Introduction to Satellite Communications Joe Montana IT 488

Introduction to Satellite Communications Joe Montana IT 488 - Fall 2003 1

The class notes used in this course are based on two different sets of class notes provided by Dr. Jeremy Allnutt and Dr. James W. La. Pean when teaching “Satellite Communications” courses. Material from Leila Ribeiro is also used. All material is used with the permission of the author. 2

Agenda • History • Overview and Basic concepts of Satellite Communications • Spectrum Allocation • Satellite Systems Applications • System Elements • System Design Considerations 3 • Current Developments and Future Trends

Important Milestones (before 1950) Putting the concepts together 1600 Tycho Brache’s experimental observations on planetary motion. 1609 -1619 Kepler’s laws on planetary motion 1926 First liquid propellant rocket lauched by R. H. Goddard in the US. 1927 First transatlantic radio link communication 1942 First successful launch of a V-2 rocket in Germany. 1945 Arthur Clarke publishes his ideas on geostationary satellites for worldwide communications (GEO concept). 4

Propulsion Rocket motors produce thrust in a process which can be explained by Newton's third law (for every action there is an equal but opposite reaction). In the case of rocket engines, the reactionary force is produced by the combustion of fuel in a combustion chamber. This force then acts upon the rocket nozzle, causing the reaction which propels the vehicle. Since rocket motors are designed to operate in space, they require an oxidizer in order for combustion to take place. This oxidizer is, in many cases, liquid oxygen. There are three different types of rocket engines: · 1. Solid propelled rockets · 2. Liquid propelled rockets · 3. Nuclear rockets The advantages and disadvantages of each type are shown below. Solid Fueled Rockets In solid fueled rockets, the fuel and oxidizer both in solid form and thoroughly mixed during manufacture. The section where the fuel is stored is also the combustion chamber. One end of the chamber is closed (the payload of the rocket would be attached to this end) and the other end of the chamber is a rocket nozzle. Advantages of solid fuel rockets include simplicity and reliability, since there are no moving parts and high propellant density, which results in a smaller sized rocket Among the disadvantages are these: once you turn on a solid rocket motor, you can't shut it off. You have to wait for the fuel to run out. Also, the thrust of a solid fuel rocket decreases greatly during its burn time. Liquid Fueled Rockets In liquid fueled rockets the fuel and oxidizer are stored in liquid form and pumped into the combustion chamber. There are two types of liquidpropellent rockets; bipropellant rockets, which have separate fuel and oxidizer, and mono-propellant rockets, which have their fuel and oxidizer combined into a single liquid. Liquid fueled rockets are superior to solid fuel rockets in many respects; they can be shut off and subsequently restarted, they generally have a higher exhaust velocity, which means lower burn times are required, and they can be throttled to produce more or less thrust, as needed. However, liquid fuel rockets are highly complex, and therefore have a lower rate of reliability. Nuclear Rockets Nuclear rockets work by routing hydrogen through a nuclear reactor. The reactor is at a high temperature, which causes the hydrogen fuel to expand as it leaves the nozzle, producing a high amount of thrust. Nuclear rockets do not need an oxidizer, and they require much less fuel per pound of payload than liquid or solid fuel rockets. This allows a vehicle using a nuclear rocket to be more versatile than one which uses chemical rockets. Disadvantages of nuclear rockets include radiation effects caused by the nuclear reactor, and the high weight of the engine assembly. Odysseus Recent studies have shown nuclear propulsion for Mars missions offers several major advantages over all-chemical propulsion systems. Therefore, a nuclear engine was selected for the Odysseus program. The Oddyseus II engine will produce 1, 112, 500 Newtons of thrust at a weight of 9100 kg. The engine will be approximately 3 m in diameter and 6 meters long. 5

V 2 Rocket 6

Important Milestones (1950’s) Putting the pieces together 1956 - Trans-Atlantic cable opened (about 12 telephone channels – operator). 1957 First man-made satellite launched by former USSR (Sputnik, LEO). 1958 First US satellite launched (SCORE). First voice communication established via satellite (LEO, lasted 35 days in orbit after batteries failed). 7

Sputnik - I 8

Explorer - I 9

Important Milestones (1960’s) First satellite communications 1960 First passive communication satellite launched into space (Large balloons, Echo I and II). 1962: First non-government active communication satellite launched Telstar I (MEO). 1963: First satellite launched into geostationary orbit Syncom 1 (comms. failed). 1964: International Telecomm. Satellite Organization (INTELSAT) created. 1965 First communications satellite launched into geostationary orbit for commercial use Early Bird (re-named INTELSAT 1). 10

