Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02957065 2017-02-06
TITLE
Synthetically recreating the geostationary satellite orbital arc array
with preferred elliptical orbit parameters
TECHNICAL FIELD
This invention discloses the use of elliptical orbits for satellite
telecommunication and
broadcasting, and ways to maximize efficiency of associated communication
networks.
CA 02957065 2017-02-06
DESCRIPTION
Two new quasi-geostationary satellite orbit operating regions are invented and
described herein. The geosynchronous & geostationary orbit (GSO) arc that is
currently
utilized by communications satellites ¨ commonly referred to as the Clarke
Belt after its
inventor Sir Arthur C. Clarke ¨ is recreated in alternate ideal locations in
the sky by
utilizing enhanced inclined elliptical satellite orbits to create a plurality
of fixed
repeatable ground tracks with apogee peaks in the northern and southern
hemispheres
spaced optimally to address current overcrowding and future communication
supply
shortfalls, and to incrementally furnish supplemental in-orbit satellite
capacity at local
daypart periods when fluctuations in demand peaks.
This invention specifies a system and methods to incrementally add additional
quantum
of transmission data throughput using preferred angular separation orbital
spacing
parameters; preferred orbit parameters, and the use additional overlapping
ground
tracks to achieve higher data throughput to enable a constellation array of
many
satellites that are utilized to create essentially two additional Clarke Belt
GSO arcs
(dubbed 'Clarke Belt 2.0'); enabling more than double the total amount of
usable
spectrum per degree of longitude than previously disclosed methods. The
resulting
number of satellites able to be operated coincidentally in the new Clarke Belt
2.0 orbit
space significantly surpasses the current total quantum, effective
transmission capacity
and throughput arising from the inherent 180 satellites per spectrum band
limitation of
the Clarke Belt GSO arc. An arrayed constellation of satellites employing
multiple
spectrum bands can utilize the aforementioned preferred embodiments whilst
maintaining the sufficient angular separation necessary for discrimination of
efficient
satellite communication links. This invention also describes system and
methods to
incrementally supplement transmission data throughput to regions during peak
usage
periods, for example internet usage peaks, in dayparts to meet demand peaks.
The Clarke Belt geostationary satellite orbital arc is overcrowded and
inefficient.
Geostationary satellite communications systems have been operating
commercially for
more than 4 decades. The original concept of geosynchronous repeaters in
space, or
geostationary satellites, was invented by Sir Arthur C. Clarke in 1945. The
orbital arc
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used for geosynchronous operation of geostationary communication and
broadcasting
satellites is commonly known as the "Clarke Belt". Orbital spectrum is a
scarce and
valuable resource which is closely governed by the countries operating
satellites and by
International Telecommunications Union (ITU) treaties and regulations.
However,
certain technical regulatory decisions were made long before newer
technologies and
widespread, critical and constant usage of communications was envisioned.
Satellite communications began almost 50 years ago by utilizing large
parabolic
communication antennas, known as 'dishes' for their shape, which were
expensive due
to their novelty, and their sheer parabolic dish size and resulting support
frames and
foundations. Large antennas provide higher gain because of more signal
directivity.
More antenna directivity allowed satellites to be placed in the more closely
together in
the Clarke Belt; to agreed limits where discrimination between operating
satellites could
be achieved with large dishes. That spacing, or orbital slot separation,
generally agreed
by the ITU's spacefaring nations was some 2 degrees of orbit separation.
Data usage per user has been doubling every 12-18 months. If satellites wish
to
reutilize and share the same, popular, spectrum bands, then a maximum of 180
orbital
slots are derived from 2 degree separation (360 degrees around the equator
divided by
two). Historically, orbital slots over the most populated and prolific regions
(those with
highest Gross Domestic Product or 'GDP') developed quickly, to where the best
orbital
positions have now been occupied for many decades. Growth of the satellite
communications industry forced additional spectrum band(s) to be utilized;
which
followed the same patterns of deployment and constriction over the highest GDP
regions and continents, to the point where they too became overcrowded and
limited
growth.
Direct-To-Home and Direct Broadcast Satellite (DTH & DBS) TV distributors that
sought
to utilize smaller (cheaper) dishes for their service and successfully
petitioned the ITU to
enforce larger orbital slot separation; eventually settling on 9-10 degrees of
orbital slot
separation as sufficient to maximize reliable and efficient distribution of
large amounts of
data broadcasted to dish sizes of 45 to 70 cm.
Today's Clarke Belt is overcrowded and inefficient due to legacy networks,
satellites
may provide service for more than 20 years before retiring, and while newer
and more
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efficient technologies have been developed, it is difficult to maintain
service to legacy
customers during expensive upgrades and concurrent multi-generational service.
Non-geostationary satellite communication orbits allow growth.
Several non-geostationary satellite orbits have been invented to enable
satellite
communication. These include highly elliptical orbits (HEO) such as Molniya
and
Tundra; medium-earth and low-earth orbits (ME0 or LEO) that may utilize polar
or
equatorial orbits, as well as others.
