WO1997018128A1

WO1997018128A1 - Applications of sun-synchronous satellite orbits to communications satellites

Info

Publication number: WO1997018128A1
Application number: PCT/US1996/014958
Authority: WO
Inventors: Emeric I. Podraczky; Paul S. Palmiter
Original assignee: Iridium, Ip Llc
Priority date: 1995-11-13
Filing date: 1996-09-18
Publication date: 1997-05-22

Abstract

A satellite-based telecommunications system comprises a constellation of satellites in circular, sun-synchronous orbits (8) at specific altitudes and inclination angles so as to orbit the earth (10) an integer number of times per solar day. These orbits (8) are unique in that any given satellite in the constellation services the same region on earth at the same time every day. Hence, the capacity of each satellite can be tailored to meet time of day dependent peak traffic requirements characteristic of cellular communications. In addition, if any one satellite in the constellation experiences a surge in cellular traffic at the same time each day to the point of reaching capacity saturation, only that particular satellite needs to be replaced with a larger capacity satellite in order for the constellation to continue to meet current demands.

Description

APPLICATIONS OF SUN-SYNCHRONOUS SATELLITE ORBITS TO COMMUNICATIONS SATELLITES

DESCRIPTION

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention generally relates to a satellite based telecommunications system and, more particularly, to sun-synchronous satellite constellations with repeat earth tracks facilitating the implementation of satellite systems with unique operational characteristics.

Descnption of the Related Art

Modern communication satellites have been in service since the late 1950s with the launching of the first man-made satellite. In the 1960s commercial telephonic communication satellites revolutionized global communications with the launch of Telestar I. Most modern communication satellites are placed in a geosynchronous orbit uniquely situated at 22,753 miles (35,786 km) above the equator. In a geosynchronous orbit, the satellite orbits the earth above the equator at the earth's rotational speed such that the satellite remains stationary with respect to the earth's surface. Hence, a given region of the earth may be continuously serviced by the same communication satellite in a geosynchronous orbit. Geosynchronous orbits are effective, and typically used, for television transmissions, and similar applications which require continuous, uninterrupted satellite service from a fixed point in the sky. Since the geosynchronous orbit is a high earth orbit enabling a large beam, a minimum of only three satellites is needed to provide coverage that encircles the entire globe. Satellite orbits and constellation geometries have been studied since the early 1960's. A good description of satellite orbit calculations can be found in the textbook by Wilbur L. Prichard et al., "Satellite Communication Systems Engineering", Prentice-Hall, Inc. , 1986, which is herein incoφorated by reference. A satellite constellation comprises a plurality of orbital planes each having a plurality of satellites orbiting therein. An overview of various satellite constellation geometry can be found in the article by Robert A. Nelson, "Satellite Constellation Geometry," Via Satellite, March 1995, pgs. 110-122, which is herein incorporated by reference. The configuration of a constellation is defined by the number of orbital planes and the number of satellites per plane. Ideally, the number of planes and the number of satellites should be kept to an optimum level consistent with effective coverage for a geographical region at the minimum cost. There are three major factors to consider when designing a constellation, that is, altitude, inclination angle, and eccentricity.

The altitude of the orbital planes is the primary factor to consider when defining a constellation geometry. Such factors as signal propagation time, satellite complexity, antenna size, atmospheric drag, Van Allen radiation belt exposure, signal power, and coverage or footprint area, are all functions of altitude.

The inclination angle is chosen considering either the latitudes of coverage required or some other parameters such as the precession of the orbital nodes. Polar constellations have inclination angles of near 90°, which permits global coverage.

Eccentricity determines the orbital plane shape. A circular orbit has an eccentricity of 0 and the satellite moves at all points in the orbit at a constant speed. An elliptical orbit has an eccentricity of between 0 and 1 and moves at different speeds at various points in the orbit. A satellite in an elliptical orbit moves fastest at perigee, the point in the orbit closest to earth, and slowest at apogee, the point in the orbit furthest from earth. By adjusting the position of the apogee, the dwell time over a given point on earth can be maximized.

Over the last decade there has been much attention paid to medium earth orbit (MEO) and low earth orbit (LEO) satellites for use in mobile communication for general services including hand-held cellular telephony for two-way voice and data communication, and paging services. Because MEO and LEO satellites are positioned at a considerably lower altitude than a geosynchronous orbit, multiple satellites, typically arranged in a constellation, are needed to cover the entire globe since lower altitude satellites tend to have a smaller beam coverage or "footprint" on the earth's surface.

