbits around the earth, and it is a satellite. However, when most people think about satellites, they usually think of the man-made machines that send radio waves down to Earth and are used for communication.
The first artificial (man-made satellite) was a Soviet satellite named Sputnik.
Satellite Types
MILSTAR:A communication satellit
Anti-Satellite weapons, sometimes called "Killer satellites" are satellites designed to destroy "enemy" satellites, other orbital weapons and targets. Some are armed with kinetic rounds, while others use energy and/or particle weapons to destroy satellites, ICBMs, MIRVs. Both the U.S. and the USSR had these satellites. Links discussing "Killer satellites", ASATS (Anti-Satellite satellite) include USSR Tests ASAT weapon and ASAT Test. See also IMINT
Astronomical satellites are satellites used for observation of distant planets, galaxies, and other outer space objects.
Biosatellites are satellites designed to carry living organisms, generally for scientific experimentation.
Communications satellites are satellites stationed in space for the purpose of telecommunications. Modern communications satellites typically use geosynchronous orbits, Molniya orbits or Low Earth orbits.
Miniaturized satellites are satellites of unusually low weights and small sizes. New classifications are used to categorize these satellites: minisatellite (500–200 kg), microsatellite (below 200 kg), nanosatellite (below 10 kg).
Navigational satellites are satellites which use radio time signals transmitted to enable mobile receivers on the ground to determine their exact location. The relatively clear line of sight between the satellites and receivers on the ground, combined with ever-improving electronics, allows satellite navigation systems to measure location to accuracies on the order of a few metres in real time.
Reconnaissance satellites are Earth observation satellite or communications satellite deployed for military or intelligence applications. Little is known about the full power of these satellites, as governments who operate them usually keep information pertaining to their reconnaissance satellites classified.
Earth observation satellites are satellites intended for non-military uses such as environmental monitoring, meteorology, map making etc. (See especially Earth Observing System.)
Solar power satellites are proposed satellites built in high Earth orbit that use microwave power transmission to beam solar power to very large antennae on Earth where it can be used in place of conventional power sources.
Space stations are man-made structures that are designed for human beings to live on in outer space. A space station is distinguished from other manned spacecraft by its lack of major propulsion or landing facilities — instead, other vehicles are used as transport to and from the station. Space stations are designed for medium-term living in orbit, for periods of weeks, months, or even years.
Weather satellites are satellites that primarily are used to monitor Earth's weather and climate.
Orbit types
Main article: List of orbits
Centric Classifications
Galacto-centric Orbit - An orbit about the center of a galaxy. Earth's sun follows this type of orbit about the galactic center of the Milky Way.
Heliocentric Orbit - An orbit around the Sun. In our Solar System, all planets, comets, and asteroids are in such orbits, as are many artificial satellites and pieces of space debris. Moons by contrast are not in a heliocentric orbit but rather orbit their parent planet.
Geocentric Orbit - An orbit around the planet Earth, such as the Moon or artificial satellites. Currently there are approximately 2465 artificial satellites orbiting the Earth.
Areocentric Orbit - An orbit around the planet Mars, such as moons or artificial satellite
Types of Orbits
Different orbits serve different purposes. Each has its own advantages and disadvantages. There are several types of orbits:
1. Polar
2. Sun Synchronous
3. Geosynchronous
Sun Synchronous Orbits
These orbits allows a satellite to pass over a section of the Earth at the same time of day. Since there are 365 days in a year and 360 degrees in a circle, it means that the satellite has to shift its orbit by approximately one degree per day. These satellites orbit at an altitude between 700 to 800 km. These satellites use the fact since the Earth is not perfectly round (the Earth bulges in the center, the bulge near the equator will cause additional gravitational forces to act on the satellite. This causes the satellite's orbit to either proceed or recede. These orbits are used for satellites that need a constant amount of sunlight. Satellites that take pictures of the Earth would work best with bright sunlight, while satellites that measure longwave radiation would work best in complete darkness.
Polar Orbits
The more correct term would be near polar orbits. These orbits have an inclination near 90 degrees. This allows the satellite to see virtually every part of the Earth as the Earth rotates underneath it. It takes approximately 90 minutes for the satellite to complete one orbit. These satellites have many uses such as measuring ozone concentrations in the stratosphere or measuring temperatures in the atmosphere.