ECHO I 11

Telstar I 12

Intelsat I 13

Important Milestones (1970’s) GEO applications development 1972 First domestic satellite system operational (Canada). INTERSPUTNIK founded. 1975 First successful direct broadcast experiment (one year duration; USA-India). 1977 A plan for direct-to-home satellite broadcasting assigned by the ITU in regions 1 and 3 (most of the world except the Americas). 1979 International Mobile Satellite Organization (Inmarsat) established. 14

Important Milestones (1980’s) GEO applications expanded 1981 First reusable launch vehicle flight. 1982 International maritime communications made operational. 1983 ITU direct broadcast plan extended to region 2. 1984 First direct-to-home broadcast system operational (Japan). 1987 Successful trials of land-mobile communications (Inmarsat). 1989 -90 Global mobile communication service extended to land mobile and aeronautical use (Inmarsat) 15

Important Milestones (1990’s) 1990 -95: - Several organizations propose the use of non-geostationary (NGSO) satellite systems for mobile communications. - Continuing growth of VSATs around the world. - Spectrum allocation for non-GEO systems. - Continuing growth of direct broadcast systems. Direct. TV created. 1997: - Launch of first batch of LEO for hand-held terminals (Iridium). - Voice service telephone-sized desktop and paging service pocket size mobile terminals launched (Inmarsat). 1998: Iridium initiates services. 1999: Globalstar Initiates Service. 2000: ICO initiates Service. Iridium fails and system is sold to Boeing. 16

Iridium 17

Overview and Basic concepts of Satellite Communications 18

Main orbit types: GEO 36, 000 km MEO 5, 000 – 15, 000 km LEO 500 -1000 km 19

USEFUL ORBITS 1: GEOSTATIONARY ORBIT In the equatorial plane Orbital Period = 23 h 56 min. 4. 091 s = one Sidereal Day (defined as one complete rotation relative to the fixed stars) Satellite appears to be stationary over a point on the equator to an observer Radius of orbit, r, = 42, 164. 57 km NOTE: Radius = orbital height + radius of the earth Average radius of earth = 6, 378. 14 km 20

USEFUL ORBITS 2: Low Earth Orbit (>250 km); T 92 minutes Polar (Low Earth) Orbit; earth rotates about 23 o each orbit; useful for surveillance Sun Synchronous Orbit(example, Tiros. N/NOAA satellites used for search and rescue operations) 8 -hour and 12 -hour orbits Molniya orbit (Highly Elliptical Orbit (HEO); T 11 h 38 min; highly eccentric orbit; inclination 63. 4 degrees 21

MOLNIYA VIEW OF THE EARTH (Apogee remains over the northern hemisphere) 22

Molniya Variants (HEO’s) Tundra Orbit – Lies entirely above the Van Allen belts. The Russian Tundra system, which employs two satellites in two 24 -hour orbits separated by 180 deg around the Earth, with an apogee of 53, 622 km and a perigee of 17, 951 km. The Molniya orbit crosses the Van Allen belts twice for each revolution, resulting in a reduction of satellite life due to impact on electronics the Russian Molniya system employs three satellites in three 12 -hour orbits separated by 120 deg around the Earth, with an apogee of 39, 354 km and a perigee of 1000 km. 23

Molniya Variants (HEO’s) The LOOPUS orbit. The LOOPUS system employs three satellites in three eight-hour orbits separated by 120 deg around the Earth, with an apogee of 39, 117 km and a perigee of 1238 km. The ELLIPSO orbit 24

A Highly Elliptical Orbit (HEO) A satellite in HEO typically has a perigee at about 500 km above the surface of the Earth and an apogee as high as 50, 000 km. The orbit is usually inclined at 63. 4 deg to provide communications services to locations at high northern latitudes. This inclination value is selected to avoid rotation of the apses; thus, a line from the Earth's center to the apogee always intersects the Earth's surface at a latitude of 63. 4 deg North. Orbit period varies from eight to 24 hours. Owing to the high eccentricity of the orbit, a satellite spends about two-thirds of the orbital period near apogee, during which time it appears to be almost stationary to an observer on the Earth (a phenomenon known as `apogee dwell'). During the brief time the satellite is below the local horizon, a hand-off to another satellite in the same orbit is required in order to avoid loss of communications. Free space loss and propagation delay for this type of orbit are comparable to that of geosynchronous satellites. However, due to the comparatively great movement of a satellite in HEO relative to an observer on the Earth, satellite systems using this type of orbit must cope with large Doppler shifts. 25

