U.S. patent application number 11/275566 was filed with the patent office on 2006-08-31 for virtually geostationary satellite array with optimized parameters.
This patent application is currently assigned to VIRTUAL GEOSATELLITE LLC. Invention is credited to Jay Brosius, David Castiel.
Application Number | 20060192056 11/275566 |
Document ID | / |
Family ID | 27663162 |
Filed Date | 2006-08-31 |
United States Patent
Application |
20060192056 |
Kind Code |
A1 |
Castiel; David ; et
al. |
August 31, 2006 |
Virtually geostationary satellite array with optimized
parameters
Abstract
A plurality of satellites are placed into a virtually
geosynchronous orbit, in which a first part of the orbit, that is
near apogee, has a similar movement to the rotation of the earth,
and therefore the orbit appears virtually stationary relative to
the earth. Different satellites in the orbit are caused to have
specified standardized parameters, and also defined according to an
orbital position at a date certain. The different satellites and
therefore be assigned to different owners according to these
parameters.
Inventors: |
Castiel; David; (Washington,
DC) ; Brosius; Jay; (Frederick, MD) |
Correspondence
Address: |
Rubinstein Law Group, Professional Corporation;David Bogart Dort
1700 Diagonal Road, Suite 300
Alexandria
VA
22314
US
|
Assignee: |
VIRTUAL GEOSATELLITE LLC
4400 Massachusetts Avenue, 385
Washington
DC
|
Family ID: |
27663162 |
Appl. No.: |
11/275566 |
Filed: |
January 17, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10354933 |
Jan 29, 2003 |
|
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11275566 |
Jan 17, 2006 |
|
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60352984 |
Jan 29, 2002 |
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Current U.S.
Class: |
244/158.4 |
Current CPC
Class: |
B64G 1/1085 20130101;
B64G 1/1007 20130101; H04B 7/185 20130101; H04B 7/19 20130101; H04B
7/1851 20130101; B64G 1/242 20130101 |
Class at
Publication: |
244/158.4 |
International
Class: |
B64G 1/10 20060101
B64G001/10 |
Claims
1. A method, comprising: setting a plurality of standardized
parameters for a satellite which is in a non geosynchronous orbit
and which communicates with different positions on the earth at
different times; placing limits on in-track and cross-track offsets
applicable at all times within active arcs, in which the satellite
is actively communicating with points on the earth; and defining
said limits as not applying during times when the satellite is
actually active.
2. A method as in claims 1, further comprising defining a specified
satellite in the orbit based on its position at a specified
date.
3. A method as in claims 2, wherein said position on said specified
date comprises a specified mean anomaly on a specified date.
4. A method as in claim 3, further comprising enabling licensing
one of said satellites in said orbit to a different owner then
another of said satellites in said orbit based on said standardized
parameters and said position.
Description
PRIORITY DOCUMENTS
[0001] This patent application claims priority under 35 U.S.C.
.sctn.120 and is a divisional of U.S. patent application Ser. No.
10/354,933, filed Jan. 29, 2003 and entitled Virtually
geostationary satellite array with optimized parameters, which
claims benefit under 35 USC .sctn.119(e) to U.S. Provisional
Application No. 60/352,984, filed Jan. 29, 2002. Both of these
applications are fully incorporated by reference for all
purposes.
BACKGROUND
[0002] Satellites in geostationary orbits are at virtually the same
location relative to the earth, at points and times in the earth's
rotation. Geosynchronous orbits require specified parameters
(22,300 miles; 0.degree. inclination) to make this work. Hence
there is only one orbital track or "orbit" which can be used for a
geosynchronous satellite. Within that orbit, there are only a
limited number of available geostationary slots. However, the
demand for satellite space increases in line with the demand for
bandwidth. In recent years, demand for bandwidth has been
increasing exponentially.
[0003] The arrangement that has been adopted over time uses
multiple slots within the single ground track orbit, each slot
having approximately 2 degrees of width relative to an
earth-centered angle. This allows for communications with a minimum
of electronic interference using directed antennae. The geo ring
around the equator hence has a total of 180 slots (360 degrees
divided by 2 degrees).
[0004] There are a limited number of geosynchronous slots that
remain available.
SUMMARY
[0005] The present invention teaches an array of virtually
geostationary satellites which address this problem, and provides a
totally new area for a plurality of satellites in new slots. These
new slots have many of the advantages of geostationary orbits.
