Abstract:
Various embodiments provide a satellite communication system including a first transceiver and a second transceiver geographically fixed on the earth, and a first satellite configured to communicate with the first transceiver through a first link. The system further includes a second satellite configured to communicate with the second transceiver through a second link and communicate with the first satellite through a laser communication crosslink. The first satellite and the second satellite are at a low earth orbit or medium earth orbit below the geostationary earth orbit of approximately 36000 km.

Description:
BACKGROUND 
       [0001]    This disclosure pertains generally to networked communication satellite systems and in particular to an extensible high bandwidth global space communication satellite system. 
         [0002]    Satellite constellations are used as a network of a plurality communication satellites to provide coordinated ground coverage. The plurality of networked satellites operate together under shared control and are synchronized so that their communication coverage may overlap to complement each other while minimizing interference between the satellites&#39; coverage. 
         [0003]    Low earth orbiting satellites (LEDs) are often deployed in satellite constellations. Because the coverage area provided by a single LEO satellite only covers a small area that moves as the satellite travels at high angular velocity needed to maintain its orbit, a plurality of LEO satellites are often needed to maintain continuous coverage over a larger area. Examples of satellite constellations include the Global Positioning System (GPS), Galileo and GLONASS constellations for navigation and geodesy, the Iridium® and Globalstar satellite telephony services, the Disaster Monitoring Constellation and RapidEye for remote sensing, the Orbcomm messaging service, Russian elliptic orbit Molniya and Tundra constellations, the large-scale Teledesic and Skybridge broadband constellation proposals of the 1990s, and the proposed LEO global backhaul constellation named COMMStellation™. 
         [0004]    One benefit of using LEO satellites is the ability to provide low latency broadband telecommunications compared to geostationary earth orbit (GEO) satellites. Indeed, the latency of Earth-to-LEO satellite communication or LEO satellite-to-earth communication is about 1 ms to 5 ms, whereas the one-way latency of Earth-to-GEO satellite communication is about 120 ms. A LEO satellite constellation can also provide more system capacity by frequency reuse across its coverage, with spot beam frequency use being similar to the frequency reuse of cellular radio towers. 
         [0005]    Medium Earth orbit (MEO) satellite constellations may also provide a solution for overcoming the latency challenge in GEO constellations. MEO is the region of space around the Earth above low Earth orbit (altitude of 2,000 km) and below geostationary orbit (altitude of 35,786 km). The most common use for satellites in this region is for navigation, such as the GPS Glonass, and Galileo constellations. 
         [0006]    Presently, there are various conventional commercial type satellite communication systems. One satellite communication system intended for the internet is Teledesic which has never been fully implemented. Teledesic successfully launched only one satellite. Another satellite communication system intended for voice and data communication is Iridium® which has a total of 66 satellites. Iridium® is a LEO satellite constellation. Iridium® has low latency communication and supports inter-satellite links only between satellites orbiting in the same direction using RF based cross-links. The cross-links have a limited bandwidth capacity which can be sufficient for voice, but insufficient for real-time video feed. 
         [0007]    GEO satellite constellations such as the European Data Relay Satellite (EDRS) system are based on geostationary earth orbit satellites which are at a higher altitude from the earth&#39;s surface. EDRS satellites relay information between satellites and ground stations. Certain GEO constellations utilize a laser cross-link to provide communication between satellites. However, due to the position of the satellites at relatively higher altitude (about 36000 km above sea level) the communication latency from a ground base station to the satellite is about 120 ms for a one-way trip. Furthermore, deploying GEO satellites is far more expensive than deploying LEO or MEO satellites. 
         [0008]    Prior satellite communication systems have not been able to support a relatively broad communication bandwidth while providing a real-time, extendible or reconfigurable communication network. 
         [0009]    With the increasing need for satellite communications, there is a persistent need in the art for a system and method for providing an extensible high bandwidth communication satellite network. 
