Satellite constellation

from Wikipedia, the free encyclopedia
GPS satellite constellation

A satellite constellation is an arrangement of satellites that serve a common goal. A satellite constellation in which the satellites fly in the same direction at a constant distance is called a satellite formation . In many cases, a satellite constellation is used to provide global coverage for a service (e.g. satellite navigation , satellite communications, and others). Global coverage means here that the illumination zones of the satellites cover the Earth's surface completely so that is always available at any place on earth at least one satellite (but still depends on the given local conditions).

background

history

The first people to produce publications on satellite constellation for global coverage were L. Vargo (1960: "Orbital Patterns for Satellite Systems" ), D. Lüders (1961: "Satellite Networks for Continuous Zonal Coverage" ) and R. Easton, R Brescia (1969: “Continuously Visible Satellite Constellations” ). Based on the work of JG Walker (1970s) and his notation on the constellation with circular satellite orbits at different orbit heights and with different orbital inclinations , this was named after him: the Walker constellation. Thereafter, constellations that provide global service coverage, with four satellites on elliptical orbits, one with three satellites and, in the new millennium, one with two satellites were published.

Design of a satellite constellation

Service coverage of the Globalstar satellite constellation
Service coverage of the Iridium satellite constellation

The challenge in designing a constellation is choosing the appropriate parameters. The various orbit parameters, such as orbit height, shape, eccentricity, inclination, etc., can be different for the satellites of a constellation, with the result that the geometric complexity of the constellation increases. The orbit parameters and their dependencies are diverse, so that four essential parameters are only briefly shown:

One of the first questions concerns service coverage. This takes into account the areas on earth on which an organization would like to offer a service. So z. B. the polar ice caps are of less interest because they are not populated enough than the rest of the earth's surface (see Globalstar vs. Iridium ). On the other hand, only service coverage can be of particular importance for one state. The type of service coverage, whether global or partial , has a decisive influence on the type of constellation.

From a financial point of view, the number of satellites plays an important role due to their construction and transport. The cost of setting up the Iridium communication system with 66–93 satellites is estimated at around US $ 5 billion and the successor system with 72–81 satellites is estimated at US $ 2.9 billion. The number of satellites influences the orbit required to cover a service or the geometric shape of the constellation. However, the number of satellites is not the only cost driver; the technologies to be used, the orbit altitude (environmental conditions) or the ground infrastructure also play an important role. This is u. a. can be recognized by the satellite navigation system Galileo , which, despite the smaller number of 30 satellites, causes costs of 6.7 to 6.9 billion euros.

If the desired scope of service coverage is known, the orbit height with the constellation type largely determines the number of satellites required. However, as the orbit height increases, the radiation increases due to the decrease in the strength of the earth's magnetic field . This increases the development costs of the satellite type. Furthermore, the required transmission power increases with increasing orbit height and the time offset due to the communication path. Using different orbit shapes, such as circular, elliptical and their alignment, the number of satellites can be reduced by increasing the geometric constellation complexity. Due to the large number of parameters, this optimization is carried out numerically in practice .

The constellation pattern or type determine the service coverage by varying the number of orbital planes and their inclinations. So is z. For example, service coverage of the polar caps is not possible in a Walker constellation with a low orbit and medium inclination (~ 60 °), whereas a polar constellation (inclination ~ 90 °) covers this area. The orbit levels and their alignment in turn influence the ground infrastructure, so for each orbit level at least one ground station (depending on the type of service) must be available that can establish contact with the satellites in this orbit. Alternatively, a relay satellite in a neighboring or higher orbit can also be used for communication with a ground station (see e.g. European Data Relay Satellite ).

Satellite constellations

LEO constellations

This type of satellite constellation is intended for low earth orbits . The background to this is the increasing radiation load that acts on the satellite as the orbit height increases. This increases the development and production costs and / or reduces the service life of a satellite or a satellite constellation. The two most famous constellations with circular orbits are the Walker and polar satellite constellations.

The Walker constellation

Walker constellation 54 °: 18/3/1
Example of a polar and a Walker constellation

The Walker Constellation, also called Walker Delta Pattern Constellation , describes the distribution of the satellites in the various circular orbits. The orbits all have the same inclination (inclination) relative to the reference plane. Typically the reference plane is the equatorial plane. The notation of this constellation is given as follows:

i: inclination [°], t: number of satellites, p: number of orbits (evenly distributed), f: phase parameters (0 to p-1)

The phase parameter can be interpreted as follows:

: true anomaly (see satellite orbit elements )

The true anomaly of satellite 2 (the nearest eastern satellite of satellite 1) is higher than the true anomaly 1 of satellite 1 by the additional amount, whereby satellites 1 and 2 are on different orbits. Ie f specifies the phase shift of the satellite distribution to the reference plane (mostly equator). For f = 0, one satellite per orbit crosses the equatorial plane at the same time, for f> 0 any satellite crosses the equatorial line first (figure: "1"), followed by the next western satellite (figure: "2") which in turn is followed by the next western satellite (Figure: "3").

