Optical Satellite Communications

Presented to: Prof. Dr. Hossam Shalaby Presented by: Eng. Islam Mohamed Salah Kotb Course: Laser and Electro-Optics ( EE-106381)

Faculty Of Engineering Alexandria University

January 2009

2

Contents: page 1. Free Space Optical Communications………………………………………… 3 2. Intersatellite links……………………………………………………………... 4 3. Global Achievements………………………………………………………… 15 4. Optical Satellite Network – The future……………………………………….. 21 5. References……………………………………………………………………... 29

3

Free Space Optical Communications:

Optical data transmission on Earth is in most cases done via optical fibers, because these allow the transmission over relatively large distances without excessive power losses, alignment issues, and disturbing influences of the atmosphere. However, it is also possible to transmit data optically via free space, not exploiting any kind of waveguide structure. This kind of optical communications has early origins, e.g. the “photo phone” patent by Alexander Graham Bell in the 1870s and the optical telegraph and is now increasingly used, both in space and on Earth. Generally, it requires an unobstructed line of sight between sender and receiver, and normally also some special free-space optics such as telescopes. Space-based, free-space optical communications is a concept that has been around for many years. In the last few years, however, there has been impressive activity to bring the concept to fruition in civilian and government non-classified projects. Today's market for space-based optical communications is primarily intersatellite links (ISLs) which are the main focus of this study. There is also a place for high data rate (many Gbps) space-earth links, though propagation effects due to the atmosphere and weather make this a much more difficult link. Some activity in space-earth optical communications will also be covered here.

One significant factor for Optical system is that the optical system will typically have a much narrower beamwidth than the RF system. This has both a positive and negative side. On the positive side, a narrower beamwidth means that the potential for interference to or from adjacent satellites will be reduced. This is particularly important in large LEO constellations. On the negative side, the requirements for more accurate pointing, acquisition and tracking (PAT) and the impact that this may have on the spacecraft could impose an unwelcome burden. Accurate PAT is critical to the acceptance of optical ISLs.

A secondary, though not unimportant, fact about optical communications is that, unlike the RF spectrum which is regulated by national and international agencies, the optical spectrum is currently unregulated.

A third factor is the reliability of optical communications systems, particularly their lasers, has been a concern in the past. This issue is being overcome by advances in optical and laser technology but needs documented space validation for wider acceptance.

Finally, the basic advantage of the optical technology over radio links is that the much shorter wavelength allows for a much more directional sending and receiving of information. In technical terms, the antenna gain can be much higher. This is particularly important for bridging interplanetary distances.

4

2. Intersatellite Links (ISL):

Some space applications require large amount of data to be transferred. An example is the transmission between different Earth-orbiting satellites (inter-satellite communications), which was first demonstrated by ESA in 2001 (ESA). It is possible to transmit tens of megabits per second or more over many thousands of kilometers, using moderate laser average powers of the order of a few watts.

Space-Earth links have been, and continue to be, primarily RF. Because of the advantages of optical systems related earlier, Japanese, European and U.S. researchers are investigating optical space-earth links from LEO as well as the far reaches of outer space. Optical links face a severe disadvantage due to the effects of the atmosphere and weather. Solutions include adaptive optics, spatial diversity, and onboard storage with burst transmission under good conditions. The first applications are likely to be in scientific satellites but as operational methodologies are developed, space-earth optical links will work their way into commercial systems.

Data can also be exchanged between a more remote spacecraft and a station on or near Earth. For example, planetary probes can generate a lot of image data, and a major challenge is to send large amount of data back to Earth. Until recently, radio links operating e.g. in the X band or Ka band were the only available technology. Currently, optical data links are considered particularly for the downlink, where the desired data volumes are much larger than for the uplink, and optical communications could greatly expand the transmission capacity to hundreds of kbit or even several megabits per second. The spacecraft then has a pulsed laser source (employing pulse position modulation, for example) and an optical telescope of moderate size targeting the receiver. The latter can be a large ground-based telescope or a transceiver in an Earth orbit.

Intersatellite links can be considered as particular beams of multibeam satellites; the beams in this case are directed not towards the earth but towards other satellites. Three classes of intersatellite link can be distinguished:

• Links (GEO-LEO) between geostationary earth orbit (GEO) and low earth orbit (LEO) satellites; also called inter-orbital links (IOL).

• Links between geostationary satellites (GEO-GEO). • Links between low orbit satellites (LEO-LEO).

