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How not to get lost in space?



The Roman philosopher Seneca said: "If a person does not know where he is going, then there is no fair wind for him." In fact, what is the use of engines, flywheels or solenoids for us if we do not know the position of the device in space? This story is about devices that allow us not to get lost in space.

Technical progress has made orientation systems small, cheap and affordable. Now, even a student microsatellite can boast an orientation system that space astronaut pioneers could only dream of. Limited opportunities generated ingenious solutions.

Asymmetrical answer: no orientation


The first satellites and even interplanetary stations flew non-oriented. Data transmission to Earth was conducted over the radio channel, and several antennas, so that the satellite was in communication at any position and any tumbling, weighed much less than the orientation system. Even the first interplanetary stations flew unoriented:
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Luna-2, the first station to reach the surface of the moon. Four antennas on the sides provide communication at any position relative to the Earth

Even today it is sometimes easier to cover the entire surface of the satellite with solar batteries and install several antennas, rather than create an orientation system. Moreover, some tasks are undemanding to orientation - for example, cosmic rays can be fixed in any position of the satellite.

Advantages:

Disadvantages:


Solar sensor


By the middle of the 20th century, solar cells became a familiar and familiar thing, so it is not surprising that they went into space. The obvious beacon for such sensors was the sun. Its bright light hit the photosensitive element and allowed to determine the direction:


Various schemes of operation of modern solar sensors, below is a photosensitive matrix


Another design option, here the matrix is ​​curved


Modern solar sensors

Advantages:

Disadvantages:

Only one axis on which solar sensors can stabilize the device does not interfere with their active use. First, the solar sensor can be supplemented with other sensors. Secondly, the solar sensor spacecraft with solar panels makes it easy to organize the spin mode on the Sun, when the device rotates aimed at it, and the solar panels operate in the most comfortable conditions.
Spaceships "Vostok" wittily used the solar sensor - the axis on the Sun was used when building the orientation to decelerate the ship. Also, solar sensors were highly demanded at interplanetary stations, because many other types of sensors cannot work outside the earth's orbit.
Due to its simplicity and low cost, solar sensors are now very common in space technology.

Infrared vertical


Devices that fly in Earth orbit often need to determine the local vertical - the direction to the center of the Earth. Visible photovoltaic cells are not very suitable for this - on the night side the Earth is much worse lit. But, fortunately, in the infrared range, the warm Earth shines almost equally in the day and night hemispheres. At low orbits, sensors determine the position of the horizon, at high orbits - scan the space in search of the warm circle of the Earth.
Structurally, as a rule, infrared plotters of the vertical contain a system of mirrors or a scanning mirror:


Infrared vertical assembly with a flywheel. The unit is designed for accurate orientation to Earth for geostationary satellites. Clearly visible scan mirror


An example of the field of view of the infrared vertical. Black Circle - Earth


Domestic infrared verticals produced by VNIIEM OJSC

Advantages:

Disadvantages:

The fact that orientation is based on only one axis does not prevent the widespread use of infrared verticals. They are very useful for geostationary satellites who need to aim their antennas at Earth. IKVs are also used in manned astronautics, for example, on modern modifications of the Soyuz spacecraft, braking orientation is made only according to its data:


Ship "Union". Duplicate sensors IKV are shown by arrows

Giroorbitant


In order to produce a braking impulse, it is necessary to know the direction of the orbital velocity vector. The solar sensor will give the correct axis about once a day. This is normal for cosmonaut flights; in the event of an emergency, a person can manually orient the ship. But the ships "Vostok" had "twin brothers", reconnaissance satellites "Zenith", which also needed to issue a braking impulse in order to return the film taken from orbit. The limitations of the solar sensor were unacceptable, so I had to invent something new. This decision was giroorbitant. When the infrared vertical works, the ship rotates because the axis to the Earth is constantly turning. The direction of the orbital motion is known, therefore, by which direction the ship turns, you can determine its position:



For example, if the ship constantly rolls to the right, then we fly right side forward. And if the ship flies stern ahead, it will constantly raise its nose up. With the help of a gyroscope, which seeks to maintain its position, this rotation can be determined:



The stronger the arrow is deflected, the more pronounced is the rotation along this axis. Three such frames allow measuring the rotation along three axes and turning the ship accordingly.
Giroorbitants were widely used in the 60-80s, but now extinct. Simple angular velocity sensors made it possible to efficiently measure the rotation of the apparatus, and the on-board computer would easily determine the position of the ship from this data.

Ion sensor


It was a beautiful idea to supplement the infrared vertical with an ion sensor. At low Earth orbits come across molecules of the atmosphere, which can be ions - to carry an electric charge. Putting sensors that fix the flow of ions, you can determine which side the ship is flying forward in orbit - there the flow will be maximum:


Scientific equipment for measuring the concentration of positive ions

The ion sensor worked faster — it took almost a whole orbit to build the orientation with the gyro orbiter, and the ion sensor was able to build an orientation in ~ 10 minutes. Unfortunately, in the region of South America is the so-called "ion pit", which makes the work of the ion sensor unstable. According to the law of meanness, it is in the region of South America that our ships should build a braking orientation for landing in the Baikonur region. Ion sensors stood on the first "Soyuz", but soon they were abandoned, and now they are not used anywhere.

