Earth satellite from earth's orbit. Geostationary orbit. Artificial Earth satellites. Geostationary and geosynchronous satellite orbits
The boundary between the Earth's atmosphere and space runs along the Karman line, at an altitude of 100 km above sea level.
Space is very close, do you realize?
So, the atmosphere. An ocean of air that splashes above our heads, and we live at its very bottom. In other words, the gas shell rotating with the Earth is our cradle and protection from destructive ultraviolet radiation. Here's what it looks like schematically:
Scheme of the structure of the atmosphereTroposphere. Extends to an altitude of 6-10 km in polar latitudes, and 16-20 km in the tropics. In winter the limit is lower than in summer. The temperature drops with altitude by 0.65°C every 100 meters. The troposphere contains 80% of the total mass atmospheric air. Here, at an altitude of 9-12 km, passenger planes fly aircraft. The troposphere is separated from the stratosphere ozone layer, which serves as a shield that protects the Earth from destructive ultraviolet radiation (absorbs 98% of UV rays). There is no life beyond the ozone layer.
Stratosphere. From the ozone layer to an altitude of 50 km. The temperature continues to drop and, at an altitude of 40 km, reaches 0°C. For the next 15 km the temperature does not change (stratopause). They can fly here weather balloons And *.
Mesosphere. Extends to an altitude of 80-90 km. The temperature drops to -70°C. They burn in the mesosphere meteors, leaving a luminous trail in the night sky for several seconds. The mesosphere is too rarefied for aircraft, but at the same time too dense for artificial satellite flights. Of all the layers of the atmosphere, it is the most inaccessible and poorly studied, which is why it is called the “dead zone.” At an altitude of 100 km there is the Karman line, beyond which it begins open space. This officially marks the end of aviation and the beginning of astronautics. By the way, the Karman line is legally considered upper limit countries below.
Thermosphere. Leaving behind the conditionally drawn Karman line, we go out into space. The air becomes even more rarefied, so flights here are only possible along ballistic trajectories. Temperatures range from -70 to 1500°C, solar radiation and cosmic radiation ionize the air. Particles at the north and south poles of the planet solar wind, falling into this layer, cause visible in low latitudes of the Earth. Here, at an altitude of 150-500 km, our satellites And spaceships , and a little higher (550 km above the Earth) - beautiful and inimitable (by the way, people climbed to it five times, because the telescope periodically required repairs and maintenance).
The thermosphere extends to an altitude of 690 km, then the exosphere begins.
Exosphere. This is the outer, diffuse part of the thermosphere. Consists of gas ions flying into outer space, because. The force of gravity of the Earth no longer acts on them. The exosphere of the planet is also called the “corona”. The Earth's "corona" is up to 200,000 km high, which is about half the distance from the Earth to the Moon. In the exosphere they can only fly unmanned satellites.
*Stratostat – a balloon for flights into the stratosphere. The record height for lifting a stratospheric balloon with a crew on board today is 19 km. The flight of the stratospheric balloon “USSR” with a crew of 3 people took place on September 30, 1933.
Stratospheric balloon **Perigee is the point of the orbit of a celestial body (natural or artificial satellite) closest to Earth.
***Apogee is the most distant point in the orbit of a celestial body from the Earth
2007
main idea
This site is dedicated to surveillance issues artificial earth satellites(Further satellite
). Since the start space age(On October 4, 1957, the first satellite, Sputnik-1, was launched). Humanity has created a huge number of satellites that circle the Earth in all kinds of orbits. Currently, the number of such man-made objects exceeds tens of thousands. Mainly " space debris" - fragments of artificial satellites, spent rocket stages, etc. Only a small part of them are operational satellites.
Among them there are research and meteorological satellites, communications and telecommunications satellites, and military satellites. The space around the Earth is “populated” by them from altitudes of 200-300 km and up to 40,000 km. Only some of them are accessible for observation using inexpensive optics (binoculars, telescopes, amateur telescopes).
By creating this site, the authors set themselves the goal of collecting together information about methods of observing and filming satellites, showing how to calculate the conditions for their flight over a certain area, and describing the practical aspects of the issue of observation and filming. The site presents mainly original material obtained during observations by participants in the “Cosmonautics” section of the astronomy club “hν” at the Minsk Planetarium (Minsk, Belarus).
And yet, answering the main question - “Why?”, the following must be said. Among the various hobbies that people are interested in are astronomy and astronautics. Thousands of astronomy lovers observe planets, nebulae, galaxies, variable stars, meteors and other astronomical objects, photograph them, and hold their own conferences and “master classes.” For what? It's just a hobby, one of many. A way to get away from everyday problems. Even when amateurs perform work of scientific significance, they remain amateurs who do it for their own pleasure. Astronomy and astronautics are very “technological” hobbies where you can apply your knowledge of optics, electronics, physics and other natural science disciplines. Or you don’t have to use it - and just enjoy contemplation. The situation with satellites is similar. It is especially interesting to monitor those satellites, information about which is not distributed in open sources - these are military reconnaissance satellites different countries. In any case, satellite observation is hunting. Often we can indicate in advance where and when the satellite will appear, but not always. And how he will “behave” is even more difficult to predict.
Thanks:The described methods were created on the basis of observations and research in which members of the astronomy club "hν" of the Minsk Planetarium (Belarus) took part:
- Bozbey Maxim.