2.1 Links between geostationary and low earth orbit satellites (GEO-LEO):

This type of link serves to establish a permanent relay via a geostationary satellite between one or more earth stations and a group of satellites proceeding in a low earth orbit at an altitude of the order of 500 to 1000 km.

5

This concept is currently operated in the NASA tracking network by means of the tracking and data relay satellites (TDRS). Also it is used by the European satellite (ARTEMIS) which provides communications between the ground and low earth orbit spacecraft.

2.2 Links between geostationary satellites (GEO-GEO):

There are many advantages for using (GEO-GEO) satellite links among which are

2.2.1 Increasing the capacity of a system:

If the traffic demand increases and exceeds the capacity of the satellite it is therefore necessary to replace this satellite by another one with greater capacity which faces many problems due to the cost and finding the suitable launcher. Instead of this we use a second satellite identical to the first one and the intersatellite link carries the excess traffic of region 1. (Figure 2.1)

Figure 2.1 Use of an intersatellite link to increase the capacity of a system without heavy investment in the earth segment. (a)1 satellite network. (b) 2nd satellite is launched to increase the capacity of the system; stations must be equipped with 2 antennas.. (c) With an ISL only stations of the most heavily loaded region must be equipped with 2 antennas. (d) The stations are distributed between the 2 satellites. The ISL carries the traffic between the 2 groups of stations.

6

2.2.2 Extension of the coverage of a system: An intersatellite link permits earth stations of 2 networks to be interconnected and hence the geographical coverage of the 2 satellites to be combined. (Figure 2.2)

Figure 2.2 Extension of system coverage. (a) Interconnection of the stations of each coverage by a satellite link. (b) Interconnection without an intersatellite link by a station common to the 2 networks. (c Interconnection without an intersatellite link by a terrestrial network. 2.2.3 Increase of the minimum elevation angle of the earth stations:

Long distance link by a single satellite requires earth stations with a small elevation angle which causes degradation of G/T for receiving station and increases risk of interference with terrestrial microwave arrays. By using 2 interconnected satellites we can use angles of about 20o. (Figure 2.3) 2.2.4 Reduction of the constraints on orbital position The orbital position of a satellite is often the result of a conflict , resolved by means of a procedure called co-ordination , between the desire of the satellite operator to ensure coverage of the service are under the best conditions and he need to avoid interference with established systems. Intersatellite links when they permit traffic to be shared among several satellites in different orbital positions, provide the operator with some latitude in the positioning of his satellite.

7

Figure 2.3 Increase of the minimum elevation angle of earth stations. 2.2.5 Satellite clusters The principal is to locate several separate satellites in the same orbital position with a separation of around 100 km and interconnection by intersatellite links. The satellites are thus all in the main lobe of an earth station antenna and appear equivalent to a single large capacity satellite. 2.2.6 A global network: Figure 2.4 shows the design of a global network based on 9 geostationary STAR satellites, which establish a basis for worldwide communication, and a set of local satellites connected to these regional satellite links.

Figure 2.4 A Global Network

8

2.3 Links between low orbit satellites (LEO-LEO): The advantages of low orbit satellites and the increasing congestion of geostationary satellite orbits suggest the future development of orbiting satellites. In fact the disadvantages of an orbiting satellite (limited duration of communication time and relatively small coverage) can be reduced in a network containing a large number of satellites which are interconnected by intersatellite links and equipped with a means of switching between beams. Motorola Inc. has planned to construct, launch and operate such a network, called IRIDIUM, for worldwide cellular personal communication services. The system incorporates a constellation of 66 satellites, designed initially with 77 satellites. 2.4 Frequency bands: Table 2.1 indicates the frequency bands allocated to intersatellite links by the Radiocommunication Regulations. These frequencies correspond to strong absorption by the atmosphere and have been chosen to provide protection against interference between intersatellite links and terrestrial systems. Intersatellite service Frequency Bands

22.55-23.55 GHz 24.45-24.75 GHz 32-33 GHz

Radio Frequency

54.25-58.2 GHz 0.8-0.9 µm (AIGaAs laser diode) 1.06 µm (Nd:YAG laser) 0.532 µm (Nd:YAG laser)

Optical

10.6 µm (CO2 laser) Table 2.1Frequency bands for intersatellite links 2.5 Radio-frequency links: Propagation losses reduce to free space losses since there is no passage through the atmosphere. Antenna pointing error can be maintained at around a tenth of the beamwidth and this leads to a pointing error loss of the order of 0.5 dB. For practical applications antenna dimensions of the order of 1 to 2 m should be considered. The development of high capacity RF intersatellite links between geostationary satellites will imply re-use of frequencies from one beam to another. In view of the small angular separation of the satellites, it would be preferable to use narrow beam antennas with reduced side lobes in order to avoid interference between systems. Consequently, and in view of the limited antenna size imposed by the launcher and the technical complexity of the deployable antennas which may be deployed, the use of high frequencies is indicated. The use of optical links may be usefully considered.

