U.S. patent application number 10/079411 was filed with the patent office on 2002-09-26 for system and method for satellite communications.
Invention is credited to Christopher, Paul F., Draim, John E..
Application Number | 20020136191 10/079411 |
Document ID | / |
Family ID | 25016285 |
Filed Date | 2002-09-26 |
United States Patent
Application |
20020136191 |
Kind Code |
A1 |
Draim, John E. ; et
al. |
September 26, 2002 |
System and method for satellite communications
Abstract
A satellite communication system that includes a terrestrial
base station and a satellite. The satellite communicates with the
terrestrial base station using a signal that has a frequency in the
range from the S band to visible light. The satellite is configured
in a COBRA orbit.
Inventors: |
Draim, John E.; (Vienna,
VA) ; Christopher, Paul F.; (Leesburg, VA) |
Correspondence
Address: |
ARNOLD & PORTER
IP DOCKETING DEPARTMENT; RM 1126(b)
555 12TH STREET, N.W.
WASHINGTON
DC
20004-1206
US
|
Family ID: |
25016285 |
Appl. No.: |
10/079411 |
Filed: |
February 22, 2002 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10079411 |
Feb 22, 2002 |
|
|
|
09750047 |
Dec 29, 2000 |
|
|
|
Current U.S.
Class: |
370/344 ;
370/319 |
Current CPC
Class: |
B64G 1/242 20130101;
B64G 1/1085 20130101; B64G 1/1007 20130101; H04B 7/195
20130101 |
Class at
Publication: |
370/344 ;
370/319 |
International
Class: |
H04B 007/204 |
Claims
We claim:
1. A satellite communication system comprising: a terrestrial base
station; and a first satellite communicating with said terrestrial
base station using a signal, in the frequency range from the S band
to visible light wherein said first satellite is configured in a
COBRA orbit.
2. The satellite communication system of claim 1, wherein said
communicating includes transmitting said signal between said
terrestrial base station and said first satellite; and wherein an
optimal location for transmitting said signal is determined based
on a frequency of said signal and the attenuation of said signal at
said frequency.
3. The satellite communication system of claim 2, wherein said
attenuation is based on the cloud water content persistent in a
region including said optimal location.
4. The satellite communication system of claim 3, wherein said
optimal location is defined by longitude and latitude.
5. The satellite communication system of claim 3, wherein said
cloud water content is determined based on an exceedance
probability.
6. The satellite communication system of claim 3, wherein said
cloud water content is determined based on a cloud water content
formula.
7. The satellite communication system of claim 3, wherein said
optimal location is based on the probability density function of an
elevation angle.
8. The satellite communication system of claim 8, wherein said
communicating includes receiving said signal between said
terrestrial base station and said first satellite; and wherein an
optimal location for receiving said signal is determined based on a
frequency of said signal and the attenuation of said signal at said
frequency.
9. The satellite communication system of claim 8, wherein said
attenuation is based on the cloud water content persistent in a
region including said optimal location.
10. The satellite communication system of claim 9, wherein said
optimal location is defined by longitude and latitude.
11. The satellite communication system of claim 9, wherein said
cloud water content is determined based on an exceedance
probability.
12. The satellite communication system of claim 9, wherein said
cloud water content is determined based on a cloud water content
formula.
13. The satellite communication system of claim 9, wherein said
optimal location is based on the probability density function of an
elevation angle.
14. A terrestrial base station communication system comprising: a
terrestrial base station communicating with a first satellite using
a signal in the frequency range from the S band to visible light,
wherein said first satellite is configured in a COBRA orbit.
15. The terrestrial base station communication system of claim 14,
wherein said communicating includes transmitting said signal
between said terrestrial base station and said first satellite; and
wherein an optimal location for transmitting said signal is
determined based on a frequency of said signal and the attenuation
of said signal at said frequency.
16. The terrestrial base station communication system of claim 15,
wherein said attenuation is based on the cloud water content
persistent in a region including said optimal location.
17. The terrestrial base station communication system of claim 16,
wherein said optimal location is defined by longitude and
latitude.
18. The terrestrial base station communication system of claim 16,
wherein said cloud water content is determined based on an
exceedance probability.
19. The terrestrial base station communication system of claim 16,
wherein said cloud water content is determined based on a cloud
water content formula.
20. The terrestrial base station communication system of claim 16,
wherein said optimal location is based on the probability density
function of an elevation angle.
21. The terrestrial base station communication system of claim 14,
wherein said communicating includes receiving said signal between
said terrestrial base station and said first satellite; and wherein
an optimal location for receiving said signal is determined based
on a frequency of said signal and the attenuation of said signal at
said frequency.
22. The terrestrial base station communication system of claim 21,
wherein said attenuation is based on the cloud water content
persistent in a region including said optimal location.
23. The terrestrial base station communication system of claim 22,
wherein said optimal location is defined by longitude and
latitude.
24. The terrestrial base station communication system of claim 22,
wherein said cloud water content is determined based on an
exceedance probability.
25. The terrestrial base station communication system of claim 22,
wherein said cloud water content is determined based on a cloud
water content formula.
26. The terrestrial base station communication system of claim 22,
wherein said optimal location is based on the probability density
function of an elevation angle.
27. A satellite communication system comprising: a terrestrial base
station; a first satellite communicating with said terrestrial base
station using a signal in the frequency range from the S band to
visible light; wherein said first satellite is configured in a
COBRA orbit; wherein said communicating includes transmitting said
signal between said terrestrial base station and said first
satellite; wherein an optimal location for transmitting said signal
is determined based on a frequency of said signal and the
attenuation of said signal at said frequency; and wherein said
attenuation is based on the cloud water content persistent in a
region including said optimal location.
28. A satellite communication system comprising: a terrestrial base
station; a first satellite communicating with said terrestrial base
station using a signal from in the frequency range the S band to
visible light; wherein said first satellite is configured in a
COBRA orbit; wherein said communicating includes receiving said
signal between said terrestrial base station and said first
satellite; wherein an optimal location for transmitting said signal
is determined based on a frequency of said signal and the
attenuation of said signal at said frequency; and wherein said
attenuation is based on the cloud water content persistent in a
region including said optimal location.
Description
RELATED INVENTION
[0001] This application is a continuation-in-part of application
Ser. No. 09/750,047 filed Dec. 29, 2000, titled "A System and
Method For Implementing a Constellation of Non-Geostationary
Satellites That Provides Simplified Satellite Tracking,"
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention is generally related to satellite
communications systems and, more particularly, to a constellation
of non-geostationary satellites that communicate with ground
stations using high frequency communications.
BACKGROUND OF THE INVENTION
[0003] Geostationary ("geo") satellites for telecommunications
applications were first proposed many years ago by the author
Arthur C. Clark. Today, there are numerous communications systems
employing geo satellites for such diverse applications as telephone
and data trunking, television distribution, direct-to-home
broadcasting, and mobile communications. Geo satellites operate on
the physical principle that a satellite, in circular orbit at the
proper altitude above the equator, will orbit the earth at the same
angular velocity as the earth's rotation. These satellites
therefore, appear to be fixed relative to a point on the earth.
This characteristic of geo satellites facilitates their use for
communication applications by allowing communications terminals on
the earth to simply point their antennas at essentially one
position in the sky.
[0004] There are however, a number of distinct drawbacks associated
with geostationary satellite systems. One major drawback is the
high cost of raising a satellite into geo orbit. Geostationary
orbits have a radius from the earth center of approximately 36,000
kilometers. Typically, a geo satellite is launched first into an
elliptical transfer orbit having an apogee at geostationary
altitude, and then its orbit is circularized by using a kick motor
to impart the necessary additional momentum to the satellite at
apogee. The apogee kick motor, before it is fired, typically weighs
as much as the satellite itself, meaning that the launch vehicle
must initially launch a payload twice as heavy as the satellite in
final orbit. Accordingly, the cost of putting a satellite into the
high circular orbit required for geostationary operation is
significantly greater than for non-geostationary satellites. The
cost associated with deployment of satellites must generally be
amortized over the lifetime of the satellite, making use of geo
satellites more expensive.
[0005] The high altitude of the geostationary orbit also adds to
the size and weight of geo satellites. Path loss, the attenuation
suffered by radio signals traveling in free space, is proportional
to the square of the distance between the source and the receiver.
This means that the antenna size and transmitted power of a geo
satellite must be greater than those of a satellite in lower orbit
in order to achieve the same communications link performance. This
is particularly true in mobile and other direct-to-user
applications where the size and power of the user terminal are
constrained by practical considerations and the burden of providing
acceptable link performance falls largely on the satellite. The
generally larger size and weight of geo satellites adds further to
the cost of launch as compared to satellites intended to operate in
lower orbits.
[0006] Another problem associated with the altitude at which geo
satellites orbit is the delay in the round trip transmission to and
from the satellite. For a pair of diverse communications terminals
located within the coverage area of a geo satellite, the path
length from terminal-to-satellite-to terminal is at least 70,000
kilometers. For the average satellite "hop" the associated
transmission delay is approximately one-quarter of a second. For
voice communications by satellite, the delay is noticeable to some
users, and may require the use of special circuitry for echo
control. For data communications, the delay complicates the use of
protocols that are predicated on the characteristics of terrestrial
circuits.
[0007] Other problems arise from the geometry of coverage of geo
satellite systems. A geostationary satellite system intended to
provide "global" services would include three geo satellites spaced
equal along the equatorial arc at 120-degree intervals. The
coverage area of each of these satellites describes a circle on the
surface of the earth with its center on the equator. At the
equator, the coverage areas of two adjacent geo satellites overlap
approximately 40 degrees in longitude. However, the overlap
decreases as latitude increases, and there are points on the earth,
north and south of the coverage areas, from which none of the geo
satellites is visible. For example, many points in Alaska, Canada
and Scandinavia cannot even see the geo satellites, these
satellites being below their visible horizon.
[0008] For a geo system, in which the satellites are in orbit above
the equator, earth stations in the equatorial regions generally
"see" the satellites at high elevation angles above the horizon.
However, as the latitude of an earth station increases, the
elevation angle to geo satellites from the earth station decreases.
For example, elevation angles from ground stations in the United
States to geostationary satellites range from 20 to 50 degrees. Low
elevation angles can degrade the satellite communications link in
several ways. The significant increase in path length through the
atmosphere at low elevation angles exacerbates such effects as rain
fading, atmospheric absorption and scintillation. For mobile
communications systems in particular, low elevation angles increase
link degradation due to blockage and multi-path effect.
[0009] Another, and perhaps more significant, problem resulting
from the specific geometry of the geo orbit, is the limited
availability of orbital positions (or "slots") along the
geostationary orbital arc. The ring of geostationary satellites
that has grown up over time generally occupies multiple slots
spaced two degrees apart and identified by their longitudinal
positions. This arrangement has been adopted internationally to
allow for satellite communications with a minimum of interference
between adjacent satellites operating in the same frequency bands.