- Dremin Gennady.
- Kenko Zoya.
- Mechinsky Vitaly.
Members of the astronomy club "hν" also provided great assistance. Lebedeva Tatyana, Povalishev Vladimir And Tkachenko Alexey. Special thanks Alexander Lapshin(Russia), profi-s (Ukraine), Daniil Shestakov (Russia) and Anatoly Grigoriev (Russia) for help in creating paragraph II §1 “Satellite Photometry”, Chapter 2 and Chapter 5, and Elena (Tau, Russia) also for consultations and writing several calculation programs. The authors also thank Mikhail Abgaryan (Belarus), Yuri Goryachko (Belarus), Anatoly Grigoriev (Russia), Leonid Elenin (Russia), Victor Zhuk (Belarus), Igor Molotov (Russia), Konstantin Morozov (Belarus), Sergei Plaksa (Ukraine), Ivan Prokopyuk (Belarus) for providing illustrations for some sections of the site.
Some of the materials were received during the implementation of an order from the Geographic Information Systems Unitary Enterprise of the National Academy of Sciences of Belarus. The presentation of materials is carried out on a non-commercial basis in order to popularize the Belarusian space program among children and youth.
Vitaly Mechinsky, Curator of the “Cosmonautics” section of the “hν” astroclub.
Site news:
- 09/01/2013: Significantly updated subparagraph 2 "Photometry of satellites during flight" p. II §1 - information has been added on two methods of photometry of satellite tracks (method of photometric track profile and method of isophote photometry).
- 09/01/2013: Subclause II §1 was updated - information on working with the "Highecl" program for calculating probable outbreaks from the GSS was added.
- 01/30/2013: Updated "Chapter 3"-- added information on working with the "MagVision" program to calculate the drop in penetration from illumination from the Sun and Moon.
- 01/22/2013: Updated Chapter 2. Added animation of satellites moving across the sky in one minute.
- 01/19/2013: Subsection updated "Visual observations of satellites" paragraph 1 "Determination of satellite orbits" §1 of Chapter 5. Added information about heating devices for electronics and optics to protect against dew, frost and excessive cooling.
- 01/19/2013: Added to "Chapter 3" information about the drop in penetration when illuminated by the Moon and twilight.
- 01/09/2013: Added sub-item "Flashes from the lidar satellite "CALIPSO" subclause “Photography of flashes”, paragraph II “Photometry of satellites” §1 of Chapter 5. Information on the features of observing flashes from the laser lidar of the satellite “CALIPSO” and the process of preparing for them are described.
- 11/05/2012: The introductory part of §2 of Chapter 5 has been updated. Information has been added on the required minimum equipment for radio observations of satellites, and a diagram of the LED signal level indicator, which is used to set a safe input audio signal level for the voice recorder, is provided.
- 11/04/2012: Sub-clause updated "Visual observations of satellites" paragraph 1 "Determination of satellite orbits" §1 of Chapter 5. Information has been added about the Brno star atlas, as well as about the red film on the LCD screens of electronic devices used in observations.
- 04/14/2012: Updated sub-item of the sub-item "Photo/video shooting of satellites" clause 1 "Determination of satellite orbits" §1 of Chapter 5. Added information about working with the "SatIR" program for identifying satellites in photographs with a wide field of view, as well as determining coordinates ends of satellite tracks on them.
- 04/13/2012: Sub-clause updated "Astrometry of satellites on the received images: photos and videos" subsection "Photo/video shooting of satellites" clause 1 "Determination of satellite orbits" §1 of Chapter 5. Added information about working with the "AstroTortilla" program to determine the coordinates of the center of the field of view of images of areas of the starry sky.
- 03/20/2012: Subclause 2 “Classification of satellite orbits by semimajor axis” §1 of Chapter 2 has been updated. Information has been added about the magnitude of GSS drift and orbital disturbances.
- 03/02/2012: Added sub-item "Observing and filming rocket launches at a distance" subparagraph “Photo/video shooting of satellites”, paragraph I “Determination of satellite orbits” §1 of Chapter 5. Information on the features of observing the flight of launch vehicles at the launch stage is described.
- "Converting astrometry to IOD format" subsection "Photo/video shooting of satellites" paragraph I "Determination of satellite orbits" §1 of Chapter 5. Added description of working with the program "ObsEntry for Window" for converting satellite astrometry into IOD format - an analogue of the "OBSENTRY" program, but for the OS Windows.
- 02/25/2012: Subclause updated "Sun-synchronous orbits" paragraph 1 "Classification of satellite orbits by inclination" §1 of Chapter 2. Added information on calculating the inclination value i ss of a sun-synchronous satellite orbit depending on the eccentricity and semi-major axis of the orbit.
- 09.21.2011: Subclause 2 “Photometry of satellites during a flight” has been updated, clause II “Photometry of satellites” §1 of Chapter 5. Information has been added about the synodic effect, which distorts the determination of the rotation period of satellites.
- 09.14.2011: Sub-clause updated "Calculation of orbital (Keplerian) elements of the satellite's orbit based on astrometric data. One flyby" subclause "Photo/video shooting of satellites" of paragraph I "Determination of satellite orbits" §1 of Chapter 5. Information has been added about the "SatID" program for identifying a satellite (using received TLE) among satellites from a third-party TLE database, and also a method for identifying a satellite in program "Heavensat" based on the observed flyby near the guide star.
