MXPA96006591A - Radiocommunication system using geostationary and non-geostationary satellites - Google Patents

Abstract

A radiocommunication system and method is disclosed using a combination of geostationary satellites (GEO) and a plurality of deorbit satellites moderately close to the earth (MEO). First, a GEO satellite is launched to provide initial system capacity. Then, the MEO satellites are launched successively into positions where they can supplement the GEO satellite coverage during peak traffic hours. Finally, when a sufficient number of MEO satellites are in place, the GEO satellite can provide the supplementary capacity

Description

"RADIOCOMMUNICATION SYSTEM USING GEOSTATIONARY AND NON-GEOSTATIONARY SATELLITES" BACKGROUND The present invention generally relates to methods and systems for providing radio communications and, more particularly, to methods and systems using satellites to provide radio communications. In the past, satellite systems to provide global coverage have been of one of three types, broadly classifiable by geostationary orbit (GEO), near-Earth orbit (LEO) and orbit close to Earth (MEO) ). An example of a geostationary satellite communication system is the INMARSAT system (International Maritime Satellite Organization). An advantage of geostationary satellites is that they remain in a fixed position relative to the earth, and only four of these satellites are required to illuminate the entire earth. A disadvantage of geostationary satellites is that they are very distant requiring high transmit power and large antennas to provide communication capacity and incurring approximately one quarter of a second signal propagation delay. An example of a LEO system is the IRIDIUM system proposed by Motorola. One advantage of LEO systems is that satellites are much closer to the earth, thus providing improved communications. Since the satellites are closer to the ground, less transmission power is needed for both the satellite and a transceiver of the individual user. A disadvantage is that approximately 70 satellites are required to provide 24-hour coverage to most points on the globe. In addition, satellites in orbit close to the earth move quite rapidly relative to the earth, thus causing high Doppler shifts and frequent communications deliveries from one satellite to the next. An example of the MEO commitment system is the ODYSSEY satellite system proposed by TRW. The orbital altitude of the MEO satellites is between the orbits of GEO and MEO, providing better communication quality in a GEO system, with less movement and Doppler shift than a LEO system. In addition, MEO systems provide more or less 24-hour coverage to most points on the globe using between 8 and 18 satellites which is much more economical than the solution of approximately 70 LEO satellites. Even though the MEO solution represents a good compromise between conflict requirements, it suffers from a practical disadvantage that almost all satellites must be in place before coverage is sufficient (in percentage of available time) to be considered attractive to sub-subscribers. This lesson was learned from the GPS satellite navigation system, which is also an MEO solution. Therefore, a considerable investment is needed that covers a long-term program before considerable revenue can be expected when an MEO system is implemented. Accordingly, it would be desirable to provide radiocommunication systems and methods that overcome the aforementioned drawbacks of conventional LEO, MEO and GEO solutions.

COMPENDIUM In accordance with exemplary embodiments of the present invention, a hybrid GEO / MEO solution begins with a launch of a geostationary satellite that provides radio communication coverage to a predominantly expected region of traffic growth, but has a limited capacity that is sufficient to support only an initial number of subscribers. This is followed by the successive launch of a number of MEO satellites. The MEO satellites, initially, can supplement the geostationary satellite's coverage. Subsequently, once a sufficient number of MEO satellites are in orbit, the primary traffic charge can be relegated to the MEO satellites, with the GEO satellite carrying out a supplementary role. Finally, if desired, enough MEO satellites can be launched to provide the full capacity of the desired system. Thus, a major inconvenience of the MEO systems, specifically the extended period between initial launch and sufficient capacity to achieve remunerability, is exceeded since the systems in accordance with the present invention provide instantaneous incapacity by first launching a geostationary satellite.

BRIEF DESCRIPTION OF THE DRAWINGS The objects, features and advantages mentioned above and others of the present invention will be more readily understood upon reading the following detailed description together with the drawings, in which: Figure 1 illustrates a geostationary satellite placed in orbit in the earth in accordance with the present invention; Figure 2 illustrates a geostationary satellite and several orbiting satellites moderately close to the earth, according to another exemplary embodiment of the present invention; and Figure 3 shows a geostationary satellite and many orbiting satellites moderately close to the earth, according to an exemplary mode.

