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Page 47 Neptune in infra-red light, Neptune in infra-red light, Keck Telescope adaptive optics Keck Telescope adaptive optics Without adaptive optics With adaptive optics

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Page 48 Telescopes can Telescopes can “ “ see see ” ” infrared light as infrared light as well as visible light well as visible light • • Infra-red image shows new stars forming Infra-red image shows new stars forming • • Not visible in Not visible in “ “ visible light visible light ” ” image because they are image because they are deeply embedded in clouds of dust deeply embedded in clouds of dust Page 49 Movie of volcanoes on Jupiter Movie of volcanoes on Jupiter ’ ’ s moon Io, s moon Io, from Keck Telescope adaptive optics from Keck Telescope adaptive optics Page 50 Concept Question Concept Question • • The Keck Telescope in Hawaii has a diameter The Keck Telescope in Hawaii has a diameter of 10 m, compared with 5 m for the Palomar of 10 m, compared with 5 m for the Palomar Telescope in California. The light gathering Telescope in California. The light gathering power of Keck is larger by a factor of power of Keck is larger by a factor of a a 2 b 4 c 15 d 50 2 b 4 c 15 d 50 • • By what factor is Keck By what factor is Keck ’ ’ s angular resolution s angular resolution better than that of Palomar, assuming that better than that of Palomar, assuming that both are using their adaptive optics systems? both are using their adaptive optics systems? a a 2 b 4 c 15 d 50 2 b 4 c 15 d 50 Page 51 Reflecting telescopes work fine at Reflecting telescopes work fine at radio wavelengths radio wavelengths • • The radio telescope at Green Bank, NC The radio telescope at Green Bank, NC Page 52 Largest radio telescope fills a whole Largest radio telescope fills a whole valley in Puerto Rico valley in Puerto Rico Page 53 Interferometry Interferometry is a method to is a method to improve spatial resolution improve spatial resolution Page 54 The The “ “ Very Large Array Very Large Array ” ” radio radio interferometer in New Mexico interferometer in New Mexico Page 55 Spectroscopy Spectroscopy • • A spectrograph A spectrograph separates the separates the different different wavelengths of wavelengths of light before they light before they hit the detector hit the detector Diffraction grating breaks light into spectrum Detector records spectrum Light from only one star enters Page 56 Spectroscopy Spectroscopy • • Graphing Graphing relative relative brightness of brightness of light at each light at each wavelength wavelength shows the shows the details in a details in a spectrum spectrum Page 57 Timing Timing • • A light curve represents a series of brightness A light curve represents a series of brightness measurements made over a period of time measurements made over a period of time Page 58 Timing: Dust devils on Mars seen Timing: Dust devils on Mars seen from Spirit Rover from Spirit Rover Page 59 Want to buy your own telescope? Want to buy your own telescope? • • Buy binoculars first e.g. 7x35 - you get much Buy binoculars first e.g. 7x35 - you get much more for the same money. more for the same money. • • Ignore magnification sales pitch Ignore magnification sales pitch • • Notice: aperture size, optical quality, weight Notice: aperture size, optical quality, weight and portability. and portability. • • Product reviews: Astronomy, Sky Telescope, Product reviews: Astronomy, Sky Telescope, Mercury Magazines. Also amateur astronomy Mercury Magazines. Also amateur astronomy clubs. clubs. Page 60 Why do we need telescopes in Why do we need telescopes in space? space? Page 61 Why do we need telescopes in Why do we need telescopes in space? space? a a Some wavelengths of light don Some wavelengths of light don ’ ’ t get through t get through the Earth the Earth ’ ’ s atmosphere s atmosphere • • Gamma-rays, x-rays, far ultraviolet, long infrared Gamma-rays, x-rays, far ultraviolet, long infrared wavelengths wavelengths • • The only way to see them is from space The only way to see them is from space b b Going to space is a way to overcome blurring Going to space is a way to overcome blurring due to turbulence in Earth due to turbulence in Earth ’ ’ s atmosphere s atmosphere c c Planetary exploration: spacecraft can actually Planetary exploration: spacecraft can actually go to the planets, get close-up information go to the planets, get close-up information Page 62 Depth of light penetration into Depth of light penetration into atmosphere at different wavelengths atmosphere at