Abstract:
A Space Tracking and Identification (STI) method and system uses low-cost identification and location beacon devices situated on each satellite. Preferably, these beacon devices are substantially independent of the mission-specific and satellite-specific navigation and communication systems, thereby allowing their use on any satellite or other space object. The beacon preferably includes a GPS receiver, an on-board processor, and a transmitter that transmits an identifier of the satellite and location information, and optionally other navigation-related information, to a relay satellite or directly to a ground-based system. The ground system delivers the received information, or a processed version thereof, to a recipient associated with the satellite identifier. The beacon preferably uses a Sensor Enabled Notification System (SENS) transmitter that uses Code Phase Division Multiple Access (CPDMA™) to assure low-cost, low-bandwidth, and virtually unlimited extensibility.

Description:
[0001]    This application claims the benefit of U.S. Provisional Patent Application 61/158,350, filed 7 Mar. 2009. 
     
    
     BACKGROUND AND SUMMARY OF THE INVENTION 
       [0002]    This invention relates to the field of satellite tracking, and in particular to a method and system that provides for satellite tracking that does not require the complex infrastructure that is conventionally used to track satellites. 
         [0003]    The number of satellites orbiting the earth continues to increase, as does the infrastructure required to track these satellites. Current methods of tracking and identification require users to monitor space traffic from the ground and develop track performance and conjunction analysis results. These systems are manpower intensive and require significant infrastructure. 
         [0004]    In the late 1950s, the U.S. government changed their satellite-active tracking system, to an earth-station-active system, because the satellite-active techniques required active participation on the part of the satellites, including transmissions at particular frequencies, and the first Sputnik did not follow the international agreement on satellite transmitting frequencies. Since then, satellite-passive systems have been commonly used to track satellites, and other objects in orbit, including the accumulated ‘space junk’ created over the years. 
         [0005]    An example satellite tracking system is the U.S. Space Surveillance Network (SSN), a globally distributed network of interferometer, radar and optical tracking systems that currently tracks over 8,000 orbiting objects, using hundreds of land-based sites to perform this active tracking. The SSN uses a combination of active-tracking techniques, including phased-array radars, conventional radars, and the Ground-Based Electro-Optical Deep Space Surveillance System (GEODSS). 
         [0006]    Because of the infrastructure costs and other limitations, the SSN does not continuously monitor the position of each orbiting object. Rather, SSN determines the orbital parameters associated with each object based on observation samples, then uses predictive techniques based on Kepler&#39;s equations of orbital motion, and other algorithms, to determine where each object is located at any particular time. These Keplerian elements are updated based on subsequent observation samples, up to 80,000 satellite observations each day. The data is transmitted directly to USSPACECOM&#39;s Space Control Center (SCC) via multiple communication means, including satellite, ground wire, microwave and phone to ensure reliable and continuous communications. 
         [0007]    The Air Force Satellite Control Network (AFSCN) is used to control select spacecraft, generally those operated by or for the U.S. government, and others of high importance to the U.S. This network uses sub-carrier transmitter signals, in addition to the orbital parameters, to provide range information and thus a more accurate location prediction. This system also requires a substantial infrastructure and significant manpower resources. 
         [0008]    Although the SSN database of orbital parameters is available to the providers of satellite services, the fact that any particular satellite is only one of the thousands of objects that the SSN is monitoring limits the options available to the service provider regarding real-time tracking and reporting. The fact that in early 2009, an Iridium satellite collided with a Cosmos satellite at over 20,000 miles per hour, resulting in a loss of tens of millions of dollars, amply demonstrates the limitations of current satellite tracking systems. 
         [0009]    Although satellite service providers may provide their own infrastructures to provide more timely and accurate satellite location determinations, the costs of such an infrastructure, in terms of capital investment and operational costs are extremely high. 
         [0010]    It would be advantageous to enable a satellite service provider to obtain real-time tracking information. It would be also be advantageous to provide this real-time tracking information using an existing communication infrastructure. It would also be advantageous to enable the satellite service provider to customize the parameters associated with the real-time reporting, including the rate of real-time updates, and other parameters. It would also be advantageous to enable the receipt of real-time tracking information from among the hundreds of currently active satellites, and potential thousands of future satellites, without requiring a significant bandwidth requirement. 
