Satellite Communication

Satellite Communication (SATCOM)

• True global connectivity
• Cellular is dependent on someone building a cell tower near the area you want to communicate on
  • Remote areas are out of luck!
• Satellites are like moving cell towers

SATCOM Challenges

• Longer distances lead to path loss, and thus extra latency (LEO is 160-2000km from ground)
• Large deployment areas, means we share a lot of bandwidth. There are many handoffs.
• Deployment is expensive and coordination is difficult.
• Making a satellite is difficult!
  • Radiation in space is a lot higher (no ozone to protect it).
  • Need to use older technology

Orbits

Low Earth Orbit (LEO)

• 160-2000km
• Includes all current human spaceflight. ISS is at 400 km.
• Roughly 90 min for complete orbit
• Last about 5 years (fuel limitations)

Geostationary Orbit (GEO)

• 35768 km
  • bandwidth is lower than LEO, also larger service area = more shared bandwidth
• Exactly 24 hours per complete orbit
• Result: fixed location in the sky over a portion on Earth
  • very few satellites can cover all of earth
  • operator can choose to only service a specific region
• Last about 20 years

Medium-Earth Orbit (MEO)

• roughly between LEO and GEO
• roughly 12 hours per complete orbit
• GNSS satellites are here (GPS, Galileo, etc.)
  • smaller constellation, longer lifetime, signal is a little stronger
• radiation belts make this area more difficult to use

Path Loss and Latency to Orbit

• Distance contributes significantly to signal strength loss
• Need higher frequency. But higher frequency $⟹$ smaller antenna $⟹$ less energy connected $⟹$ weaker signal. This is part of the same signal-budget story discussed in Signal Qualities.
• Being at an angle on the horizon increases total distance and path loss.
• Speed of light gives
  • LEO: $\approx 30-50$ ms
  • MEO: $\approx 125-325$ ms
  • GEO: $\approx 600-800$ ms
• Huge coverage areas share bandwidth among many users
  • Cellular ideas can apply here by providing “cells of coverage” on the ground.
• Moving satellites lead to many handoffs. They move at around $7$ km/s.

Compared with Wide Area Networks, satellite systems push coverage much farther but pay for it with more latency, more shared bandwidth, and higher deployment complexity.

Cost Considerations

• Costs have dropped a lot in the last few years
• It is about $1000 per kg.
• Starlink v1.0: 260 kg, Starlink v2.0: 1250 kg, GPS: 1000-2000 kg