A Medium-Earth Orbit (MEO) By setting the altitude parameters at 10, 000 km, you generated a medium-Earth orbit (MEO). This one happens to be an Intermediate Circular Orbit (ICO), since the apogee and perigee are equal. Its orbit period measures about seven hours. The maximum time during which a satellite in MEO orbit is above the local horizon for an observer on the Earth is a few hours. A global communications system using this type of orbit requires relatively few satellites in two to three orbital planes to achieve global coverage. MEO systems operate similarly to LEO systems. In MEO systems, however, hand-over is less frequent, and propagation delay and free space loss are greater. Examples of MEO (specifically ICO) systems are Inmarsat-P (10 satellites in 2 inclined planes at 10, 355 km), and Odyssey (12 satellites in 3 inclined planes, also at 10, 355 km). A Low-Earth Orbit (LEO) By selecting a relatively short period (90 minutes), we have generated a satellite in low-Earth orbit (LEO). A typical LEO is elliptical or, more often, circular, with a height of less than 2000 km above the surface of the Earth. The orbit period at those altitudes ranges between 90 minutes and two hours. The radius of the footprint of a communications satellite in LEO ranges between 3000 and 4000 km. The maximum time during which a satellite in LEO is above the local horizon for an observer on the Earth is 20 minutes. A global communications system using this type of orbit requires a large number of satellites, in a number of different orbital planes. When a satellite serving a particular user moves below the local horizon, it must hand over its duties to a succeeding one in the same orbit or in an adjacent one. Due to the comparatively great movement of a satellite in LEO relative to an observer on the Earth, satellite systems using this type of orbit must cope with large Doppler shifts. Satellites in LEO are also affected by atmospheric drag that causes the orbit to gradually deteriorate. Examples of major LEO systems are Globalstar. TM (48+8 satellites in 8 orbital planes at 1400 km) and Iridium® (66+6 satellites in 6 orbital planes at 780 km). There also a number of small LEO systems, such as Po. Sat, built by SSTL in 1993 and launched into an 822 by 800 km orbit, inclined at 98. 6 deg. 26

Geosynchronous & Geostationary Orbits A geosynchronous orbit is defined as an orbit with a period of one sidereal day (1436. 1 minutes). A geostationary orbit is a special case of a geosynchronous orbit with zero inclination and zero eccentricity, i. e. , an equatorial, circular orbit. A satellite in a geostationary orbit appears fixed above a location on the surface of the Earth. In practice, a geosynchronous orbit typically has small non-zero values for inclination and eccentricity, causing the satellite to trace out a small figure eight in the sky. The footprint or service area of a geosynchronous satellite covers almost one-third of the Earth's surface (from about 75 deg South to about 75 deg North latitude), so that near-global coverage can be achieved with as few as three satellites in orbit. A disadvantage of a geosynchronous satellite in a voice communication system is the round-trip delay of approximately 250 milliseconds. A Polar Orbit The plane of a polar orbit is inclined at about 90 deg to the equatorial plane, intersecting the North and South poles. The orbit is fixed in space, and the Earth rotates underneath. Thus, in principle, the coverage of a single satellite in a polar orbit encompasses the entire globe, although there are long periods during which the satellite is out of view of a particular ground station. This gap in coverage may be acceptable for a store-and-forward communications system. Accessibility can, of course, be improved through the deployment of two or more satellites in different polar orbits. Most small LEO systems employ polar or near-polar orbits. An example is the COSPAS-SARSAT Maritime Search and Rescue system, which uses eight satellites in near polar orbits: four SARSAT satellites moving in 860 km orbits inclined at 99 deg (which makes them Sunsynchronous) and four COSPAS satellites moving in 1000 km orbits inclined at 82 deg. A Sun-Synchronous Orbit In a Sun-synchronous or helio-synchronous orbit, the angle between the orbital plane and Sun remains constant, resulting in consistent light conditions for the satellite. This can be achieved by careful selection of orbital altitude, eccentricity and inclination, producing a precession of the orbit (node rotation) of approximately 1 deg eastward each day, equal to the apparent motion of the Sun. This condition can be achieved only for a satellite in a retrograde orbit. A satellite in Sun-synchronous orbit crosses the equator and each latitude at the same time each day. This type of orbit is therefore advantageous for an Earth observation satellite, since it provides constant lighting conditions. 27