An inclined elliptical orbit may be created that mimics the geostationary
orbit, and this
style of orbit is utilized in this invention and the improvements herein which
disclose;
preferred elliptical orbit parameters, preferred embodiments to achieve more
data
throughput and efficient use of spectrum, and methods to incrementally add
communications capacity timed to meet increased demand periods, as well as
other
preferred embodiments.
Nine (9) degree orbit spacing provides technical and economic benefits versus
two (2) degree orbit spacing.
Orbital spacing of satellites is utilized to allow a valuable resource in
spaced to be
utilized by different entities. In order to balance the efficient use of space
with
maximizing the performance several advancements were discovered over the past
few
decades, which show several benefits over early satellite implementations. One
of
these was orbital spacing versus antenna size tradeoffs. Use of "two-degree
spacing"
in satellites is considerable less efficient that "nine-degree spacing". As
briefly
discussed above, 2 degree spacing requires larger antennas (typically 2 metres
or
larger) for sufficient sensitivity and discrimination of adjacent satellites,
in order to
enable sharing of spectrum; which in-turn may increase remote user site costs
and
thereby reduce customer uptake, especially when compared to smaller antenna
sizes of
less than 1 metre.
Orbital slot spacing as commonly utilized and approved for DTH/DBS satellites
(9-10
degrees) allows adjacent satellites to operate at higher transmit powers
without
interference to each other ¨ as well as enabling utilization of smaller/lower
cost user
antenna diameters of 45 to 70 cm ¨ the combination of which provides higher
efficiency
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from increased network throughput and overall economic benefits to consumers
and
DTH/DBS operators.
This invention describes improved parameters and preferred embodiments having
technical and economic benefits. One of the benefits of this invention is the
overall
number of satellites allowed in the new orbit, each satellite further
benefitting from better
technical and economic parameters from increased spacing or separation.
Use of overlapping Ground Tracks surpasses the number of 'equivalent slots'
available in the GS0 Clarke Belt (480 vs 180).
This invention's preferred embodiment teaches a method to create sixteen
overlapping
ground tracks, eight overlapping ground tracks interleaved in each of the
northern and
southern hemisphere, where each ground track may produce 30 'equivalent slots'
in
each orbit (16 x 30 = 480 slots). This preferred embodiment provides a number
of other
benefits and improved characteristics, including; increased overall data
throughput, less
costly user site antennae, less obtrusive and less costly user installations
(wall or roof
mounting versus concrete foundation), more pleasing aesthetics, and increased
sales.
With this invention the GS0 Clarke Belt's current 180 x 2 degree spacing
orbital slots
may be effectively synthetically recreated in two alternate regions of the sky
each
serving the northern and southern hemispheres with supplemental satellite
capacity.
This invention teaches a method to develop 240 orbital slots in each
hemisphere (480
worldwide), each slot having superior performance characteristics (-9 degree
versus 2
degree spacing, improved path loss, less latency, etc.) when compared to GSO
Clarke
Belt parameters. Methods to minimize interference from conjunctions or
crossovers of
the repeating ground tracks are possible by choosing when satellites are
placed into
their orbits, thus phasing when satellites are active may be considered as
well as other
means to reduce overlap times or signal path interference from sharing common
spectrum bands.
Threefold increase in data throughput accrues from this invention.
This invention provides more than twice the number of slots and can typically
provide
threefold increases in overall user data throughput accrues from having more
satellites
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(more than double), higher powered satellites, less overall adjacent satellite
interference, less noise floor, improved user elevation angles from satellites
operating
more directly overhead (resulting in less signal attenuation and less line-of-
sight view to
satellite obstructions), more available satellites in view, more users sharing
transmission
capacity (user distribution contention smoothing), amongst other technical and
economic benefits.
Preferred Elliptical Orbit Parameters have a number of other benefits.
The scarce number of unused available GSO orbit slots, along with a resulting
increased and renewed interest in non-geostationary orbit (NGSO) orbits by low-
earth-
orbiting (LEO) systems, means the use of space for satellite communications is
increasingly popular. Current ITU global regulatory submissions for LEO
satellites far
exceed 20,000 satellites; the U.S. FCC alone has received applications that
exceed
10,000 LEO satellites. LEO NGSO communication satellites commonly operate at
constant altitude heights between 800 and 1,400 km. Inclined elliptical
satellite orbits
(HEO orbits) that traverse through the 800-1,400 km altitudes to their perigee
have
potential for collisions with the more than 30,000 planned LEO NGSO
satellites; or
otherwise may force HEO operators to manoeuvre satellites with rockets to
avoid
collisions, or affix protective shielding. Adding rockets or shielding
capabilities to
inclined elliptical orbit satellites increases their mass and expense to
launch.
