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There are some type of space applications that require very large amount of data to be transmitted for instance the transmission between different earth orbiting satellite and this type of communication also called (inter-satellite communication) which was established in 2001 by ESA. These links have the ability to transmit tens of megabits per second and even more over many thousands of kilometers by using adequate laser average powers of the order of a few watts. Exchanging of data can be done between more than one spacecraft and a station on or near earth, an example is the ability of planetary probes to generate a lot of data image and the big challenge here is while sending large amount of data back to earth. Before radio links which operates in X or ka bands were the only available technology, however optical data links are considered for the downlink at this time, whereas any wanted data volume is much larger than the uplink.

With optical communication, transmission capacity can be expand greatly to hundreds of Kbit or even many megabits per second. The spacecraft then has a pulsed laser source and a moderate size of optical telescope targeting the receiver. A large ground based telescope can be the latter or a transceiver in an earth orbit.

One of the main advantages of using optical communication rather than radio links is that the much shorter wavelength permits for a much more directional sending and receiving of information, as a result having much higher rates and lower power necessities. Technically, the Gain of the antenna can be much higher. This is essential for bridging interplanetary distances, however optical links are so sensitive and can be easily affected by weather conditions.

In today’s market, free space optical communication is mainly inter-satellite links (ISLs) and there is also another type of links which is the high data rate (many Gbps) space/earth links, however this type of link is very difficult because of propagation effects that results from the weather and atmosphere. Below is a detailed discussion about inter-satellite links and its types

Inter-satellite links

In order to have an autonomous formation of satellite constellation, satellites must be able to communicate directly with each other through inter-satellite links (ISLs). Inter-satellite links are two direction communication paths between different satellites. These links are able to provide flexibility in space segment whereas reducing the cost of the system’s earth segment.

Inter-satellite links are considered as a specific beam of multi beam satellite, the beam here is pointed towards other satellites rather than pointing towards the earth. Inter satellite links can be classified into three classes and they are as below:

· Links between geostationary and low earth orbit satellite (GEO-LEO):

This link creates a permanent relay between one or more earth stations and a group of satellite by a geostationary satellite resulting in a low earth orbit at a high order of 500 to 1000 km. This type of link is presently used in the NASA tracking network for the purpose of tracking and data relay satellite (TDRS). It is also used by the European satellite (ARTEMIS) in order to provide a communication between a low earth orbit spacecraft and the ground.

· Links between geostationary satellite (GEO-GEO):

There are many benefits from using this type of links; some of these advantages are listed below:

1. Increasing system’s capacity:

In case if the demand of the traffic increases and exceeds satellite’s capacity, replacing the satellite by another satellite which has a greater capacity is necessary. This satellite will face many problems because of finding the proper launcher and because of the cost as well. As a replacement for this, it is better to use another identical satellite to the first one and the excess traffic of the origin will be carried by the inter-satellite link.

Figure1: The figure above shows how the use of inter-satellite links increases system’s capacity without heavy investment in the earth segment.

(a) Satellite network.

(b) Satellite is started on to increase system’s capacity; here stations should be set with two antennas.

(c) With Inter-satellite Links only in most heavily loaded region, stations should be set with two antennas.

(d) Stations are distributed between the two satellites and the traffic between the two groups of stations is carried by the Inter-Satellite Link.

2. Extension of the coverage of a system:

The inter-satellite link permits two different network’s earth stations to be connected with each other as a result combining the geographical coverage of the two satellites.

Figure2: the above figure shows system’s coverage extension.

a) Interconnection of the each coverage’s station with a satellite link.

b) Interconnection without an inter-satellite link by a station common to the two networks.

c) Interconnection without an inter-satellite link with a terrestrial network.

3. Increase of the minimum elevation angle of the earth station:

Having a single satellite with a long distance link needs an earth station with a small elevation angle. This then will cause degradation of G/T for receiving station and will increase the risk of intervention with terrestrial microwave arrays. It’s possible to use angles of about only by using two interconnected satellites.

Figure 3: geostationary satellite increase the minimum elevation angle of the earth station

4. Decreasing the constraints and bonds on orbital position:

Usually the orbital position of a satellite is the result of a conflict; this is determined and solved by a technique called co-ordination. This coordination happens between the desire of the operator satellite in order to ensure whether the coverage of the service is under the proper conditions or not. And avoiding interference with established systems is needed. When inter-satellite links allows sharing of traffic amongst many satellites in different orbital positions, it provides the operator with some of latitude in the positioning of the satellite.

· Links between low orbit satellite (LEO-LEO):

All the benefits of the low orbit satellite and the growing congestion of geostationary satellite orbits suggest the development of the future of orbiting satellite. Like anything this type of link has some advantages for instance limiting the time duration while communication and relatively small coverage. However these disadvantages of the orbit satellite can be limited and reduced in a network having a huge number of satellites which are connected with each other by inter-satellite links and prepared with a mean of switching between beams.

Motorola which is one of the biggest corporations has planned to build, promote and operate such a network called IRIDIUM, for worldwide cellular personal communication services. This system contains a constellation of 66 satellites which are interconnected with each other, initially designed with 77 satellites.

The worldwide development of space based optical communication has been supported by government agencies and the main funding agencies of this development are the European agency, the Japanese government, NASA and the DOD in the United States. This is changing like the commercial satellite world integrates optical ISLs, and corporations will be ready to take partnership and develop more and more of their independent projects and development funds.

- Japan:

The Japanese has a very strong optical communication program. The agency of science and technology has designed the communication research laboratory (CRL) of the ministry of posts and telecommunication as an optical communications center of excellence. Therefore the government has specified that optical communication and technologies which include sensing. These are very important issues for Japan. As a center of excellence, researches have been collected around the world and dedicated a lot of money for the purpose of developing this area.

Figure 4: Japanese optical communication system

- Europe:

In Europe the main driver in the development of optical communication is the European Space Agency (ESA) and there are a number of national efforts as well. (Artemis) which is the abbreviation of the advanced relay and technology mission of the European space agency handles the payloads for the demonstration and improvement of the advanced technologies and services, especially data relay, land mobile communication and navigation.

Artemis was launched abroad Ariane 5 12th of July in 2001, however they failed to reach the geostationary orbit because of the malfunction which happened on the launcher. After 300 days they again started the operation using electric propulsion for orbit control and finally on 31 of January in 2003, Artemis successfully placed into the geostationary orbit.

Figure 5: The Advanced Relay and Technology Mission (Artemis).

Space based optical communication between satellites at very high rates (10 GB/s) is now possible. It is also reasonable to believe that as more space packages are built and widespread on orbit operation experience expands in the next few years, the cost of higher rate optical cross-links will be basically lower than their microwave functional equivalent. The next step with having such a powerful technology is the awareness of an optical satellite system of worldwide extent. In fact this optical satellite network could make a revolution in the architecture of the space system which may be used as a serious subsystem. Offering communication services and remote sensing are two examples of these space systems.

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