Parameters Determining Orbit Size and Shape Parameter Definition Semimajor Axis Half the distance between the two points in the orbit that are farthest apart Apogee/Perigee Radius Measured from the center of the Earth to the points of maximum and minimum radius in the orbit Apogee/Perigee Altitude Measured from the "surface" of the Earth (a theoretical sphere with a radius equal to the equatorial radius of the Earth) to the points of maximum and minimum radius in the orbit Period The duration of one orbit, based on assumed two-body motion Mean Motion The number of orbits per solar day (86, 400 sec/24 hour), based on assumed two-body motion Eccentricity The shape of the ellipse comprising the orbit, ranging between a perfect circle (eccentricity = 0) and a parabola (eccentricity = 1) 28

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Orientation of Orbital Plane in Space Parameter Definition Inclination The angle between the orbital plane and the Earth's equatorial plane (commonly used as a reference plane for Earth satellites) Right Ascension of the Ascending Node The angle in the Earth's equatorial plane measured eastward from the vernal equinox to the ascending node of the orbit Argument of Perigee The angle, in the plane of the satellite's orbit, between the ascending node and the perigee of the orbit, measured in the direction of the satellite's motion Longitude of the Ascending Node The Earth-fixed longitude of the ascending node The ascending node (referenced in three of the above definitions) is the point in the satellite's orbit where it crosses the Earth's equatorial plane going from south to north. 30

Parameters determining orbit orientation 31

Satellite Location parameters To specify the satellite's location within its orbit at epoch. Parameter Definition True Anomaly The angle from the eccentricity vector (points toward perigee) to the satellite position vector, measured in the direction of satellite motion and in the orbit plane. Mean Anomaly The angle from the eccentricity vector to a position vector where the satellite would be if it were always moving at its angular rate. Eccentric Anomaly An angle measured with an origin at the center of an ellipse from the direction of perigee to a point on a circumscribing circle from which a line perpendicular to the semimajor axis intersects the position of the satellite on the ellipse. Argument of Latitude The sum of the True Anomaly and the Argument of Perigee. Time Past Ascending Node The elapsed time since the last ascending node crossing. Time Past Perigee The elapsed time since last perigee passage. 32

Parameters determining satellite position 33

Orbital Velocities and Periods Satellite System Orbital Height (km) INTELSAT 35, 786. 43 3. 0747 23 56 4. 091 ICO-Global 10, 255 4. 8954 5 55 48. 4 1, 469 7. 1272 1 55 17. 8 780 7. 4624 1 40 27. 0 Skybridge Iridium Orbital Velocity (km/s) Orbital Period h min s 34

GSO AND NGSO FACTORS NGSO OPTIONS: LEO MEO HEO AVOID RADIATION BELTS IF POSSIBLE 35

Coverage vs. Altitude Satellite Altitude (km) 36

LEO, MEO and GEO Orbit Periods 37

Minimum Delay for two hops 38

Why do satellites stay moving and in orbit? v (velocity) F 2 F 1 (Gravitational Force) (Inertial-Centrifugal Force) 39

Spectrum Allocation 40

Frequency Spectrum concepts: • Frequency: Rate at which an electromagnetic wave reverts its polarity (oscillates) in cycles per second or Hertz (Hz). • Wavelength: distance between wavefronts in space. Given in meters as: = c/f Where: c = speed of light (3 x 108 m/s in vacuum) f = frequency in Hertz • Frequency band: range of frequencies. • Bandwidth: Size or “width” (in Hertz) or a frequency band. • Electromagnetic Spectrum: full extent of all frequencies from zero to infinity. 41

Radio Frequencies (RF) RF Frequencies: Part of the electromagnetic spectrum ranging between 300 MHz and 300 GHz. Interesting properties: Efficient generation of signal power Radiates into free space Efficient reception at a different point. Differences depending on the RF frequency used: - Signal Bandwidth - Propagation effects (diffraction, noise, fading) - Antenna Sizes 42

