Space Navigation Systems

FIELDS OF STUDY: Aerospace Engineering; Space Technology

ABSTRACT: Many factors influence space travel and must be considered before spacecraft are launched. A global navigation satellite system (GNSS) is a network of artificial satellites (unmanned spacecraft that orbit celestial objects) that have been launched into orbit around Earth to help guide spacecraft (as well as provide location information to electronic devices such as smartphones). The global satellite system, which consists of the Global Positioning System (GPS) in the United States and GLONASS in Russia as well as other developing systems around the world, is used in conjunction with other technology by teams of scientists and engineers on the ground and by those aboard space vessels or by the vessels themselves.

Since ancient times, people have looked to the stars to help them navigate from place to place on Earth. In the twenty-first century, much technology has been developed to aid navigation both on Earth and in space. Teams of scientists, engineers, and astronauts use sophisticated networks of systems that include antennas, computers, radios, satellites, and more to navigate vessels through space.

In addition, mathematical calculations are used to determine any potential conditions that would affect the course of a spacecraft as it travels through space. Mathematical models account for things such as interferences, gravity, and solar radiation pressure (radiation from objects in space). Even the smallest of forces can greatly alter a spacecraft’s mapped path, or trajectory. Accurate mathematical models are crucial for accounting for every possibility that could occur while navigating.

On manned spacecraft, navigation is done both by astronauts on the craft and teams controlling navigation systems on the ground. For unmanned spacecraft, navigation happens solely on the ground. Timing is key for space navigation. Radio signals are sent back and forth between the spacecraft and Earth. On Earth, a team of scientists and engineers use these signals to track the location and speed of the spacecraft and make adjustments to the flight course as necessary. In space, astronauts are responsible for communicating with these teams on the ground, sometimes referred to as ground control. Some unmanned space vessels use software that enables them to navigate through space and transmit data such as images and trajectory information back to Earth, eliminating much of work on the part of ground control.

The National Aeronautics and Space Administration (NASA) Christopher C. Kraft, Jr. Mission Control Center, also known as Mission Control, Houston, is in charge of manned space missions and missions to the International Space Station (ISS). Since NASA’s Space Shuttle program ended in 2011, Mission Control focuses much of its attention on ISS missions. Teams at Mission Control check on crews aboard the ISS and monitor spacecraft to ensure everything operates as planned. The teams can respond to any issues that develop over the course of missions.

Teams at NASA’s Jet Propulsion Laboratory, which is located near Pasadena, California, are responsible for the direction of unmanned space voyages. Each team has specific roles in the navigation of spacecraft and can respond to any problems that arise. Before missions, the teams plot navigation courses, calculating the positions of space objects that could interfere with the spacecraft’s trajectory. During the mission, the teams use large dish antennas, radio transmissions, and images to determine the location of a spacecraft. They also ensure the spacecraft is on the correct trajectory. To overcome factors that interfere in space navigation, most spacecraft have onboard systems which use algorithms based on statistics and control theory. They correct spacecraft’s trajectories when there is a delay or issue in communicating with other navigation systems.

Obstacles to Navigating Space

Many obstacles exist that affect space navigation. First, everything in space is in motion, which affects a vessel moving through space. Earth rotates, and it orbits around the sun. Destinations such as planets, moons, asteroids, and stars are constantly moving. This motion affects calculating and planning missions.

Distance is another factor that affects navigation. Spacecraft must often travel far from Earth to reach targets that are of scientific interest. This distance is measured in astronomical units (AU); 1 AU is 150 million kilometers (93 million miles). Mars is only about 0.52 AU from Earth on average, but Jupiter is 4.2 AU away and Neptune 29.09 AU. Distance must be precisely calculated and considered when planning space missions, as it affects factors such as fuel use and communications.

While the development of advanced technology has helped to make communication to and from Earth easier, communicating while in space still has limitations. Spacecraft do not have much power available for radio communication. Most vessels have solar panels that pull energy from the sun. However, spacecraft that travel far from the sun cannot rely on its energy and therefore do not have as much power. This means any radio signals sent to Earth may take as long as a few hours to reach those on the ground.

The most important factor that affects space navigation is gravity. Gravity from the sun, planets, moons, and other celestial objects has a pull on the trajectory of a spacecraft. While it can be used to propel a spacecraft to save fuel, it must be calculated and timed perfectly to avoid potential problems.

Inertial Navigation Systems (INS)

Early spacecraft, such as those used in the Apollo missions in the 1960s, used inertial navigation systems (INS) to keep track of their location. INS uses accelerometers and gyroscopes, instruments that use the degree of inertia they display to a change in speed or direction to measure the amount of that change. A computer then uses these measurements to calculate the spacecraft’s location relative to a known starting point. These systems required very little computing power. The Apollo Navigation and Guidance System had less computing power than a twenty-first century pocket calculator.

In the twenty-first century, INS is still used in spacecraft, though often in combination with other forms of navigation, such as satellite navigation. INS is particularly useful for satellites in low Earth orbit, for which satellite navigation cannot be used.

People can use the stars to determine their position on Earth, but this system has its limits. It is not exact and depends on clear skies. Scientists instead developed satellites that can nearly pinpoint an object’s precise position. The satellites send signals to a receiver, which determines the receiver’s distance in relation to the satellite by measuring the time it takes for the signal to reach the receiver. The more satellites used, the better a position can be pinpointed.

Satellite navigation quickly became an important tool, especially in space. Scientists have placed navigational satellites in medium Earth orbit (MEO), which gave them stability and afforded exact orbit predictions. These systems, known as global navigation satellite systems (GNSS), relay their locations in space and time to networks of ground control stations, which have teams of receivers that calculate their exact positions. These systems are not always accurate, however, and signals can be influenced by a variety of factors, such as atmospheric changes and even tall buildings. GNSSs are not only used in space but also on the ground by drivers, law enforcement officials, energy response teams, telecommunication companies, surveyors, and more. These satellites can control air traffic, computer networks, and power grids.

As of 2020 there are four fully functional GNSSs: the Global Positioning System (GPS) of the United States, the GLObal NAvigation Satellite System (GLONASS) of Russia, BeiDou of China, and Galileo of Europe. GPS, originally known as Navstar Global Positioning System, has around thirty-two satellites in orbit, while GLONASS has twenty-four satellites in orbit. Galileo has around twenty-four and BeiDou has thirty-five. These satellites interact with a network of ground stations to provide information on position, time, and velocity anywhere in the world regardless of weather conditions.

Other satellite systems are in development and operate regionally, with plans to be globally operational in the future. These include India’s Indian Regional Navigation Satellite System (IRNSS) and Japan’s Quasi-Zenith Satellite System. Once these systems become fully functional, they will give access to signals from dozens more satellites around the globe.

In space, navigation is difficult because spacecraft need some reference point to guide and correct their trajectory. To overcome this, NASA has invented a space navigation technique known as Station Explorer for X-ray Timing and Navigation Technology (SEXTANT). It uses X-ray technology and pulsars as guide stars, which is similar to GPS technology in terms of its precision.

PRINCIPAL TERMS

  • astronomical unit: a unit of length equal to the average distance between the Earth and the sun, about 150 million kilometers (93 million miles); often used to measure distances within the solar system.
  • GPS: Global Positioning System, a system of US-owned navigation satellites in medium Earth orbit.
  • inertia: the resistance a physical object displays to a change in speed or direction.
  • satellite navigation: navigation, on Earth or in space, with the aid of satellites that pinpoint an object’s position using radio signals.
  • trajectory: the path a spacecraft follows in space.

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