Lecture

Week 35 Lesson Plan Addendum

Satellites



Satellites and Satellite Images at Space.com: Zoomable images from space of landmarks like New York City, Noah's Ark, and the Egyptian Pyramids



A satellite is defined as any object that orbits any other object. Satellites can be celestial, such as a moon orbiting a planet in the solar system, or a planet in the solar system orbiting the sun. Satellites can also be man-made. Man-made satellites are typically launched into outer space from earth to collect data, photos and other information about Earth and all the many things that exist around it.

A satellite is basically any object that revolves around a planet in a circular or elliptical path. The moon is Earth's original, natural satellite, and there are many manmade (artificial) satellites, usually closer to Earth.

  • The path a satellite follows is an orbit. In the orbit, the farthest point from Earth is the apogee, and the nearest point is the perigee.

  • Artificial satellites generally are not mass-produced. Most satellites are custom built to perform their intended functions. Exceptions include the GPS satellites (with over 20 copies in orbit) and the Iridium satellites (with over 60 copies in orbit).

  • Approximately 23,000 items of space junk -- objects large enough to track with radar that were inadvertently placed in orbit or have outlived their usefulness -- are floating above Earth. The actual number varies depending on which agency is counting. Payloads that go into the wrong orbit, satellites with run-down batteries, and leftover rocket boosters all contribute to the count. This online catalog of satellites has almost 26,000 entries!

Although anything that is in orbit around Earth is technically a satellite, the term "satellite" is typically used to describe a useful object placed in orbit purposely to perform some specific mission or task. We commonly hear about weather satellites, communication satellites and scientific satellites.

Go to this web site for more info


http://science.nasa.gov/realtime/


or this site for trajectory hands on of satellites


http://lectureonline.cl.msu.edi/~mmp/kap7/orbiter/orbit.htm

http://www.cnes.fr/web/1071-a-satellites-trajectory.php




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, where the distance around Earth is greatest and so rotation is fastest. To make a rough estimate of the equatorial boost, 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 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 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).

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 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.




Orbital Velocity and Altitude

A rocket must accelerate to at least 25,039 mph (40,320 kph) to completely escape Earth's gravity and fly off into space.

Earth's escape velocity is much greater than what's required to place an Earth satellite in orbit. With satellites, the object is not to escape Earth's gravity, but to balance it. Orbital velocity is the velocity needed to achieve balance between gravity's pull on the satellite and the inertia of the satellite's motion -- the satellite's tendency to keep going. This is approximately 17,000 mph (27,359 kph) at an altitude of 150 miles (242 km). Without gravity, the satellite's inertia would carry it off into space. Even with gravity, if the intended satellite goes too fast, it will eventually fly away. On the other hand, if the satellite goes too slowly, gravity will pull it back to Earth. At the correct orbital velocity, gravity exactly balances the satellite's inertia, pulling down toward Earth's center just enough to keep the path of the satellite curving like Earth's curved surface, rather than flying off in a straight line.

The orbital velocity of the satellite depends on its altitude above Earth. The nearer Earth, the faster the required orbital velocity has to be. At an altitude of 124 miles (200 kilometers), the required orbital velocity is just over 17,000 mph (about 27,400 kph). To maintain an orbit that is 22,223 miles (35,786 km) above Earth, the satellite must orbit at a speed of about 7,000 mph (11,300 kph). That orbital speed and distance permits the satellite to make one revolution in 24 hours. Since Earth also rotates once in 24 hours, a satellite at 22,223 miles altitude stays in a fixed position relative to a point on Earth's surface. Because the satellite stays right over the same spot all the time, this kind of orbit is called "geostationary." Geostationary orbits are ideal for weather satellites and communications satellites.

The moon has an altitude of about 240,000 miles (384,400 km), a velocity of about 2,300 mph (3,700 kph) and its orbit takes 27.322 days. (Note that the moon's orbital velocity is slower because it is farther from Earth than artificial satellites.)

