Satellite constellation
Under a satellite constellation is meant an array of satellites that serve a common goal. A constellation of satellites, wherein the satellites fly in the same direction with a constant distance is referred to as a satellite formation. In many cases, a constellation of satellites for global coverage of a service is used ( eg, satellite navigation, satellite communication and others). A global coverage means that the illumination zones of satellites covering the surface completely, so that at any time at any place on Earth, a satellite is accessible (but still dependent on the prevailing local conditions ).
- 2.1 LEO constellations 2.1.1 The Walker constellation
- 2.1.2 Polar satellite constellation
- 2.2.1 Molniya constellation
Background
Historical Summary
The first people who brought out 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 "). Due to the work of JG Walker ( 70s), whose notation, constellation with circular satellite orbits at different orbit heights and with different orbital inclinations, it was named after him: the Walker constellation. Then constellations were having a global service coverage, with four satellites in elliptical orbit lanes, a published with three satellites and in the new millennium with a two satellites.
Design of a satellite constellation
The challenge in the design of a constellation consists in choosing the appropriate parameters. In this case, the various orbital parameters, such as orbit height, shape, eccentricity, inclination, etc., differ for the satellite of a constellation, which results in that the geometric complexity of the constellation increases. The orbit parameters and their dependencies are varied so that four key parameters are only briefly presented:
One of the first issues refer to the service cover. This takes into account the areas on the Earth where an organization wants to offer a service. Thus, for example, be the polar caps of lesser interest, since too low populated than the rest of the surface (see Globalstar Vs. Iridium ). On the other hand, only the service coverage of a state of particular importance can be. The type of service coverage, whether globally or partially, significantly influenced the type of constellation.
From a financial perspective plays the number of satellites, due to the construction and transport, an essential role. Thus, the cost of building the Iridium communications system with 66-93 satellites in U.S. $ or the successor system about 5 billion is estimated to be 72-81 satellites at U.S. $ 2.9 billion. The number of satellites affects the required orbit to cover a service or the geometric shape of the constellation. However, the number of satellites is not the only cost drivers, the technologies to be used, the orbit height ( environmental conditions ) or the ground infrastructure play a more significant role. This is, inter alia, be recognized by the Galileo satellite navigation system, which causes despite the lower number of satellites of 30 pieces costs in the amount of 6.7 to 6.9 billion euros.
Once the desired scope of service coverage known, determines the orbit height with the Konstellionstyp significantly the required number of satellites. With increasing orbit height but increases the radiation due to the decrease of the strength of the geomagnetic field. This increases the cost of development of the satellite type. Furthermore, the required transmit power increases with orbit height and the time offset due to the communications path. Using different orbit shapes such as circular, elliptical and the orientation, the number of satellites can be reduced by increasing the geometric constellation complexity. Due to the large number of parameters, this optimization is performed numerically in practice.
The constellation pattern and type determine the service cover by means of varying the number of orbit planes and their inclinations. For example, is a service cover the polar ice caps in a Walker constellation with low and medium orbit inclination (~ 60 ° ) is not possible, whereas a polar constellation (inclination ~ 90 ° ) covering this area. The orbit planes and their orientation in turn affect the ground infrastructure, it must be at least one ground station to each orbital plane (depending on the type of service) are available, which can make contact with the satellite in this orbit.
Satellite constellations
LEO constellations
This type of satellite constellations is intended for low earth orbit. Background is the rising radiation load applied with increasing altitude orbit at the satellite. This increases the design and manufacturing costs and / or reduce the life of a satellite or a constellation of satellites. The two best known constellations with circular orbits are the Walker and the polar satellite constellation.
The Walker constellation
Walker constellation in English also Walker Delta Pattern Constellation, describes the distribution of the various satellites in circular orbits. The orbits have it all the same orbital 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: Satellite Number, p: Number of orbits (evenly distributed ), f: Phase parameter (0 to p-1)
Of the phase parameter can be interpreted as follows:
The true anomaly of the satellite 2 ( the next eastern satellite of Satellite 1 ) lies to the additional amount higher than the true anomaly of the satellite 1 1, wherein the satellites 1 and 2 are on different orbital paths. That F is the phase shift of the satellite to the reference plane of distribution (usually the equator). For f = 0 in each case always one satellite per orbit at the same time crossing the equatorial plane, wherein f > 0 first, any satellite exceeds the equator line (Figure " 1"), followed by the next western satellite (Figure " 2") which in turn is followed by its closest western satellites ( Figure " 3").
