Abstract:
A system for transmitting non-GPS information for reception by a global positioning system (GPS) receiver, the system including a processor, a memory coupled to the processor and including computer-readable instructions configured to, when executed by the processor, cause the processor to receive the non-GPS information, determine an available pseudo-random noise (PRN) code, spread the non-GPS information using the available PRN code to provide a spread signal, modulate a GPS carrier frequency using the spread signal to produce a GPS compatible signal, and a terrestrial transmitter configured to transmit the GPS compatible signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims any and all benefits as provided by law of U.S. Provisional Application No. 60/889,033 filed Feb. 9, 2007, which is hereby incorporated by reference in its entirety. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0002]    Not Applicable 
       REFERENCE TO MICROFICHE APPENDIX 
       [0003]    Not Applicable 
       BACKGROUND 
       [0004]    Global positioning system (GPS) receivers are widely used and have many potential applications. Many electronic devices now include GPS receivers such as mobile phones, in-car navigation systems, and vehicular-guidance systems. An electronic device containing a GPS receiver is capable of precisely determining the location (plus or minus a few centimeters) of the electronic device, anywhere in the world. Generally, using a GPS device, a user is able to obtain position information in terms of latitude, longitude, and altitude. The position information can then be processed into other forms of information, such as a location on a map or a Postal Code. 
         [0005]    GPS receivers can use signals from a combination of satellite-based transmitters and ground-based transmitters to calculate the receiver&#39;s position. Referring to  FIG. 1 , orbiting the Earth is a constellation of twenty-four satellites (A, B, C, D, E, F) in six planes. Each of the satellites transmits signals modulated by a pseudo-random noise (PRN) code towards the Earth&#39;s surface. A unique PRN code (also known as a “gold code”) is assigned to each GPS satellite, with several spare PRN codes available. The signals can carry information that includes a coarse/acquisition code, a precision code (P-code), and a navigation message. GPS receivers calculate location information using the signals and information from at least three of the GPS satellites. By comparing the amount of time that it took for the signal transmitted by each satellite to reach the GPS receiver, and using the data contained in the signals, the GPS receiver is able to precisely calculate the location of the GPS receiver. The ground-based transmitters can monitor the GPS signals, and correct for any drift in the orbits of the GPS satellites by updating the ephemeris constant and/or the base clock offset of each of these satellites. In this manner, a user can use a GPS receiver to precisely determine the location of the GPS receiver. 
         [0006]    The GPS satellites transmit signals over several frequencies such as the L1 carrier frequency (1575.42 MHz) and the L2 carrier frequency (1227.6 MHz), and in the future, the L5 carrier frequency (1176.45 MHz). The GPS satellites use Direct Sequence Spread Spectrum (DSSS) modulation, which is a type of code-division multiple-access (CDMA) modulation, to modulate the signals transmitted by each of the GPS satellites. The signals transmitted by each of the GPS satellites (e.g., the P-code, the coarse/acquisition signal, etc.) are “spread” by the PRN code corresponding to an individual satellite. The spread signal is used to modulate a carrier frequency (e.g., the L1 and/or L2 frequencies). The modulated spread signal is broadcast to GPS receivers. The use of DSSS can increase the signal&#39;s resistance to interference. Since each signal is nearly uncorrelated with respect to each other, the DSSS modulated GPS signals can be demodulated using standard CDMA techniques. 
         [0007]    The navigation message is a 50 Hz signal that includes data bits describing the GPS satellite orbits, clock corrections, and other system parameters. A complete navigation message is sent over the course of a 12.5-minute cycle using twenty-five 1500-bit frames. A single 1500-bit frame is sent every thirty seconds (yielding an effective throughput of 50 bps). Each 1500-bit frame is divided into five 300-bit sub-frames. The first sub-frame of each 1500-bit frame includes satellite-specific clock-correction information. The second and third sub-frames include satellite-specific ephemeris data information. The fourth and fifth sub-frames include system data, or almanac data. Combining twenty-five consecutive corresponding sub-frames (e.g., twenty-five consecutive fourth sub-frames, twenty-five consecutive fifth sub-frames, etc.) yields an entire navigation message. 
