Patent Publication Number: US-7583225-B2

Title: Low earth orbit satellite data uplink

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application No. 60/801,764 filed on May 18, 2006 and entitled “Generalized high performance, low-cost navigation system” which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to navigation and, more particularly, to satellite-based navigation techniques. 
     BACKGROUND 
     Performance of a navigation system can be determined by the error distribution in navigation measurements (e.g., accuracy) provided by the system. System performance may also depend on its ability to provide timely warnings to users when it should not be used (e.g., integrity). Performance may also be measured by how long a navigation system takes to achieve its first position fix from a cold start (e.g., time to first fix). In addition, system performance may depend on the fraction of time or particular circumstances in which specified performance parameters fall within specified limits (e.g., availability). 
     Unfortunately, the navigation signals provided by various existing navigation systems often do not provide satisfactory system performance. In particular, the signal power, bandwidth, and geometrical leverage of such navigation signals are generally insufficient to meet the needs of many demanding usage scenarios. 
     Existing navigation approaches based, for example, on Global Positioning System (GPS) signals often provide insufficient signal power or geometry to readily penetrate buildings or urban canyons. Such signals may also be susceptible to jamming in hostile environments, and can prevent their usage in safety-of-life applications. Other navigation approaches based, for example, on cellular telephone or television signals typically lack vertical navigation information. 
     SUMMARY 
     In accordance with one embodiment of the invention, a method of performing navigation includes receiving a low earth orbit (LEO) signal from a LEO satellite; decoding a navigation signal from the LEO signal; receiving first and second ranging signals from first and second ranging sources, respectively; determining calibration information associated with the first and second ranging sources; and calculating a position using the navigation signal, the first and second ranging signals, and the calibration information. 
     In accordance with another embodiment of the invention, a navigation device includes an antenna adapted to receive a LEO signal from a LEO satellite and receive first and second ranging signals from first and second ranging sources, respectively; a receiver processor adapted to downconvert the LEO signal for further processing; and a navigation processor adapted to decode a navigation signal from the LEO signal, and adapted to calculate a position of the navigation device using the navigation signal, the first and second ranging signals, and calibration information associated with the first and second ranging sources. 
     In accordance with another embodiment of the invention, a navigation device includes means for receiving a LEO signal from a LEO satellite; means for decoding a navigation signal from the LEO signal; means for receiving first and second ranging signals from first and second ranging sources, respectively; means for determining calibration information associated with the first and second ranging sources; and means for calculating a position using the navigation signal, the first and second ranging signals, and the calibration information. 
     In accordance with another embodiment of the invention, a method of providing a LEO signal from a LEO satellite includes providing a plurality of transmit channels over a plurality of transmit slots, wherein the transmit channels comprise a set of communication channels and a set of navigation channels; generating a first pseudo random noise (PRN) ranging overlay corresponding to a navigation signal; applying the first PRN ranging overlay to a first set of the navigation channels; combining the communication channels and the navigation channels into a LEO signal; and broadcasting the LEO signal from the LEO satellite. 
     In accordance with another embodiment of the invention, a LEO satellite includes an antenna adapted to broadcast a LEO signal from the LEO satellite; and a processor adapted to: provide a plurality of transmit channels over a plurality of transmit slots, wherein the transmit channels comprise a set of communication channels and a set of navigation channels, generate a first PRN ranging overlay corresponding to a navigation signal, apply the first PRN ranging overlay to a first set of the navigation channels, and combine the communication channels and the navigation channels into the LEO signal. 
     In accordance with another embodiment of the invention, a LEO satellite includes means for providing a plurality of transmit channels over a plurality of transmit slots, wherein the transmit channels comprise a set of communication channels and a set of navigation channels; means for generating a first PRN ranging overlay corresponding to a navigation signal; means for applying the first PRN ranging overlay to a first set of the navigation channels; means for combining the communication channels and the navigation channels into a LEO signal; and means for broadcasting the LEO signal from the LEO satellite. 
     In accordance with another embodiment of the invention, a method of providing a data uplink to a LEO satellite includes determining position information using a LEO signal received from the LEO satellite, a first ranging signal received from a first ranging source, and a second ranging signal received from a second ranging source; determining a timing advance parameter using a local clock reference and a LEO satellite clock reference; preparing a data uplink signal comprising uplink data to be broadcast to the LEO satellite; synchronizing the data uplink signal with the LEO satellite using the timing advance parameter; and broadcasting the data uplink signal to the LEO satellite. 
     In accordance with another embodiment of the invention, a data uplink device includes an antenna adapted to: receive a LEO signal from a LEO satellite, receive first and second ranging signals from first and second ranging sources, respectively, and broadcast a data uplink signal to the LEO satellite; and a processor adapted to: determine position information using the LEO signal, the first ranging signal, and the second ranging signal, determine a timing advance parameter using a local clock reference and a LEO satellite clock reference, prepare the data uplink signal comprising uplink data to be broadcast to the LEO satellite, and synchronize the data uplink signal with the LEO satellite using the timing advance parameter. 
     In accordance with another embodiment of the invention, a data uplink device includes means for determining position information using a LEO signal received from the LEO satellite, a first ranging signal received from a first ranging source, and a second ranging signal received from a second ranging source; means for determining a timing advance parameter using a local clock reference and a LEO satellite clock reference; means for preparing a data uplink signal comprising uplink data to be broadcast to the LEO satellite; means for synchronizing the data uplink signal with the LEO satellite using the timing advance parameter; and means for broadcasting the data uplink signal to the LEO satellite. 
     In accordance with another embodiment of the invention, a navigation signal comprises at least a portion of a LEO signal provided by a LEO satellite, a method of performing localized jamming of the navigation signal includes filtering a noise source into a plurality of frequency bands to provide a plurality of filtered noise signals in the frequency bands, wherein the navigation signal is spread over a plurality of channels of the LEO signal, wherein the channels are distributed over the frequency bands and a plurality of time slots; generating a PRN sequence corresponding to a modulation sequence used by the LEO satellite to spread the navigation signal over the channels; modulating the filtered noise signals using the PRN sequence to provide a plurality of modulated noise signals; and broadcasting the modulated noise signals over an area of operations to provide a plurality of jamming bursts corresponding to the navigation signal, wherein the jamming bursts are configured to substantially mask the navigation signal in the area of operations. 
     In accordance with another embodiment of the invention, a navigation signal comprises at least a portion of a LEO signal provided by a LEO satellite, a jamming device configured to perform localized jamming of the navigation signal includes a noise source adapted to provide a noise signal; a plurality of filters adapted to filter the noise signal into a plurality of frequency bands to provide a plurality of filtered noise signals in the frequency bands, wherein the navigation signal is spread over a plurality of channels of the LEO signal, wherein the channels are distributed over the frequency bands and a plurality of time slots; a PRN sequence generator adapted to provide a modulation sequence used by the LEO satellite to spread the navigation signal over the channels; a plurality of oscillators adapted to modulate the filtered noise signals using the PRN sequence to provide a plurality of modulated noise signals; and an antenna adapted to broadcast the modulated noise signals over an area of operations to provide a plurality of jamming bursts corresponding to the navigation signal, wherein the jamming bursts are configured to substantially mask the navigation signal in the area of operations. 
     In accordance with another embodiment of the invention, a navigation signal comprises at least a portion of a LEO signal provided by a LEO satellite, a jamming device configured to perform localized jamming of the navigation signal includes means for filtering a noise source into a plurality of frequency bands to provide a plurality of filtered noise signals in the frequency bands, wherein the navigation signal is spread over a plurality of channels of the LEO signal, wherein the channels are distributed over the frequency bands and a plurality of time slots; means for generating a PRN sequence corresponding to a modulation sequence used by the LEO satellite to spread the navigation signal over the channels; means for modulating the filtered noise signals using the generated PRN sequence to provide a plurality of modulated noise signals; and means for broadcasting the modulated noise signals over an area of operations to provide a plurality of jamming bursts corresponding to the navigation signal, wherein the jamming bursts are configured to substantially mask the navigation signal in the area of operations. 
     The scope of the invention is defined by the claims, which are incorporated into this section by reference. A more complete understanding of embodiments of the present invention will be afforded to those skilled in the art, as well as a realization of additional advantages thereof, by a consideration of the following detailed description of one or more embodiments. Reference will be made to the appended sheets of drawings that will first be described briefly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  provides an overview of an integrated high-performance navigation and communication system in accordance with an embodiment of the invention. 
         FIG. 2  provides a further overview of the system of  FIG. 1  in accordance with an embodiment of the invention. 
         FIG. 3  illustrates an overall operational configuration of the system of  FIG. 1  in accordance with an embodiment of the invention. 
         FIG. 4  illustrates an approach for implementing low earth orbit signals in accordance with an embodiment of the invention. 
         FIG. 5  illustrates an autocorrelation function associated with low earth orbit signals in accordance with an embodiment of the invention. 
         FIG. 6  illustrates a process of decoding a military navigation component of a low earth orbit signal in accordance with an embodiment of the invention. 
         FIG. 7  illustrates a block diagram of a correlator of a navigation device in accordance with an embodiment of the invention. 
         FIG. 8  illustrates a process of decoding a commercial navigation component of a low earth orbit signal in accordance with an embodiment of the invention. 
         FIG. 9  illustrates an alternate process of decoding a commercial navigation component of a low earth orbit signal in accordance with an embodiment of the invention. 
         FIG. 10  illustrates a process of decoding a civil navigation component of a low earth orbit signal in accordance with an embodiment of the invention. 
         FIG. 11  illustrates a comparison between navigation components of a low earth orbit signal and GPS codes in accordance with an embodiment of the invention. 
         FIG. 12  illustrates a block diagram of a jamming device that may be used to perform localized jamming of navigation signals in accordance with an embodiment of the invention. 
         FIG. 13  provides a frequency and time domain representation of the operation of the jamming device of  FIG. 12  in accordance with an embodiment of the invention. 
         FIG. 14  illustrates a process of generating pseudo random noise in accordance with an embodiment of the invention. 
         FIG. 15  illustrates a process of constructing uniformly distributed integers of a modulo range from a channel selection pool in accordance with an embodiment of the invention. 
         FIG. 16  illustrates a process of converting a channel selection pool to a list of random non-overlapping channels in accordance with an embodiment of the invention. 
         FIG. 17  illustrates a frequency hopping pattern generated by the process of  FIG. 16  in accordance with an embodiment of the invention. 
         FIG. 18  illustrates a block diagram of a receiver processor configured to receive and sample navigation signals for downconversion in accordance with an embodiment of the invention. 
         FIG. 19  illustrates a block diagram of a navigation processor configured to perform ranging processing in accordance with an embodiment of the invention. 
         FIG. 20  illustrates various state variable definitions used by the navigation processor of  FIG. 19  in accordance with an embodiment of the invention. 
         FIG. 21  illustrates a block diagram of a tracking module configured to perform signal tracking in accordance with an embodiment of the invention. 
         FIGS. 22-29  illustrate various uses of a navigation system to perform navigation in different environments in accordance with various embodiments of the invention. 
         FIG. 30  illustrates a generalized frame structure for a low earth orbit satellite uplink in accordance with an embodiment of the invention. 
         FIG. 31  illustrates a ground infrastructure to synchronize a low earth orbit satellite data uplink in accordance with an embodiment of the invention. 
         FIG. 32  illustrates an implementation of a low level data uplink signal in accordance with an embodiment of the invention. 
         FIG. 33  illustrates a block diagram of a transmitter to support a low earth orbit satellite data uplink in accordance with an embodiment of the invention. 
         FIG. 34  illustrates a block diagram of various components of a low earth orbit satellite configured to support a data uplink in accordance with an embodiment of the invention. 
     
