WO2000011811A1

WO2000011811A1 - Satellite communication system utilizing a ratcheting footprint

Info

Publication number: WO2000011811A1
Application number: PCT/US1999/018758
Authority: WO
Inventors: Michael Delachapelle; David C. Morse; Leonard Jon Quadracci
Original assignee: The Boeing Company
Priority date: 1998-08-20
Filing date: 1999-08-18
Publication date: 2000-03-02
Also published as: AU5777399A

Abstract

The present invention relates to a method for allocating beams transmitted from and received at positions in Earth orbit to portable, mobile, and fixed terminals and gateways. The method includes the steps of forming a plurality of footprints using antenna elements which generate a plurality of beams. The antenna elements are carried on board a plurality of satellites flying in orbits below geosynchronous altitude and each of the footprints is electronically steered along azimuth and elevation by a two-dimensional phased array front end. One of the plurality of satellites, including a first-serving satellite and a second satellite, is positioned next to said first serving satellite. The footprints illuminate portions of the Earth's surface with a plurality of beams, wherein at least one temporary Earth-fixed cell falls within one of the plurality of the footprints.

Description

SATELLITE COMMUNICATION SYSTEM UTILIZING A RATCHETING FOOTPRINT

Backεround of the Invention Field of the Invention

The present invention relates to the field of satellite communications and, more particularly, to an apparatus and method for satellite communications that manages the switching of antenna beams.

Background Information

Public telephone networks currently utilize a combination of land lines, microwave repeaters, undersea cables, and cellular systems. As an alternative to current systems, greater levels of communication services can be achieved by employing large satellites that operate in geosynchronous orbit. This approach, however, is expensive and is unsuitable for providing direct service to telephone customers using portable and mobile terminals because of the extremely high power levels that would be required to communicate with satellites flying at an altitude of 22,300 miles. Another major drawback of geosynchronous systems is that they hand off and manage individual users rather than groups of users. In an effort to overcome the limitations of geosynchronous systems, communication systems employing low Earth orbit (LEO) satellites have been proposed. These systems, however, provide their own set of problems because of the rapid motion of satellites over the Earth's surface.

U.S. Patent No. 5,408,237 ('237 patent) describes an Earth-fixed cell beam management system for satellite communications. In the system, beams sent by LEO satellites are precisely controlled so they illuminate "Earth-fixed cells," as opposed to "satellite-fixed cells." The "Earth-fixed cell" is a stationary region mapped to an "Earth- fixed grid." The Earth's surface is initially mapped into unchanging permanent fixed boundaries defined by the "Earth-fixed grid" which each satellite can accurately locate from its position data. Each satellite is capable of steering, transmitting and receiving beams conveying packets of information to the Earth-fixed grid. The beams are continually adjusted to compensate for the effects of the motion of the satellite, attitude changes, and the rotation of the Earth.

Unfortunately, the Earth-fixed cells described in the '237 patent require an antenna that is extremely expensive and difficult to manage. Further, the processing required to steer all the beams to keep them on the Earth-fixed cell requires unproven technology.

Direct worldwide telephone services via satellite that are currently available to persons using portable, mobile and fixed terminals are extremely limited and too expensive for use by all but a few. Providing an economically viable satellite network for voice, data, and video, which can be used by subscribers around the globe, has presented a major challenge to the communications business. The development of a high powered reliable, and relatively inexpensive satellite communication system which can transmit and receive radio signals to portable, mobile, and fixed terminals on the land, sea, and air, without the intermediate steps of routing traffic through land-based equipment, would cause a major technological advance and would satisfy a long-felt need within the electronics and telephone industries.

Summary of the Invention

In accordance with one aspect, the present invention relates to a method for allocating a plurality of beams transmitted from and received at positions in Earth orbit to a plurality of portable, mobile, and fixed terminals and gateways. The method includes the steps of forming a plurality of footprints using antenna elements which generate a plurality of beams, the antenna elements being carried on board a plurality of satellites flying in orbits below geosynchronous altitude. Each of the plurality of footprints is electronically steered along azimuth and elevation by a two-dimensional phased array front end. One of the plurality of satellites includes a first- serving satellite and a second satellite, which is positioned next to said first serving satellite. The plurality of footprints illuminate portions of the Earth's surface with the plurality of beams, wherein at least one temporary Earth-fixed cell falls within one of the plurality of footprints.

