Satellite Technologies
Satellite systems use microwave dishes aligned to orbital satellites that can either relay signals between sites directly or via another satellite.
- widespread use of satellite television receivers allows for Internet connectivity
Geostationary Orbital Satellite Internet Access
- Historically, data communications satellites were placed in high geostationary orbit
- one drawback is increased latency
- signal must travel over thousands of miles more than terrestrial connections
- e.g.,
- if accessing a web server over DSL has 10-20 ms round trip time (RTT) delay on the link
- accessing same site over satellite link could involve 600-800 ms RTT delay
- issue for real-time applications
- e.g., video conferencing, VoIP, multiplayer gaming
- one drawback is increased latency
- transfer rates vary between providers and access packages
- 2 or 6 Mbps up and 30 Mbps down are typical
Round Trip Time (RTT)
RTT is the two-way latency, or the time take for a probe to be sent and a response to be received.
Implementation
- To create a satellite Internet connection
- ISP installs a very small aperture terminal (VSAT) satellite dish antenna at the customer’s premises
- aligns it with the orbital satellite
- no need for any realignment
- Because the satellite does not move relative to the dish
- antenna is connected via coaxial cabling to a digital video broadcast satellite (DVB-S) modem
- ISP installs a very small aperture terminal (VSAT) satellite dish antenna at the customer’s premises
Low Earth Orbit Satellite Internet Access
A different type of service uses an array of satellites positioned in low Earth orbit (LEO).
- support better bandwidth
- around 70–100 Mbps
- are lower latency
- 100–200 ms RTT
- drawback
- satellites move relative to the surface of Earth
- customer’s premises antenna needs a motor to periodically align with the array
- dish construction uses a technology called phased array to:
- connect to different satellites as they pass overhead
- minimize the amount of mechanical realignment required
- must have a clear view of the whole sky
- satellites move relative to the surface of Earth
Global Positioning System
- Satellites are also used to implement the Global Positioning System (GPS)
- allows a device with a suitable sensor to triangulate its position using signals from orbital satellites
- triangulation process can be slow
- most smartphones and laptops use Assisted GPS (A-GPS)
- used to:
- obtain coordinates from the nearest cell tower
- adjust for the device’s position relative to the tower
- uses cellular data
- used to:
- most smartphones and laptops use Assisted GPS (A-GPS)
- GPS satellites are operated by US government
- some GPS sensor use signals from other satellites operated by
- EU (Galileo)
- Russia (GLONASS)
- China (BeiDou)
- some GPS sensor use signals from other satellites operated by
- GPS signals can be jammed or spoofed
- can be used to defeat geofencing