The closer proximity to the Earth also greatly reduces signal delay from a LEO satellite to ground stations and allows for smaller receivers on the ground. While minimizing signal delay is not vital for DTH services or corporate networks, it makes the orbit ideal for voice traffic being sent directly to handheld devices. These lower orbits are challenging since the satellites constantly move in and out of view of individual ground receivers. If it is necessary to maintain a continuous link despite the movement of the satellites, a fleet of spacecraft is required to form a constellation.
Telecommunications technology has made the world a much more intimate place. This is in no small part due to satellites circling the globe providing communications, scientific research, broadcasting, navigation, imagery, and support for national defense efforts. The first satellite, Sputnik, was launched by the Soviet Union in 1957 and served to demonstrate that man-made objects can reach and maintain a simple orbit. This small craft with limited instrumentation did little more than measure the density of the upper atmosphere and provide information on how radio waves propagate through the ionosphere.
In March 2011, the U.S. Air Force launched the ## test flight of the top secret X-37B space plane. Originally scheduled to land after ## days, the mission was extended and the spacecraft was still in orbit at the end of 2011. The 2010 maiden flight of the X-37B lasted for ## days. The military has not divulged specifics about the space plane’s cargo or mission, but it is speculated that it carries advanced Air Force experiments, sensors, and other research payloads. Some space technology experts believe the X-37B is a reconnaissance tool, given its ability to land, change payloads, and alter its orbit more rapidly than a LEO satellite.
The national defense needs of many countries help to drive demand for satellite capacity. Armed forces and international peacekeeping organizations from across the globe lease capacity from various commercial satellite operators or acquire Earth imagery services from commercial companies. Military forces are also building more of their own dedicated satellites due to increased capacity demands and the need for secure connectivity for deployed troops.
Earth observation satellites produce the largest amounts of data to be transmitted to Earth on a regular basis and thus require dedicated data processing ground stations. While all satellites require ground stations to keep track of them and relay commands, Earth observation satellites are specifically intended to gather large amounts of data through a variety of sensors and then transmit that data back down for interpretation and storage.
An unmanned U.S. Air Force space plane, the X-37B, was launched in April 2010 aboard an Atlas V rocket. The X-37B remained in orbit for ## days, testing its capabilities and conducting a variety of experiments on behalf of the Air Force. Some international observers expressed concerns that the secrecy shrouding this vehicle could be interpreted by other nations as evidence that the U.S. was developing a space-based weapon. Other space technology experts believe the most likely mission of the X-37B is reconnaissance, given its ability to land, change payloads, and alter its orbit more rapidly than a LEO satellite.
One classification of satellite is based not just on the spacecraft’s capabilities. Military satellites are generally characterized by the end users they are built to serve, not the type of service provided. Although they may perform the same functions as their non-defense counterparts, such as communication or remote sensing, they are instead operated by national intelligence or defense personnel. Armed forces from across the globe also rely on leased capacity from commercial satellite operators.
The closer proximity to the Earth also greatly reduces signal delay from a LEO satellite to ground stations and allows for smaller receivers on the ground. While these attributes are beneficial, these lower orbits are challenging in that these satellites constantly move in and out of view of individual ground receivers. If it is necessary to maintain a continuous link, a fleet of spacecraft is required to form what is called a satellite constellation. LEO is home to communications constellations belonging to mobile satellite services companies such as Iridium and Globalstar.
Most modern satellites are specialized machines designed typically to serve a single specific mission, such as communications, remote sensing, scientific observation, or navigation. While the general trend over the past several decades has been to make larger and more powerful spacecraft, there has also been a growing interest in launching extremely small objects, often measuring no more than 10 centimeters (4 inches) on a side. Such spacecraft, called cubesats, have been developed by many universities and other organizations for scientific experimentation and technology development.
An essential element of space infrastructure, ground stations transmit commands to and receive data from spacecraft. They also often contain facilities to process that data, particularly in the case of Earth observation satellites. The data sent from ground stations includes command and control data, software upgrades, and other mission-critical instructions. Satellites send information such as tracking and telemetry data in addition to imagery and scientific observations.