GB2319700A

GB2319700A - Satellite communication system using satellites at different altitudes

Info

Publication number: GB2319700A
Application number: GB9722728A
Authority: GB
Inventors: Kenneth Lee Sowles; James Powers Redden; Peter Joseph Armbruster; Daniel Richard Taylo
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 1996-11-25
Filing date: 1997-10-29
Publication date: 1998-05-27
Also published as: IT1295433B1; IL121764A0; GB9722728D0; KR19980042704A; JPH10163947A; BR9705135A; TW399375B; RU97119431A; CA2221003A1; FR2756439A1; NL1007404A1; CN1184385A; DE19751160A1; ITRM970628A1

Abstract

In a space-based communication system 10, a first satellite 22 orbits earth at a first altitude, while second satellites 20 coupled to the first via telecommunication links orbit at a second altitude different to the first. Control information destined for one of the second satellites is communicated from base station 40 to first satellite 22 and from first satellite 22 to the second satellite. Thus control information on cross-links between second satellites is minimised in order to maximise available bandwidth for revenue generating traffic (e.g. voice, fax, data). The first satellite 22 may be in geosynchronous orbit while the second satellites may be in low or medium earth orbits. Alternatively the first satellite is in medium earth orbit while the second satellites are in low earth orbits. Control information may include satellite commands, telemetry, traffic routing vectors, connection establishment, release messaging, cell channel frequency, satellite manoeuvring commands or telephony radio link management data.

Description

1 SPACE-BASED COMMUNICATION SYSTEMS

Technical Field

2319700 This invention relates generally to space-based telecommunication systems and, in particular, to satellites in different orbits communicating satellite control information to each other.

Background of the Invention

Some of the satellites in conventional satellite telecommunication systems have the ability to communicate with adjacent satellites that share the same orbiting altitude.. This inter-satellite connection is referred to as cross-links, whereby the crosslinks carry voice and/or data from one satellite to another satellite. If a base station located on earth transmits command information destined for a satellite other than the first satellite receiving the information, the command information may have pass through many cross-links before the satellite that is selected to receive the command information actually receives it. Such relaying of data or information from one satellite to another satellite is commonly referred to as hopping. Hopping consumes valuable energy and bandwidth, especially when command information is being relayed over many different satellites. Accordingly, there is a significant need for a satellite telecommunication system that reduces the number of hops it takes before satellite control information or commands are received by the destined satellite. Additionally, use of crosslinks for satellite control information may consume crosslink bandwidth which could be used to carry revenue generating traffic. Such traffic includes, but is not limited to voice, data, or fax information. Accordingly, there is a significant need for a satellite telecommunication system that minimizes the satellite control traffic carried on the crosslinks, therefore maximizing the available bandwidth for revenue generating traffic.

Brief Description of the Drawings

FIG. 1 shows a high level diagram of a satellite telecommunication system according to a preferred embodiment of the present invention; and satellite.

FIG. 2 shows an example of a highly simplified diagram of a Description of the Preferred Embodiments

The present invention has utility in that a network of satellites in geosynchronous orbit can communicate with a network of low-earth orbit satellites. By transferring sateRite control information and commands through the geosynchronous satellites, no low-earth orbit cross-links are necessary for transferring this information. A base station (i.e., satellite control facility) communicates directly to a geosynchronous satellite which in turns passes it directly to the individual low-earth or medium earth orbit satellites. A geosynchronous satellite will have visibility and hence direct communication with several low-earth orbit satellites at one time. By linking three or four geosynchronous satellites, every satellite in the low-earth orbit constellation would be visible.

FIG. 1 shows a highly simplified diagram of satellite telecommunication system 10. As shown in FIG. 1, telecommunication system 10 comprises at least two sateWtes 20 and 22, any number of subscriber units 30 and at least one base station 40. Generally, satellites 20 and 22, subscriber units 30 and base station 40 of telecommunication system 10 may be viewed as a network of nodes. All nodes of telecommunication system 10 are or may be in data communication with other nodes of telecommunication system 10 through telecommunication links. In addition, all nodes of telecommunication system 10 are or may be in data communication with other telephonic devices dispersed throughout the world through public switched telephone networks (PSTNs) and/or conventional terrestrial communication devices coupled to a PSTN through conventional terrestrial base stations.

A "satellite" as used throughout this description means a man made object or vehicle intended to orbit the earth. A "constellation" means a number of satellites arranged in orbits for providing specified coverage (e.g., radio communication, remote sensing, etc.) of a portion, portions or all of the earth. A constellation typically includes multiple rings (or planes) of satellites and may have an equal number of satellites in each plane, although this is not essential.