       SUMMARY 
       [0010]    One or more embodiments of the present disclosure provide a satellite communication system including a first transceiver and a second transceiver geographically fixed on the earth, and a first satellite configured to communicate with the first transceiver through a first link. The system further includes a second satellite configured to communicate with the second transceiver through a second link and communicate with the first satellite through a laser communication crosslink. The first satellite and the second satellite are at a low earth orbit or medium earth orbit below the geostationary earth orbit of approximately 36000 km. 
         [0011]    These and other features and characteristics of the present disclosure, as well as the methods of operation and functions of the related elements of structure and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures. In one embodiment of this disclosure, the structural components illustrated herein are drawn to scale. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the inventive concept. As used in the specification and in the claims, the singular form of “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    In the accompanying drawings: 
           [0013]      FIG. 1  is a schematic block diagram showing a satellite communication system, according to one embodiment; 
           [0014]      FIG. 2  is a block diagram of components of a satellite of satellite system shown in  FIG. 1 , according to one embodiment; 
           [0015]      FIG. 3  shows a constellation of plurality satellites over the terrestrial globe, according to one embodiment; 
           [0016]      FIG. 4  is a diagram showing communication channels between a plurality of transceivers on unmanned aerial vehicles and a satellite, according to one embodiment; 
           [0017]      FIG. 5  is a schematic diagram showing a path of a satellite in relation to a plurality of ground base stations, according to one embodiment; 
           [0018]      FIG. 6A  depicts Other-Three-Billion (O3b)-like constellation of satellites, according to a one embodiment; 
           [0019]      FIG. 6B  depicts a sun-synch constellation of satellites, according to another embodiment; and 
           [0020]      FIG. 6C  depicts an Iridium®-like constellation of satellites, according to yet another embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]      FIG. 1  is a schematic block diagram showing a satellite communication system, according to one embodiment. Satellite communication system  10  comprises plurality of satellites  12 . Although two satellites  12 A and  12 B are shown in  FIG. 1 , any number of satellites  12  can be employed in satellite communication system  10 . Satellite communication system  10  is configured to provide real time, global communication access, from multiple areas in a large region of the globe or around the globe as a whole. Satellite communication system  10  further includes one or more ground based transceiver or ground base station  14 . For example, in one embodiment, one or more ground based transceivers  14  can be configured to send (uplink) or receive data from one or more of the plurality of satellites  12 . One or more ground based transceivers  14  can be positioned in multiple area of a large region of the globe so as to be able to communicate with satellite  12 B for increased communication coverage. One or more ground based transceivers  14  can be in communication with one, two or more satellites  12 . For example, as shown in n  FIG. 1 , ground based transceiver or ground base station  14  is shown receiving data from satellite  12 A via downlink  13 . Satellite communication system  10  further comprises one or more transceivers  16 . In one embodiment, transceivers  16  are provided on airborne assets  16 A such as, but not limited to, unmanned airborne vehicles (UAVs) or other types of aircraft. However, one or more transceivers  16  can also be positioned on any moveable vehicle or provided stationary on the ground surface. As shown in  FIG. 1 , one or more plurality of airborne assets (e.g., UAVs)  16  are shown transmitting data to satellite  12 B via links  15 . 
         [0022]    In general, UAVs  16  are provided with antennas configured to acquire and track a moving LEO satellite  12  (e.g., satellite  12 B). Once the LEO satellite  12  is out of view, the antennas of UAV  16  acquire and track the next viewable LEO satellite  12 . In the case of a GEO satellite, the UAVs  16  are also provided with antennas for acquiring and tracking the GEO satellite. However, because the GEO satellite is stationary, the antenna of the transceiver on UAV  16  only acquires and tracks one satellite (the GEO satellite) which is stationary. However, due to considerably higher latency for the GEO systems, the GEO system may not be the appropriate configuration for real-time data transmission. 