Brief Description of the Drawin2s

An appreciation of the aims and objectives of the present invention, and a more

complete and more comprehensive understanding of this invention, may be achieved by studying the following description of a preferred embodiment and by referring to the

following drawings: Figure 1 shows a ratcheting footprint antenna of the present invention;

Figure 2A shows a satellite footprint consisting of a square beam lattice;

Figure 2B shows a satellite footprint consisting of a triangular beam lattice;

Figure 3 shows a footprint from a center-scanned position of a satellite in polar

orbit;

Figure 4 shows a footprint coverage areas for two adjacent satellites in a

common orbit plane;

Figure 5 shows how beam registration on the Earth is redefined upon a satellite

plane handover; and

Figure 6 shows how the ratcheting angle of a satellite decreases with the

increasing satellite antenna elevation angle. Detailed Description of the Invention

The present invention, as shown in Figs. 1 and 2A, relates to an uplink management scheme for a non-geocentric satellite orbit (NGSO) satellite communication system and is used to facilitate the management of a constellation of broadband low- Earth orbit (LEO) satellites. The management scheme includes control of a multiple beam antenna 30 that scans a footprint 22 on the Earth and defines temporary Earth- fixed cells 26 and hands these cells off from satellite to satellite. The multi-beam antenna 30 ratchets the entire footprint 22 and scans all beams 20 in the footprint 22 as a group, rather than individually. Further, in the management scheme for the multi-beam antenna 30, the temporary Earth-fixed cells 26 are redefined for each orbit plane. The use of the ratcheting footprint 22 of the present invention that defines temporary Earth- fixed cells 26 is simpler and cheaper than prior art individual beam ratcheting. As shown in Fig. 1, a plurality of beams are formed by the multi-beam antenna

(MBA) 30. The MBA 30 emits the plurality of fixed beams 20 via its antenna elements 40. Movement of the footprint in two dimensions, i.e., azimuth and elevation, is accomplished by placing a two-dimensional phased array front end 34 onto the MBA 30. The MBA 30 can be of a number of designs, such as a Rotman lens or a Butler Matrix. Each of the antenna elements 40 includes a plurality of low-noise amplifiers (LNA) 36, and a corresponding number of phase shifters 38.

As shown in Fig. 2A, the present invention uses a satellite footprint, such as the square satellite footprint 22, to achieve full coverage of an area by arranging a large number of the beams 20 generated by the MBA 30. A triangular satellite footprint 24 consisting of triangular beam lattices, as shown in Fig. 2B, may also be used to create the satellite footprint of the present invention. In an alternative embodiment, the beams 20 may also be overlapped (not shown) to compensate for ratcheting angle differences between center and edge beams.

As shown in Fig. 2A, the present invention keeps the temporary cells 26, which are located in one of the footprints 22, fixed to a region of the Earth for as long as the temporary cells 26 remain in a specified orbital plane of satellites. In other words, the assignment of each of the temporary Earth-fixed cells 26 endures for the time that the temporary cells 26 remain within the coverage region for a plane of satellites and is redefined when the temporary cells 26 are handed off to the next orbital satellite plane. The temporary cells 26 are typically handed over to an adjacent orbital plane. The temporary Earth-fixed cells 26 are designated as temporary since they only exist for a limited period of time, i.e., before being handed over to the adjacent orbital plane.

The satellite footprint 22 moves in small motions about a nominal nadir position of the satellite by electronically tilting the phase front using the phase shifters 38 in each of the antenna elements 40 as shown in Fig. 1. The entire footprint 22, which includes all the beams 20 in the footprint 22, is scanned as a group. This arrangement provides a much simpler and less expensive antenna design compared to an antenna that individually scans each beam 20, such as described in the prior art concept of utilizing Earth-fixed cells.

As shown in Fig. 2 A, the movement of the MBA 30 is described as "ratcheting" because the footprint 22 tracks the position of the temporary Earth-fixed cells 26. For example, cell 26 A is tracked for a small movement range, i.e. about plus or minus one half-beam width, which lasts approximately ten seconds, and then the footprint 22 rapidly ratchets forward to pick up new cells 26, in this case cell 26B in a new row 28, which is handed over from the footprint 22 of a leading adjacent satellite (s). Every time the MBA 30 ratchets, the new row of cells 28 is handed over to the leading edge of the footprint 22 and a row of the temporary cells 26 is handed off to the footprint 22 of a trailing satellite T in the orbit plane. The temporary cells 26 move down the footprint 22, row by row, for up to approximately five minutes, depending on design, until they reach the row of the temporary cell 26C, where they are picked up by the footprint 22 of the trailing satellite T.

