What satellites do
"Right now, high above us there are many dozens of satellites orbiting the earth. What are they there for? What do they do?"
"Many satellites take pictures of our earth..."
"...some for weather forecasting..."
"...some for determining the health of crops..."
"...others to track pollution."
"Some satellites take pictures of space..."
"...and our sun."
"Most of NASA's satellites gather information about the world around us so that we can better understand that world."
How satellites send us their information
"Once a satellite gathers information from its vantage point in space, it must transmit that data back to earth. NASA's tracking stations receive this data and forward it to scientists for detailed study and analysis."
"Satellites transmit their information to Earth in much the same way a radio or television station transmits a TV show or music to the TV or radio in your home. The picture or music is converted to electrical signals and then sent through the air to your home where your TV or radio converts the electrical signal back into pictures or music."
"A television station desires to reach as many homes as possible. It sends its signal in all directions with equal strength. This is called omni-directional transmission."
"Much of this signal energy does not reach anyone's home and is therefore wasted."
"This is not a serious problem for the television station since it has access to as much energy as it needs. The strength of the electrical signal fed into the transmitting antenna is measured in watts. A typical television station transmits about fifty-thousand watts."
"For a satellite however, the supply of energy is very limited. A typical satellite's solar panels can produce about one-thousand watts to run all of its systems. The amount energy allocated for transmitting information to the ground might be as little as twelve watts."
Signal strength fading
"Although a television station transmits a lot of energy, its signal cannot reach everyone."
"As the signal travels away from the transmission tower, its energy is spread evenly over a larger and larger area. This is depicted here by the spheres increasing in diameter as they get farther from the source. The energy at any one point on the sphere gets smaller as the sphere gets larger since the energy that was originally transmitted is spread over the surface of increasingly larger spheres. The equation shows the mathematical relationship between the strength of the signal and the distance from the source. As you can see, the signal strength one-hundred miles from the source is only one ten-thousandth that of the signal one mile from the source. A low strength television signal shows up as static; the picture becomes more difficult to see clearly. Similarly, a low strength satellite signal makes it difficult to accurately re-create the information that was gathered."
"Because television stations want to reach as many homes as possible, an omni-directional antenna is appropriate."
"Often, satellites don't need to send the information they've gathered to many locations at the same time. In this case, a satellite can use a directional antenna. This type of antenna concentrates the transmitted energy into a smaller area."
"The panel antenna improves upon the omni-directional antenna by concentrating most of the energy into a hemisphere."
"The Yagi antenna does even better, concentrating most of the energy into a cone of about twenty degrees."
"With a parabolic antenna beam widths of less than one degree can be achieved for maximum concentration of energy."
"Concentrating energy in smaller beam widths achieves greater distances with the same electrical power."
"When a highly directional antenna is used, it must be accurately pointed at the intended recipient of the signal. In order to point its antenna, the satellite must know precisely where it is in space and where the signal target is. The satellite's on-board computer then performs complex mathematical calculations to generate pointing commands to drive the antenna. The locations of the satellite itself and its ground target are periodically sent to the satellite from computers on the ground."
"In the same way signal transmission energy can be focused to achieve greater distance, signal reception can be enhanced by the use of a parabolic antenna as well..."
"...This is exactly the same as when a reflecting telescope is used to magnify the light from a distant planet or star."
"In fact, visible light and satellite and television transmission signals are just different names for essentially the same thing: electromagnetic waves. Their only difference is their wavelength, or the distance between the peaks of their waves."
"A satellite can only send the information it has gathered to earth when it is in view of a ground station. A ground station's horizon, shown by the green circle, limits how much of the satellite's orbit it can view; the higher the orbit, the more the ground station can view. At altitudes of about 150 to 400 miles, a satellite takes about ninety minutes to make one orbit around the earth. Satellites in these altitudes are referred to as low-earth-orbiters, or LEO [pronounced "lee-oh"]. Many of NASA's satellites are LEOs: Hubble Space Telescope, and the International Space Station to name a few. If a LEO satellite passes directly over a ground station, it will be in view for about fifteen minutes."
"Here we can see that the satellite will only be in view of the ground station for two or three consecutive orbits. In reality, the plane of the satellite's orbit remains fixed while the earth rotates under it. Sometimes the satellite will pass close to the edge of the visibility circle and be in view for only two to three minutes. When the satellite flies over the ground station from south to north, we say it is ascending."
"Twelve hours after the satellite flies over the ground station in the ascending direction, the earth will have rotated half way around its axis. The ground station will now see the satellite two or three more times on the descending side of its orbit. With a single ground station, a satellite can send its information back to earth only forty to fifty minutes each day. When a satellite is not in view of a ground station, it must record the information it gathers. It plays back this information very fast whenever it is in view."
Tracking and Data Relay Satellite
"To overcome the limited view-time imposed by ground stations for low-earth-orbiters, NASA developed a satellite that tracks other satellites: the Tracking and Data Relay Satellite, or TDRS [pronounced "tee-driss"], shown here in the higher, violet colored orbit."
