How to locate Mars?

For modern people, it is so easy to determine the location, because there are at least four global positioning systems above us, namely GPS, GLONASS, Beidou and Galileo. Can eliminate the positioning error within 10 meter, but it can only stay on the earth. This positioning technology will fail when it goes out of near-earth orbit, and detectors performing deep space missions will need more advanced positioning methods.

Inertial positioning

In fact, the principle of inertial positioning is very simple, that is, take an inertial measurement unit (accelerometer+gyro) before departure, then detect the changes of gyro along the way, and get the speed and position through integral calculation.

One advantage of inertial navigation is that it can locate itself without any contact with the outside world, which is very obvious for intercontinental missiles or probes performing tasks in deep space. Of course, it also has one of the biggest shortcomings, that is, cumulative error, which will eventually reach incompetent positioning over time.

Triangular differential one-way ranging based on ultra-long baseline measurement and positioning technology

In everyone's impression, this technology is used to observe celestial bodies. For example, the shooting of M87* black hole adopts the ultra-long baseline observation technology. Half of the millimeter wave/submillimeter wave radio telescopes on the earth add up, and the observation data takes about ten days. Finally, it will take several years to get photos of black holes.

So in everyone's impression, a very long baseline can only be used for observation, but the principle of observing celestial bodies is the same as that of observing detectors. The radio signal sent by the detector reaches different VLBI members on the earth, so a positioning technology based on VLBI is developed: triangular differential unidirectional ranging, that is, Delta differential unidirectional ranging. This development and the positioning method in the 1970s have achieved good results in lunar exploration and positioning of Jupiter-orbiting travelers.

Its accuracy depends on the gain of the signal receiving antenna and the baseline length between VLBI antennas. The farther the distance, the higher the accuracy. But there is a problem. If it is too far away, it will be blocked by the ground because of the curvature of the earth. Therefore, if the antenna continues to be on the ground, the positioning accuracy will decrease with the increase of the target distance, but it can reach the meter-level accuracy in the lunar orbit.

The technology of DOR was first studied by Americans, and the technology is also the strongest. ESA started its research in 1986, and Japan space agency started its research in 2003. At present, the two companies are still cooperating with NASA. DOR positioning technology in China began in July 2004 to prepare paving stones for the Chang 'e project.

Astronomical navigation based on astronomical positioning

Astronomical navigation should be familiar to everyone, and it is an indispensable navigation technology in modern navigation. According to the exact time, we can determine the angle of ordinary stars and calculate the current position of the ship in the ocean. In the solar system, small changes in the position of the detector are not as easy to locate as on the earth's sphere, so the navigation of distant stars is not feasible in the solar system, but it is difficult for astronomers.

1, navigation based on the position of the sun and planets.

Of course, this requires a catalog, that is, a database of the positions and movements of the major planets in the solar system. By measuring the angles with these planets with space-borne telescopes, the current position of the detector itself can be calculated. This does not depend on the earth base station, and it is also one of the ways of autonomous navigation. Other missions even use the position of asteroids to measure their own position, but this is only occasionally used by missions in the asteroid belt.

The composite photo of the solar system taken by Voyager 1 in February 1990.

Generally speaking, positioning is not a single use, but a comprehensive multi-party data before determining their own position, so the accuracy will be higher.

2. Realize its precise positioning based on pulsar position.

However, the radio band antenna of pulsar is too large, so if it is necessary to locate pulsar in deep space, at least X-ray band is needed, which can reduce the antenna size and make it easy for deep space detectors to carry. But pulsars are not celestial bodies with obvious signals, so pulsar positioning is perfect in theory, but it is not generally difficult to operate in practice.

The gold-plated record of Voyager 1 is the position of the solar system to 14 pulse star. Theoretically, as long as the antenna gets enough signals, there is no problem in self-positioning.

In the Milky Way, if a catalogue of pulsars around the Milky Way is established, the positioning problem in this range is not particularly big in theory. However, due to the expansion of the scale, the celestial bodies that are positioning themselves are also moving, and the positioning accuracy will be reduced. However, on a larger scale, this is not the main problem. If we go out of the Milky Way, we can only refer to the angle of the galaxy inside our own galaxy, or even the larger supercluster. However, it is still too early to consider such a problem now.