U.S. patent application number 12/216391 was filed with the patent office on 2008-10-30 for method and apparatus for selectively operating satellites in tundra orbits.
Invention is credited to Paul D. Marko.
Application Number | 20080268837 12/216391 |
Document ID | / |
Family ID | 23721775 |
Filed Date | 2008-10-30 |
United States Patent
Application |
20080268837 |
Kind Code |
A1 |
Marko; Paul D. |
October 30, 2008 |
Method and apparatus for selectively operating satellites in tundra
orbits
Abstract
A satellite system provides geosynchronous satellites in
elliptical orbits in respective elliptical orbital planes separated
by 120 degrees. The satellites traverse a common figure-eight
ground track comprising northern and southern loops. The satellites
are controllably switched to operate the satellite currently
traversing the northern loop to deliver a selected signal (e.g., a
selected frequency signal) to satellite receivers.
Inventors: |
Marko; Paul D.; (Pembroke
Pines, FL) |
Correspondence
Address: |
ROYLANCE, ABRAMS, BERDO & GOODMAN, L.L.P.
1300 19TH STREET, N.W., SUITE 600
WASHINGTON,
DC
20036
US
|
Family ID: |
23721775 |
Appl. No.: |
12/216391 |
Filed: |
July 3, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11543916 |
Oct 6, 2006 |
7406311 |
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12216391 |
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10171619 |
Jun 13, 2002 |
7136640 |
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11543916 |
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09433849 |
Nov 4, 1999 |
6442385 |
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10171619 |
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Current U.S.
Class: |
455/428 |
Current CPC
Class: |
H04B 7/18534 20130101;
H04B 7/195 20130101 |
Class at
Publication: |
455/428 |
International
Class: |
H04Q 7/20 20060101
H04Q007/20 |
Claims
1. A method for controlling first and second geosynchronous
satellites in elliptical orbits in respective orbital planes in a
satellite communication system, the satellite communication system
providing at least one of a first frequency signal and a second
frequency signal to receivers, the satellites each traversing a
common ground track having a northern loop, a southern loop, and a
crossover point between the northern and southern loops, the method
comprising the steps of: selecting said first frequency satellite
signal to be transmitted from whichever of said first and second
geosynchronous satellites is approaching and traversing said
northern loop; monitoring the orbital positions of said first and
second geosynchronous satellites with respect to entering said
northern loop and exiting said northern loop; switching via a
satellite command system said first frequency signal to one of said
first and second geosynchronous satellites as said satellite
approaches said northern loop; powering down said first and second
geosynchronous satellites when they are descending said southern
loop below the equator; and powering on said first and second
geosynchronous satellites when they are ascending said southern
loop from the equator.
2. A method as claimed in claim 1, further comprising the step of
transmitting said second frequency satellite signal from a third
geosynchronous satellite.
Description
[0001] This application is a continuation of U.S. patent
application Ser. No. 11/543,916, filed Oct. 6, 2006, which is a
continuation of U.S. patent application Ser. No. 10/171,619, filed
Jun. 13, 2002 (now issued as U.S. Pat. No. 7,136,640), which is a
continuation of Ser. No. 09/433,849, filed Nov. 4, 1999 (now issued
as U.S. Pat. No. 6,442,385).
FIELD OF THE INVENTION
[0002] The invention relates to a method and apparatus for
selectively operating tundra orbit satellites in a satellite
broadcast system.
BACKGROUND OF THE INVENTION
[0003] Radio frequency transmissions are often subjected to
multipath fading. Signal blockages at receivers can occur due to
physical obstructions between a transmitter and the receiver or
service outages. For example, mobile receivers encounter physical
obstructions when they pass through tunnels or travel near
buildings or trees that impede line of sight (LOS) signal
reception. Service outages can occur, on the other hand, when noise
or cancellations of multipath signal reflections are sufficiently
high with respect to the desired signal.
[0004] Communication systems can incorporate two or more
transmission channels for transmitting the same program or data to
mitigate the undesirable effects of fading or multipath. For
example, a time diversity communication system delays the
transmission of program material on one transmission channel by a
selected time interval with respect to the transmission of the same
program material on a second transmission channel. The duration of
the time interval is determined by the duration of the service
outage to be avoided. The non-delayed channel is delayed at the
receiver so that the two channels can be combined, or the program
material in the two channels selected, via suitable receiver
circuitry. One such time diversity system is a digital broadcast
system (DBS) employing two satellite transmission channels.