- 09.12.2011: Updated sub-item "Calculation of orbital (Keplerian) elements of the satellite's orbit based on astrometric data. Several flights" of the sub-item "Photo/video shooting of satellites" of paragraph I "Determination of satellite orbits" §1 of Chapter 5. Added information about the TLE recalculation program -elements for the required date.
- 09/12/2011: Added sub-item "Entry of an artificial satellite into the Earth's atmosphere" subsection “Photo/video shooting of satellites”, paragraph I “Determination of satellite orbits” §1 of Chapter 5. Information on working with the “SatEvo” program for predicting the date of entry of satellites into the dense layers of the Earth’s atmosphere is described.
- "Flashes from geostationary satellites" subclause “Photography of flashes”, p. II “Photometry of satellites” §1 of Chapter 5. Information has been added about the period of visibility of GSS flashes.
- 09/08/2011: Sub-clause updated "Change in the brightness of an satellite during its flight" subparagraph 2 "Photometry of satellites during the flight" paragraph II "Photometry of satellites" §1 of Chapter 5. Added information about the form of the phase function for several examples of reflective surfaces.
- subparagraph 1 "Observation of artificial satellite flares" paragraph II "Satellite photometry" §1 of Chapter 5. Added information about the unevenness of the time scale along the image of the satellite track on the photodetector matrix.
- 09/07/2011: Sub-clause updated "Photometry of satellites during flight" p. II "Photometry of satellites" §1 of Chapter 5. Added an example of a complex light curve of the satellite "NanoSail-D" (SCN:37361) and modeling of its rotation.
- "Flashes from low-orbit satellites" subparagraph 1 “Observation of satellite flares”, paragraph II “Photometry of satellites” §1 of Chapter 5. A photograph and photometric profile of a flare from the LEO satellite “METEOR 1-29” have been added.
- 09/06/2011: Sub-clause updated "Geostationary and geosynchronous satellite orbits"§1 of Chapter 2. Added information on the classification of geostationary satellites, information on the shape of GSS trajectories.
- 09/06/2011: Sub-clause updated "Shooting the passage of satellites: equipment for shooting. Optical elements" subclause “Photo/video shooting of satellites”, paragraph I “Determination of satellite orbits” §1 of Chapter 5. Added links to reviews of domestic lenses as applied to shooting satellites.
- 09/06/2011: Sub-clause updated "Phase angle" Section II "Satellite Photometry" §1 Chapter 5. Added animation of satellite phase changes depending on the phase angle.
- 13.07.2011: Completed completion of all chapters and sections of the site.
- 07/09/2011: Finished writing the introductory part to paragraph II "Satellite Photometry"§1 Chapter 5.
- 07/05/2011: Finished writing the introductory part to §2 "Radio observations of satellites" Chapters 5.
- 07/04/2011: Sub-clause updated "Processing observations" p. I "Reception of satellite telemetry" §2 of Chapter 5.
- 07/04/2011: Finished writing Section II "Obtaining cloud images"§2 Chapter 5.
- 07/02/2011: Finished writing Section I "Reception of satellite telemetry"§2 Chapter 5.
- 07/01/2011: Finished writing the subparagraph "Photo/video shooting of satellites" clause I §1 Chapter 5.
- 06/25/2011: Finished writing Applications.
- 06/25/2011: Finished writing the introductory part to Chapter 5: “What and how to observe?”
- 06/25/2011: Finished writing the introductory part to §1 "Optical observations" Chapters 5.
- 06/25/2011: Finished writing the introductory part to paragraph I "Determination of satellite orbits"§1 Chapter 5.
- 06/25/2011: Finished writing Chapter 4: "About the time".
- 01/25/2011: Finished writing Chapter 2: "What kind of orbits and satellites are there?".
- 01/07/2011: Finished writing Chapter 3: "Preparing for Observations".
- 01/07/2011: Finished writing Chapter 1: "How do satellites move?"
The launch of the world's first Soviet artificial Earth satellite on October 4, 1957 marked the beginning of the era of creating artificial celestial bodies. When launching the first artificial Earth satellites (AES), the task was to obtain information regarding density and temperature upper layers atmosphere and distribution of mass within the Earth. Satellite launches have made it possible to establish that at high altitudes the air density is many times greater than expected before the launch of satellites, and that it varies greatly during the day. Based on data from monitoring the movement of satellites, scientists have more accurately determined the shape and size of the Earth, as well as the distances between the continents. The range of tasks that are posed and solved with the help of artificial satellites is constantly expanding. To date, the number of launched satellites reaches several thousand. Satellites are now launched not only for scientific purposes, but also to solve many practical problems.
For example, a number of countries are launching meteorological and communications satellites. For several years, the US Navy has been using the Transit satellite navigation system, consisting of five satellites, on a trial basis.
In connection with the rapid growth of air traffic, developments began to be made on the use of artificial satellites to prevent aircraft collisions in the air, control air traffic and provide aircraft navigation.
The introduction of satellite aircraft navigation and communication systems should solve the problem of reliable air traffic control and navigation support for flights in the context of the expected increase in aircraft traffic by the end of this century. The use of satellite aircraft navigation and communication systems will make it possible to ensure aircraft flights in all weather conditions. Given the exceptional reliability of these systems, it will be possible to reduce lateral and vertical separation standards and thereby improve the use of airspace. Air traffic controllers and pilots will be able to communicate with any point on Earth and in the airspace.