         [0023]    Satellite system  10  is also configured so that plurality of satellites  12  communicate with each other through cross-links. For example, as shown in  FIG. 1 , satellite  12 A is depicted in communication with satellite  12 B through crosslink  17 . One possible communication scenario would be that, for example, airborne assets  16  send data to satellite  12 B via uplink  15 . Satellite  12 B in turn relays the received data to satellite  12 A through crosslink  17 . Satellite  12 A then transmits the data received from satellite  12 B to ground based station  14  through downlink  13 . Although  FIG. 1  depicts the use of two satellites, three or more satellites  12  can be used, in which case a plurality of cross-links  17  can be used to transmit information or data from the satellite that receives the uplink to the satellite that sends downlinks the data to the ground base station. 
         [0024]    In one embodiment, the communication from transceivers  16  on airborne assets  16 A to satellite  12 B or from satellite  12 A to ground based station  14  can be performed in real time, i.e., with low latency less than 100 ms. In one embodiment, satellite communication system  10  supports a relatively broad communication bandwidth, greater than about 1 gigabit per second (1 Gbps) in uplink  15  or downlink  13  to provide relatively large sets or streams of data to be uplinked from, for example, transceiver  16 A on airborne asset  16  to satellite  12 B and downlinked from, for example, satellite  12 A to ground base station  14 . 
         [0025]    In one embodiment, the satellite communication system  10  is extendible or reconfigurable in that a constellation of satellites in orbit can be upgraded without any lost connectivity, i.e., provides a seamless extensibility or seamless upgrade. In one embodiment, the constellation can be extended or reconfigured or upgraded by adding one or more satellites  12  either for redundancy by filling in nodes of the network or can be software upgraded on orbit by uploading desired software or hardware. In one embodiment, for example, the added satellites can be provided with more capable equipment for increasing data rates to improve the overall bandwidth of the network. 
         [0026]    In one embodiment, the communication crosslink  17  between satellite  12 A and  12 B is a laser communication link. In one embodiment, two different wavelengths can be used for sending and for receiving so as prevent any ambiguity and/or to prevent cross-talk. In one embodiment, a single wavelength can be used but with different polarizations for sending and receiving. In one embodiment, the two wavelengths are proximate to each other. However, in another embodiment, the two wavelengths may be widely separated when desired. In one embodiment, one laser wavelength is around 1.550 μm and the other laser wavelength is around 1.564 μm. However, as it can be appreciated, any practical and convenient wavelengths can be used. In one embodiment, the communication data rate or bandwidth is about 10 gigabits per second for the crosslink  17 . However, the communication data rate or bandwidth can also be increased to 40 gigabits per second or more. Alternatively, the communication data rate can also be lower than 10 gigabits per second if desired. In one embodiment, the laser communications crosslink  17  can employ a plurality of wavelengths for sending and a plurality of wavelengths for receiving. For example, this can be done to increase the data rate by means of wavelength division multiplexing. Uplink  15  from transceiver  16  on airborne asset  16 A to satellite  12 A, and downlink  13  from satellite  12 A to ground base station  14  are radio frequency (RF) based communication channels. In one embodiment, each satellite  12  receives over 1 gigabits per second at a frequency of about 29 GHz for one uplink  15 . Therefore, for four uplinks  15  (e.g., 4 UAVs uplinking data to satellite  12 B), each link would have a data rate or bandwidth of about 274 Mbps. In another embodiment, the data rate or bandwidth of each satellite can be extended above (4×274 Mbps) to, for example, 16×274 Mbps by providing satellites with more power. 
         [0027]    Ground base station  14  includes antenna  14 A. Antenna  14 A is relatively large (e.g., about 12 m in diameter). Antenna  14 A can be arranged or oriented to allow communication with multiple orbit planes of satellites simultaneously. In one embodiment, downlink  13  to the ground base station  14  can have a data rate or bandwidth of about one gigabit per second at an RF frequency of about 20 GHz. However, the data rate or bandwidth can be doubled (2×1 Gbps), for example, by using dual polarization, e.g., left hand circular polarization (LHCP) and right hand circular polarization (RHCP). 