different wavelengths Page 63 X-ray mirrors also concentrate light X-ray mirrors also concentrate light and bring it to a focus and bring it to a focus • • X-ray mirrors X-ray mirrors Page 64 Chandra spacecraft: x-rays Chandra spacecraft: x-rays Page 65 Hubble Space Telescope: clearer vision Hubble Space Telescope: clearer vision above atmospheric turbulence above atmospheric turbulence Hubble can see UV light that doesnʼt penetrate through atmosphere Page 66 Example of robotic planet exploration: Example of robotic planet exploration: Galileo mission to Jupiter Galileo mission to Jupiter Artists conception Artists conception Page 67 Types of space missions Types of space missions • • Earth orbiters Earth orbiters • • Planetary fly-bys Planetary fly-bys – – Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune so Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune so far far – – New Horizons flyby of Pluto arrives there July 14 2015 New Horizons flyby of Pluto arrives there July 14 2015 • • Planetary orbiters Planetary orbiters – – Venus, Mars, Jupiter, Saturn so far Venus, Mars, Jupiter, Saturn so far • • Probes and Probes and landers landers – – Mars rovers: Spirit and Mars rovers: Spirit and Opportunity Opportunity – – Mars landers: e.g. Phoenix Mars landers: e.g. Phoenix – – Probes Probes sent from orbiters of Venus, Mars, Jupiter sent from orbiters of Venus, Mars, Jupiter – – Titan Titan lander lander Huygens probe from Huygens probe from Cassini Cassini spacecraft spacecraft Page 68 Space missions carry telescopes, Space missions carry telescopes, other instruments as well other instruments as well • • Typically planetary fly-bys and orbiters carry Typically planetary fly-bys and orbiters carry small telescopes small telescopes – – If you are close, you don If you are close, you don ’ ’ t need super-good angular t need super-good angular resolution resolution • • Other instruments: Other instruments: – – Particle collectors and analyzers, radio antennae, Particle collectors and analyzers, radio antennae, spectrographs, laser altimeters, dust detectors, ..... spectrographs, laser altimeters, dust detectors, ..... – – Mars rovers: probes to get rock samples and Mars rovers: probes to get rock samples and analyze them analyze them Page 69 Spirit Rover on Mars Spirit Rover on Mars Page 70 Concept Question Concept Question • • You are trying to decide whether to observe a new You are trying to decide whether to observe a new comet from a 10m telescope on the ground without comet from a 10m telescope on the ground without adaptive optics, or from the Hubble Space Telescope adaptive optics, or from the Hubble Space Telescope diameter 2.4m. diameter 2.4m. • • Which of the following would be better from the Which of the following would be better from the ground, and which from space ground, and which from space a a Ability to make images in ultraviolet light Ability to make images in ultraviolet light b b Spatial resolution of images in infrared light Spatial resolution of images in infrared light c c Ability to record images of a Ability to record images of a very very faint distant comet faint distant comet Page 71 The Main Points The Main Points • • Telescopes gather light and focus it Telescopes gather light and focus it – – Larger telescopes gather more light Larger telescopes gather more light – – Telescopes can gather Telescopes can gather “ “ light light ” ” at radio, infrared, visible, ultraviolet, x- at radio, infrared, visible, ultraviolet, x- ray wavelengths ray wavelengths • • Telescopes can be on ground, on planes, in space Telescopes can be on ground, on planes, in space • • If Earth If Earth ’ ’ s atmosphere weren s atmosphere weren ’ ’ t turbulent, larger telescopes would t turbulent, larger telescopes would give higher spatial resolution give higher spatial resolution – – Adaptive optics can correct for blurring due to turbulence Adaptive optics can correct for blurring due to turbulence • • Every new telescope technology has resulted in major new Every new telescope technology has resulted in major new discoveries and surprises discoveries and surprises A Brief History of Satellite Laser Ranging: 1964 – present Jan McGarry Tom Zagwodzki NASA GSFC 694 With special thanks to John Degnan for additional background information and slides. From the GSFC perspective 3292005 • Unambiguous time-of-flight measurement • 1 to 2 mm normal point precision • Passive space segment reflector • Simple refraction model • Night Day Operation • Near real-time global data availability • Satellite altitudes from 400 km to 20,000 km GPS, GLONASS and the Moon • Centimeter accuracy satellite orbits ~ 1-2 cm LAGEOS ~ 2-3 cm GPS SLR generates unambiguous centimeter accuracy orbits SLR generates unambiguous centimeter accuracy orbits Observable: The precise measurement of the roundtrip time-of-flight of an ultrashort 500 psec laser pulse between an SLR ground station and a retroreflector- equipped satellite which is then corrected for atmospheric refraction using ground-based meteorological sensors. 