Many of the Mobile Satellite Services (MSS) have so called "peak" usage times when the demand for satellite services is greater than the average level of service required. In particular, cellular telephones have become common place to the point of being thought of as a necessity by many cellular subscribers, leading to ever increasing cellular traffic. A characteristic of mobile cellular telecommunications is that the peak usage times tend to occur at the same time each day, usually corresponding to the business work day and centered around the noon hour when cellular phone use seems to peak. Not suφrisingly, peak cellular usage is also concentrated into specific geographic regions corresponding to the urban population centers of the world. Accordingly, telecommunication satellites passing over these population centers at peak usage times experience and must cope with heavy Mobile Satellite Service (MSS) demands. U.S. Patent 5,326,054 to Turner discloses an Apogee at

Constant time-of-day Equatorial orbit, or "ACE" orbit for short, which has an eccentricity of 0.49 and takes advantage of the maximized dwell time of an elliptical orbit. The ACE orbit is a type of elliptical, sun- synchronous orbit where a single satellite orbits the globe completing five or six revolutions per day. Each apogee of the single satellite is selected to occur at the same time of day, every day, at the same longitude above the equator during peak telecommunication usage times. The satellite at apogee remains visible overhead for about a two- hour period. Five or six preselected apogee points above the equator may be chosen with the proviso that the points are equally spaced by one-fifth the circumference of the earth, or 72° longitude. Unfortunately, since the satellite moves so fast at perigee it is only visible overhead for a short time. Therefore, the perigees must, as much as possible, be selected to correspond to uninhabited parts of the globe, or urban centers during off-peak times of day. Furthermore, a single satellite in an ACE orbit can only service a very small portion of the world at any given time.

U.S. Patent 5,433,727 to Horstein et al. discloses a medium earth orbit (MEO) satellite-based cellular telecommunications system employing a constellation of satellites in a plurality of inclined orbits approximately 5,600 to 10,000 nautical miles above the earth. This region roughly corresponds to the area between the first and second

Van Allen radiation belts. The characteristic parameters, such as the altitude of each orbit, the number of orbits, the inclination angle of each orbit, the number of satellites in each orbit, are tailored to maximize the coverage area of each satellite and their related line of sight elevation angles, while minimizing propagation time delays, the number of beam-to-beam and satellite-to-satellite handovers, and the overall number of satellites in the constellation.

MEO satellite constellations, such as that described above, require earth tracking stations and are not sun-synchronous, that is, a different satellite passes overhead for any given time from one day to the next. Hence, all of the satellites in the constellation will at some time be called upon to handle peak traffic over some part of the world. For this reason, all of the satellites in the constellation must be of a sufficiently large capacity to handle the most demanding peak traffic. This tends to be expensive, particularly since all of these high capacity satellites will spend a large part of their life not being used to capacity. Furthermore, if only one geographic population center in the world experiences growth coupled with increased satellite demand, every satellite in the constellation must be upgraded to cope with the increased traffic.

Low earth orbits (LEOs) lie in a range between the outer limits of the earth's atmosphere and the first Van Allen radiation belt in a range of about 600 Km to 2000 Km. Several companies have applied to the FCC for a special "Pioneers Preference" license for the right to use LEO technology for cellular communications. The largest are Motorola's Iridium system and Loral/Qualcom's Globalstar system. These system's altitudes range from approximately 780 to 1390 Km and have different inclination angles. LEO satellites, like the MEO satellites, require earth tracking stations and generally suffer from the same requirement of needing a constellation wide increase of satellite capacities even if the increased demand for services is very localized. Specifically, all of the satellites in the constellation will at some time be called upon to handle peak traffic in all parts of the world and, therefore, all of the satellites in the constellation must be of a sufficiently large capacity to handle the most demanding peak traffic. Since the world has relatively few population centers which experience peak cellular traffic at only a few known times of the day, it would be advantageous and cost effective to design a satellite constellation which, while simultaneously servicing the entire globe, would require only selected satellites to handle peak traffic loads and which could be selectively upgraded as communication traffic increases.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a circular, sun-synchronous constellation of communication satellites wherein the constellation is in the same configuration at the same time each solar day.