The two-degree spacing is achieved by using high gain, directional
antennas at the ground stations accessing the satellites. The geo
ring around the equator thus provides a total of 180 slots (360
degrees/two degrees per slot) Most of the geo slots are now
occupied, making it difficult to find positions for more geo
satellites. Frequency, polarization and beam diversity have been
used to multiply capacity, but capacity in the geostationary arc
remains limited. Moreover, not all geo orbital positions are
equally useful or attractive for various applications.
[0010] Various non-geostationary satellite systems have been
implemented in the past to overcome some of the drawbacks of gee
satellites. An early example is the Russian Molniya system, which
employed satellites in elliptical 12-hour orbits to provide
coverage to the northern latitudes in the Soviet Union. The Iridium
and Globalstar systems use satellites in low circular orbits to
significantly reduce transmission delay and allow acceptable link
performance with very small user terminals. However,
non-geostationary systems operate in inclined orbits, and thus pose
a potential for interference with geo satellites operating at the
same frequencies as they cross the geostationary ring.
[0011] In January 1999, an application was filed before the Federal
Communications Commission (FCC) by Virtual Geosatellite LLC for the
construction of a global broadband satellite communications system
based on the teachings of U.S. Pat. Nos. 5,845,266 and 5,957,409,
issued on Dec. 21, 1998 and Sep. 28, 1999, respectively, to an
inventor of the present invention and two other individuals. The
system proposed in the FCC application employs three arrays of
satellites in elliptical orbits, two arrays covering the northern
hemisphere and one covering the southern hemisphere, each array
having five eight hour satellites emulating many of the
characteristics of geo satellites. The satellites appear to "hang"
in the sky because their angular velocity at or near apogee
approximates the rotation rate of the earth. Nine so-called "active
arcs" are created with centers located at the apogee points of the
satellite orbits. The satellites in each of the three arrays move
in a repeating ground track from one active arc to the next, so
that there is always one active satellite available in each active
arc. While in their active arcs, the satellites move very slowly,
averaging only about eight degrees per hour, with respect to
terrestrial antennas. Between arcs, the satellites are deactivated.
The active arcs occupy different portion of the sky than any of the
geo satellites located near the equator. As a result, the virtual
geo satellites are visible from most parts of the northern, and
southern hemispheres, but do not interfere with satellites in the
geo arc.
[0012] Although the prior art virtual geo satellite constellation
described above addresses many of the shortcomings of geostationary
satellites, it requires that ground terminals track the satellites
as they slowly traverse the active arcs. Moreover, as one satellite
leaves the end of an active arc and is deactivated, the ground
station antenna must quickly re-point, or slew, 90-50 degrees to
pick up the satellite that has just arrived at the beginning of the
active arc to take the place of the first satellite. For large
antennas, such rapid slewing may prove, impractical, and actually
require the use of two antennas at each site. Phased array antennas
can provide rapid re-pointing, but the commercial availability of
affordable designs, especially for the consumer market, is unclear.
Some form of data buffering to cover the outage period is another
possible alternative, although also likely to be complex and
expensive.
OBJECTIVES
[0013] Therefore, it is an objective of the present invention to
provide a system of non-geostationary satellites that significantly
simplifies the tracking requirements and reduces the cost for
satellite ground stations.
[0014] It is another objective of the present invention to provide
a system of satellites that materially increases global
communications satellite capacity without interfering with the
existing geostationary satellite ring.
[0015] It is further objective of the present invention to provide
a global system of communications satellites with higher average
elevation angles and lower transmission delay than existing
geostationary satellites.
[0016] It is further objective of the present invention to provide
a global system of communications satellites utilizing
communication frequency bandwidths which reduce the amount of
signal attenuation between satellites and ground stations.
[0017] It is yet a further objective of the present invention to
provide a total global communications system of satellites and
ground facilities with lower construction and implementation costs
than existing geostationary systems.
[0018] The above-stated objectives, as well as other objectives,
features and advantages, of the present invention will become
readily apparent from the following detailed description, which is
to be read in conjunction with the appended drawings.
SUMMARY OF THE INVENTION
[0019] The present invention is directed to a constellation of
non-geostationary satellites that may be deployed and utilized in a
manner that materially increases global communications capacity,
does not interfere with satellites in the existing geostationary
ring, and provides simplified satellite tracking. A system
embodiment includes first and second pluralities of satellites in
inclined elliptical orbits, each plurality of satellites forming a
repeating ground track that brings the satellites over the same
points on the earth everyday. In the preferred embodiment the
satellites have a mean motion of 3, meaning they orbit the earth
three times per day, but other integer values of mean motion, such
as 2 and 4 are applicable.
[0020] Each orbiting satellite has communications equipment on
board for communicating with ground stations. The communications
equipment on each satellite in the constellation is enabled, or
activated (e.g., powered) only during a portion of the orbit when
the satellite is near apogee, the point in the orbit where the
satellite altitude is greatest and the satellite is moving most
slowly from the viewpoint of the earth stations. Preferably, the
portion of the orbit during which the satellite is enabled is
symmetrically disposed about the apogee of the orbit. In the
preferred embodiment, with mean motion 3, each of the satellites is
enabled near its apogee for a duration of 4 hours, which is 50
percent of its total orbit period.
[0021] Each of the satellite ground tracks has a number of active
arcs corresponding to the portion of the satellite orbits during
which the communications equipment on the satellites is enabled to
communicate. The orbits of the first plurality of satellites are
configured such that each of the active arcs of the first ground
track begins and ends at points that fall on the same meridian of
longitude. This is accomplished by selecting an argument of perigee
that "leans" the satellite orbits toward the equator, placing the
satellite apogee at about 40 degrees latitude. The "argument of
perigee" is an orbital parameter that indicates the angular
position in the plane of the orbit where perigee occurs. Arguments
of perigee between zero degrees and 180 degrees locate the position
of perigee in the Northern Hemisphere, and hence concentrate
satellite coverage in the Southern Hemisphere. Conversely arguments
of perigee between 180 degrees and 360 degrees locate the perigee
in the Southern Hemisphere and hence concentrate coverage on the
Northern Hemisphere.
[0022] At the same time, the orbits of the second plurality of
satellites have an argument of perigee that is the supplementary
angle of the argument of perigee of the first plurality of
satellites, causing the satellite orbits of the second plurality of
satellites to lean by an equal amount in the opposite direction.
The orbits of the second plurality of satellites are further
configured such that each active arc of the second ground track
begins at a point coincident with the ending point of one of the
active arcs of the first ground track, and ends at a point
coincident with the beginning point of the same one of the first
active arcs. The result, as viewed from a ground station, is a
closed path formed by an active arc of the first ground track and a
corresponding active arc of the second ground track. For the
preferred embodiment with an orbital mean motion of three, the
closed path is repeated three times around the earth, at equal,
120-degree intervals.
[0023] In addition to the constellation of satellites, the system
embodiment of the present invention typically includes a plurality
of ground stations, each having communications equipment configured
to communicate with the communications equipment on the first and
second plurality of satellites, and located at positions on the
earth from which they can track satellites in one of the first
active arcs and satellites in the one second active arc that has
coincident beginning and ending points.
[0024] In another aspect of the invention, the orbits of the first
and second pluralities of satellites are configured such that at
all times there is at least one satellite in either each of the
active arcs of the first ground track, or each of the active arcs
of the second ground track. Preferably, there are an equal number
of satellites in the two ground tracks, and the orbits of the
satellites are further configured such that when one satellite is
at the end of an active arc in one ground track and in the process
of being deactivated, another satellite is at or near the beginning
of the corresponding active arc in the other ground track and being
reactivated. At the changeover point the two satellites must be
near enough to allow a ground station to follow what appears to be
a single active satellite in a closed path in the sky overhead
without having to break lock and slew to a new position when
satellite changeovers occur. However, the orbital parameters of the
satellites in the first and second ground tracks are preferably
selected such that at the points where the ground tracks cross, the
satellites are far enough apart in space that they do not actually
collide.
[0025] Preferably, the satellites in each ground track are equally
spaced in mean anomaly to achieve the greatest number of satellites
enabled at the same time. "Mean anomaly" represents the fraction of
an orbit period that has elapsed since the satellite passed through
perigee, as expressed in degrees. For example, the mean anomaly of
a satellite two hours into an 8-hour orbit is 90 degrees (one
quarter of the period).
[0026] Continuous communication at the preferred 50 percent duty
cycle requires a minimum of three evenly spaced satellites in each
ground track. Adding more basic groups of six satellites to the two
ground tracks creates additional orbital capacity. In the preferred
embodiment, the orbital parameters allow up 12 satellites to be
placed in each active arc of the ground track while maintaining a
minimum angular spacing between satellites of at least 2
degrees.
[0027] In another aspect of the invention, each of the satellites
in the constellation has an orbital height lower than the height
necessary for geostationary orbits. This aspect of the invention
has the benefit of reducing satellite size and weight for a given
communications capacity, reducing launch requirements, and reducing
satellite transmission delay. Also launching into elliptical orbits
requires less energy than circular orbits, further reducing launch
vehicle costs.
[0028] To minimize perturbation effects caused by the earth's
shape, the present invention also preferably uses the critical
orbital inclination of 63.4 degrees. This is the inclination of the
orbital plane that results in a stable elliptical orbit whose
apogee always stays at the same latitude in the same
hemisphere.
[0029] In another aspect of the present invention, the orbits of
satellites are configured such that the portion of the satellites's
orbits during which communications equipment is enabled, is
separated from the earth's sequatorial plane by at least a
predetermined amount. This feature avoids potential interference
with existing satellites in the geostationary ring and allows the
communications frequencies allocated to geostationary satellites to
be reused for the non-geostationary constellation of the present
invention.
[0030] In a further aspect of the present invention, each satellite
has a power system configured to generate an amount of power less
than that required when the communications equipment on the
satellite is enabled, and more than that required when the
communications equipment is not enabled. The power system can store
the excess power generated when the communications equipment is not
enabled, and use the stored power to supplement the generated power
to meet the requirements of the communications equipment when it is
enabled. For the preferred embodiment with a duty cycle of 50
percent, satellite weight saving resulting from this power
conservation scheme can be significant.
[0031] In a further aspect of the present invention, the
attenuation associated with communications between satellites of
the present invention and the plurality of ground stations is
optimized by adopting high frequency bands. One frequency band that
has been utilized by satellites in the elliptical orbits described
herein has been the Ku band, the frequency range between 10.9 to 17
GHz. The present invention operates at frequency bands above the Ku
band to optimize the attenuation and enable efficient satellite
communications.
