14-760: Adv. Real World NetWorKs

1. 14-760: Adv. Real World NetWorKs Lecture 7 * Satellite Communication * Spring 2019 (Kesden)

2. Why Do We Need Satellites? Terrestrial Radio Orbital Radio

3. Orbits: Low Earth Orbit (LEO) • 1200 miles or less above surface • Approximately 90 minutes to orbit Earth • Less energy to launch • Less energy to transmit • Higher resolution scanning of earth • Higher data rates and lower latency, generally • Move quickly across earth’s surface, “covering a lot of ground” quickly. • Applications • All (human occupied) space stations • Most observational satellites, e.g. photographing, spying, etc • Constellation-based communication systems, e.g. Iridium • Upside: Reach everywhere • Downside: Many hand-offs

4. Orbits: Medium Earth Orbit (MEO) • 1200 to 22,236 miles above surface • 2 – 8 hours to orbit earth • Larger coverage areas, lower resolution/throughput, higher latency • Higher cost • Applications • Same basic uses as LEO • Except – not used for (human occupied) space stations

5. Orbits: Geostationary Orbit • 22,236 miles above Earth’s surface, specifically the equator • Rotates along with Earth, maintaining constant relative position • Can’t be seen near poles • Area of coverage is constant • Antennas don’t even need to change angle to maintain contact • Less stable orbit (asymmetry of earth causes longitudinal drift) • Fuel required to maintain orbit • Fixed number of satellite slots • One ring • Minimum spacing between satellites • Best application • Latency tolerant communication that can’t be interrupted • 1/8 second each way, 1/4 secondround trip – not so good for humans • Non-interactive data don’t care

6. Orbits: Molniya • Used mainly by Russians • Communication in Russian and former Soviet areas • Spying on US/Canada, esp. early warning of missile launches • Orbit is about 12 hours • 6 – 9 hours of use per every other revolution • The other revolution it is around wrong pole • 3 satellites provide 24 hour coverage https://en.wikipedia.org/wiki/Molniya_orbit

7. Satellite Modes and Multiplexing • “Bent pipe” • Transmissions come from Earth to satellite and are broadcast back down • Addressed communication or some form of multiplexing • Requires a ground station in each satellite coverage area, possibly with ground relaying • Intersatellite links • Novel with Iridium • Satellites relay messages through space to ground station(s)

8. Satellite based telephone: A Perspective • 1856 first trans-Atlantic telegraph line • 1866 – the first one that worked for more than a month • 1956 TAT-1, first trans-Atlantic telephone line came into service • 36 channels a 4khz, later reallocated to 48 channels at 3khz • 1969 Intelsat, global organization promoting and organizing satellite usage, had enough coverage to connect the globe • 1978 TAT-1 retired, but by then there were plenty, plenty more • Satellite-based long distance overlaps cable-based long distance. • Greater latency means lower quality • Reaches places cables didn’t or don’t.

9. Satellite Phones Today • Used for global coverage • Remote areas • Example: Iridium • Low earth orbit • Relatively low call quality • Interruptions due to satellite movement • But, satellite will come to you – even if restricted view of the sky from terrain, etc • Most other providers • Geostationary • Often higher call quality and/or data rate • Not truly global, depends on satellites • Line-of-site can be a problem in some terrain

10. Satellite-based broadcast • Very natural for many-to-one communications • Television, e.g. DirectTV • Radio, e.g. SiriusXM

11. Satellite Based Radio • SiriusXM • Geostationary satellites • 2x XM • 2x Sirius • 2 spare

12. Satellite TV • 1960s - mid-1970s: Experiments, small deployments • 1970s: Networks used satellites to distribute their content from each other • 1976: Taylor Howard became world’s first telephone pirate • Built a receiving dish to get HBO in his back yard. • Tried to pay HBO, but they didn’t want to deal with individuals • He wrote a book describing how, and stealing satellite TV became “a thing” until providers scrambled it. • 1980s: Distributors began scrambling content and eventually, under regulatory pressure, agreed to sell descramblers and subscriptions – but they sure weren’t cheap • By 1986: Distributors scrambling signals was putting satellite folks out of work • John MacDougall lost his business installing and servicing satellites and was working as a part-time operator at a satellite company • He pointed their satellite at the one carrying HBO’s content, over-rode their signal, and delivered a message for 90 seconds • By the 1990s, most satellite TV, e.g. Dish, was digital, not analog