[0006] A new geo like space called the virtual geo space is
disclosed according to the present invention. This provides new
real estate in the satellite sector.
[0007] The space includes a plurality of satellites in elliptical
orbits, which satellites are active during an "active arc"
occurring during their apogee portions. Multiple satellites can be
placed in each orbit to trace the same ground track. The same
number of satellites, at least one, is in the active arc apogee
portions at any one time.
[0008] Another aspect relates to the findings different satellites
within the different arcs/ground tracks in a way that enables
different satellites within the same arc to be operated by
different owners. In the past, allocating authorities such as the
FCC have typically assigned a specific satellite real estate to one
owner. In this system, the satellite may shift within the arc, but
still be assigned to the same owner, and different owners may be
assigned to different satellites, which continually shift in
position within the arc.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] These and other aspects of the invention will be described
in detail with reference to the accompanying drawings, wherein:
[0010] FIGS. 1 and 2 show the orbits and orbital space of the
satellite array of the present application;
[0011] FIG. 3 shows multiple satellites in a ground track;
[0012] FIG. 4 shows separate satellites in separate slots in active
orbits, and
[0013] FIG. 5 shows the positions of those satellites in the active
arc of the orbit;
[0014] FIG. 6 shows a configuration with four ground tracks;
[0015] FIG. 7 shows the satellites in one of the ground tracks of
the FIG. 6 embodiment;
[0016] FIG. 8 shows the FIG. 7 (single ground track) satellites
from a hypothetical north view, showing the virtually geostationary
area;
[0017] FIG. 9 shows the ground tracks of the satellites of FIGS. 7
and 8;
[0018] FIG. 10 shows illustrates active arcs occupied with
satellites placed at an approximate 2-degree spacing;
[0019] FIG. 11 shows the view from earth from one satellite;
[0020] FIG. 12 shows a chart that charts the number of satellites
in an arc vs. eccentricity and spacing;
[0021] FIG. 13 illustrates the differences in mean anomaly
separation between the different degrees of separation; and
[0022] FIG. 14 illustrates the variation in included zenith angle
with time between two adjacent satellites spaced at 2 degrees at
apogee
DETAILED DESCRIPTION
[0023] FIGS. 1 and 2 show the orbits and orbital space of the
satellite array of the present application. Each of a plurality of
satellites are placed into elliptical orbits of a special type. The
preferred orbits are inclined at inclination of around 63 degrees,
e.g., 63.435 or 116.565 degrees. The satellites are posigrade,
elliptical orbits having three revolutions per sidereal day.
[0024] The argument of perigee refers to the location of the lowest
altitide 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 of near 270 or ninety 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.
[0025] The orbits also have an eccentricity of around 0.65 to
0.66.
[0026] The satellite may also have an apogee altitude of 26 967.6
km, perigee altitude of 798.3 km, argument of perigee at or near
270 or 90 degrees, eccentricity of about 0.66, altitude over the
equator of approximately 5430.6 km, altitude at start and end of
the active arcs of 17,787.7 km, 45.1 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
20250-20230 km.
[0027] Orbits having an integer number of revolutions per day will
have a ground track that passes over the same points on the earth
every (siderial) day. Such ground tracks are referred to in this
specification as repeating ground tracks.
[0028] The satellites are only active during part of their time of
orbit. The time when the satellites are active is referred to as
active arcs. The active arcs are defined to be around the orbital
apogee, where the satellites travel most slowly. This maximizes the
time that a satellite spends in the active arc. In this embodiment,
each earth communicating satellite remains in each active arc for
4.8 hours. After leaving the active arc, each earth communicating
satellite becomes inactive and non-radiating, and spends 3.2 hours
transiting to its next active arc. The satellite then enters
another active arc and begins communications again. This means that
each satellite is active for 4.8/(4.8+3.2).apprxeq.60% of the
time.
[0029] Each of the satellites include communication equipment which
communicates with corresponding communication equipment located on
the earth. Therefore, the satellites may communicate with various
points on the earth.
[0030] 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 hang, or loiter over the earth. Since the
argument of perigees are at the southern- 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 geo-stationary orbit.
[0031] A first set of satellites have apogees in the Northern
Hemisphere forming the space 100. Those satellites are also shown
in the ground track map of FIG. 2, with their respective apogees
200 shown being bolded in FIG. 2. A second set of satellites has
apogees in the Southern Hemisphere forming the space 110.