         [0011]    These advantages, and others, can be realized by a Space Tracking and Identification (STI) method and system that uses low-cost identification and location beacons situated on each satellite. Preferably, these beacons are substantially independent of the mission-specific and satellite-specific navigation and communication systems, thereby allowing their use on any satellite. The beacon preferably includes a GPS receiver, an on-board processor, and a transmitter that transmits an identifier of the satellite and location information, and optionally other navigation-related information, to a relay satellite or directly to a ground-based system. The ground system delivers the received information, or a processed version thereof, to a recipient associated with the satellite identifier. The beacon preferably uses a Sensor Enabled Notification System (SENS) transmitter that uses Code Phase Division Multiple Access (CPDMA™) to assure low-cost, low-bandwidth, and virtually unlimited extensibility. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The invention is explained in further detail, and by way of example, with reference to the accompanying drawings wherein: 
           [0013]      FIG. 1  illustrates an example block diagram of a system in accordance with this invention. 
           [0014]      FIG. 2  illustrates an example block diagram of a processing center in accordance with this invention. 
           [0015]      FIG. 3  illustrates an example flow diagram of a method in accordance with this invention. 
           [0016]      FIG. 4  illustrates an example block diagram of a communication system for an embodiment of this invention. 
       
    
    
       [0017]    Throughout the drawings, the same reference numerals indicate similar or corresponding features or functions. The drawings are included for illustrative purposes and are not intended to limit the scope of the invention. 
       DETAILED DESCRIPTION 
       [0018]    In the following description, for purposes of explanation rather than limitation, specific details are set forth such as the particular architecture, interfaces, techniques, etc., in order to provide a thorough understanding of the concepts of the invention. However, it will be apparent to those skilled in the art that the present invention may be practiced in other embodiments, which depart from these specific details. In like manner, the text of this description is directed to the example embodiments as illustrated in the Figures, and is not intended to limit the claimed invention beyond the limits expressly included in the claims. For purposes of simplicity and clarity, detailed descriptions of well-known devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail. 
         [0019]    The invention is presented using the paradigm of low-earth-orbit (LEO) satellites, although one of skill in the art will recognize that the principles of this invention are applicable to determining the location of any space vehicle, independent of whether the vehicle is traveling in any particular orbit. 
         [0020]      FIG. 1  illustrates an example block diagram of a system in accordance with this invention. In this example embodiment, satellites  110  include a beacon device  120  that is configured to communicate location information, from which the satellite&#39;s location can be determined. The beacon device  120  is preferably configured to operate autonomously, and may be separate from, or integrated with, other electronic devices on the satellite. The beacon device  120  may be installed in the satellite before launch, or attached to an existing satellite via on-orbit rendezvous, docking, and mating techniques, including, for example, mechanical, magnetic, and adhesive techniques. 
         [0021]    The beacon device  120  communicates the location information to a ground station  140 , optionally through one or more relay satellites  130 . The transmission of the location information preferably includes an identification of the particular beacon device, allowing the identification of the satellite to which it is assigned, as well as the information required to determine the location of the satellite. If the identification of the beacon device  120  or the satellite  110  is included in the transmitted location information, the identification of the satellite may be deduced from the reported location of the satellite at the particular time of the communication from the device  120 , based on the known orbital parameters associated with the satellite  110 . 
         [0022]    The ground station  140  provides a preliminary filtering of all of the messages received, and forwards the appropriate messages to the processing center  150 . A processor at the processing center  150  determines the identification and location of the satellite  110  based on the location information provided by the beacon device  120 , and, based on its internal database, determines the intended recipients  181 - 184  of this information. 
         [0023]    In a preferred embodiment of this invention, the users/customers of the services of the processing center  150  register with the provider of the processing center  150 , and inform the provider of the identification of the satellite of interest, and the identification of one or more recipients that are to be informed of the location of this satellite. The user may also specify conditional rules for sending the information to one or more of the recipients. For example, the communication of the location information may be on an ‘exception’ basis, so that, for example, the information is only sent to the identified recipients in the event that the reported location differs by a specified amount from the predicted location of the satellite. Also optionally, the processing center  150  may access other processing systems  160  to provide additional information, such as a more precise determination of the position of the satellite  110  based on Doppler and other effects, as well as a determination of the presence of other orbital objects in the vicinity of the satellite  110 , based either on a reported location of the object based on this invention, or an estimated location of the object based on available orbital parameters. 