Example: 54 °: 18/3/1

This Walker constellation (see figure) contains 18 satellites, which are distributed over 3 orbit planes, i.e. 6 satellites per orbit plane, with each orbit plane having an inclination of 54 ° (not shown in the figure). The phase shift between the satellite planes is 20 °.

Depending on the inclination of the orbit orbit, the polar caps cannot be covered in a Walker constellation.

Polar satellite constellation

A polar constellation, also called Walker Polar Star Pattern Constellation , is characterized by an angle of inclination of approximately 90 °, i.e. H. the satellites of the constellation cross the polar ice caps. A Walker Delta Pattern Constellation with an inclination of approximately 90 ° is therefore a polar constellation. This achieves coverage of the polar regions, which are, however, rather insignificant from a commercial point of view (insufficient population). Such communication systems are of great interest for scientific research missions to the polar ice caps. In contrast to Globalstar, the satellite constellation Iridium is a polar system. For this reason, the Iridium communication system is preferred for scientific missions to the northern and southern latitudes . This use was also a reason for the postponement of the shutdown of the system due to the bankruptcy in August 2000 and the subsequent economic continuation by Iridium Satellite LLC from 2001.

Highly elliptical constellations

Molniya orbit constellation

Molnija constellation

A Molnija constellation is characterized by the use of the Molnija orbit ( highly elliptical orbit ) orbit type . A Molnija orbit has the advantage that a satellite can provide a service below the apogee area for a relatively long time . This type was used for Russian communication satellites because the transmission power of geostationary satellites would be too great for the northern latitudes of Russia and a communication link to a satellite in a polar constellation is too short or would require too many satellites. An example of such a constellation is the Satellite Data System (SDS) (see figure on the right) of the United States Armed Forces , which has been used by the United States Armed Forces since 1976 starting with SDS-1.

MEO satellite constellations

MEO constellations are preferably used by navigation satellite systems. Due to the altitude, fewer satellites are required than in the LEO, but a higher transmission power is required. Furthermore, these systems are located in the Van Allen Belt , which means that they have to be designed for a higher radiation dose .

Geostationary satellite constellations

Geostationary satellite constellation

The advantage of stationing a satellite constellation in the GEO is the minimal number of satellites required for global service coverage. In theory, two satellites would be enough to reach over 80% of all places on earth (if the earth were a perfect and smooth sphere). In practice, however, accessibility is not guaranteed at the interfaces or at high latitudes due to the local conditions such as hills, mountains, buildings and other obstacles. The transmission power also plays an important role, so that the Russian communications satellites did not use a GEO station, but a Molnija orbit. For this reason, GEO constellations have at least three satellites (see illustration). The NASA uses such a constellation type to support their space missions in LEO. This constellation is known as the TDRS system (Tracking and Data Relay Satellite System).

Orbit combinations

As mentioned earlier, increasing the constellation complexity can reduce the number of satellites. So z. B. different orbit types, such as LEO and MEO, can be used for a constellation, whereby an inter-satellite connection for the satellites of this constellation must exist on the two orbit types. Furthermore, orbits and their orientation can be used to z. B. to generate polygonal constellations. The possibilities are quite diverse, so that only brief reference is made here.

Others

What is not described in more detail in this context are inter-satellite links (eng .: inter-satellite link, abbr .: ISL) and their use in satellite networks. Inter-satellite connections are relevant for forwarding the received data. If the satellites in a constellation cannot establish a connection with one another, as is the case with Globalstar , the presence of a ground station in the footprint is required, which transmits the data forwarded by the satellite to the terrestrial network. ISL offers an alternative, as is the case with Iridium . Using these connections, data can only be transmitted through the satellite constellation, without an intermediate step via a ground station. The continuation of this technology leads to satellite networks. These still theoretical systems could one day provide an infrastructure similar to the Internet in space.

Applications

Satellite constellations are used in various areas, such as B .:

See also

Wiktionary: Constellation  - explanations of meanings, word origins, synonyms, translations
Commons : Satellite Constellation  - collection of images, videos and audio files

Web links

Simulation software for satellite constellations:

swell

Individual evidence

  1. ^ Robert A. Nelson: Satellite Constellation Geometry. (PDF; 640 kB) March 1995, accessed on September 2, 2011 (English).
  2. ^ Lloyd Wood: Satellite Constellation Networks. (PDF; 348 kB) Retrieved September 2, 2011 (English).
  3. ↑ Iridium's NEXT Satellites: Global Reach, New Partnerships. Defense Industry Daily, May 1, 2011, accessed August 30, 2011 .
  4. ↑ Iridium Announces Comprehensive Plan For Next-Generation Constellation. (No longer available online.) Iridium, June 2, 2010, archived from the original on September 6, 2011 ; accessed on August 30, 2011 (English). Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot / investor.iridium.com
  5. ↑ Galileo costs explode - Berlin clings to a bottomless pit. handelsblatt.com, October 7, 2010, accessed September 5, 2011 .
  6. ^ Bau, Jason H .: Topologies for Satellite Constellations in a Cross-linked Space Network Backbone. (PDF) MIT , July 31, 2002, accessed on May 14, 2016 .
  7. ^ Sauter, Luke M. (2002): "Satellite Constellation design for Mid-Course Ballistic Missile Intercept", United States Air Force Academy