In general, the higher the orbit, the longer the satellite can stay in orbit. At lower altitudes, a satellite runs into traces of Earth's atmosphere, which creates drag. The drag causes the orbit to decay until the satellite falls back into the atmosphere and burns up. At higher altitudes, where the vacuum of space is nearly complete, there is almost no drag and a satellite can stay in orbit for centuries (take the moon as an example).


Satellites usually start out in an orbit that is elliptical. The ground control station controls small onboard rocket motors to provide correction. The goal is to get the orbit as circular as possible. By firing a rocket when the orbit is at the apogee of its orbit (its most distant point from Earth), and applying thrust in the direction of the flight path, the perigee (lowest point from Earth) moves further out. The result is a more circular orbit.


What is Inside a Typical Satellite?
























Most satellites serve one or more functions:

  • Communications

  • Navigation

  • Weather Forecasting

  • Environmental Monitoring

  • Manned Platforms

Communications Satellites:


Communications satellites have a quiet, yet profound, effect on our daily lives. They link remote areas of the Earth with telephone and television. Modern financial business is conducted at high speed via satellite. Newspapers such as USA Today and The Wall Street Journal are typeset and then transmitted to printing plants around the country via satellite.

Radio signals near the microwave frequency range are best suited to carry large volumes of communications traffic, because they are not deflected by the Earth's atmosphere as lower frequencies are. Basically, they travel in a straight line, known as "line-of-sight communication." If someone in San Francisco tried to beam a microwave signal directly to Hawaii, it would never get there; it would disappear into space or dissipate into the ocean. Over short distances, we erect microwave towers every 25 miles or so to act as "repeaters" to repeat and boost the signal. Think of a geostationary communications satellite as a repeater in the sky.

Navigation Satellites

Satellites for navigation were developed in the late 1950s as a direct result of surface ships and submarines needing to know exactly where they were at any given time. In the middle of the ocean out of sight of land, one can't determine an accurate position by looking out the window.

The idea of using satellites for navigation began with the launch of Sputnik 1 on October 4, 1957. Monitoring that satellite, scientists at Johns Hopkins University's Applied Physics Laboratory noticed that when the transmitted radio frequency was plotted on a graph, a curve characteristic of the Doppler shift appeared. By studying this apparent change of radio frequency as the satellite passed overhead, they were able to show that the Doppler shift, when properly used, described the orbit of the satellite.

Most navigation systems use time and distance to calculate location. Early on, scientists recognized the principle that, given velocity and the time required for a radio signal to be transmitted between two points, the distance between the two points can be computed. To do this calculation, a precise, synchronized departure time and measured arrival time of the radio signal must be obtained. By synchronizing the signal transmission time between two precise clocks, one in a satellite and one at a ground-based receiver, the transit time could be measured and then multiplied by the speed of light to obtain the distance between the two positions.

This three-dimensional satellite navigational system (NAVSTAR) enables a traveler to obtain his or her position anywhere on or above the planet. Data transmitted from the satellite provides the user with time, precise orbital position of the satellite, and the position of other satellites in the system. Currently, there is a full constellation of 24 orbiting satellites devoted to navigation.

Using a commercial Global Positioning System (GPS) locator, the user can calculate distance by measuring the time it takes for the satellite's radio transmissions, traveling at the speed of light, to reach the receiver. Once distance from four satellites is known, position in three dimensions (latitude, longitude, and altitude) can be calculated by triangulation, and velocity in three dimensions can be computed from the Doppler shift in the received signal. The new GPS receivers do all of the work; a traveler simply turns on the unit, makes certain that it's locked onto at least four satellites, and the precise position of the GPS unit is displayed automatically. One innovative application of GPS technology is to determine Earth's ground movement after an earthquake. Referencing a network of these sensitive receivers can lead to a remarkably accurate assessment of plate movement.