3292005 Satellite and Lunar Laser Ranging • Terrestrial Reference Frame SLR – Geocenter motion – Scale GM – 3-D station positions and velocities 50 • Solar System Reference Frame LLR – Dynamic equinox – Obliquity of the Ecliptic – Precession constant • Earth Orientation Parameters EOP – Polar motion – Length of Day LOD – High frequency UT1 • Centimeter Accuracy Orbits – Testcalibrate microwave navigation techniques e.g., GPS, GLONASS, DORIS, PRARE – Support microwave and laser altimetry missions e.g., TOPEXPoseidon, ERS 12, GFO-1, JASON, GLAS, VCL – Support gravity missions e.g. CHAMP, GRACE, Gravity Probe B • Geodynamics – Tectonic plate motion – Regional crustal deformation • Earth Gravity Field – Static medium to long wavelength components – Time variation in long wavelength components – Mass motions within the solid Earth, oceans, and atmosphere • Lunar Physics LLR – Centimeter accuracy lunar ephemerides – Lunar librations variations from uniform rotation – Lunar tidal displacements – Lunar mass distribution – Secular deceleration due to tidal dissipation in Earth’s oceans – Measurement of GM E + M M • General Relativity – Testevaluate competing theories – Support atomic clock experiments in aircraft and spacecraft – Verify Equivalence Principle – Constrain β parameter in the Robertson-Walker Metric – Constrain time rate of change in G • Future Applications – Global time transfer to 50 psec to support science, high data rate link synchronization, etc French L2T2 Experiment – Two-way interplanetary ranging and time transfer for Solar System Science and improved General Relativity Experiments Asynchronous Laser Transponders 3292005 ~60 satellites tracked since 1964 3292005 SLR Retro-Reflector Array RRA: GPS 35,36 32 cubes – each 28mm Aluminum coated reflective surfaces Array shape: planar square Array size: 239 x 194 x 37 mm Array mass: 1.27 kg 3292005 9 cubes – each 32 mm - 1 nadir pointing and 8 on sides Research grade radiation resistant suprasil quartz, silver coated Arrray shape: hemispherical Array size: 16cm diameter Array mass: 731 gm 3292005 • There are 5 retro-reflectors arrays: 3 Apollo and 2 Luna. • Apollo RRA’s have 3.8 cm cubes. Apollo 11 14 have 100, Apollo 15 has 300. • Regularly tracked by only a few stations. NASA funded University of Texas MLRS has successfully ranged continuously since 1970s. Ranges are accurate to a few centimeters. GSFC invented and developed Satellite Laser Ranging and continues to advance SLR RD, however, there have been many contributors to SLR over the years, including SAO U.Texas in U.S. and many international groups. 1960s 1970s 1980s 1990s GODLAS STALAS MOBLAS 4-8 SLR2000 524 GSFC Codes 810 OPS 723 RD 901 CDP 920 453 OPS 694 RD 48 INCH SLR at GSFC TLRS 3292005 • 1964: First successful demonstration of SLR to Beacon Explorer 22-B satellite at GSFC 3 m ranging • 1968-1976: NASA, CNES, and SAO SLR systems carried out first meter level global geodetic and gravity field measurements using reflectors on remote sensing satellites • 1969: NASA Apollo 11 mission places first retroreflector array on Moon to begin international Lunar Laser Ranging LLR effort. • 1975-1976: CNES and NASA launched first passive satellites dedicated to SLR Starlette and LAGEOS to begin modern space geodetic era • 1975-1979: NASA builds up SLR network for POD support of GEOSAT and SEASAT ocean altimetric missions 10 - 20 cm ranging. • 1979-1993: NASA Crustal Dynamics Project CDP provides focus for further technology development 1 cm ranging and international cooperation in defining contemporary tectonic plate motions, regional crustal deformation, Earth Orientation Parameters, Earth gravity field etc. GSFC Firsts a 40+ year history • 1992-present: Various US and European remote sensing missions e.g. ERS-1 2, TOPEXPoseidon, GFO rely heavily on centimeter orbits provided by SLR. NASA provides the world’s data most precise data and until budget cuts in 2003, provided half of the global SLR data. • 1998-present: GSFC selected as the Central Bureau CB for the new International Laser Ranging Service ILRS. The CB is responsible for overseeing global operations of 40 international stations providing cm- accuracy orbits for 20 artificial satellites and the Moon and ensuring that all ILRS stations, operations, data, and analysis centers adhere to ILRS standards. formerly Goddard Optical Research Facility GORF  Located ~ 3 miles from GSFC in middle of BARC on Springfield Road.  