         [0024]      FIG. 2  illustrates an example block diagram of a processing center  150 . The processing center includes a receiver  210  for receiving the messages from the beacon devices on the satellites ( 110 - 120  of  FIG. 1 ), typically via a ground station ( 140 ). A message discriminator and decoder  220  provides each message to a location determinator  230 , and the identification and location of the satellite is provided to a processor  240  that processes this information based on the user/customer&#39;s specified requirements, stored in database  250 . If a customer message is to be sent, a message generator  260  creates one or more messages, based on information provided by the processor  240 , and sends the messages to the intended recipients, preferably via an Internet access device  270 . The Internet access device  270  is also preferably used to communicate the aforementioned customer requirements to the processor  240 , for storage in the database  250 . 
         [0025]    The operation of the processing center  150  is presented in more detail in the flow diagram of  FIG. 3 , with reference to elements of  FIGS. 1 and 2 . 
         [0026]    At  310 , the messages from the satellites are decoded. The particular decoding process will be dependent upon the process used by the beacon device  120 . As detailed further below, in a preferred embodiment of this invention, a Code Phase Division Multiple Access (CPDMA) technique is preferably used. U.S. Pat. No. 6,128,469, “SATELLITE COMMUNICATION SYSTEM WITH A SWEEPING HIGH-GAIN ANTENNA”, issued 3 Oct. 2000 to Ray Zenick, John Hanson, Scott McDermott, and Richard Fleeter; U.S. Pat. No. 6,396,819, “LOW-COST SATELLITE COMMUNICATION SYSTEM”, issued 28 May 2002 to Richard Fleeter, John Hanson, Scott McDermott, and Ray Zenick; U.S. Pat. No. 6,317,029, “IN-SITU REMOTE SENSING” issued 13 Nov. 2001 to Richard Fleeter; U.S. Pat. No. 7,227,884, “SPREAD-SPECTRUM RECEIVER WITH PROGRESSIVE FOURIER TRANSFORM” issued 5 Jun. 2007 to Scott A. McDermott; and U.S. Pat. No. 7,433,391, “SPREAD-SPECTRUM RECEIVER WITH FAST M-SEQUENCE TRANSFORM, issued 7 Oct. 2008 to James F. Stafford and Scott A. McDermott, disclose systems and methods that facilitate the reception and processing of messages from a large number of preferably low-cost transmitters using CPDMA, and each is incorporated by reference herein. 
         [0027]    The loop  320 - 380  is repeated for each of the received and decoded messages. At  330 , the satellite identification and location are determined. The determination process is based on the information in the received message provided by the beacon device  120 . In a preferred embodiment, the message includes a unique identification of the satellite, and a location determined via the Global Positioning System (GPS). Depending upon the configuration of the beacon device  120 , a determined latitude, longitude, and elevation may be included in the message, or the raw GPS timing information provided by the GPS satellites is included in the message, leaving the determination of the latitude, longitude, and elevation to be performed at the processing center  150 . 
         [0028]    Optionally, the beacon device  120  may be configured to transmit a sequence of location information, from which the processing center  150  can determine the velocity, and optionally the acceleration, of the satellite, as well as the reported location. Also optionally, the processing center  150  may access one or more auxiliary processing systems  160  to further enhance the accuracy of the determined location of the satellite, taking into account, for example, errors introduced by the velocity of the satellite and other factors. Optionally, GPS-Doppler compensation can be performed by the beacon device  120  to facilitate accurate location determination. 