9

2.6 Optical Links 2.6.1 Establishing a link: Two aspects should be indicated: -

• The small diameter of the telescope which is typically of the order 0.3 m. In this way one is freed from congestion problems and aperture blocking of other antennas in the payload.

• The narrowness of the optical beam which is typically 5 microradians. Notice that this width is several orders of magnitude less than that of a radio beam and this is an advantage for protection against interference between systems. But it is also a disadvantage since the beamwidth is much less than the precision of satellite altitude control (typically 0.1 0 or 1.75 mrad). Consequently an advanced pointing device is necessary; this is probably the most difficult technical problem.

There are three basic phases to optical communications: • Acquisition: the beam must be a wide as possible in order to reduce the

acquisition time. But this requires a high power laser transmitter. A laser of lower mean power can be used which emits pulses of high peak power with a low duty cycle. The beam scans the region of space where the receiver receives the signal, it enters a tracking phase and transmits in the direction of the received signal. On receiving the return signal from the receiver, the transmitter also enters the tracking phase. The typical duration of this phase is 10 seconds.

• Tracking: the beams are reduced to their nominal width. Laser transmission becomes continuous. In this phase which extends throughout the following, the pointing error control device must allow for movements of the platform and relative movement of the two satellites. In addition, since the relative velocity pf the 2 satellites is not zero, a lead-ahead angle exist between the receiver line of sight and the transmitter line of sight. As will be demonstrated below, the Lead-ahead angle is larger than the beamwidth, and must be accurately determined.

• Communications: information is exchanged between the two ends. 2.6.2 Lead-ahead angle:

Consider two satellites, S1 and S2 respectively moving with velocity vectors VS1

and VS2, whose components orthogonal to the line joining S1 and S2 at time t are respectively the two vectors represented in figure 2.5 by VT1 and VT2. The propagation time of a photon from S1 to S2 is tp=d/c, where d is the distance between the two satellite at time t and c the speed of light (c=3 x 108 m/s). The lead-ahead angle β is given by:

C

VV TT 212−

=β (2.1)

Where |VT1-VT2| is the modulus of the difference vector VT1-VT2.

10

Figure 2.5 Lead-ahead angle for intersatellite between two satellites Sl and S2 with velocity vector components VT1 and VT2 in a plane perpendicular to the line joining S1 and S2 at time t; tp is the propagation time of a photon from S1 to S2. Two situations will now be considered:

1. Intersatellite links between two geostationary satellites 2. Interorbital links between a geostationary satellite and a low earth orbiting

satellite.

2.6.2.1 Between two GEO satellite separated by angle α (Figure 2.6)

As both satellites are on the same circular orbit, the velocity vectors VS1 and VS2, which are tangential to the orbit, have equal modulus, i.e.:

|VS1|=|VS2|=ω(RO+RE)=3075 m/s

Where: ω is the angular velocity of a geostationary satellite =7.293x10-5 rad/ s RO is the altitude of a geostationary satellite = 35786 km RE is the earth radius = 6378 km

CRR

CVV EOTT )2/sin()(4||2 21 αω

β+

=−

= (2.2)

11

Figure 2.6 Lead head angle for intersatellite links between 2 geostationary satellites 2.6.2.2 Between a GEO satellite and an LEO satellite with circular orbit ( Figure 2.7):

The relative velocity of the 2 satellites varies with time and so does the value of

the lead-ahead angle. Its maximum value is obtained when the LEO satellite crosses the equatorial plane. Denoting as i the LEO satellite orbit inclination, then :

2/1

212

22

121 }cos||||2|||{||| iVVVVVV SSSSTT −+=− (2.3) Where |VS1|= ωGEO(RO+RE)=3075 m/s |VS2|= ωGEO(h+RE)

And h is the LEO satellite altitude, ωLEO+µ1/2(h+RE)-3/2 is the LEO satellite angular rate (µ=3.986 x 1014 m3/s2) From equation 2.1

2/121

22

21 }cos||||2|||){|2( iVVVV

C SSSS −+=β (rad) (2.4)