Space debris from earlier LEO satellite collisions and anti-satellite missile
tests have
created over 10,000 pieces of cataloged space debris that NASA tracks in the
800 km
to 1,000 km orbit height, as well as an estimated 1 million additional
particles too small
to track at ever-spreading orbit heights, but still deadly to satellites.
Orbits which have
perigee heights above 1,500 km avoid the regions of space where collisions are
likely.
Higher perigee is achieved by reducing the orbit eccentricity. Elevating
perigee height
also has a benefit of less orbital drag and therefore extends satellite
lifetimes or reduces
mass requirements from less fuel, and subsequently lowers satellite launch
costs if less
mass. Furthemore, elliptical orbits with less eccentricity (essentially
flatter) provide
additional benefits, as reducing the apogee height provides improved technical
parameters accrued from lower height/shorter path to satellites; specifically
reducing
apogee heights generates less path loss attenuation and less round-trip
latency.
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Another benefit from this invention specifies preferred elliptical orbit
height parameters
of 1,525 km; although other perigee height values above 1,500 km are possible
and still
preferred.
Less eccentricity is another specified preferred embodiment; as an increased
perigee
altitude above 1,500 km height reduces potential collisions and reduces drag
(reduced
drag generates longer satellite lifetimes and less launch weight/mass); and
lower
apogee delivers improved technical performance of less latency and less path
loss.
Ability to add incremental capacity to meet demand peaks is another benefit.
Internet usage peak demands may occur in response to the number of users at
peak
times, or from the nature of the traffic being communicated, or a combination
of the two.
Consumption of video related traffic may constitute half of the typical
traffic of internet
data distributed, the majority of which are consumed during evenings. Internet
traffic
loads may vary considerably throughout the day; and at peak times may be
triple or
quadruple off-peak times. Additional capacity may also be sought for other
business
purposes which may include but are not limited to; off-peak data store-and-
forward,
business users seeking video connectivity or broadcasting, mobile cellphone or
data
consumption, live sporting events, point-of-sale and banking transactions; or
consumer
social media and web browsing ¨ amongst others.
Another benefit of this invention is the ability to incrementally add
satellite distribution
capacity specifically timed at a repeatable time of day around the world to
meet user
demand peak(s). This invention teaches a system and method to provide
incremental
satellite capacity timed to supplement capacity at a repeatable portion of
day; known as
a `daypart. By utilizing any satellite orbit having a repeatable ground track
that occurs
in the same portion of the sky overhead each day, incremental capacity can be
provided
to be in the active arc area for transmission at specific dayparts by
utilizing several
means that are disclosed herein.
A preferred embodiment to this invention is the ability to add incremental
capacity in
chosen dayparts by adding capacity to any specified orbit. The means to
increment
capacity may include the following methods to cause additional capacity to be
timed to
coincide with any desired time periods requiring additional capacity,
including; causing
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higher capacity satellites to coincide with the desired periods, or causing
additional
single or multiple satellites to be interspersed to arrive coincident with the
desired
periods. Higher capacity satellites may be formed by several methods,
including;
employing additional spectrum bands, higher power satellites, utilizing larger
antennas
on satellites, employing more spectrum or frequency reuse, spot-beam
transmissions,
higher-order modulation efficiencies, on-board signal processing, or other
techniques
developed to cause more capacity over a satellite communications link.
There are additional methods specified to utilize the inclined elliptical
orbits.
There are other benefits of the invention and preferred embodiments disclosed
herein.
A number of invented methods are disclosed in the claims and description;
which
disclose certain inventions and specified combinations of methods to improve
use of
elliptical orbits herein. These inventions generally disclose, but are not
limited to;
single-hop hemispheric communication links and applications, communication
link
improvements, methods to mitigate and improve satellite positioning and
control effects
generally including artificial intelligence, providing secure communications
during the
portion of the non-active arc approaching perigee where higher speed orbit
transiting
occurs, and methods to maintain link connection during handoffs.
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DETAILED DESCRIPTION
BACKGROUND
Satellites situated in geostationary orbits (GSO) remain essentially at the
same position
relative to the earth. Geosynchronous orbits require specified parameters at
42,164 km
from the center of the earth (35,786 km above sea level) and positioned above
the
equator at 00 inclination to make this work, therefore there is only one
orbital track
(orbit) which can be utilized for GSO geosynchronous satellite operation.
Within the
GSO orbit there are a finite and small number of available geostationary
slots. In recent
years, global demand for delivery and consumption of data has been doubling
every two
years, and demand for satellite capacity and bandwidth increases in line with
the
demand for data.
The parameters governing utilization of GSO were globally adopted when
commercial
satellites were established during the 1970's, which allowed for the use of
large
receiving dishes on the ground. That created an environment which allows
multiple slots
within the single GSO ground track orbit, each slot having approximately 2
degrees of
separation in the GSO orbital arc in order to enable larger directional
antennas to
communicate with a minimum of electronic interference; while later on
approximately 9
degrees of orbital spacing was mandated and utilized for higher powered
satellites to
communicate with smaller antennas. The GSO ring around the equator hence has a
total of 180 operational orbit slots (360 degrees divided by 2 degree
separation spacing)
for large antenna operation, or 40 slots (360 degrees divided by 9 degree
separation
spacing) for small antenna operation due to wider aperture beamwidth of small
antennas.