1 5 The present invention is applicable to space-based telecommunication systems 10 that assign particular regions on the earth to specific cells on the earth, and preferably to systems 10 that move cells across the surface of the earth. Satellites 20 may be a single satellite or one of many satellites 20 in a constellation of satellites orbiting earth. The present invention is also applicable to space-based telecommunication systems 10 having satellites 20 which orbit earth at any angle of inclination including polar, equatorial, inclined or other orbital patterns. The present invention is applicable to systems 10 where full coverage of the earth is not achieved (i.e., where there are "holes" in the telecommunication coverage provided by the constellation) and to systems 10 where plural coverage of portions of the earth occur (i.e., more than one satellite is in view of a particular point on the earth's surface).

In the preferred embodiment, satellites 20 have a low-earth orbit, while satellite 22 is a geosynchronous, satellite. In alternative embodiment, satellites 20 may be in medium-earth orbit, while satellite 22 is a geosynchronous, satellite.][n another alternative embodiment, satellites 20 may be in low-earth orbit, while satellite 22 may be in medium-earth orbit. There may be more than one satellite 22 which services satellites 20 and these satellites 22 may be able to communicate with each other. Low-earth orbit satellite are typically at an altitude range of 70OKm to MOOKm (400 to 800 miles) altitude, while medium-earth orbit satellite at an altitude of about 10,00OKm (6200 miles), and geosynchronous satellites are at an altitude of about 3600OKm (23,000 miles).

Each satellite 20 communicates with other nearby satellites 20 via crosslinks. These cross-links form a backbone of space-based mobile telecommunication system 6. Thus, a call or user communication, including but not limited to voice, fax and data from subscriber unit 30 located at any point on or near the surface of the earth may be routed through satellite 20 or a constellation of satellites to within range of substantially any other point on the surface of the earth. A communication may be routed down to subscriber unit 31 (which is receiving the call) on or near the surface of the earth from another satellite. How satellite 20 physically communicates (e.g., spread spectrum technology) with subscriber units 30 and base station 40 is well known to those of ordinary skill in the art.

Satellite 20 communicates with satellite 22 via a cross-link when satellite 20 is in view of satellite 22. Satellite 20 is not always in view of satellite 22, but is in view of satellite 22 for a period of time during its orbit around the earth. In alternative embodiments, there are multiple satellites 22, where each satellite 20 is able to communicate with one of the satellites 22 no matter where they are in their orbit around earth. Moreover, each satellite 22 may be able to communicate with adjacent satellites 22.

Cross-links between satellites 20 and 22 carry satellite control information, including but not limited to, satellite commands, telemetry, traffic routing vectors, traffic connection establishment, release messaging, cell channel frequency, satellite maneurvering commands, cell shutdown schedules. Satellite control information may additionally include telephony radio link management data such as time-tagged beam on/off tables, time-tagged beam related radio channel data such as allocation and deallocation of radio resources, and time-tagged beam related broadcast channel data.

Satellite 22 receives satellite control information for one or more satellite from base station 40 and communicates it to appropriate satellites 20. Since satellite 22 communicates satellite control information to satellites 20, the cross-links between satellites 20 only carry user information (e.g., voice, fax and data) to support a can from one subscriber unit 30 to another subscriber unit 30.

Subscriber units 30 may be located anywhere on the surface of earth or in the atmosphere above earth. Mobile telecommunication system 10 may accommodate any number of subscriber units 30. Subscriber units 30 are preferably communication devices capable of receiving voice and/or data from satellites 20 and/or base stations 40. By way of example, subscriber units 30 may be hand-held, mobile satellite cellular telephones adapted to transmit to and receive transmissions from satellites 20 and/or base stations 40. Moreover, subscriber units 30 may be computers capable of sending email messages, video signals or facsimile signals just to name a few.

How subscriber units 30 physically transmit voice and/or data to and receive voice and/or data from satellites 20 is well known to those of ordinary skill in the art. In the preferred embodiment of the present invention, subscriber unit 30 communicates with satellite 20 using a limited portion of the electromagnetic spectrum that is divided into numerous channels. The channels are preferably L-Band, K-Band, 5-band frequency channels or combination thereof, but may encompass Frequency Division Multiple Access (FDMA) and/or Time Division Multiple Access (TDMA) and/or Code Division Multiple Access (CDNLA) communication or any combination thereof. Other methods may be used as known to those of ordinary skill in the art.

Base station 40 communicates with and controls satellites 20 via satellite 22. There may be multiple base stations 40 located at different regions on the earth. For example, there may be one base station located in Honolulu, another base station located in Los Angeles and another base station in Washington, D.C. Base stations 40 may provide satellite signalling commands to satellite 22 so that satellites 22 and 20 maintain their proper position in their orbit and perform other essential house keeping tasks. Base stations 40 may be additionally responsible for receiving voice and/or data from satellites 20. How base station 40 physically communicates (e.g., spread spectrum) with satellites 22 and 20 and/or subscriber units 30 is well known to those of ordinary skill in the art.