12

Figure 2.7 Lead-ahead angle at a geostationary (GEO) satellite for interorbital links between it and a low earth orbiting (LEO) satellite 2.6.3 Transmission: Laser sources operate in single and multi-frequency modes. In single frequency mode spectral width varies between 10 kHz and 10 MHz. In multi-frequency mode it is from 1.5 to 10 nm. The power emitted depends on the type of laser. Table 2.2 gives orders of magnitude. Type of Laser Wavelength Transmitted power Solid state AlGaAs 0.8-0.9 µm About 100 mW InPAaGa 1.3-1.5 µm About 100 mW ND:YAG 1.06 µm 0.5-1 w Nd:YAG 0.532 µm 100 mW Gas Laser CO2 10.6 µm Several tens of watts Table 2.2 Typical values of transmitted power for Lasers.

Modulation can be internal or external. Internal modulation implies direct modification of the operation of the laser. External modulation is a modification of the light beam after its emission by the laser. The intensity, the frequency, the phase and the polarisation can be modulated. Phase and polarisation modulation are external. Intensity and frequency modulation can be internal or external. 2.6.4 Transmission Loss: Transmission loss reduces to free space loss: L=(λ/4πR)2 (2.5) Where λ is the wavelength and R is the distance between transmitter and reveiver.

13

2.6.5 Reception: The receiving gain of the antenna is given by: GR=(πDR/λ)2 Where DR is the effective diameter of the receiver antenna. The receiver can be of the direct detection type (Figure 2.8) or a coherent detection receiver (Figure 2.9). With direct detection, the incident photons are converted into electrons by a photodetector. The subsequent baseband electric current at the photodetected output is amplified then detected by a matched filter. With coherent detection, the optical signal field associated with the incident photons is mixed with the signal from a local laser. The resulting optical field is converted into a bandpass electric current by a photodetector and is subsequently amplified by an intermediate frequency amplifier. The demodulator detects the useful signal either by envelope detection or by coherent demodulation. The receiving losses include optical transmission losses, and for coherent detection, losses associated with the degradation of the wavefront (the quality of the wavefront is an important characteristic for optimum mixing of the received signal field and the local oscillator field at the photodetector front end). Filtering reject out-of-band photons, also introduces losses, since the transmission coefficient reduces with bandwidth. A typical filter width is from 0.1 to l00nm.

Figure 2.8 Optical ISL direct detection receiver

Figure 2.9 Optical ISL coherent detection receiver. The signal-to-noise power ratio at the detector output depends on the type of detection. Coherent detection confers a higher value of S/N than direct detection. Unless high data rates are involved, there is no advantage in weight or power from using coherent

14

detection techniques for communications along with a separate direct detection receiver for acquisition and tracking. 2.7 Conclusion: Intersatellite links permit the following:

• The use of a geostationary satellite as a relay for permanent links between low orbit satellites and a network of a small number of earth stations.

• An increase in system capacity by combining the capacities of several geostationary satellites.

• The planning of systems with a higher degree of flexibility. • Consideration of systems providing a permanent link and worldwide coverage

using low orbit satellites as an alternative to systems using geostationary satellites.

Optical technology is more advantageous in terms of mass and power consumption for high capacity links.

15

3. Global achievements:

Space-based optical communications development around the world has been primarily supported by government agencies. The European Space Agency, the Japanese government, and NASA and the DOD in the United States have been the main funding agencies. This is changing as the commercial satellite world integrates optical ISLs, and companies will be willing to form partnerships and invest more of their own independent research and development funds.

3.1 Japan

The Japanese have a strong program in optical communications. The Science and Technology Agency has designated the Communications Research Laboratory (CRL) of the Ministry of Posts and Telecommunications as a Center of Excellence for Optical Communications and Sensing. Thus the government has determined that optical communications and optical technologies, including sensing, are extremely important issues for Japan. As a Center of Excellence, the CRL has gathered researchers from around the world and devoted a lot of money for developments in this area. An overview of the types of links and systems being considered, from ISLs to space-earth links, is shown in Figure 3.1. A comment was made during the site visit to CRL that all ISLs of the future would be optical.

Figure. 3.1. Japanese Optical Communications System Plan (CRL).