Geosynchronous Orbits
Also known as geostationary orbits,
satellites in these orbits circle the Earth at the same rate as the Earth spins. The Earth actually takes 23 hours, 56 minutes, and 4.09 seconds to make one full revolution. So based on Kepler's Laws of Planetary Motion, this would put the satellite at approximately 35,790 km above the Earth. The+ satellites are located near the equator since at this latitude, there is a constant force of gravity from all directions. At other latitudes, the bulge at the center of the Earth would pull on the satellite.
Geosynchronous orbits allow the satellite to observe almost a full
hemisphere of the Earth.
These satellites are used to study large scale phenomenon such as
hurricanes, or cyclones.
These orbits are also used for communication satellites.
The disadvantage of this type of orbit is that since these satellites are
very far away, they have poor resolution.
The other disadvantage is that these satellites have trouble
monitoring activities near the poles.
Altitude Classifications
Low Earth Orbit (LEO) –
Geocentric orbits ranging in altitude from 0 - 2,000 km
Medium Earth Orbit (MEO) -
Geocentric orbits ranging in altitude from 2,000 km
-to just below geosynchronous orbit at 35,786 km
(22,240 miles).
-Also known as an intermediate circular orbit.
High Earth Orbit (HEO) –
Geocentric orbits above the altitude of geosynchronous
orbit 35,786 km (22,240 miles).
Eccentricity Classifications
Circular Orbit - An orbit that has an eccentricity of 0 and whose path traces a circle.
Hohmann transfer orbit - An orbital maneuver that moves a spacecraft from one circular orbit to another using two engine impulses. This maneuver was named after Walter Hohmann.
Elliptic Orbit - An orbit with an eccentricity greater than 0 and less than 1 whose orbit traces the path of an ellipse.
Geosynchronous Transfer Orbit - An elliptic orbit where the perigee is at the altitude of a Low Earth Orbit (LEO) and the apogee at the altitude of a geosynchronous orbit.
Geostationary Transfer Orbit - An elliptic orbit where the perigee is at the altitude of a Low Earth Orbit (LEO) and the apogee at the altitude of a geostationary orbit.
Molniya Orbit - A highly elliptic orbit with inclination of 63.4° and orbital period of ½ of a sidereal day (roughly 12 hours). Such a satellite spends most of its time over a designated area of the planet.
Tundra Orbit - A highly elliptic orbit with inclination of 63.4° and orbital period of one sidereal day (roughly 24 hours). Such a satellite spends most of its time over a designated area of the planet.
Hyperbolic orbit - An orbit with the eccentricity greater than 1. Such an orbit also has a velocity in excess of the escape velocity and as such, will escape the gravitational pull of the planet and continue to travel infinitely.
Parabolic Orbit - An orbit with the eccentricity equal to 1. Such an orbit also has a velocity equal to the escape velocity and therefore will escape the gravitational pull of the planet and travel until its velocity relative to the planet is 0. If the speed of such an orbit is increased it will become a hyperbolic orbit.
Escape Orbit (EO) - A high-speed parabolic orbit where the object has escape velocity and is moving away from the planet.
Capture Orbit - A high-speed parabolic orbit where the object has escape velocity and is moving toward the planet.
Synchronous Classifications
Synchronous Orbit - An orbit where the satellite has an orbital period equal to the average rotational period (earth's is: 23 hours, 56 minutes, 4.091 seconds) of the body being orbited and in the same direction of rotation as that body. To a ground observer such a satellite would trace an analemma (figure 8) in the sky.
Semi-Synchronous Orbit (SSO) - An orbit with an altitude of approximately 20,200 km (12544.2 miles) and an orbital period of approximately 12 hours
Geosynchronous Orbit (GEO) - Orbits with an altitude of approximately 35,786 km (22,240 miles). Such a satellite would trace an analemma (figure 8) in the sky.
Geostationary orbit (GSO): A geosynchronous orbit with an inclination of zero. To an observer on the ground this satellite would appear as a fixed point in the sky.
Clarke Orbit - Another name for a geostationary orbit. Named after the writer Arthur C. Clarke.
Supersynchronous orbit - A disposal / storage orbit above GSO/GEO. Satellites will drift west. Also a synonym for Disposal Orbit.
Subsynchronous orbit - A drift orbit close to but below GSO/GEO. Satellites will drift east.
Graveyard Orbit - An orbit a few hundred kilometers above geosynchronous that satellites are moved into at the end of their operation.