[0032] In yet a further aspect of the invention, satellites are
added to the constellation to form additional pairs of ground
tracks having the same shapes as the first and second ground
tracks, but displaced in longitude by a predetermined amount. The
amount of longitudinal displacement is such that at all times each
of the satellites in the active arcs of the additional ground track
pairs is separated by at least a predetermined angle from any of
the active satellites in other ground track pairs. The preferred
embodiment, with satellites in orbits having a mean motion of three
and operating at a 50 percent duty factor, can accommodate four
pairs of ground tracks having 24 active arcs (i.e., 12 closed
paths) in each hemisphere, or a total of 48 active arcs worldwide.
If each arc is filled with a maximum of 12 active satellites, the
total number of equivalent non-geostationary satellite slots that
the present invention can support is 576, or more than three times
as many as the existing geo stationary ring, assuming minimum
two-degree satellite spacing.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] FIG. 1 shows the basic characteristics of an elliptical
satellite orbit including the bunching together of satellites near
apogee.
[0034] FIG. 2 shows a perspective view of five elliptical orbits
having one satellite in each orbit according to the prior art.
[0035] FIG. 3 is a Cartesian plot showing a ground track for the
elliptical orbits of FIG. 2 according to the prior art.
[0036] FIG. 4 is a Cartesian plot showing a left-leaning repeating
ground track according to the present invention.
[0037] FIG. 5 is a Cartesian plot showing a right-leaning repeating
ground track according to the present invention.
[0038] FIG. 6 is a Cartesian plot showing the left- and right-right
leaning ground tracks of FIGS. 4 and 5 combined to form closed
teardrop patterns according to the present invention.
[0039] FIGS. 7A-7B show equatorial and polar perspective views of
six elliptical orbits having one satellite in each orbit according
to the present invention.
[0040] FIG. 8 is a Cartesian plot showing the basic six satellite
system of FIGS. 7A-B.
[0041] FIG. 9 is a Cartesian plot showing the same basic
six-satellite satellite system as FIG. 8 at a later point in
time.
[0042] FIG. 10 is a Cartesian plot showing a constellation of
twenty-four satellites according to the present invention having
four active satellites in each closed teardrop pattern.
[0043] FIG. 11 is a Cartesian plot showing 12 teardrop patterns in
the Northern Hemisphere and 12 teardrop patterns in the Southern
Hemisphere according to the present invention.
[0044] FIGS. 12A-12B show equatorial and polar perspective views of
the 24 teardrop patterns of FIG. 11.
[0045] FIG. 13 is a Cartesian plot showing a constellation of
satellites according to the present invention having twelve
teardrop patterns in the Northern Hemisphere, one of which has
twenty-four active satellites.
[0046] FIGS. 14A-14B are block diagrams showing a layout of typical
satellite and ground station communications equipment used
according to the present invention.
[0047] FIG. 15 is a flowchart showing a power consumption
methodology of a satellite according to the present invention.
[0048] FIG. 16 is a chart representing signal attenuation at
various frequencies for ground fog and rain.
[0049] FIG. 17 is a chart representing signal attenuation at 22.2
GHz for various locations at a zenith path.
[0050] FIG. 18 is a chart representing signal attenuation at 49.5
GHz for various locations at a zenith path.
[0051] FIG. 19 is an equation calculating signal attenuation at any
frequency for various locations at a zenith path.
[0052] FIG. 20 is a chart representing cloud attenuation at 22.2
GHz for various locations at a zenith path.
[0053] FIG. 21 is a chart representing cloud water content, in
gm/m.sup.2, for various locations at a zenith path and 99%
"non-rainy"conditions.
[0054] FIG. 22 is an equation calculating cloud water content at a
zenith path, given a location (in latitude and longitude, degrees)
and an exceedance probability.
[0055] FIG. 23 is a chart representing cloud attenuation at the
infrared wavelength of 10 microns, with a 99% "non-rainy"
condition.
[0056] FIG. 24 is a chart representing cloud water content for
various locations, at a zenith path and 90%
"non-rainy"conditions.
[0057] FIG. 25 is a chart representing cloud attenuation at the
infrared wavelength of 10 microns, with a 90% "non-rainy"
condition.
[0058] FIG. 26 is a chart representing cloud attenuation at 10
micron wavelengths, with 10% exceedance, for various Earth
locations at six selected attenuation levels: 10 dB, 20 dB, 30 dB,
40 dB, 100 dB, and 110 dB.
[0059] FIG. 27 is a chart representing cloud attenuation at 1
micron wavelengths, with 10% exceedance, for various Earth
locations at six selected attenuation levels: 10 dB, 20 dB, 30 dB,
40 dB, 100 dB, and 200 dB.
[0060] FIG. 28 is a chart representing cloud water content for
various locations at a zenith path and 80% "non-rainy"
conditions.
[0061] FIG. 29 is a chart representing cloud attenuation at the
infrared wavelength of 10 microns, with a 80% "non-rainy"
condition.
[0062] FIG. 30 is a chart representing cloud attenuation at 10
micron wavelengths, with 20% exceedance, for various Earth
locations at six selected attenuation levels: 10 dB, 20 dB,30 dB,
40 dB, 100 dB, and 110 dB.
[0063] FIG. 31 is a chart representing cloud attenuation at 1
micron wavelengths, with 20% exceedance, for various Earth
locations at six selected attenuation levels: 10 dB, 20dB, 30 dB,
40 dB, 100 dB, and 110 dB.
[0064] FIG. 32 is a graph showing probability density functions for
a COBRA satellite system, as a function of latitude.
[0065] FIG. 33 is a graph showing the average of the cosecant of
the elevation angle of a COBRA satellite system as compared to the
latitude of the COBRA satellite system.
[0066] FIG. 34 is a graph showing attenuation for a COBRA satellite
system at 30 GHz.
[0067] FIG. 35 is a graph comparing the average attenuation of
geosynchronous satellite systems and COBRA satellite systems at 30
GHz.
[0068] FIG. 36 is a graph of the prior art showing attenuation, at
45 GHz, for ninety-nine percent "non-rainy" conditions assuming
satellite s at the zenith position.
[0069] FIG. 37 is a graph of attenuation at 45 GHz for satellites
at the zenith position in the Northern Hemisphere.
[0070] FIG. 38 is a graph of attenuation for COBRA satellite
systems, operating in the Northern Hemisphere, at 45 GHz.
[0071] FIG. 39 is a graph comparing the average attenuation of
geosynchronous satellite systems and COBRA satellite systems at 45
GHz.
[0072] FIG. 40 is a graph of attenuation at 75 GHz for satellites
at the zenith position in the Northern Hemisphere.
[0073] FIG. 41 is a graph of attenuation for geosynchronous
satellite systems, operating in the Northern Hemisphere, at 75
GHz.
[0074] FIG. 42 is a graph of attenuation for COBRA satellite
systems, operating in the Northern Hemisphere, at 75 GHz.
[0075] FIG. 43 is a graph comparing the average attenuation of
geosynchronous satellite systems and COBRA satellite systems at 75
GHz.
[0076] FIG. 44 is a graph of attenuation at 90 GHz for satellites
at the zenith position in the Northern Hemisphere.
[0077] FIG. 45 is a graph of attenuation for geosynchronous
satellite systems, operating in the Northern Hemisphere, at 90
GHz.
[0078] FIG. 46 is a graph of attenuation for COBRA satellite
systems, operating in the Northern Hemisphere, at 90 GHz.
[0079] FIG. 47 is a graph comparing the average attenuation of
geosynchronous satellite systems and COBRA satellite systems at 90
GHz.
[0080] FIG. 48 is a graph of Net Loss (Loss minus Gain at constant
antenna aperture) for a COBRA satellite system showing three
locations.
[0081] FIG. 49 is a graph showing optimal frequencies for the
Northern Hemisphere for a COBRA satellite system.
DETAILED DESCRIPTION OF THE INVENTION
[0082] The present invention is directed to a communications system
including ground stations and a constellation of satellites in
elliptical orbits that emulate many of the characteristics of
geostationary satellites from the viewpoint of the ground stations
on the earth and that communicate with the ground stations using
high frequency signals. As explained in greater detail below, the
satellites of the present invention are in elliptical orbits and
operate in a portion of their orbits near apogee. This portion of
their orbits can be referred to as their active arcs. The orbital
parameters of the satellites are adjusted such that these active
arcs are in the Northern and Southern Hemispheres, outside of the
equatorial region.
[0083] The satellite constellations of the present invention are
known as "Communications Orbiting Broadband Repeating Array," on
"COBRA" satellites. At its basic sense, a satellite is a COBRA
Orbit utilizes a learning eight-hour elliptic orbit. A
six-satellite array using 8-hour left-learning (or right-learning)
elliptic orbits is capable of continuously covering virtually all
the Northern Hemisphere. With this COBRA array, there are three
complete cycles (orbits) per day, there are three `loops` in the
ground track. Apogees are all in the Northern Hemisphere, where the
orbital velocities are lower and more closely match the earth's
srotational velocity. For this reason, the Northern Hemisphere
loops are narrower than loops in the Southern Hemisphere, where the
velocities are much higher with satellites traversing a wider range
of longitude in less time. The three distinct loops, as well as the
three apogee (and three perigee) locations all lie exactly 120
degrees apart in longitude, as one would expect. In the left
learning loops, the satellites at 15.degree. N have a mean anomaly
(MA of 90.degree., while those at 60.degree. N have an MA of
270.degree.. This situation is just reversed for the right-leaning
loops. Each of these short arcs represents exactly one-half of an
orbital period. In terms of mean anomaly, these active arcs lie
between 90.degree. and 270.degree. MA. Since this translates into
exactly one-half of the orbital period, the payload duty cycle is
thus 50%.
[0084] Another type of COBRA orbit involves a left-leaning COBRA
track containing three satellites in combination with a right
learning COBRA track that also contains three satellites. These six
satellites are time-phased, for each teardrop, to meet at two
crossover points in space. The resultant effect is to create an
inverted (in the Northern Hemisphere) teardrop shaped closed-path
trace. To a mid-latitude observer on the ground in or anywhere near
the teardrop, it will appear that one active satellite is
continuously circling nearly overhead. It should be noted that this
closed path is only possible because the left-leaning COBRA active
arcs travel from South to North, while the right leaning arcs
travel in the reverse direction.