    
    
     Embodiments of the present invention and their advantages are best understood by referring to the detailed description that follows. It should be appreciated that like reference numerals are used to identify like elements illustrated in one or more of the figures. 
     DETAILED DESCRIPTION 
     In accordance with various embodiments discussed herein, a navigation system employing Low Earth Orbiting (LEO) satellites may be used to implement various navigation signals to provide high integrity navigation. Passive ranging signals from LEO satellites and other non-LEO transmitters (e.g., spaceborne and/or terrestrial), may be integrated into the system. 
     A reference network of monitor stations may estimate the clock bias, signal structure, and transmitter location or ephemeris of the various platforms from which the passive ranging signals are transmitted. This estimated information (also referred to as calibration information) may be conveyed to various navigation devices through a data link with LEO satellites or other data links. 
     The navigation devices may be configured to blend the broadcast information and the several different types of signals together to perform high-accuracy navigation. The broadcast LEO signal may be implemented with military, commercial, and civil navigation signals to permit partitioning of users among the different navigation signals and to enable infrastructure cost sharing. An integrated spread spectrum, low probability of intercept and detection (LPI/D) data uplink may also be provided as also described herein. 
     Referring now to the figures wherein the showings are for purposes of illustrating embodiments of the present invention only, and not for purposes of limiting the same,  FIG. 1  provides an overview of an integrated high-performance navigation and communication system  100  (also referred to as an iGPS system) in accordance with an embodiment of the invention. System  100  may include a navigation device  102  (also referred to as user equipment, a user device and/or a user navigation device) implemented with appropriate hardware and/or software to receive and decode signals from a variety of space and terrestrial ranging sources to perform navigation. Such signals may include, for example, satellite broadcasts from GPS, LEO (e.g., Iridium or Globalstar), Wide Area Augmentation System (WAAS), European Geostationary Navigation Overlay Service (EGNOS), Multi-functional Satellite Augmentation System (MSAS), Galileo, Quasi-Zenith Satellite System (QZSS), and/or Mobile Satellite Ventures (MSV) satellites. Such signals may also include terrestrial broadcasts from cellular towers, TV towers, WiFi, WiMAX, National Vehicle Infrastructure Integration (VII) nodes, and other appropriate sources. In one embodiment, navigation device  102  may be implemented in accordance with various embodiments set forth in U.S. patent application Ser. No. 11/268,317 filed on Nov. 7, 2005 which is incorporated herein by reference 
     In the example shown in  FIG. 1 , navigation device  102  may be configured to receive global positioning system (GPS) signals  106  (e.g., protected and/or unprotected GPS signals) from conventional navigation satellites. In addition, navigation device  102  may further receive signals  104  from various low earth orbit (LEO) satellites  108 . In this regard, each of LEO signals  104  (also referred to as iGPS signals) may be configured as a composite signal including a communication signal  104 A, a military navigation signal  104 B, a commercial navigation signal  104 C, and a civil navigation signal  104 D. Such an implementation allows LEO satellites  108  to simultaneously service military, commercial, and civil users, and allows such users to share the costs of operating system  100 . 
     In one example, LEO satellites  108  may be implemented by satellites of an existing communication system (e.g., Iridium or Globalstar) that have been modified and/or reconfigured to support system  100  as described herein. As also shown in  FIG. 1 , LEO satellites  108  may be implemented to support crosslink signals  110  between the various LEO satellites  108 . 
     Using GPS signals  106  and/or LEO signals  104 , navigation device  102  may calculate its position (and accordingly the position of an associated user) to high accuracy. Once determined, the calculated position data (and other data as may be desired) may then be uplinked to LEO satellites  108  using a spread spectrum data uplink described herein. 
     Navigation device  102  may be further configured to receive and perform navigation using broadcasts of other space and terrestrial ranging sources as may be desired in particular embodiments. In addition, navigation device  102  may be configured with an inertial measurement unit (IMU) implemented, for example, as a microelectromechanical system (MEMS) device to provide jamming protection as described herein. 
     Navigation device  102  may be implemented in any desired configuration as may be appropriate for particular applications. For example, in various embodiments, navigation device  102  may be implemented as a handheld navigation device, a vehicle-based navigation device, an aircraft-based navigation device, or other type of device. 
       FIG. 2  provides a further overview of system  100  in accordance with an embodiment of the invention. In particular,  FIG. 2  illustrates LEO satellites  108  and GPS satellites  202  in orbit around the earth.  FIG. 2  further illustrates various aspects of infrastructure subsystems of system  100 . For example, system  100  may include a reference network  204  configured to receive LEO signals  104  or other ranging signals, GPS ground infrastructure  206 , and LEO ground infrastructure  208 . It will be appreciated that additional spaceborne and/or terrestrial components may also be provided in various embodiments of system  100 . 
       FIG. 3  illustrates an overall operational configuration of system  100  in accordance with an embodiment of the invention. It will be appreciated that although a variety of subsystems are illustrated in  FIG. 3 , all of such subsystems need not be provided in all embodiments of system  100 . 
     As shown in  FIG. 3 , LEO satellites  108  exhibit rapid angle motion relative to navigation devices  102  and various illustrated terrestrial subsystems. Advantageously, this rapid angle motion can aid the terrestrial subsystems in solving for cycle ambiguities. In addition, LEO signals  104  may be implemented with high power relative to conventional navigation signals  106 . As such, LEO signals  104  may also enable penetration through interference or buildings. 
     LEO signals  104  may include a ranging and data link to the various ground terminals. As shown in  FIG. 3 , such terminals may include a geographically diverse reference network  204  and navigation devices  102  (illustrated in this example as a cell phone handset, MEMS device, and an automobile). 
     A variety of satellites are also illustrated, including GPS satellites  202 , Galileo satellites  306 , WAAS satellites  302 , and QZSS/MSV  304  satellites, any of which may be configured to broadcast ranging and data downlinks to reference network  204  and navigation devices  102  in accordance with various embodiments. 
     It will be appreciated that for purposes of clarity, some ranging signals are not shown in  FIG. 3 . For example, in one embodiment, all of the illustrated satellites may be configured to broadcast to all of navigation devices  102  and reference network  204 . 
     As also shown in  FIG. 3 , a variety of ranging signals  318  from a plurality of ranging signal sources  310  may be monitored by reference network  204  and navigation devices  102 . Reference network  204  may be configured to characterize each ranging signal source  310  to provide calibration information associated with each ranging signal source. Such information may be passed to LEO satellite  108  over an appropriate data uplink  320 , encoded by LEO satellite  108  into one or more of military, commercial, or navigation signals  104 B/ 104 C/ 104 D of LEO signal  104 , and broadcast to navigation devices  102  as part of LEO signal  104 . The calibration information can then be used by navigation devices  102  to interpret ranging signals  318  in order to perform navigation in combination with a ranging measurement performed using LEO signal  104 . 
     In general, a variety of transmitters can provide timing and (and therefore ranging) data. In this regard, for a generalized ranging source, its associated ranging signal may be received by reference network  204  and navigation devices  102 . Reference network  204  may determine calibration information associated with the ranging signal, and telemeter such calibration information to navigation devices  102  through a data uplink with LEO satellites  108  and/or through terrestrial links to navigation devices. 
     For example,  FIG. 3 , illustrates GPS signals  106  being received by one of ranging signal sources  310  implemented as a WiFi node. If the capability to measure the timing (equivalent to range if multiplied by the speed of light) of pre-defined attributes of a WiFi signal is implemented within a GPS receiver, the receiver can measure the received WiFi and GPS signal times concurrently. The difference between these quantities can be calculated, time tagged, and transferred to reference network  204  to provide calibration information associated with the WiFi node. Additional calibration information may be determined by reference network  204  in response to receiving GPS signals  106  and other types of ranging signals  318 . In each case, reference network  204  may telemeter real-time calibration information associated with the WiFi node to navigation devices  102  through LEO satellite  104  over uplink  320  and LEO signal  104  (e.g., over space-based links). Calibration information may also be provided to navigation devices  102  over terrestrial links. Advantageously, each ranging signal source  310  does not necessarily need to be in view of all nodes of reference network  204  if a network  316  (e.g., the Internet) is present between the various terrestrial nodes. 
     As discussed, LEO satellites  108  may be implemented as communication satellites (for example, Iridium or Globalstar satellites) that have been modified and/or reconfigured as described herein to support navigation features of system  100 . Tables 1 and 2 below identify various attributes of Iridium and Globalstar communication satellites, respectively, that may be used as LEO satellites  108  in accordance with various embodiments: 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
            
