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How Do Satellites Work?

Satellites stay in orbit due to the balance between gravitational pull and centrifugal force.

Satellite Orbits

  • LEO: Low Earth Orbit
    • Earth Observation, Weather Forecasting, Geographic Area Surveying, Satellite Phone Calls
    • 160-2000km → 1.5hrs Orbital Period
  • MEO: Medium Earth Orbit
    • GPS
    • 20200km → 12hrs
    • 24 satellites in MEO can cover the entire Earth
  • GEO: Geosynchronous Earth Orbit
    • Broadcasting
    • 35786km → 23hrs 56min Orbital Period (same rotational speed as Earth)
    • GEO Satellites can cover ⅓rd of the Earth’s surface → 3 satellites to cover the entire Earth
    • Geo-Stationary Belt: Concentric to the equator, remain stationary with respect to the Earth
      • Satellites here are ideal for television broadcasting since the angle of the satellite dish does not need to be adjusted
      • The GSB is very crowded with Satellites and managed by ITU
  • Van Allen Belt: a dangerous region between LEO and MEO where particles are highly charged which can damage satellite electronics
  • Orbits by Inclination:
    • Equatorial Orbit: parallel to the earth
    • Inclined Orbit: some degrees (between 0-180) above or below the equator
    • Polar Orbit: perpendicular to the equatorial orbit
  • Orbits by Shape:
    • Circular Orbit: eccentricity (how much a conic shape deviates from perfectly circular) of 0
      • Ex. GEO, Equatorial, Polar
    • Elliptical Orbit: eccentricity of between 0 and 1. Sometimes they are closer to Earth while other times they are farther (closest/farthest point = Perigee/Apogee)
      • Ex. Highly Elliptical Orbit

Components of Communication Satellites

  • Transponders: change the frequency of the received signal, filter out any signal noise, and amplify the signal power
  • Batteries and Solar Panels: solar panels are usually used for energy generation but during eclipse times batteries are used
    • Sun Sensor: helps angle the solar panels in the right direction
  • Reflector Antenna: receives signals from Earth stations
  • Thruster: used to fix deviations from intended orbit (resulting from non-uniform distribution of Earth’s mass and the gravitational pull from Sun/Moon) as well as to avoid space junk
    • Fuel Tank: stores fuel for the thruster in the body of the satellite
  • Multi-Layer Films: protects the components from temperatures varying from -150 to 200℃ and radiation from the sun
  • Ground Stations: not on the satellites. Stations on Earth perform Uplink (sending microwave signals up) and Downlink (receiving microwave signals back)

GPS Satellite

  • Atomic Clock: measures time by monitoring the vibration of atoms
  • L Band Antenna: a very efficient, small, low bandwidth operating antenna

Earth Observation Satellite

  • Sensors, Cameras, other case-specific equipment, etc

Satellite Retirement

  • When a satellite is no longer functional thrusters are fired to send the satellite to the Graveyard Orbit
  • Thrusters firing horizontally → satellite rotational speed accelerates → more centrifugal force → satellite travels a few kilometers above the GEO

Case Study: Starlink

  • May 24th, 2019: the first cohort of 60 satellites sent up via Falcon 9 (12,000 planned over the decade)
  • Goal: provide high-quality broadband internet to regions before-unserved and low-latency connectivity to already established regions
    • Consumers only need to buy a pizza-box sized antenna at $200 (developed in-house by SpaceX) to be able to access the Starlink network
    • Current market receivers are around $30,000
  • Krypton Thrusters: an ion thruster that autonomously avoids space debris and deorbits
    • Used to raise Starlink satellites from initial orbit (440km) to final orbit (550km)
  • Control Momentum Gyroscopes: navigate the satellite while in orbit
  • Each Starlink satellite costs $300,000. At scale, Starlink will generate $30-50 billion/year
  • Information is transmitted from Satellite to Satellite in a vacuum via light (light travels 47% slower in glass than in a vacuum)
  • All satellites eventually are connected via lasers allowing a smooth transfer of information in the Starlink net (K-th Nearest Neighbor model)
Koko Xu

Lover of international cuisines and Class B chess player.