         [0028]    Although downlink  13  and uplink  15  are described in the above paragraph as being RF based links, downlink  13  and uplink  15  are not limited to RF but can encompass other wavelengths of the electromagnetic spectrum. For example, alternatively or in addition to the RF uplink and RF downlink, laser communication uplinks and downlinks can also be provided, for example, in the similar manner as for crosslink  17 . 
         [0029]    In one embodiment, satellite system  10  further includes ground control command station  18 . Ground control station  18  sends data up or uplinks data via uplink  19  to which ever one of satellites  12  (e.g., satellite  12 A or satellite  12 B) ground station  18  can communicate at the time. The data is then distributed over the network of satellites  12  by using crosslink  17  between satellites  12 . 
         [0030]    In one embodiment, ground control station  18  would determine what is the routing path for transmitting data from a satellite receiving data from a UAV (e.g., satellite  12 B) to the satellite (e.g., satellite  12 A) downlinking the data to the ground base station based on the current data communication loads through the network. Ground control station  18  can specify within the uplinked data a path to route through between satellites  12 . In another embodiment, ground control station  18  may not be needed. In which case, satellite system  10  may be provided the control function for example by embedding the control function in satellites  12 . In this case, system  10  may monitor itself by, for example, each satellite  12  communicating its status to neighboring satellites  12 , and re-route or select an appropriate communication path in real time. 
         [0031]      FIG. 2  is a block diagram of a satellite of satellite system  10 , according to one embodiment. Each satellite  12  (e.g., satellite  12 A or satellite  12 B) has plurality of RF receivers (RF Rx)  20  and one RF transmitter (RF Tx)  22 .  FIG. 2  shows that each satellite  12  has four RF receivers. However, one or more RF receivers can be provided in satellite  12 . Satellite  12  further comprises plurality of laser communication (lasercom) transceiver  24 . In this embodiment, each satellite  12  has six laser communication transceivers  24 . However, each satellite  12  can be provided with two or more transceivers  24  (e.g., for example four laser communication transceivers). In one embodiment, the plurality of RF receivers  20  in satellite  12 B are configured to receive data from the plurality of transceivers  16  on airborne assets (e.g., UAVs)  16 A. One of the laser communication transceivers  24  of satellite  12 B is configured to transmit the received data to one of the laser communication transceivers  24  of satellite  12 A. RF transmitter  22  of the satellite  12 A is configured to transmit the data to ground base station  14 . 
         [0032]    Each satellite  12  further includes processor  26  configured to control RF receivers  20 , RF transmitter  22 , and laser communication transceivers  24  and to control routing of data received by RF receivers  20  to RF transmitter  22  for transmitting to desired destinations through the network. Network data processor  26  can also be configured to operate at higher network protocol layers, performing such functions as Layer  2  switching, Layer  3  Internet Protocol routing, and quality-of-service management. 
         [0033]      FIG. 3  shows a constellation of satellites  12  over the terrestrial globe, according to one embodiment. In one embodiment, satellites  12  form a constellation (Genesis constellation) that includes constellation of satellites  30  corresponding to an Iridium®-like constellation and constellation of satellites  31  that are added to the Iridium®-like constellation of satellites  30 . Satellites  30  which are represented by small dots are connected via cross-links  32  represented by solid lines. Satellites  31  which are represented by larger dots are positioned on the same network of satellites as satellites  30  on the Iridium®-like constellation. Satellites  31  are connected via cross-links  33  represented by dashed lines. In addition, satellites  31  are also connected through cross-links  32 . Hence, satellites  12  of the Genesis constellation form a more connected network than the Iridium®-like constellation of satellites  30 . In addition, satellites  31  may be added to the Iridium®-like constellation at a future point in time to incorporate evolution upgrades with seamless integration. 