[0085] More specifically a COBRA orbit satellite system
encompasses, as shown in U.S. application Ser. No. 09/709,280 and
which is hereby incorporated by reference, a plurality of
satellites in orbits around the earth having apogees and perigees,
each of the satellites having communications equipment thereon
configured to communicate only during a predetermined portion of
the satellite's sorbit proximate to apogee, the orbits of the
plurality of satellites being configured to form at least two
ground tracks on the earth displaced from each other
longitudinally, each of the ground tracks repeating daily and
having a number of active arcs, each active arc corresponding to
the portion of the orbit of each satellite during which the
communications equipment on the satellite is enabled to
communicate, the orbits of the plurality of satellites being
further configured such that at all times there are at least two of
the satellites in each of the active arcs and such that at all
times each of the satellites in any one of the active arcs is
separated by at least a predetermined angle, as observed from the
earth, from each other satellite in the same active arc and from
any satellite in any other active arc.
[0086] The constellation may have the orbit of each of the
plurality of satellites with a mean motion that is one of 2, 3 and
4; and the orbit of each of the plurality of satellites may be
inclined at critical inclination. The argument of perigee of the
orbits of each of the plurality of satellites may be in the range
of 195 degrees to 345 degrees for apogees in the northern
hemisphere and in the range of 15 degrees to 165 degrees for
apogees in the southern hemisphere. Each of the plurality of
satellites may have throughout its orbit a orbital height lower
than a height necessary for geostationary orbits, and the
satellites in each of the two or more ground tracks are equally
spaced in mean anomaly.
[0087] The COBRA orbit satellite system may also encompass, as
shown in U.S. application Ser. No. 09/750,047 (from which the
present invention is a continuation-in-part), a system of
satellites comprising a first plurality of satellites in orbits
around the earth having apogees and perigees, each of the first
plurality of satellites being configured to be active only during a
predetermined portion of the satellite's sorbit proximate to
apogee, the orbits of the first plurality of satellites having a
first argument of perigee and being configured to form a first
common ground track, the first common ground track repeating daily
and having a number of first active arcs each corresponding tot he
predetermined portion of each satellite's sorbit during which the
satellite is active, the orbits of the first plurality of the first
active arcs begins and ends at points on a same meridian of
longitude; and a second plurality of satellites in orbits around
the earth having apogees and perigees, each of the second plurality
of satellites configured to be active only during a predetermined
portion of the satellite's sorbit proximate to apogee, the orbits
of the second plurality of satellites having a second argument of
perigee being a supplementary angle to the first argument of
perigee, and being configured to form a second common ground track,
the second common ground track repeating daily and having a number
of active arcs corresponding to the predetermined portion of each
satellite's sorbit during which the satellite is active, the orbits
of the second plurality of satellites being further configured such
that each of the second active arcs begins at a point coincident
with the ending point of one of the first active arcs, and end at a
point coincident with the beginning point of the same one of the
first active arcs.
[0088] The system may have orbits of the first plurality of
satellites and the second plurality are further configured such
that at all times there is at least one satellite in at least one
of (i) each of the first active arcs and (ii) each of the second
active arcs.
[0089] The first plurality of satellites may have a first number of
satellites and the second plurality of satellites may have a second
number of satellites equal to the first number of satellites, and
the orbits of the first plurality of satellites and the second
plurality may be further configured such that at any time one of
the first plurality of satellites is at a beginning point of one
the first active arcs, one of the second plurality of satellites is
simultaneously at or near a coincidental ending point of one of the
second active arcs. The orbits of the first plurality of satellites
and the second plurality may be further configured such that
satellites do not collide at points where the first and second
ground tracks cross, and the satellites in each of the first ground
track and the second ground track may be equally spaced in mean
anomaly.
[0090] The system of the first plurality of satellites and the
second plurality of satellites may be configured such that at all
times each of the satellites in any one of the first active arcs
and the second active arcs is separated by at least a predetermined
angle, as viewed from the earth, from each other satellite in the
same active arc. The orbit of each satellite of each of the
pluralities of satellites may be inclined at critical inclination,
and each satellite of each of the pluralities of satellites may
have throughout its orbit a orbital height lower than a height
necessary for geostationary orbits. The orbits the first and second
pluralities of satellites may have a mean motion that is one of 2,
3 and 4.
[0091] The satellite system may also encompass a third plurality of
satellites in orbits around the earth, each of the third plurality
of satellites configured to be active only during a same portion of
the satellite's sorbit as the predetermined portion of the orbit of
each of the first plurality of satellites, the orbits of the third
plurality of satellites being configured to form a third common
ground track with third active arcs, the third common ground track
having a same shape as, and displaced in longitude by a
predetermined amount from, the first common ground track; and a
fourth plurality of satellites in orbits around the earth, each of
the fourth plurality of satellites configured to be active only
during a same portion of the satellite's sorbit as the
predetermined portion of the orbit of each of the second plurality
of satellites, the orbits of the third plurality of satellites
being configured to form a fourth common ground track with fourth
active arcs, the fourth common ground track having a same shape as,
and displaced in longitude by the predetermined amount from, the
second common ground track; wherein the predetermined amount of
longitudinal displacement is such that at all times (i) each of the
satellites in any of the third active arcs is separated by at least
a predetermined angle, as observed from the earth, from any
satellite in any of the second active arcs, and (ii) each of the
satellites in any of the forth active arcs is separated by at lest
a predetermined angle, as observed from the earth, from any
satellite in any of the first active arcs.
[0092] COBRA constellations offer several benefits over prior art
systems in which satellites activated in an active arc. In these
prior systems, as a satellite is turned off at the end of an active
arc, it is replaced by another satellite that is being turned on at
the beginning of the arc. Accordingly, any ground station that has
been tracking the satellite as it moves slowly through its active
arc, must quickly re-point its antenna beam to the beginning of the
arc as the replacement satellite arrives.
[0093] In contrast, COBRA constellations permit a ground station to
continuously track the active satellites without having to slew the
ground station antenna beam between the turn-off position of the
departing satellite and the turn-on position of the arriving
satellite. As explained in detail below, this is accomplished by
using a "left-leaning" elliptical ground track active arc together
with a "right-leaning" elliptical ground track active arc. The
parameters of the satellite orbits are adjusted such that the end
points of the two active arcs coincide, the turnoff point of one
arc being the same as the turn-on point of the other. As shown in
detail below, the combination of the two active arcs creates an
upside-down, teardrop shaped closed pattern in the Northern
Hemisphere, or an upright teardrop pattern in the Southern
Hemisphere. The active arcs of the left-leaning satellite tracks
exhibit satellite motion from south to north, in the Northern
Hemisphere, while the right leaning arcs contain active satellites
moving from north to south. An observer on the ground in the
Northern Hemisphere region served by satellites in these ground
tracks observes the active satellites moving slowly in a
counter-clockwise direction looking upwards into the sky, at
generally high elevation angles. At the changeover points, which
occur about every four hours for satellites in the 8-hour orbits of
the preferred embodiment, the ground station antenna momentarily
"sees" two satellites at virtually the same azimuth and elevation
location, so that no "slewing" maneuver is required when switching
from the satellite in one ground track being deactivated to
satellite in the other ground track being activated. No
discontinuity in active satellite position or antenna beam pointing
direction, will be observed at the ground antenna. The ground
station antenna will experience changes only in the angular azimuth
and elevation tracking rates as the switchover is made from one
ground track to the other. In the preferred embodiment, the
switchover is controlled from a master ground control station and
can be accomplished without an interruption in service.
[0094] COBRA orbits take advantage of the fact than satellites in
elliptical orbits spend more time near the apogees of their orbits,
when they are farther from the earth, than near their perigees.
FIG. 1 shows a typical elliptical orbit 10 having two foci 11 and
12. The satellite orbits along the path of the ellipse 10, with the
center of the earth being at focus position 12 (the "occupied
focus").
[0095] The apogee 14 and perigee 16 of the orbit are defined by the
points on-the ellipse farthest from and closest to the occupied
focus, respectively. The major axis of the ellipse 18 runs through
the two foci of the ellipse, from apogee 14 to perigee 16. One-half
of the major axis is referred to as the semi-major axis, a; it is
from this parameter that the orbital period is uniquely determined.
The two lengths along the semi-major axis, from the apogee 14 and
perigee 16 to the occupied focus 12 are called the "radius of
apogee" and the "radius of perigee" respectively. The amount of
difference between these distances defines the eccentricity of the
ellipse. In terms of the semi-major axis, a, and eccentricity, e,
the radius of apogee, r.sub.a, and the radius of perigee, r.sub.p,
are:
r.sub.a=a(1+e); (1)
[0096] and
r.sub.p=a(1-e) (2)
[0097] The greater the eccentricity, the less the ellipse resembles
a circle.
[0098] The position of a satellite in an elliptical orbit follows
Kepler's ssecond law of motion, which states that the orbiting
satellite will sweep out equal areas of the orbit in equal times.
This results in the satellite moving rapidly when it is at or near
perigee and moving slowly when it is at or near apogee. For an
8-hour orbit, for example, a satellite will spend more than four
hours near apogee. The circles on the ellipse of FIG. 2, for
example, mark off even time intervals in the motion of a satellite
about the orbit, and show clearly how the satellite slows down and
dwells for an extended period of time near apogee.
[0099] In COBRA systems, a constellation of satellites is chosen to
operate such that the desired point on-the earth always tracks and
communicates with a satellite at or near apogee. By using prograde
orbits, those in which the satellite is rotating in the same
directional sense as the earth, the satellites at apogee can be
made to appear to move very slowly, or even stop momentarily, in
the sky.
[0100] Although the satellites in COBRA orbits resemble
geostationary satellites in that they appear virtually stationary
when at or near apogee, typically moving at a rate of less than
eight degrees per hour, each satellite does eventually leave its
active arc, and, as explained in further detail below, is replaced
by another satellite that enters the complementary active arc at
the same time, and within view of the same ground stations. This
characteristic means that unlike geo satellites, each satellite in
a COBRA orbit does not operate 100 percent of the time. Outside of
their active arcs, the satellites are typically not using their
transmit and receive capability, and hence do not use a large
portion of their power capacity.
[0101] Since each satellite is fully powered only part of the time,
the satellite can be generating and storing power during the period
when it is not active and then use it while in its active arc.
Hence the satellite power source, typically arrays of solar cells,
can be sized to provide only a fraction of the power needed during
operation with the balance coming from the energy stored, typically
in rechargeable batteries, during the inactive parts of its orbit.
For example, since a satellite in the preferred embodiment is
operating only 50 percent of the time, its power system can, in
principle, be designed to generate 50 percent of the full load
power from its solar arrays (plus whatever power is required to
maintain housekeeping functions). This mode of operation can result
in a significant saving in the weight and size of the
satellites.
[0102] Before describing in detail the preferred satellite
arrangement according to the present invention, the nomenclature
utilized herein to describe the characteristics of satellite orbits
will be first defined.