Home to MOBLAS-7, 48” telescope, VLBI MV3, GPS, and numerous other facilities and experiments.  GGAO has been the site of all NASA SLR system development, testing and collocations. The Italian MLRO system, the Saudi SALRO, and other ILRS systems have also been developed and tested at site. GODLAS GSFC records first SLR returns ever on Oct 31, 1964 GSFC team lead by Henry Plotkin BE-B: first satellite with retro-reflectors - Developed in early 1970s as a “stationary laser” system at GORF. - X-Y mount with 61cm 24” telescope. - Initial system had 1 Hz ruby 694nm laser. MOBLAS - 7: ~1980 with Jack Waller -Systems built in 1978 by Contraves telescope mount, and BFEC electronics. - 76 cm 30” diameter telescope. - Laser now: 5Hz, 532nm, 100mJ. -5 systems, all still operating, now located in: California Mon.Peak Australia South Africa Maryland GGAO Tahita. - Built in 1973-74 by Kollmorgen Corporation as multi-user facility - Arcsecond precision tracking - RD Facility used by many groups: • Field testing of bread board for optical heterodyne spectrometers in 1970s 1980s M.Mumma colleagues • Automated guiding and two-color refractometry D. CurrieUMd, D. WellnitzUMd: 1970s. • Lunar laser ranging test facility C. AlleyUMd: 1980s. • Comparison of one way propagation times of laser pulses East-West vs West-East by C. Alley and R. Nelson in 1983. • Single and two-color satellite laser ranging test bed Zagwodzki, Degnan, McGarry: 1980s 1990s.  MLA Earthlink Calibration Experiment: May 2005. and many others… 48” Telescope Facility aka 1.2 m Telescope Located at GGAO Code 723: ~1985 Who are these people and why do they look so young? DEGNAN COYLE RALL ZAGWODZKI MCGARRY PACINI UNGER ABSHIRE Prototype at GGAO: ~2002 • SLR2000 was concept developed by John Degnan in the 1990s, with many innovative ideas requiring RD efforts. • System designed in the mid 1990s, development began in late 1990s. Technical development is now in code 694 SLR OPS is in code 453. • Currently tracking LEO satellites. Nearing the completion of major technical challenges. Expect to have a working semi-automated system by end of this year. NASA plans to build 12 new SLR systems contracted out and will replace all of its existing SLR Network with systems using technology developed on this prototype. OMC ranging plot Satellite returns SLR Precision has improved from meter to sub-cm level. NASA operating costs have decreased reductions in manpower. And global data volume has been increasing. NASA SLR is part of the International Laser Ranging Service ILRS The International Laser Ranging Service ILRS began in 1998 and provides global satellite and lunar laser ranging data and their related products to support geodetic and geophysical research activities as well as IERS products important to the maintenance of an accurate International Terrestrial Reference Frame ITRF. The service develops the necessary global standardsspecifications and encourages international adherence to its conventions. The ILRS is one of the space geodetic services of the International Association of Geodesy IAG. GSFC has been chosen to run the Central Bureau of the ILRS. The Central Bureau is responsible for overseeing global operations of 40 international stations 3292005 The Future of Laser Ranging: Asynchronous Transponders SLR Ground Station Spacecraft Transponder 532nm uplink 1064nm downlink Ground System Analysis computes Ranges Fire and Return Event Times sent via Internet Fire and Return Event Times sent via Comm Link 1R2 signal loss for transponders, 1R4 signal loss for RRA ranging. Transponders are the only viable option for ranging to planetary distances. 