3.1.1Engineering Test Satellite VI (ETS-VI)

ETS-VI was intended to go into GEO. It did not achieve this, however, and lasted from 1994 to 1996, its lifespan a result of the effects of being in the wrong orbit. CRL and NASA's Jet Propulsion Laboratory (JPL) were able to do some space-earth experiments during the life of the spacecraft. It provided a bi-directional link at 1.024 Mbps using intensity modulation and direct detection (IM/DD). The spacecraft used a 7.5 cm diameter telescope. The downlink used a 0.83 µm, 13.8 mW AlGaAs laser diode. The

16

uplink was at 0.51 µm using an argon laser from a 1.5 m telescope in Tokyo. The Laser Communications Experiment (LCE) is shown in Figure 3.2. Its mass was 22.4 kg and it consumed 90 W max

Figure. 5.2. ETS-VI LCE.

3.1.2 Optical Inter-Orbit Communications Engineering Test Satellite (OICETS)

OICETS, which was launched into LEO in 2000 carrying an optical terminal is compatible with the European SILEX terminal and will communicate with the ESA ARTEMIS satellite in GEO. The Laser Utilizing Communications Experiment (LUCE) will have a 26 cm telescope with a 50 Mbps intensity modulated 0.847 µm, 200 mW laser diode link to ARTEMIS and a 2.048 Mbps direct detection link at 0.819 µm from ARTEMIS.

3.2 Europe

In Europe, ESA has been a primary driver in the development of optical communications although there have been a number of national efforts also.

The Advanced Relay and Technology Mission (Artemis) of the European Space Agency (ESA) carries payloads for the demonstration and promotion of advanced technologies and services, in particular data relay, land mobile communications and navigation.

Artemis was launched aboard Ariane 5 on July 12, 2001, but failed to reach geostationary orbit due to a malfunction on the launcher. After 300 days of operation using electric propulsion for orbit control Artemis was placed successfully into geostationary orbit on January 31, 2003.

17

Figure. 3.3. ARTEMIS.

Figure. 3.4. ARTEMIS.

18

3.2.1 Optical Ground Station has sights set on Artemis

The OGS, located at the Instituto de Astrofisica de Canarias' (IAC) Observatorio del Teide in Tenerife, will be used for the commissioning and later for periodic checkouts of the laser communication terminal onboard Artemis.

To perform laser communication from ground to space, the sky must be perfectly clear, without any clouds, and the path through the atmosphere must be as short as possible. Both conditions are satisfied at the Observatorio del Teide, which is situated above cloud level at 2400 metres altitude and is the closest point to the equator in an ESA member state.

Figure 3.5OGS telescope 3.2.2A world first : Data transmission between European satellites using laser light

On 22 November 2001, for the first time, a data link between satellites was established using a laser beam as signal carrier. On board ESA's Artemis satellite - launched last July 2001 by an Ariane 5 - is the SILEX system. This system provides an optical data transmission link with the CNES Earth observation satellite SPOT 4, which is orbiting the earth at an altitude of 832 km while Artemis is temporarily in a parking orbit at 31 000 km. Through the laser data link, images taken by SPOT 4 can be transmitted in real time to the image processing centre at Spot Image in Toulouse, France, via Artemis, thus drastically reducing the time between taking the picture and its delivery to the centre. This is possible whenever the two satellites are in line of sight. Without the Artemis relay the images are stored on board in SPOT 4's memory and dumped to the ground stations.

The experiment performed last night consisted in establishing the link four times: in the course of four successive SPOT 4 orbits, the SILEX terminal on board Artemis activated its optical beacon to scan the area where SPOT was expected to be. When contact was made, SPOT 4 responded by sending its own laser beam to Artemis. On receiving the SPOT 4 beam, Artemis stopped scanning and the optical link was maintained for a pre-programmed period lasting from 4 to 20 minutes.

During the period when the two satellites were "communicating", test data were transmitted from SPOT 4 to the ground via Artemis at a rate of 50 000 000 bits per second (50 Mbps). The extremely high accuracy of the data stream was confirmed at ESA's test station in Redu (Belgium) and the SPOT 4 receive station in Toulouse. The main challenge in establishing an optical link between satellites is to point a very

19

narrow beam with extreme accuracy to illuminate the partner spacecraft flying at a speed of 7000 m/s. Last night's experiment was performed under worst-case conditions since Artemis is not in its nominal geostationary position but in an lower parking orbit, circling the Earth every 19 hours.

This experiment was preceded by a series of tests a week earlier, during which a link was established between Artemis and ESA's optical ground station in Tenerife. Thoses tests demonstrated the correct operation of the SILEX terminal and paved the way for the subsequent steps.

The SILEX system consists of two terminals: one on board Artemis, the other on SPOT 4. Both terminals were designed and built by Astrium. The definition and procurement of the system were conducted in close cooperation between ESA and the French space agency, CNES.