Currently, it is possible to create a unified global navigation system. But this requires international cooperation. The use of satellites for aircraft navigation is a further development of the methods of aviation astronomy. Let's consider some concepts related to the navigational use of satellites.
Elements of the satellite's orbit.
The artificial satellite of the Earth is considered to be spacecraft, launched into outer space, the movement of which in its orbit is subject to natural forces. The path of an artificial satellite in space is called an orbit. In accordance with the laws of celestial mechanics, the plane of the Earth's satellite's orbit always passes through the center of mass of the Earth and the satellite. Therefore, all possible satellite orbits are located in the cross-sectional planes of the Earth in a large circle. As a result, the satellite can move, for example, in the equatorial plane, but cannot move in planes parallel to the Earth.
In order for a body to become an artificial satellite, it is necessary to give it a speed relative to the Earth that is no less than a circular speed, which is called the first cosmic speed. For a satellite moving in a circle near the Earth's surface, it is equal to 7.912 km/s. The circular speed decreases with increasing altitude. For example, at an altitude of 1000 km, the satellite’s circular speed is 7.356 km/s. If an artificial satellite receives a speed greater than the circular speed corresponding to its height above the earth's surface, then it will move in an elliptical orbit. At an orbital speed of 11.19 km/s, the artificial satellite enters an elliptical orbit relative to the Sun, i.e., it ceases to be a satellite of the Earth.
For navigation purposes, stationary, synchronous and non-synchronous satellites are used.
A satellite having an equatorial circular orbit with a period of revolution equal to the period of rotation of the Earth is called stationary. It is located in space always above the same point of the equator. To achieve this condition, the satellite must move from west to east at an altitude of 35,800 km at a speed of 3.076 km/s. In this case, the angular velocity of the satellite will be equal to angular velocity rotation of the Earth.
A satellite having an orbital period an integer number of times less or greater than the Earth's rotation period is called synchronous. Such a satellite is characterized by the fact that, under the first condition, it passes over the same point on the Earth at the same time every day.
A satellite whose orbital period is not a multiple of the Earth's rotation period is called asynchronous.
Knowing the elements of the satellite’s orbit, it is possible to determine its position in space for any moment in time. The elliptical orbit of the satellite is shown in Fig. 7.20. In this figure, I is the perigee of the orbit (the point of the satellite’s orbit closest to the Earth); A - apogee of the orbit (the most distant point of the satellite’s orbit from the Earth); i is the angle of inclination of the satellite’s orbital plane to the plane of the celestial equator; - ascending node of the orbit (the point in the orbit at which the satellite crosses the plane of the celestial equator, moving from the Southern Hemisphere to the Northern); 15 - descending node of the orbit; T - point of vernal equinox; Q - right ascension of the ascending node of the orbit; co - angular distance of perigee along the orbit from the ascending node; a - right ascension of the satellite; - satellite declination. To fully determine a satellite's orbit, six elements must be known. The elements Q, i, с are called corner elements. The spatial elements of the orbit include: the semimajor axis of the ellipse a and the orbital eccentricity, i.e. the ratio of the focal length to the semimajor axis of the ellipse. The semimajor axis and eccentricity characterize the size and shape of the elliptical orbit. The sixth element is the time of passage of perigee.
Satellite position on celestial sphere determined by declination and right ascension. But these elements change very quickly, since the satellite has a short orbital period. If the motion of the satellite were not affected by disturbing forces, then the position of its orbit in space, as well as the size and shape of the orbit, would remain unchanged.
Rice. 7.20. Elements of satellite orbit
In reality, the motion of the satellite experiences complex and varied disturbances. The influence of the gravitational forces of the Sun, Moon and planets, the inhomogeneity of the Earth’s gravitational field, the influence of atmospheric resistance forces and electromagnetic forces change the parameters of the satellite’s orbit.
Without knowledge of the trajectory elements and coordinates of an artificial satellite, it cannot be used for aircraft navigation. Therefore, satellite navigation systems include a computer that calculates the ephemeris (coordinates) of the satellite. The calculated coordinates are transmitted to the satellite, and from there to the aircraft, where they are used in processing the measurement results.
Satellite navigation systems.
The main task of aircraft navigation comes down to determining the position of the aircraft. IN modern conditions this problem can be solved with the help of satellites, which are new promising means of aircraft navigation. Artificial satellites, being celestial bodies, have a number of advantages over natural celestial bodies - they are equipped with transceiver equipment, which makes it possible to measure not only angular coordinates AES, but also to use the properties of radio waves to determine the range to them.
Let us briefly consider the principle of operation of a satellite navigation system using the Navstar satellite navigation system as an example. Its composition (Fig. 7.21) includes: one or more satellites; network of tracking stations; computer center; transmitting center; aircraft avionics. The number of satellites, their altitude and the position of their orbits is determined by practical considerations to ensure flights in the required areas.
A network of tracking stations monitors satellites and determines their exact position. The number of stations depends on the required tracking duration. Stations are located at points with precisely known coordinates. Data from tracking stations arrives at a computer center, where a computer is used to calculate satellite ephemerides, which
then transmitted by the transmitting center to the satellite, and from there sent to subscribers as part of the navigation signal.
