For centuries mariners have looked to the stars to cross the oceans. Mariners at sea carried instruments such as cross staffs and sextants to determine the position of celestial objects in the sky. When combined with an accurate time reference, this would allow a marine navigator to determine the position of a ship on the Earth’s surface. Similar techniques updated with modern technology can be used in space. Early human missions to the Moon even carried sextants and celestial charts that could be used to determine position.
The art and science of space navigation have continued to be employed to ensure that spacecraft arrive at their destinations. Special tools and techniques have been developed to find the way across the vast distances of the solar system. Early flyby missions needed only to arrive in the correct vicinity, but more recent missions orbiting other planets required highly precise trajectories to be successful. Performing this type of navigation requires a network of Earth-based communication stations, accurate time keeping equipment, and many complex calculations.
Probes in the first generation of planetary spacecraft were often equipped with star trackers to maintain attitude and keep antennas pointed at the Earth. Star trackers are small optical devices that maintain a lock on a bright star. The most common star used for this purpose was Canopus, visible in the southern hemisphere. It is the second brightest star in the sky after Sirius, although Sirius is not well-suited for tracking because it is a binary star. The star tracker would signal if a spacecraft drifted away from its correct alignment, and thrusters would be fired to correct the position.
As a spacecraft approaches its intended destination, other kinds of optical techniques are used to refine its position. Cameras on the spacecraft are used to obtain images of nearby planetary bodies with stars in the background. Because the positions of the stars are well known, these images can be used to determine the position of the spacecraft relative to the planetary body. In this way the position of the spacecraft can be determined at higher accuracy than is possible from Earth.
Deep Space Network
Since the early days of space exploration a network of communications stations has been required to track and navigate planetary spacecraft. Goldstone, the first US station for this purpose, became operational in 1958 in the Mojave Desert in California to support the Pioneer missions to the Moon. Stations in Australia, South Africa, and Spain were later added. This array of facilities, known as the Deep Space Network, today consists of three globally distributed stations to enable communication with missions at all times. The Deep Space Network is managed by the Jet Propulsion Laboratory for NASA. Most data from planetary missions are returned through this network, and different missions must share the system.
Each station in the Deep Space Network is equipped with at least four dish antennas used to transmit and receive data from operational spacecraft. The largest antenna at each station is a 70 meter diameter dish. Others are about 30 meters in diameter. Large dishes are needed to concentrate weak signals from distant spacecraft.
Tracking a distant spacecraft from Earth requires predictions to be made of its position. Communications are not maintained around the clock. Instead, the Deep Space Network re-acquires a spacecraft at specified times. The spacecraft continues to move between communication sessions, so a prediction must be made to point the antenna at the correct point in the sky. Thousands of measurements of the spacecraft’s distance and velocity are made. These are used to update a model predicting the movement of the spacecraft.
One of the first tasks required in space navigation is determining velocity. The speed at which a spacecraft moves can be found by measuring the Doppler shift of its radio transmissions. All spacecraft are designed to transmit at certain radio frequencies. On Earth, the received signal is slightly shifted to a lower frequency because the spacecraft is moving away. This is the same process that causes light from stars and distant galaxies to be “red-shifted”. Measuring the amount of Doppler shift enables engineers to determine the rate at which the spacecraft is receding.
The range, or distance, of a spacecraft must also be determined. This can also be accomplished using radio transmissions. The amount of time for a signal to travel between Earth, the probe, and back can be used to determine the distance because the speed of light is precisely known. The amount of time required for the signal to be processed on the spacecraft must also be known. This is done by testing engineering copies of the spacecraft radio equipment used for testing. Movements of the Earth between the time of transmission and when the signal was received must also be known.
The angular position of the spacecraft must also be determined. This is a measure of the probe’s location as seen from Earth, and along with range and velocity allows the position and movement to be calculated in three dimensions. Angular position can be crudely measured by using pointing information of an Earth-based antenna. More precise measurements are possible by using transmissions received simultaneously at two different Earth-based stations.
Timing is Everything
All the space navigation techniques have one important aspect in common: they rely on accurate timekeeping. Atomic clocks are used at each tracking station. These provide a stable timing reference to maintain constant transmission frequencies and allow data to be combined from more than one station.
To measure the velocity using Doppler shift, frequencies of radio transmissions must be precisely controlled. On board spacecraft, ultra-stable oscillators are used to maintain a consistent frequency in radio transmissions. These stable transmissions can be used to determine position, but not with a great deal of accuracy. Highly stable time standards are heavy and consume a great deal of power, so installing them in planetary spacecraft is not an option. This situation is different with GPS satellites, all of which carry their own atomic clocks. To obtain a stable timing reference for planetary spacecraft, a stable time standard known as a hydrogen maser is used to produce a stable transmission frequency on Earth. This signal is sent to a spacecraft, which is then programmed to mathematically alter the frequency and return the signal. This way the velocity of a spacecraft can be determined to an accuracy of a few millimeters per second.
To determine the range, the travel time of a signal must be known to a small fraction of a second. Atomic clocks at the tracking station measure the amount of time required to send and receive signals moving at the speed of light. Atomic clocks are also used to compare signals received at different stations. This allows space navigators to precisely determine the angular position.
The Goldstone station is located in the Mojave Desert of California. The location was chosen for favorable topography and open space that limited radio interference. Goldstone contains several individual sites, each with its own set of large dish antennas. Each site is commonly named for the mission or destination for which it was first used. The first to begin operation was the Pioneer site, used to receive data from the Pioneer probes to the Moon. The Uranus site was first used to receive data from the Voyager 2 flyby of Uranus.