It is yet another object of the present invention to provide a constellation of communication satellites wherein larger capacity communication satellites service selected regions at peak traffic times and smaller capacity, less expensive communication satellites in the constellation service regions during off-peak traffic times. It is yet another object of the present invention to provide a constellation of communication satellites wherein when particular satellites begin to reach capacity saturation at the same time each day, only those satellite needs to be replaced with higher capacity communication satellites. According to the invention, a satellite-based telecommunications system comprises a constellation of satellites in sun-synchronous orbit at specific altitudes and inclination angles so as to orbit the earth with a period that is an integer number of times per solar day. These orbits are unique in that any given satellite in the constellation services the same region on earth at the same time every day. Hence, the capacity of each satellite can be tailored to meet time of day dependent peak traffic requirements characteristic of mobile communications. Typically, peaks always occur at certain times of the business day while others may occur during data exchanges between businesses at the end of the day. Sun-synchronous orbits are therefore suitable to constellation designs where the capacity of satellites in different planes can be matched with the cellular traffic requirements at specific times of the day.

To establish the constellation, a plurality of satellites are launched in a sun-synchronous, circular, inclined orbit plane around the earth. The footprints of the satellites on the earth slightly overlap and march around the globe drifting from west to east, returning to their starting point exactly one solar day later. Additional such orbit planes are separated in their right ascending nodes such that the footprints of the satellites in each additional plane lags the previous plane footprints by an angle dependent on the number of planes used for coverage in a global constellation. Further, satellites may be interconnected by intersatellite links both in plane and across planes.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which: Figure 1 is a sun-synchronous orbit orientation at various times during one earth year cycle; Figure 2 is an orbit plane shown drifting eastward across the equator;

Figure 3 is a map of the earth showing footprints from an exemplary single plane of the sun-synchronous communication satellite constellation according to the present invention; Figure 4 is a map of the earth showing footprints from two neighboring planes of the sun-synchronous communication satellite constellation according to the present invention; and

Figure 5 is a map of the earth showing footprints from a full sun-synchronous communication satellite constellation according to the present invention;

Figure 6 is a globe showing the footprints of a 13 orbit per day satellite constellation;

Figure 7 is a globe showing the footprints of a 12 orbit per day satellite constellation.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Referring now to the drawings, and more particularly to Figure 1 , there is shown a sun-synchronous orbit orientation at various times during one earth year cycle. Only one orbit 8 is shown for clarity. The earth 10 orbits the sun 12 in an elliptical orbit path 14. The earth 8

10 is closest to the sun 12 at the winter solstice, and furthest from the sun at the summer solstice. The orbital plane in a circular sun- synchronous orbit is always aligned in the direction of the sun.

The earth is not a perfect sphere, but rather is oblate since it bulges at the equator which is about 16 miles greater the radius of the earth at the poles. Oblateness has two effects on a satellite orbit. First, it causes the major axis of the orbital plane to rotate about the orbit normal. Second, and more importantly to a circular sun- synchronous orbit, oblateness causes the ascending node of the orbit to drift. For inclination angles of less than 90°, the ascending node drifts westward. For inclination angles of greater than 90°, the ascending node drifts eastward. This drifting effect is illustrated in Figure 2. For a non-rotating earth, the orbit plane 8 around the earth 10 is shown drifting eastward across the equator. The drifting effect makes sun-synchronous orbits possible. If the altitude, inclination, and eccentricity are chosen so that the ascending node drifts eastward at the same rate as the earth revolves around the sun, then the earth-sun line maintains a constant orientation with respect to the orbital plane. If the orbital plane is initially peφendicular to the direction of the sun, the satellite will always be illuminated. The orbital period is an integer fraction of a mean solar day. Therefore, the satellite maintains the same time-of-day schedule over a given point on earth.

As explained further in Nelson, supra, the E-sat satellite system currently uses a sun-synchronous orbit to provide data services to public utilities and petroleum companies, direct-to-home television broadcast services, and the financial broadcast service industry. The sun- synchronous orbit is advantageous for these types of services since the satellite maintains the same time-of-day schedule, the ground trace will repeat itself every day, and a known satellite passes overhead at a scheduled time. Integer Altitude Altitude Inclination Orbits (Km) (nm) (°) per day

7 5165.4 2789.1 142.1

8 4182.2 2258.2 125.3

9 3384.79 1827.6 116.0

10 2722.5 1470.0 110.1

11 X 2162.2 1167.5 105.9

12 X 1680.9 907.61 102.9619

13 X 1262.1 681.49 100.7

14 X 893.83 482.63 99.01

15 566.92 306.12 97.66

16 274.44 148.19 96.58

The above table shows a list of altitudes and inclination angles that will satisfy the sun-synchronous criteria. The table may be calculated by selecting the orbital periods to be equal to 86,400 seconds/n, where n is an integer between 7 to 16. 86,400 is the seconds in an average solar day. The orbital inclination angle is selected to result in an easterly rotation of the right ascension of the ascending nodes of the orbital planes of 360° per solar year (0.9856° per solar day).

