Patent Document

This application 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 U.S. patent application Ser. No. 09/433,849, filed Nov. 4, 1999, now issued as U.S. Pat. No. 6,442,385. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a method and apparatus for selectively operating tundra orbit satellites in a satellite broadcast system. 
     BACKGROUND OF THE INVENTION 
     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. 
     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. 
     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 f 1  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 f 2  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. 
     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). 
     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 . 
     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. 
     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. 
     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. 
     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. 
     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 
     In accordance with the present invention, satellites in a satellite broadcast system are operated in separate tundra orbits 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 satellites are selectively switched based on their position with respect to the ground track. For example, the satellites are selectively switched at or near the crossover point such that when each satellite is in an orbital position corresponding to a point along the northern loop, the satellite provides a late satellite signal. Each satellite is switched from late to early satellite operation mode when in orbital positions that are both above the equator and correspond to the southern loop of the ground track. Alternatively, the satellites are 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). 
     In accordance with another aspect of the present invention, at least three satellites are each operated in tundra orbits which traverse a common figure-eight ground track comprising northern and southern loops. The satellites are in three orbital planes spaced essentially evenly about the earth (e.g., approximately 120 degrees apart), resulting in an eight hour orbital position separation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a block diagram of a conventional time diversity satellite broadcast system; 
         FIG. 2  illustrates orbital elements of a satellite constellation; 
         FIG. 3  illustrates a ground track corresponding to satellites in a tundra orbit and with respect to an exemplary geographic region; 
         FIG. 4  illustrates components of an exemplary ground track for a satellite in a tundra orbit; 
         FIG. 5  illustrates a crossover situation for satellites in tundra orbits with a common ground track; 
         FIG. 6  illustrates a switchover situation for satellites in tundra orbits with a common ground track; 
         FIG. 7  is a block diagram of a time diversity satellite broadcast system constructed in accordance with an embodiment of the present invention; 
         FIG. 8  is a graph illustrating elevation angles for satellites in tundra orbits with respect to each other; and 
         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. 
     
    
    
     Throughout the drawing figures, like reference numerals will be understood to refer to like parts and components. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     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. 
     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. 
     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 f 1  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 f 2  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 . 
     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. 
       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. 
       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. 
       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 . 
     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 . 
     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 . 
     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 . 
     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 . 
     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.

Technology Category: h