Aug 7, 2023
LOW-EARTH ORBIT (LEO) – Understanding Network and Security for Far-Edge Computing
LEO satellites are positioned in orbit around the Earth at an altitude of up to 2,000 kilometers (1,200 miles). Because of this, they are in constant motion relative to an observer.
LEO satellites are known for their ability to provide coverage over a large area of the Earth’s surface since they orbit the Earth relatively quickly (compared to GEO satellites). This allows them to provide communication and other services to a large number of users, as well as to track the movement of objects on the surface of the Earth.
The primary technical advantage of LEO-based SATCOM systems is their much lower latency than GEO (as low as ~20ms RTT). The main disadvantage is caused by the fact that they are in constant motion concerning any given point on the ground. They must use mechanisms such as motorized tracking antennas (or complex phased-array antennas) and constellations of a sufficient size to ensure users on the ground can always reach at least one satellite.
Here are some examples of LEO-based SATCOM services:
Certus 700: An L-band service from Iridium that supports speeds as high as 704 Kbps. It is served by 66 cross-linked satellites in LEO.
Starlink Roam: A Ka/Ku-band service from Starlink that supports speeds up to 200 Mbps. It is served by over 3,50020 cross-linked satellites in LEO, with plans to grow to as many as 12,000.
Global Navigation Satellite System (GNSS)
GNSS is an overarching term that includes all of the systems that use timing signals from satellite constellations to determine a position on the ground for navigation purposes.
GNSS for positioning
Trilateration
All satellite-based navigation systems discussed in this section determine a terminal’s position using trilateration. Unlike triangulation, it measures distance – not angles. Satellites in these systems repeatedly broadcast their current position and local time, derived from multiple onboard atomic clocks.
The following figure demonstrates a point on the ground receiving the same broadcast from four satellites:
Figure 3.42 – Trilateration using four satellites
From these four pieces of data, a terminal can calculate its position within a margin of error that varies from centimeters to hundreds of meters, depending on the circumstances.
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