On-board equipment, depending on the type of aircraft, may include an aircraft transceiver, a radio sextant, Doppler system equipment for working with satellites, a radar receiver for a protractor-rangefinder system and a digital computer(digital computer) with the help of which satellite signals are processed and the aircraft’s position is automatically determined and displayed on the indicator.
Rice. 7.21. General principle operation of the satellite navigation system
To reduce the cost of on-board equipment, some systems provide for determining the location of the aircraft on the ground using a computer. Then data about the aircraft's position through relay stations on the satellite is transmitted by the air traffic controller on board the aircraft for the crew to solve navigation problems.
Satellite navigation systems can be of the following types: goniometer, rangefinder and Doppler.
Goniometer satellite systems are based on on-board measurements of the angular heights of two satellites. In the process of measuring altitudes, equipment installed on satellites transmits their coordinates to the aircraft. Measured altitudes and received information about the position of satellites are automatically processed by on-board computer equipment, which provides current geographical coordinates airplane seats.
Rangefinding satellite systems are based on measuring the range of two satellites and the altitude of the aircraft. Measuring ranges to two satellites allows you to get earth's surface two circles of equal distances. The intersection of these circles gives the location of the airplane. The center of the circle of equal ranges is the geographic location of the satellite. The aircraft's position is determined by the on-board automatic computing device based on the known trajectory elements of the satellites, flight altitude and ranges to two satellites.
Doppler satellite systems are based on the principle of determining the Doppler frequency shift of satellite signals received by aircraft. Such a system consists of one or more satellites, the position of their orbits in time is precisely known. The satellite is equipped with a transmitter with a strictly stabilized frequency of radiation of navigation signals. These signals are transmitted at a set time interval. Using a receiver on an airplane, the Doppler frequency shift is determined. Integrating the resulting frequency shift provides the range to the satellite. Three such measurements allow you to determine the position of the aircraft, as in a rangefinder system. The Doppler system does not provide continuous positioning of the aircraft. But at the same time, it is possible to do without angular measurements, which require stabilization of the antenna platform on the aircraft, which significantly complicates the on-board equipment.
Satellite navigation systems have undeniable advantages over other systems, and their implementation will improve the reliability and safety of flights over any region of the globe.
The trajectory of an artificial satellite is called an orbit. During the free flight of the satellite, when its onboard jet engines are turned off, the movement occurs under the influence gravitational forces and by inertia, and main force is the gravity of the Earth.
If we consider the Earth to be strictly spherical, and the action gravitational field The Earth is the only force acting on the satellite, then the movement of the satellite obeys the well-known Kepler laws: it occurs in a stationary (in absolute space) plane passing through the center of the Earth - the orbital plane; the orbit has the shape of an ellipse (Figure 3.1) or a circle ( special case ellipse).
When the satellite moves, the total mechanical energy (kinetic and potential) remains unchanged, as a result of which, as the satellite moves away from the Earth, its speed of movement decreases. In the case of an elliptical orbit, the perigee point is the point of the orbit corresponding to lowest value radius vector r = rп, the apogee point is the point corresponding highest value r = ra (Fig. 3.2).
The earth is located at one of the foci of the ellipse. The quantities included in formula (3.1) are related by the relations: The distance between the foci and the center of the ellipse is ae, i.e., proportional to the eccentricity. Satellite height above the Earth's surface
Where R- radius of the Earth. The line of intersection of the orbital plane with the equatorial plane (a - a in Fig. 3.1) is called the line of nodes, the angle i between the orbital plane and the equatorial plane is the orbital inclination. Based on inclination, equatorial (i = 0°), polar (i = 90°) and inclined orbits (0°
The satellite's orbit is also characterized by the apogee longitude d - the longitude of the sub-satellite point (the point of intersection of the radius vector with the Earth's surface) at the moment the satellite passes the apogee and the orbital period T (the time between two successive passages of the same orbital point).
For communications and broadcasting systems, it is necessary that there is a clear line of sight between the satellite and the associated earth stations for a communication session of sufficient duration. If the session is not round-the-clock, then it is convenient for it to be repeated every day at the same time. Therefore, synchronous orbits with a period of revolution equal to or a multiple of the time the Earth rotates around its axis, i.e., a sidereal day (23 hours 56 minutes 4 s), are preferred.
A high elliptical orbit with an orbital period of 12 hours was widely used, when Molniya satellites were used for communication and hanging systems (perigee altitude 500 km, apogee altitude 40 thousand km). The movement of the satellite at high altitude - in the apogee region - slows down, and the satellite passes through the perigee region, located above the southern hemisphere of the Earth, very quickly. The visibility zone of an artificial satellite in a Molniya type orbit during most of the orbit is large due to its significant altitude. It is located in the northern hemisphere and is therefore convenient for northern countries. Service of the entire territory of the former USSR by one of the satellites is possible for at least 8 hours, so three satellites replacing each other were enough for round-the-clock operation. Currently, in order to eliminate interruptions in communication and broadcasting, simplify pointing systems for antennas of earth stations on satellites and other operational advantages, a transition has been made to the use of geostationary orbits (GSO) of Earth satellites.