[0032] Two ground tracks are illustrated having active arcs in the
Northern Hemisphere, and one ground track is illustrated having
active arcs in the Southern Hemisphere.
[0033] 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 move very little
relative to the earth. The space in which these apogees occur moves
at a similar rotational rate to the Earth, thus becoming the
"virtual geostationary space", in which active arcs derived from
virtual geosationary orbits, lay.
[0034] The virtual space may therefore include a plurality of
satellites, each of which is in a highly elliptical orbit with its
apogee over a Hemisphere, either the Northern or Southern
Hemisphere.
[0035] In the embodiment disclosed herein, the satellite is active
over substantially 4.8 hours (e.g. 4.8 hours 310 percent) out of
every 8 hour orbit, this orbit repeating itself 3 times per day (24
hour day) and defining a repeating ground track. More generally,
each of the satellites is active for substantially 60 percent of
the time it is in orbit, but more generally can be active within 45
and 80 percent of the time that it is in orbit.
[0036] Several satellites may occupy the same ground track, such as
210. The satellites in the single ground track are timed so that as
soon as one satellite leaves each active arc of the ground track,
another enters that active arc. 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. 3 shows 72 satellites in each ground
track 300, 310. The active arc includes the top part of the curve.
FIG. 3 shows how the satellites bunch up in this area, 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.
[0037] With these parameters, a ground track with three active arcs
provides a coverage of substantially three times fifty degrees of
longitude, or 150 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 150 degrees of
occupied longitude. This provides a total of 300 degrees of
longitude occupied by the active arcs (or less if the two ground
tracks overlap in areas of high demand for example). The active
arcs may be placed, as shown, to maximize the viewing angles to
Continental areas.
[0038] The satellites tend to bunch up at the regions near apogee
since this is the time when the satellites move the most slowly.
FIG. 3 shows this effect, with most of the satellites being bunched
at the apogee areas. However, if the satellites are placed in this
way, as described herein, their ground tracks will not cross one
another during their active periods. If two groups of satellites
are displaced by 45 degrees from one another as in FIG. 2, the
satellites appear as six, distinct, parabolic shaped active arcs in
the Northern Hemisphere on a Cartesian map. Six active arcs in the
Southern Hemisphere could also be used. The active arcs in the
Southern Hemisphere could be the inverse of the Northern Hemisphere
active arcs.
[0039] Since the satellites are only communicating near
apogee--active arcs, these create the virtual Geo space as shown in
FIG. 1. The satellites actually trace a complete path which is not
shown in FIG. 1. However, the space 100 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.
[0040] 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 .theta. is the minimum separation
desired in the active arc occurring at apogee, and to is the time
necessary for a satellite to move that distance at that
location.
[0041] Table 1 summarizes some exemplary slot parameters.
TABLE-US-00001 TABLE 1 Mathematical Relationships between Desired
Orbital Separation Angle, Relative Mean Anomaly, and Relative Right
Ascension Satellite Separation, Relative Mean Anomaly between a
Satellite leading and Separation, Earth following satellite
Relative Right Central Angle in a ground track Ascension [] -360
(t.sub.0/P) 360 (t.sub.0/86,400) (following relative (following
relative to leading to leading (following (following satellite has
lower satellite's orbit value) has higher RAAN)
[0042] Where:
[0043] 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,
[0044] Relative Right Ascension is the difference in degrees
between the RAANs of two orbits,
[0045] t.sub.0 is the time required to move .quadrature. degrees
true anomaly at apogee in seconds,
[0046] P is satellite orbital Period in seconds
[0047] The table provides mathematical relationships between Right
Ascension of the Ascending Node and Mean Anomaly for satellites
flying in the same ground track, but separated by a minimum of
.theta. 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 virtual
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.
[0048] 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.
[0049] 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. Each active arc spans
around 50 degrees of longitude at the highest portion of the orbit.
All three active arcs therefore occupy around 150 degrees of
longitude.
[0050] 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. The two ground tracks
are spaced so as to maintain a minimum separation between all
active arcs (distributing the remaining 60 degrees of longitude as
spacing) while providing optimum position and coverage
characteristics for the active arcs. Active arcs can be freely
placed to maximize viewing angles to desired service areas.