               
                   
                 Based on GSM Cell Phone Architecture 
               
               
                   
                 Both FDMA and TDMA 
               
               
                   
                 41.667 kHz channel divisions 
               
               
                   
                 10.5 MHz downlink allocation 
               
               
                   
                 40% Root Raised Cosine QPSK modulation at 25,000 sps 
               
               
                   
                 90 ms frame 
               
               
                   
                 Time Slots: (1) simplex down, (4) 8.28 ms duplex up, (4) 8.28 ms 
               
               
                   
                 duplex down 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
             
            
               
                   
                 Based on CDMA IS-95 Cell Phone Architecture 
               
               
                   
                 Both FDMA and CDMA 
               
               
                   
                 1.25 MHz channel divisions 
               
               
                   
                 16.5 MHz downlink allocation 
               
               
                   
                 Bent-Pipe Transponder 
               
               
                   
                   
               
            
           
         
       
     
     In one example where Iridium communication satellites are used to implement LEO satellites  108 , flight computers of the Iridium communication satellites can be reprogrammed with appropriate software to facilitate the handling of navigation signals. In another example where Globalstar communication satellites are used to implement LEO satellites  108 , the satellite bent pipe architecture enables ground equipment to be upgraded to enable a variety of new signal formats. 
     In embodiments where LEO satellites  108  are implemented using communication satellites, the communication satellites may be configured to support communication signals as well as navigation signals. In this regard, such navigation signals may be implemented to account for various factors such as multipath rejection, ranging accuracy, cross-correlation, resistance to jamming and interference, and security, including selective access, anti-spoofing, and low probability of interception. 
       FIG. 4  illustrates an approach for implementing LEO signals  104  in accordance with an embodiment of the invention. In particular, blocks  410 ,  420 , and  430  of  FIG. 4  illustrate the structure of signals transmitted and received by LEO satellites  108  to provide support for communication and navigation signals, where LEO satellites  108  are implemented using existing Iridium communication satellites. In blocks  410 ,  420 , and  430 , frequency is shown in the horizontal axis, time is shown in and out of the page, and power spectral density is shown in the vertical axis. 
     In one embodiment, LEO satellite  108  may be configured to support a plurality of channels implemented as a plurality of transmit slots  402  and a plurality of receive slots  404  configured in a time division multiple access (TDMA) fashion over a 90 ms frame width, and further configured in a frequency division multiple access (FDMA) fashion over a 10 MHz frequency bandwidth. In this regard, it will be appreciated that each channel may correspond to a particular transmit or receive slot of a frame provided in a particular frequency band. For example, in one embodiment, LEO satellite  108  may be implemented to support the transmission of approximately 960 channels, with 240 frequency bands providing 4 time slots per frame (e.g., 240 frequency bands×4 time slots=960 channels). 
     As shown in block  410 , some of the transmit slots  402  and receive slots  404  may be associated with existing communications (e.g., shown in  FIG. 4  as telephone calls  440 ). The used transmit slots  402  may correspond to the data provided over communication signal  104 A of LEO signal  104  transmitted by LEO satellite  108 . 
     It will be appreciated that in the embodiment shown in block  410 , a plurality of transmit slots  402  remain unused. In accordance with various embodiments of the invention, the unused communication capacity of unused transmit slots  402  may be leveraged to support navigation signals as described herein. 
     As shown in block  420 , a ranging overlay  422  of pseudo random noise (PRN) may be introduced in each of the remaining unused transmit slots  402 . Ranging overlay  422  can be run at low average power on a channel-by-channel basis, but with the aggregate ranging overlay  422  exhibiting high power to overcome jamming. In contrast, block  430  shows ranging overlay  422  implemented using a maximum power spot beam provided by LEO satellite  108 . 
     In one embodiment, ranging overlay  422  may be implemented using a combination of frequency hopping and direct sequence PRN. For the frequency hopping component, a subset of frequencies may be chosen on a pseudo-random basis each burst. Then, within each burst, the data bits are also chosen on a pseudo-random basis. 
     In one embodiment, telephone calls  440  may be given priority in transmit slots  402  over ranging overlay  422 , with ranging overlay  422  being little affected by occasional missing or corrupted bursts. In another embodiment, ranging overlay  422  may be given priority in transmit slots  402  over telephone calls  440 , with telephone calls  440  similarly being little affected by occasional missing or corrupted bursts. 
     In one embodiment, ranging overlay  422  may be implemented with as wide a bandwidth as possible subject to spectrum regulations. In this case, all available channels may be used, and various methods of frequency, time, and code division multiple access (CDMA) may be employed to create a downlink signal that tends to look like flat white noise unless the user knows the code. The flatness provides a signal that is well suited for accuracy, jam resistance, and multipath rejection. Cross correlation can be minimized by using an appropriate encryption algorithm made possible by fast digital signal processing in navigation device  102 . 
     In one embodiment, LEO signal  104  may be implemented as a complex signal s(t) versus time t as shown in the following equation: 
     