Microwave Frequencies • Sub-range of the RF frequencies approximately from 1 GHz to 30 GHz. Main properties: - Line of sight propagation (space and atmosphere). - Blockage by dense media (hills, buildings, rain) - Wide bandwidths compared to lower frequency bands. - Compact antennas, directionality possible. - Reduced efficiency of power amplification as frequency grows: Radio Frequency Power OUT Direct Current Power IN 43

Spectrum Regulation International Telecommunication Union (ITU): Members from practically all countries around the world. • Allocates frequency bands for different purposes and distribute them around the planet. • Creates rules to limit RF Interference (RFI) between countries that reuse same RF bands. • Mediates disputes and creates rules to deal with harmful interference when it occurs. • Meets bi-annually with its members, to review rules and allocations: World Radio Communication Conference (WRC). • There also the Regional Radio Communication Conferences (RCC), which happen less often. 44

Radio Frequency Spectrum Commonly Used Bands 45

Space-Earth Frequency Usability Resonance frequencies below 100 GHz: • 22. 2 GHz (H 20) • 53. 5 -65. 2 GHz (Oxygen) Atmospheric attenuation effects for Space-to-Earth as a function of frequency (clear air conditions). (a) Oxygen; (b) Water vapor. [Source: ITU © 1988] 46

Insights on Frequency Selection: (Part 1: Lower frequencies, stronger links) LEO satellites need lower RF frequencies: Omni-directional antennas on handsets have low gain - typically G = 0 db = 1 Flux density F in W/m 2 at the earth’s surface in any beam is independent of frequency Received power is F x A watts , where A is effective area of antenna in square meters For an omni-directional antenna A = G 2/ 4 = 2/ 4 At 450 MHz, A = 353 cm 2, at 20 GHz, A = 0. 18 cm 2 Difference is 33 d. B - so don’t use 20 GHz with an 47

Insights on Frequency Selection: (Part 2: Higher frequencies, higher capacity) GEO satellites need more RF frequencies High speed data links on GEO satellites need about 0. 8 Hz of RF bandwidth per bit/sec. A 155 Mbps data link requires 125 MHz bandwidth Available RF bandwidth: C band occupied) 500 MHz Ku band slots occupied) (All GEO slots 750 MHz Ka band (Most GEO 2000 MHz (proliferating) Q/V band ? 48

Satellite Systems Applications 49

Initial application of GEO Satellites: Telephony 1965 circuits 1968 Early Bird 34 kg 240 telephone Intelsat III 152 kg 1500 circuits 1986 Intelsat VI 1, 800 kg 33, 000 circuits 2000 Large GEO 3000 kg 8 - 15 k. W power 1, 200 kg payload 50

Current GEO Satellite Applications: Broadcasting - mainly TV at present Direc. TV, Prime. Star, etc. Point to Multi-point communications VSAT, Video distribution for Cable TV Mobile Services Motient (former American Mobile Satellite), INMARSAT, etc. 51

Satellite Navigation: GPS and GLONASS GPS is a medium earth orbit (MEO) satellite system GPS satellites broadcast pulse trains with very accurate time signals A receiver able to “see” four GPS satellites can calculate its position within 30 m anywhere in world 24 satellites in clusters of four, 12 hour orbital period “You never need be lost again” Every automobile and cellular phone will eventually have a GPS location read-out 52

LEO Satellites in year 2000 Several new systems are just starting service Circular or inclined orbit with < 1400 km altitude Satellite travels across sky from horizon to horizon in 5 - 15 minutes Earth stations must track satellite or have omnidirectional antennas Constellation of satellites is needed for continuous communication. Handoff needed. 53

System Elements 54

Satellite System Elements Space Segment Satellite Earth Stations Coverage Region SCC TT&C Ground Station Ground Segment 55

Space Segment – – Satellite Launching Phase Transfer Orbit Phase Deployment Operation TT&C - Tracking Telemetry and Command Station: Establishes a control and monitoring link with satellite. Tracks orbit distortions and allows correction planning. Distortions caused by irregular gravitational forces from non-spherical Earth and due to the influence of Sun and Moon forces. SSC - Satellite Control Center, a. k. a. : – OCC - Operations Control Center – SCF - Satellite Control Facility Provides link signal monitoring for Link Maintenance and Interference monitoring. – Retirement Phase 56