         [0029]    Having identified the satellite associated with the message, the customer data is accessed, at  340 , to determine the appropriate actions to take, if any. In a straightforward embodiment of this invention, the customer data includes a list of e-mail addresses to forward the location of the satellite, and the location is sent as an e-mail message. In an optional embodiment, the list includes other types of Internet addresses and a corresponding protocol and/or format for composing the location message. For example, with regard to  FIG. 1 , in addition to sending an e-mail message to a PC  181 , the location information can be formatted for compatibility with a cell phone  182 , a personal data assistant (PDA)  183 , or a portal  184  to another processing system or subnetwork, such as existing satellite tracking networks. 
         [0030]    Optionally, the location information can be provided as standard Earth-centered orbital Keplerian Two Line Elements (TLEs) or Vector Covariance Message (VCM) in a format that is compatible with the U.S. Space Surveillance Network (USSSN and AFSSN) and the Air Force Satellite Control Network (AFSCN), to further augment these systems. 
         [0031]    As noted above, in a preferred embodiment of this invention, the user/customer is provided the option of setting parameters for determining when to notify some or all of the recipients of the satellite&#39;s location. These parameters may include, for example, notifying select recipients at less frequent intervals than others, notifying some or all of the recipients only when the reported location differs by a given threshold from a predicted location of the satellite, notifying some or all of the recipients if the reported location and velocity indicates a potential collision with another space object, and so on. 
         [0032]    The intended recipients and their requirements are determined at  350 , and the appropriate messages are prepared, at  360 . As noted above, these messages may be provided in any number of forms, based on the particular customer requirements. 
         [0033]    At  370 , the messages are communicated to the recipients. As noted above, in a preferred embodiment, the Internet is used to provide this communication, although one of skill in the art will recognize that any means of communication may be used. 
         [0034]    Each received message is processed similarly, via the loop  320 - 380 , and the next group of messages is received and processed, at  310 . One of skill in the art will recognize that the sequential process of  FIG. 3  may be embodied using alternative processes, such as parallel processing, event-triggered processing, and so on, and the particular sequence of steps may differ from that illustrated in  FIG. 3 . 
         [0035]    As will be evident to one of skill in the art, the communication of this location information from potentially thousands of satellites can consume a significant amount of bandwidth and other resources. In particular, a conventional system that requires synchronization among the receivers and transmitters would introduce a significant amount of overhead to coordinate the communications from these hundreds or thousands of transmitters. As noted above, in a preferred embodiment of this invention, the beacon devices  120  are configured to use a Code Phase Division Multiple Access (CPDMA) technique. 
         [0036]      FIG. 4  illustrates an example block diagram of a communications system that is well suited for use in this invention, with reference to the elements of  FIGS. 1 and 2 . Illustrated are a set of beacon devices  120   a - 120   c  that are situated on satellites  110 , and the receiver  210  and message discriminator and decoder  220  of the processing system  150 . 
         [0037]    The beacon devices  120   a - c  each includes a transmitter  480   a - c  and a location detecting device  490   a - c , such as a GPS receiver. The transmitters  480   a - c  each provide a transmit signal  481   a - c  comprising a message  482   a - c  that includes the location information from the locator device  490   a - c  and is encoded using a spreading-code  402 . The message  482   a - c  also preferably includes a unique identifier of the satellite  110 . A “maximal length sequence” or “M-Sequence” is preferably used as the spreading code. Maximal length sequences are simple to generate using maximal linear feedback shift registers. 
         [0038]    Each transmitter  480   a - c  is substantially autonomous, and each transmitter  480   a - c  uses the same encoding and communications parameters, including the same spreading-code  402 , and the same nominal carrier frequency to provide the transmit signal  481   a - c  over the same communications channel. By using the same spreading code and carrier frequency, the beacon devices  120   a - c  can be produced at a substantial cost savings, compared to conventional CDMA devices that use a plurality of selectable codes. These transmit signals  481   a - c  form a composite signal  481  within this common communications channel at the nominal carrier frequency. 
         [0039]    If two or more transmitters  480   a - c  transmit at the same time and at the same code-phase and essentially the same frequency, a collision results and these transmissions will not be distinguishable within the composite signal  481 . If only one transmitter  480   a - c  is transmitting at a given code-phase with respect to the receiver, the transmitted message  482   a - c  will be decodable at this code-phase, even though it is at the same carrier frequency of other signals. A typical code  402  includes a sequence of hundreds or thousands of bits, thereby forming hundreds or thousands of code-phases for each message. The likelihood of two transmitters  480   a - c  transmitting at exactly the same code-phase at the same time with respect to the receiver  210  is slight, particularly if the message duration is relatively short. 