Disposal Orbit - A synonym for graveyard orbit.
Junk Orbit - A synonym for graveyard orbit.
Areosynchronous Orbit - A synchronous orbit around the planet Mars with an orbital period equal in length to Mars' sidereal day, 24.6229 hours.
Areostationary Orbit (ASO) - A circular areosynchronous orbit on the equatorial plane and about 17,000 km(10557 miles) above the surface. To an observer on the ground this satellite would appear as a fixed point in the sky.
Heliosynchronous Orbit - An heliocentric orbit about the Sun where the satellite's orbital period matches the Sun's period of rotation. These orbits occur at a radius of 24.360 Gm (0.1628 AU) around the Sun, a little less than half of the orbital radius of Mercury.
Special Classifications
Sun-synchronous Orbit - An orbit which combines altitude and inclination in such a way that the satellite passes over any given point of the planets's surface at the same local solar time. Such an orbit can place a satellite in constant sunlight and is useful for imaging, spy, and weather satellites.
Moon Orbit - The orbital characteristics of earth's moon. Average altitude of 384,403 kilometres (238,857 mi), elliptical-inclined orbit.
Pseudo-Orbit Classifications
Horseshoe Orbit - An orbit that appears to a ground observer to be orbiting a certain planet but is actually in co-orbit with the planet. See asteroids 3753 (Cruithne) and 2002 AA29.
Exo-orbit - A maneuver where a spacecraft approaches the height of orbit but lacks the velocity to sustain it.
Orbital Spaceflight - A synonym for Exo-orbit.
Lunar transfer orbit (LTO) -
Prograde Orbit - An orbit with an inclination of less than 90°. Or rather, an orbit that is in the same direction as the rotation of the primary.
Retrograde orbit - An orbit with an inclination of more than 90°. Or rather, an orbit counter to the direction of rotation of the planet. Almost no satellites are launched into retrograde orbit because the quantity of fuel required to launch them is much greater than for a prograde orbit. This is because when the rocket starts out on the ground, it already has an eastward component of velocity equal to the rotational velocity of the planet at its launch latitude.
What is Inside a Typical Satellite?
Satellites come in all shapes and sizes and play a variety of roles. For example:
Weather satellites help meteorologists predict the weather or see what's happening at the moment. Typical weather satellites include the TIROS, COSMOS and GOES satellites. The satellites generally contain cameras that can return photos of Earth's weather, either from fixed geostationary positions or from polar orbits.
Communications satellites allow telephone and data conversations to be relayed through the satellite. Typical communications satellites include Telstar and Intelsat. The most important feature of a communications satellite is the transponder -- a radio that receives a conversation at one frequency and then amplifies it and retransmits it back to Earth on another frequency. A satellite normally contains hundreds or thousands of transponders. Communications satellites are usually geosynchronous.
Broadcast satellites broadcast television signals from one point to another (similar to communications satellites).
Scientific satellites perform a variety of scientific missions. The Hubble Space Telescope is the most famous scientific satellite, but there are many others looking at everything from sun spots to gamma rays.
Navigational satellites help ships and planes navigate. The most famous are the GPS NAVSTAR satellites.
Rescue satellites respond to radio distress signals (read this page for details).
Earth observation satellites observe the planet for changes in everything from temperature to forestation to ice-sheet coverage. The most famous are the LANDSAT series.
Military satellites are up there, but much of the actual application information remains secret. Intelligence-gathering possibilities using high-tech electronic and sophisticated photographic-equipment reconnaissance are endless. Applications may include:
Relaying encrypted communications
Nuclear monitoring
Observing enemy movements
Early warning of missile launches
Eavesdropping on terrestrial radio links
Radar imaging
Photography (using what are essentially large telescopes that take pictures of militarily interesting areas)
Despite the significant differences between all of these satellites, they have several things in common. For example:
All of them have a metal or composite frame and body, usually known as the bus. The bus holds everything together in space and provides enough strength to survive the launch.
All of them have a source of power (usually solar cells) and batteries for storage.
Arrays of solar cells provide power to charge rechargeable batteries. Newer designs include the use of fuel cells. Power on most satellites is precious and very limited. Nuclear power has been used on space probes to other planets . Power systems are constantly monitored, and data on power and all other onboard systems is sent to Earth stations in the form of telemetry signals.
All of them have an onboard computer to control and monitor the different systems.