There are a scarce number of geosynchronous slots that remain available for
development ¨ particularly overhead of the most populous regions of the earth
which
were developed first.
The present invention teaches a synthetic array of essentially geostationary
satellites
which address the scarcity problem, and provides two totally new dimensions or
regions
of the sky for several hundred satellites in new equivalent orbital slots.
These new slots
have many of the advantages of geostationary orbits; however, due to their
closer
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proximity to the earth and higher elevation angles for communication, among
others,
they provide superior communications link performance.
A new GSO-like synthetic space called the Clarke Belt 2.0 is disclosed
according to the
present inv.ention. This provides new orbital slot real estate in the
satellite sector.
The newly created orbital slot space can include a number of satellites in
elliptical orbits,
which satellites are active during an "active arc" occurring during their
apogee portions.
Multiple overlapping ground tracks may be created, each with multiple
satellites placed
in each orbit to trace the same ground track, where multiple satellites can
operate one
after the other in an orbit appearing as a "cars on a freeway" which may bunch
up or
spread out according to their speed and the ebb and flow of traffic. The same
number of
satellites, at least one, is in the active arc apogee portions at any point in
time.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other aspects of the invention will be described in detail with
reference to the
accompanying drawings, wherein:
FIG. 1 shows the orbits of the satellite array of the present application;
FIG. 2 shows a typical 24 hour repeating ground track, consisting of three 8
hour
repeating ground track loops, each with a bolded active arc;
FIG. 3 shows multiple overlapping ground track loops equally spaced 15 degrees
of
longitude apart in the northern hemisphere, and FIG. 4 shows multiple
overlapping
ground track loops equally spaced 15 degrees of longitude apart in both the
northern
and southern hemispheres;
FIG. 5 shows a preferred embodiment displaying 8 ground tracks in the northern
hemisphere and 2 ground tracks in the southern hemisphere, mimicing the
population
ratio of the northern versus southern hemisphere, and locating southern ground
tracks
over populated regions; and
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FIG. 6 shows multiple satellites optimally arrayed in one of the ground track
loops of
the preferred embodiment.
FIG. 7 shows multiple overlapping ground tracks produced by interleaving 8
ground
tracks in the northern hemisphere with apogee peaks every 15 degrees of
longitude;
indicating points of orbit conjunction.
DETAILED DESCRIPTION:
FIG. 1 shows the orbits of the satellite array of the present application.
Each of a
number of satellites is placed into elliptical orbits of a special type. The
preferred orbits
are inclined at inclination of around 63 degrees, e.g., 63.435 degrees. The
satellites are
in posigrade elliptical orbits having three revolutions per sidereal day.
The argument of perigee refers to the location of the lowest altitude portion
of the orbit
around the orbit from the point in the orbit where the orbiting satellite
crosses the
equator in a northward direction. The orbits preferably have an argument of
perigee
near 270 or 90 degrees, which has the effect of placing the apogee or highest
point of
the orbit over the northern-most or southern-most portion of the orbit
respectively.
The orbits have an eccentricity of around 0.60 to 0.62, e.g., 0.61.
The satellites may also have an apogee altitude of approximately 26,250 km,
perigee
altitude of approximately 1,525 km, argument of perigee at or near 270 or 90
degrees,
eccentricity of about 0.61, altitude over the equator of approximately 5400
km, altitude
at start and end of the active arcs of around 12,000 km, approximately 25
degrees
(north or south) latitude at start and end of the active arcs, and nominal
latitude of
63.435 degrees. The orbit semi major axis is approximately 20,150 km.
Orbits having an integer number of daily revolutions have a ground track that
passes
over the same point(s) on the earth every (sidereal) day. Such ground tracks
are
referred to in this specification as repeating ground tracks.
The satellites are only active during part of their time of orbit. The time
and positions
when the satellites are active is referred to as active arcs. The active arcs
are defined to
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be centred on the orbital apogee peak, where the satellites are higher and
travel most
slowly to maximize the time that a satellite spends in the active arc. In this
embodiment,
each earth communicating satellite remains in each active arc for around 5.5
hours.
After leaving the active arc, each earth communicating satellite becomes
inactive and
non-emitting, and spends 2.5 hours transiting to its next active arc whose
apogee peak
is 120 degrees of longitude to the west. The satellite then enters another
active arc and
begins communications again. This means that each satellite is active for
5.5/(5.5+2.5)70% of the time.
Each of the satellites includes communication equipment which communicates
with
corresponding communication equipment located on the earth. Therefore, the
satellites
may communicate with various points on the earth, or each other, or other
satellites, or
mobile objects.