         [0040]    Additionally, even if more than one transmitter  480   a - c  is transmitting simultaneously at the same code-phase with respect to the receiver, component variations and other factors may cause each signal to be transmitted at slightly different carrier frequencies, and will be decodable if the receiver is able to distinguish these different carrier frequencies. Accordingly, even if the hundreds or thousands of transmitters  480   a - c  are transmitting concurrently, the likelihood of a collision of relatively short messages will be very slight. Further, even if a collision occurs, the likelihood of repeated collisions will be extremely slight. 
         [0041]    In the case of reporting satellite position information, each particular message is relatively insignificant, because the likelihood of the satellite veering from its predictable course is very low. That is, for example, if the beacon device  120  is configured to send a location report every minute, the absence of one or two reports between received reports will have relatively little impact on the use of these reports. 
         [0042]    Further, the likelihood of the message being received can be increased by repeating the transmission of the message, or sending a plurality of location messages during each reporting period. The sending of a plurality of location messages will also facilitate determination of the satellites current velocity and/or acceleration. 
         [0043]    Because the messages  281   a - c  are discernible based on code-phase and frequency, and do not require synchronization among the transmitters and receivers, the overhead associated with the transmissions from potentially hundreds or thousands of transmitters is substantially less than the overhead incurred in conventional wireless transmission systems, such as the conventional IEEE 802.11 communication standard. 
         [0044]    In this example embodiment, a satellite  130  receives the composite signal  281   a - c  from all of the transmitters within view of the satellite  130  and relays the composite information to a ground station  140 , in either a ‘store-and-forward’ mode, when the remote stations  480   a - c  and the ground station  140  are not contemporaneously in view of the satellite  130 , or in a ‘bent-pipe’ mode, wherein the satellite  130  receives the information from the remote stations  480   a - c  and merely retransmits the information to the ground station  140 , typically at a different transmission frequency. Because the satellite  130  and ground station  140  can be configured with directional antennas, a significant gain in signal to noise ratio can be achieved by such a configuration, without requiring a directional antenna at each beacon device  120   a - c.    
         [0045]    For the purposes of this invention, the signal  481  that is received at the ground station  140  and forwarded to the processing center  150  is considered to be the composite of the individual transmissions  481   a - c , regardless of whether this composite  481  is relayed through one or more relays, such as a satellite  130 , and regardless of whether it is received by a single receiver or multiple receivers. 
         [0046]    As noted above, messages from transmitters  480   a - c  that may transmit at the same code-phase with respect to the receiver  210  can be distinguished within the composite signal  481  if their carrier frequencies differ by a distinguishable amount. The receiver  210  receives the composite signal  481  and down-converts the composite signal  481  to a plurality of baseband signals  411 , each down-conversion frequency being within a given range of the nominal carrier frequency, the range being dependent upon the expected variance of frequencies among the transmitters  480   a - c . The preferred number of down-converters is based on the given range and the selectivity/bandwidth of each down-converter, to assure that the entire range is adequately covered. One of ordinary skill in the art will recognize that alternative schemes can be used to down-convert signals from transmitters that are transmitting within the given range of a nominal frequency; for example, a single down-converter can be used if there is sufficient time to down-convert each required frequency in a sequential manner. 
         [0047]    The receiver  210  provides the baseband signals  411  to the message discriminator and decoder  220 . Within the message discriminator  220 , a phase detector  430  corresponding to each baseband signal (i.e. each transmit frequency) determines the code-phase(s)  435  that contain(s) substantial signal energy. Each phase detector  430  provides this (these) code-phase(s)  435  to a demodulator  450 , along with the input baseband signal  411 . The demodulator  450  thereby receives each (frequency, code-phase) pair that indicates the presence of a message from one of the remote transmitters  480   a - c . The demodulator  450  receives the baseband signal  411  that is provided by a particular down-converter  415 , and the phase(s)  435  at which substantial energy was detected within this particular baseband signal  411 . 