[0051] This process is repeated in the Southern Hemisphere using
orbital arguments of perigee of around 90 degrees.
[0052] Since earth communicating satellites using these active arcs
are in orbit at over 17,000 to over 27,000 km, from these high
vantage points each satellite in an active arc can see ground area
encompassing several active arcs. FIG. 11, for example, shows a
satellite earth view and ground track 1100 for a single satellite,
with its apogee over North America.
[0053] 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 times 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 backdrop of 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.
[0054] FIG. 4 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.
[0055] 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.
[0056] FIG. 5 illustrates the relative positions of the satellites
shown in FIG. 4 within one active arc of one ground track.
[0057] FIG. 6 illustrates a possible configuration of four ground
tracks over the earth. FIG. 7 shows 72 satellites placed in just
one of the above ground tracks.
[0058] FIG. 8 shows the same 72 satellites seen from the North,
effectively, the FIG. 7 view, from the north. Satellites follow
behind each other in the paths illustrated in these figures, while
maintaining a separation of at least two degrees earth central
angle from all other satellites. The active portion of the ground
track occurs in the higher, flattened portion 800 of the satellite
paths shown in FIG. 8. These portions, or "petals" in this view,
are geo-synchronous creating a "virtual geostationary" arrangement
for placing earth communicating satellites. FIG. 9 shows the
satellites of FIGS. 7 and 8 in their ground track.
[0059] If the satellites are spaced so as to maintain at least, for
example, two-degree intervals at apogee within the active arc, on
the order of 72 satellites can be placed in each ground track,
comprising 15 in each of three active arcs and 9 in transit between
active arcs in each ground track. Each satellite travels in its own
orbit, as shown in FIG. 4. The 72 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 by 5 degrees over that of the preceding
satellite and its mean anomaly adjusted to be 15 degrees less than
the preceding satellite.
[0060] Since this embodiment uses four ground tracks, each with
three active arcs, this embodiment can accommodate 4 ground
tracks.cndot.15 satellites per active arc.cndot.3 active arcs per
ground track=180 active arc satellite slots. FIG. 10 illustrates
active arcs occupied with satellites placed at an approximate
2-degree spacing.
[0061] In this embodiment, the apogee of the satellites lies at
around 27,000 kilometers above the surface of the earth, or around
three-quarters the altitude of satellites in the geostationary
orbit. The lower 27,000 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 60 percent or 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.
[0062] FIG. 1 shows a 40 degree separation between the active arcs
and the Geo band. 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.
[0063] 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 two ground tracks in the
Northern Hemisphere, with six active arcs, and a single ground
track in the Southern Hemisphere, and the 3 active arcs of that
single ground track. In the embodiment of FIG. 2, there are 3 other
ground tracks. This includes one ground track for the Northern
Hemisphere and two for the Southern Hemisphere. 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.
[0064] One advantage of this system is that this may avoid
interference between the virtual geo satellites, and the geo ring
of satellites. The disclosed system may have more than 40 degrees
of separation between the satellites and the geo ring.
[0065] 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 45.1 degrees latitude, on either side
of the apogee. Anything greater than that, and specifically,
anything greater than 50 degrees latitude, may be preferred.
[0066] The mean anomaly spacing to ensure 2 degrees of satellites
separation near apogee is 1080/72 or 15 degrees. Each daily ground
track covers 3.times.360 degrees or 1080 degrees. Therefore, there
can be 72 satellites per ground track. With 4 possible ground
tracks, this can produce an effective possibility of 288 slots,
using the 15 degree mean anomaly spacing. Each arc therefore may
have 72/3=24 satellites. Between 4 and 5 satellites out of the 24
within each active arc is in active duty at any one time.
Conversely, between 19 out of those 24 stay in standby mode in each
loop at any time.
[0067] While 288 satellites may be the maximum theoretical numbers,
280 satellites total, or 70 per ground track, provides 14
satellites per active arc rather than 14 and a fraction. This may
avoid phasing problems between different users that might otherwise
occur. This provides a mean anomaly spacing of 1080 degrees divided
by 70 equals 15.428 degrees and about 2.06 degrees width slots at
apogee.