Near apogee, where the satellite's progress slows, its motion almost matches
the
rotational speed of the earth. Therefore the earth-communicating satellites in
the active
arcs will therefore appear to move very slowly, or linger overtop the earth.
Since the
argument of perigees are at the southern-most or northern-most ends of the
orbits, the
active arcs straddle the apogees, and the corresponding active portions of the
ground
tracks, are hence displaced at a large angle to the North or South
respectively from the
equator and the geostationary orbit.
A first set of satellites have apogees in the Northern Hemisphere forming the
space.
Those satellites are also shown in the ground track map of FIG. 2, with their
respective
apogees shown being bolded in HG. 2. A second set of satellites has apogees in
the
Southern Hemisphere forming the space.
FIG. 2 shows the active parts of the arc in bold. While the satellites are in
these active
parts of the arc, they have a similar rotational rate relative to the earth,
and therefore
appear to move very little relative to the earth. The space in which these
apogees may
occur moves at a similar rotational rate to the Earth, thus becoming the
"synthetic
geostationary space", in which active arcs derived from the synthetic
geostationary
orbits lay.
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The synthetic space may therefore include a number of satellites, each of
which is in an
inclined elliptical orbit with its apogee over a Hemisphere, either the
Northern or
Southern Hemisphere.
In the embodiment disclosed herein, the satellite is active over substantially
5.5 hours
out of every 8 hour orbit, this orbit repeating itself 3 times per day (-24
hour sidereal
day) and defining a repeating ground track. Generally, each of the satellites
is active for
substantially 70 percent of the time it is in orbit, but more generally can be
active within
60 and 80 percent of the time that it is in orbit.
Several satellites may occupy the same ground track. The satellites in the
single ground
track are minimally timed so that as soon as one satellite leaves each active
arc of the
ground track, another enters that active arc and provides effectively
continuous service.
Each ground track has three active arcs around the earth, and, if continuous
coverage
is desired, enough satellites are placed in the ground track, spaced evenly in
time, so
that there is always one satellite in each active arc per system. For example,
FIG.
6 shows 14 satellites in each repeating ground track loop. The active arc
includes the
top part of the curve. FIG. 6 shows how the satellites bunch up in this area
as they slow
down, and that there are relatively fewer satellites in the other, non-active,
areas. The
satellites are preferably placed within the active arc in a way such that
there is at least
one satellite in each active arc per system at all times, but preferably more
than one.
The active arcs in the Southern Hemisphere could be the inverse of the
Northern
Hemisphere active arcs.
With these parameters, a ground track with three active arcs provides coverage
of
substantially three times sixty degrees of longitude, or 180 degrees of
longitude. A
second ground track may be interleaved to create a second set of active arcs
in the
Northern Hemisphere to thereby provide another 180 degrees of occupied
longitude. A
maximum of 36 active arc apogee peaks (12 repeating ground tracks) can be
overlapped and developed in each hemisphere whilst providing a desired level
of
discrimination between satellites. If less is required the active arcs may be
placed at any
preferred longitudes to maximize the viewing angles to continental landmass
areas. In
this preferred embodiment 8 overlapping repeating ground tracks are disclosed,
each
having apogee peaks occurring approximately every 15 degrees of longitude.
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Since the satellites are only communicating near apogee in their active arcs,
these
create the synthetic orbit space. The satellites actually trace a complete
path which is
not shown in FIG. 1. However, the space is formed only by the active arcs of
the
satellites. The satellites actually travel in other positions besides these
active arcs, but
communicate only within these active arcs.
Multiple earth communicating satellite systems may use the same active arcs
disclosed
above, to place its earth communicating satellites in the same ground tracks
as above.
However, this system times the entry of its satellites to differ from those of
other
systems by at least to, where 0 is the minimum separation desired in the
active arc
occurring at apogee, and tA is the time necessary for a satellite to move that
distance at
that location.
Preferred slot parameters may include relationships between Desired Orbital
Separation, Relative Mean Anomaly, and Relative Right Ascension. Where:
o RMA is relative mean anomaly or Mean Anomaly difference between two
satellites in degrees relative to a common epoch (reference time), as measured
in each respective orbit,
O Relative Right Ascension is the difference in degrees between the RAANs
of
two orbits,
O Tta is the time required to move a desired number of degrees true anomaly
at
apogee in seconds,
o P is satellite orbital Period in seconds
The preferred mathematical relationships between Right Ascension of the
Ascending
Node and Mean Anomaly for satellites flying in the same ground track may be
specified,
but separated by a minimum of e degrees earth central angle. Each entry time
differing
from its neighbor by to constitutes a slot in the active arc. Each satellite
in each active
arc occupies one slot in that arc. A "protected" interval may exist around the
satellite
which travels with the satellite. In a geostationary orbit, a slot is defined
by the longitude
of the point on the earth under it. In this synthetic geostationary orbit, a
slot is defined as
an active arc entry time stated for a specified epoch day. Ground tracks and
active arcs
may be created, with one satellite in each active arc at all desired times.