DETAILED DESCRIPTION In accordance with the present invention, the Figure 1 shows the first satellite 10 to be established in a geostationary orbit to provide the initial capacity of the system. Although limited in its capacity, this geostationary satellite can provide sufficient capacity for service to a limited number of subscribers within a predefined geographical coverage or traffic area 20. Thus, for example, if a terminal unit remains within of area 20 of geographic coverage, and is one of the limited number of subscribers to whom service is provided by geostationary satellite 10, the terminal unit 30 would be expected to receive a good service except possibly in times of maximum use. During a second phase of development of the system, successive satellites 40 can be launched into an orbit fairly close to the earth as seen in Figure 2. For example, this orbit could be called a harmonically synchronous orbit by which the satellite is placed in orbit on Earth an integral number of times in a sidereal day in such a way that the terrain tracks are repeated. For example, an orbital radius of 16,756 kilometers (orbital height of 10,386 kilometers) provides 4 orbits per sidereal day. Note that each of the satellites 40 in orbit moderately close to the earth could have a higher nominal capacity than the geostationary satellite 10 because of their relative proximity. Of course, a single satellite 40 in orbit moderately close to the earth, will not cover any region on the earth for more than a fraction of a day, for example, for two hours once a day, but that coverage can be selected to occur during a period of maximum traffic on the day during at least one predominant traffic location and thus supplements the limited capacity of the GEO satellite allowing the number of subscribers to expand.

A desirable feature of exemplary embodiments according to the present invention is that the modulation and multiple access method (e.g., FDMA, TDMA, CDMA or hybrid thereof) used for bidirectional exchange of radio signals between a terminal unit and the satellites, regardless of whether there is access to geostationary satellites or orbiting satellites moderately close to the earth. It is also desirable that the orbital period of the MEO satellite be a submultiple of a day such that it repeatedly flies through the selected service area during the designated maximum traffic period. As more 40 MEO satellites are launched, more and more of the global traffic from a continuously expanding subscriber base will be adopted by the MEO satellites. Finally, if enough MEO satellites have been launched, the 10 GEO satellite can be discarded. However, the number of 40 MEO satellites required to provide 100 percent coverage of 100 percent of locations is significantly greater than if these percentages relaxed, particularly when secondary criteria are added, such as a mobile elevation angle. - Satellite greater than 20 degrees, or visibility of two satellites from each mobile phone for at least 90 percent of the time, to provide diversity reception. When a greater number of MEO satellites have been launched as shown in Figure 3, there will still be "holes" in the coverage when the secondary criteria are not filled in specific locations for a certain amount of time. In accordance with the present invention, the geostationary satellite or satellites are kept in operation in this phase in order to fill the holes, thereby allowing the secondary criteria to be filled with a smaller final number of MEO satellites. It can be seen that geostationary satellite 40 originally launched has a different role in three different program phases: PHASE 1: The geostationary satellite only provides service to a limited initial subscriber base. PHASE 2: The geostationary satellite has its capacity supplemented during periods of maximum traffic by one or more MEO satellites. PHASE 3: The geostationary satellite "fills" the holes in the coverage provided by a limited constellation of the MEO satellites.

In order to function effectively in all three roles, the geostationary satellite 10 must possess certain characteristics. In particular, to fill its phase 3 role, the geostationary satellite must be able to direct the capacity of regions that need it (v. Gr, the holes) by means of electronically or mechanically directed antenna rays or commutated rays. An exemplary technical solution to address this capability would be to use a phased array satellite transponder as disclosed in US Patent Application Serial No. 08 / 179,953, issued to Paul W. Dent "A Cellular Satellite Communications System with Improved Frequency Reuse "presented on January 11, 1994 whose exposition is incorporated, in its entirety, in this by reference. An alternative arrangement, however, is the multi-beam parabolic antenna system driven by a so-called matrix power amplifier as used in current INMARSAT-III satellites and as described in US Patent Number 3,917,998 issued to Welti whose exposure it is also incorporated herein by reference. The arrangement of the matrix power amplifier that allows either the power of each transmission amplifier to drive an associated beam, or, with greater flexibility, and on a signal-by-signal basis, the multi-stage power of the power amplifier that they will accumulate in a single beam, if that is where capacity is where it is most needed today. Although the present invention has been described in terms of exemplary embodiments mentioned above, these embodiments are intended to be illustrative in all respects rather than restrictive of the present invention. For example, even though the aforementioned exemplary embodiments only have one geostationary satellite and a plurality of orbiting satellites moderately close to the earth, two or more geostationary satellites could be provided. Furthermore, even though it would be desirable to launch the geostationary satellite first to provide instantaneous capacity, one or more of the orbiting satellites could be launched fairly close to the ground before the geostationary satellite. Those skilled in the art will readily appreciate that many modifications and adaptations are proposed by means of the present invention whose scope is defined by the appended claims including all equivalents thereof.