[0005] With reference to FIG. 1, a DBS 10 with time diversity is
shown. An uplink facility comprises a splitter 12 for providing
multiple channel time division multiplexed (TDM) content 11 to each
of two transmission channels 14 and 16. The first transmission
channel 14 is transmitted to a first satellite 20 at a first
frequency f1 via uplink components indicated at 18. The second
transmission channel 16 is delayed by a selected time interval, as
indicated at 22, prior to being transmitted to a second satellite
24 at a second frequency f2 via uplink components indicated at 26.
A dual arm receiver receives the early and late signals from the
satellites 20 and 24, respectively, at a downconverter 28. A delay
unit 30 delays the early signal from the satellite 20 via a time
interval corresponding to the time interval used to delay the
second transmission channel at the transmitter. The delay is
applied to all of the channels in the multiple channel TDM content
11. The delayed output from the delay unit 30 can then be
synchronized with the late signal and combined, as indicated at 32.
A channel selector 34 extracts content corresponding to a
particular one of the channels in the multiple channel TDM content
in response to a user input, for example.
[0006] In a particular implementation of a DBS with time diversity,
three satellites 20, 24 and 36 operate in respective ones of tundra
orbits 50, 52 and 54, as illustrated in FIG. 2. In other words, the
satellites 20, 24 and 36 are in respective ones of three inclined,
elliptical orbits which are each separated by approximately 120
degrees. The combination of the 120 degree separation and the
rotation of the earth yields a common ground track 60 for all three
orbits which is illustrated in FIG. 3. In addition to an
approximately 120 degree spatial separation, the orbits 50, 52 and
54 are temporally separated by T/3 or one-third of an orbit period
T (e.g., one-third or eight hours of a 24 hour geosynchronous
orbit).
[0007] With continued reference to FIG. 3, the satellite ground
track 60 is a figure-eight, having a northern loop 62 that is
smaller than the southern loop 64. The northern and southern loops
62 and 64 share a common ground track point hereinafter referred to
as the crossover point 66, as shown in FIG. 4. At the crossover
point, satellites descending from the northern loop 62 to the
southern loop 64 have the same orbital position as satellites
ascending from the southern loop 64 to the northern loop 62. Each
satellite 20, 24 and 36 spends approximately one-third (e.g., eight
hours) of its orbit time south of the equator 68 and,
correspondingly, two-thirds (e.g., sixteen hours) of its orbit time
north of the equator. Thus, when one satellite 20 is at perigee, as
shown in FIG. 5, the subsatellite points of the other two
satellites 24 and 36 cross paths and are therefore in the same sky
position at the crossover point 66.
[0008] As indicated in FIG. 6, when one satellite 36 is at apogee,
the other two satellites 20 and 24 are at essentially equal
latitude near the equator 68. Of these two satellites, (e.g.,
satellites 20 and 24 in FIG. 6), one satellite 20 appears to be
rising in the southeast, while the other satellite 24 appears to be
setting in the southwest. The rising satellite commences
transmitting, while the setting satellite ceases transmitting to
comply with international coordination and interference concerns
with respect to the allocation of bandwidth for satellite
operations. By symmetry of the elliptical orbit, this situation of
two satellites at nearly the same latitude occurs halfway through
an orbit following the time of perigee, that is, at time T/2 (e.g.,
24/2 or 12 hours) past perigee.
[0009] In a time diversity system as described above in connection
with FIG. 1, the satellites 20, 24 and 36 operate as either the
"early" satellite (i.e., the satellite transmitting the nondelayed
channel 14) or the "late" satellite (i.e., the satellite
transmitting the delayed channel 16), depending on the position of
the satellite along the satellite ground track 60. For example,
when the satellites 20, 24 and 36 are located along the ground
track 60 as depicted in FIG. 6, the satellite 20 is the late
satellite for illustrative purposes and is switched on shortly
after it ascends past the equator along the southern loop 64.
Correspondingly, the satellite 24 is switched off for its travel
along the portion of the southern loop 64 that is below the equator
68. The satellite 36 is the early satellite.
[0010] When each satellite commences its ascent north of the
equator along the southern loop 64, the satellite is switched from
"early" to "late", or "late" to "early", depending on its "early"
or "late" status during its traverse of the previous northern loop
62. Thus, the "early" or "late" status of a satellite changes in an
alternate manner after the completion of the period during which
the satellite is switched off, that is, while traversing the
southern loop 64 when the orbital position of the satellite is at a
latitude below the equator 68. Accordingly, in the previous
example, when the late satellite 36 reaches a latitude near the
equator while descending in the southern loop 64, the early
satellite 20 is at apogee, and the satellite 24 is switched on and
is commencing its ascent above the equator, approximately eight
hours later. The satellite 36 is therefore switched off and the
satellite 24 is the late satellite. The uplink components 18 and 26
are each controlled using data from a telemetry, tracking and
command (TTC) system 27 which monitors and controls the flight
operations of the satellites 20, 24 and 36, as shown in FIG. 1. In
accordance with this TTC system data, the uplink components 18 and
26 are commanded to transmit the content on the transmission
channels 14 and 16, respectively, to the selected ones of the
satellites, depending on their orbital positions and corresponding
positions along the ground track 60. Each satellite is capable of
receiving either of the frequencies corresponding to the late or
early satellite signals as commanded by the TTC system.