Those orbits in the table marked with an "X" have been deemed the most probable orientations for the present invention when atmospheric drag and radiation exposure are considered. Sun- synchronous orbits which orbit the earth 15 revolutions per solar day or more have altitudes which may be too low and atmospheric drag becomes a problem requiring additional fuel to maintain the orbit. Similarly, sun-synchronous orbits which orbit the earth 11 revolutions or less per solar day have altitudes which may be too high and the first Van Allen radiation belt becomes a problem unless the satellites are properly shielded from the damaging radiation.

According to the invention, a constellation of communication satellites in circular, sun-synchronous orbits with repeat earth track can provide significant cost savings to a Mobile Satellite Services (MSS) provider. The low altitude of these orbits minimizes the time delay associated with the propagation of radio waves to and from the satellites and allows the operator to selectively tailor the capacity of the individual satellites.

Referring now to Figure 3, there is shown a global map of the earth with a latitude and longitude grid overlay. Footprints from an exemplary first plane (Pl) of the sun-synchronous communication satellite constellation are shown circling the earth. In this example, the orbital plane contains 1 1 satellites having footprints labeled SOI to Sl l . The constellation is designed to orbit the earth 14 times per solar day. Hence, from the table above, the altitude of the orbital plane above the earth is 893.83 Km, and the inclination angle is 99.01 °. As shown in Figure 3, the satellites in plane 1 orbit the earth in a nearly polar orbit having footprints which move in the direction indicated by the arrows. Since Figure 3 is a still figure, it is not readily apparent that in fact the footprints drift in an eastwardly direction due to the oblateness of the earth. In this example, the sun-synchronous footprints will circle the globe exactly 14 times in the course of one solar day, drifting to the east while the earth completes a revolution such that they are in the same place at the same time each solar day. The sun-synchronous orbit of the present invention is particularly suitable for the establishment and maintenance of constellations for mobile communication satellite systems where the demand for services in most regions are dependent on the time of the day. If, for example, when it is twelve noon in New York, satellite P1S01 services that part of the United States at that same time every day. Hence, satellite P1S01 will need to be a high capacity telecommunications satellite to handle the peak load. Satellites P1S02-

P1S1 1 will soon thereafter serve that region and may therefore also have to be of a larger capacity if they pass during the peak time. Hence, unless a particular satellite services some part of the world during a peak time, it may be a less expensive, smaller capacity communication satellite.

Referring now to Figure 4, a second plane (P2) comprising 11 communication satellites, having footprints labeled P2S01-P2S1 1 is launched in a sun-synchronous orbit plane about 30° to the west of the first plane (Pl). As seen in the figure, each footprint overlaps all of its neighbors so as not to leave any dead spots. For a 14 orbit per day system, it may take about two planes of satellites to service a region during a peak time.

Additionally, each of the satellites in the constellation may communicate with other satellites in the constellation or gateway stations via inteφlane-links and cross-links. An inteφlane-link permits a particular satellite to communicate with adjacent satellites in the same plane. Similarly, cross-links allows a particular satellite to communicate with adjacent satellites in neighboring planes. Figure 5 shows a 14 orbit per day sun-synchronous communication satellite constellation of the present invention having global coverage. The constellation comprises six planes, P1-P6, each comprising 11 satellites labeled as S01-S1 1 for a total of 66 communication satellites. At any given time of day, the constellation will have the same footprint pattern as the day before. Therefore, when initially launching the constellation, each satellite can be tailored to the individual demand that will be placed upon it according to known peak global traffic demand times. Likewise, if any one of the 66 satellites begins to experience a surge in cellular traffic at a particular time each day to the point of reaching saturation, only that particular satellite needs to be replaced with a larger capacity satellite in order for the constellation to continue to meet the current demands, rather than having to replace the whole system.

Figure 6 is a computer generated example of what the footprints of a 13 orbit per day satellite constellation looks like projected on the earth's surface. For this configuration, there are five planes of satellites, each containing 9 satellites, for a total of 45 communication satellites.

Figure 7 is yet another computer generated example of what the footprints of a 12 orbit per day satellite constellation looks like projected on the earth's surface. For this configuration, there are four planes of satellites, each containing 8 satellites, for a total of 32 communication satellites. For both of the constellations shown in Figures 5 and 6, the individual satellites may have a capacity tailored to meet its particular service demands.

