Satellite Communication

1. Satellite Communication

2. 17.2 Satellite Networks Orbits Three Categories of Satellites GEO Satellites MEO Satellites LEO Satellites

3. Figure 17.13Satellite orbits

4. Example 1 What is the period of the moon according to Kepler’s law? Solution The moon is located approximately 384,000 km above the earth. The radius of the earth is 6378 km. Applying the formula, we get Period = (1/100) (384,000 + 6378)1.5 = 2,439,090 s = 1 month

5. Example 2 According to Kepler’s law, what is the period of a satellite that is located at an orbit approximately 35,786 km above the earth? Solution Applying the formula, we get Period = (1/100) (35,786 + 6378)1.5 = 86,579 s = 24 h A satellite like this is said to be stationary to the earth. The orbit, as we will see, is called a geosynchronous orbit.

6. Figure 17.14Satellite categories

7. Figure 17.15Satellite orbit altitudes

8. Table 17.1 Satellite frequency band

9. Figure 17.16Satellites in geosynchronous orbit

10. Figure 17.17Triangulation

11. Figure 17.18GPS

12. Figure 17.19LEO satellite system

13. Figure 17.20Iridium constellation

14. Note: The Iridium system has 66 satellites in six LEO orbits, each at an altitude of 750 km.

15. Note: Iridium is designed to provide direct worldwide voice and data communication using handheld terminals, a service similar to cellular telephony but on a global scale.

16. Figure 17.21Teledesic

17. Note: Teledesic has 288 satellites in 12 LEO orbits, each at an altitude of 1350 km.

18. Satellite Components • Satellite Subsystems • Telemetry, Tracking, and Control • Electrical Power and Thermal Control • Attitude Control • Communications Subsystem

19. Satellite Orbits • Equatorial • Inclined • Polar

20. Orbital Mechanics Without Force Effect of Gravity Gravity

21. Here’s the Math… • Gravity depends on the mass of the earth, the mass of the satellite, and the distance between the center of the earth and the satellite • For a satellite traveling in a circle, the speed of the satellite and the radius of the circle determine the force (of gravity) needed to maintain the orbit

22. But… • The radius of the orbit is also the distance from the center of the earth. • For each orbit the amount of gravity available is therefore fixed • That in turn means that the speed at which the satellite travels is determined by the orbit

23. Let’s look in a Physics Book… • From what we have deduced so far, there has to be an equation that relates the orbit and the speed of the satellite: T is the time for one full revolution around the orbit, in seconds r is the radius of the orbit, in meters, including the radius of the earth (6.38x106m).

24. The Most Common Example • “Height” of the orbit = 22,300 mile • That is 36,000km = 3.6x107m • The radius of the orbit is 3.6x107m + 6.38x106m = 4.2x107m • Put that into the formula and …

25. The Geosynchronous Orbit • The answer is T = 86,000 sec (rounded) • 86,000 sec = 1,433 min = 24hours (rounded) • The satellite needs 1 day to complete an orbit • Since the earth turns once per day, the satellite moves with the surface of the earth.

26. Assignment • How long does a Low Earth Orbit Satellite need for one orbit at a height of 200miles = 322km = 3.22x105m • Do this: • Add the radius of the earth, 6.38x106m • Compute T from the formula • Change T to minutes or hours

27. GEO Coverage • Altitude is about 6 times the earth’s radius • Three satellite can cover the surface of the earth

28. Orbit Examples • Geostationary • Equatorial and Geosynchronous • Inclined Geosynchonous • Satellite moves north/south relative to the earth station • Polar LEO • Satellite group covers the entire earth

29. LEOS Coverage • Altitude is 1/6 of the earth’s radius

30. Communication Frequencies • Uplink (Earth to Satellite) • C Band: around 6 GHz • Ku Band: around 14 GHz • Ka Band: around 30 GHz • Downlink (Satellite to Earth) • C Band: around 4 GHz • Ku Band: around 12 GHz • Ka Band: around 20 GHz

31. Sputnik I Sputnik I -- 60 cm (about 2 ft.) diam. sphere with straight-wire antennas

32. Explorer I Explorer I -- 1 m. long and 20 cm in diam., spin stabilized (like a gyroscope), with flexible antennas

33. A generic military/meteorological/communications satellite 1-3 m. on each side, stabilized with internal gyroscopes or external thrusters

34. Dual-spin stabilized satellite 1-3 m. in diameter, up to several meters tall; lower section spins to provide gyroscopic stability, upper section does not spin

35. LIONSATLocal IONospheric Measurements SATellite • will measure ion distrib. in ram and wake of satellite in low orbit • student-run project • (funded by Air Force, NASA and AIAA) • www.psu.edu/dept/aerospace/lionsat

36. Hubble Space Telescope http://www.stsci.edu/hst/proposing/documents/cp_cy12/primer_cyc12.pdf

37. Propulsion • Provides force needed to change satellite’s orbit. • Includes thrusters and propellant.

38. Spacecraft Propulsion Subsystem • Uses of onboard propulsion systems • Orbit Transfer • (Low Earth Orbit) LEO to (Geosynchronous Earth Orbit) GEO • LEO to Solar Orbit • Drag Makeup • Attitude Control • Orbit Maintenance

39. Types of Propulsion • Chemical Propulsion • Performance is energy limited • Propellant Selection • Electric Propulsion • Electrostatic—Ion Engine • Electrothermal—ArcJet • Electomagnetic—Rail gun

40. Types of Propulsion • Solar Sails • Would use large (1 sq. km.) reflective sail (made of thin plastic) • Light pushes on the sail to provide necessary force to change orbit. • Still on the drawing board, but technologically possible! • Nuclear Thermal

41. Power • Provides, stores, distributes, and controls electrical power. • Need power for (basically everything) communications, computers, scientific instruments, environ. control and life support, thermal control, and even for propulsion (to start the rocket engine)

42. Power • Solar array: sunlight  electrical power • max. efficiency = 17% (231 W/m2 of array) • degrade due to radiation damage 0.5%/year • best for missions less than Mars’ dist. from Sun • Radioisotope Thermoelectric Generator (RTG): nuclear decay  heat  electrical power • max. efficiency = 8% (lots of waste heat!) • best for missions to outer planets • political problems (protests about launching 238PuO2) • Batteries – good for a few hours, then recharge

43. Power • Dynamic Power Sources • Like power plants on Earth. • Fuel Cells • Think of these as refillable batteries. • The Space Shuttle uses hydrogen-oxygen fuel cells.

44. Power • The design is highly dependent on: • Space Environment (thermal, radiation) • Shadowing • Mission Life

45. Thermal • Thermal Control System • Purpose—to maintain all the items of a spacecraft within their allowed temperature limits during all mission phases using minimum spacecraft resources.

46. Thermal • Passive • Coatings (control amt of heat absorbed & emitted) • can include louvers • Multi-layer insulation (MLI) blankets • Heat pipes (phase transition)

47. Thermal • Active (use power) • Refrigerant loops • Heater coils

48. Communications • Transmits data to ground or to relay satellite (e.g. TDRS) • Receives commands from ground or relay satellite

49. Communications • Radios (several for redundancy) • voice communications if humans onboard • data sent back to Earth from scientific instruments • instructions sent to s/c from Earth • Video (pictures of Earth, stars, other planets, etc.) • various antennas: dish, dipole, helix

50. Attitude Sensing and Control • Senses and controls the orientation of the spacecraft.