Figure 3.6 Artemis and SPOT 4 communicating via the SILEX system - Artist's impression. 3.2.3Successful optical data relay link between OICETS and Artemis

On 9 December 2005 the first bi-directional optical link between KIRARI, the Japanese satellite officially called OICETS (which stands for "Optical Inter-orbit Communications Engineering Test Satellite"), and ESA’s Artemis was made on Friday 9 December 2005. KIRARI (meaning “glitter” or “twinkle” in Japanese) is the second optical data relay satellite using Artemis, following the world premiere laser link with SPOT-4 in November 2001 in the framework of the SILEX development, an innovative payload which provides a laser beam as a data signal carrier. This optical service has operated regularly since then and accumulated more than 1100 links totalling 230 hours to date.

The KIRARI experiment is the result of a long standing agreement between JAXA, the Japan Aerospace Exploration Agency, and ESA for cooperation on data relay services, and builds upon the SILEX development.

KIRARI has, for the first time, used an optical link for both data and command transmission. The quality of the link was excellent.

20

The optical link experiment between KIRARI and Aretemis will continue through 2006, making several optical links under different environmental conditions to completely qualify and characterize the KIRARI technology.

KIRARI, which is in low earth orbit at an altitude of about 600 km. As with the SILEX development, the KIRARI link carries data at 50 Mbps in the return direction between KIRARI and the ground, and 2 Mbps in the forward direction between ground and satellite. The transmissions through Artemis are linked to the ESA ground station at Redu, Belgium, which is connected via data lines to the KIRARI control centre at Tsukuba, Japan.

3.2.4Another world first for Artemis: a laser link with an aircraft

On 18 December 2006 Artemis, successfully relayed optical laser links from an aircraft. These airborne laser links, established over a distance of 40 000 km during two flights at altitudes of 6000 and 10 000 meters, represent a world first. The relay was set up through six two-way optical links between a Mystère 20 equipped with the airborne laser optical link LOLA (Liaison Optique Laser Aéroportée) and the SILEX laser link payload on board ARTEMIS in its geostationary orbital position at 36 000 kilometres altitude: a feat equivalent to targeting a golf ball over the distance between Paris and Brussels.

Figure 3.7Artemis laser link to aircraft

Figure 3.8 LOLA telescope assembly

21

4. Optical Satellite Networks- The future:

This work is built on the premise that optical space communication at very high rates ( 10 Gb/s) between satellites is now feasible. It is reasonable to believe as more space packages are built and extensive on-orbit-operation experience develops in the next few years, the cost of high-rate optical crosslinks will be substantially lower than their microwave functional equivalent. A natural next step with such a powerful enabling technology is the realization of an optical satellite network of global extent. This optical satellite network can in turn revolutionalize space system architectures that may use the network as a critical subsystem. Examples of these space systems include those offering communication services or remote sensing. We will summarizes briefly the state-of-the-art of optical crosslink technology, examines architectures (from the physical layer to the application layer) that combine other technologies to form an integrated space and terrestrial network, and finally explores the space of possible revolution in network performance and applications that are enabled by such a key technology innovation.

Figure 4.1 illustrates the general concept of an optical space network and its possible user community. It can simultaneously serve a number of applications such as data readout of space sensors, support of space exploration, and science missions and act as the conduit for data communications for fixed ground terminals and mobile spaceborne, airborne, seaborne, and ground vehicles. The primary links through the atmosphere are assumed to be microwave due to poor optical propagation in bad weather conditions. However, we will mention technologies for special applications where optical downlinks from satellite to airborne and ground terminals are viable if absolute all-weather availability is not required. Note that this space backbone can be in high/geosynchronous (GEO; 40 000 Km altitude), medium (MEO; 5000–15 000 Km), or low (LEO; 1000–2000 Km) earth orbits. Figure 4.2 specifically gives an example of two LEO constellations interconnected by optical crosslinks and connected to terrestrial user terminals and interconnected with fiber and wireless networks via “gateways” forming a global heterogeneous network. Data networks based on such LEO, MEO, or GEO constellation topologies will be an important future alternative to provide global data networking services, especially in areas of poor or congested terrestrial infrastructure deployment, and in mobile and quick deployment application scenarios.

22

Figure 41. General concept of an optical space network and its possible user community

Figure 4.2. Example of an integrated satellite and terrestrial network.

23

Space networking as an application area spans many different aspects of communication and relay services. Figure 4.3 depicts these applications in a logical tree structure. The important forefronts as highlighted are data networking services for near-earth applications with emphasis on bursty computer traffic in both access and trunking.