3292005 For further information on the History of Satellite Laser Ranging, see: “Thirty Years of Satellite Laser Ranging”, John Degnan, Keynote Speech, Proceedings of the Ninth International Workshop on Laser Ranging Instrumentation, Canberra, Australia, November 7-11, 1994. Direct Broadcast Satellite: Architecture and Evaluation Venkata N. Padmanabhan padmanabcs.berkeley.edu Daedalus Retreat, June 1996 Overview • Geostationary satellite broadcasts directly to user premises – 24 inch dish antenna, ISA adaptor card • Asymmetric Internet access – users typically receive more data than they send – 12 Mbps satellite downlink; target rate of 400 Kbps per user – slow uplink: SLIPPPP over a modem line • Easy to deploy at short notice even in remote locations User Subnet SLIPPPP Internet Uplink Site Internet Server Asymmetric Routing • Outgoing traffic over the SLIPPPP line; Incoming traffic over the satellite hop • Option 1: Encapsulation – outgoing packets use DBS source address – packets sent encapsulated over the SLIP line – works for a single host but not for a subnet • Option 2: Home agent-based routing – outgoing packets use home source address – home agent tunnels incoming packets over DBS – a more general solution Transport Issues • Large bandwidth-delay product – TCP sender and receiver need to maintain large windows to keep the “data pipe” full – 500-1000 Kbps times 1 second = 50-100 KB • Asymmetric bandwidth – Uplink has much smaller bandwidth than the downlink – TCP acknowledgements stream might throttle the flow of data packets UDP Throughput – Throughput tapers off beyond an offered load of about 1.4 Mbps 200 400 600 800 1000 1200 1400 1600 200 400 600 800 1000 1200 1400 1600 1800 2000 Offered Load Kbps Throughput Kbps Day Night UDP Loss Rate 0.05 0.1 0.15 0.2 0.25 0.3 0.35 200 400 600 800 1000 1200 1400 1600 1800 2000 Night Day Offered Load Kbps Loss Rate – High loss rate due to Internet during the day – Sharp upswing for offered load beyond 1.4 Mbps TCP Throughput – Poor throughput for the 8-32 KB buffers used by most Internet servers – Comparable to UDP when loss rate is low TCP Buffer Size bytes 100 200 300 400 500 600 700 20000 40000 60000 80000 100000 120000 140000 Night Day Throughput Kbps TCP Dynamics 5000 10000 15000 20000 25000 30000 20 40 60 80 100 120 140 Time seconds Congestion Window KB Fast retransmissions – 2 MB transfer with 130 KB buffers – Poor throughput due to fluctuation in congestion window size though not timeouts at source TCP Throughput Internet only – Best throughput for 16-32 KB buffers – Deterioration for large buffers presumably due to increased burstiness of the source TCP Buffer Size bytes 200 400 600 800 1000 1200 20000 40000 60000 80000 100000 120000 140000 Throughput Kbps Day Night Conclusions • Performance of DBS system is often limited by the Internet • Large TCP windows needed to keep data pipe full – buffer sizes typically used by servers on the Internet are too small – but large buffer sizes could increase source burstiness and lead to Internet losses. Future Work • Improve performance of data transport – install host close to uplink to evaluate the satellite hop in isolation – split-connection approach for instance, in conjunction with a Web proxy cache – reduce burstiness of TCP source • Evaluate application-specific solutions – plentiful downlink bandwidth, large latency – suitable for predictive prefetching of Web data [PM96] Status of the UCB Testbed • One DirecPC host fully functional – BSDOS driver for the ISA adaptor card Keith Sklower • Home agent-based routing • Web browsing and video dissemination demonstrated Stay Current In Your Field • Broaden Your Knowledge • Increase Productivity 349 Berkshire Drive • Riva, Maryland 21140 888-501-2100 • 410-956-8805 Website: www.ATIcourses.com • Email: ATIATIcourses.com Boost Your Skills With ATIcourses.com ATI Provides Training In: • Acoustic, Noise Sonar Engineering • Communications and Networking • Engineering Data Analysis • Information Technology • Radar, Missiles Combat Systems • Remote Sensing • Signal Processing • Space, Satellite Aerospace Engineering • Systems Engineering Professional Development ATI Provides Training In: • Acoustic, Noise Sonar Engineering • Communications and Networking • Engineering Data Analysis • Information Technology • Radar, Missiles Combat Systems • Remote Sensing • Signal Processing • Space, Satellite Aerospace Engineering • Systems Engineering Professional Development Check Our Schedule Register Today The Applied Technology Institute ATIcourses.com specializes in training programs for technical professionals. Our courses keep you current in state- of-the-art technology that is essential to keep your company on the cutting edge in todays highly competitive marketplace. Since 1984, ATI has earned the trust of training departments nationwide, and has presented On-site training at the major Navy, Air Force and NASA centers, and for a large number of contractors. Our training increases effectiveness and productivity. Learn From The Proven Best Register online at www.ATIcourses.com or call ATI at 888.501.2100 or 410.956.8805 Vol. 97 – 53 A comprehensive, quantitative tutorial designed for satellite professionals Course Outline