       
         
           
             
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     In the above equation, A is the signal amplitude, n is the symbol index, p is the direct-sequence pseudo-random noise value given as ±1, h is the symbol impulse response, m is the channel frequency index, f0 is the spread spectrum broadcast span, and N is the number of channel frequencies forming the spread spectrum broadcast span. 
     In another embodiment where LEO satellites  108  are implemented by Globalstar satellites, a low-power direct-sequence code may be provided on each of the 1.25 MHz channels that is orthogonal to telephony traffic. 
       FIG. 5  illustrates an autocorrelation function  502  that may be implemented by navigation device  102  to lock on to LEO signal  104  in accordance with an embodiment of the invention. In  FIG. 5 , τ is the autocorrelation argument, R is the autocorrelation function of the basic 40% root raised cosine symbol impulse response, N is the number of channels allowable by LEO satellite&#39;s  108  spectrum allocation (e.g., a maximum of 240 in one embodiment), f 0  is the allowable frequency span (related to N by the channel spacing such that f 0 =[41.667 kHz]N in one embodiment), and φ m  is the satellite phase bias for each channel. 
     In addition,  FIG. 5  provides plots  504  and  510  of autocorrelation function  502  using different scales. In plot  504 , an envelope  506  of autocorrelation function  502  is shown as being formed by the effective correlation length of the 25 ksps direct sequence data. In this embodiment, autocorrelation is formed by the aggregation of the broadband channels separated by 41.667 kHz. For example, for a 10 MHz wide broadcast, the effective direct sequence chip length may be that of Y code, namely 30 m. For comparison, an example GPS coarse/acquisition (C/A) code  512  and an example GPS military (M) code  514  are also shown superimposed on plot  510 . As shown in plot  510 , the side lobes of autocorrelation function  502  are as readily manageable as those for GPS M-code  514 . In this regard, the side lobes of autocorrelation function  502  are either highly attenuated or clearly distinguishable. 
     As previously described, LEO signal  104  may include various navigation signals including military navigation signal  104 B, commercial navigation signal  104 C, and civil navigation signal  104 D. As such, navigation devices  102  may be configured to decode one or more of these signals to perform navigation. 
     For example,  FIG. 6  illustrates a process of decoding military navigation signal  104 B of LEO signal  104  in accordance with an embodiment of the invention. It will be appreciated that the process of  FIG. 6  may be performed by navigation device  102  in response to receiving LEO signal  104 . 
     In various applications, it is desirable to implement military navigation signal  104 B as a high power signal to overcome possible jamming. Accordingly, as shown in step  1  of  FIG. 6 , LEO signal  104  may include several parallel channels  602  (shown as 12 channels in  FIG. 6 ) configured to carry military navigation signal  104 B. In one embodiment, a pseudo-random process may be used to determine the particular channels  602  activated for each broadcast burst from LEO satellites  108 . Also shown in step of  FIG. 6 , a string of quadrature phase-shift key (QPSK) symbols  604  are illustrated for each parallel burst on channels  602 , with time going into the page. QPSK symbols  604  are modulated with the PRN direct sequence encoding and also exhibit bias and rotation based on their frequency offset in LEO signal  104 . 
     In step  2  of  FIG. 6 , the PRN encoding is despread by rotating each burst to baseband, subtracting off inter-channel bias, and stripping off the PRN direct sequence pattern to provide a set of bursts carrying data associated with military navigation signal  104 B, as represented by modified QPSK symbols  606 . 
     In step  3  of  FIG. 6 , low-bit rate data is demodulated according to a set of M possible orthogonal macro symbols  608 . If quarter cycle ambiguities from the QPSK modulation are present, the combined ambiguities and macro symbols may not be perfectly orthogonal. Once the data is estimated, a hard decision algorithm strips off the estimated data leaving only unmodulated carrier  610 . 
     In step  4  of  FIG. 6 , the carrier is averaged over the entire burst and then over each channel. As a result, an in phase and quadrature measurement  612  of the instantaneous tracking error can be provided. A phase locked loop (PLL) of navigation device  102  is then used to track the satellite carrier. 
       FIG. 7  illustrates a block diagram of a correlator of navigation device  102  that may be used to perform the process of  FIG. 6  in accordance with an embodiment of the invention. A numerically controlled oscillator  702  generates a carrier that downconverts the incoming LEO signal  104  (e.g., received through an antenna of navigation device  102 ) to a baseband signal  714 . Baseband signal  714  is provided to an upper path  704  that performs punctual code carrier tracking. Baseband signal  714  is also provided to a lower path  706  that performs early minus late detection. 
     In lower path  706 , a bank of synthesizers  708  and PRN generators  710  replicate each channel of LEO signal  104 . In upper path  704 , replicated signals  712  are mixed with baseband signal  714  to remove all code and phase rotation for each channel separately. A hypothesis generator  716  computes the signal associated with each of the possible macro symbols  608  and quarter cycle ambiguities, if any. A processor  718  uses a maximum a posteriori (MAP) algorithm to provide a data estimate  720  identifying which of the macro symbol hypotheses is most likely. As shown, data estimate  720  is passed to lower path  706  for use in early minus late detection. To perform punctual detection in upper path  704 , processor  718  strips off the data and outputs the resulting bursts to summing block  722  that integrates the aggregate bursts over time to arrive at the in phase and quadrature tracking error  724 . 
     In lower path  706 , replicated signals  712  are further modulated by an early minus late block  726  and a data generator block  728  (using data estimate  720  received from upper path  704 ). As shown, the resulting modulated signals are summed together to form a composite early minus late replica signal  730  that is mixed with baseband signal  714  and sent to summing block  732  for time averaging to provide an early minus late discriminator  734 . Accordingly, given carrier lock and a sufficient averaging interval, early minus late discriminator  734  provides a measure of the instantaneous tracking error. 
       FIG. 8  illustrates a process of decoding commercial navigation signal  104 C of LEO signal  104  in accordance with an embodiment of the invention. It will be appreciated that the process of  FIG. 8  may be performed by navigation device  102  in response to receiving LEO signal  104 . 
     As shown, the process of  FIG. 8  is similar to the process of  FIG. 6 , with steps  1 - 4  of  FIG. 8  generally corresponding to steps  1 - 4  of  FIG. 6 . However, it will be appreciated that in the process of  FIG. 8 , fewer channels  802  (e.g., 2 channels in the illustrated embodiment) are used in comparison with channels  602  of  FIG. 6 . Because of the fewer number of channels  802  used, commercial navigation signal  104 C of LEO signal  104  may be implemented with lower power and lower bandwidth than military navigation signal  104 B. 
       FIG. 9  illustrates an alternate process of decoding commercial navigation signal  104 C of LEO signal  104  in accordance with an embodiment of the invention. As shown, the process of  FIG. 9  is similar to the process of  FIG. 8 , with steps  1 - 2  of  FIG. 9  generally corresponding to steps  1 - 2  of  FIG. 8 . However, in step  3  of  FIG. 9 , it is assumed that downlink data (e.g., calibration information) can be received by a navigation device  102  in a manner other than LEO signal  104  (for example, from a link to reference network  204  or one or more of nodes  310  shown in  FIG. 3 ). Further processing can then be performed in steps  4  and  5  of  FIG. 9 , similar to steps  3  and  4  of  FIG. 8 , respectively. Advantageously, the insertion of step  3  in the process of  FIG. 9  can provide higher sensitivity in indoor environments. In this regard, navigation device  102  can receive a reliable representation of downlink data from one or more reference stations of reference network  204 , without requiring navigation device  102  to perform downlink data and/or quarter cycle stripping, thereby reducing the processing required by navigation device  102  and improving signal processing gain. 
       FIG. 10  illustrates a process of decoding civil navigation signal  104 D of LEO signal  104  in accordance with an embodiment of the invention. In various embodiments, the use of civil navigation signal  104 D may be generally focused on carrier-only navigation. As a result, civil navigation signal  104 D may be implemented with relatively narrow bandwidth (for example, approximately 1 MHz) and may be publicly known. As such, channels  1002  used for civil navigation signal  104 D may be implemented without significant spectrum spread. In this regard, it will be appreciated that channels  1002  illustrated in step  1  of  FIG. 10  are closely grouped in comparison with channels  602  and  802  illustrated in step  1  of each of  FIGS. 6 ,  8 , and  9 . It will be appreciated that the operation of steps  1 - 4  of  FIG. 10  will be understood from the operation steps  1 - 4  of  FIG. 6  previously discussed. 
     In view of the above discussion, it will be appreciated that in certain embodiments military, commercial, and civil navigation signals  104 B,  104 C, and  104 D of LEO signal  104  may be implemented with the following attributes identified in the following Table 3: 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Signal 
                 Power 
                 Bandwidth 
               
