Unlike rain fade which only persists during precipitation events in the atmosphere, operators of Low Earth Orbit (LEO) systems will find that many of their customers will not be able to receive 99.9 percent availability services due to the presence of objects on the Earth. These objects block the view to LEO satellites, which cross the sky at lower elevations below the height of mountains, towers, buildings, trees, light poles, cables, and other objects. I define the loss of signal due to these terrestrial obstructions as “Earth fade.”
One of the reasons why the Geostationary Orbit (GEO) was selected by satellite communications pioneers is because in this orbit, satellites do not move relative to the fixed earth station antennas on the Earth. This allows antennas on the ground to be simple and without tracking systems, since the satellite positions do not change.
However, in LEO, providing uninterrupted services will become significantly more complicated due to Earth fade. Because the Earth rotates as the satellites revolve around in their respective orbits, the elevation and look angle will constantly vary to each user as the satellites rise and fall across the visible sky. Since the position of each ground-serving LEO satellite can be totally random from the perspective of the user terminal, users would have to make sure that their LEO network has at least one satellite above the tallest object near the antenna, in order to receive uninterrupted services. I define this as the Minimum Clearance Elevation (MCE).
Relatively flat regions (such as plains and hills) with no tall towers or buildings would require low MCE. These places would have no problems receiving signals from most LEO systems. However, in suburban areas with many tall buildings, mountainous regions, or people living in areas surrounded by tall trees, they would require a relatively high MCE.
For LEO satellite constellations, the number of satellites, their operating altitudes, and number of orbit planes will determine if the MCE required by each user terminal will avoid Earth fade. Constellations with hundreds of satellites operating in few orbit planes (sparse constellations) would provide lower MCEs, while systems operating thousands of satellites in multiple orbit planes (dense constellations) would provide Higher MCEs, and would thereby provide higher availability of service to greater number of end users.
The operators of LEO constellations will need to calculate the MCE required for each installation based on observing the physical environment of end user installations and compare it against their system characteristics, to determine if Earth fade can be avoided. If it is the latter, the Service Level Assurance (SLA) will be affected and the end users must agree to a form of Earth fade interruption of services. Even if the LEO constellation can mitigate Earth fade due to alternate satellites present in the user’s field of view, user traffic control gateways will require sophisticated algorithms to reroute user traffic to alternate satellites when signals are interrupted. LEO systems with intersatellite links and user terminals that are capable of communicating with multiple satellites should be able to recognize when their user terminals have experienced Earth fade. This systems will better manage the service hand-offs to alternate satellites than the systems without crosslinks.
Earth fade is a pressing issue for LEO systems. To mitigate its effects, LEO systems will require thousands (if not tens of thousands) of cross-linked satellites. This will further add to its complexity and cost. Until then, it seems clear that people who are living in mountainous or urban areas will find it difficult to receive reliable communications services via LEO broadband satellite networks. VS