1. Mission Analysis. Kepler’s laws. Circular and

elliptical satellite orbits. Altitude regimes. Period of revolution. Geostationary Orbit. Orbital elements. Ground trace.

2. Earth-Satellite Geometry. Azimuth and elevation.

Slant range. Coverage area.

3. Signals and Spectra. Properties of a sinusoidal

wave. Synthesis and analysis of an arbitrary waveform. Fourier Principle. Harmonics. Fourier series and Fourier transform. Frequency spectrum.

4. Methods of Modulation. Overview of modulation.

Carrier. Sidebands. Analog and digital modulation. Need for RF frequencies.

5. Analog Modulation. Amplitude Modulation AM.

Frequency Modulation FM.

6. Digital Modulation. Analog to digital conversion.

BPSK, QPSK, 8PSK FSK, QAM. Coherent detection and carrier recovery. NRZ and RZ pulse shapes. Power spectral density. ISI. Nyquist pulse shaping. Raised cosine filtering.

7. Bit Error Rate. Performance objectives. EbNo.

Relationship between BER and EbNo. Constellation diagrams. Why do BPSK and QPSK require the same power?

8. Coding. Shannon’s theorem. Code rate. Coding gain.

Methods of FEC coding. Hamming, BCH, and Reed- Solomon block codes. Convolutional codes. Viterbi and sequential decoding. Hard and soft decisions. Concatenated coding. Turbo coding. Trellis coding.

9. Bandwidth. Equivalent noise bandwidth. Occupied

bandwidth. Allocated bandwidth. Relationship between bandwidth and data rate. Dependence of bandwidth on methods of modulation and coding. Tradeoff between bandwidth and power. Emerging trends for bandwidth efficient modulation.

10. The Electromagnetic Spectrum. Frequency bands

used for satellite communication. ITU regulations. Fixed Satellite Service. Direct Broadcast Service. Digital Audio Radio Service. Mobile Satellite Service.

11. Earth Stations. Facility layout. RF components.

Network Operations Center. Data displays.

12. Antennas. Antenna patterns. Gain. Half power

beamwidth. Efficiency. Sidelobes.

13. System Temperature. Antenna temperature. LNA.

Noise figure. Total system noise temperature.

14. Satellite Transponders. Satellite communications

payload architecture. Frequency plan. Transponder gain. TWTA and SSPA. Amplifier characteristics. Nonlinearity. Intermodulation products. SFD. Backoff.

15. The RF Link. Decibel dB notation. Equivalent

isotropic radiated power EIRP. Figure of Merit GT. Free space loss. WhyPower flux density. Carrier to noise ratio. The RF link equation.

16. Link Budgets. Communications link calculations.

Uplink, downlink, and composite performance. Link budgets for single carrier and multiple carrier operation. Detailed worked examples.

17. Performance Measurements. Satellite modem.

Use of a spectrum analyzer to measure bandwidth, CN, and EbNo. Comparison of actual measurements with theory using a mobile antenna and a geostationary satellite.

18. Multiple Access Techniques. Frequency division

multiple access FDMA. Time division multiple access TDMA. Code division multiple access CDMA or spread spectrum. Capacity estimates.

19. Polarization. Linear and circular polarization.

Misalignment angle.