[0068] Up to 14 independent systems can use the fourteen slowly
moving, active, satellites in each active arc. This may provide a
total of 42 slots for the three active arcs in each one ground
track. With 70 satellites in a single ground track, continuous
coverage may be provided underneath all three active arcs. If only
a single ground track is used, then there may be a triangular
outage region along the equator, midway between peaks of the active
arcs, jumping to about 20 degrees of latitude. However, the second
ground track provides continuous coverage of the entire Hemisphere
including all the equatorial regions.
[0069] 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 geosatellites.
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.
[0070] The apogees at the active arcs may be placed at specific
longitudes to concentrate the capacity over land masses. These
satellites may use primarily Ku and C bands, but may also use the
Ka Band.
[0071] Another embodiment relates to a technique that allows
defining VGSO allocations by an allocating authority (e.g. the FCC)
and their tolerances. The following embodiment describes a way in
which virtually geosynchronous satellites could be allocated by the
allocating authority. For example, this system may describe
tolerances which enable different satellites within the same active
arc or ground track to be assigned to different owners or users. A
technique is disclosed to enable satellites in the same arc to be
assigned to different users. These satellites may shift in position
within the arc at different times, since they are not
geosynchronous relative to the arc. However, the satellites are
defined in a way that allows different users to own different
satellite positions within the arc.
[0072] Simulation studies have shown that variations in orbital
elements interact (as would be expected) to produce a net effect in
satellite movement, as seen from Earth Stations. As expected, small
perturbations in right ascension, argument of perigee, or mean
motion alone, for example, can produce significant movement out of
track and out of timing for a VGSO satellite. However, further
analysis demonstrates that certain combinations of orbital
perturbations can substantially counteract each other and result in
relatively small net movements over much (but usually not all) of
the active arcs. An example is certain combinations of
perturbations to mean anomaly and argument of perigee. Therefore it
does not appear easily feasible to specify easily measurable,
two-dimensional parameters as seen from the ground at specific
times (such as azimuth and elevation parameters at a specified
active arc entry and/or exit time) and guarantee good performance
over the entire active arc in the face of perturbations to the
satellite's orbit.
[0073] It might suffice to specify a full set of orbital parameters
and place tolerances on each of them, but that approach then does
not lead to easily observable, measurable, or verifiable
characteristics without doing a full orbital mechanics analysis.
Therefore, to avoid overly esoteric tolerance specifications while
protecting against poorly performing but in-spec possibilities, the
inventors postulate what they believe is a very workable approach
to specifying tolerances. In this embodiment, limits are placed on
in-track and cross-track offsets applicable at all times within the
active arcs. This has the desirable effect of ensuring good
satellite placement while ignoring any perturbations that are not
relevant to that objective.
[0074] Note that any tolerance specification should only concern
measurement within the active arcs. At other times the satellites
are quiescent, hence interference is not an issue. Moreover, when
quiescent, satellites may not be able to participate in ranging,
telemetry or other activities designed to aid in position
determination.
[0075] The inventors have found that the following parameters may
define and assign allocations within the VGSO operating
environment. The tolerances below yield generous station-keeping
boxes while ensuring tight-enough tolerances on satellite movement
so as not to contribute significantly to adjacent satellite
interference levels over nominal values. While the following
represents preferred values, it should be understood that other
similar values could be alternatively selected. TABLE-US-00002 Mean
3.000 Motion Inclination: 63.435'', specifically that required to
ensure a fixed argument of perigee in a posigrade orbit
Eccentricity: 0.630 Argument 270.degree. for Northern arcs (ground
tracks 1a and 2a) of 90.degree. for Southern arcs (ground tracks 1b
and 2b) perigee: (see 2 below) Longitude 65.degree. West (ground
tracks 1a or 1b, occurring at of Apogee 180.degree. Mean Anomaly),
or 125.degree. West (ground over tracks 2a or 2b, occurring at
180.degree. Mean Anomaly), Americas: as assigned
[0076] TABLE-US-00003 III. LONGITUDE OF II. AGREEMENT OF APOGEE
OVER I. GROUND TRACK PERIGEE AMERICAS 1a 270.degree. 65.degree. W
1b 90.degree. 65.degree. W 2a 270.degree. 125.degree. W 2b
90.degree. 125.degree. W
[0077] Each satellite may operate over an active arc spanning
72.degree. to 2880 of Mean Anomaly within its orbit, plus the three
minutes of time preceding 720 Mean Anomaly and 3 minutes of time
following 288.degree. of Mean Anomaly. At all other times each
satellite must suppress all radiation by at least 60 decibels below
that authorized during operation in the active arc.