FIG. 2 shows an example of a highly simplified diagram of satellite or 22. Satellite 20 comprises at least two transceivers 30, processor 34 and memory 36. Some transceivers 30 of satellite 20 are capable of sending and receiving satellite control information from satellite 99, while other transceivers 30 are capable of sending and receiving user information (e.g., voice, fax and data) from other adjacent satellites 20, subscriber units 30 and base stations 40. Some transceivers 30 of satellite 22 are capable of sending and receiving satellite control information from satellite 22 and from base station 40. Processor 34 controls the whole operation of satellite 20, 79 and may be responsible for executing software application programs. Memory 36 stores the software programs executed by processor 34. Although one processor 34 and one memory unit 36 are shown in FIG. 2, those skilled in the art understand that more than one processor and memory can be used in satellite 20 and 22. The number of processor and memory size is unimportant to the present invention.

The advantages of the present invention is to eliminate the need to use low-earth orbit satellite crosslinks to provide control information to other low-earth orbit satellites. Another advantage is that only one hop is needed to communicate satellite control information from satellite 22 to satellite 20. Thus, satellite control commands can reach the designated low-earth orbit satellite much faster. Yet another advantage is that bandwidth previously used for control information can be carried by 5 satellite 22, which allows more bandwidth for revenue-bearing traffic.

Accordingly, it is intended by the appended claims to cover all modifications of the invention which fall within the true spirit and scope of the invention. For example, the following combinations are possible: geosynchronous satellite 22 communicating with medium-earth or low- earth orbit satellites 20, or medium-earth orbit satellite 22 communicating with low-earth orbit satellites 20.

Claims

CLAIMS What is claimed is:

1. A space-based communication system comprising:

a first satellite orbiting earth at a first altitude; and a second satellite coupled to the first satellite via a telecommunication link and orbiting the earth at a second altitude different than the first altitude, wherein the first satellite and second satellite communicate 10satellite control information to each other.

2. A space-based communication system as recited in claim 1, wherein the first satellite is in geosynchronous orbit around earth.

3. A space-based communication system as recited in claim 2, wherein the second satellite is in a medium-earth orbit around the earth.

4. A space-based communication system as recited in claim 2, wherein the second satellite is in low-earth orbit around the earth.

5. A space-based communication system as recited in claim 1, wherein the telecommunication link is a radio frequency link.

6. A space-based communication system as recited in claim 1, 25 wherein the telecommunication link is an optical link.

7. A space-based communication system as recited in claim 1, further comprising: a base station capable of transmitting and receiving first radio frequency signals; and at least one subscriber unit capable of transmitting and receiving second radio frequency signals; and wherein the first satellite includes, a first transceiver capable of receiving and transmitting the first radio frequency signals to the base station; and a second transceiver capable of receiving and transmitting third radio frequency signals to the second satellite; and wherein the second satellite includes, first transceiver capable of receiving and transmitting the third radio frequency signals to the first satellite; and second transceiver capable of receiving and transmitting the second radioIrequency signals to the subscriber unit.

8. A space-based communication system as recited in claim 1, further comprising:

a base station capable of transmitting and receiving first radio frequency signals; and at least one subscriber unit capable of transmitting and receiving second radio frequency signals; and wherein the first satellite includes, a first transceiver capable of receiving and transmitting the first radio frequency signals to the base station; and a second transceiver capable of receiving and transmitting optical signals to the second satellite; and wherein the second satellite includes, a first transceiver capable of receiving and transmitting the optical signals to the first satellite; and a second transceiver capable of receiving and transmitting the second radio frequency signals to the subscriber unit.

9. A space-based communication system as recited in claim 1, further comprising:

a base station capable of transmitting and receiving optical signals; and at least one subscriber unit capable of transmitting and receiving first radio frequency signals; and wherein the first satellite includes, first transceiver capable of receiving and transmitting the optical signals to the base station; and second transceiver capable of receiving and transmitting second radio frequency signals to the second satellite; and wherein the second satellite includes, first transceiver capable of receiving and transmitting the second radio frequency signals to the first satellite; and second transceiver capable of receiving and transmitting the first radio frequency signals to the subscriber unit.

10. A space-based communication system as recited in claim 1, further comprising: a base station capable of transmitting and receiving first optical signals; and at least one subscriber unit capable of transmitting and receiving radio frequency signals; and wherein the first satellite includes, a first transceiver capable of receiving and transmitting the first optical signals to the base station; and a second transceiver capable of receiving and transmitting second optical signals to the second satellite; and wherein the second satellite includes, first transceiver capable of receiving and transmitting the second optical signals to the first satellite; and second transceiver capable of receiving and transmitting the radio frequency signals to the subscriber unit.