Download
1 / 79
800 likes | 998 Views
Satellite Communicatio n. Modified by Sunantha Sodsee. Satellites. The basic component of a communications satellite is a receiver-transmitter combination called a transponder.
E N D
Satellite Communication Modified by Sunantha Sodsee
Satellites • The basic component of a communications satellite is a receiver-transmitter combination called a transponder. • A satellite stays in orbit because the gravitational pull of the earth is balanced by the centripetal force of the revolving satellite. • Satellite orbits about the earth are either circular or elliptical.
Satellite Orbits Satellite orbits. (a) Circular orbit. (b) Elliptical orbit.
Satellite Orbits Angle of elevation.
Orbit Shapes • Only some of the satellites have circular orbits. • Others have elliptical orbits. These orbits have further classifiers: • Perigee: point on orbit when satellite is closest to earth. • Apogee: point on orbit when satellite is farthest from earth.
Putting a Satellite in Orbit • A rocket must accelerate to at least 25,039 mph 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 objective is not to escape Earth's gravity, but to balance it.
Different Roles for Satellites • Weather satellites help meteorologists predict the weather or see what's happening at the moment. • The satellites generally contain cameras that can return photos of Earth's weather. • Communications satellites allow telephone and data conversations to be relayed through the satellite. • 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.
Different Satellites (Cont’d) • 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, e.g., GPS.
Different Satellites (Cont’d) • Rescue satellites respond to radio distress signals. • Earth observation satellites observe the planet for changes in everything from temperature to forestation to ice-sheet coverage. • Military satellites are up there, but much of the actual application information remains secret.
Transponder • Some satellites have (hundreds of) transponders for communication purposes. • A transponder • receives transmissions from earth (uplink); • changes signal frequency; • amplifies the signal; and • transmits the signal to earth (downlink).
Satellite Dish • Ground stations feature large parabolic dish antennas with high gain and directivity for receiving the weak satellite signal. Satellite signals The larger the dish is the higher the received signal power.
LEO (Iridium) GEO (Inmarsat) Earth 1000 km MEO (ICO) 10,000 km HEO 35,768 km Not drawn to scale Orbits of Different Satellites
Satellite Costs • Satellite launches don't always go well; there is a great deal at stake. The cost of satellites and launches to name one. • For example, a recent hurricane-watch satellite mission cost $290 million. A missile-warning satellite cost $682 million. • A satellite launch can cost anywhere between $50 million and $400 million. Russian launches are generally the cheapest and the French launches are the most expensive. • A shuttle mission pushes toward half a billion dollars (a shuttle mission could easily carry several satellites into orbit).
How can I see an Overhead Satellite? • This satellite tracking Web site (http://www.heavens-above.com/) shows how you can see a satellite overhead, thanks to the German Space Operations Center. • You will then need your coordinates for longitude and latitude, available from the USGS Mapping Information Web site (http://geonames.usgs.gov/).
Locating an Overhead Satellite • Satellite-tracking software is available for predicting orbit passes. The above websites will help with this. Note the exact times for the satellites. • Use binoculars on a clear night when there is not a bright moon. • Ensure that your watch is set to exactly match a known time standard. • A north-south orbit often indicates a spy satellite!
Recall: What it is • GPS: Global Positioning System is a worldwide radio-navigation system formed from a constellation of 24 satellites and their ground stations. • Uses the principle of triangulation and time- of-arrival of signals to determine the location of a GPS receiver.
Typical GPS Applications • Location - determining a basic position • Navigation - getting from one location to another • Tracking - monitoring the movement of people and things. • Mapping - creating maps of the world • Timing - bringing precise timing to the world
Triangulation Requirements • To triangulate, a GPS receiver measures distance using the travel time of radio signals. • To measure travel time, GPS receiver needs very accurate timing. • Along with distance, receiver need accurate data on where satellites are in space. • System will also need to correct for any delays the signal experiences as it travels through atmosphere.
Components of GPS System • Control Segment: five ground stations located on earth. • Space Segment: satellite constellation (24 active satellites in space). • User Segment: GPS receiver units that receive satellite signals and determine receiver location from them.
