typical satellite system topologies

Slide Number 1Rev -, July 2001

Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada

Volume 3





Typical Satellite System Topologies

Section 1

Vol 3: Satellite Communication Principles



3.1.1.1: LEO and MEO Schemes

LEO and MEO SchemesWhen initially conceived, LEO and MEO schemes were developed primarily to provide satellite phone service around the world. Many LEO and MEO configurations were considered.

In addition to phone service, new LEO concepts, such as the Teledesic plan, are being designed primarily for broadband IP service.

The primary advantage of a LEO system is its proximity to Earth. As a result, very low power is needed to transmit signals to the satellite within the field of view.

The second advantage of a LEO system is its improved Quality of Service provided by low transmission delays.

In most LEO constellations, 3 or 4 satellites can be visible at one time.

Vol 3: Satellite Communication Principles, Sec 1: Typical Satellite System Topologies

Part 1: Ideas for Earth Coverage




LEO and MEO SchemesThe orbital period for LEOs ranges from 90 to 120 minutes, and satellite visibility above the horizon does not exceed 20 minutes.

Any call connection exceeding the visibility margin will have to undergo a hand-over process to maintain connectivity.

As the satellite travels at a high speed relative to the terrestrial observer, the radio interface must make allowance for large Doppler shifts in frequency.

The radius of the serviced area is between 3000-4000 km. As a consequence, a global system requires a constellation of many orbits, with many satellites on each of the orbits.






LEO and MEO Schemes

A LEO constellation may include on-board processing to enable direct satellite-to-satellite switching, or may employ the “bent pipe” concept, whereby the satellite simply acts like a repeater.

A “bent-pipe” concept offers the operator the flexibility and option to add wide band data links to meet new requirements.

On the other hand, LEO and MEO constellations that incorporate on board processing do not have the same flexibility to easily offer wide band data links. They do, however, offer greater switching capability.








Globalstar ICO Iridium TeledesicOrbit Class LEO MEO LEO LEO

Altitude (Km) 1410 10390 780 695 - 705

Number of Satellites 48 10 66 840

Number of Planes 8 2 6 21

Inclination ( ) 52 45 86.4 98.16

Period (Minutes) 114 360.9 100.1 98.8

Satellite Visibility Time(Minute s)

16.4 115.6 11.1 3.5

Minimum Mobile TerminalElevation Angle ( )

10 10 8.2 40

Minimum Earth-Space Link One-Way Propagation Delay (Ms)

4.63 34.5 2.6 2.32

Maximum Earth-Space LinkOne-Way Propagation Delay(Ms)

11.5 48.0 8.22 3.40

Minimum Earth StationElevation Angle ( )

110 - - 40

Coverage -70 to +70 Global Global Global (minus2 at each

pole)



GlobalstarTM Constellation


Figure 3.1.1.1a Globalstar Constellation



Used by Permission by SaVi, The Geometry Center,University of Minnesota




Globalstar Footprint

Figure 3.1.1.1b Globalstar Footprint



Used by Permission by SaVi, The Geometry Center, University of Minnesota

Spot Beams, Coverage Overlap Area Not Covered

Covered Area




Iridium Constellation

Figure 3.1.1.1c Iridium Constellation







Iridium Footprint

Figure 3.1.1.1d Iridium Footprint





Covered Area




Teledesic Constellation

Figure 3.1.1.1e Teledesic Constellation







Teledesic Footprint

Figure 3.1.1.1f Teledesic Footprint





Covered Area




New ICO



ICO Global Communications, founded in 1995, also declared bankruptcy and re-emerged as New ICO. The controlling shareholder in the reformed company is Teledesic, or more accurately, ICO-Teledesic Global Limited.

The New ICO is planning to put a high bandwidth data connectivity scheme into effect by 2003. The scheme is based on 10 MEOs, orbiting at 10,390 kilometers, and a fixed network of ground sites, collectively called ICONET.

The satellites are of bent pipe design, with the necessary data switching and handling capacity located on the ground.

Since this is effectively a Teledesic project as well, it could be said that this is a combined LEO and MEO internet access scheme.