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The orbital parameters described above may be varied somewhat while still
preserving
the characteristic of stationary active arcs over the northern or southern
hemisphere.
However, all satellites to be slotted together into active arcs in a
coordinated fashion
using this scheme may agree to use at least the same Mean Motion,
eccentricity,
inclination, argument of perigee, and ground track. The right ascension of the
ascending
node and mean anomaly of each satellite are preferably also adjusted together
in order
to place the satellite on the specified ground track at the satellite's
specified time of
active arc entry. This yields a coordinated motion among all such satellites
where
minimum separation criteria among them can be guaranteed.
Orbital parameters are adjusted to create ground tracks that repeat daily. In
this
preferred embodiment, each ground track has three active arcs in the Northern
Hemisphere using orbital arguments of perigee of around 270 degrees. Each
active arc
spans over 50 degrees of longitude at the highest portion of the orbit. All
three active
arcs in a ground track therefore occupy over 150 degrees of longitude.
A second ground track interleaved with the first creates a second set of
active arcs in
the Northern Hemisphere accounting for another 150 degrees of occupied
longitude, for
a total of 300 degrees of longitude occupied by active arcs. Additional ground
tracks can
be equally interspersed, up to a practical maximum where active arcs are
spaced each
degrees of longitude, or 12 ground tracks in each hemisphere. In this
preferred
embodiment the optimal spacing is 8 ground tracks in each hemisphere; creating
24
repeating ground track loops or active arcs, spaced each 15 degrees of
longitude in
each hemisphere. Ground tracks may be optimally spaced so as to maintain a
minimum separation between all active arcs while providing optimum position
and
coverage characteristics for the active arcs. Active arcs may also be freely
placed on
any line of longitude to maximize viewing angles to desired service areas.
This process is repeated in the Southern Hemisphere using orbital arguments of
perigee of around 90 degrees.
Since earth communicating satellites using these active arcs are in orbit at
over 12,000
to over 26,000 km, from these high vantage points each satellite in an active
arc can
see ground area encompassing several active arcs, or indeed several
continents. FIG.
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1, for example, shows a view from space centred over North America with many
satellites in northern hemisphere orbits, with their apogee peaks overhead. An
inverse,
or upside down, orbits would be created if southern hemisphere orbits were
depicted.
In order to place multiple satellites onto the same ground track passing at
spaced time
intervals, the planes of the orbit of following satellites may be rotated
about the earth's
axis by the amount and in the direction the earth has rotated in the interval
between the
time(s) the satellites pass over a given point. Larger time intervals between
satellites in
a ground track may cause more orbital rotation of the following satellite
about the earth's
axis to keep the satellite over the same ground track. This angle, when
measured
relative to a celestial reference point, e.g., the position of the sun against
the stars at the
time of the Vernal Equinox, is known as the orbit's right ascension of the
ascending
node (RAAN). If all satellites moved in the same orbit, rather than orbits
that have been
adjusted for earth rotation, following satellites would travel in ground
tracks further to the
west of those of the preceding satellite, since the surface of the earth is
constantly
moving around to the east relative to the stars. Hence to follow a common
ground track
and share active arcs, each satellite should occupy its own orbit having its
own RAAN.
FIG. 6 illustrates the satellites occupying successive slots in one active arc
and the
separate orbits and relative positions in orbits which allow each satellite to
follow the
active arc properly.
Spacing in space can be assured by ensuring a constant separation of the
points in the
ground tracks under each satellite, and if necessary adjusting orbits to
ensure differing
altitudes at ground track crossings. Satellites in adjacent ground tracks may
be phased
or timed to stagger their distance from adjacent ground tracks. FIG.9
illustrates ground
track orbit crossover points, known as conjunctions, for overlapping ground
tracks in the
preferred embodiment of 8 ground track loops in the northern hemisphere. By
timing
the satellites in each adjacent ground track properly, for example every other
ground
track equally staggered, conjunctions may be completely avoided and desired 9
degree
angular separation between satellites may be maintained or optimized.
FIG. 4 illustrates a possible configuration of 16 ground tracks, or 48
repeating ground
track loops, over the earth. FIG. 5 shows a preferred embodiment of 8 northern
and 2
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southern ground tracks, mimicking the ratio of population in the northern and
southern
hemispheres.
FIG. 6 shows the satellites following behind each other in the paths
illustrated in these
figures, while maintaining a separation of at least nine degrees earth central
angle from
all other satellites. The active portion of the ground track occurs in the
higher, flattened
portion of the satellite paths shown in FIG. 6. These portions are slow moving
and
essentially geo-synchronous creating a "synthetic geostationary" arrangement
for
placing earth communicating satellites.