The orbit of a geostationary satellite is a circular (eccentricity e = 0), equatorial (inclination i = 0°), synchronous orbit with an orbital period of 24 hours, with the satellite moving in an easterly direction. Back in 1945, the GSO orbit was calculated and proposed to be used for communication satellites by the English engineer Arthur Clarke, later known as a science fiction writer. In England and many other countries, the geostationary orbit is called the “Clark Belt”
The orbit has the shape of a circle lying in the plane of the Earth's equator with a height above the Earth's surface of 35,786 km. The direction of rotation of the satellite coincides with the direction of the daily rotation of the Earth. Therefore, to an observer on earth, the satellite appears motionless at a certain point in the celestial hemisphere.
The geostationary orbit is unique in that with no other combination of parameters it is possible to achieve immobility of a freely moving satellite relative to an earthly observer. It is necessary to note some advantages of geostationary satellites. Communication is carried out continuously, around the clock, without transitions (satellite entering another); on the antennas of earth stations, automatic satellite tracking systems have been simplified, and on some even eliminated; the mechanism for driving (moving) the transmitting and receiving antennas is lightweight, simplified, and made more economical; a more stable value of signal attenuation on the Earth-Space path has been achieved; the visibility zone of a geostationary satellite is about one third of the earth's surface; three geostationary satellites are enough to create a global communication system; there is no (or becomes very small) frequency shift due to the Doppler effect.
The Doppler effect is a physical phenomenon that involves a change in the frequency of high-frequency electromagnetic oscillations when the transmitter and receiver move mutually. The Doppler effect is explained by
the difference in distance in time. This effect can also occur when an satellite moves in orbit. On communication lines through a strictly gestational satellite, the Doppler shift does not occur, on real geostationary satellites it is little significant, and on highly elongated elliptical or low circular orbits it can be significant. The effect manifests itself as instability of the carrier frequency of oscillations relayed by the satellite, which is added to the hardware frequency instability that occurs in the equipment of the onboard repeater and earth station. This instability can significantly complicate signal reception, leading to a decrease in reception noise immunity.
Unfortunately, the Doppler effect contributes to changes in the frequency of modulating oscillations. This compression (or expansion) of the spectrum of the transmitted signal cannot be controlled by hardware methods, so if the frequency shift exceeds acceptable limits (for example, 2 Hz for some types of frequency division equipment), the channel becomes unacceptable.
The delay of a radio signal during its propagation along the Earth - satellite - Earth line also has a significant impact on the properties of communication channels.
When transmitting simplex (unidirectional) messages (television programs, sound broadcasting and other discrete (intermittent) messages, this delay is not felt by the consumer. However, with duplex (two-way) communication, a delay of several seconds is already noticeable. For example, an electromagnetic wave from the Earth to the GEO and back “travels” for 2...4 s (taking into account the signal delay in the satellite equipment) and to ground equipment. In this case, it makes no sense to transmit exact time signals.
The launch of a geostationary satellite into orbit is usually carried out by a multi-stage rocket through an intermediate orbit. A modern launch vehicle is a complex spacecraft that is propelled by the reactive force of a rocket engine.
The launch vehicle consists of the rocket and head blocks. The rocket unit is an autonomous part of a composite rocket with a fuel compartment, propulsion system and elements of the stage separation system. The head unit includes a payload and a fairing that protects the structure of the satellite from the power and thermal effects of the oncoming air flow during flight in the atmosphere and serves for mounting on its inner surface elements that are involved in preparation for launch, but do not function in flight. The main fairing makes it possible to lighten the design of the satellite and is a passive element, the need for which disappears after the launch vehicle exits the dense layers of the atmosphere, where it is dropped. The payload of the spacecraft consists of relay communication and broadcasting equipment, radio telemetry systems, the satellite body itself with all auxiliary and support systems.
The operating principle of an expendable multistage launch vehicle is as follows: while the first stage is operating, the rest, along with the true payload, can be considered as the first stage payload. After its separation, the second one begins to work, which, together with subsequent stages and the actual payload, forms a new independent rocket. For the second stage, all subsequent ones (if any), together with the actual payload, play the role of a payload, and so on, i.e., its flight is characterized by several stages, each of which is like a step for imparting the initial speed to other single-stage rockets included in the its composition. In this case, the initial speed of each subsequent single-stage rocket is equal to the final speed of the previous one. The first and subsequent stages of the carrier are rejected after complete burnout of the fuel in the propulsion system.
The path that the launch vehicle takes when launching an artificial satellite into orbit is called the flight path. It is characterized by active and passive sections. The active phase of the flight is the flight of the launch vehicle stages with the engines running, the passive phase is the flight of spent rocket units after their separation from the launch vehicle.
The carrier, starting vertically (section 1, located at an altitude of 185... 250 km), then enters the curved active section 2 in the eastern direction. In this section, the first stage ensures a gradual decrease in the angle of inclination of its axis relative to the local horizon. Sections 3, 4 are the active flight sections of the second and third stages, respectively, 5 is the satellite orbit, 6, 7 are the passive flight sections of the first and second stage rocket units (Fig. 3.4). When launching an artificial satellite into the appropriate orbit, the time and place of launch of the launch vehicle play an important role. It has been calculated that it is more advantageous to locate the cosmodrome as close to the equator as possible, since when accelerating in the eastern direction the launch vehicle gains additional speed. This speed is called the peripheral speed of the cosmodrome Vк, i.e. the speed of its movement around the Earth’s axis due to the daily rotation of the planet. i.e. That is, at the equator it is 465 m/s, and at the latitude of the Baikonur Cosmodrome - 316 m/s. In practice, this means that a heavier satellite can be launched from the equator by the same launch vehicle.