All of them have a radio system and antenna. At the very least, most satellites have a radio transmitter/receiver so that the ground-control crew can request status information from the satellite and monitor its health. Many satellites can be controlled in various ways from the ground to do anything from change the orbit to reprogram the computer system.
All of them have an attitude control system. The ACS keeps the satellite pointed in the right direction.
The Hubble Space Telescope has a very elaborate control system so that the telescope can point at the same position in space for hours or days at a time (despite the fact that the telescope travels at 17,000 mph/27,359 kph!). The system contains gyroscopes, accelerometers, a reaction wheel stabilization system, thrusters and a set of sensors that watch guide stars to determine position.
Launching Satellites
A satellite is often launched from the inside of a rocket. The satellite may be folded up in the rocket’s nosecone. The engines fire, and the rocket blasts off. Then, the first stage of the rocket falls away from the rest of the rocket. The first stage includes the main rockets. Next, the satellite is launched. Its thrusters blast it into orbit. The satellite parts, including solar energy panels and antennas, unfold. Now the satellite is in its orbit and is tested and starts its working time in space.
How is a Satellite Launched into an Orbit?
All satellites today get into orbit by riding on a rocket or by riding in the cargo bay of the Space Shuttle. Several countries and businesses have rocket launch capabilities, and satellites as large as several tons make it safely into orbit on a regular basis.
For most satellite launches, the scheduled launch rocket is aimed straight up at first. This gets the rocket through the thickest part of the atmosphere most quickly and best minimizes fuel consumption.
After a rocket launches straight up, the rocket control mechanism uses the inertial guidance system to calculate necessary adjustments to the rocket's nozzles to tilt the rocket to the course described in the flight plan. In most cases, the flight plan calls for the rocket to head east because Earth rotates to the east, giving the launch vehicle a free boost. The strength of this boost depends on the rotational velocity of Earth at the launch location. The boost is greatest at the equator (welye †iLv), where the distance around Earth is greatest and so rotation is fastest.
How big is the boost from an equatorial launch? To make a rough estimate, we can determine Earth's circumference by multiplying its diameter by pi (3.1416). The diameter of Earth is approximately 7,926 miles (12,753 km). Multiplying by pi yields a circumference of something like 24,900 miles (40,065 km). To travel around that circumference in 24 hours, a point on Earth's surface has to move at 1,038 mph (1,669 kph). A launch from Cape Canaveral, Florida, doesn't get as big a boost from Earth's rotational speed. The Kennedy Space Center's Launch Complex 39-A, one of its launch facilities, is located at 28 degrees 36 minutes 29.7014 seconds north latitude. The Earth's rotational speed there is about 894 mph (1,440 kph). The difference in Earth's surface speed between the equator and Kennedy Space Center, then, is about 144 mph (229 kph). (Note: The Earth is actually oblate -- fatter around the middle -- not a perfect sphere. For that reason, our estimate of Earth's circumference is a little small.)
Considering that rockets can go thousands of miles per hour, you may wonder why a difference of only 144 mph would even matter. The answer is that rockets, together with their fuel and their payloads, are very heavy. For example, the February 11, 2000 lift-off of the Space Shuttle Endeavor with the Shuttle Radar Topography Mission required launching a total weight of 4,520,415 pounds (2,050,447 kg). It takes a huge amount of energy to accelerate such a mass to 144 mph, and therefore a significant amount of fuel. Launching from the equator makes a real difference.
Once the rocket reaches extremely thin air, at about 120 miles (193 km) up, the rocket's navigational system fires small rockets, just enough to turn the launch vehicle into a horizontal position. The satellite is then released. At that point, rockets are fired again to ensure some separation between the launch vehicle and the satellite itself.
Inertial Guidance Systems
A rocket must be controlled very precisely (wbf©~j fv‡e )to insert a satellite into the desired orbit. An inertial guidance system (IGS) inside the rocket makes this control possible. The IGS determines a rocket's exact location and orientation by precisely measuring all of the accelerations the rocket experiences, using gyroscopes and accelerometers. Mounted in gimbals, the gyroscopes' axes stay pointing in the same direction. This gyroscopically-stable platform contains accelerometers that measure changes in acceleration on three different axes. If it knows exactly where the rocket was at launch and it knows the accelerations the rocket experiences during flight, the IGS can calculate the rocket's position and orientation in space.
In the context of spaceflight, a satellite is any object which has been placed into orbit by human endeavor. They are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon.