13. Iridium Example • Constellation of satellites in low earth orbit • 77 were originally planned, Iridium element has an atomic number of 77 • Only 66 proved necessary in practice. Current constellation is 66 + spares • 100 minute orbit • Low orbit means low latency and low power, esp. for handsets. http://www.icao.int/anb/panels/acp/wg/m/iridium_swg/ird-08/ird-swg08-ip05 - ams(r)s manual part ii v4.0.pdf (Figure 2-2, Page 5)

14. Iridium Example • 3 phase beam antennas produce 48 spot beams on earth • 48-beam configuration provides a 4,700 km radius • When satellites “cramp” due to orbit, outer rings are deactivated. • 4 crosslink antennas to route traffic through constellation • This is magic. Only one grounds station needed • Others for backup, US military, etc. • Each satellites supports up to 1100 phones calls at 2400bps. http://www.icao.int/anb/panels/acp/wg/m/iridium_swg/ird-08/ird-swg08-ip05 - ams(r)s manual part ii v4.0.pdf (Figure 2-3, Page 6)

15. Iridium: Intersatellite Links • Relay messages along “intersatellite links (ISL)” from one satellite to another • Routing among satellites is a complex, dynamic shortest-path like problem • To my knowledge, algorithm has never been published • Very much a special sauce, and likely a secret one. http://www.kt.agh.edu.pl/~brus/satelity/Iridium-Leo.pdf

16. Iridium: Completing Calls • Handset turns on and transmits “Ready to receive” • Routing across inter-satellite links as needed and down to subscriber’s home Earth gateway • Now home gateway knows about handset’s location, etc. • All call setup goes to/from a user, including satellite to satellite go through home gateway • On hook, Off hook, ringing, forwarding, etc • Satellite-to-satellite call data need not pass through Earth

17. Iridium: Motion • Satellites move quickly • About 16655 Mph • Users are relatively stationary – even if moving in an airplane • Commercial plane is about 3% of satellite speed • Orbit of each satellite is about 100 minutes • Constellation is the same over Earth about every 1,440 minutes • Satellites are in approximately the same position and direction over sky every 24 hrs • Worse case for 1 satellite failure is 37 minute outage/24 hrs

18. Iridium: Frequency Division • 1616 – 1625.5Mhz Frequencies • 10.5Mhz bandwidth • 240 channels @ 41.67Khz each • 500 Khz of guard bandwidth, ~ 2k between each pair of channels http://www.kt.agh.edu.pl/~brus/satelity/Iridium-Leo.pdf

19. Iridium: Frequency Division • Frequency re-use factor of 12 • 20 channels per cluster • 240 channels / 12 cells/cluster = 20 channels/cluster http://www.kt.agh.edu.pl/~brus/satelity/Iridium-Leo.pdf

20. Iridium: Time Division • Time Division Multiple Access (TDMA) • 90ms frame • 4 full-duplex channels • 50kbs • Voice is 4800bps • 2400bps each way • 4800 b/s * 90ms = 432 bits/user/time slot • 432 bits/sec / 50 kb/s = 8.64 ms • 8 simultaneous users • 8 users * 8.64 ms = 69.12 ms for user data frames • 20.88ms for framing, etc. http://www.kt.agh.edu.pl/~brus/satelity/Iridium-Leo.pdf

21. Iridium: Capacity • 3,168 users per satellite • 48 cells/satellite * 66 satellites • Reduced to 2,150 due to overlapping satellites • 2,150 users/satellite • 80 users/cell • 20 frequencies / cell • 8 users/ frequency • 172,000 simultaneous users • Maximum theoretical

22. Iridium Next • Launched over last 2 years • SpaceX did the launches • 12 satellites, 12 months (Wow!) • Satellite construction < 5 weeks/satellite (Wow!) • 66 satellites + 6 orbit spares + 9 ground spares • Same basic architecture • Backward compatible • Additional frequency space • Speeds up to 1.4Mbps