         [0008]    The signals transmitted by the GPS satellites travel line of sight, but can have a hard time passing through solid objects such as building structures and mountains. For example, if a user has a GPS receiver inside of a 50-story building, the user may not be able to receive any GPS satellite signals. The lack of a GPS satellite signal can have disastrous consequences such as an inability for 911 call centers to locate a caller or an inability to communicate with an object and/or a vehicle, such as an automated or guided vehicle, en-route to a destination. 
         [0009]    The Federal Communications Commission (FCC) has established a wireless Enhanced 911 (“E911”) plan. The E911 program is divided into two parts—Phase I and Phase II. Phase I requires wireless carriers to report the telephone number of a wireless 911 caller and the location of the carrier&#39;s antenna that received the call. Phase II of the E911 regulations require wireless carriers to provide far more precise location information, within 50 to 300 meters in most cases. To comply with the wireless E911 plan, many wireless carriers have integrated GPS receivers into mobile phones, and other mobile communication devices. In the event of a 911 call by a mobile phone user, the GPS enabled mobile phone can relay location information provided by the GPS receiver to a 911 call center for use in determining the location of the mobile phone. 
       SUMMARY 
       [0010]    Various aspects of the invention can provide one or more of the following capabilities. Virtually any type of information can be transmitted to GPS receivers using the existing GPS infrastructure. Information can be transmitted to GPS receivers (e.g., a device with an antenna, a radio receiver that can receive GPS signals and information, and a processor for use the worldwide GPS system) using the navigation message of a GPS signal. Information can be transmitted to GPS receivers using a terrestrial GPS transmitter such as a pseudolite (e.g., a terrestrial transmitter that can provide services typically provided by a satellite such as a GPS signal), a mobile transmitter, an airborne transmitter, a satellite or existing GPS satellites. Communication with and reprogramming of electronic devices coupled to a GPS receiver can be accomplished. GPS containing devices, vehicles and ordnances can be reprogrammed or redirected using information received in a GPS signal. By using the GPS receiver to sent information to these GPS containing devices, vehicles and ordnances, the space, weight and cost of providing a separate receiver for this information can be avoided. 
         [0011]    Standard GPS receivers can continue to operate successfully even in the presence of information signals containing supplemental information. A transmission source can utilize signal PRN codes which are unused either in the entire GPS satellite constellation or at least with respect to the “visible” satellites at the time of the transmission. Information can be addressed to a specific GPS receiver. Information can be provided to an electronic device having no communication capability apart from an attached GPS receiver device, without redesigning the electronic device. Information can be provided to a GPS receiver using a non-interfering duty-cycle, for example, a duty cycle that is less than about 30% of existing GPS satellite transmissions. Information can be provided to a GPS receiver by modulating the information using an orthogonal code different from any of the GPS satellites. Existing GPS receivers and attached electronic devices can be reprogrammed using information transmitted in a GPS signal. Information such as, system control information or course-correction information can be transmitted to vehicular guidance systems and ordnances. Information can be transmitted to mobile and aerial vehicles and devices. Covert communication can be accomplished. 
         [0012]    These and other capabilities of the invention, along with the invention itself, will be more fully understood after a review of the following figures, detailed description, and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a schematic diagram of a constellation of GPS satellites. 
           [0014]      FIG. 2  is a schematic diagram of a GPS system including terrestrial transmitters. 
           [0015]      FIG. 3  is a schematic diagram of a portion of a navigation message included in a GPS signal. 
           [0016]      FIG. 4  is a flowchart of a process for transmitting and receiving information using the system shown in  FIG. 2 . 
           [0017]      FIG. 5  is a flowchart of a process for transmitting and receiving information using the system shown in  FIG. 2 . 
           [0018]      FIG. 6  is a schematic diagram of an implementation of a GPS system using terrestrial transmitters. 
           [0019]      FIG. 7  is a flowchart of a process for transmitting information to a guide vehicle. 
           [0020]      FIG. 8  is a schematic diagram of a waveform related to the principal of orthogonality. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    Embodiments of the invention provide techniques for transmitting information, such as data, to a GPS receiver device without substantially interfering with standard GPS satellite signals. GPS receiver devices include electronic devices with the capability to receive GPS satellite signals, such as GPS-enabled mobile phones, in-vehicle navigation systems, vehicular guidance systems, aviation navigation systems, maritime navigation systems, etc. A transmission source transmits information to a GPS receiver using GPS-like signals. The information can be transmitted to a GPS receiver, for example, using signals with a lower (or complementary) duty-cycle than existing GPS signals and/or by modulating the signal using available PRN codes (for example, one of the spare or unused PRN codes). Depending on the chosen transmission method, the GPS receiver can use CDMA demodulation techniques to demodulate and extract the information contained in the GPS-like signals broadcast to the GPS receiver. 