If the satellites are spaced so as to maintain at least, for example, nine-
degree intervals
at apogee within the active arc, on the order of 14 satellites can be placed
in each
ground track, comprising 10 in each of three active arcs and 4 in transit
between active
arcs in each ground track. Each satellite travels in its own orbit. The
similar orbits differ
only by their RAAN and mean anomaly (MA), whereby in this example the RAAN of
the
orbit of each immediately following satellite in the ground track is increased
over that of
the preceding satellite and its mean anomaly adjusted to be less than the
preceding
satellite.
Since this invention discloses 16 ground tracks, each with three active arcs,
this
invention can accommodate 16 ground tracks x 10 satellites per active arc x 3
active
arcs per ground track = 480 equivalent active arc satellite slots. FIG. 6
illustrates an
active arc occupied with satellites placed at approximately 9-degree spacing.
In this preferred embodiment, the apogee of the satellites lays at around
26,000
kilometers above the surface of the earth, or around three-quarters the
altitude of
satellites in the geostationary orbit. The lower 26,000 kilometer apogee
altitude of this
embodiment leads to savings in satellite costs, since the shorter path to and
from the
satellite yields less path loss, on the order of 7dB less than that of a
geostationary
satellite. The consequent reduced power requirements for a given link
translate into
savings in satellite weight and cost for a given capability. In addition, the
orbit used in
this preferred implementation requires less than half the launch energy
required for
launch into the geostationary orbit, yielding additional savings. These
savings offset the
costs of satellite time spent outside of active arcs.
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The chosen active arc regions minimally deliver a 25 degree separation between
the
active arcs and the geostationary Clarke Belt arc. However, other degrees of
separations can also be used, simply by setting the amount of time or length
of active
arc of communicating with the satellites.
Different numbers of satellites may be used, as described herein. In an
embodiment,
the satellites form two different ground tracks in each of the Hemispheres.
Each of the
ground tracks has three distinct active arcs. FIG. 2 shows one ground track in
the
Northern Hemisphere, with three active arcs. These ground tracks can be
populated by
satellites. In this embodiment, the peak of the active arcs, or apogees, is at
63.4
degrees latitude.
One advantage of this system is that this may avoid interference between the
synthetic
geostationary inclined elliptical orbit satellites, and the GSO Clarke Belt
ring of satellites.
The disclosed system may have more than 25 degrees of angular separation
between
the satellites and the GSO ring.
Other modifications of these parameters can of course be used. While the above
has
described the peak of the active arcs being at 63.4 degrees, the minimum
latitude for
the active portions of the active arcs is about 25 degrees latitude, on either
side of the
apogee. Generally anything greater than 25 degrees, and more generally
anything
greater than 30 degrees latitude, may be preferred.
Several independent system operators can use the slowly moving, active,
satellites in
each active arc. This may provide a total of 30 slots for the three active
arcs in each one
ground track. With 42 satellites in a single ground track, continuous coverage
may be
provided underneath all three active arcs. This system may have multiple
advantages.
By making the satellites active during only part of their orbits, the
satellites create no
interference with each other or with the GSO. The satellites are also much
lower in
altitude than the geo satellites. Hence latency may be better than geos, the
satellites
may be smaller, less expensive, require a smaller antenna, are less expensive
to launch
and allow more frequency reuse. The apogees at the active arcs may be placed
at
specific longitudes to concentrate the capacity over land masses. These
satellites may
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use C, X, Ku, Ka and ON bands, but may also use the L, S Bands, or other bands
allowed by regulation.
An explanation of the constellation parameters follows:
= a. Mean Motion: The number of revolutions around the earth the satellite
makes in
1 day. An integer value of mean motion ensures that the satellite will repeat
the
same ground track each day. Since we want all satellites to follow a repeating
ground track, and wanted each satellite to visit no more than 3 active arcs,
we
selected an integer mean motion, rather than a rational mean motion, which
would
have yielded repeating ground tracks at intervals longer than one day. A mean
motion of 4 yields 4 active arcs per ground track and active arcs that are too
broad
to maintain the regional geographic coverage that we desired. A mean motion of
2
yields 2 active arcs per ground track, and very narrow active arcs. Slotting
here is
less feasible, since positions on the active arc are not well separated in
angle.
Also, its apogee altitude is high, being around 38,500 kilometers, leading to
high
latency. This is the well-known Molniya orbit.
= b. Inclination: 63.435 degrees. This figure prevents the line of apsides,
the line
connecting the apogee and perigee, from rotating around the orbit, moving the
apogee southward toward the equator. If the inclination is higher, the line of
apsides will rotate in a direction opposite to the direction of satellite
motion. If
lower, the line of apsides will rotate around the orbit in the same direction
as
satellite motion.
= c. Eccentricity 0.61. This value, when combined with the necessary mean
motion,
yields an apogee of 26,400 kilometers, and a perigee of 1,525 kilometers.
While
there is a small amount of drag at that perigee, orbit lifetimes are expected
to be
into the tens of years, since most of the orbit is spent much higher.