Figure 4.3. New and important development area for space networking.

To serve a wide variety of usage, this network probably will have a hierarchical architecture, as shown in Figure 4.4. Thus, the optical satellite network is a direct analog of the terrestrial network.

Figure 4.4. Hierarchical space network architecture—wide/metro/local-area networks.

24

As shown in chapter 2 RF links require much larger aperture size than an optical link. Moreover, since the carrier frequencies of optics are very high ( 200 THz), each optical carrier can accommodate very high data rates ( 100 Gb/s) without the nasty dispersion effects of fiber, and there is the possibility of using wavelength-division multiplexing (WDM) to further increase the data rate per optical beam. There is no doubt that optical crosslink technology will greatly revolutionalize space system architectures. We can summarize the distinguishing good properties of optical crosslinks with the following general characteristics, for high rates such as 10 Gb/s: 1) Small antenna sizes ( 30 cm), compared to several feet for RF systems; 2) Modest weight ( 100 lb) and power ( 100 W), compared to several hundred pounds and several hundred watts for RF systems; 3) Continuous operations with the sun in or near the field of view; 4) Easy multiplexing, demultiplexing, switching, and routing for network applications; 5) Much lower cost than RF systems. An optical space communication system can be partitioned into three interacting subsystems each with their separate critical design issues. These three subsystems are: 1) Opto/mechanical/thermal subsystem; 2) Spatial acquisition and tracking subsystem; 3) Communication subsystem. Note that these are logical partitions and not physical partitions. The subsystems, such as the tracking and optical subsystems, may share common physical hardware. 4.2 Optical Satellite Network Architecture: There can be two reasons an optical satellite network is economically viable. The first is that for long-distance intercontinental transmissions, it can be cost-competitive with undersea fiber systems and can become an alternative for terrestrial networks. One interesting property space optical communications has is that the power attenuation due to free-space diffraction loss is only inversely proportional to the square of the link distance, whereas optical fiber attenuation is exponential in distance and amplifiers/repeaters at regular distances are required to maintain performance. Given long enough link distance, the total attenuation seen by the fiber link will become much larger than that seen by the space optical link. In , a simple life-cycle cost comparison of equal capacities in 10-Gb/s quanta was performed. The per-year operating costs are normalized with respect to system lifetime cost ( 10 years for space systems), included nonrecurrent engineering, production, deployment, and operating costs. The crossover between the two systems is estimated to be around 5000 Km (using undersea fiber systems as a benchmark and based on best guessed optical crosslink and RF up- and downlink technology costs). The second reason is that the optical satellite network may provide unique services to space missions and open-air-interface accesses such as voice and data communications over microwave satellite systems for mobile platforms and remote users with no broadband wired access, and satellite and terrestrial distributed sensing readout.

25

4.2.1 Interfaces to RF and Other Access Links and to the Terrestrial Network: Figure 4.5 depicts the various interfaces of the optical satellite network: microwave and optical trunk connections to the terrestrial WAN, metro-area network, and LAN interconnections via microwave up- and downlinks, and microwave and optical individual user accesses. Though limited in rates ( a small number of gigabits/second using high spectral efficiency modulation and coding), microwave links will be the primary connections to the terrestrial network. With this choke point, the optical satellite network cannot be considered architecturally as a simple extension of the terrestrial network but rather a separate network with only moderate rate gateway connections ( 10 Gb/s) to the terrestrial WAN. Optical connections through the atmosphere to the ground can alleviate this choke point, but they can only operate in clear weather and need multipath diversity to provide any sensible network performance. Microwave satellite data accesses at high rates require narrow spot antenna beams for high sensitivities and small user terminals. These can be realized via a multiple beam antenna system using a microwave lens or a microwave phase array.