         [0022]    Referring to  FIG. 2  a GPS system  15  includes the GPS satellites  20 ,  30 , and  40 , transmitters  50 ,  60 ,  70 , and  80 , signals  22 ,  32 ,  42 ,  52 ,  62 ,  72 , and  82 , a land/stationary platform  90 , a mobile platform  95 , an airborne platform  100  and a GPS receiver  105 . The satellites  20 ,  30 , and  40  transmit the GPS signals  22 ,  32 , and  42  towards Earth  115  for reception by the GPS receiver  105 . Supplemental “GPS-like” signals, such as signals  52 ,  62 ,  72 , and  82 , can be used to broadcast GPS information and/or other information to GPS receivers. Other quantities and configurations of the transmitters  50 ,  60 ,  70 , and  80 , the satellites  20 ,  30 , and  40 , and/or the GPS receiver  105  are possible (e.g., five satellites, one ground-based transmitter, and four GPS receivers). 
         [0023]    The transmitters  50 ,  60 ,  70 , and  80  can be used to provide GPS signals and/or GPS-like signals to the GPS receiver  105 . Non-satellite transmitters can be stationary, mobile, airborne, and/or terrestrial. For example, the transmitter  50  is installed on the land/stationary platform  90 , the transmitter  60  is installed within a building  120 , and the transmitter  70  is installed on the mobile platform  95  (in  FIG. 2 , a truck). The land platform  90  can be a stationary object such as a pole dedicated to the transmitter  50  or another structure such as a radio antenna, a mobile-phone tower, a light pole, a roof of a building, a water tower, a bridge, a mountain top, etc. The mobile platform  95  can be a moving object, such as a car, a truck, a boat, a train, a bus, a tank, etc. The transmitter  80  is installed on an airplane  100 , although other similar aerial vehicles can be used (e.g., a helicopter, an unmanned aerial vehicle (UAV), satellite and/or a blimp). 
         [0024]    Non-satellite based transmitters (e.g., the transmitters  50 ,  60 ,  70 , and  80 ) can be used to supplement (e.g., repeat) the GPS signals transmitted by the GPS satellites, and/or to send GPS-like signals including information to the GPS receiver  105 . The type of information that can be broadcast to the GPS receiver  105  (or any GPS enabled device) is broad. The information can be non-GPS information which does not include information that is not intended to be used by the GPS for determining the position of the GPS receiver and can be used by the GPS receiver (or a device connected to the GPS receiver) for related or unrelated purposes. For example, the information can include information such as location information, text messages, image files, audio data or files, video data or files, reconfiguration instructions, firmware upgrades, encrypted signals, software updates, anti-virus updates, Web pages, navigation information, navigation files, e-mails, map files, document files, etc. The information can include covert communications that are encrypted or otherwise hidden, such as using steganographic methods. The information can also include information of significance to vehicular or ordnance guidance or control systems such as speed, direction destination information and updates, coordinate information and updates, operational instructions and updates, etc. Transmissions of other types of information are possible. 
         [0025]    Information can be sent to a GPS receiver  105  using standard GPS signal formats such as the navigation message embedded in the GPS signals  22 ,  32 ,  42 ,  52 ,  62 ,  72 , and  82 . The navigation message can be replaced with other information, which can result in a bandwidth of approximately 50 bits-per-second (bps). Other data rates are possible. Other portions of a standard GPS signal can be replaced with other information. More than one of the unused PRN codes can be used to transmit data. 
         [0026]    Referring also to  FIG. 3 , a navigation message  400  includes frames  405  and sub-frames  410   1  through  410   5 . Twenty-five of the frames  405  make up a single navigation message  400 , although other quantities of frames  405  can make up an entire navigation message  400  (e.g., 50 of the frames  405  can make up a single navigation message  400 ). Each of the frames  405  includes five sub-frames  410   1  through  410   5 , although other quantities of sub-frames  410  can make up a single one of the frames  405  (e.g., ten sub-frames can make up a single one of the frames  405 ). 