[0011] In view of the above-described system for operating early
and late satellites in tundra orbits, a compromise exists between
the elevation angle and the availability of spatial and/or time
diversity. When elevation angles to one or two satellites are
greatest, at least one method of diversity is less available. This
tradeoff situation is presented every T/3 or eight hours where T is
a 24 hour orbit period. For example, in the crossover situation
depicted in FIG. 5, one satellite 20 is at perigee and is not
visible from locations in the United States. The other two
satellites 24 and 36 are in essentially the same position in the
sky. No spatial diversity is available at such orbital positions
for approximately one hour, although time diversity is available.
In the switchover situation depicted in FIG. 6, two satellites have
nearly the same elevation angle, but different azimuths. The
elevation angle for these rising and setting satellites 20 and 24,
respectively, is nearly as small as the minimum elevation angle for
any satellite visible at that location during the orbit period. The
elevation angle of the third satellite 36, however, is the greatest
elevation angle for that United States location. Since the setting
and rising satellites 24 and 20 are relatively low with respect to
the horizon, the rising satellite that is switched on is likely to
be obscured by terrestrial obstruction. Thus, a reduced
availability of spatial and time diversity exists at such times.
This situation exists for approximately one hour and occurs
approximately every eight hours. For places in the eastern United
States, this situation begins prior to the switchover described
with reference to FIG. 6, whereas the situation commences after
switchover for places in the western United States.
[0012] The tradeoff situations described above emphasize the
importance of time diversity. The receiver, as stated previously,
stores all of the channels in the multi-channel TDM content signal
11 for a selected period of time. Thus, if both of the satellites
are obstructed momentarily, the signal 11 can be recovered from the
delayed portion of early received signal. Additionally, since the
output of the signal combiner 32 contains the combined early and
late signals from all of the channels, the user may change the
channel selector 34 and immediately receive the new channel
contents from the combined TDM signal. Such storage, however,
requires significant memory which increases the cost of the
receiver. A need therefore exists for a satellite broadcast system
which reduces the memory requirements of the receiver in a time
diversity satellite broadcast system. A need also exists for a
satellite broadcast system that selectively switches signals
transmitted from satellites in selected tundra orbit positions to
improve reception of the signals (e.g., by increasing elevation
angle).
SUMMARY OF THE INVENTION
[0013] In accordance with the present invention, first and second
geosynchronous satellites are operated in elliptical orbits in
respective orbital planes and follow a common figure-eight ground
track having northern and southern loops connected via a crossover
point, that is, each satellite traverses the crossover point when
in orbital positions corresponding to the descent of the satellite
from the northern loop to the southern loop and to the ascent of
the satellite from the southern loop to the northern loop. The
first and second satellites are selectively switched based on their
position with respect to the ground track. For example, the
satellites are selectively switched at or near (e.g. approaching)
the crossover point such that when each satellite is in an orbital
position corresponding to a point along the northern loop or near
the crossover point, the satellite provides a first frequency
signal. Each satellite is powered down when below the equator. The
satellites can be selectively switched to improve reception of a
signal of a particular frequency (e.g., to transmit a selected
frequency signal from whichever satellite is traversing the
northern loop).
[0014] In accordance with another aspect of the present invention,
a third geosynchronous satellite can transmit a second frequency
satellite signal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The various aspects, advantages and novel features of the
present invention will be more readily comprehended from the
following detailed description when read in conjunction with the
appended drawings, in which:
[0016] FIG. 1 is a block diagram of a conventional time diversity
satellite broadcast system;
[0017] FIG. 2 illustrates orbital elements of a satellite
constellation;
[0018] FIG. 3 illustrates a ground track corresponding to
satellites in a tundra orbit and with respect to an exemplary
geographic region;
[0019] FIG. 4 illustrates components of an exemplary ground track
for a satellite in a tundra orbit;
[0020] FIG. 5 illustrates a crossover situation for satellites in
tundra orbits with a common ground track;
[0021] FIG. 6 illustrates a switchover situation for satellites in
tundra orbits with a common ground track;
[0022] FIG. 7 is a block diagram of a time diversity satellite
broadcast system constructed in accordance with an embodiment of
the present invention;
[0023] FIG. 8 is a graph illustrating elevation angles for
satellites in tundra orbits with respect to each other; and
[0024] FIGS. 9, 10, 11 and 12 illustrate ground track positions of
three satellites at different times during an orbital period and
their selection as early and late satellites in a time diversity
system in accordance with an embodiment of the present
invention.