Figure 4.5 Interfaces of the optical satellite network 4.2.2 Physical Network Topology The primary goal of a backbone constellation is to provide the coverage as required by the users. Users are in LEO, MEO, GEO, and the relevant parts of highly elliptical orbits, as well as airborne and on the ground. These coverage requirements can be met by a variety of constellations, with different altitudes, number of orbital planes, arrangement of the orbits, and arrangement of satellites within the orbits. All of these factors will influence the complexity and performance of the overall system. Given that GEO is the configuration of choice for the backbone of a satellite network, the capacities of the links in this backbone may reach 100 Gb/s and optical links are the only viable candidates. An interesting benefit of a free-space link is that when the traffic load shifts,

26

the physical connection topology can be easily changed via pointing the telescopes to a new satellite, and load balancing can be implemented more easily. 4.2.3 Spacecraft Node Switching Architecture: The backbone optical satellite network nodes must be designed to support necessary network functions just like their terrestrial network counterparts. The current terrestrial networks have backbone routers that deal with the long haul traffic and access routers for aggregation. These data network services may themselves co-exist on the same fiber plant with circuit-switched oriented services. Figure 4.6 shows the connection architecture for an optical satellite network backbone node. A significant difference from a terrestrial backbone node is that the satellite node will also have to deal with accesses from individual users and thus have an aggregation function as well.

Figure 4.6 Optical satellite node connection architecture. Figure 4.7 depicts a backbone constellation with WDM access links and optical wavelength switching at the relay nodes. The node will also have to deal with RF access links. These can be demodulated and multiplexed onto an optical wavelength or can be subcarrier-modulated onto a wavelength, losing some power efficiency but gaining hardware simplification in the process. 4.2.4 Intraspacecraft Optical Network (Spacecraft-LAN) All-optical switching and routing at an optical satellite network node is format-insensitive to the traffic but will lose some link quality in the form of signal-to-noise ratio degradation. Format insensitivity may be a significant advantage for optical satellite systems since space systems are designed and deployed years ahead of user services. Analog space links with high linearity and low distortions can be supported easily by all-optical switching onboard the spacecraft. This gain must be traded off against the loss of link performance over a fully regenerative relay node. A likely architecture for the satellite relay node is that the core backbone has circuit provisioning and switching and

27

only accesses are routed and electronically processed. Thus, all relay nodes are logically one hop away from all the other nodes when viewed at Layer 3, the network layer. Some wavelength efficiency may be sacrificed, but it is a good trade for electronic processing hardware complexity.

Figure 4.7 Optical satellite network WDM switching node. 4.2.5 Optical Links to the Ground and Aircraft—Clear Air Links: It is possible in clear weather conditions to communicate optically to a user terminal from space, especially if the terminal is located on a high-flying aircraft such as an airliner (Figure 4.8). It is also the only viable link (albeit at lowrates due to significant phase front distortions as the optical beam propagate through the plasma generated) to the space shuttle during its RF blackout period during reentry.

Spatial, temporal, and frequency diversity architectures are candidates for fade mitigation of the received signal. A diversity system statistically guarantees that at least one of the independent paths has good quality. In contrast to spatial diversity for wireless systems, spatial diversity for atmospheric optical systems can be readily implemented since the intensity and phase coherence length is on the order of centimeters, i.e., multiple transmitters or receivers only need to be placed centimeters apart to see approximately independent channel fades. 4.2.6 Optical Multiple Access: In a number of specialized applications, multiple users can be within the same field of view of the satellite receiving telescope. From geosynchronous orbit, users more than 100 m apart can be resolved by multiple focal plane detectors of a modest telescope of 10 cm in diameter. However, for many lower rate random access users, such as those

28

on airliners, it is prohibitively costly to assign one optical receiver per user even if it is dynamically scheduled. Thus, an optical multiple-access receiver combining the signals of a larger group of users at the same detector makes sense.

Figure 4.8 Optical links over clear atmosphere and boundary layer turbulence. 4.3 Summary: Not very often in the history of communications and networking have there been truly transforming inventions that result in quantum leaps in the nature of services or costs to the end users. Optical satellite communications will likely be classified as one such transforming technology if its architectural implications are fully exploited. Not only will the satellite network become economically viable, but also its deployment and the extraordinary services it can offer are capable of radically transforming space system architectures. To make a space network cost competitive, it is imperative that the best designs be used. An unimaginative network will use the optical link as a backbone or a high-speed entrance link, which by itself may to some be transforming enough. The opportunity is now to use imagination and creativity to bring home this new breakthrough in satellite networking and none too soon, given the state of stagnation in the aerospace community.

29

5. References: 1. G.Maral, M. Bousquet “ Satellite Communications Systems: Systems, Techniques

and Technology” 3rd ed., Wiley Series in Communications And Distributed Systems.

2. Vincent W.S. Chan, Fellow IEEE, Fellow, OSA “ Optical Satellite Networks”, IEEE Jornal Of Lightwave Technology , Vol.12, No. 11, Nov 2003 pp.2811-2827

3. “Encyclopedia of laser Physics and Technology”, www.rp-photonics.com 4. European Space Agency. www.ESA.org