[0103] The term "mean motion", n, is a value indicating the number
of complete revolutions per day a satellite makes. If this number
is an integer, the ground tracks of the satellites repeat each day
and each ground track for that day overrides the tracks of the
preceding day. Mean motion is conventionally defined as the hours
in a day (24) divided by the number of hours that it takes a
satellite to complete a single orbit. For example, a satellite that
completes an orbit every eight hours (an "8-hour satellite") has a
mean motion of three. Integral mean motions of two, three and four
are of particular applicability, but the present invention does not
exclude higher values. As explained in further detail below, a
small deviation from the exact integer value of mean motion is
usually necessary to compensate for the perturbing effect due to
non-sphericity of the earth, which is called "regression of the
line of nodes."
[0104] The "elevation angle".sigma., is the angle from an
observer's shorizon up to the satellite. A satellite on the horizon
would have-zero degrees elevation while a satellite directly
overhead would have 90 degrees elevation. Geo satellites orbit near
the equator, and usually have a 20-50 degree elevation from points
in the United States.
[0105] The "inclination", I or i, is the angle between the orbital
plane of the satellite and equatorial plane. Prograde orbit
satellites orbit in the same orbital sense (clockwise or counter
clockwise) as the earth. For prograde orbits, inclination lies
between zero degrees and 90 degrees. Satellite retrograde orbits
rotate in the opposite orbital sense relative to the earth, so for
retrograde orbits the inclination lies between 90 degrees and 180
degrees.
[0106] The "critical inclination" for an elliptical orbit is the
particular planar inclination that results in a zero apsidal
rotation rate. This inclination results in a stable elliptical
orbit whose apogee always stays at the same latitude in the same
hemisphere. Two inclination values satisfy this condition: 63.435
degrees for prograde orbits or its supplement 116.565 degrees for
retrograde orbits.
[0107] The "ascending node" is the point on the equator where the
satellite passes from the southern hemisphere to the northern
hemisphere. The right ascension of the ascending node ("RAAN") is
the angle measured eastward in the plane of the equator from a
fixed inertial axis in space (the vernal equinox) to the ascending
node.
[0108] For the present invention, the longitudinal spacing between
the ascending nodes of different satellites in the constellation is
called "S", and is uniform in the preferred embodiment.
[0109] The "argument of perigee" is a value that indicates the
angular position in the plane of the orbit where perigee occurs.
Arguments of perigee between zero degrees and 180 degrees locate
the position of perigee in the northern hemisphere, and hence
concentrate satellite coverage in the southern hemisphere.
Conversely arguments of perigee between 180 degrees and 360 degrees
locate the perigee in the `southern hemisphere and hence
concentrate coverage on the northern hemisphere.
[0110] "Mean anomaly", M, represents the fraction of an orbit
period that has elapsed since the satellite passed through perigee,
as expressed in degrees. For example, the mean anomaly of a
satellite two hours into an 8-hour orbit is 90 degrees (one quarter
of the period). The total mean anomaly over the period of a day for
a satellite with mean motion n is simply n times 360 degrees.
[0111] FIG. 2 depicts the basic elliptical-orbit satellite array,
generally designated 20, of a prior art, virtual geo system
proposed by Virtual Geosatellite LLC in an application filed before
the Federal Communications Commission (FCC) in January 1999. The
entire system proposed in the FCC application employs three such
arrays of satellites, two arrays covering the northern hemisphere
and one covering the southern hemisphere, each array having five
8-hour satellites emulating many of the characteristics of geo
satellites.
[0112] In FIG. 2, virtual geo satellite 22 is shown in elliptical
orbit 24 around the earth. The communications equipment on
satellite 22 communicates with earth ground stations 26 and 28.
Virtual geo satellite 32, shown in a separate elliptical orbit 34,
is at the same time in communication with ground stations 36 and
38.
[0113] Like geo-based systems, the virtual geo satellites are
virtually continuously in the same general location or region in
the sky. Unlike geo-based systems, however, the ground
communications equipment of the virtual geo system does not always
communicate with the same satellite. For example in the illustrated
embodiment, ground stations 26 and 28 are initially in
communication with satellite 12, but are later in communication
with satellite 32 that is in elliptical orbit 34.The virtual geo
satellites move slightly relative to the earth when they are at or
near apogee. However, virtual geo satellite 22, for example, which
is shown at apogee, later moves to perigee, and still later to
other locations over other areas of the earth including, for
example, ground stations 36 and 38. The virtual geo system allows
for operation over specific geographic locations that are
preferentially covered. For example, continental landmasses can be
covered by the constellation to the exclusion of other areas, such
as the oceans between the continents. In the illustrated prior art
embodiment, for example, the United States, Europe and portions of
Asia and Russia are preferentially covered.
[0114] To preclude interference with satellites in the geo ring,
the communications equipment on the satellites of the virtual geo
system is disabled when the satellites are within a predetermined
distance 30 from the earth's sequatorial plane. As discussed in
further detail below, this provides an angular separation of at
least 40 degrees, as viewed from the earth, between geo satellites
and those of the virtual geo system.
[0115] The five satellites depicted in FIG. 2 are in orbits that
have the same values for radius of apogee, radius of perigee,
argument of perigee, inclination and mean motion, but are spaced in
RAAN and in mean anomaly such that they all follow a common ground
track. FIG. 3 shows a plot of the ground track 50 in Cartesian
coordinates superimposed on an equidistant cylindrical projection
of the earth, for the prior art. five-satellite array of FIG. 2.
(Note that the plot of the single ground track 50 actually "folds
over" from the left edge of the world map to the right edge, giving
it the appearance of multiple traces.) In the virtual geo system,
the satellites have a mean motion of three, thus making three
orbits of the earth each day. The orbits are equally spaced around
the axis of the earth, and are equally spaced in mean anomaly for
the five satellites the orbital spacing in longitude, S, is set
equal to 72 degrees. In order to have the five satellites in the
five different orbits all follow the same ground track, their
spacing in mean anomaly must be n times S, or 216 degrees. As can
be seen from FIG. 3, the satellites, having mean motion three, make
three loops around the world. In general, the number of loops in
the ground track will be the same as the mean motion. The positions
of the loops can be shifted east or west in longitude to target
different coverage areas by adjusting the RAANs of all of the
orbits of the array while maintaining their relative spacing. In
the virtual geo system depicted, the argument of perigee is 270
degrees, which makes the loops symmetrical about the apogee of the
orbits. As the apogee is in the northern hemisphere, the virtual
geo system shown favors coverage of the northern hemisphere. As can
be seen, there is one satellite 40, 44, 46 in each of the active
arcs at the top of the loops near apogee, and two inactive
satellites 42, 48 in positions between the active arcs. In this
particular case the ends of the active arcs are both at 45.1
degrees north latitude and the middle at 63.4 degrees north
latitude, which is the same as the angle of inclination. This
provides a very large separation (approximately 40 degrees) between
the active arcs and the geostationary ring. The duty cycle of each
satellite shown in FIG. 4 is 60 percent, meaning that each
satellite is active for 60 percent of time, centered around its
apogee. When an active satellite is about to leave one end point of
an active arc, one of the inactive satellites appears at the other
end point to take its place and is switched from an inactive state
to an active state.
[0116] The virtual geo system offers the opportunity to add more
satellites to each active arc and to insert a second ground track
with an equal number of satellites, between the loops of the
original ground track in each hemisphere. Each orbital position in
each of the active arcs constitutes, in effect, an orbital slot,
which in the prior art system has been dubbed a "V-slot". However,
the possible number of such virtual slots for any orbital
configuration is ultimately limited by the spacing between
satellites at apogee within each active arc, and the spacing
between satellites in the vicinity of the points where the active
arcs of adjacent ground tracks intersect. It has been determined
that the virtual geo system can accommodate a maximum of 14
satellites in each active arc while still maintaining minimal 2
degree satellite spacing. If in addition, a second ground track is
added to the southern as well as the northern hemisphere, raising
the total number of active arcs to 12, then the maximum potential
number of virtual sots is 14.times.12 or 168 virtual slots.
[0117] Because all of the active satellites in the prior art system
are moving in one direction, essentially from west to east, it is
necessary to slew each earth station antenna beam from the end of
the active arc to the beginning of the active arc when the active
satellite being tracked reaches its turn-off point. The present
invention overcomes this shortcoming while--also providing a
significant capacity improvement over the prior art system.
[0118] In contrast to other satellite systems the COBRA
constellation or basic array of the present embodiment utilizes
elliptical-orbit satellites deployed in one or more pairs of
repeating ground tracks. FIG. 4 shows a first basic 24-hour
repeating ground track 130 that is one track in a pair of repeating
ground tracks of the COBRA system. In this embodiment, the argument
of perigee falls between 180 degrees and 270 degrees, such that the
orbital ellipse is no longer aligned with the axis of the earth,
but is "leaning over" towards the equator. In the embodiment shown,
the apogee occurs in the vicinity of 40 degrees latitude, which has
the benefit of being close to high population densities at the
middle latitudes, where such a system is likely to have greatest
use. With a mean motion of three for the satellite orbits, there
are three loops in the ground track. However, because the argument
of perigee makes the orbits lean, the loops of the ground track are
not symmetrical about a meridian of longitude, as they would be if
the argument of perigee were 90 degrees or 270 degrees, but appear
in the figure to lean to the left. The active arcs 132, 134, 136,
highlighted in black, are on the left, or westward side, side of
each loop and, in contrast to the prior art system, oriented in a
predominantly north-south direction. In the left leaning ground
track shown, the satellites traverse the active arcs in a
south-to-north direction.
[0119] In the embodiment shown, the active arcs represent the
portion of each satellite orbit lying within one-quarter of an
orbital period on either side of apogee. This means that the
satellites are active 50 percent of the time and inactive during
the other 50 percent of the time, and hence, have a 50 percent duty
cycle. Satellites in these active arcs are turned on at the lower
end of the arcs, at about 20 degrees north latitude, and move in a
northerly direction until they reach the turn-off points, at about
61.5 degrees north latitude. The lower end of arcs, at about 20
degrees latitude, provides more than adequate angular separation
from geostationary satellites in the equatorial plane. Note that
the orbital parameters of the satellites forming ground track 130
have been carefully adjusted so that for each of the active arcs
132, 134, 136, the beginning and end points lie on the same
meridian of longitude.
[0120] FIG. 5 shows a second basic 24-hour repeating ground track
140 of the embodiment of present invention shown in FIG. 4, which
is a right-leaning version of the first basic ground track shown in
FIG. 4. To make the northern loops of the second ground track lean
to the right, or eastward, an argument of perigee between 270
degrees and 360 degrees is selected. In the preferred embodiment,
the arguments of perigee selected for the satellites of the left-
and right-leaning ground tracks are supplementary angles, that is,
the ground tracks lean left and right by the same amount. The three
active arcs 142, 149, 146 in FIG. 5 are highlighted in black. In
this instance, the active arcs are on the right side of the ground
track's northern loops, so that the satellites in these active arcs
are moving in a southerly direction. As in the previous case, the
orbital parameters of the satellites have been carefully tailored
so that the start and end points of each active arc lie along the
same meridian.