Claims (14)

CLAIMS:

1. A satellite communication system comprising: at least one geostationary satellite and at least one satellite in orbit moderately close to the ground to provide radiocommunications to a plurality of terminals distant from the subscriber, wherein at least one geostationary satellite provides radio communications to the remote terminals of the subscriber during periods when the remote subscriber terminals can not access at least one of the orbiting satellites moderately close to the ground.

2. A satellite communications system comprising: a geostationary satellite that provides radiocommunication service to a plurality of remote subscriber terminals until a first satellite of orbit fairly close to the ground is capable of operation, wherein the first satellite of Orbit moderately close to the Earth supplements the radio communication service of the geostationary satellite after the first orbit satellite is capable of operating moderately close to the Earth.

3. A system according to claim 2, wherein the first satellite of orbit fairly close to the ground is placed in a position to cover or cover at least one area of maximum traffic during maximum periods of time of the day.

4. A system according to claim 2, further comprising: a plurality of additional satellites of moderately cerebellar orbit to the earth that become functional after the first satellite of orbit fairly close to the earth operates, wherein the satellite geostationary directs its ability to service certain areas based on the traffic and service capacity of orbiting satellites moderately close to the ground. A system according to claim 1, wherein the geostationary satellite is followed successively by an increased number of orbiting satellites moderately close to the earth, the geostationary satellite providing one of two: being unique in serving the subscribers, provide service to subscribers supplemented during peak periods by means of at least one satellite in orbit moderately close to the earth, and service for free spaces in coverage provided by orbiting satellites moderately close to the ground. 6. The system according to claim 5, wherein the service provided by the geostationary satellite is determined based on a number of orbiting orbiting satellites in the vicinity of the earth. 7. A method for providing radiocommunication to a plurality of terminal units comprising the steps of: launching a satellite into a geostationary orbit; provide radiocommunication service to the plurality of terminal units using only the geostationary satellite before launching the additional satellites; throwing a plurality of satellites into an orbit fairly close to the earth; and providing radiocommunication service using both the geostationary satellite, at least one of the plurality of satellites in orbit fairly close to the earth. The method according to claim 7, wherein the second step further comprises the steps of: providing radiocommunication service using at least the plurality of orbiting satellites moderately close to the ground to supplement the service of the geostationary satellite until a predetermined number of the orbiting satellites moderately close to the earth has been launched, after which provide a communication service using the geostationary satellite to supplement the service of orbiting satellites moderately close to the earth. 9. A satellite communications system to service a number of terrestrial-based terminals, with variable activity levels comprising: a geostationary satellite placed in order to be visible from the service area, 24 hours a day; at least one sub-synchronous satellite in an orbit that has a synchronized repeat terrestrial track to cover the service area during periods of maximum expected activity of the ground-based terminals. The communication system according to claim 9, comprising a central ground station in communication with the geostationary satellite and with the sub-synchronous satellite when it is visible to transmit signals between the public switch telephone and the terminals through at least one of the satellites. The communications system according to claim 10 further comprising a control means for directing the focused beams in the geostationary satellite antenna to momentarily locations that do not adequately provide service by at least one sub-synchronous satellite. The communications system according to claim 11, wherein the control means is a beamforming computer. The communications system according to claim 12, wherein the beamforming computer is placed at the central station. 14. A communication system for providing telephone communications between portable wireless terminals and the public telephone system through orbiting satellites comprising: at least one geostationary relay satellite comprising an electronically steerable antenna; at least one sub-synchronous relay satellite. a tracking network to follow the instantaneous position on the satellite and feed the information to the communications access stations; at least one access station for communications connected with PSTN and in communication with at least one geostationary relay satellite, the access station comprises: a control means for controlling the area illuminated by the beams of the steerable antenna; a means of route to send signals between the portable terminals and PSTN through a satellite and a controlled antenna beam based on the information of the tracking network.