         [0034]      FIG. 3  shows six lines corresponding to the various communication paths or cross-links are originating from each satellite  12 . These cross-links include both cross-links  32  (solid lines) and cross-links  33  (dashed lines). Each crosslink  32  or  33  is associated with one laser communication transceiver (e.g., laser communication transceiver  24  shown in  FIG. 2 ). Therefore, in one embodiment, there is provided six laser communication transceivers on each satellite  12  of the Genesis constellation. On the other hand, Iridium®-like constellation of satellites  30  only use four crosslink transceivers for communication with neighboring satellites. Indeed, as shown in  FIG. 3 , there are two neighboring satellites  30  to each satellite  30  in the Iridium®-like constellation that satellite  30  does not have connections to. Therefore, the Genesis constellation of satellites  12  which includes satellites  30  and satellites  31  is more interconnected than the Iridium®-like constellation of satellites  30 . 
         [0035]      FIG. 4  is a diagram showing communication channels between plurality of transceivers  16  on UAVs  16 A and satellite  12 B, according to one embodiment. In  FIG. 4 , UAVs  16  are shown over South Asia collecting image surveillance data. The coverage of satellite  12 B is represented by solid circle  40 . The center of circle  40  represents the position of satellite  12 B which orbits around the globe around orbit  42 . The coverage of other satellites are also depicted by dotted circles. These dotted circles overlap with solid circle  40  to define overlapping coverage with coverage of satellite  12 B. Within coverage  40  of satellite  12 B (i.e., within circle  40 ) are shown a plurality of UAVs  16  represented as dots communicating with satellite  12 B. The solid ovals represent the RF uplink beam width from UAVs  16  on satellite  12 B. A ground base station (not shown in  FIG. 4 ) can be located outside coverage  40  of satellite  12 B. Therefore, in this case, satellite  12 B sends the data uplinked to satellite  12 B through a path between successive satellites  12  (i.e., two or more satellites  12 ) to a destination satellite (e.g., satellite  12 A) having a coverage or footprint that contains the ground base station (e.g., ground base station  14 ) intended to receive the data. Destination satellite  12 A has a coverage that contains the ground base station  14  and would then downlink the data to the ground base station  14 , as shown in  FIG. 1 . 
         [0036]    In one embodiment, if only four RF receivers (e.g., RF Rx  20  in  FIG. 2 ) are provided on satellite  12 , then only 4 UAVs can be serviced or supported. Hence, additional RF receivers may be needed in order to be able to support up to 16 UAVs. In one embodiment, if there are four transceivers on each satellite  12 , UAV  16  that is under coverage of satellite  12 B is instructed by command control station  18  (shown in  FIG. 1 ) to transmit to satellite  12 B that passes over the UAV  16  which of two channels (frequencies) and which of two polarizations (RHCP or LHCP) to use during a period of contact (e.g., about 9 minutes) with satellite  12 B. To accomplish this, a receiving control channel is provided on each UAV so as to enable each UAV  16  to receive the instruction from command control station  18 . 
         [0037]      FIG. 5  is a schematic diagram showing a path of satellite  12 A in relation to a plurality of ground base stations  14  and  41 , according to one embodiment. Although two ground base stations  14  and  41  are shown in communication with satellite  12 A along orbit path or orbit plane  50  of satellite  12 A, more than two ground stations can be provided to communicate with satellite  12 A along orbit plane  50  of satellite  12 A. Therefore, orbit plane  50  of satellite  12 A would be sharing a plurality of ground base stations (e.g., ground base stations  14  and  41 ). Each of the ground base stations  14  and  41  may have one, two or more antennas. Satellite  12 A can be configured such that its orbit plane  50  would pass over at least two ground stations (e.g., ground stations  14  and  41 ). With an appropriate arrangement or orientation of the ground antennas of ground station  14 , the ground station  14  can have a field of regard (FOR 1 ) such that satellite  12 A can have a period of time which depends on the antennas position and arrangement and the orbit speed of the satellite during which to downlink data to the ground station  14  via downlink  13  during transit of satellite  12 A. For example, in one embodiment, the period of time can be approximately 20 minutes for satellites at 780 km with an earth view time of about 11 minutes for direct flyovers when starting and ending 8 degrees from the horizon. In other words, ground base station  14  comprises a plurality of antennas configured and arranged so that a field of regard (FOR 1 ) of the antennas provides a period of time sufficient for satellite  12 A to downlink data to the ground base station  14  during orbit transit of satellite  12 A within the field of regard of satellite  12 A. When satellite  12 A is out of the field of regard (FOR 1 ) of ground station  14 , satellite  12 A may “enter” field of regard (FOR 2 ) of ground station  41 . In this case satellite  12 A may downlink the data to ground station  41  if desired. In one embodiment, there may be 5 other orbit planes  50 . Each satellite along each of these orbit paths  50  is able to downlink to a ground station (e.g., ground station  14 ). 