         [0027]    The information can be a single 50 bit payload which is sent in a single one of the sub-frames  410 , or can be a larger message that is split up over multiple sub-frames  410  or multiple navigation messages sent on the same or multiple PRN codes. For example, a 2000-bit message can be split up over forty consecutive sub-frames  410 . The 2000-bit message could be split up over forty consecutive corresponding sub-frames (e.g., forty consecutive  410   2  sub-frames). Other combinations are possible. The GPS receiver can reconstruct information that has been split up over multiple sub-frames, or alternatively a processor located externally from the GPS receiver can reconstruct information split up over multiple sub-frames  410 . 
         [0028]    The information transmitted by the non-satellite based transmitters can be broadcast using existing GPS frequencies such as the L1 and L2 bands, and in the future, the L5 band, although other frequency bands can be used. Because the GPS satellites can transmit on the same frequency bands as the transmitters  50 ,  60 ,  70 , and  80 , the signals transmitted by the transmitters  50 ,  60 ,  70 , and  80  can interfere with existing GPS signals. To reduce, or even eliminate interference, information can be broadcast to GPS receivers (e.g., the GPS receiver  105 ) using an available PRN code to encode the information and/or using different or lower duty-cycle transmissions. Varying the duty-cycle of the transmissions (e.g., using a duty cycle of 10-30%) can reduce interference with existing GPS signals by improving the signal-to-noise ratio of the information transmitted relative to existing GPS signals. Other techniques can be used. 
         [0029]    In operation, referring to  FIG. 4 , with further reference to  FIG. 2 , a process  200  for transmitting information using an available PRN code and the GPS system  15  includes the stages shown. The process  200 , however, is exemplary only and not limiting. The process  200  can be altered, e.g., by having stages added, removed, or rearranged. 
         [0030]    At stage  205  an available PRN code is identified. An available PRN code is a PRN code such as one of the spare PRN codes and/or a PRN code in use by a GPS satellite  5  that is out of view of the GPS receiver  105 . If one of the spare PRN codes is chosen, the likelihood of interference with another of the GPS satellites can be reduced or even eliminated. Alternatively, a tracking module (e.g., a computer processor running the necessary software) can track the GPS satellites to determine which of the satellites are “in-view” of the GPS receiver  105  at any given time. The tracking module can select a PRN code corresponding to one of the GPS satellites that is not in-view of the GPS receiver  105  to modulate the information being broadcast by the transmitters  50 ,  60 ,  70 , and/or  80 . As the GPS satellites orbit the Earth  115 , the availability of a particular PRN code can change. For example, in  FIG. 2 , the GPS satellite  30  (here, acting as one of the GPS satellites) is shown in-view of the GPS receiver  105  making its code unavailable for use in the DSSS modulation process. As the satellite  30  orbits the Earth  115 , the satellite  30  can disappear over a horizon of the Earth  115 , which can make its PRN code available for use by a ground based transmitter. Once the satellite  30  is again in-view of the GPS receiver, however, its PRN code becomes unavailable. The tracking module can track and/or predict which PRN code will be available at any given time. 
         [0031]    At stage  210 , the information can be sent using DSSS and the selected available PRN code. Portions of the information can be sent using one or more of the available PRN codes. For example, multiple information streams can be sent using different PRN codes, or a single information stream can be split into multiple streams that are sent using different PRN codes. 
         [0032]    At stage  215 , the sent information can be amplified and broadcast by a transmitter (e.g., the satellites  20 ,  30 , and/or  40 , and/or the transmitters  50 ,  60 ,  70 , and/or  80 ) for reception by a GPS receiver (e.g., the GPS receiver  105 ). When the GPS satellites are used to broadcast non-GPS signals, cooperation by the entity operating the satellite (e.g., the United States Government) may be required. 
         [0033]    At stage  220 , the sent information can be received and amplified by a GPS receiver (e.g., the GPS receiver  105 ). The transmitted information can be demodulated to substantially recover the sent information. Error correction, such as a cyclic redundancy check (CRC) code with error correction capability, can be used during transmission process. At stage  225  the recovered information is output by the GPS receiver. 