               
                   
                   
               
             
            
               
                   
                 Military 
                 Maximum 
                 Maximum 
               
               
                   
                 Commercial 
                 Moderate 
                 High 
               
               
                   
                 Civil 
                 Moderate 
                 Moderate 
               
               
                   
                   
               
            
           
         
       
     
     In another embodiment of the invention, system  100  can be implemented to permit military use of military navigation signal  104 B while simultaneously denying use of commercial and/or civil navigation signals  104 C and  104 D to adversaries in a particular area of operations, without compromising use of commercial and civil navigation signals  104 C and  104 D outside the area of operations. 
     For example, in one embodiment, the decoding of commercial navigation signal  104 C may be conditioned on the use of a distributed encryption key that may be permitted to expire over the area of operations. In another embodiment, the broadcasting of commercial navigation signal  104 C by LEO satellites  108  may be selectively interrupted over the area of operations (for example, individual spot beams from LEO satellites  108  may be independently turned off). 
     In another embodiment, commercial navigation signal  104 C and/or civil navigation signal  104 D may be locally jammed within the area of operations. In this regard,  FIG. 11  illustrates a comparison between military navigation signal  104 B, civil navigation signal  104 D, and GPS C/A code  512 , and GPS M-code  514 . 
     As shown in  FIG. 11 , GPS C/A code  512  can be jammed for military purposes by jamming the C/A code band. As also shown in  FIG. 11 , civil navigation signal  104 D can be viewed as a subset of military navigation signal  104 B in both power spectral density and bandwidth. If ranging overlay  422  is implemented using both FDMA and TDMA, it can be seen that civil navigation signal  104 D is manifested in frequency hopping bursts as shown in  FIG. 11 . 
       FIG. 12  illustrates a block diagram of a jamming device  1200  that may be used to perform localized jamming of civil and commercial navigation signals  104 C and  104 D in accordance with an embodiment of the invention. As shown in  FIG. 12 , a white noise source  1202  (for example, created using Brownian motion) is processed by a filter  1204  to provide a noise signal  1206  having a bandwidth corresponding approximately to a transmission channel of LEO satellite  108 . 
     A military receiver device  1208 , generator  1210 , and oscillators  1212 / 1214  are configured to provide multiple channels  1216  corresponding to the instantaneous frequency of civil navigation signal  104 D as determined by a predefined, published civil PRN sequence. Channels  1216  are used to modulate noise signal  1206  which is then upconverted using additional illustrated components to emit jamming bursts at precisely the times, durations, and frequencies of civil navigation signal  104 D received from LEO satellites  108  as part of LEO signal  104 . It will be appreciated that the above approach can also be used to provide jamming of commercial navigation signal  104 C as may be desired in particular implementations. 
       FIG. 13  provides a frequency and time domain representation of the operation of the jamming device of  FIG. 12  in accordance with an embodiment of the invention. As shown in  FIG. 13 , individual noise bursts  1302  provided by jamming device  1200  are focused in a narrow frequency band  1304  corresponding to civil navigation signal  104 D. Advantageously, military navigation signal  104 B components (represented by dark rectangles  1306 ) is effectively unchanged and is fully available for military operations. 
     The generation of ranging overlay  422  at LEO satellite  108  will now be described in relation to  FIGS. 14-17 . In this regard, various processes described in relation to  FIGS. 14-17  may be performed by appropriate processors of LEO satellite  108 . In addition, LEO satellite  108  may be configured with appropriate software and hardware to modulate and broadcast communication signals (e.g., telephony bursts) in QPSK format. 
       FIG. 14  illustrates an approach to generating pseudo random noise in accordance with an embodiment of the invention. The embodiment shown in  FIG. 14  uses a counter-based pseudo-random number generator  1400 . In this regard, a counter value  1402  is combined with a 128-bit encryption traffic key  1404  to provide a 128-bit cipher. By associating counter value  1402  with cipher  1406 , the various PRN elements of ranging overlay  422  can be constructed. In one embodiment, counter input  1402  and cipher may each be implemented as 128-bit words using the Advanced Encryption Standard (AES) process. 
     As shown in  FIG. 14 , each counter value  1402  may include a type flag  1412  that identifies each counter value  1402  as specifying either a channel selection (e.g., if type flag  1412  is set to a “1”) or direct sequence chips (e.g., if type flag  1412  is set to a “0”). If type flag  1412  is set to channel selection, then other bits of counter value  1402  may specify which channels of a channel selection pool  1408  through which to broadcast data burst chips. If type flag  1412  is set to direct sequence, then other bits of counter value  1402  may correspond to a chip block index  1414  (e.g., specifying a particular one of direct sequence chips  1410  to be broadcast) and a burst count  1416  (e.g., specifying a frame number of the particular direct sequence chip  1410  to be broadcast). 
     In one embodiment, cipher  1406  can be used to select a value from a channel selection random number pool  1408  that directs frequency hopping. In another embodiment, cipher  1406  can be used to select direct sequence chips  1410  that fill up the QPSK data bits. 
       FIG. 15  illustrates a process of constructing uniformly distributed integers of a modulo range from channel selection pool  1408  in accordance with an embodiment of the invention. It will be appreciated that the process of  FIG. 15  may be used in conjunction with channel selection pool  1408  previously described in relation to  FIG. 14 . 
       FIG. 16  illustrates a process of converting channel selection pool  1408  to a list of random non-overlapping channels in accordance with an embodiment of the invention. The process of  FIG. 16  can be used for military navigation signal  104 B, commercial navigation signal  104 C, and civil navigation signal  104 D, by selecting different parameters for M and N (shown in  FIG. 16 ) in accordance with values provided in the following Table 4: 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 Signal 
                 Power (N) 
                 Bandwidth (M) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Military 
                 Large 
                 240 
               
               
                   
                 Commercial 
                 1 or 2 
                 &gt;100 
               
               
                   
                 Civil 
                 1 or 2 
                 8-32 
               
               
                   
                   
               
            
           
         
       