A satellite is an object that orbits around something else. For example, the moon orbits around the earth, and it is a satellite. However, when most people think about satellites, they usually think of the man-made machines that send radio waves down to Earth and are used for communication.
The first artificial (man-made satellite) was a Soviet satellite named Sputnik.
Satellite Types
MILSTAR:A communication satellit
Anti-Satellite weapons, sometimes called "Killer satellites" are satellites designed to destroy "enemy" satellites, other orbital weapons and targets. Some are armed with kinetic rounds, while others use energy and/or particle weapons to destroy satellites, ICBMs, MIRVs. Both the U.S. and the USSR had these satellites. Links discussing "Killer satellites", ASATS (Anti-Satellite satellite) include USSR Tests ASAT weapon and ASAT Test. See also IMINT
Astronomical satellites are satellites used for observation of distant planets, galaxies, and other outer space objects.
Biosatellites are satellites designed to carry living organisms, generally for scientific experimentation.
Communications satellites are satellites stationed in space for the purpose of telecommunications. Modern communications satellites typically use geosynchronous orbits, Molniya orbits or Low Earth orbits.
Miniaturized satellites are satellites of unusually low weights and small sizes. New classifications are used to categorize these satellites: minisatellite (500–200 kg), microsatellite (below 200 kg), nanosatellite (below 10 kg).
Navigational satellites are satellites which use radio time signals transmitted to enable mobile receivers on the ground to determine their exact location. The relatively clear line of sight between the satellites and receivers on the ground, combined with ever-improving electronics, allows satellite navigation systems to measure location to accuracies on the order of a few metres in real time.
Reconnaissance satellites are Earth observation satellite or communications satellite deployed for military or intelligence applications. Little is known about the full power of these satellites, as governments who operate them usually keep information pertaining to their reconnaissance satellites classified.
Earth observation satellites are satellites intended for non-military uses such as environmental monitoring, meteorology, map making etc. (See especially Earth Observing System.)
Solar power satellites are proposed satellites built in high Earth orbit that use microwave power transmission to beam solar power to very large antennae on Earth where it can be used in place of conventional power sources.
Space stations are man-made structures that are designed for human beings to live on in outer space. A space station is distinguished from other manned spacecraft by its lack of major propulsion or landing facilities — instead, other vehicles are used as transport to and from the station. Space stations are designed for medium-term living in orbit, for periods of weeks, months, or even years.
Weather satellites are satellites that primarily are used to monitor Earth's weather and climate.
Orbit types
Main article: List of orbits
Centric Classifications
Galacto-centric Orbit - An orbit about the center of a galaxy. Earth's sun follows this type of orbit about the galactic center of the Milky Way.
Heliocentric Orbit - An orbit around the Sun. In our Solar System, all planets, comets, and asteroids are in such orbits, as are many artificial satellites and pieces of space debris. Moons by contrast are not in a heliocentric orbit but rather orbit their parent planet.
Geocentric Orbit - An orbit around the planet Earth, such as the Moon or artificial satellites. Currently there are approximately 2465 artificial satellites orbiting the Earth.
Areocentric Orbit - An orbit around the planet Mars, such as moons or artificial satellite
Types of Orbits
Different orbits serve different purposes. Each has its own advantages and disadvantages. There are several types of orbits:
1. Polar
2. Sun Synchronous
3. Geosynchronous
Sun Synchronous Orbits
These orbits allows a satellite to pass over a section of the Earth at the same time of day. Since there are 365 days in a year and 360 degrees in a circle, it means that the satellite has to shift its orbit by approximately one degree per day. These satellites orbit at an altitude between 700 to 800 km. These satellites use the fact since the Earth is not perfectly round (the Earth bulges in the center, the bulge near the equator will cause additional gravitational forces to act on the satellite. This causes the satellite's orbit to either proceed or recede. These orbits are used for satellites that need a constant amount of sunlight. Satellites that take pictures of the Earth would work best with bright sunlight, while satellites that measure longwave radiation would work best in complete darkness.
Polar Orbits
The more correct term would be near polar orbits. These orbits have an inclination near 90 degrees. This allows the satellite to see virtually every part of the Earth as the Earth rotates underneath it. It takes approximately 90 minutes for the satellite to complete one orbit. These satellites have many uses such as measuring ozone concentrations in the stratosphere or measuring temperatures in the atmosphere.