"TDRS orbits the earth at an altitude of twenty-two thousand miles, directly above the equator. At this altitude it orbits at the same rate as the earth's rotation.. This is called a geostationary orbit. The advantage of this orbit is that TDRS can always be in view of its ground station."
"From its perch at geostationary altitude, TDRS can see a LEO satellite for about half of its orbit."
"With two TDRS satellites, a LEO satellite can be tracked for nearly its entire orbit. The region in which the LEO satellite is not in view of either TDRS is called the zone of exclusion, or ZOE, shown here by the red shaded area."
"Since the first TDRS satellite was launched in 1982, this space-based tracking network has grown. First, with the addition of spare satellites in view of the ground station in Las Cruces, New Mexico, and later, with the addition of satellites over the ZOE and a ground station on the island of Guam. Now, communications can be maintained over the entire orbit of a LEO satellite."
"This is particularly important when the information to be sent to earth cannot be recorded for later playback. Voice communications and video from the International Space Station are now possible without interruption during critical mission phases."
"As a spacecraft travels farther away from the Earth, it becomes more challenging to communicate with it. NASA developed the Deep Space Network (DSN) to track and communicate with spacecraft that make interplanetary trips and those that go beyond our solar system."
"As a spacecraft travels farther away from the Earth, we need bigger antennas to send the commands and pick up the returning signals. The DSN dish antennas come in different sizes. The largest one has a diameter measuring 70 meters, almost the size of a football field."
"The three ground stations consisting of the DSN are located in Goldstone, California; Madrid, Spain; and Canberra, Australia. They are separated by equal distances in order to be able to receive signals from deep space within the 24 hour period."
"Let's give an example here of the communication techniques behind the Mars Exploration Rover mission. On its way to Mars, as referred to the cruise stage, communication was dependent on the two antennas onboard the spacecraft. The low-gain antenna was omni-directional and was used when the spacecraft was near the Earth. The medium-gain antenna was a directional antenna that had to be pointed toward the Earth for communications, but had more power to communicate when the spacecraft was farther away from the Earth."
"After landing and throughout the mission, the rovers use their UHF antennas to send data to Mars Odyssey, a satellite orbiting the red planet, which relays the data back to the DSN control center."
"As telecommunication technology evolves, it enables us to explore into frontiers that our ancestors could only dream about. Being able to communicate with objects so far away from home is a crucial part of achieving success in space exploration."
"Satellite communications - playing a vital role in NASA's missions."
"This is the Fly It! Module of the Space Communications training for the Space Operations Learning Center (SOLC). We have prepared a mission for you to accomplish using what you've learned from the Flight Training section. Are you ready?"
Your mission is to gather imagery data of a dangerous and fast moving hurricane approaching a major city. You'll be using a LEO satellite to gather the data and send it back to the appropriate ground stations on Earth. You need to do this within a certain amount of time in order to determine whether all of the citizens of this city will need to be evacuated."
You'll need to use the control buttons in the right sequence to operate and control the satellite, ground stations and satellite tracking views in order to achieve the mission objectives. The Mission Status display box and various elements of the display will present you with data, give you instructions, and show your progress. "
"Afterwards, you'll see how using TDRSS satellites would work in this mission too as an extra bonus."
"If you're ready to take on the mission, click Start!"
"In order to complete your mission, you'll first need to run a simulation to find out which ground stations are able to communicate with the satellite, and for how long, in the next 24 hours. The total amount of time the satellite can communicate to a ground station will be shown as "Contact Time". Note: Remember that this simulation is running many times faster than real time, so keep that in mind while you are watching."
Video shows simulation of LEO satellite contacting each of the ground stations and verifying their transmission rate, contact time and online status.
"In this phase, you'll see the satellite gathering the imagery data of the hurricane. The satellite will basically "capture" an image of the hurricane with it's equipment. During this phase, you'll see a red beam coming out of the LEO satellite and scanning the ground. This red beam is how the satellite captures the imagery data. The data is then stored, and can then be sent to ground stations all over the world for processing."
Video shows LEO satellite capturing hurricane imagery data.
"During this phase, you'll fly the real mission. You only have 24 hours to download the image captured by the satellite. Using the data from the simulation, you need to determine which combination of two ground stations can download the hurricane data to earth for analysis within 24 hours."
Video shows satellites downloading imagery data to two ground stations and the progress bar increases to 100% before the time limit is complete.
"Good job, you've selected the two correct ground stations and the hurricane imagery was downloaded and analyzed in time to evacuate all of the citizens! Now you'll see how using TDRSS satellites would have made the download even faster."
Video shows TDRSS satellites interacting with LEO satellites and ground stations on earth and download progress increases much faster than with just LEO satellites.
"You've completed the Fly It! Module for Space Communications. Good job! We hope you've learned a lot about satellites and how they gather important data and send that data back to ground stations back on earth. At the Space Operations Learning Center, we have more interesting subjects that you can learn about. Just visit us at the SOLC Home Page."