Ground Monitor Stations Falcon AFB Colorado Springs, CO Master Control Monitor Station Kwajalein Monitor Station Hawaii Monitor Station Diego Garcia Monitor Station Ascension Island Monitor Station
Important Terminology • Satellite transmits Ephemeris and Almanac Data to GPS receivers. • Ephemeris data contains important information about status of satellite (healthy or unhealthy), current date and time. This part of signal is essential for determining a position. • Almanac data tells GPS receiver where each GPS satellite should be at any time throughout day. Each satellite transmits almanac data showing orbital information for that satellite and for every other satellite in the system.
TOA Concept • GPS uses concept of time of arrival (TOA) of signals to determine user position. • This involves measuring time it takes for a signal transmitted by an emitter (satellite) at a known location to reach a user receiver. • Time interval is basically signal propagation time.
TOA Concept (Cont’d) • Time interval (signal propagation time) is multiplied by speed of signal (speed of light) to obtain satellite to receiver distance. • By measuring propagation time of signals broadcast from multiple satellites at known locations, receiver can determine its position.
Measuring Distance using a PRC Signal • At a particular time (let's say midnight), the satellite begins transmitting a long, digital pattern called a pseudo-random code (PRC). • The receiver begins running the same digital pattern also exactly at midnight. • When the satellite's signal reaches the receiver, its transmission of the pattern will lag a bit behind the receiver's playing of the pattern.
Measuring Distance • The length of the delay is equal to the signal's travel time. • The receiver multiplies this time by the speed of light to determine how far the signal traveled. • Assuming the signal traveled in a straight line, this is the distance from receiver to satellite.
Differential GPS • Technique called differential correction can yield accuracies within 1-5 meters, or even better, with advanced equipment. • Differential correction requires a second GPS receiver, a base station, collecting data at a stationary position on a precisely known point. • Because physical location of base station is known, a correction factor can be computed by comparing known location with GPS location determined by using satellites.
Using GPS Data • A GPS receiver essentially determines the receiver's position on Earth. • Once the receiver makes this calculation, it can tell you the latitude, longitude and altitude of its current position. To make the navigation more user- friendly, most receivers plug this raw data into map files stored in memory.
Using GPS Data (Cont’d) • You can • use maps stored in the receiver's memory, • connect the receiver to a computer that can hold more detailed maps in its memory, or • simply buy a detailed map of your area and find your way using the receiver's latitude and longitude readouts. • Some receivers let you download detailed maps into memory or supply detailed maps with plug-in map cartridges.
Using GPS Data (Cont’d) • A standard GPS receiver will not only place you on a map at any particular location, but will also trace your path across a map as you move. • If you leave your receiver on, it can stay in constant communication with GPS satellites to see how your location is changing. • This is what happens in cars equipped with GPS.
Using GPS Data With this information and its built-in clock, the receiver can give you several pieces of valuable information: • How far you've traveled (odometer) • How long you've been traveling • Your current speed (speedometer) • Your average speed • A "bread crumb" trail showing you exactly where you have traveled on the map • The estimated time of arrival at your destination if you maintain your current speed
WiFi • WiFi is the wireless way to handle networking. • It is also known as 802.11 networking. • The big advantage of WiFi is its simplicity. • You can connect computers anywhere in your home or office without the need for wires. The computers connect to the network using radio signals, and computers can be up to 100 feet or so apart.
Wireless Networking Standards • WiFi refers to the protocols that allow wireless networking. • These protocols are codified in standards. • Standards are mutually agreed upon rules adopted by the industry on how the wireless networks operate. • There are several standards that enable wireless local area networks (WLANs).
Walkie-Talkie Network • If you want to understand wireless networking at its simplest level, think about a pair of walkie-talkies. • These are small radios that can transmit and receive radio signals. • Recall, when you talk into a Walkie-Talkie, your voice is picked up by a microphone, encoded onto a radio frequency and transmitted with the antenna.
Walkie-Talkie Network (Cont’d) • Another walkie-talkie can receive the transmission with its antenna, decode your voice from the radio signal and drive a speaker. • Simple walkie-talkies like this transmit at a signal strength of about 0.25 watts, and they can transmit about 500 to 1,000 feet.