A lower eccentricity will yield lower apogees, higher perigees and even less
atmospheric drag and LEO orbit intersection, but slightly lower declinations
(angle
above the equator from the center of the earth) for the lowest part of the
active
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arcs. Coverage area will be also reduced, due to lower operational satellite
altitudes at active arc end points.
= d. Argument of Perigee: 270 degrees for northern ground tracks and 90
degrees
for southern ground tracks. These values are important as they determine where
the apogees are, where satellite motion is slowest. These figures place the
apogees at the furthest angles in declination from the equator, and keep the
active
arcs, which span 216 degrees of Mean Anomaly, well separated from the
equatorial arc. As the Argument of Perigee departs from these values, the ends
of
the active arcs will move toward the equator. Some slight variation in
argument of
perigee from the cited value, on the order of one degree, might be desirable
to
ensure good satellite spacing at orbit crossings. Otherwise little flexibility
exists in
these numbers.
= e. Longitude of Apogees: This measure specifies where the peaks of the
active
arcs are located over the surface of the earth in coordinates relative to the
rotating
earth. For a Mean Motion of 3, a satellite's ground track will pass through
three
apogee longitudes, spaced 120 degrees from each other in longitude. Therefore,
for a given ground track, specifying one Longitude of Apogee specifies the
other
two as well. For convenience therefore, specifying the location of the active
arcs in
the Americas region from 0 degrees West Longitude to 120 degrees West
Longitude is sufficient to locate a ground track in any Longitude orientation.
A
given ground track may have any Longitude of Apogee in this 0-120 degree
range.
Good coverage of important markets may be an important criterion for selecting
the locations of the Longitude of Apogees. The second ground track should have
an Apogee of Longitude that places the active arcs between those of the first,
without crossing and maintaining a preferred separation from those of the
first,
depending on desired coverage versus active arc separations.
= f. Active Arc Span: 2 hours and 40 minutes (or 120 degrees of Mean
Anomaly) to
each side of apogee, plus -8 minutes per side for housekeeping, and
switchover.
The ratio of active satellites to total satellites per ground track per system
determines this span. This choice derives from 4 active satellites and 6 total
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satellites per ground track. It is however possible to design an arrangement
using
3 active arcs one active satellite per arc, and 4 total satellites rather than
6. In this
case the active arcs extend down to 28 degrees North declination at a minimum
operational altitude of around 11,900 kilometers rather than the 17,500-18,000
kilometers of the present design. The satellites would have to cope with a
greater
variation in orbital altitude, but would be in operation for 75 percent of the
time
each. Coverage areas may suffer, since the extensions of the active arcs are
at
relatively low altitudes.
= g. Mean Anomaly at epoch: selected to place each satellite at an
appropriate
interval from its neighbor. The absolute number is not so important here as
the
relative MA. Absolute MA will determine when the satellite passes a point on
the
earth. Relative MA will determine the separations among satellites. Mean
Anomaly
spacing and minimum included zenith angles of the satellites are related.
Each Repeating Ground Track Loop (RGTL) is one active arc created over an 8
hour
period on one ground track. Each ground track has three RGTLs. A communication
system may provide substantial Northern Hemisphere coverage from 5 satellites
in one
Northern ground track, providing service from the equator northward everywhere
under
the active arc. At the worst-case Longitude exactly between active arcs,
coverage from
a single ground track exists north of 30 degrees North. Coverage of the
Southern
Hemisphere is similar, using a single Southern ground track. Global landmass
coverage
pole to equator to pole may be attained using two Northern and one Southern
ground
tracks and 15 satellites, with good RGTL placements. Full-time coverage from a
ground
track requires a minimum of 5 satellites per ground track.
Services offered by many prospective operators will concentrate on regional
markets, or
for example on markets primarily on land-masses. Ground track occupancy and
visibility
requirements can be reduced in that case. An operator seeking to service
specific
regions would place satellites in the ground tracks with active arcs serving
those
regions. A consortium of operators may share in the development, construction,
and
launch costs of satellites serving a particular ground track and its three
RGTLs. Since
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each satellite visits all active arcs daily in the ground track, a satellite
loss is spread over
three markets rather than one, and results in a 20% time-outage rather than a
100%
outage. In-orbit sparing may be reduced (e.g., 1 for 5 rather than 1 for 1),
and risk may
be spread amongst operators (akin to an insurance pool), and losses are
relieved.
FIG. 6 presents the number of satellite systems that can be accommodated in
each
ground track. Each satellite may belong to a different system. If a given
system requires
only one ground-track and places an active satellite in each of the three
RGTLs, each
ground track would, for example, accommodate 14 satellites per RGTL at a
minimum
required 9 degree zenith angle between satellites. The zenith angle may be
measured
from the surface of the earth. Each such system may moreover be viewed as
equivalent
to three regional systems, one per satellite per RGTL for distinct regional
operations.
Although only a few embodiments have been disclosed in detail above, other
modifications are possible.
All such modifications are intended to be encompassed within the following
claims, in
which:
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