While the invention has been described in terms of a single preferred embodiment, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims.

Claims

CLAIMSWe claim:

1. A satellite constellation based telecommunications system, comprising: at least a first plane of satellites placed in a circular sun- synchronous orbit around earth, said first plane of satellites following a repeat track orbit wherein said satellites orbit the earth an integer number of times per solar day; said repeat track orbit causing said first plane of satellites to be in the a same position relative to the planet at a same time each solar day; said first plane of satellites having the feature that particular satellites in said first plane of satellites that are positioned over a region of the earth during peak telecommunication usage times each solar day may have a larger telecommunication capacity than other satellites in said first plane.

2. A satellite constellation based telecommunications system as recited in claim 1 further comprising: a plurality of orbital planes each containing a plurality of satellites, each of said plurality of orbital planes following repeat track orbits wherein said plurality of satellites orbit the earth an integer number of times per solar day, said repeat track orbits causing each of said plurality of satellites to be in a same position relative to the earth at a same time each solar day; particular ones of said plurality of satellites that are positioned over a region of the earth during peak telecommunication usage times each solar day may have a larger telecommunication capacity than other ones of said plurality of satellites.

3. A satellite constellation based telecommunications system as recited in claim 1 wherein said satellites orbit the earth 7 to 14 times per solar day.

4. A satellite constellation based telecommunications system as recited in claim 1 wherein said satellites orbit the earth 12 to 14 times per solar day.

5. A satellite constellation based telecommunications system as recited in claim 1 wherein said satellites orbit the earth 14 times per solar day.

6. A satellite constellation based telecommunications system as recited in claim 1 wherein said satellites orbit the earth 12 times per solar day.

7. A satellite constellation based telecommunications system as recited in claim 1 wherein said satellites orbit the earth 13 times per solar day.

8. A satellite constellation based telecommunications system as recited in claim 1 wherein said repeat track orbit is calculated from the criteria: OP= average solar day in seconds/n, where OP is orbital period and n is an integer between 7 and 16; and an orbital inclination angle is selected to result in an easterly rotation of the right ascension nodes of the orbital planes equal to 360° in an average solar year.

9. A satellite based telecommunications system as recited in claim 2 wherein each of said plurality of orbital planes comprises 7 to 12 satellites.

10. A satellite based telecommunications system as recited in claim 1 wherein each of said plurality of satellites can communicate with adjacent satellites is said first plane.

11. A satellite based telecommunications system as recited in claim 2 wherein each of said plurality of satellites can communicate with adjacent satellites a same plane and with adjacent satellites in neighboring planes.

12. A satellite-based telecommunication system, comprising: a constellation of communication satellites following a repeat track orbit around earth wherein said constellation of satellites orbit the earth an integer number of times per solar day, said constellation of communication satellites comprising higher capacity telecommunication satellites and lower capacity telecommunication satellites; a plurality of telecommunication devices positioned throughout a plurality of geographical regions on the earth, said plurality of geographical regions having peak telecommunication device usage times and off-peak telecommunication device usage times during a solar day, said repeat track orbit causing said higher capacity communication satellites to cover ones of said plurality of geographical regions during said peak telecommunication device usage times.

13. A satellite-based telecommunication system as recited in claim 12 wherein said constellation comprises a plurality of orbital planes, each containing a plurality of communication satellites.

14. A satellite-based telecommunication system as recited in claim 13 wherein said constellation orbits the earth 14 times per solar day and comprises 6 orbital planes each comprising 11 satellites.

15. A satellite-based telecommunication system as recited in claim 13 wherein said constellation orbits the earth 13 times per solar day and comprises 5 orbital planes each comprising 9 satellites.

16. A satellite-based telecommunication system as recited in claim 13 wherein said constellation orbits the earth 12 times per solar day and comprises 4 orbital planes each comprising 8 satellites.

17. A method for configuring a constellation of telecommunication satellites in a satellite-based telecommunication system, comprising the steps of: launching individual telecommunication satellites into a repeat track orbit over earth to form a constellation of telecommunication satellites, said constellation of telecommunication satellites orbiting the earth an integer number of times per solar day; assessing ones of said individual telecommunication satellites which reach near telecommunication saturation capacity during peak telecommunication usage times each solar day; replacing only ones of said individual telecommunication satellites in said constellation which have been assessed as reaching near telecommunication saturation capacity during peak telecommunication usage times each solar day.