There are two available radio signals that GPS receivers can use: the Standard Positioning Service (SPS) for civilians, and the Precise Positioning Service (PPS) for military and other authorized personnel. The most significant cause of errors in positioning is the deliberate effort by the Department of Defense to decrease the accuracy of user systems for national security reasons. Selective Availability (SA) refers to the purposeful degradation of the information broadcast by the satellites. SA affects the accuracy of the SPS, but not PPS. With SA, a GPS system will be accurate 95% of the time to within 328 feet (100 meters) horizontally and 512 feet (156 meters) vertically.

For those who require positions with higher accuracy, Differential Global Positioning Systems (DGPS) add a new element to GPS. DGPS places a GPS stationary receiver at a known location on or near the Earth's surface. This reference station receives satellite signals and adjusts for transmission delays and Selective Availability, using its own known latitude, longitude, and altitude. The stationary receiver sends out a correction message for any suitably-equipped local receiver. A DGPS-compatible receiver adjusts its position calculations using the correction message. DGPS reference stations are constructed, operated, and maintained by the United States Coast Guard.
 

Weather Satellites

Weather satellites have been our eyes in the sky for more than 30 years, since the April, 1960 launch of Tiros I. Today, satellite images showing the advance of weather fronts are regular elements of the evening news. This meteorological information is also available to anyone with a personal computer. A network of American, European, Japanese, and Russian satellites orbits the Earth in various configurations to provide "real-time" monitoring of our environment.

Polar Orbiting Satellites

TIROS polar orbiting satellites (NOAA-class), launched and operated by the United States, are the principal sources of environmental data for the 80% of the globe that is not covered by conventional monitoring equipment. These satellites measure temperature and humidity in the Earth's atmosphere, record surface ground and surface sea water temperatures, and monitor cloud cover and water/ice boundaries. They have the capability to receive, measure, process, and retransmit data from balloons, buoys, and remote automatic stations distributed around the globe. These satellites also carry Search and Rescue (SAR) transponders, which help locate downed airplanes or ships in distress. Polar orbiting satellites send back pictures to Earth via Automatic Picture Transmission (APT) or High Resolution Picture Transmission (HRPT) formats.

NOAA (National Oceanic and Atmospheric Administration) class satellites and Russian Meteor class satellites orbit very close to the poles on each revolution of the Earth. At an altitude of 860 km. (600 miles), the sensors scan the Earth's entire surface over a 24-hour period. The sensors are sensitive to visible light and infrared (IR) radiation. As each NOAA polar-orbiting satellite orbits the Earth, it sends back a constant stream of data.

Instruments on board the satellite scan the Earth's surface from side to side (perpendicular to the ground track), with each scan covering an area about 2 km. high and 3,000 km. wide. Typically, the lower resolution APT imagery is transmitted at 2 lines/second, or 120 lines/minute. In a pass lasting 12 minutes, this translates into an image approximately 5,800 km. long and 3,000 km. wide. As an example, the entire east coast of the United States would be visible in one image, from southern Florida north up to Hudson Bay, and from the Atlantic Ocean to west of the Great Lakes.

During the day, this data stream consists of one visible and one infrared image. At night, both channels are infrared. Imagery in both the visible and infrared formats is transmitted simultaneously. Students are familiar with the visible image because it is similar to one from a conventional camera. Understanding what the infrared imagery represents is sometimes harder to grasp. Various land and water bodies absorb heat differentially, so they reflect different levels of heat energy. The Gulf Stream offers an excellent example: on an infrared image, the warmer temperatures of the Gulf Stream are clearly delineated as the darker portions of the image, while the cooler temperatures of the surrounding Atlantic are lighter in color. With readily-available computer software, students can use a mouse to place a cursor anywhere on the image and accurately measure the surface water temperature to within 2 degrees Fahrenheit.

Currently, four NOAA-class satellites, which transmit both APT and HRPT imagery, are available for classroom use. NOAA 14 passes over Maine in the middle of the day. NOAA 12 is considered the primary early morning and early evening satellite. In addition to the United States' NOAA satellites, Russian Meteor class satellites transmit weather satellite imagery in the APT format as well.