As shown in Fig. 3, when satellites utilizing the present communication scheme are in polar orbits, the ratcheting movement is generally north to south. However, there is a small component in the east to west direction to compensate for the Earth's rotation. Accordingly, for satellites heading south, the satellite's footprint 22 tracks north with a small easterly component, and then ratchets directly south to pick up a new row. Figure 4 shows two adjacent satellites, #1 satellite 41 and #2 satellite 43, in the same orbit plane with adjacent footprint coverage areas. For the polar orbiting satellites, #1 satellite 41 is directly north of #2 satellite 43. However, the ground tracks of the satellites 41 and 43 have a slight east-to-west tilt because of the Earth's rotation. Even through the satellite ground track in Fig. 4 appears to cause random beam registrations on the temporary Earth-fixed cells 26, the footprint scanning motion depicted in Fig. 3 assures the exact same beam registration as achieved by tracking a temporary cell 26 and then ratcheting forward to register to a new one.

Figure 4 also shows that the beam column's registration is maintained when rows are handed over to the trailing satellite (T). Therefore, the beam registration can be maintained until handed over to a new orbit plane.

The beam registration on the Earth is redefined on satellite plane handover, as shown in Fig. 5. Here, the curvature of the Earth forces the convergence of satellite footprints 22A and 22B in the adjacent orbital planes when at higher latitudes. This convergence creates an overlap between the footprints 22 A and 22B, which increases with latitude. At least one cell overlap between the footprints 22A and 22B is required for successful handover between orbit planes. Unfortunately, when the entire footprint 22 is ratcheted, it is not possible to exactly track and perfectly register on all temporary Earth-fixed cells 26 in the footprint 22 because the optimum ratcheted angle is different for the center beams 33 at the nadir than for the edge beams 35, as shown in Fig. 2A and Fig. 6. For equally spaced cell centers, the change in elevation angle to register on the next row of cells decreases with the increasing elevation angle.

One solution for compensating for the above-discussed registration problem is to choose a ratcheting angle that optimizes registration for the nadir, i.e. center beams 33, causing the ratcheting angle to be excessive for the edges. The edge beams 35 are then oversized to compensate. This results in a substantial G/T antenna loss.

Another approach for solving the performance problem of the present invention is to select a ratchet angle to optimize the ratchet for beams halfway between the center and the edge. Unfortunately, this also causes a substantial G/T antenna loss.

A preferred approach for solving the above-described performance problem of the present invention is to choose the ratcheting angle that optimizes registration for the edge cells. If this is done, the ratchet angle will be insufficient for the center beams 33. However, this is compensated for by making the center beams larger than required. This solution will assure that the center beams 33 encompass the temporary Earth-fixed cells 26. Since the slant range is less and the range loss is significantly diminished, the performance loss is greatest at the nadir, where there is generally considerable link margin relative to the edge of the coverage. For Ka-band systems, there is usually several decibels of link margin available for the nadir, i.e. center beams 33. Therefore, it is possible to have no overall performance degradation when optimizing registration for the edge cells. The present invention provides a low-cost approach to uplink management by ratcheting entire footprints and is more simple and cost-effective than the prior art method of ratcheting individual beams. Except as otherwise disclosed herein, the system's components shown in outline or block form are individually well-known and the internal construction operations are not critical, either to the making or using of this invention, or to the description of the best mode of the invention.

While a detailed description of the above has been expressed in terms of specific examples, those skilled in the art will appreciate that many other configurations could be used to accomplish the purpose of the disclosed invention. Accordingly, it will be appreciated that various equivalent modifications of the above-described embodiments may be made without departing from the sprit and scope of the invention.

Claims

1. A method for allocating a plurality of beams transmitted from and received at positions in Earth orbit to a plurality of portable, mobile, and fixed terminals and gateways comprising the steps of: forming a plurality of footprints using a plurality of antenna elements which generate the plurality of beams, said plurality of antenna elements being carried on board a plurality of satellites flying in orbits below geosynchronous altitude, wherein each of said plurality of footprints is electronically steered along azimuth and elevation by a two-dimensional phased array front end; and illuminating portions of said Earth's surface with said plurality of beams of said plurality of footprints, wherein at least one temporary Earth-fixed cell falls within one of said plurality of footprints.