[0121] FIG. 6 shows the results of combining the left- and
right-leaning ground tracks 130, 140 of FIGS. 5 and 6. Again, the
active arc portions of the ground tracks are highlighted in black
for emphasis. The reason for beginning and ending the active arcs
on the same meridian now becomes apparent, as the active arcs join
to form a closed path. To allow continuous tracking of active
satellites around the closed path, it is necessary to time phase
the satellites in the system such that a northerly moving satellite
that is just ending its active arc portion in the left-leaning
ground track is met at the same location by a southerly moving
satellite that is just beginning its active arc in the
right-leaning ground track. In this way, as the master ground
control station commands the simultaneous shut-down of the former
satellite and turn-on of the latter, the antenna on a ground
station tracking the former satellite merely begins receiving
signals from the latter satellite and follows the motion of this
active satellite until the next succeeding switchover occurs. A
similar changeover occurs at the end of the active arcs closer to
the equator, where the departing right-leaning track satellite is
replaced by the arriving left-leaning track satellite.
[0122] The embodiment of the present invention shown in FIG. 6,
having satellites with a mean motion of 3, actually produces
"teardrop" patterns covering three distinct areas in the Northern
Hemisphere separated by 120 degrees of longitude, in this case the
heavily populated regions of Japan and East Asia, the North
American continent, and Eastern Europe. Of course, a similar, but
inverted set of these teardrop patterns could be placed in the
Southern Hemisphere, giving similar coverage to selected geographic
regions.
[0123] FIG. 7A depicts the orbits of a system, according to the
present invention, of six satellites, generally designated 60, that
are properly time-phased to provide each cloth-path teardrop
pattern with one active satellite always visible at high elevation
angles to a ground antenna in that surrounding geographic region
(note that FIG. 7A is an earth-centered, fixed projection, i.e.,
with the earth not rotating). The six satellites are in elliptical
orbits around the earth all having the same radius of apogee,
radius of perigee, inclination and mean motion. The orbits of the
three satellites forming the left-leaning ground track share the
same values for argument of perigee and mean anomaly, and have
RAANS that are evenly spaced at 120-degree intervals. Similarly,
the orbits of the three satellites forming the right-leaning ground
track share common values of argument of perigee and mean anomaly,
which however, are different from those of the left leaning
satellites The difference between the two groups in mean anomaly is
180 degrees, so that the left-leaning satellites are near apogee,
for example, the right leaning satellites are near perigee. The
arguments of perigee of the two groups of satellites are
supplementary angles, meaning that the sum of the two arguments of
perigee is 180 degrees. This can be seen from Table 1, below, which
lists the orbital elements for the basic six-satellite array
depicted in FIG. 7A. The orbital elements were developed by
iteration and may be further refined.
1TABLE 1 Oribital Elements Sat # a (km) e I (deg) RAAN (deg) M
(deg) 1 20261 0.6458 63.41 138.5 232 180 2 20261 0.6458 63.41 18.5
232 180 3 20261 0.6458 63.41 28.5 232 180 4 20261 0.6458 63.41
100.2 308 0 5 20261 0.6458 63.41 340.2 308 0 6 20261 0.6458 63.41
220.2 308 0
[0124] In the basic, six-satellite system depicted in FIG. 7A, the
communications system on satellite 62, for example, which is in
orbit 64, communicates with earth ground stations 72 and 74 while
it is in its active arc near apogee. Satellite 66, shown in
separate elliptical orbit 68, is in the other ground track and
inactive, but at a time half an orbital period later will be in
communication with ground stations 72 and 74.
[0125] FIG. 7B shows the same basic six-satellite array of FIG. 7A
looking up from the South Pole (note that FIG. 7B is a projection
in inertial space, i.e., with the earth rotating). This perspective
shows more clearly that while all of the elliptical orbits have the
same basic shape, three lean in one direction and three in the
other--in the preferred embodiment, as discussed above, the
satellite being turned off at the end of its active arc in one
ground track, is at the same position in space as the satellite
being turned on at the beginning of its active arc in the
complementary ground track. To avoid the possibility of a mid-space
collision, remote as it may seem, a slight deviation can be added
to either the right ascension, the argument of perigee or the mean
anomaly of the array of satellites forming one of the ground
tracks. Varying one or more orbital parameters slightly in this
manner causes the satellites to miss each other, although the
orbital paths do cross. Varying the argument of perigee slightly
causes the orbital paths to pass above and below one another, and
is preferable because it avoids the possibility of a collision
entirely. Clearly the deviation need only be very small to achieve
the desired result without affecting the ability of a ground
station antenna to "see" both satellites simultaneously, as they
appear to cross in the sky.
[0126] FIG. 8 shows in Cartesian coordinates, the basic six
satellite system of FIGS. 7A-7B, at the time that the three
satellites 152, 154, 156 in the left-leaning ground track 130 are
in the active communication mode at approximately 48 degrees north
latitude, and are traveling in a northerly direction. The other
three satellites 162, 169, 166, at about 24 degrees north latitude,
are in the inactive portion of their orbits, and in the process of
moving toward their next turn-on positions. It can be seen, by
tracing the ground tracks, that each satellite visits each of the
three closed-loop teardrop patterns once per day, remaining active
in that teardrop for about a four-hour active arc duration. This
feature allows what is termed "graceful degradation" in the event
of a satellite failure. In contrast with geostationary system,
where loss of a single satellite serving a giving geographical area
would result in a total loss of communications for the affected
area, a single satellite failure in the present system would result
in only a four hour outage in each coverage area during each
24-hour day. During each day, the outage would rotate among the
three teardrop patterns, and not affect all of the coverage areas
at the same time.
[0127] FIG. 9 shows a similar picture as FIG. 8 at a later time,
when the southerly moving group of satellites 162, 164, 166 has
become active. In this view-, the three inactive satellites 152,
154, 156 are actually in the Southern Hemisphere at 32 degrees
south latitude, and closer to perigee than were the inactive
satellites in the previous figure.
[0128] FIG. 10 shows a satellite constellation for Northern
Hemisphere coverage, according to the present invention, that has
four active satellites in each teardrop pattern, or a total of 12
satellites. At the instant shown, the six satellites at about 42
degrees north latitude are at apogee. At each end of each teardrop
pattern, there are actually two satellites in or near the same
position. One has just been turned on, and the other has just been
turned off. In addition, there are six satellites at about 47
degrees south latitude that are at their perigee points. Note that
this perigee point is not at 63.4 degrees south because the
argument of perigee is not equal to 270 degrees, the same reason
that the apogee is not at 63.4 degrees north.
[0129] Although FIG. 10 shows a system with four active satellites
per tear drop pattern, a larger number of the above-described
basic, six-satellite groups can be placed in the single pair of
left- and right-leaning ground tracks. In the teardrop pattern, the
critical region in terms of inter-satellite spacing, appears to be
at the cusp, the point at which the active arcs meet closest to the
equator. It has been found, by iteration, that up to 12 active
satellites can be supported in the northerly-moving satellite path,
plus a corresponding 12 active satellites in the southerly-moving
path, for a total of 24 active satellites per teardrop pattern.
This maximum capacity per teardrop is achieved on the basis that
all the satellites in each ground track, both active and inactive,
are equally spaced in mean anomaly.
[0130] The location of the closed loop teardrop patterns may be
shifted at will in longitude by incrementing the RAANs of the
satellites that form the paired ground tracks. Thus, selected
market areas may be addressed, recognizing that this will be a
global system, and that each system will cover three similar areas
spaced 120 degrees apart in longitude. Although the latitudinal
coverage pattern may not be shifted, the system inherently covers
the mid- and higher latitudes much more effectively than a geo
system, as the satellites are more directly overhead for these
latitudes.
[0131] Communications system capacity can be multiplied for the
present invention by creating additional ground track pairs
displaced in longitude around the earth. However, spacing between
teardrop patterns must be maintained so that one teardrop pattern
does not approach closer than two degrees to its nearest adjacent
neighbor. It has been found that for the preferred embodiment, up
to 12 "teardrops" can be placed in the Northern Hemisphere without
any mutual interference, and a like number of 12 can be placed in
an inverted manner in the Southern Hemisphere. Thus, in all a total
of 24.times.24, or 576 active satellites could be placed in such a
system. If this number of active positions, or slots, is compared
to the 180 possible slots in the geo ring, the present invention
represents a potential global communication satellite capacity that
is greater by a factor of 3.2 (576/180).
[0132] FIG. 11 is a plot in Cartesian coordinates showing the 12
teardrop closed paths 180 in the Northern Hemisphere, and the 12
teardrop patterns 182 in the Southern Hemisphere, discussed
above.
[0133] FIG. 12A illustrates the same teardrop patterns in inertial
space from a perspective slightly above the earth's sequatorial
plane. This view shows clearly the angular separation between the
geo ring 189 and the teardrop patterns 180, 182.
[0134] FIG. 12B illustrates the teardrop patterns in inertial space
from the perspective of an observer above the earth's North Pole.
Because the perspective is from a distance of 130,000 miles above
the north pole, the teardrops in the Southern Hemisphere, which are
further away, appear smaller, although in reality are the same size
as the Northern Hemisphere teardrops. This view also shows more
clearly that the orbital altitudes of the satellites according to
the present invention are well within the altitude necessary for
geostationary orbit.
[0135] FIG. 13 illustrates the 12 teardrop closed paths that can be
accommodated in the Northern Hemisphere, with 24 active satellites
170 shown occupying one of the teardrop patterns. The other
patterns each have one active satellite shown.
[0136] It should be noted that all elliptical orbits, including
those described herein are subject to effects of long-term
perturbations, which if not compensated, cause the desired
satellite coverage to drift off with the passage of time. These
perturbation effects result from the earth's J2 rotation harmonic,
which reflects the fact that the earth is not a perfect sphere, but
actually bulges at the equator. The two principal effects are
regression of the line of nodes for posigrade orbits (I>90
degrees), and rotation of the line of apsides. For inclinations
greater than critical (I between 63.4 degrees and 116.6 degrees)
the line between the perigee and the apogee for each satellite (the
line of apsides) will regress; for other inclinations (I<63.4
degrees or >116.6 degrees) the line of apsides will progress. At
exactly the critical angles of 63.4 degrees or 116.6 degrees, the
line of apsides will remain stable, a very desirable effect which
is used to advantage in the preferred embodiment for maintaining
apogee at a selected latitude. For inclined elliptical orbits there
will be a regression of the line of nodes that must be compensated
by a small adjustment in orbital period. All the satellites in a
given array design are affected similarly. The effect is to cause
the plane of the orbit to rotate clockwise as seen looking down on
the North Pole. If that happens, the satellite would pass over a
selected meridian at a slightly earlier time each day. Fortunately
this effect can be compensated by slightly decreasing the period of
each satellite in the array to effectively stretch out the
trajectory ground track and cause the ground track to repeat
exactly over the life of the satellite.