Types of Satellite Stabilization Spin Stabilization Satellite is spun about the axis on which the moment of inertia is maximum (ex. , HS 376, most purchased commercial communications satellite; first satellite placed in orbit by the Space Shuttle. ) Three-Axis Stabilization Bias momentum type (ex. , INTELSAT V) Zero momentum type (ex. , Yuri) 57

Satellite Subsystems Communications Antennas Transponders Common Subsystem (Bus Subsystem) Telemetry/Command (TT&C) Satellite Control (antenna pointing, attitude) Propulsion Electrical Power Structure Thermal Control 58

Ground Segment Collection of facilities, users and applications. FSS – Fixed Satellite Service MSS – Mobile Satellite Service Earth Station = Satellite Communication Station (air, ground or sea, fixed or mobile). 59

System Design Considerations 60

Basic Principles Satellite Uplink Downlink Earth Station Tx Source Information Output Information Rx 61

Signals: Carried by wires as voltage or current Transmitted through space as electromagnetic waves. Analog: • Voltage or Current proportional to signal; e. g. , Telephone. Digital: Generated by computers. Ex. Binary = 1 or 0 corresponding to +1 V or – 1 V. 62

Separating Signals Up and Down: FDD: Frequency Division Duplexing. f 1 = Uplink f 2 = Downlink TDD: Time Division Duplexing. t 1=Up, t 2=Down, t 3=Up, t 4=Down, …. Polarization V & H linear polarization RH & LH circular polarizations 63

Separating Signals (so that many transmitters can use the same transponder simultaneously) Between Users or “Channels” (Multiple Access): FDMA: Frequency Division Multiple Access; assigns each transmitter its own carrier frequency f 1 = User 1; f 2 = User 2; f 3 = User 3, … TDMA: Time Division Multiple Access; each transmitter is given its own time slot t 1=User_1, t 2=User_2, t 3=User_3, t 4 = User_1, . . . CDMA: Code Division Multiple Access; each transmitter transmits simultaneously and at the same frequency and each transmission is modulated by its own pseudo randomly coded bit stream Code 1 = User 1; Code 2 = User 2; Code 3 = User 3 64

Digital Communication System TRANSMITTER Source Data Source Coding Channel Coding Modulator RF Channel Output Data Source Decoding Channel Decoder Demodulator RECEIVER 65

Current Developments and Future Trends 66

Current Trends in Satellite Communications Bigger, heavier, GEO satellites with multiple roles More direct broadcast TV and Radio satellites Expansion into Ka, Q, V bands (20/30, 40/50 GHz) Massive growth in data services fueled by Internet Mobile services: May be broadcast services rather than point to point Make mobile services a successful business? 67

The Future for Satellite Communications – 1 Growth requires new frequency bands Propagation through rain and clouds becomes a problem as RF frequency is increased C-band (6/4 GHz) Rain has little impact 99. 99% availability is possible Ku-band (10 -12 GHz) Link margin of 3 d. B needed for 99. 8% availability Ka-band (20 - 30 GHz) Link margin of 6 d. B needed for 99. 6% availability 68

The Future for Satellite Communications - 2 Low cost phased array antennas for mobiles are needed Mobile systems are limited by use of omni-directional antennas A self-phasing, self-steering phased array antenna with 6 d. B gain can quadruple the capacity of a system Directional antennas allow frequency re-use 69

Homework #1 Answer the questions below for Dish Network’s direct-to-home digital television broadcasting. Reference the text pages 7, 443, 445 and Dish Network’s Web site, and section 11. 2 page 441. 1) How many satellites does Dish Network have in the sky? Name them? 2) How many transponders are on each of these satellites? What frequency band is used? 3) What orbit are these satellites in (LEO, MEO, GEO)? 4) Why are two heads used on the Dish Network antenna (text page 445)? 5) On what date was Echostar I launched? Echostar V? 6) Are these satellites spin or three axis stabilized? See page 443 7) Go to the Website and download the azimuth and elevation application( products – installation) and follow the directions to aim the dish antenna to receive a signal for the zip code where you live. State the azimuth, elevation and skew angles and longitude for each satellite. 8) See page 443. If the frequency band were C rather than Ku, how would this affect the size of the receive antenna you would need on your rooftop? 9) What is a transponder? Why does a satellite have multiple transponders and not just one? 10) Extra credit – Go to the Air & Space Museum and view Explorer I, Sputnik I, the V 2 rocket, TIROS II, TIROS-N and ITOS. Briefly describe the purpose of each. 70