         [0038]      FIG. 6A  depicts an Other-three-billion (O3b)-like constellation of satellites, according to a one embodiment. In the O3b-like constellation, for example, 8 satellites in one orbit plane at an altitude of about 8000 km at 0 deg. orbital inclination are provided. In the O3b-like constellation, coverage over vast regions is obtained with only one orbit plane as the orbit plane of the 8 satellites is positioned at the equator and the satellites orbit at relatively high altitude 8000 km. For example, coverage of South America, Africa, India, China, etc. is achieved, as well as the United States and most of Europe. In this case, the number of transceivers can be reduced because not more than two forward transceivers and two aft transceivers may be needed. This will enable connections from one satellite to the two closest satellites in front of the satellite and the two closest satellites behind the satellite. As a result, multiple redundancy can be achieved in case of failure. 
         [0039]      FIG. 6B  depicts a sun-synch constellation of satellites, according to another embodiment. In this constellation, for example, 45 satellites in 5 orbit planes at an altitude of about 1250 km at 100.6 deg. orbital inclination are employed. In this case, each orbit plane has 9 satellites. In this case, the total number of satellites is greater than in the O3b-like constellation. However, this number of satellites may be needed in some applications such as climate monitoring applications and remote sensing applications where a satellite goes over a given part of the globe at the same time every day. 
         [0040]      FIG. 6C  depicts an Iridium®-like constellation of satellites, according to yet another embodiment. In this constellation, for example, 66 satellites in 6 orbit planes at an altitude of 780 km at 86.4 deg. orbital inclination can be employed. Therefore, in the Iridium®-like constellation, each orbit plane has 11 satellites. 
         [0041]    The types of constellations that can be implemented is not limited to the above exemplary constellations but can be extended to other types of constellations. For example, a network mesh or some other kind of network architecture that changes over time can also be implemented. 
         [0042]    As it can be appreciated from the above paragraphs, satellite system  10  is easily extendable by adding satellites to an already existing constellation of satellites. This can be done to replenish old nodes or create new nodes with either historic or enhanced capabilities. For example, new satellites can be inserted into an existing orbit plane and easily connect to neighboring satellites using steering capability of the laser communication transceivers. 
         [0043]    Furthermore, the use of lightweight, low-power laser communication transceivers provides a high-speed network in space with redundant paths. The use of laser communication transceivers provides relatively large angle (e.g., ±30 degree or more) steering with low weight, power, and disturbance impacts. The use of laser crosslink transceivers enables pointing the laser line-of-sight and perform look-ahead, dispersion and jitter compensation, as well as track neighboring satellites to maintain a continuous connection during desired intervals. 
         [0044]    Although the inventive concept has been described in detail for the purpose of illustration based on various embodiments, it is to be understood that such detail is solely for that purpose and that the inventive concept is not limited to the disclosed embodiments, but, on the contrary, is intended to cover modifications and equivalent arrangements that are within the spirit and scope of the appended claims. For example, it is to be understood that the present disclosure contemplates that, to the extent possible, one or more features of any embodiment can be combined with one or more features of any other embodiment. 
         [0045]    Furthermore, since numerous modifications and changes will readily occur to those with skill in the art, it is not desired to limit the inventive concept to the exact construction and operation described herein. Accordingly, all suitable modifications and equivalents should be considered as falling within the spirit and scope of the present disclosure.