Star sensor


One axis on the Sun is often not enough. For navigation, you may need another bright object, the direction to which, together with the axis of the sun will give the desired orientation. This object was the star Canopus - it is the second brightest in the sky and is far from the sun. The first device that used the star for orientation was Mariner 4, which was launched to Mars in 1964. The idea turned out to be successful, although the star sensor drank a lot of blood from the MCC - while building the orientation, it was aimed at the wrong stars, and had to “jump” around the stars for several days. After the sensor finally aimed at Canopus, he began to lose it all the time - the rubbish flying near the probe sometimes flashed brightly and restarted the star search algorithm.
The first star sensors consisted of photocells with a small field of view that could only be directed to one bright star. Despite their limited capabilities, they were actively used at interplanetary stations. Now technical progress, in fact, has created a new class of devices. Modern stellar sensors use a matrix of photocells, work in tandem with a computer with a catalog of stars and determine the orientation of the vehicle according to the stars that are visible in their field of view. Such sensors do not need preliminary construction of a rough orientation by other devices and are able to determine the position of the device, regardless of the part of the sky to which they are directed.


Typical star sensors


The larger the field of view, the easier it is to navigate.


Illustration of the sensor operation - by the relative position of the stars, according to the catalog, the direction of sight is calculated

Advantages:

Disadvantages:

Now stellar sensors are used where you need to know the position of the device very accurately - in telescopes and other scientific satellites.

Magnetometer


A relatively new direction is the construction of orientation in the Earth’s magnetic field. Magnetometers for measuring the magnetic field were often placed at interplanetary stations, but were not used to build the orientation.


Earth's magnetic field allows you to build an orientation along all three axes.


"Scientific" magnetometer probes "Pioneer-10" and -11


The first digital magnetometer. This model appeared on the Mir station in 1998 and was used in the Phila boarding module of the Rosette probe.

Advantages:

Disadvantages:

The simplicity and low cost of magnetometers made them very popular in microsatellites.

Gyro-stabilized platform


Historically, spacecraft often flew unoriented or in solar spin mode. Only in the mission target area did they include active systems, build orientation along three axes, and perform their task. But what if we need to maintain an arbitrary orientation for a long time? In this case, we need to "remember" the current position and fix our turns and maneuvers. And for this, humanity did not invent anything better than gyroscopes (measure angles of rotation) and accelerometers (measure linear accelerations).
Gyroscopes
The property of a gyroscope is widely known to strive to maintain its position in space:



Initially, the gyroscopes were only mechanical. But technological progress has led to the emergence of many other types.
Optical gyros . Very high accuracy and the absence of moving parts are different optical gyroscopes - laser and fiber optic. In this case, the Sagnac effect is used - the phase shift of waves in a rotating ring interferometer.


Laser gyro

Solid-state wave gyroscopes . In this case, the precession of the standing wave of a resonating solid is measured. They contain no moving parts and are characterized by very high accuracy.

Vibration gyros . The Coriolis effect is used for work - oscillations of one part of the gyroscope reject the sensitive part when turning:



Vibration gyroscopes are produced in MEMS-performance, they are distinguished by low cost and very small size with relatively good accuracy. It is these gyroscopes that stand in telephones, quadcopters, and the like. MEMS-gyroscope can work in space, and they are put on microsatellites.

The size and accuracy of gyros clearly:



Accelerometers
Structurally, accelerometers are scales - a fixed load changes its weight under the influence of accelerations, and the sensor converts this weight into an acceleration value. Now accelerometers, in addition to large and expensive versions, have got MEMS-analogs:


An example of a “large” accelerometer


Micrograph of MEMS accelerometer

The combination of three accelerometers and three gyroscopes allows you to fix the rotation and acceleration on all three axes. Such a device is called a gyrostabilized platform. At the dawn of cosmonautics, they were possible only on a gimbal, were very complex and expensive.


Gyro-stabilized platform ships Apollo. The blue cylinder in the foreground is a gyroscope. Video Test Platform

The pinnacle of the mechanical systems were the beskardannye systems when the platform hung motionless in gas flows. It was a high-tech, the result of the work of large teams, very expensive and secret devices.


The sphere in the center is a gyrostabilized platform. Peacekeeper ICBM guidance system

But now the development of electronics has led to the fact that the platform with suitable for simple satellites accuracy fits in the palm of your hand, it is developed by students, and even publish the source code.



MARG-platforms have become an interesting innovation. In them, data from gyroscopes and accelerometers are complemented by magnetic sensors, which allows to correct the accumulated error of gyroscopes. A MARG sensor is probably the most suitable option for microsatellites - it is small, simple, cheap, has no moving parts, consumes little energy, provides orientation along three axes with error correction.
In "serious" systems, stellar sensors are commonly used to correct errors in the orientation of a gyro-stabilized platform.
The trajectory error, as a rule, is corrected by the systems of radio monitoring of the orbit - antennas on the Earth can determine its position and speed very accurately from signals from the apparatus. In low orbits for this, a cheap analogue has recently appeared - GPS / GLONASS.

Additional sources of information


Lecture “Designing an Orientation and Stabilization System”.
Abstract "orientation sensors and actuators".

By the tag "inconspicuous difficulties" - publications about launch vehicles, launch facilities, orientation systems.

Source: https://habr.com/ru/post/365759/

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