20. Rain Loss. Rain attenuation. Crane rain model.

Effect on GT. IIn nssttrru ucctto orr Dr. Robert A. Nelson is president of Satellite Engineering Research Corporation, a consulting firm in Bethesda, Maryland, with clients in both commercial industry and government. Dr. Nelson holds the degree of Ph.D. in physics from the University of Maryland and is a licensed Professional Engineer. He is coauthor of the textbook Satellite Communication Systems Engineering, 2nd ed. Prentice Hall, 1993 and is Technical Editor of Via Satellite magazine. He is a member of IEEE, AIAA, APS, AAPT, AAS, IAU, and ION. March 16-18, 2009 Boulder, Colorado June 15-17, 2009 Beltsville, Maryland 1740 8:30am - 4:30pm Register 3 or More Receive 100 00 each Off The Course Tuition. Testimonials “Great handouts. Great presentation. Great real-life course note examples and cd. The instructor made good use of student’s experiences. “Very well prepared and presented. The instructor has an excellent grasp of material and articulates it well” “Outstanding at explaining and defining quantifiably the theory underlying the concepts.” “Fantastic It couldn’t have been more relevant to my work.” “Very well organized. Excellent reference equations and theory. Good examples.” “Good broad general coverage of a complex subject.” Additional Materials In addition to the course notes, each participant will receive a book of collected tutorial articles written by the instructor and soft copies of the link budgets discussed in the course. 1 Via Satellite, October, 1998 Earth Station High Power Amplifiers KPA, TWTA, or SSPA? by Robert A. Nelson The high power amplifier HPA in an earth station facility provides the RF carrier power to the input terminals of the antenna that, when combined with the antenna gain, yields the equivalent isotropic radiated power EIRP required for the uplink to the satellite. The waveguide loss between the HPA and the antenna must be accounted for in the calculation of the EIRP. The output power typically may be a few watts for a single data channel, around a hundred watts or less for a low capacity system, or several kilowatts for high capacity traffic. The choice of amplifier is highly dependent on its application, the cost of installation and long term operation, and many other factors. This article will summarize the technologies, describe their important characteristics, and identify some issues important to understanding their differences and relative merits. TYPES OF AMPLIFIERS Earth station terminals for satellite communication use high power amplifiers designed primarily for operation in the Fixed Satellite Service FSS at C-band 6 GHz, military and scientific communications at X-band 8 GHz, fixed and mobile services at Ku-band 14 GHz, the Direct Broadcast Service DBS in the DBS portion of Ku-band 18 GHz, and military applications in the EHFQ-band 45 GHz. Other frequency bands include those allocated for the emerging broadband satellite services in Ka-band 30 GHz and V-band 50 GHz. Generally, the frequency used for the earth-to-space uplink is higher than the frequency for the space-to-earth downlink within a given band. An earth station HPA can be one of three types: a klystron power amplifier KPA, a traveling wave tube amplifier TWTA, or a solid state power amplifier SSPA. The KPA and TWTA achieve amplification by modulating the flow of electrons through a vacuum tube. Solid state power amplifiers use gallium arsenide GaAs field effect transistors FETs that are configured using power combining techniques. The klystron is a narrowband, high power device, while TWTAs and SSPAs have wide bandwidths and operate over a range of low, medium, and high powers. The principal technical parameters characterizing an amplifier are its frequency, bandwidth, output power, gain, linearity, efficiency, and reliability. Size, weight, cabinet design, ease of maintenance, and safety are additional considerations. Cost factors include the cost of installation and the long term cost of ownership. KPAs are normally used for high power narrowband transmission to specific satellite transponders, typically for television program transmission and distribution. TWTAs and SSPAs are used for wideband applications or where frequency agility is required. Originally, TWTAs provided high power but with poor efficiency and reliability. Compared to a KPA, these disadvantages were regarded as necessary penalties for wide bandwidth. SSPAs first became available about 20 years ago. They were restricted to low power systems requiring only a few watts, such as small earth stations transmitting a few telephone channels. Within the past decade, however, TWTA and SSPA technologies have both advanced considerably. Today there is vigorous competition between these two technologies for wideband systems. KPA The klystron power amplifier KPA is a narrowband device capable of providing high power and high gain with relatively high efficiency and stability. The bandwidth is about 45 MHz at C-band and about 80 MHz at Ku-band. Thus a separate KPA is usually required for each satellite transponder. In a klystron tube an electron beam is formed by accelerating electrons emitted from a heated cathode through a positive potential difference. The electrons enter a series of cavities, typically five in number, which are tuned around the operating frequency and are connected by cylindrical drift tubes. In the input cavity the electrons are velocity-modulated by a time-varying electromagnetic field produced by the input radio frequency RF signal. The distribution in velocities results in a density modulation further down the tube as the electrons are bunched into clusters when higher velocity electrons catch up with slower electrons in the drift tubes. Optimum bunching of electrons occurs in the output cavity. Large RF currents are generated in the cavity wall by the density- modulated beam, thereby generating an amplified RF output signal. The energy of the spent electron beam is dissipated as heat in the collector. The intermediate cavities are used to optimize the saturated power, gain, and bandwidth characteristics. Additional bunching of electrons is provided, yielding higher gain. The gain is typically 15 dB per cavity, so that a five-cavity klystron can provide a total gain of about 75 dB if synchronously tuned. However, by stagger tuning the individual cavities to slightly different frequencies, the bandwidth can be increased with a reduction in gain. A typical gain is on the order of 45 dB. For a cavity device like a klystron, the bandwidth is a fixed percentage of the