[0025] Throughout the drawing figures, like reference numerals will
be understood to refer to like parts and components.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0026] With reference to FIG. 7, a time diversity satellite
broadcast system 100 is provided having at least three satellites
102, 104 and 106 in a non-geostationary constellation. Each
satellite 102, 104 and 106 is preferably placed in a tundra orbit
whereby each satellite is in an elliptical orbit inclined 63.4
degrees relative to the equator. Each satellite 102, 104 and 106 is
preferably geosynchronous with a nominal 24-hour period. In
addition, the three orbit planes for the satellites 102, 104 and
106 are spaced evenly about the earth by approximately 120 degree
increments, as illustrated by satellites 20, 24 and 36 in FIG. 2,
resulting in approximately an eight hour orbital position
separation. In accordance with the present invention, two of the
three satellites 102, 104 and 106 are selectively operated in early
and late satellite service modes to transmit to, for example, the
48 contiguous United States coverage area at any time.
[0027] The satellites 102, 104 and 106 follow a common ground track
such as the ground track 60 illustrated in FIG. 3. As stated above,
satellite ground track 60 is a figure-eight, having a northern loop
62 that is smaller than the southern loop 64. The northern and
southern loops 62 and 64 share a crossover point 66, as shown in
FIG. 4. Each satellite 102, 104 and 106 spends approximately
one-third (e.g., eight hours) of its orbit time south of the
equator 68. Each satellite is preferably switched off during this
time period. Each satellite spends two-thirds (e.g., sixteen hours)
of its orbit time north of the equator 68. In addition, each
satellite spends eight of those sixteen hours in the smaller
northern loop 62. In addition, the orbital positions of a satellite
that correspond to the northern loop 62 of the ground track 60
provide the highest elevation angles when compared to orbital
positions corresponding to the southern loop 64. As shown in FIG.
8, the satellites 102, 104 and 106 achieve maximum elevation angles
for respective eight hour periods in each 24-hour orbital period.
The present invention takes advantage of these eight hour periods
of improved elevation angles by switching the satellite 102, 104 or
106 that is entering the northern loop of its ground track to late
satellite operation. Correspondingly, the satellite that is in the
southern loop 64 of the ground track 60 and is above the equator is
operated as the early satellite.
[0028] With continued reference to FIG. 7, a transmitter or uplink
center 110 in the system 100 provides a signal such as a
multi-channel TDM content signal 112 to a splitter 114. The
splitter 114, in turn, provides the signal to each of two
transmission channels 116 and 118. The first transmission channel
116 is transmitted at a first frequency f1 via an uplink component
indicated at 120. The second transmission channel 118 is delayed by
a selected time interval, as indicated at 122, prior to being
transmitted to a second satellite at a second frequency f2 via an
uplink component indicated at 124. A TTC unit 126 is provided which
tracks the flight operations of the satellites 102, 104 and 106.
Data from the TTC unit 126 is used to direct the dish 128
associated with the uplink component 120 and the dish 130
associated with the uplink component 124 to the satellite
traversing the southern loop 64 (i.e., when the satellite is above
the equator) and the satellite traversing the northern loop 62,
respectively, of the ground track 60.
[0029] The satellites are depicted in exemplary ground track
positions in FIGS. 9, 10, 11 and 12 for illustrative purposes. In
FIG. 9, the satellite 102 is ascending the southern loop 64 from
the equator 68 and is powered on. The satellite 104 is at apogee
and operated as the late satellite in accordance with the present
invention. The satellite 106 is descending the southern loop 64
below the equator 68 and is therefore being powered down. Prior to
reaching an orbital position near the equator, the satellite 106 is
operated as the early satellite in accordance with the present
invention.
[0030] FIG. 10 depicts the ground track positions of the satellites
102, 104 and 106 in the illustrated example after four hours of the
24-hour orbital period have elapsed since the positions depicted in
FIG. 9. Once the satellite 102 reaches the crossover point 66, the
uplink component 124 is commanded using data from the TTC unit 126
to re-point its beam from the satellite 104, which has now also
reached the crossover point 66, to the satellite 102 to operate the
satellite 102 as the late satellite while it traverses the northern
loop 62. Correspondingly, the uplink component 120 is commanded to
re-point its beam from the satellite 102 to the satellite 104 to
switch its operation from late to early satellite operation.