3.1.1.2: Mixing LEO or MEO with HEO

Mixing LEO or MEO with HEO

Figure 3.1.1.2 LEO, MEO, HEO



EARTH

GEO Orbit

LEO Orbit

MEO Orbit

HEO Orbit

Image Courtesy of Image Courtesy of Telesat CanadaTelesat Canada



Mixing LEO or MEO with HEOLEO and MEO satellite systems constellations could theoretically provide global coverage; many do not because constellations are chosen to serve specific groups of users located in certain regions.

MEO may be circular or elliptical in shape. Circular MEO is also called intermediate circular orbit (ICO). The satellite period approaches six hours; therefore, a constellation of 10 to 20 satellites distributed on two or three orbits is required to provide global coverage.

HEO was originally used for Soviet television broadcast, since much of the Soviet Union lies at high latitudes beyond easy reach of geostationary orbit.






Mixing LEO or MEO with HEOHEO inclination must be 63.435 at which the line of apsis is stationary.

Apsis is the point in an astronomical orbit at which the distance of the body from the center of attraction is either greatest or least.

Stationary apsis indicates that the apogee, which has the least attraction, and the perigee, which has the greatest attraction, remains the same for each orbital pass.

An inclination of 63.435 degrees provides a stationary apogee in the Northern Hemisphere.

An inclination of –63.435 degrees provides a stationary apogee in the Southern Hemisphere.

HEOs have perigees and apogees approaching 450 to 600 km and 40,000 km, respectively.






Mixing LEO or MEO with HEOMolniya satellites are synchronized with the Earth's rotation, making two complete revolutions each day.

The laws of orbital mechanics dictate that the spacecraft orbital velocity is greatly reduced near apogee, allowing broad visibility of the Northern Hemisphere for periods up to eight hours at a time. Therefore, by carefully spacing 3-4 Molniya spacecraft, continuous communications can be maintained.

A HEO satellite will spend about two thirds of the orbital period near apogee, and during that time it appears to be almost stationary for an observer on the Earth (this is referred to as apogee dwell).






Mixing LEO or MEO with HEOAfter the apogee period of orbit, a switchover needs to occur to another satellite in the same orbit in order to avoid loss of communications to the user.

Due to the relatively large movement of a satellite in HEO with respect to an observer on the Earth, satellite systems using this type of orbit need to be able to cope with large Doppler shifts.

The Russian Molniya system employs 3 satellites in three 12 hour orbits separated by 120 degrees around the Earth, with apogee distance at 39,354 km and perigee at 1000 km.

The Russian Tundra system employs 2 satellites in two 24 hour orbits separated by 180 degrees around the Earth, with apogee distance at 53,622 km and perigee at 17,951 km. Two satellites in separate orbits are necessary for full, continuous coverage of the chosen point.






GEOs in Global Coverage SchemesAll geostationary orbits comply with the following parameters:

3.1.1.3: GEOs in Global Coverage Schemes





GEOs in Global Coverage SchemesA geostationary orbit is one where the orbit has the same period as its primary's rotation period, and remains stationary over a single point on the Earth's surface.

A geosynchronous orbit only has the first restriction: that is, geosynchronous orbits can be elliptical, but geostationary ones have to be circular and stationed over the equator.

Geostationary orbits are circular orbits that are oriented in the plane of the Earth’s equator.

In a geostationary orbit, the satellite appears stationary to an observer on Earth.






GEOs in Global Coverage SchemesMore technically, a geostationary orbit is a circular prograde orbit in the equatorial plane with an orbital period equal to that of the Earth. This is achieved with an orbital radius of 6.6107 (equatorial) Earth radii, or an orbital height of 35,786 km.

The orbital location of geostationary satellites is called the Clarke Belt in honor of Arthur C. Clarke who first published the theory of locating geosynchronous satellites in Earth’s equatorial plane for use in fixed communications purposes.

In practice, the orbit has small non-zero values for inclination and eccentricity, causing the satellite to trace out a small figure of eight in the sky.






GEOs in Global Coverage SchemesThe figure 8 drift results from external forces. While there are hundreds of external forces acting on the satellite, the primary forces are as follows:

• The gravitational pull of the sun.

• The gravitational pull of other objects in the solar system.

• The uneven distribution of land mass on the surface of the Earth.

The footprint, or service area, of a geostationary satellite covers almost 1/3 of the Earth’s surface, from about 81.4 degrees south to 81.4 degrees north.

