Low Earth Orbit Communications
Satellites provide a range of services: from broadcast television, phone, and broadband internet services, to Earth and weather observations. Remote sensing satellites that take pictures of the Earth help advance scientific goals, track natural disasters like forest fires, and support many other tasks. GPS satellites provide precise positioning, navigation, and timing information to civilians and the military. From a military perspective, satellites also provide secure communications, missile warnings, and intelligence.
Low Earth orbit (often known as LEO) encompasses Earth-centered orbits with an altitude of 2,000 km (1,200 mi) or less. Low Earth orbit is considered the area in Earth orbit near enough to Earth for convenient transportation, communication, observation, and resupply. This is the area where the International Space Station currently orbits and where many proposed future platforms will be located.
The second most populated orbital location is geosynchronous Earth orbit (GEO), around 36,000 kilometers (22,320 miles) from Earth’s surface. Satellites in geosynchronous Earth orbit have speeds that match the rotation of the Earth, so they are able to orbit above the same longitude at all times. These satellites provide many of the same services as LEO satellites, trading precise image resolution for wider fields of view. Lastly, in between these orbital positions (i.e., from 2,000 to about 36,000 kilometers above the Earth’s surface, or 1,240 to 22,320 miles) is medium Earth orbit, which contains the fewest satellites. The GPS satellite constellation orbits in medium Earth orbit.
The most well-known are two types of Very Eliptical Orbits [VEOs] are “Molniya” type and “Tundra” type orbits. The first orbit is a 12-hour orbit with an apogee altitude of about 40 thousand km and an inclination of about 63 degrees. – was widely used in the USSR by the Molniya satellites, which is where it got its name.
In GEO systems the cost of traffic is lower, but subscriber terminals are very expensive; in low-orbit systems, on the contrary, the terminals are more accessible, but the cost of traffic is an order of magnitude higher. With high-orbit systems, both existing and those being designed for the future, access to the satellite is possible only through an earth station with a narrowly directed antenna, preferably a “tracking” antenna,
Several factors contribute to the success of LEO satellites communication. The lower orbital altitude produces lower signal propagation delays , which is beneficial in the case of internet access or other communications services (and is critical for adequate voice quality of service), requires lower transmit power from the handset to the satellite, and reduction in satellite deployment costs due to multiple satellites deployed per orbital launch. This shorter lag comes at the cost of a smaller field of view and faster orbital speed, such that a greater number of satellites is needed to cover the same service area. These factors combined with advancements in satellite design and production are enabling LEO communication to become a reality.
LEO satellites can be separated into three classes, Little LEOs, Big LEOs and Broadband LEOs. Little LEO systems are those which support non-voice services such as messaging, e-mail, and remote monitoring. Systems that fall into this class are Orbcomm and Leo One. Big LEO systems consist of larger non-geostationary satellites and are adapted primarily for voice but also provide data services. Examples of such systems are Iridium and Globalstar. Broadband LEO systems provide high speed data, video conferencing, and other high speed applications. Teledesic and SkyBridge are examples of such systems.
A new generation of communications satellites operating in the low earth orbit (LEO) provides cellular-like telephone and broadband data services anywhere in the world. Under development for the past decade, these satellites are now being deployed in large numbers. A number of competing LEO satellite systems plan to blanket the globe from a combined constellation of over 500 satellites by 2004. The new satellite systems amalgamate some of the attributes of cellular telephone systems with those of traditional satellite communication networks to provide users with seamless global communications. The network is divided into two elements, the satellite constellation and the terrestrial segment, where interconnection between the two is provided via terrestrially-based gateway stations that are strategically positioned within each particular network. Voice communications are provided via hand-held telephones where users will be able to place or receive calls by way of the local terrestrial service provider or, in the absence of such service, directly to the satellite. Data services will also be accessible via hand-held devices or computers.
There were almost 5,500 active satellites in orbit as of spring 2022 [mosty American], and one estimate predicts the launch of an additional 58,000 by 2030. Large constellations of satellites in low Earth orbit are the primary drivers of the increase. Satellites provide important services, but there are potential environmental and other effects that this trend could produce.
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