         [0034]    The stages  220  and/or  225  (including sub-portions of the stages  220  and/or  225 ) can be accomplished by a GPS receiver (e.g., the GPS receiver  105 ), or another device external to the GPS receiver. For example, the GPS receiver  105  itself can demodulate the received modulated information. Alternatively, the GPS receiver  105  (for example, a GPS receiver in a mobile device) can receive the information stream and retransmit it via a wired or wireless network to a remote processor, such as one operated by mobile phone network operator. The remote processor can then demodulate the sent information and transmit the recovered information to the GPS receiver  105  and/or the attached mobile device. 
         [0035]    In operation, referring to  FIG. 5 , with further reference to  FIG. 2 , a process  300  for transmitting information using reduced duty-cycles and/or non-interfering duty-cycles, or PRN codes, using the GPS system  15  includes the stages shown. The process  300 , however, is exemplary only and not limiting. The process  300  can be altered, e.g., by having stages added, removed, or rearranged. While the process  300  describes the process of transmitting information, the process  300  can also be used to transmit standard GPS signals. 
         [0036]    At stage  305 , a transmitter (e.g., the satellites  20 , 30 , and/or  40 , and/or the transmitters  50 ,  60 ,  70 , and/or  80 ) broadcasts the information stream using a duty cycle of about 10-30%. Other duty cycles can be used. The information stream is a modified navigation message, as described above, although other forms of the information stream are possible. Broadcasting information using a lower duty cycle than standard GPS signals can reduce, or possibly eliminate interference with standard GPS signals. The information stream is encoded using an existing PRN code. The PRN code used to encode the information stream can be a PRN code in-use by a GPS satellite for transmitting GPS signals, although unused PRN codes can be used in addition to or instead of the in-use PRN code. The encoded information stream can be broadcast at a power level higher than existing GPS signals, subject to saturation effects in the GPS transmitter and/or receiver. 
         [0037]    At stage  310 , a GPS receiver (e.g., the GPS receiver  105 ) receives the lower duty-cycle broadcast. The GPS receiver can be configured to detect, receive, and/or process the lower duty-cycle broadcast to recover the information contained therein. For example, correlation and integration can be used to recover the lower duty-cycle broadcast when the signal strength is below the noise floor. The GPS receiver processes the lower-duty cycle information stream such that simultaneous detection of existing GPS signals is possible. At stage  315 , the GPS receiver outputs the recovered information using standard GPS spread spectrum processing (as described herein). 
         [0038]    The stages  310  and/or  315  can be accomplished by a GPS receiver (e.g., the GPS receiver  105 ), or another device external to the GPS receiver. For example, the GPS receiver  105  can be configured to process the lower duty-cycle broadcast to recover the information. Alternatively, the GPS receiver can receive the lower-duty cycle broadcast and retransmit it to a remote processor using, for example, wired, cellular or other wireless transmission technology. The remote processor can process the received broadcast to recover the information, and transmit the recovered information to the GPS receiver  105  and/or the attached mobile device. 
         [0039]    The GPS system  15  of  FIG. 2  can be used to provide information to GPS receivers (here, the GPS receiver  105 ). When the GPS receiver is able to receive standard GPS signals, the GPS system  15  can be used to augment the standard GPS signals by providing information to the GPS receiver  105 . Alternatively, in locations where the GPS receiver  105  is unable to receive standard GPS signals (e.g., within a building or a cave), the GPS system  15  can be used to relay the standard GPS signals and/or provide other information to the GPS receiver  105 . 
         [0040]    The information can be used to communicate with GPS enabled guided vehicles or ordnances. For example, some guided vehicles and ordnances can receive GPS signals such as a Global Positioning System Aided Munition (GAM). Because some GPS enabled guided vehicles are programmed with target coordinates prior to being launched, it can be desirable to be able to transmit information (e.g., updated target or destination coordinates, abort, or other control information) to the guided vehicle or ordnance after being launched while the guided vehicle is en route to a target. 