Theoretically, only four or five satellites would be needed to cover the entire land area of the Earth between 81° north latitude and 81° south latitude.






The following figure illustrates the figure of eight drift:


GEOs in Global Coverage Schemes

Figure 3.1.1.3a External Forces Acting on A Satellite



Image Courtesy of Telesat CanadaImage Courtesy of Telesat Canada




GEOs in Global Coverage Schemes

Figure 3.1.1.3b GEO Coverage Area




°

°



Initially, geostationary satellites provided a cost-effective way to broadcast analog TV signals to hard-to-reach communities. The TV signal was interfaced from the receive-only (TVRO) Earth Station to a VHF or UHF transmitter for re-transmission to the community.

Broadcast

3.1.2.1 BroadcastPart 2: The Single GEO


More recently, with the advent of digital TV transmissions and the improvement in video compression techniques, the TV signals are broadcast directly to homes or cable head-ends. Figure 3.1.2.1 Broadcast System




Very powerful broadcast geostationary satellites are used to transmit strong TV signals directly to small antennas.

Moreover, the subscriber can receive IP and multimedia services along with the TV signal. The upstream return path is transmitted to the satellite.

Broadcast satellites are also used to transmit CD quality music channels directly to moving vehicles.

Broadcast

3.1.2.1 BroadcastPart 2: The Single GEO




3.1.2.2.1 Star Topology

3.1.2.2 Star and Mesh Topology

Figure 3.1.2.2. Star Topology Drawing and TDM/TDMA Satellite Access

Part 2: The Single GEO


A star topology is employed when a corporate head office needs to communicate to its multitude of regional offices, and vice versa.

Hub

Hub

Outroute TD M

Inroute TD MA

Hub





The remote terminal is sized with a small antenna and SSPA to send data upstream towards the hub at rates from 64 to 128 kbps. The upstream data rates can be increased for videoconference or streaming applications.

The upstream path employs a TDMA access method, which can be prioritized and sized according to throughput requirements.

A star topology normally requires a complex HUB strategically located at a convenient gateway Earth Station. The HUB will be interfaced to a terrestrial medium in order to provide interconnectivity to the corporate office.







Star topology satellite network also requires a Network Control Facility (NCF) to manage HUB and remote configurations and specific throughput data rates.

The Network Control Facility is equipped with specialized utilities to monitor system performance of the entire network or sub-network, and provide throughput statistics to the end user.

The Network Control Facility will be capable of broadcasting software downline loads to all remote terminals, or transmit specific software downline loads to individual remotes.

An alarm facility is provided to proactively monitor the health of each remote, and the HUB sub-assemblies as well.







TELEPORT

TELCO

HOMES

CORPORATE

TRANSPORTABLE

CELLULAR

SATELLITE NETWORK OPERATIONS CENTRE

PSTN

DAMA

4

Figure 3.1.2.2 Mesh Topology Drawing



3.1.2.2.2 Mesh Topology





As in the Star Topology, the Network Control Facility can provide the following functionality:

• Space segment resource management

• Earth segment resource management

• DAMA processing (for voice/data call connectivity)

• PAMA processing (for voice/data call connectivity)

• Network Configuration database management

• Software downline load capability

• System performance statistics

• Call Detail Reports

• Network Operator GUI interface






A Mesh network can support a mixture of voice and data interfaces.

Gateway terminals are used to provide access to the PSTN. A remote Earth Station could be an end-office serving a community, or a simple subscriber or payphone kiosk.

The available satellite bandwidth is divided into a series of discrete satellite channels consisting of two types:


• Traffic Channels

• Control Channels







Traffic channels would normally employ QPSK or BPSK modulation schemes for voice and data connections, depending on the information and FEC coding rates to be supported. Data Connections:Data connections could range from 4800 to 128000 bps. The following type of data connections could be employed:

• Permanently-assigned multiple access (PAMA), primarily used for synchronous data transmissions.

• Asynchronous DAMA initiated by HAYES based commands.

• V.25 bis DAMA synchronous data connections, normally used to extend LAN networks.