frequency of operation. The bandwidth is proportional to the frequency and inversely proportional to the Q quality factor, which is defined as 2 π times the ratio of the energy stored and the average energy lost in one cycle. Thus at C-band 6 GHz, a typical bandwidth is 45 MHz. But at Ku- band 14 GHz the bandwidth is about 80 MHz. These bandwidths are well suited for C-band and Ku-band satellite transponders. By adding a sixth, filter cavity the KPA bandwidth can be doubled. Thus 80 MHz KPAs are also available at C-band. 2 Klystrons can be made with extended interaction circuits in one or more cavities that increase the bandwidth substantially. This technology can provide a bandwidth of 400 MHz at 30 GHz. Output powers up to 1 kW can also be achieved at different bandwidths. Although the bandwidth is relatively small, a conventional klystron can be mechanically tuned over a wide frequency range. A klystron can be capacitively or inductively tuned. All satcom klystrons are inductively tuned because of better efficiency and repeatability. The inductance is varied by moving a wall in the cavity sliding short. The output power of a KPA is about 3 kW at C-band and 2 kW at Ku-band. The lowest power KPA offered for commercial satellite communications is around 1 kW, although for certain applications powers under 1 kW are available. TWTA The traveling wave tube amplifier TWTA consists of the traveling wave tube TWT itself and the power supply. The TWT can have either a helix or coupled-cavity design. The TWT is a broadband device with a bandwidth capability of about an octave, which easily covers the 500 MHz bandwidth typical of satellites in the FSS. It also covers the typical 800 MHz DBS bandwidth requirement, as well as even broader bandwidths in Ka-band and higher bands. The TWT, like the klystron, is an example of a device based on modulating the flow of electrons in a linear beam, but differs from the klystron by the continuous interaction of the electrons with the RF field over the full length of the tube instead of within the gaps of a few resonant cavities. The TWT has a heritage of over half a century. The original concept was proposed in 1944 by Rudolf Kompfner, who investigated experimental laboratory microwave tubes while working for the British Admiralty during World War II. The first practical TWT was developed at the Bell Telephone Laboratories in 1945 by John Pierce and L.M. Field. Bell Labs was interested in the technology for its possible application to communication. By the early 1960s, the TWT was adapted for use in satellite power amplifiers in the Telstar program. In a TWT, amplification is attained by causing a high density electron beam to interact with an electromagnetic wave that travels along a slow-wave structure, which usually takes the form of a helical coil. A helix is the widest bandwidth structure available. The electrons are emitted from a heated cathode and are accelerated by a positive voltage applied to an aperture that forms the anode. The electrons are absorbed in a collector at the end of the tube. The RF signal is applied to the helix. Although the signal travels at nearly the speed of light, its phase velocity along the axis of the tube is much slower because of the longer path in the helix, as determined by the pitch and diameter of the coil, and is nearly equal to the velocity of the electrons. For example, if the electrons are accelerated by a 3,000 volt potential difference on the anode, the speed of the electrons is about one tenth the speed of light. Thus the length of the helix wire should be about ten times the axial length of the tube to bring about synchronism between the RF traveling wave and the electron beam. The electrons interact with the traveling wave and form clusters that replicate the RF waveform. Midway down the tube, an attenuator, called a sever, absorbs the RF signal and prevents feedback, which would result in self-oscillation. On the other side of the attenuator, the electromagnetic field of the electron clusters induces a waveform in the helix having the same time- dependence as the original signal but with much higher energy, resulting in amplification. The gain is typically on the order of 40 to 60 dB. The beam-forming optics are critical parts of the tube. To minimize heat dissipation caused by electrons striking the helix, the beam must be highly focused and the transmission from one end of the tube to the other must be close to 100 percent. When the electrons reach the end of the tube, they impact with the walls of the collector, where most of the heat is generated. The efficiency of the tube can be improved by applying a negative potential to the collector, which retards the electron beam as the electrons enter it. A collector designed to operate in this way is called a depressed collector. Less energy is converted to heat as the electron beam impinges on the collector, and consequently less energy is lost as thermal waste. However, the distribution of electron energies is not uniform. In a multi-stage depressed collector, high energy electrons are directed to stages with high retarding fields and low energy electrons are directed to stages with low retarding fields. This configuration improves the efficiency further, but with greater complexity. Another means of achieving greater efficiency is through improving beam synchronization. As the electrons travel along the tube and interact with the RF signal, they give up energy and lose velocity. Thus with an ordinary helix, they tend to fall behind the signal. This problem can be mitigated by brute force by increasing the accelerating potential but at the expense of degrading the TWT linearity. A more elegant method is through the use of a tapered helix, in which the pitch of the helix decreases along the tube. The signal velocity is thus retarded to compensate for beam slowing. The selection of optimum helix configurations has been made possible through advanced computer modeling techniques. Another type of TWT is a coupled- cavity device, used for high power applications. In this case a series of cavity sections are connected to form the slow- wave structure and is similar to the klystron in this respect. However, in the klystron the cavities are independent, while in the TWT the cavities are coupled by a slot in the wall of each cavity. The output power of a helix TWTA at C-band ranges from a few watts to about 3 kW, while power levels of 10 kW can be attained with coupled-cavity TWTAs. Helix TWTAs at Ku-band have less power, with a maximum power of around 700 W. Higher frequency TWTAs are also