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Sci Tech    H4'ed 6/20/21

Satellite Constellations to dominate the Stars: PART I

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Letter #25 Satellite Constellations to dominate the Stars
PART I: to whom does it make sense?

A falcon 9 rocket can launch 60 Starlink satellites at a time.
Photo credit: Bill Jelen

by Miguel Coma

The sky: a part of divinity and life

We humans have always looked to the sky to find peace of mind or connect to something larger than ourselves. Crucial elements required to create life can be found inside the furnace of stars: we are stardust. Since life began on Earth, living creatures have evolved in relation to electro-magnetic fields emitted by the Sun, the Earth and lightning.

The atmosphere: our tiny habitat

The Earth's atmosphere helps create the right temperature, pressure and chemical conditions for life. It also protects us from the sun's harmful ultraviolet emissions. If Earth was the size of an apple, the atmosphere would be 1/20th of the apple's skin. Space begins at the Ka'rma'n line, only 100 km (62 miles) above sea level. If you ever cross deserted land, the closest human beings might be astronauts in the International Space Station, passing 420 km above your head.

Constellations of satellites

A satellite is an object that follows an orbit (path) around a planet or a star. The Moon is the Earth's only natural satellite. Since the USSR launched Sputnik-1 in 1957, artificial satellites have orbited the Earth. They offer services in telecommunications (voice, data, radio, television), navigation (such as GPS), scientific monitoring of the Earth (weather, climate, life), astronomy, map making, military and intelligence. A satellite constellation uses multiple satellites to cover larger areas on the ground. The further from the Earth, the wider the area that the satellite can cover. Yet, most constellations use Low Earth Orbits (LEO, at altitudes of 160 km - 2,000 km); some use Medium Earth Orbits (MEO, at 2,000 km - 36,000 km). Geostationary Orbit (GEO) satellites keep a stationary position 36,000 km above the same spot on Earth. A TV satellite dish always points to the same GEO broadcasting satellite. The most recent Internet service constellations use LEO orbits.

Satellite constellations can have notable differences. Let's look at two extremes: O3B, the MEO constellation built by the French Thales Group and Starlink, the LEO constellation built by Elon Musk's SpaceX. Starlink offers a better latency (20 ms) than O3B (125 ms). (Latency is the time used to transport data.) But low latency with LEO comes with a high price: each satellite can only service a small part of the Earth's surface. O3B covers most of the Earth's inhabited ground and sea areas with just 20 satelliteswhile Starlink's plans include 42,000 satellites!

To connect to a satellite constellation, every user needs a device, such as a satellite phone (i.e. Globalstar, Iridium) or a satellite dish (O3B, Starlink, OneWeb). Lynk's LEO constellation promises text messaging using regular, unmodified mobile phones.

How do they work?

Each constellation is different. To connect to the Internet, Starlink users need their own satellite dish. O3B and OneWeb customers can use their smartphone or laptop on a satellite-connected cruise ship, offshore oil platform or via a cell phone antenna. The dish and satellite contain hundreds of tiny antennas called a phased array or a massive Multiple Input-Multiple Output antenna (mMIMO). They produce concentrated beams of radiofrequencies (microwave radiation) steered in the users' direction. MEO O3B satellites beams have a diameter of 700 km on the ground, while LEO Starlink beams might be just 14 km.

For long distance communication (i.e. a Zoom call between London and Chicago, or a text message between Hong Kong and Mexico City), satellites must communicate with each other. Inter-Satellite Links (ISL) using direct laser beams are ideal, but still experimental. Satellites can also interconnect through ground stations using radiofrequency signals (microwave radiation) between 12 gigahertz (GHz) and 75 GHz. These frequencies are higher than those used for Wi-Fi or 4G cell phones in order to allow a wider "data pipe:" they provide more spectrum and bandwidth and can transport data faster.

Is this 5G?

Telecommunication satellite constellations are often associated with terrestrial 5G mobile networks. Indeed, they share the same technology: satellite constellations and terrestrial 5G mobile networks use millimeter wave frequencies and massive MIMO beamforming antennas. However, a mobile network primarily uses fibre optic cables to connect its cell towers. (The connection from cell towers to end users are wireless.) Only rural areas that lack high-speed optical fibre might benefit from satellites to connect their cell towers. While satellites and 5G share technologies, there is no common definition of "5G"so questions about 5G will always be debatable.

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Katie Singer writes about nature and technology in Letters to Greta. She spoke about the Internet's footprint in 2018, at the United Nations' Forum on Science, Technology & Innovation, and, in 2019, on a panel with the climatologist Dr. (more...)
 

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