     
       FIG. 17  illustrates a frequency hopping pattern generated by the process of  FIG. 16  in accordance with an embodiment of the invention. As shown in  FIG. 16 , various random channel selections (associated with corresponding transmission frequencies) are provided for successive transmission bursts. It will be appreciated that each frequency and chip is generated in a pseudo random manner using a common key (for example, a 128-bit key) known in advance by LEO satellite  108  and navigation device  102 . 
       FIGS. 18-21  illustrate various aspects of navigation device  102  that may be implemented in accordance with various embodiments of the invention. For example,  FIG. 18  illustrates a block diagram of a receiver processor  1800  of navigation device  102  configured to receive and sample signals for downconversion in accordance with an embodiment of the invention. As shown in  FIG. 18 , navigation signals received by an antenna  1802  are filtered by multi-band filters  1804  (to preselect desired frequency bands), amplified by amplifier  1806 , and sampled by sample and hold circuitry  1808  to provide raw digital RF samples  1816 . 
     Receiver processor  1800  also includes an oscillator  1810  and synthesizer  1812  that may be used to synchronize sample and hold circuitry  1808 . In various embodiments, the sample rate of sample and hold circuitry  1808  may be chosen to prevent overlap among aliased, pre-selected frequency bands. 
     Receiver processor  1800  also includes an IMU  1814  implemented as a 3-Axis MEMS gyro and accelerometer having measurement time tags synchronized to the common clock of the receiver, and may be used to provide raw digital motion samples  1818 . It will be appreciated that other receiver implementations may alternatively be used to facilitate single or multiple-step down conversion. 
       FIG. 19  illustrates a block diagram of a navigation processor  1900  of a navigation device  102  configured to perform ranging processing in accordance with an embodiment of the invention. As shown in  FIG. 19 , a Hilbert transform block  1902  converts raw digital RF samples  1816  into complex samples  1904 . A plurality of tracking modules  1906  are provided. Each tracking module  1906  is associated with a different signal provided in complex samples  1904 , and can be used to track either satellite or terrestrial ranging sources. 
     Navigation processor  1900  provides feed forward commands  1908  to tracking modules  1906  based on raw digital motion samples  1818  processed by inertial processor  1916  and extended Kalman filter  1914 . Aiding information  1908  drives tracking modules  1906  to a small fraction of a wavelength. The raw code and carrier phase measurements  1910  from tracking modules  1906  are read into navigation preprocessor  1912 , processed by extended Kalman filter  1914 , and combined to provide a position fix  1918 . 
       FIG. 20  illustrates various state variable definitions employed by extended Kalman filter  1914  of navigation processor  1900  in accordance with an embodiment of the invention. In one embodiment, a navigation processing method disclosed by the previously referenced U.S. patent application Ser. No. 11/268,317 may be used to perform navigation using a plurality of ranging sources. 
     In  FIG. 20 , equation  2002  is a model of an integrate and dump correlator. The output tracking error Δy is modeled by averaging over time T the difference between the actual phase and the phase predicted by the filter. Equation  2004  is a continuous time update model of the complete navigation system, including inertial, clock, and all timing and ranging sources, both terrestrial and space based. The estimator state vector variables are cumulative correlator phase, user position, velocity, attitude, accelerometer bias, gyro bias, range bias, range bias rate, clock bias, and clock bias rate. Equation  2006  is the carrier phase observation model, showing time transfer feed forward to the user from the reference site taking into account geometry and atmospheric error. 
       FIG. 21  illustrates a block diagram of one of tracking modules  1906  in accordance with an embodiment of the invention. Tracking module  1906  receives feed forward commands  1908  to preposition both the code and carrier phase for the particular ranging signal being tracked by tracking module  1906 . Downconverter  1950  rotates the carrier provided in complex samples  1904  to baseband as a first processing step. Next, the downconverted signal  1952  signal is split and passed to a matched early minus late filter  1954  and a matched punctual filter  1956 . 
     The signal waveform for each ranging signal in view is either pre-stored in user memory or, optionally, refreshed via a data link with a LEO satellite  108  or a network (e.g., cellular, WiFi, WiMAX, or VII) node. The data link update enables extension of the architecture to be used with virtually any transmitted signal. This impulse response (analogous to PRN code for a GPS satellite) forms a basis for matched filter processing. The impulse response of a terrestrial signal such as cellular, WiFi, WiMAX, VII, or television may be tailored by retaining the deterministic portion of the reference signal. Any portion of the signal that contains non-deterministic characteristics, such as unknown data, is nulled out in the reference signal. Each of these matched filters is then provided with the reference signal structure impulse response for implementation in the matched filter/correlator. As a result, filters  1954  and  1956  provide in-phase and quadrature representations of early minus late tracking errors  1958  and punctual tracking errors  1960 , respectively. 
     Various data structures may be used to encode ranging sources in accordance with various embodiments of the invention. For example, in one embodiment, a ranging signal can be represented by the following code: 
     
       
         
           
               
               
             
               
                   
               
             
            
               
                 struct ranging_signal { 
                 /* Generalized Ranging Source 
               
               
                 Parameters */ 
               
            
           
           
               
               
               
               
            
               
                   
                 impulse_response 
                 broadcast_signal; 
                 /* signal structure of 
               
            
           
           
               
            
               
                 ranging source */ 
               
            
           
           
               
               
               
               
            
               
                   
                 double 
                 broadcast_frequency; 
                 /* ranging source 
               
            
           
           
               
            
               
                 frequency */ 
               
            
           
           
               
               
               
               
            
               
                   
                 position 
                 broadcast_location; 
                 /* phase center of ranging 
               
            
           
           
               
            
               
                 source */ 
               
            
           
           
               
               
               
               
            
               
                   
                 time 
                 broadcast_clock; 
                 /* clock bias of ranging 
               
            
           
           
               
            
               
                 source */ 
               
            
           
           
               
               
            
               
                   
                 }; 
               
               
                   
                   
               
            
           
         
       
     
     In the code above, the signal reference waveform is encoded as an impulse response parameter whose time origin is tied to the broadcast clock. The broadcast frequency is the carrier frequency of the ranging source. The broadcast location is encoded as a precision ephemeris for space vehicles and as a Cartesian static coordinate for terrestrial ranging sources. A clock correction calibrates the ranging source against system time based on Coordinated Universal Time (UTC) (e.g., provided by the United States Naval Observatory (USNO)). 
     In various embodiments, appropriate ground stations may be configured to decipher new ranging signal codes employed by LEO satellites  108  in near real-time. In this regard, such ground stations may provide the deciphered codes to navigation devices  102 , thereby permitting navigation devices  102  to perform navigation using virtually any signal, cooperative or not. 
       FIGS. 22-29  illustrate various uses of system  100  to perform navigation in different environments services in accordance with various embodiments of the invention. For example,  FIG. 22  illustrates the use of system  100  to provide indoor positioning in accordance with an embodiment of the invention. In this regard, it will be appreciated that in  FIG. 22 , navigation device  102  may be positioned inside a building or other structure. 
     As shown in  FIG. 22 , navigation device  102  (for example, a handheld user navigation device) may receive LEO signal  104  either directly from LEO satellite  108  and additional ranging signals  318  from nodes  310 . As also shown, reference stations of reference network  204  may also receive ranging signals  318 . As previously discussed, reference network  204  may be configured with appropriate hardware or software to determine calibration information associated with each ranging signal source  310 , passed to LEO satellite  108  over data uplink  320 , encoded by LEO satellite  108  into LEO signal  104 , and broadcast to navigation device  102  as part of LEO signal  104 . The calibration information can then be used by navigation devices  102  to interpret ranging signals  318  in order to perform navigation in combination with a ranging measurement performed using LEO signal  104 . As a result, navigation device  102  may utilize LEO signal  104  and ranging signals  318  to perform navigation. 
     Military navigation signal  104 B (e.g., provided by LEO satellite  108  as part of LEO signal  104 ) as well as ranging signals  318  (e.g., provided by ranging signal sources  310  such as cellular or television signal sources) may be implemented as high power signals capable of penetrating building materials to reach navigation device  102  when positioned in indoor environments. Accordingly, by using such high power signals in the approach shown in  FIG. 22 , navigation device  102  may perform navigation indoors and acquire quickly from a cold start. 
       FIG. 23  illustrates the use of system  100  to provide indoor positioning in accordance with another embodiment of the invention. It will be appreciated that the implementation shown in  FIG. 23  generally corresponds with the implementation of  FIG. 22  previously discussed. However, in the embodiment shown in  FIG. 23 , navigation device  102  may also optionally communicate with reference network  204  or nodes  312  or  314  through network  316 . 
     In addition, system  100  may be configured to employ on-tether commercial signal processing as described herein with regard to  FIG. 8 . In this case, a lower power commercial navigation signal  104 C may be used to obtain increased processing gain by transmitting a replica of the navigation data encoded in commercial navigation signal  104 C over ranging signals  318 . Because the navigation data is removed using the process of  FIG. 8 , tracking loop bandwidth may be significantly reduced. 
     In one embodiment, navigation device  102  may determine its final position fix by forming a vector of pseudoranges for each ranging source, k, then linearizing about an initial guess for user position, x, and user clock bias τ. 
     