Example: 54 °: 18/3/1
This Walker constellation ( see illustration) contains 18 satellites that orbit spread over 3 levels, a total of 6 satellites per orbit plane, each orbital plane has an inclination of 54 ° ( not shown in figure). The phase shift between the satellite planes is 20 °.
Depending on the inclination of the orbit tracks the polar caps can not be covered in a Walker constellation.
Polar satellite constellation
A polar constellation, also known in English Walker Polar Star Constellation pattern, characterized by an inclination angle of about 90 °, ie the satellites in the constellation cross the polar ice caps. A Walker Delta Pattern Constellation is therefore a polar constellation with an inclination of approximately 90 °. This coverage of the polar regions is achieved, but from a commercial perspective rather insignificant ( for thinly populated ). For scientific research missions to the polar caps, however, such communication systems are of high interest. The Iridium satellite constellation is in contrast to Globalstar a polar system. For this reason, the Iridium communications system is preferably used 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.
High elliptic constellations
Molniya constellation
A Molniya constellation is characterized by the use of the orbit type Molniya orbit ( orbit hochelliptischer ) from. A Molniya orbit has the advantage that a satellite, a relatively long time can provide a service under the domain of the apogee. This type is used for russian communication satellite, since the transmission power of the geostationary satellites for the northern latitudes Russia would be too large and a communication link to a satellite constellation is a polar briefly and would require too many satellites. An example of such a situation is the Satellite Data System (SDS ) ( see figure at right ) of the armed forces of the United States that is used starting from 1976 with SDS -1 through this.
Molniya orbit with hour markers
MEO satellite constellations
MEO constellations are preferably used by navigation satellite systems. Due to the height to be less than in the LEO satellites, but requires a higher transmission power. Furthermore, these systems are located in the Van Allen Belt, which has the consequence that they must be designed for a higher radiation dose.
Geostationary satellite constellations
The advantage to the deployment of a constellation of satellites in GEO is the minimum number of satellites required for a global service coverage. Theoretically, a maximum of two satellites would be needed in order to reach all places on earth can (when the earth is a perfect and smooth ball would be ). Practically, however, is at the interfaces as well as in high latitudes an availability not guaranteed, due to the local conditions such as hills, mountains, buildings and other obstacles. The transmit power plays an important role, so that the Russian communications satellite did not use GEO- deployment, but a Molniya orbit. For this reason, GEO constellations have at least three satellites ( see figure). NASA uses such a constellation type to support their space missions in LEO. This constellation is as TDRS system (English: Tracking and Data Relay Satellite System) known.
Orbit combinations
As previously mentioned, the number of satellites can be reduced by increasing the constellation complexity. Thus, for example, different types of orbit, such as LEO and MEO used for a constellation where an inter-satellite link to satellite constellation that must exist on the two types of orbit. Furthermore, orbits and their alignment can be used to generate, for example, polygonal configurations. The possibilities are quite varied, so that reference is made here only briefly.
Others
What is not described in detail in this context, inter-satellite links are ( eng.: inter -satellite link, Abbr: ISL) and their use in satellite networks. Inter- satellite links are relevant to the routing of the received data., The satellites of a constellation no interconnection prepared as is the case with Globalstar, the presence of a base station in the footprint is required, which transmits the data forwarded from the satellite to the terrestrial network. An alternative is to use ISL, as with iridium. By these compounds can only data through the satellite constellation, without an intermediate step of a ground station is transmitted. The continuation of this technology leads to satellite networks. This rather theoretical systems could one day provide an infrastructure similar to the Internet in the space available.
Applications
Satellite constellations are used in various areas of their application, such as:
- Audio communication: Globalstar, Inmarsat, Iridium
- Data communication: Orbcomm
- Satellite Navigation: GPS, GLONASS, Beidou, Galileo
- Remote Sensing: Disaster Monitoring Constellation, RapidEye
- And other