The final stage of the launch vehicle's flight is the launch of the satellite into orbit, the shape of which is determined by the kinetic energy imparted to the satellite by the rocket, i.e., the final speed of the carrier. In the case when the satellite is given an amount of energy sufficient to launch it into GEO, the launch vehicle must launch it to a point 35,875 km away from the Earth and give it a speed of 3075 m/s.
The orbital speed of a geostationary satellite is easy to calculate. The height of the GEO above the Earth's surface is 35,786 km, the radius of the GSO is 6366 km larger (the average radius of the Earth), i.e. 42,241 km. Multiplying the value of the GSO radius by 2l (6.28), we obtain its circumference - 265,409 km. If we divide it by the length of the day in seconds (86,400 s), we obtain the orbital speed of the satellite - on average 3.075 km/s, or 3075 m/s.
Typically, the launch of a satellite by a launch vehicle is carried out in four stages: entry into the initial orbit; entering a “waiting” orbit (parking orbit); entry into transfer orbit; entering the final orbit (Fig. 3.5). The numbers correspond to the following stages of launching a satellite into GEO: 1 - initial transfer orbit; 2 - first activation of the apogee engine to enter an intermediate transfer orbit; 3 - determination of position in orbit; 4 - second activation of the apogee engine to enter the initial drift orbit; 5 - reorientation of the orbital plane and error correction; 6 - orientation perpendicular to the orbital plane and error correction; 7 - stopping the satellite platform, opening the panels, complete undocking with the rocket; 8 - opening the antennas, turning on the gyrostabilizer; 9 - position stabilization: orientation of antennas to the desired point on the Earth, orientation of solar panels to the Sun, turning on the on-board repeater and establishing its nominal operating mode.
What is geostationary orbit? This is a circular field, which is located above the Earth’s equator, along which an artificial satellite rotates with the angular velocity of the planet’s rotation around its axis. It does not change its direction in the horizontal coordinate system, but hangs motionless in the sky. Geostationary Earth orbit (GEO) is a type of geosynchronous field and is used to place communications, television broadcasting and other satellites.
The idea of using artificial devices
The very concept of geostationary orbit was initiated by the Russian inventor K. E. Tsiolkovsky. In his works, he proposed populating space with the help of orbital stations. Foreign scientists also described the work of cosmic fields, for example, G. Oberth. The man who developed the concept of using orbit for communication is Arthur C. Clarke. In 1945, he published an article in Wireless World magazine, where he described the advantages of the geostationary field. For his active work in this field, in honor of the scientist, the orbit received its second name - the “Clark Belt”. Many theorists have thought about the problem of implementing high-quality communication. Thus, Herman Potochnik in 1928 expressed the idea of how geostationary satellites could be used.
Characteristics of the “Clark Belt”
For an orbit to be called geostationary, it must meet a number of parameters:
1. Geosynchrony. This characteristic includes a field that has a period corresponding to the rotation period of the Earth. A geosynchronous satellite completes its orbit around the planet in a sidereal day, which is 23 hours, 56 minutes and 4 seconds. The Earth needs the same time to complete one revolution in a fixed space.
2. To maintain a satellite at a certain point, the geostationary orbit must be circular, with zero inclination. An elliptical field will result in a displacement either east or west, as the craft moves differently at certain points in its orbit.
3. The “hovering point” of the space mechanism must be at the equator.
4. The location of satellites in geostationary orbit should be such that the small number of frequencies intended for communication does not lead to overlap of frequencies of different devices during reception and transmission, as well as to avoid their collision.
5. Sufficient amount of fuel to maintain a constant position of the space mechanism.
The geostationary orbit of the satellite is unique in that only by combining its parameters can the device remain stationary. Another feature is the ability to see the Earth at an angle of seventeen degrees from satellites located in the space field. Each device captures approximately one-third of the orbital surface, so three mechanisms are capable of covering almost the entire planet.
Artificial satellites
Satellites have been launched by many countries and companies. The world's first artificial device was created in the USSR and flew into space on October 4, 1957. It was named Sputnik 1. In 1958, the United States launched a second spacecraft, Explorer 1. The first satellite, which was launched by NASA in 1964, was named Syncom-3. Artificial devices are mostly non-returnable, but there are those that are partially or completely returned. They are used to conduct scientific research and solve various problems. So, there are military, research, navigation satellites and others. Devices created by university employees or radio amateurs are also launched.
"Standing point"
Geostationary satellites are located at an altitude of 35,786 kilometers above sea level. This altitude provides an orbital period that corresponds to the Earth's rotation period relative to the stars. The artificial vehicle is motionless, therefore its location in geostationary orbit is called the “standing point”. Hovering ensures constant long-term communication, once oriented the antenna will always be pointed at the desired satellite.
Movement
Satellites can be transferred from low-altitude orbit to geostationary orbit using geotransfer fields. The latter are an elliptical path with a point at a low altitude and a peak at an altitude that is close to the geostationary circle. A satellite that has become unsuitable for further work is sent to a disposal orbit located 200-300 kilometers above GEO.
