page image

CHAPTER II - Spectrum Use-Cases

There is a variety of orbit types for satellites. Each type corresponds to certain performance characteristics which require differing regulatory regimes. The most important distinction is between geosynchronous orbit (GSO, also referred to as GEOs) and non-geosynchronous orbits (NGSO). Geosynchronous satellites appear fixed in the sky, as they move at the same angular velocity as the Earth and orbit parallel to the Earth’s rotation. One common subcategory of GSO satellites is geostationary satellites, which are parked above the equator. GSO satellites provide coverage over a fixed area of the Earth.

In order to maintain a stable orbit, the satellite must be at a relatively far distance from Earth. GSO satellites are roughly 36,000 kilometers (22,000 miles) from Earth’s surface—about a tenth of the distance to the moon. As the name implies, non-geo or NGSO satellites are simply not at the specific height needed to maintain the same rotational speed as the Earth without falling into the planet’s gravitational pull. Usually the NGSO orbits of interest are significantly closer to the Earth’s surface compared to GSO.

One category of NGSO satellites that is seeing particularly strong growth today is Low Earth Orbit or LEO satellites. LEOs orbit at about 2,000 kilometers (1,200 miles) or less. In between LEO and GSO orbits is the Medium Earth Orbit (MEO). There are a number of satellites in MEO orbit, perhaps the most important is the Global Positioning System, or GPS satellite constellation.

GSO and LEO satellites have different characteristics that result in strengths and weaknesses for different applications. When it comes to coverage, a single GSO can cover a very large area with constant service. LEO satellites require a constellation of multiple overlapping orbits to achieve the same coverage, and have increasing complexity of the devices on both ends and potentially resulting gaps in the area served if the constellation is not engineered to provide continuous coverage. GSO satellites appear fixed in the sky, so earth stations do not have to move to track the transmitting satellite, which is particularly important in higher frequency bands due to their narrow beam widths. On the other hand, LEO satellites require more expensive antenna systems to track the transmissions. The frequent handoffs between LEO satellites can result in variability in service that may not be suitable for some applications. GSOs can more easily coordinate spectrum use according to their specific location in orbit using static analyses. Their stability, large coverage and relative simplicity have made GSO satellites the traditional workhorse for many applications, and their orbital slots are highly sought after.

However, despite the advantages of GSOs, one of the main drawbacks of GSO satellites for communications purposes is the long delay it takes for information to travel the distance up to the high orbit and back. Jennifer Manner, Senior Vice President of Regulatory Affairs at EchoStar Corporation, noted that the high-orbit GEOs have about 600 milliseconds of latency. Many broadband applications are relatively insensitive to this level of latency, but for some applications it is a noticeable delay. Such a delay can be somewhat disruptive to interactive voice communications, for example. Other applications that depend on feedback loops on either end of the communication, such as some video games, are virtually unusable at such latencies. According to Professor Dennis Roberson, in some ways latency is “becoming the new digital divide.” Many of the newest innovations—such as augmented reality—depend on very low latency, to provide real-time feedback to the user.

The lower orbit of LEO satellites allows communications with a fraction of the latency of GSO satellites, leading to a great deal of excitement around potential LEO use for broadband and other communications purposes. Companies such as SpaceX, Kepler, Telesat, LeoSat and others have acquired new authorization from the FCC to provide satellite service within the United States with constellations of thousands of satellites.

 
 
Title Goes Here
Close [X]