3.1.2.2.3 Mesh Topology, Traffic Channels




Voice Connections:Voice connections could employ the following type voice coding methodologies:

• G711 64 Kbps PCM

• G726 32 Kbps ADPCM

• G728 16 Kbps LDCELP

• G729 8 Kbps VCELP



3.1.2.2.3 Mesh Topology, Traffic Channels




Dedicated control channels are used to pass call connectivity and administrative messages between the Network Control Facility and the remote terminals.

The Reverse Order Wire (ROW) control channels are used in a load-sharing, random access manner by all the remote terminals.

Specifically, a random Aloha access method is used. Collisions are inherent in this access method, and when they occur, message content will be destroyed.

Mechanisms are usually put in place to ensure the successful transmission of the message from the remote terminal to the Network Control Facility in order to set up a call request in a timely manner.



3.1.2.2.4 Mesh Topology, Control Channels




The Forward Order Wire (FOW) control channel is used to broadcast data from the Network Control Facility to the remote terminals.

Using an addressing mechanism, messages can be received by all remote terminals in a broadcast manner, or messages can be directed to a particular remote terminal.

The FOW data is transmitted on a dedicated SCPC channel, and the messages are contained in high-level data link control (HDLC) frames.



3.1.2.2.4 Mesh Topology, Control Channels



Point to Point Systems

3.1.2.3 Point to Point Systems

Point-to-point topologies are used to provide the simplest form of satellite communications.

This type of satellite network does not require any complex network control facility because the bandwidth and power requirements normally remain fixed throughout the service life cycle.

Remote access of the point-to-point modem can be provided by a supervisory control and data acquisition (SCADA) satellite circuit in order to manually change frequency, bandwidth, or power requirements.






3.1.2.3 Point to Point Systems

Examples of a point-to-point service are:

• T1 or E1 transmission (PSTN service restoration)

• MegaStream transmission

• Ship-to-shore transmission (oil rig)

• Corporate Network connectivity (telephony and WAN)

• Wireless Local Loop (WLL) gateway access

• Emergency Deployment Networks





Rem o te

S a te lli te M ode m

W e b Br o w s e r &V id e o

C o n fe r e n cin g

1024 KBS

Point-to-Point Sa te l l i te Ne tw ork

S a te lli te M ode m

100/10Bas e T

LAN

8 or 16W a tt SS P A

Co rp o rateG atew ay

W e b Br o w s e r &V id e o

C o n fe r e ncin g

Sa te llite M ode m

100/10Bas e T

8 o r 16W a tt S SP A

Internet

Se r ve r

Route r

Route rRoute r

Sa te llite M ode m

Te le p h on eove r IP

T e le p h o n eo ve r IP

Te le p h o n eo ve r IP


3.1.2.3 Point to Point SystemsPart 2: The Single GEO


Figure 3.1.2.3 Point-to-Point SystemsImage Courtesy of Telesat CanadaImage Courtesy of Telesat Canada



3.1.3 Co-location of Satellites

Placement of several satellites near each other in orbit is a common practice used to increase the total communications payload as referenced by an Earth Station antenna.

This allows a single fixed antenna to receive signals from all of the co-located satellites without the need of a satellite tracking mechanism.

There are co-location satellite programs available in the market that access a time-ordered best-knowledge databases maintained by the satellite operator GeoControl system to compute the assigned places of satellites (ephemerides) of up to five satellites and evaluate their relative motion.

Co-location of Satellites

Sect 1: Typical Satellite System Topologies




The user, according to his requirements, can specify the monitoring interval.

Short-term predictions based on the most recent orbit and maneuver information may, for example, be used to assess a possible collision risk in the near future and decide on necessary corrective measures.

Prediction over several station-keeping cycles may, on the other hand, be used in a post analysis, to verify proper operations of co-located spacecraft in the past.

The graphical representation of the satellite motion displays the relative inter-satellite distance together with the motion of each individual spacecraft in longitude, latitude and distance.


3.1.3 Co-location of SatellitesSect 1: Typical Satellite System Topologies





3.1.3 Co-location of SatellitesSect 1: Typical Satellite System Topologies

Arabsat, for example, operates two co-located satellites as shown in the diagram below.

Arabsat-2A operates the C-band transponders.

Arabsat-3A operates the Ku-Band transponders.

Therefore, inter-spectral interference is non-existent. Figure 3.1.3 Co-location of Arabsat Satellites


Used by Permission of ARABSAT

typical satellite system topologies

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