Geostationary orbit altitude
A satellite in a given field keeps a certain distance from the Earth, neither approaching nor moving away. It is always located above some point on the equator. Based on these features, it follows that the forces of gravity and centrifugal force balance each other. The altitude of the geostationary orbit is calculated using methods based on classical mechanics. In this case, the correspondence of gravitational and centrifugal forces is taken into account. The value of the first quantity is determined using Newton's law of universal gravitation. The centrifugal force indicator is calculated by multiplying the mass of the satellite by the centripetal acceleration. The result of the equality of gravitational and inertial mass is the conclusion that the orbital altitude does not depend on the mass of the satellite. Therefore, the geostationary orbit is determined only by the altitude at which the centrifugal force is equal in magnitude and opposite in direction to the gravitational force created by the Earth's gravity at a given altitude.
From the formula for calculating centripetal acceleration, you can find the angular velocity. The radius of the geostationary orbit is also determined by this formula or by dividing the geocentric gravitational constant by the angular velocity squared. It is 42,164 kilometers long. Taking into account the equatorial radius of the Earth, we obtain a height equal to 35,786 kilometers.
Calculations can be carried out in another way, based on the statement that the orbital altitude, which is the distance from the center of the Earth, with the angular velocity of the satellite coinciding with the rotational motion of the planet, gives rise to a linear velocity that is equal to the first cosmic velocity at a given altitude.
Speed in geostationary orbit. Length
This indicator is calculated by multiplying the angular velocity by the field radius. The value of the speed in orbit is 3.07 kilometers per second, which is much less than the first cosmic speed on the near-Earth path. To reduce the rate, it is necessary to increase the orbital radius by more than six times. The length is calculated by multiplying the number Pi and the radius, multiplied by two. It is 264924 kilometers. The indicator is taken into account when calculating the “standing points” of satellites.
Influence of forces
The parameters of the orbit along which the artificial mechanism rotates can change under the influence of gravitational lunar-solar disturbances, inhomogeneity of the Earth's field, and ellipticity of the equator. The transformation of the field is expressed in such phenomena as:
- The displacement of the satellite from its position along the orbit towards points of stable equilibrium, which are called potential holes in the geostationary orbit.
- The angle of inclination of the field to the equator grows at a certain speed and reaches 15 degrees once every 26 years and 5 months.
To keep the satellite at the desired “standing point,” it is equipped with a propulsion system, which is turned on several times every 10-15 days. Thus, to compensate for the increase in orbital inclination, a “north-south” correction is used, and to compensate for the drift along the field, a “west-east” correction is used. To regulate the satellite's path throughout its entire lifespan, a large supply of fuel on board is required.
Propulsion systems
The choice of device is determined by the individual technical features of the satellite. For example, a chemical rocket engine has a displacement fuel supply and operates on long-stored high-boiling components (dianitrogen tetroxide, unsymmetrical dimethylhydrazine). Plasma devices have significantly less thrust, but due to prolonged operation, which is measured in tens of minutes for a single movement, they can significantly reduce the amount of fuel consumed on board. This type of propulsion system is used to maneuver the satellite into another orbital position. The main limiting factor in the service life of the device is the fuel supply in geostationary orbit.
Disadvantages of an artificial field
A significant drawback in interaction with geostationary satellites is large delays in signal propagation. Thus, at the speed of light of 300 thousand kilometers per second and an orbital altitude of 35,786 kilometers, the movement of the Earth-satellite beam takes about 0.12 seconds, and the Earth-satellite-Earth beam takes 0.24 seconds. Taking into account the signal delay in the equipment and cable transmission systems of terrestrial services, the total delay of the “source-satellite-receiver” signal reaches approximately 2-4 seconds. This indicator significantly complicates the use of devices in orbit for telephony and makes it impossible to use satellite communications in real-time systems.
Another disadvantage is the invisibility of the geostationary orbit from high latitudes, which interferes with communications and television broadcasts in the Arctic and Antarctic regions. In situations where the sun and the transmitting satellite are in line with the receiving antenna, there is a decrease, and sometimes complete absence of signal. In geostationary orbits, due to the immobility of the satellite, this phenomenon manifests itself especially clearly.
Doppler effect
This phenomenon consists of a change in the frequencies of electromagnetic vibrations with the mutual movement of the transmitter and receiver. The phenomenon is expressed by a change in distance over time, as well as the movement of artificial vehicles in orbit. The effect manifests itself as low stability of the satellite's carrier frequency, which is added to the hardware instability of the frequency of the onboard repeater and earth station, which complicates the reception of signals. The Doppler effect contributes to a change in the frequency of modulating vibrations, which cannot be controlled. In the case when communication satellites and direct television broadcasting are used in orbit, this phenomenon is practically eliminated, that is, there are no changes in the signal level at the receiving point.
Attitude in the world towards geostationary fields
The birth of space orbit has created many questions and international legal problems. A number of committees, in particular the United Nations, are involved in their resolution. Some countries located on the equator made claims to the extension of their sovereignty to the part of the space field located above their territory. The states stated that the geostationary orbit is a physical factor that is associated with the existence of the planet and depends on the Earth's gravitational field, so the field segments are an extension of the territory of their countries. But such claims were rejected, since the world has a principle of non-appropriation of outer space. All problems related to the operation of orbits and satellites are resolved at the global level.