1. © 2008 The McGraw-Hill Companies 1 Satellite Communicatio n Modified by Sunantha Sodsee

2. © 2008 The McGraw-Hill Companies 2Satellites  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.

3. © 2008 The McGraw-Hill Companies 3 Satellite Orbits Satellite orbits. (a) Circular orbit. (b) Elliptical orbit.

4. © 2008 The McGraw-Hill Companies 4 Satellite Orbits Angle of elevation.

5. © 2008 The McGraw-Hill Companies 5 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.

6. © 2008 The McGraw-Hill Companies 6 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.

7. © 2008 The McGraw-Hill Companies 7 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.

8. © 2008 The McGraw-Hill Companies 8 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.

9. © 2008 The McGraw-Hill Companies 9 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.

10. © 2008 The McGraw-Hill Companies 10Transponder  Some satellites have (hundreds of) transponders for communication purposes.  A transponder 1) receives transmissions from earth (uplink); 2) changes signal frequency; 3) amplifies the signal; and 4) transmits the signal to earth (downlink).

11. © 2008 The McGraw-Hill Companies 11 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.

12. © 2008 The McGraw-Hill Companies 12 Orbits of Different Satellites Earth 1000 km 35,768 km 10,000 km LEO (Iridium)GEO (Inmarsat) HEO MEO (ICO) Not drawn to scale

13. © 2008 The McGraw-Hill Companies 13 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).

14. © 2008 The McGraw-Hill Companies 14 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/).

15. © 2008 The McGraw-Hill Companies 15 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!

16. © 2008 The McGraw-Hill Companies 16 GPS

17. © 2008 The McGraw-Hill Companies 17 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.

18. © 2008 The McGraw-Hill Companies 18 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

19. © 2008 The McGraw-Hill Companies 19 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.

20. © 2008 The McGraw-Hill Companies 20 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.

21. © 2008 The McGraw-Hill Companies 21 Ground Monitor Stations Falcon AFB Colorado Springs, CO Master Control Monitor Station Hawaii Monitor Station Ascension Island Monitor Station Diego Garcia Monitor Station Kwajalein Monitor Station

22. © 2008 The McGraw-Hill Companies 22 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.

23. © 2008 The McGraw-Hill Companies 23 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.

24. © 2008 The McGraw-Hill Companies 24 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.

25. © 2008 The McGraw-Hill Companies 25 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.

26. © 2008 The McGraw-Hill Companies 26 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.

27. © 2008 The McGraw-Hill Companies 27 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.

28. © 2008 The McGraw-Hill Companies 28 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.

29. © 2008 The McGraw-Hill Companies 29 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.

30. © 2008 The McGraw-Hill Companies 30 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.

31. © 2008 The McGraw-Hill Companies 31 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

32. © 2008 The McGraw-Hill Companies 32Reference  Shalinee Kishore, LUCID Summer Workshop on Wireless Communications, Lehigh University, July 26-August 3, 2004 32