Geosynchronous Orbits
Also known as geostationary orbits,
satellites in these orbits circle the Earth at the same rate as the Earth spins. The Earth actually takes 23 hours, 56 minutes, and 4.09 seconds to make one full revolution. So based on Kepler's Laws of Planetary Motion, this would put the satellite at approximately 35,790 km above the Earth. The+ satellites are located near the equator since at this latitude, there is a constant force of gravity from all directions. At other latitudes, the bulge at the center of the Earth would pull on the satellite.
Geosynchronous orbits allow the satellite to observe almost a full
hemisphere of the Earth.
These satellites are used to study large scale phenomenon such as
hurricanes, or cyclones.
These orbits are also used for communication satellites.
The disadvantage of this type of orbit is that since these satellites are
very far away, they have poor resolution.
The other disadvantage is that these satellites have trouble
monitoring activities near the poles.
Altitude Classifications
Low Earth Orbit (LEO) –
Geocentric orbits ranging in altitude from 0 - 2,000 km
Medium Earth Orbit (MEO) -
Geocentric orbits ranging in altitude from 2,000 km
-to just below geosynchronous orbit at 35,786 km
(22,240 miles).
-Also known as an intermediate circular orbit.
High Earth Orbit (HEO) –
Geocentric orbits above the altitude of geosynchronous
orbit 35,786 km (22,240 miles).
Eccentricity Classifications
Circular Orbit - An orbit that has an eccentricity of 0 and whose path traces a circle.
Hohmann transfer orbit - An orbital maneuver that moves a spacecraft from one circular orbit to another using two engine impulses. This maneuver was named after Walter Hohmann.
Elliptic Orbit - An orbit with an eccentricity greater than 0 and less than 1 whose orbit traces the path of an ellipse.
Geosynchronous Transfer Orbit - An elliptic orbit where the perigee is at the altitude of a Low Earth Orbit (LEO) and the apogee at the altitude of a geosynchronous orbit.
Geostationary Transfer Orbit - An elliptic orbit where the perigee is at the altitude of a Low Earth Orbit (LEO) and the apogee at the altitude of a geostationary orbit.
Molniya Orbit - A highly elliptic orbit with inclination of 63.4° and orbital period of ½ of a sidereal day (roughly 12 hours). Such a satellite spends most of its time over a designated area of the planet.
Tundra Orbit - A highly elliptic orbit with inclination of 63.4° and orbital period of one sidereal day (roughly 24 hours). Such a satellite spends most of its time over a designated area of the planet.
Hyperbolic orbit - An orbit with the eccentricity greater than 1. Such an orbit also has a velocity in excess of the escape velocity and as such, will escape the gravitational pull of the planet and continue to travel infinitely.
Parabolic Orbit - An orbit with the eccentricity equal to 1. Such an orbit also has a velocity equal to the escape velocity and therefore will escape the gravitational pull of the planet and travel until its velocity relative to the planet is 0. If the speed of such an orbit is increased it will become a hyperbolic orbit.
Escape Orbit (EO) - A high-speed parabolic orbit where the object has escape velocity and is moving away from the planet.
Capture Orbit - A high-speed parabolic orbit where the object has escape velocity and is moving toward the planet.
Synchronous Classifications
Synchronous Orbit - An orbit where the satellite has an orbital period equal to the average rotational period (earth's is: 23 hours, 56 minutes, 4.091 seconds) of the body being orbited and in the same direction of rotation as that body. To a ground observer such a satellite would trace an analemma (figure 8) in the sky.
Semi-Synchronous Orbit (SSO) - An orbit with an altitude of approximately 20,200 km (12544.2 miles) and an orbital period of approximately 12 hours
Geosynchronous Orbit (GEO) - Orbits with an altitude of approximately 35,786 km (22,240 miles). Such a satellite would trace an analemma (figure 8) in the sky.
Geostationary orbit (GSO): A geosynchronous orbit with an inclination of zero. To an observer on the ground this satellite would appear as a fixed point in the sky.
Clarke Orbit - Another name for a geostationary orbit. Named after the writer Arthur C. Clarke.
Supersynchronous orbit - A disposal / storage orbit above GSO/GEO. Satellites will drift west. Also a synonym for Disposal Orbit.
Subsynchronous orbit - A drift orbit close to but below GSO/GEO. Satellites will drift east.
Graveyard Orbit - An orbit a few hundred kilometers above geosynchronous that satellites are moved into at the end of their operation.