[0137] As will be apparent to a person of skill in the art, the
system of the present invention has applicability to a broad
variety of satellite communications services, including telephone,
broadband data, television distribution, direct broadcasting and
mobile communications, as well as to non-communications services,
such as meteorology and earth resources monitoring. FIGS. 14A-14B
provide, by way of example, block diagrams of the satellite and
ground stations that can be used for television distribution and
data transmission services in accordance with the present
invention. The block diagrams show elements that can be used, for
example, to carry out communication between the ground station 62,
satellite 42, and ground station 64 of FIG. 8A.
[0138] Referring to FIG. 14B, video information to be distributed
is received as video input 200, and input to a video codes 202
which processes digital coded video information. This digital coded
video is multiplexed with a number of other channels of video
information by video multiplexes 204. The resultant multiplexed
video 206 is modulated and appropriately coded by element 208 and
then up converted by transmitter element 210. The up-converted
signal is transmitted by antenna 212 via link "A". Antenna 212 is
pointed at satellite 42 and is controlled by pointing servos
213.
[0139] Referring now to FIG. 14A, the transmission from antenna
21.2 is received by phased array antenna 214 of satellite 42. The
received signal is detected by one of the receivers 216, from which
it is input to multiplexes 218. The output of multiplexes 218 feeds
the transponders 250, where the received signal may be translated
in frequency, switched among a number of transponder channels, or
otherwise processed in some way, either at RF, IF or baseband. From
the transponders, the signal typically goes through power amplifier
252 and multiplexer 254 to feed beam former 256. Beam former 256
drives a transmit, steerable phased array antenna 260 which
transmits the signal in a current geo frequency band to steerable
antenna 262 in the remote user terminal 64, of FIG. 14B. This
signal preferably uses the same frequency that utilized by current
geo satellites for such services. The phased array antenna 260 is
steered by an on-board computer that follows a preset and repeating
path, or from the ground. At user terminal 64 in FIG. 14B, the
signal on link "D" is received by receiver 264 through steerable
antenna 262, demodulated at 266, demultiplexed at 267 and decoded
at 268 to produce the video output 270.
[0140] In the alternative, user terminal 64 may include an antenna
22_2, and transmitter and receiver 218 capable of two-way
transmission of voice and data over links "B" and "C".
Correspondingly, central ground station 62 would include a receiver
and down-converter 214, and equipment to support two-way voice and
data. Typical data applications include multi-media and Internet
services.
[0141] In addition to the communications functions, FIG. 14A
depicts an on-board processor 280, which determines the orientation
of the satellite and steering of the satellite antennas from
various parameters. Power supply 290 supplies and regulates
electrical power for all the various satellite subsystems and
components that require such power. Power supply 290 includes a
source of power, here shown as solar array 292, and an energy
storage element, here showed as a battery array 294. Importantly,
according to the present invention, the solar array 292 is sized to
provide an amount of power that is less than that required to fully
power the satellite communications functions of the satellite, the
fraction being referred to herein as the power ratio of the
satellite. The power ratio depends on the kind of orbit that the
satellites will have, and how long the satellites will be
transmitting during the elliptical orbit. The preferred embodiment
of the present invention has a nominal power ratio of 0.5, to power
a satellite that is communicating half of the time. (Some small
amount of power must be generated to maintain housekeeping
functions even when the communications equipment on a satellite is
disabled.) The other half of the time, the transmitters and
receivers on-board the satellite are disabled, allowing solar array
292 to provide power to charge battery 294.
[0142] FIG. 15 depicts this power consumption methodology of a
satellite of the present invention in a flowchart format that is
generally designated 300. Step 302 represents receiving satellite
orbit data from one or more ground station antennas tracking the
satellites. Step 304 represents calculating at any given time the
position of each of the satellites in orbit from the received
orbital data. This requires that a processor at the master ground
control station, mentioned earlier, record and process the orbital
data as it is being received. At step 306 the processor determines
whether each of the satellites is, or is not, within its respective
active arc. If at step 308, it is determined that a satellite has
just left its active arc, a command is sent at step 310 to disable
the on-board satellite communications equipment. In that case, the
satellite power supply 290 is also commanded at step 312 to use the
power generated by the solar array 292 to charge the battery array
294. If at step 314 it is determined that a satellite has just
entered its active arc, a command is sent at step 316 to enable the
satellite's son-board communications equipment, and at step 318 the
necessary power is drawn from the power supply and the battery. An
independent on-board means, such as a programmed timer, may be
included in the satellite on-board processor to ensure that the
satellite's communications equipment is disabled when it is not in
an active arc, and thereby avoid any possibility of interference
with satellites in the geo ring.
[0143] The present invention reduces the amount of signal
attenuation associated with communication between the
aforementioned satellites and a plurality of ground stations by
adopting various frequency bands. One frequency band that has been
utilized by satellites in the elliptical orbits described herein to
transmit and receive signals has been the Ku band, the frequency
range between 10.9 to 17 GHz.
[0144] The height elevation angles typical the COBRA and COBRA
system facilitate the use of higher frequencies for two-way
satellite communications. This is because the satellites are
looking almost straight down at the earth, resulting in minimum
signal attenuation. The attenuation due to rain, or severe weather
is roughly proportional to the cosecant of the elevation angle;
thus satellites at 90.degree. elevation angles would have much less
attenuation than satellites seen from the ground at a 30.degree.
elevation angle.
[0145] The present invention overcomes limitations in the prior art
satellite communications systems resulting from signal attenuation.
Satellite communications are most affected by the moisture content
in the atmosphere, especially present in cloud formations.
Satellites utilizing communications in the C Band frequencies have
little or no attenuation related to moisture content. However,
these systems, in addition to only being available on certain
satellite systems, possess the disadvantages associated with low
frequency communications, especially the amount of and size of the
equipment required for effective ground-satellite communications.
Satellites utilizing communications in the Ku Band frequencies and
subject to high degradation, and thus high signal attenuation, in
conditions with high atmospheric moisture content. This attenuation
makes effective communication more difficult, for example, in
tropical areas or other areas of high atmospheric moisture
content.
[0146] Prior satellite communication frequencies have operated at
frequency bands lower than the Ku band due to the signal
attenuation present at normal operating positions for satellites.
Signal attenuation for any given satellite communication system
depends on at least two factors: 1) the general atmospheric
condition of any given location; and 2) the atmospheric path length
at which a signal must travel between satellite and
reception/transmission station.
[0147] The present invention utilizes calculations for cloud signal
attenuation first studied in the prior art by Chu and Hogg, and
discussed in U.S. application Ser. No. 09/986,057 hereby
incorporated by reference. In these studies, Chu and Hogg derived
attenuation values for optical and infrared communications (in the
10.6 micron wavelength). Their studies showed that infrared signals
propagated better through fog than optical signals. FIG. 16 is a
chart derived from the data of Chu and Hogg, showing signal loss
(in dB) through 1 km of fog with 0.1 gm/m.sup.3liquid water density
through various frequencies. As shown in FIG. 16, where 10 microns
correlates to 30 THz frequency and 1 micron correlates to 300 THz
frequency, for fog conditions the attenuation results in
approximately 60 dB signal loss at 10 microns, and 200 dB signal
loss at 1 micron wavelengths.
[0148] The Chu and Hogg studies were for terrestrial
communications, not satellite communications. It is impossible to
derive cloud moisture content values (and thereby calculate signal
attenuation) for satellite communications, as opposed to ground fog
or rain. Because the total water content for clouds was not known
from these studies, it was presumed that the high water content
present in clouds would make neither optical (near 1 micron
wavelength) nor infrared (near 10 micron wavelength) communications
possible. As a result, satellite communications systems remained in
the mid level to high frequency bands.
[0149] An embodiment of the present invention utilizes formulations
for cloud water content. This formulation is derived from the prior
art of Chu and Hogg, described above, and prior art investigated by
F. Barbaliscia et al. This latter prior art calculates signal
attenuation values for very small aperture satellite systems with
an assumption of 99% "non-rainy" atmospheric conditions, which is
the condition just prior to the commencement of rain. In
particular, zenith attenuation maps for selected portions of Europe
and other regions, with 99% "non-rainy" conditions, exist in the
prior art, demonstrated by F. Barbaliscia, M. Boumis, and A
Martellucci, "World Wide Maps of Non Rainy Attenuation for Low
Margin Satcom Systems Operating in the SHF/EHF Bands," Ka Band
Conference, September 1998. FIG. 17 shows the graph produced by
Barbaliscia et al. showing attenuation at the frequency 22.2 GHz,
while FIG. 18 shows the graph produced by Barbaliscia et al,
showing attenuation at the frequency 49.5 GHz. The results shown in
FIGS. 17 and 18 were for satellites assumed to be at the "zenith"
position, i.e. directly overhead any given point with an elevation
angle of 90 degrees.
[0150] These attenuation graphs of the prior art do not present a
meaningful manner in which to relate signal attenuation to clouds,
apart from any atmospheric moisture effects. The representation of
attenuation shown in FIG. 17, at 22.2 GHz, largely relates to water
vapor absorption as the primary atmospheric condition. The
representation of attenuation shown in FIG. 18, at 49.5 GHz,
relates to oxygen, cloud, and water vapor absorption as the
atmospheric conditions. These maps (shown in FIGS. 17 and 18) can
be approximately solved for water vapor and cloud attenuation.
After the effects of clouds and water vapor and separated, zenith
attenuation was estimated over a wide range of frequencies with
integrated gaseous attenuation models. Calculation of zenith
attenuation functions was derived by the present inventor using
Liebe's swater vapor relations. Thus, from these two attenuation
graphs, general attenuation function was derived for a wide range
of frequencies (again, with 99% "non-rainy" conditions); this
function and results for the range up to 100 GHz were derived and
described in P. Christopher, "World Wide Millimeter Wave
Attenuation Functions from Barbaliscia's 49/22 GHz Observations,"
Ka Band Conference, Toarmina Sicily, October 1999, attached hereto
as an Appendix. In this work an equation derived from the work in
the prior art which calculates such attenuation (in dB) is shown in
FIG. 19, where fog is frequency, in GHz, and assuming "non-rainy"
zenith attenuation.