[0078] Each authorized satellite is allocated a time on a specified
date certain, e.g. the first of January 2005 at which it shall
arrive at a specified mean anomoly, e.g., 72.degree. Mean Anomaly
in its orbit within the Americas Active Arc for its assigned Ground
Track. The time of arrival at 72.degree. Mean Anomaly on other days
may be calculated by adding or subtracting an appropriate integer
number of sidereal day intervals (i.e., that time necessary for the
earth to rotate precisely once with respect to the stars, being
approximately 23 hours and 56 minutes) to result in a time within
the desired day.
[0079] 4 Allowable orbital tolerances In-Track No satellite shall
arrive at any point Tolerance within any active arc at a time more
than 45 seconds removed from that predicted by the satellite's
assignment, over the lifetime of the satellite. Cross-Track No
satellite shall move out of track by Tolerance any more than 0.1
degrees as seen from any point on the earth, from that track
predicted by the satellite's assignment, over the lifetime of the
satellite.
[0080] An explanation of the constellation parameters follows:
[0081] a. Mean Motion: The number of revolutions around the earth
the satellite makes in 1 day.
[0082] 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.
[0083] 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.
[0084] 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.
[0085] 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.
[0086] c. Eccentricity 0.66. This value is the maximum feasible
value, which, when combined with the necessary mean motion, yields
an apogee of 27,271 kilometers, and a perigee of 513 kilometers.
While there is some small amount of drag at perigee, orbit
lifetimes are expected to be well 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 arcs, per Table 1. Coverage Area will be also reduce
somewhat, due to lower operational satellite altitudes at active
arc end points. TABLE-US-00004 TABLE 1 The Effect of Eccentricity
on Orbital Parameters Declination of lowest point Altitude in
active Apogee, of ends Perigee, Eccentricity arc, degrees
kilometera of arcs, km kilometers 0.66 46.02 27,271 18,025 513 0.65
45.34 27,068 17,863 716 0.64 44.64 26,865 17,702 919 0.63 43.95
26,680 17,544 1,122
[0087] An eccentricity value in the above range may be chosen with
relatively little effect on HLSA characteristics and advantages.
Lower eccentricities move the lower ends of the active arcs closer
still to the equator, and bec ome increasingly less desirable.
[0088] 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 separotatorial arc. As
the Argument of Perigee departs from these values, the ends of the
active arcs will move tow ard 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, depending on the results of further analyses.
Otherwise little flexibility exists in these numbers.
[0089] 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.
[0090] 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 region from 0 degrees West Longitude to 120 degrees West
Longitude is sufficient to locate a ground track in any Longitude
orientation. This range may be termed the Americas Sector (the
others may be termed the EurAsian Sector and the Pacific
Sector).
[0091] 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 criteria 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 good separation from those of
the first. Once the location of the first active Arc is located,
the second may be place 60 degrees in Longitude from the first, or
slightly more or less, depending on desired coverage versus active
arc separations.
[0092] f. Active Arc Span: 2 hours and 24 minutes (or 108 degrees
of Mean Anomaly) to each side of apogee, plus x minutes per side
for housekeeping, switchover. The ratio of active satellites to
total satellites per ground track per system determines this span.
This choice derives from 3 active satellites and 5 total satellites
per ground track. It is however possible to design a VGSO
arrangement using 3 active arcs one active satellite per arc, and 4
total satellites rather than 5. 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 not
benefit much, since the extensions of the active arcs are at
relatively low altitudes.
[0093] 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 as shown in FIG. 12 shown later.
[0094] 2. In order to realize the advantages of VGSO, the FCC could
standardize the parameters shown in Table 2: TABLE-US-00005 TABLE 2
Parameters to be Standardized IV. PARAMETER V. SUGGESTED VALUE VI.