Disposal Orbit - A synonym for graveyard orbit.
Junk Orbit - A synonym for graveyard orbit.
Areosynchronous Orbit - A synchronous orbit around the planet Mars with an orbital period equal in length to Mars' sidereal day, 24.6229 hours.
Areostationary Orbit (ASO) - A circular areosynchronous orbit on the equatorial plane and about 17,000 km(10557 miles) above the surface. To an observer on the ground this satellite would appear as a fixed point in the sky.
Heliosynchronous Orbit - An heliocentric orbit about the Sun where the satellite's orbital period matches the Sun's period of rotation. These orbits occur at a radius of 24.360 Gm (0.1628 AU) around the Sun, a little less than half of the orbital radius of Mercury.
Special Classifications
Sun-synchronous Orbit - An orbit which combines altitude and inclination in such a way that the satellite passes over any given point of the planets's surface at the same local solar time. Such an orbit can place a satellite in constant sunlight and is useful for imaging, spy, and weather satellites.
Moon Orbit - The orbital characteristics of earth's moon. Average altitude of 384,403 kilometres (238,857 mi), elliptical-inclined orbit.
Pseudo-Orbit Classifications
Horseshoe Orbit - An orbit that appears to a ground observer to be orbiting a certain planet but is actually in co-orbit with the planet. See asteroids 3753 (Cruithne) and 2002 AA29.
Exo-orbit - A maneuver where a spacecraft approaches the height of orbit but lacks the velocity to sustain it.
Orbital Spaceflight - A synonym for Exo-orbit.
Lunar transfer orbit (LTO) -
Prograde Orbit - An orbit with an inclination of less than 90°. Or rather, an orbit that is in the same direction as the rotation of the primary.
Retrograde orbit - An orbit with an inclination of more than 90°. Or rather, an orbit counter to the direction of rotation of the planet. Almost no satellites are launched into retrograde orbit because the quantity of fuel required to launch them is much greater than for a prograde orbit. This is because when the rocket starts out on the ground, it already has an eastward component of velocity equal to the rotational velocity of the planet at its launch latitude.
What is Inside a Typical Satellite?
Satellites come in all shapes and sizes and play a variety of roles. For example:
Weather satellites help meteorologists predict the weather or see what's happening at the moment. Typical weather satellites include the TIROS, COSMOS and GOES satellites. The satellites generally contain cameras that can return photos of Earth's weather, either from fixed geostationary positions or from polar orbits.
Communications satellites allow telephone and data conversations to be relayed through the satellite. Typical communications satellites include Telstar and Intelsat. The most important feature of a communications satellite is the transponder -- a radio that receives a conversation at one frequency and then amplifies it and retransmits it back to Earth on another frequency. A satellite normally contains hundreds or thousands of transponders. Communications satellites are usually geosynchronous.
Broadcast satellites broadcast television signals from one point to another (similar to communications satellites).
Scientific satellites perform a variety of scientific missions. The Hubble Space Telescope is the most famous scientific satellite, but there are many others looking at everything from sun spots to gamma rays.
Navigational satellites help ships and planes navigate. The most famous are the GPS NAVSTAR satellites.
Rescue satellites respond to radio distress signals (read this page for details).
Earth observation satellites observe the planet for changes in everything from temperature to forestation to ice-sheet coverage. The most famous are the LANDSAT series.
Military satellites are up there, but much of the actual application information remains secret. Intelligence-gathering possibilities using high-tech electronic and sophisticated photographic-equipment reconnaissance are endless. Applications may include:
Relaying encrypted communications
Nuclear monitoring
Observing enemy movements
Early warning of missile launches
Eavesdropping on terrestrial radio links
Radar imaging
Photography (using what are essentially large telescopes that take pictures of militarily interesting areas)
Despite the significant differences between all of these satellites, they have several things in common. For example:
All of them have a metal or composite frame and body, usually known as the bus. The bus holds everything together in space and provides enough strength to survive the launch.
All of them have a source of power (usually solar cells) and batteries for storage.
Arrays of solar cells provide power to charge rechargeable batteries. Newer designs include the use of fuel cells. Power on most satellites is precious and very limited. Nuclear power has been used on space probes to other planets . Power systems are constantly monitored, and data on power and all other onboard systems is sent to Earth stations in the form of telemetry signals.
All of them have an onboard computer to control and monitor the different systems.