Geostationary (GOES) Satellites

In late 1966, ATS-1 was launched into a geostationary orbit over the equator south of Hawaii. For the first time, meteorologists could monitor the weather continuously during daylight. It provided images of nearly one-third of the Earth's surface every 23 minutes with 4 km. resolution.

In May of 1974, the first of a new series of GOES satellites was launched. Both visible and infrared images were acquired simultaneously by the Visible and Infrared Spin Scan Radiometer (VISSR) on board the spacecraft. The visible channel offers ground resolution of 0.8 km. for sections of the full Earth view and 6.2 km. resolution in the infrared spectra. The greatest advantage to having both visible and infrared capability is that weather systems can be monitored both day and night (at 30-minute intervals). Thus, destructive hurricanes can be tracked around the clock. Most satellite images seen on our local evening news and the Weather Channel are produced by GOES satellites. Usually, the infrared images are "loop animated" to show the progression and movement of storms.

drawing of several satellites in varying orbits

While the United States maintains and operates its GOES satellites, the European community is served by its European Space Agency (ESA) Meteosat satellite, and Japan with its GMS satellite. This network provides complete global coverage of all but the extreme north and south polar regions.

GOES satellites make day and night observations of weather in the coverage area and transmit real-time VISSR data, monitor cataclysmic weather events such as hurricanes, relay meteorological observation data from surface collection points, and perform facsimile transmission of processed graphic and imaged weather data.

The primary function of our GOES satellites to the education community is to provide imagery of varying resolution and time frames. VISSR is the most stunning example, although it requires a much more sophisticated ground station to receive and process the signal. From Hawaii to Maine, land features can be examined to 0.8 km. resolution. The snow-capped Rocky Mountains stand out nicely, as do larger lakes and reservoirs.

Mission to Planet Earth

Four Landsat satellites (launched in 1972, 1975, 1978, and 1982) were specifically designed to learn about how different parts of the planet interact. Three are still sending back data. The newest generation of environmental satellites is part of a National Aeronautics and Space Administration (NASA) initiative that aims its space instruments at the Earth instead of the stars.

This program, Mission to Planet Earth, may well take precedence over space exploration for the next few years. Its Earth Observing System (EOS) will include 17 new satellites to be launched over the next 15 years. "The idea grew out of a critical mass of scientists coming together to understand how the Earth as a system is changing," explains Robert Price, director of the Mission to Planet Earth office for NASA. "If humankind is changing the face of the Earth, it's time we started answering some of the scientific questions relating to that." EOS focuses on the remote sensing of climate change indicators such as the ozone layer in the upper atmosphere, cloud cover, and sea-ice at the poles. In addition, it follows the climatological effects of localized phenomena like volcanic eruptions and El Niño, a periodic change in wind patterns and current movements that results in decreased fisheries along the southern Pacific coast. The information provided by EOS satellites will determine the course of environmental management in the future.

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.

  • 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.

There are three basic kinds of orbits, depending on the satellite's position relative to Earth's surface:

  • Geostationary orbits (also called geosynchronous or synchronous) are orbits in which the satellite is always positioned over the same spot on Earth. Many geostationary satellites are above a band along the equator, with an altitude of about 22,223 miles, or about a tenth of the distance to the Moon. The "satellite parking strip" area over the equator is becoming congested with several hundred television, weather and communication satellites! This congestion means each satellite must be precisely positioned to prevent its signals from interfering with an adjacent satellite's signals. Television, communications and weather satellites all use geostationary orbits. Geostationary orbits are why a DSS satellite TV dish is typically bolted in a fixed position.

  • The scheduled Space Shuttles use a much lower, asynchronous orbit, which means they pass overhead at different times of the day. Other satellites in asynchronous orbits average about 400 miles (644 km) in altitude.

  • In a polar orbit, the satellite generally flies at a low altitude and passes over the planet's poles on each revolution. The polar orbit remains fixed in space as Earth rotates inside the orbit. As a result, much of Earth passes under a satellite in a polar orbit. Because polar orbits achieve excellent coverage of the planet, they are often used for satellites that do mapping and photography.