         [0048]    The demodulator  450  decodes each baseband signal  411  at each of these code-phase(s)  435  to produce a decoded signal corresponding to each of these (frequency, code-phase) pairs. Given that substantial energy has been detected in this frequency-based signal  411  at each identified code-phase  435 , each decoded signal is assumed to correspond to a segment of a particular transmitted message  482   a - c . The demodulator  450  routes each decoded signal from each (frequency, code-phase) pair into a corresponding queue  460 , thereby forming strings of messages in each queue  460 , corresponding to each transmitted message  482   a - c.    
         [0049]    Although the discriminator  220  is illustrated as containing multiple phase detectors  430 , to allow the detectors  430  to process the output of each down-converter  415  in parallel, one of ordinary skill in the art will recognize that a single phase detector can be used, if there is sufficient time to sequentially detect each phase within each down-converted signal  411 . Preferably, the efficiency of the discriminator  220  is such that it allows the detection process to be accomplished via software running on a general purpose processor, or on a signal processor, as well as via conventional hardware devices. 
         [0050]    In an example embodiment, messages  281   a - c  that contain location, velocity, and time are transmitted as bursts of 100 bps binary phase shift keying (BPSK) modulated data that is spread across 2.5 MHz of bandwidth at approximately one second per transmission. The size of the message is dependent upon the desired precision, which may depend upon the requirements of the particular user. Also in a preferred embodiment, the user/customer is provided the option of specifying the interval between transmissions, the number of bursts at each transmission period, and so on. These parameters may be set before the beacon device  120  is launched, or, depending upon the capabilities of the particular beacon device, programmable after the device  120  is deployed, as customer requirements change. To conserve power, the interval between transmissions from the beacon device is preferably substantially greater than the duration of the transmission, preferably at least 10:1, and typically in the order of 100:1 or more, thereby providing a duty cycle in the order of 1% or less. 
         [0051]    Optionally, the beacon device  120  may be configured to be triggered to send its location message based on parameters other than, or in addition to, time intervals, such as acceleration-based triggers, system-status triggers, external triggers, such as a prompt from the processing system  160 , and so on. 
         [0052]    The foregoing merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are thus within its spirit and scope. For example, in addition to location information, the messages may also include additional information that facilitates the monitoring of the satellite, such as a monitor of In-, Cross-, and Radial-Track position and velocity information, as well as satellite power, and other status information. That is, the beacon device  120  may include accelerometers, attitude control sensors (e.g., sun, star, or earth sensors), and sensors to independently monitor the health of the satellite it is connected to (e.g., RF, optical, temperature sensors). In like manner, although the invention is presented in the context of a beacon device being placed on a satellite, the beacon device can be place on any space object, such as an approaching asteroid, large ‘space junk’ items, or other objects. These and other system configuration and optimization features will be evident to one of ordinary skill in the art in view of this disclosure, and are included within the scope of the following claims. 
         [0053]    In interpreting these claims, it should be understood that: 
         [0054]    a) the word “comprising” does not exclude the presence of other elements or acts than those listed in a given claim; 
         [0055]    b) the word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements; 
         [0056]    c) any reference signs in the claims do not limit their scope; 
         [0057]    d) several “means” may be represented by the same item or hardware or software implemented structure or function; 
         [0058]    e) each of the disclosed elements may be comprised of hardware portions (e.g., including discrete and integrated electronic circuitry), software portions (e.g., computer programming), and any combination thereof; 
         [0059]    f) hardware portions may include a processor, and software portions may be stored on a computer-readable medium, and may be configured to cause the processor to perform some or all of the functions of one or more of the disclosed elements; 
         [0060]    g) hardware portions may be comprised of one or both of analog and digital portions; 
         [0061]    h) any of the disclosed devices or portions thereof may be combined together or separated into further portions unless specifically stated otherwise; 
         [0062]    i) no specific sequence of acts is intended to be required unless specifically indicated; and 
         [0063]    j) the term “plurality of” an element includes two or more of the claimed element, and does not imply any particular range of number of elements; that is, a plurality of elements can be as few as two elements, and can include an immeasurable number of elements.