All of them have a radio system and antenna. At the very least, most satellites have a radio transmitter/receiver so that the ground-control crew can request status information from the satellite and monitor its health. Many satellites can be controlled in various ways from the ground to do anything from change the orbit to reprogram the computer system.
All of them have an attitude control system. The ACS keeps the satellite pointed in the right direction.
The Hubble Space Telescope has a very elaborate control system so that the telescope can point at the same position in space for hours or days at a time (despite the fact that the telescope travels at 17,000 mph/27,359 kph!). The system contains gyroscopes, accelerometers, a reaction wheel stabilization system, thrusters and a set of sensors that watch guide stars to determine position.
Launching Satellites
A satellite is often launched from the inside of a rocket. The satellite may be folded up in the rocket’s nosecone. The engines fire, and the rocket blasts off. Then, the first stage of the rocket falls away from the rest of the rocket. The first stage includes the main rockets. Next, the satellite is launched. Its thrusters blast it into orbit. The satellite parts, including solar energy panels and antennas, unfold. Now the satellite is in its orbit and is tested and starts its working time in space.
How is a Satellite Launched into an Orbit?
All satellites today get into orbit by riding on a rocket or by riding in the cargo bay of the Space Shuttle. Several countries and businesses have rocket launch capabilities, and satellites as large as several tons make it safely into orbit on a regular basis.
For most satellite launches, the scheduled launch rocket is aimed straight up at first. This gets the rocket through the thickest part of the atmosphere most quickly and best minimizes fuel consumption.
After a rocket launches straight up, the rocket control mechanism uses the inertial guidance system to calculate necessary adjustments to the rocket's nozzles to tilt the rocket to the course described in the flight plan. In most cases, the flight plan calls for the rocket to head east because Earth rotates to the east, giving the launch vehicle a free boost. The strength of this boost depends on the rotational velocity of Earth at the launch location. The boost is greatest at the equator (welye †iLv), where the distance around Earth is greatest and so rotation is fastest.
How big is the boost from an equatorial launch? To make a rough estimate, we can determine Earth's circumference by multiplying its diameter by pi (3.1416). The diameter of Earth is approximately 7,926 miles (12,753 km). Multiplying by pi yields a circumference of something like 24,900 miles (40,065 km). To travel around that circumference in 24 hours, a point on Earth's surface has to move at 1,038 mph (1,669 kph). A launch from Cape Canaveral, Florida, doesn't get as big a boost from Earth's rotational speed. The Kennedy Space Center's Launch Complex 39-A, one of its launch facilities, is located at 28 degrees 36 minutes 29.7014 seconds north latitude. The Earth's rotational speed there is about 894 mph (1,440 kph). The difference in Earth's surface speed between the equator and Kennedy Space Center, then, is about 144 mph (229 kph). (Note: The Earth is actually oblate -- fatter around the middle -- not a perfect sphere. For that reason, our estimate of Earth's circumference is a little small.)
Considering that rockets can go thousands of miles per hour, you may wonder why a difference of only 144 mph would even matter. The answer is that rockets, together with their fuel and their payloads, are very heavy. For example, the February 11, 2000 lift-off of the Space Shuttle Endeavor with the Shuttle Radar Topography Mission required launching a total weight of 4,520,415 pounds (2,050,447 kg). It takes a huge amount of energy to accelerate such a mass to 144 mph, and therefore a significant amount of fuel. Launching from the equator makes a real difference.
Once the rocket reaches extremely thin air, at about 120 miles (193 km) up, the rocket's navigational system fires small rockets, just enough to turn the launch vehicle into a horizontal position. The satellite is then released. At that point, rockets are fired again to ensure some separation between the launch vehicle and the satellite itself.
Inertial Guidance Systems
A rocket must be controlled very precisely (wbf©~j fv‡e )to insert a satellite into the desired orbit. An inertial guidance system (IGS) inside the rocket makes this control possible. The IGS determines a rocket's exact location and orientation by precisely measuring all of the accelerations the rocket experiences, using gyroscopes and accelerometers. Mounted in gimbals, the gyroscopes' axes stay pointing in the same direction. This gyroscopically-stable platform contains accelerometers that measure changes in acceleration on three different axes. If it knows exactly where the rocket was at launch and it knows the accelerations the rocket experiences during flight, the IGS can calculate the rocket's position and orientation in space.
.
.
.