       
         
           
             
               
                 
                   
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     The method of least squares is used to refine the user position estimate: 
     
       
         
           
             
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     In another embodiment, system  100  may be implemented to provide high-accuracy, high-integrity navigation. In this regard,  FIG. 24  illustrates the use of system  100  to perform navigation using GPS signals  106  and dual band LEO signals  104  and  104 ′ in accordance with an embodiment of the invention. Specifically,  FIG. 24  shows how a single-frequency L1 GPS signal may be used with two different LEO signals  104  and  104 ′ (e.g., different LEO signals in different frequency bands from different LEO satellites  108  and  108 ′) to provide a high level of navigation performance. In the embodiment shown in  FIG. 24 , the carriers of GPS signals  106  and LEO signals  104  and  104 ′ are sufficient for navigation—the code phases from the signals need not be used. However, in another embodiment, both code and carrier are used to derive maximum information from the available observables. 
     In  FIG. 24 , stations of reference network  204  may monitor GPS signals  106  and LEO signals  104  and  104 ′, and gather continuous carrier phase information to carry out precise orbit determination of GPS satellites  202  and LEO satellites  108 . By using different LEO signals  104  and  104 ′, effects of the ionosphere can be removed, yielding a carrier phase signal that is ionosphere free. Cycle ambiguities of all GPS satellites  202  and LEO satellites  104  and  104 ′ (e.g., shown by ellipsoids  2402 ) by can be estimated by taking advantage of the large angle motion of LEO satellites  104  and  104 ′. 
     The position of navigation device  102  (e.g., an aircraft in this embodiment) can be determined in  FIG. 24  in a manner similarly described above with regard to  FIGS. 22-23 . In particular, the following notation provides the kth pseudorange measurement to determine the user position, x, at epoch m, and the tropospheric zenith delay, DZ, along with all the satellite range biases, modeled as continuous variable, b. 
     
       
         
           
             
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     Again, the method of least squares is used to solve the system of equations for the position adjustments, time biases, and vector of range biases. Even though measurements using GPS signals  106  are single frequency and subject to ionospheric bias, the resulting solution does not have an ionospheric dependence. Because measurements using LEO signals  104  and  104 ′ are ionosphere free and because LEO satellites  104  and  104 ′ exhibit rapid angle motion (compared with the virtually static motion of GPS satellites  202 ), the geometry matrix is full rank with the exception of a common mode between the clock and the ranging biases. This means that the bias estimates for GPS satellites  202  take on values that position the user correctly based on the ionosphere-free measurements using LEO signals  104  and  104 ′. 
       FIG. 25  illustrates the use of system  100  to perform navigation using GPS signals  106  and a single LEO signal  104  in accordance with an embodiment of the invention. The orbit geometry of a single LEO satellite  108  in view tends to place the LEO satellite  108  on a trajectory that aligns a position uncertainty ellipsoid  2502  with the local horizontal. In addition to LEO signal  104  and GPS signal  106 , a third signal  2504  (e.g., from Galileo satellite  306  or another satellite) may be optionally used by navigation device  102  (e.g., an aircraft in this embodiment) to determine its position. 
     The integrity of a navigation system can be measured by the system&#39;s ability to provide timely warnings to users when it should not be used. In this regard, the integrity risk of a navigation system can be characterized as the probability of an undetected hazardous navigation system anomaly. In one embodiment, system  100  can be implemented to provide high integrity using Receiver Autonomous Integrity Monitoring (RAIM). In RAIM implementations, navigation device  102  can be configured to monitor measurement self-consistency to detect navigation errors associated with a variety of failure modes. Advantageously, the rapid motion of LEO satellites  108  can facilitate such measurements. 
     With RAIM, the residual of the least squares fit is used to carry out a chi-square hypothesis detection of a system fault. In this regard, the following equation may be used:
 