[0151] To derive the total water content of the clouds themselves,
the present invention applies the formula in FIG. 19 to the earlier
prior art work to determine the net effects of cloud attenuation at
22.2 GHz. Thus, the values shown in FIGS. 17 and 18 can be
manipulated to separate the attenuation effects of the clouds
themselves from the water vapor present in those clouds. As shown
in FIG. 20, the affect of attenuation for clouds themselves (which
results in a 1% "non-rainy" condition) at 22.2 GHz can be
represented.
[0152] The present invention utilizes a satellite system that can
combine the works in the prior art to derive the water content of
clouds at 99% "non-rainy" conditions. As shown in FIG. 21, the
total water content (in gm/m.sup.2) of clouds can be shown for
variable locations on the Earth, again assuming a zenith position
(i.e., from a ground antenna pointed at zenith, or 90 degrees
elevation angle). These values can be calculated for 10 micron
wavelengths--combining the works of Chu and Hogg with Barbaliscia
et al. results in FIG. 21, which shows 10 micron wavelength cloud
attenuation at 99% "non-rainy" conditions.
[0153] The present invention utilizes a method to calculate cloud
cover attenuation derived from the above-indicated prior art for
communication frequencies. This method calculates the cloud water
content (in g/m.sup.2) for a zenith communications link according
to the following (where PR=exceedance probability, and LON and LAT
are earth locations in longitude and latitude (degrees)
respectively), and is shown in FIG. 22.
[0154] The present invention utilizes these calculations to
determine signal attenuation. The prior art of Barbaliscia et al.
determined attenuation for various conditions at certain
frequencies in the GHz range; from that prior art (and utilizing a
general attenuation function), cloud attenuation can be determined
(as opposed to attenuation from other atmospheric conditions);
applying the formula for cloud water content determines such values
at certain conditions (informed from the prior art of Chu and Hogg
with Barbaliscia et al.); and applying the values for cloud
attenuation (in dB) with the values for water content (in
g/m.sup.2), with the work of Chu and Hogg, yields cloud
attenuation. As shown in FIG. 23, the cloud attenuation for the 10
micron wavelength region has been calculated: the water content of
the clouds (in gm/m.sub.2) at a location is multiplied using the 10
micron values for "fog" in FIG. 1 (derived from work in the prior
art, where fog is assumed to be equivalent to cloud cover) (where
the values are for attenuation for atmospheric conditions in
g/m.sup.3 per km of pathlength). In this manner the attenuation can
be determined according to the present invention.
[0155] From this equation it is possible to derive cloud water
content at exceedance probabilities greater than the 1% value (99%
"non-rainy") utilized in the prior art. For example, at 10%
exceedance (90% "non-rainy"), the cloud water content for various
locations are less severe than for the previously derived 1%
exceedance value, as shown in FIG. 24. Attenuation for a 10 micron
infrared communication with 10% exceedance is shown in FIG. 25.
[0156] FIG. 26 graphs attenuation at 10 micron wavelength
communications, with 10% exceedance, for various Earth locations at
six selected attenuation levels (latitude is charted on the y axis,
longitude on the x axis, with the contours showing attenuation at
10, 20, 30, 40, 100, and 110 dB). FIG. 27 graphs similar
attenuation contours at 1 micron wavelength communications.
[0157] From this equation, it is also possible to derive cloud
water content at 20% exceedance (80% "non-rainy"). As shown in FIG.
28, the cloud water content for various locations are less severe
than for the previously derived 1% and 10% exceedance value.
Similarly, attenuation for a 10 micron infrared communication with
20% exceedance is shown in FIG. 29.
[0158] FIG. 30 graphs attenuation at 10 micron wavelength
communications, with 20% exceedance, for various Earth locations at
six selected attenuation levels (latitude is charted on the y axis,
longitude on the x axis, with the contours showing attenuation at
10, 20, 30, 40, 100, and 110 dB). FIG. 31 graphs similar
attenuation contours at 1 micron wavelength communications.
[0159] Elliptical orbits of the COBRA system can be characterized
in terms of an elevation probability density function as shown in
FIG. 32. The probability density function shown is calculated from
satellite elevations over time at selected locations on the earth's
surface. The elevation probability density function can be used to
derive average cosecant of elevation for orbiting satellites of the
present invention at various latitudes. Applying the cosecant of
elevation for each latitude for the satellite system of the present
invention yields the average cosecant of elevation shown in FIG.
33. The result of FIG. 33 is used to multiply the zenith
attenuation (dB), at each latitude, to yield the desired COBRA
attenuation (dB). Of course, it is possible to utilize many
different elliptical orbits to produce minimum elevation angles,
and thenceforth derive probability density functions and a
resultant attenuation figures. Moreover, it is possible to derive
attenuation values for a range of frequencies.
[0160] It is possible, utilizing the work of Barbaliscia et al. and
Christopher (referenced above) to compute average attenuation
figures at specific latitudes for both a geosynchronous satellite
system and an elliptical orbit satellite system. These results, at
30 GHz, utilizing an elliptical orbit of the COBRA system, are
shown in FIG. 34. FIG. 34 indicates an improvement over
geosynchronous satellite systems if one uses a COBRA elliptical
orbit satellite system and higher frequency communication links
than those used for geosynchronous satellite systems. For example,
utilizing frequencies above the Ku band results in attenuation
values shown in FIG. 35. FIG. 35 indicates that it is preferable,
in a satellite system of the preferred embodiment, to operate
communication links at frequencies higher than the Ku band.
[0161] In the present invention, the advantages of the COBRA system
start to appear at frequencies higher than 30 Ghz. For example, as
shown in FIG. 36, the zenith attenuation at 45 GHz is clearly
higher than the attenuation at 30 GHz, shown in FIG. 17. A closer
view of the 45 GHz zenith attenuation in the Northern Hemisphere is
shown in FIG. 37. For the Northern Hemisphere of the Earth, the 45
GHz zenith attenuation shown in FIG. 37 compares favorably with the
COBRA attenuation of the same geographical area, as shown in FIG.
38. Comparison with a traditional geosynchronous orbit satellite
communications yields more unexpected results, as shown in FIG. 39,
where the average attenuation for a geosynchronous satellite is
seen as increasing in a manner so as to make communications not
feasible above 45 GHz. However, even though the COBRA orbits
produce lesser attenuation than geosynchronous orbits, the
resultant derived attenuation is much higher than those for
geosynchronous orbits. A satellite system with a COBRA orbit is
able to take advantage of the 45 GHz region because of its low
attenuation. Still higher frequency bands show even more favorable
attenuation values than would normally be available in traditional
satellite communications systems, as shown in FIGS. 40-43 (for 75
GHz), and as shown in FIGS. 44-47 (for 90 GHz).
[0162] Finally, at a constant antenna aperture for satellite
systems, it is possible to calculate a `net loss` of attenuation by
taking the attenuation and subtracting any signal gain for constant
antenna aperture over a range of frequencies. These calculations
yield unexpected results, and the entire Earth can be examined for
minimum `net loss` frequencies, with optimal frequencies derived by
showing what frequency is desired for communication at any given
longitude and latitude. When examining satellite systems for Net
Loss, the possibilities of higher frequency communications exists.
For example, as shown in FIG. 48, Net Loss at New York has local
minima at 45 and 82 GHz. Either frequency would be possible for
viable satellite communications, although communications at 82 GHz
has a lower Net Loss than communications at 45 GHz. It is possible
to calculate useable frequency choices over the entire Northern
Hemisphere. FIG. 49 shows the trend of these optimum frequencies
for a satellite system utilizing COBRA orbits. These optimum
frequencies, chosen to minimize net loss at constant aperture,
might be thought of as frequencies chosen to maximize cost
effectiveness. Note that any constant aperture may be chosen for
this optimization, whether 2 meters or 10 cm, for the location of
the minimum frequency does not change regardless of aperture.
[0163] As noted, operating the satellites only in the region of
apogee prevents interference with satellites in the geostationary
ring. In the present invention, the active orbital arcs are well
away from the equator because coverage has been optimized to place
satellite apogee, where the satellites spend most of their time,
over high traffic density-areas in the Northern and Southern
Hemispheres. The present invention will allow existing geo
satellite frequency allocations to be reused many more times and
help to reduce the intense worldwide pressure on scarce spectrum
resources.
[0164] In addition to avoiding possible interference with the
geostationary ring, the present invention provides high elevation
angles to the satellites while in their active arcs. Maximizing
elevation angle materially reduces the atmospheric effects,
blockage and multi-path that often adversely affect communication
with geo satellites. These advantages are particularly attractive
for satellite communications in higher frequency bands (e.g.,
20-100 GHz) where atmospheric attenuation becomes a significant
impairment.
[0165] Although the satellite system according to the present
invention performs in many of its aspects like a geostationary
satellite system, the satellites in the system orbit at a
significantly lower altitude. A geostationary satellite orbits at
36,000-kilometer altitude, while the 8-hour satellites of the
present invention, for example operate at an altitude between
approximately 21,000 and 26,000 kilometers in their active arcs.
Because the path loss of the communications link to satellites in
these elliptical orbits is significantly less than the path loss to
geostationary orbit, both the power and antenna size of the
communications package on the satellites can be reduced
accordingly.
[0166] A lower orbital altitude also yields benefits in terms of
the cost of launching the satellites. Unlike geo satellites,
satellites in elliptical orbits do not require apogee motors to
boost them into final orbit. This factor alone reduces by
approximately half the launch vehicle lift requirement per
satellite. In addition, the reductions in size and weight of the
satellite power and communications systems, mentioned above, all
add to the benefits of present invention from viewpoint of launch
costs.
[0167] It should be noted that, unlike satellites in the
geostationary ring, satellites according to the preferred
embodiment must be added in increments of six, filling three
teardrop patterns equally spaced around the world. However, for the
reasons earlier discussed, the cost of constructing and launching
the six satellites in a basic array should compare very favorably
with that of three geo satellites providing equivalent global
services.
[0168] Use of the present invention significantly simplifies the
tracking of non-geostationary satellites. Ground station antennas
follow what appears to be a single active satellite in a roughly
circular closed path in the sky overhead without having to break
lock and slew to a new position when satellite changeovers occur,
which makes the present invention a more attractive alternative to
geo satellites. In addition, the present invention offers an
increase in available world communication capacity for a variety of
applications, does not interfere with satellites in the existing
geostationary ring, provides a global system of communications
satellites with a higher average elevation angle and lower
transmission delay than geo systems, and offers lower overall
construction and launch costs than comparable geo satellite
systems.
[0169] While this invention has been described in reference to
illustrative embodiments, the description is not intended to be
construed in a limiting sense. Various modifications and
combinations of the illustrative embodiments, as will as other
embodiments of the invention, will be apparent to persons skilled
in the art upon reference to the description. It is therefore
intended that the appended claims encompass any of such
modifications or embodiments.
* * * * *