MEAN NOTION VII. 3 (SEMIMAJOR AXIS = 20,270.421 RM) Eccentricity In
range of 0.66 to 0.68 (0.64 attractive) Inclination 63,436 degrees,
or that required to stop apsidal rotation Longitudes of Apogees 65
and 125 degrees W Longitude in Americas Sector. Further study may
suggest other locations Argument of Perigees 270 degrees for
Northern HLSAs: 90 degrees for Southern HLSAs Active Arc extents
(may be 108 degrees (plus 3 degrees specified as degrees of Mean
housekeeping) of MA each side Anomaly in orbit) of apogee Index
position in each ground 0 degrees MA at Jan. 1, 2005 track, defined
as a Mean Anomaly at a cited epoch, from which all satellite
positions are to be measured. Spacing in ground track (in 15
degrees MA Mean Anomaly, or alternatively, time of cited point
crossing) for each service type to be authorized Required minimum
sarth per existing FSS for 2 degree atation antenna pattern spacing
characteristics for each such spacing or corresponding IV.
PARAMETER V. SUGGESTED VALUE service Orbital maintenance as later
determined to be tolerances: necessary In track Cross-track
Altitude Downlink PFD limits when in [TBD] HLSA Emission
attenuation (or [TBD] maximum cirp or PFD) when outside HLSA
[0095] 4. Each High Latitude Stationary Arc (HLSA) is one active
arc on one ground track. Each ground track has three HLSAs. A
system may provide substantial Northern Hemisphere coverage from 5
satellites in one Northern HLSA 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 HLSA
ground track. Global coverage pole to equator to pole may be
attained using two Northern and one Southern ground tracks and 15
satellites, with good HLSA placements. Full-time coverage from a
HLSA requires 5 satellites per ground track.
[0096] 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
HLSAs. Since each satellite visits all active arcs in the ground
track, a satellite loss is spread over three markets rather than
one, and results in a 20 percent time-outage rather than a 100
percent outage. Sparing is cheaper (e.g., 1 for 5 rather than 1 for
1), risk is spread over several operators (similar to an insurance
pool), and loss consequences are less drastic. VGSO deployments are
therefore also well suited to regional services.
[0097] FIG. 12 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 HLSAs, each
ground track would, for example, accommodate 14 satellites per
ground track (for e=0.64) at a minimum required 2 degree included
zenith angle between satellites. The zenith angle may be measured
from the earth's center through the satellite. More may be
accommodated if measured from the surface of the earth. With four
ground tracks, the VGSO allocation scheme can accommodate 56
systems, each with a satellite at all times in each of three HLSAs.
Each such system may moreover be viewed as equivalent to three
regional systems, one per satellite per HLSA, for a total of 168
distinct regional operations possible.
[0098] In granting a license for a VGSO assignment, the Authority
would be assigning satellite deployment parameters as described in
Table 2, Parameters to be Standardized, above, plus an authorized
service and authorized spectrum. In addition, the Commission
assigns a position within a ground track to a licensee, defined as
a Mean Anomaly relative to the index position. The Commission may
wish to grant assignments in units of multiple, e.g. 5 satellites
in one ground track evenly spaced by 216 degrees in Mean Anomaly
from each other, the first of which maintains the specified Mean
Anomaly relative to the index position. This assignment places one
satellite at all times in each HLSA of the one ground track. As
above, 14 such assignments are possible given 2-degree spacing. The
Space Station would be licensed to operate in each HLSA of the
ground track. Licensees would require separate ground station
licenses for each ground station served by each HLSA of the ground
track.
[0099] FIG. 13 relates Mean Anomaly difference to the minimum
included zenith angle of adjacent satellites for two different
eccentricities. Note that differences in True Anomaly are normally
defined within the same orbit, which is not the case here. Hence,
while it is acceptable to assign slots based on Mean Anomaly
differences, minimum included zenith angle (the angle between two
satellites measured from the center of the earth) is a more
accurate measure of satellite separation than True Anomaly, since
it also accounts for orbit plane separation angles.
[0100] FIG. 14 illustrates the variation in included zenith angle
with time between two adjacent satellites spaced at 2 degrees at
apogee. The time spent in the HLSA is in the flat low-valued
region, whereas the peaks represent satellite passage through
perigee, when they separate widely and inactive.
[0101] True Anomaly, which is related to included zenith angle,
cannot be calculated from Mean Anomaly, since Mean Anomaly is
expressed as a transcendental function of true anomaly, and the
equation cannot be inverted to solve for TA. Numeric techniques are
often used for this purpose, and were used (via orbital analysis
software) to derive the values in included zenith angle in this
description.
[0102] Although only a few embodiments have been disclosed in
detail above, other modifications are possible.
[0103] All such modifications are intended to be encompassed within
the following claims, in which:
* * * * *