 R=|Δφ−H{circumflex over (x)}| 
 
     In the above equation φ corresponds to ranging measurements, H corresponds to a satellite geometry matrix, and {circumflex over (x)} corresponds to a position estimate. Following its determination of every position fix, navigation device  102  may be configured to calculate measurement residual R. If R is less than a threshold value, then system  100  is deemed to be operating properly. If R is greater or equal to a threshold value, the navigation device  102  may issue an integrity alarm. 
       FIG. 26  shows the effect of a ranging error on a position solution in accordance with an embodiment of the invention. Ordinarily, the ranging measurements are self consistent. However, should one or more of the measurements be corrupted and biased, the error could push the output solution away from the truth. RAIM is able to detect the error because the inconsistency among measurements is highly correlated with the actual position error. 
       FIG. 27  illustrates how the precision of the system carrier phase counterbalances occlusion and poor Dilution of Precision (DOP) geometry. In the two-dimensional case, the least squares fit excludes the vertical component of the position error. Advantageously, in one embodiment, system  100  may be implemented with centimeter-level carrier phase precision to provide robust navigation during occlusion. As shown, the process of  FIG. 27  may also use a pre-surveyed altitude map. 
       FIG. 28  illustrates the use of system  100  to perform navigation using signals received directly from LEO satellite  108  and GPS satellites  202  in accordance with an embodiment of the invention.  FIG. 29  illustrates a similar implementation of  FIG. 28 , but with network  316  and ranging signals  318  added to preclude momentary interruptions in LEO signals  104  and GPS signals  106  from affecting the continuity of service. 
     As previously described, system  100  may be configured to support data uplink  320  from reference stations of reference network  204  to facilitate navigation performed by navigation devices  102  using navigation signals  104 B/ 104 C/ 104 D. Data uplink  320  may also be supported by appropriately-configured navigation devices  102 . In this regard, data uplink  320  may also be used to pass any desired data from reference network  204  and/or navigation devices  102  to LEO satellite  108  for subsequent broadcast as part of communication signal  104 A of LEO signal  104 . 
     Because GPS Time and UTC are available from a precision timing function of system  100 , it is possible to establish a one-way uplink protocol that allows data uplink  320  to occur without direct two-way synchronization. The time and frequency phasing of data uplink  320  can be pre-positioned to arrive at LEO satellite  108  to exactly match the satellite&#39;s instantaneous carrier phase and frame structure on a symbol-by-symbol basis. Given a suitable multi-use protocol, it is possible to share the uplink channel among multiple navigation devices  102 . Such a multi-use protocol may be implemented by time, frequency, code, or any combination thereof. In one embodiment, data uplink  320  may be configured as a spread spectrum uplink with anti-jamming and low probability of intercept and detection (LPI/D) characteristics. In another embodiment, low power signals of data uplink  320  may be summed over many symbols to pull an aggregate macro symbol out of the noise and provide an LPI/D uplink. 
       FIG. 30  illustrates a generalized frame structure for data bursts  3002  of uplink  320  to LEO satellite  108  in accordance with an embodiment of the invention. In one embodiment, data uplink  320  may be configured to support uplink bursts on approximately 240 channels with 414 bits per burst. For data uplink  320  to be aligned properly on a symbol by symbol basis, in one embodiment, the frame structure of LEO satellite  108  may be pre-positioned in a rest state (e.g., no time shift and no frequency shift relative to a master clock of LEO satellite  108 ). In another embodiment, a reference station of reference network  204  may be configured to generate an appropriate synchronization signal for data uplink  320  to LEO satellite  108 . The effect of this synchronization signal is to pre-align the frame structure for the data symbols in a burst against the UTC or GPS Time reference. 
       FIG. 31  illustrates a ground infrastructure to synchronize data uplink  320  in accordance with an embodiment of the invention. In particular, the ground infrastructure of  FIG. 31  includes a reference station of reference network  204  that may be used to align a payload field  3104  of each data burst  3002 . In one embodiment, the reference station may be configured to not broadcast during the portion of the burst allocated to payload  3104  (this time is reserved for navigation devices  102 ). In one embodiment, each of navigation devices  102  may be authorized to uplink a single symbol within a certain time and frequency slot. In this manner, each symbol (or each orthogonal bit in the QPSK uplink frame structure) is individually addressable by any navigation device  102  that knows its position and UTC/GPS Time. Navigation devices  102  may be implemented in accordance with any appropriate multi-use protocol by which navigation devices  102  are assigned the bits in the defined fields. For example, under a CDMA protocol, multiple navigation devices  102  may even share the same bits. 
     In various embodiments, data uplink  320  may be implemented with low power signals. For example, in one embodiment, uplink  320  may be implemented using milliwatt-level broadcasts to transmit several bits of data per second to LEO satellite  108 . If this power is spread over, for example, a 10 MHz bandwidth, the resulting power flux spectral density is reasonable for LPI/D applications. Such a spread spectrum implementation of uplink  320  may also provide antijam protection. 
       FIG. 32  illustrates an implementation of a low level signal used for data uplink  320  in accordance with an embodiment of the invention. In one embodiment, LEO satellite  108  may be configured to receive each bit in a QPSK modulation along with background noise. Because QPSK can be synthesized from two orthogonal binary phase-shift key (BPSK) streams, a simplified BPSK probability distribution (pair of offset Gaussian distributions) is shown in  FIG. 32 . Normally, a detector in a demodulator of LEO satellite  108  makes a “1” or “0” (noted here as −1) decision based on a threshold value at zero, and the probability of a bit error is calculated by integrating the area under the Gaussian as a function of SNR. 
     In one embodiment, the demodulator is treated as a hard limiter. When the SNR is much less than unity, the center Gaussian curve shown in  FIG. 32  is representative. The presence of a signal (i.e., a data bit) will ever so slightly shift the curve from one side to the other, but in general, the output will be swamped by noise. However, by averaging many discrete samples together, LEO satellite  108  can detect the emergence of a signal. Calculations known to those skilled in the art place the loss of a hard limiter at about 2 dB. In other words, but for a 2 dB effective analog to digital conversion loss, the input signal is completely preserved-even if LEO satellite  108  was originally implemented as communication satellite. The above approach is not limited to particular implementations of LEO satellite  108 . 
     In various embodiments, processing of data bits can be performed by reference network  104 , navigation device  102 , or onboard LEO satellite  108 . In another embodiment, custom engineered demodulators with a multi-bit RF front end may be used to eliminate the 2 dB hard limiter loss in LEO satellites  108  implemented with analog bent pipe configurations. 
       FIG. 33  illustrates a block diagram of a transmitter  3300  configured to support data uplink  320  in accordance with an embodiment of the invention. In this regard, it will be appreciated that transmitter  3300  may be provided as part of a reference station of reference network  204  or as part of one or more navigation devices  102 . For example, in one embodiment transmitter  330  may be integrated into a handheld Defense Advanced GPS Receiver (DAGR) handheld device, cellular telephone handset, or any other compact, low-cost device. Advantageously, such navigation devices  102  may be configured to permit users of such devices to send low-latency text or status messages from anywhere in the world over data uplink  320  for further broadcast over communication signal  104 A. 
     As shown in  FIG. 33 , the position and clock of navigation device  102  (e.g., provided by navigation solution  3302 ), and the position and clock offset of LEO satellite  108  (e.g., provided by navigation preprocessor  1912 ) are differenced to form an a priori timing advance parameter τ 0  used by timing advance calculation block  3308  as shown. In this regard, τ 0  corresponds to the lead time by which the transmission of an individual data bit, d nm , should be advanced to arrive at LEO satellite  108  at precisely the right time and phasing. 
     The timing advance parameter then governs the synthesis of the signal in the baseband processor. The data to be uplinked is encoded and encrypted in block  3304  according to user preference. Data modulator block  3306  generates 40% root raised cosine pulses that are modulated by the appropriate data bit, PRN direct sequence code, and channel frequency offset provided by PRN generator block  3310  and synthesizer block  3312 . Any desired number of channels can be concurrently processed in parallel. The signals are summed, upconverted (in this case by 100 MHz), converted to real form, converted from digital to analog, and upconverted to RF for broadcast as shown by blocks  3316  through  3324  of  FIG. 33 . 
     For compact and low power operation, the baseband component may be implemented to reside in the modified baseband real estate of a DAGR or cellular handset. In one embodiment, antenna  3324  may also be used for GPS signals in a DAGR or cellular handset. In one embodiment, the power consumption and form factor of the data uplink broadcast hardware may be implemented for handset or compact use. For example, in one embodiment, such transmit hardware may be implemented by a RF2638 chip available from RF Micro Devices that provides 10 dBm of RF output power and draws 25 mA at 3V. 
       FIG. 34  illustrates a block diagram of various components  3400  of LEO satellite  108  configured to support data uplink  320  in accordance with an embodiment of the invention. In one embodiment, LEO satellite  108  may be configured to receive data bit impulses through an antenna  3402  and a receiver block  3404 , and fill the internal frame structure with the resulting decision, namely +1 or −1. PRN generator block  3406  commands frequency hopping on the uplink in a pattern known in advance by both navigation device  102  and LEO satellite  108 . The direct sequence PRN code is also applied to the incoming bits by PRN generator block  3408 . Waveforms associated with the various macro symbol hypotheses (provided by hypothesis generator block  3410 ) are mixed with the incoming signal and then processed by a processor  3412  (e.g., in the manner previously described with regard to processor  718 ) to provide the resulting data message  3414 . As with LEO signal  104  also described herein, orthogonal encoding provides excellent bit energy per noise spectral density (Eb/NO) performance for data uplink  320 . 
     Data uplink  320  also contains a built-in ranging signal by virtue of the PRN coding modulation. Optionally, a delay-locked loop (DLL) may be provided in LEO satellite  108  to estimate the range from navigation device  102  to LEO satellite  108 . As a result, it is possible to perform reverse triangulation and use multiple LEO satellites  108  to passively triangulate the position of navigation device  102 . 
     Advantageously, system  100  may be used to provide desired features in a variety of applications. For example, in one embodiment, system  100  may be implemented to provide rapid, directed rekeying. Using public-private key infrastructure techniques with system  100 , navigation devices  102  may be authenticated using a two-way data link prior to passing encrypted traffic keys over the air. In this manner, positive control can be maintained over the specific user, receiver, location, and time of rekeying. 
     In another embodiment, system  100  may be implemented to support joint blue force situational awareness. In this regard, navigation devices  102  can share position information with other friendly forces nearby, and hazard areas and information on adversary locations can be shared in real time. 
     In another embodiment, system  100  may be implemented to support communications navigation and surveillance-air traffic management. In this regard, navigation devices  102  may be implemented in aircraft (e.g., in place of the antenna and GPS card in an aircraft&#39;s Multi-Mode Receiver (MMR)) to enable Cat III landing, a built-in communication link, integrated automatic dependent surveillance, and integrated space-based air traffic control. 
     In another embodiment, system  100  may be implemented to support search and rescue. In this regard, navigation devices  102  may be configured to provide global E911 features for both military and civil purposes. The LPI/D characteristics of the military version of data uplink  320  could qualify a modified DAGR to be employed under hostile conditions. 
     In another embodiment, system  100  may be implemented to support enroute retargeting. In this regard, guided munitions may be commanded or retargeted in real time using commands issued by a modified DAGR. 
     In another embodiment, system  100  may be implemented to support battle damage assessment. In this regard, information gathered in human or sensor form, including position information, can be quickly aggregated via data uplink  320 . In another embodiment, system  100  may be implemented to support weather information correlated by position can be aggregated in real time. 
     In another embodiment, system  100  may be implemented to permit a network of navigation devices  102  to aggregate measurements of jammer power or use time or frequency characteristics in a jammer to triangulate their exact locations. 
     In another embodiment, system  100  may be implemented to support spot beam control. In this regard, an envelope of authority to control spot beam power for antijam purposes may be delegated to navigation devices  102 . For example, if jamming is experienced, navigation devices  102  may be configured to request a real-time increase in the broadcast power of LEO signal  104 . Such an implementation could be made available to military or civil safety of life users, with the envelope of authority determined by government policy. 
     In another embodiment, system  100  may be implemented to support global cellular text messaging. For example, data uplink  320  capability may be provided in navigation device  102  (e.g., a modified DAGR or cellular telephone handset) to permit text messages to be sent to and from any location worldwide. 
     Embodiments described above illustrate but do not limit the invention. It should also be understood that numerous modifications and variations are possible in accordance with the principles of the present invention. Accordingly, the scope of the invention is defined only by the following claims.