[0031] FIG. 11 depicts the ground track positions of the satellites
102, 104 and 106 in the illustrated example after another four
hours (i.e., a total of eight hours) of the 24-hour orbital period
have elapsed since the positions depicted in FIG. 9. The satellite
106 is powered on when it commences its ascent of the southern loop
64 above the equator 68. The early satellite 104 is powered down
below the equator 68. The satellite 102 continues to operate as the
late satellite which provides better elevation angles than the
other two satellites.
[0032] FIG. 12 depicts the ground track positions of the satellites
102, 104 and 106 in the illustrated example after another four
hours (i.e., a total of twelve hours) of the 24-hour orbital period
have elapsed since the positions depicted in FIG. 9. The uplink
components 120 and 124 are commanded using data from the TTC unit
126 to re-point their beams to satellites 102 and 106 to operate
the satellites 102 and 106 as the early and late satellites,
respectively. As stated above, the satellite 104 is powered down at
latitudes below the equator 68.
[0033] As can be seen from the illustrated example in FIGS. 9-12,
the satellite traversing the northern loop 62 of the ground track
60 is operated as the late satellite for as many as eight hours
until the next satellite commences the northern loop 62 of the
ground track 60. When the satellites are in orbital positions
corresponding to the northern loop 62, they have favorable
elevation angles for minimizing the effects of line of sight
obstructions, multipath fading and foliage attenuation of the
received signal at the receiver 140 in FIG. 7.
[0034] In accordance with yet another aspect of the present
invention, a satellite receiver tunes to the late satellite signal.
Since the satellite that is in the orbital positions corresponding
to the northern loop of the ground track is selected to be the late
satellite, the satellite has improved elevation angles and is
therefore less likely to be subjected to line of sight obstruction,
multipath fading and foliage attenuation. Accordingly, the receiver
can employ a relatively small buffer for storing the early
satellite signal for a selected channel in the multi-channel TDM
content signal for a predetermined period for combining purposes.
This is in contrast with conventional receivers that store all
channels in the early satellite signal for a selected period of
time prior to channel selection, as indicated at 30 in FIG. 1.
[0035] As shown in FIG. 7, a dual arm receiver is tuned to receive
the early and late signals from the early and late satellites,
respectively. The received signals are downconverted by a
downconverter 142. A channel selector 144 extracts a selected one
of the channels in the received signals. The channel selector 144
can operate, for example, in response to a user input. The early
signals for the selected channel are provided to a delay unit 146
which needs only be configured to store data from a single channel
for a period of time corresponding to the delay imposed on the
content 112 by the delay unit 122 in the transmitter 110. The late
signals for the selected channel are provided to a signal combiner
148, along with the output of the delay unit 146, and then combined
using one or more diversity combining methods to generate a user
signal 150.
[0036] The need to store all channels at the receiver for a
selected amount of time, as explained in connection with
conventional receivers and FIG. 1, is eliminated by the switching
operation of the late and early satellites described herein.
Referring to FIG. 7, when a new channel is selected via the channel
selector 144, the early signal is applied to the delay buffer 146
at the output of the channel selector, while the late signal is
simultaneously applied to the signal combiner 148. Since it is
unlikely that the late satellite signal is not received (i.e.,
since it is transmitted from a satellite at a high elevation
angle), the output of the signal combiner immediately provides the
new channel contents to the user based on the late signal
availability. If the signal from the lower elevation early
satellite was available at the output of the channel selector, it
exits the delay block 146 and is available to the signal combiner
148 for combination with the late signal after the delay period has
elapsed. The advantage of increasing the late signal availability
is evident when considering the example of early satellite signal
availability only. Under this condition with the receiver in FIG.
7, the early signal will not be available at the signal combiner
148 when a new channel is selected until after the early signal
exits the delay block 146. This results in an interruption of
service for the period of the delay block. In the system described
in FIG. 1, all of the channels must be stored (e.g., as an early
signal for a selected period of time) to overcome latency problems
such as the interruption in service that can occur when the channel
is changed, and the late channel is obstructed or severely faded.
Such conditions have a high probability of occurring when the late
satellite is transmitting from lower elevations along the lower
loop 64.
[0037] Although the present invention has been described with
reference to preferred embodiments thereof, it will be understood
that the invention is not limited to the details thereof. Various
modifications and substitutions have been suggested in the
foregoing description, and others will occur to those of ordinary
skill in the art. All such substitutions are intended to be
embraced within the scope of the invention as defined in the
appended claims.
* * * * *