Walkie-Talkie Network (Cont’d) • In order to do this, we require • Each computer is equipped with a walkie-talkie. • We would give each computer a way to set whether it wants to transmit or receive. • And we would give the computer a way to turn its binary 1s and 0s into two different beeps that the walkie-talkie could transmit and receive and convert back and forth between beeps and 1s/0s.
Walkie-Talkie Network (Cont’d) • This would actually work. • The only problem would be that the data rate would be very slow. A walkie-talkie is designed to handle the human voice (and it's a pretty scratchy rendition at that), so you would not be able to send very much data this way. Maybe 1,000 bits per second. • Another problem: the walkie-talkies could not be used to connect to the internet.
WiFi’s Radio Technology • The radios used in WiFi are not so different from the radios used in walkie-talkies. • They have the ability to transmit and receive. • They have the ability to convert 1s and 0s into radio waves and then back into 1s and 0s. • There are major differences, of course.
WiFi’s Radio Technology (Cont’d) • WiFi radios that work with the 802.11b and 802.11g standards transmit at 2.4 GHz, while those that comply with the 802.11a standard transmit at 5 GHz. • Normal walkie-talkies normally operate at 49 MHz. The higher frequency allows higher data rates. • WiFi radios use much more efficient coding techniques (process of converting 0’s and 1’s into efficient radio signals) that also contribute to the much higher data rates.
WiFi’s Radio Technology (Cont’d) • The radios used for WiFi have the ability to change frequencies. • For example, 802.11b cards can transmit directly on any of three bands, or they can split the available radio bandwidth into dozens of channels and frequency hop rapidly between them. • The advantage of frequency hopping is that it is much more immune to interference and can allow dozens of WiFi cards to talk simultaneously without interfering with each other.
WiFi Range • Regardless of which setup you use, once you turn your Wireless Access Point on, you will have a WiFi hotspot in your house. • In a typical home, this hotspot will provide coverage for about 100 feet (30.5 meters) in all directions, although walls and floors do cut down on the range. • Even so, you should get good coverage throughout a typical home. For a large home, you can buy inexpensive signal boosters to increase the range of the Hotspot.
Another Way to Amplify WiFi Signals A WiFi repeater is installed to extend coverage. Wireless Access Point
Infrastructure versus Ad Hoc • All the connections that we have talked about today require a connection from a device equipped with a wireless network interface card (NIC) to a wireless access point. • Generally, all such connections are operating in what is known as the infrastructure mode. Here the wireless network resembles a cellular architecture. • Wireless devices can also communicate directly with each other, i.e., it is not required that they communicate with an access point first.
Infrastructure versus Ad Hoc • When devices with NIC cards communicate directly with each other, the wireless network operates in ad hoc mode. • Essentially peer-to-peer communication is enabled.
Ad Hoc Mode • Ad Hoc connections can be used to share information directly between devices. This mode is also useful for establishing a network where wireless infrastructure does not exist. • Some uses, • Synchronize data between devices. • Retrieve multimedia files from one device and “play” them on another device. • Print from a computer to a printer without wires. • There are many applications of ad hoc networking in the military and in specialized networks.
How are Multiple Transmitters Supported? • Recall the method for supporting multiple transmitter is called the multiple access method. • In 802.11 systems, only one user is allowed to communicate with a receiver at a time (cannot use another frequency channel support a second or third additional user). • The way the one user is selected depends on the carrier sense multiple access with collision avoidance (CSMA/CA) random access method.
CSMA • To help illustrate the operation of CSMA, we will use an analogy of a dinner table conversation. • Let’s represent our wireless medium as a dinner table, and let several people engaged in polite conversation at the table represent the wireless nodes.
CSMA (Cont’d) • The term multiple access covers what we already discussed above: When one wireless device transmits, all other devices using the wireless medium hear the transmission, just as when one person at the table talks, everyone present is able to hear him or her. • Now let's imagine that you are at the table and you have something you would like to say. • At the moment, however, I am talking.
