Satellites

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

Earth Observation Satellite

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)