         [0041]    Referring to  FIG. 6  a targeting system  700  includes a satellite  705 , a guided vehicle (or ordnance)  710 , a transmitter  715 , a launch site  720 , and a command center  725 . The satellite  705  can be one of the GPS satellites, or can be another space vehicle such as the satellites  20 ,  30 , and/or  40 , a military satellite, a commercial satellite, a space vehicle, etc. The GPS enabled guided vehicle  710  can be launched from the launch site  720  towards a target or destination  730  by the command center  725 . If the target  730  is a mobile target (e.g., a tank, a ship, etc.) that is in motion while the guided vehicle  710  is in flight, updated or new target (or destination) coordinates can be transmitted by the command center  725  to the guided vehicle  710  using the transmitter  715  and/or the satellite  705 . If the target  730  changes, for example a new target is selected, while the guided vehicle  710  is in flight, updated or new target (or destination) coordinates can be transmitted by the command center  725  to the guided vehicle  710  using the transmitter  715  and/or the satellite  705 . In addition or alternatively, other information, such as control information, can be transmitted to the guided vehicle  710 , for example, changes in speed or direct and abort, return to base or self-destruct instructions. The command center  725  can monitor the target  730  using various methods such as radar (not shown), ground based operatives (not shown), etc. 
         [0042]    In operation, referring to  FIG. 7 , with further reference to  FIG. 6 , a process  800  for transmitting information to a guided vehicle, using the targeting system  700  includes the stages shown. The process  800 , however, is exemplary only and not limiting. The process  800  can be altered, e.g., by having stages added, removed, or rearranged. 
         [0043]    At stage  805  the guided vehicle is programmed with coordinates of a target. The coordinates can be, for example, information that represents the latitude and longitude of the target, although other location or guidance information can be used. At stage  810 , the guided vehicle  710  is launched. 
         [0044]    At stage  815 , the command center  725  determines whether the guided vehicle  710  is still in flight. If the guided vehicle  710  is no longer in flight, the process  800  ends. If the guided vehicle  710  is still in flight, the process  800  continues. 
         [0045]    At stage  820 , the location of the target  730  is monitored by, for example, the command center  725  using radar. At stage  825 , the command center  725  determines if the target has moved (or the destination has changed) from the targeting (or destination) coordinates programmed into the guided vehicle  710  in stage  805 . If the targeting information does not require updating, the process  800  returns to stage  815 . If the targeting information requires updating, at stage  830  the targeting system  700  transmits updated targeting (or destination) coordinates encoded in GPS-like signals to the guided vehicle  710  via the satellite  705  and/or the transmitter  715  using for example, the process  200  (of  FIG. 4 ) and/or the process  300  (of  FIG. 5 ). Alternatively, the targeting system  700  can transmit control information that changes the direction and/or speed of the guided vehicle  710  to control time of arrival and destination of the guided vehicle  710 . 
         [0046]    While communication with GPS enabled guided vehicles has been disclosed, other applications and types of communications are possible. For example, GPS-like signals can be used to transmit information to unmanned aerial vehicles, military aircraft, ground stations, individual troops, etc. 
         [0047]    Other embodiments are within the scope of the invention. For example, due to the nature of software, functions described above can be implemented using software, hardware, firmware, hardwiring, or combinations of any of these. Features implementing functions can also be physically located at various positions, including being distributed such that portions of functions are implemented at different physical locations. 
         [0048]    While  FIG. 1  has been described in context of a single GPS receiver (i.e., the GPS receiver  105 ), other quantities are possible. The GPS satellites  20 ,  30 , and  40  can be configured to transmit other information. The GPS receivers  105  can require upgrades/updates to use the method and systems described herein, such as software updates, firmware updates, hardware updates, etc. The PRN codes used to modulate the information can be totally orthogonal, or partially orthogonal. When two carrier frequencies are totally orthogonal to one another, the frequencies are chosen such that a receiver can reject an unwanted interfering signal, regardless of the intensity of the interfering signal. For example, when multiple modulation frequencies are used, each frequency overlaps with surrounding frequencies. When the signals are orthogonal, however, the points at which a desired frequency is measured, all other frequencies are zero (e.g., arrow  900  in  FIG. 8 ). The L1 band and/or the L5 band is preferably used for “life-critical” information (e.g., navigation information provided to a commercial airliner), although other frequency bands can be used. While some signals have been described as “GPS-like,” other formats are possible. For example, the navigation message format of a standard GPS signal can be replaced by another message format. 
         [0049]    Further, while the description above refers to the invention, the description may include more than one invention.