Abstract:
Data from GPS satellites within the field of view of a ground station are retransmitted to LEO satellites, such as Iridium satellites, and cross-linked if necessary before being transmitted to a user. The user is then able to combine the fed-forward data with data received directly from GPS satellites in order to resolve errors due to interference or jamming. Iridium and data aiding thus provides a means for extending GPS performance under a variety of data-impaired conditions because it can provide certain aiding information over its data link in real time.

Description:
CROSS-REFERENCED TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of application U.S. Ser. No. 10/873,581, filed Jun. 22, 2004, status pending, and which claims priority to U.S. Provisional Application No. 60/536,788, filed on Jan. 15, 2004. 
     
    
     BACKGROUND INFORMATION 
       [0002]    1. Field 
         [0003]    This invention relates generally to systems for enhancing global positioning system performance. 
         [0004]    2. Background 
         [0005]    GPS, or Global Positioning System, is funded by and controlled by the U.S. Department of Defense (DOD). While there are many thousands of civil users of GPS world-wide, the system was designed for and is operated by the U.S. military. GPS provides specially coded satellite signals that can be processed in a GPS receiver, enabling the receiver to compute position, velocity, and time. Four GPS satellite signals are used to compute positions in three dimensions and the time offset in the receiver clock. 
         [0006]    The GPS satellites transmit two microwave carrier signals. The L1 frequency (1575.42 MHz) carries the navigation message and the Satellite Positioning Service (SPS) code signals. The L2 frequency (1227.60 MHz) is used to measure the ionospheric delay by Precise Positioning System (PPS) equipped receivers. 
         [0007]    Three binary codes shift the L1 or L2 carrier phase. The C/A Code (Coarse Acquisition) modulates the L1 carrier phase. The C/A code is a repeating 1 MHz Pseudo Random Noise (PRN) Code. This code modulates the L1 carrier signal, spreading the spectrum over a 1 MHz bandwidth. The C/A code repeats every 1023 bits (one millisecond). Each satellite has a different PRN C/A code, and GPS satellites are often identified by their PRN number, the unique identifier for each pseudo-random-noise code. The C/A code that modulates the L1 carrier is the basis for the civil SPS. 
         [0008]    Authorized users with cryptographic equipment and keys and specially equipped receivers use the Precise Positioning System, or PPS. Authorized users include U.S. and allied military, certain U.S. Government agencies, and selected civil users specifically approved by the U.S. Government. In the PPS, the P-Code (Precise) modulates both the L1 and L2 carrier phases. The P-Code is a very long (seven days) 10 MHz PRN code. In the Anti-Spoofing (AS) mode of operation, the P-Code is encrypted into the Y-Code. The encrypted Y-Code requires a classified AS Module for each receiver channel and is for use only by authorized users with cryptographic keys. The P/Y Code is the basis for the PPS. 
         [0009]    A Navigation Message also modulates the L1-C/A code signal. The Navigation Message is a 50 Hz signal consisting of data bits that describe the GPS satellite orbits, clock corrections, and other system parameters. The GPS Navigation Message consists of time-tagged data bits marking the time of transmission of each subframe at the time they are transmitted by the SV. A data bit frame consists of 1500 bits divided into five 300-bit subframes. A data frame is transmitted every thirty seconds. Three six-second subframes contain orbital and clock data. Satellite Vehicle (SV) Clock corrections are sent in subframe one and precise satellite orbital data sets (ephemeris data parameters) for the transmitting SV are sent in subframes two and three. Subframes four and five are used to transmit different pages of system data. An entire set of twenty-five frames (125 subframes) makes up the complete Navigation Message that is sent over a 12.5 minute period. 
         [0010]    Ephemeris data parameters describe SV orbits for short sections of the satellite orbits. Normally, a receiver gathers new ephemeris data each hour, but can use old data for up to four hours without much error. The ephemeris parameters are used with an algorithm that computes the SV position for any time within the period of the orbit described by the ephemeris parameter set. 
         [0011]    The C/A code is broadcast at 1,575.42 MHz in a 2.046 MHz wide band (complete null to null), and is used for civilian operations and for initial acquisition in military operations. The P/Y code is a wider-band signal spanning 20.46 MHz that provides 10 times higher ranging precision than C/A code commensurate with its higher chipping rate. Often, C/A code is the first casualty of jamming. The 1.023 MHz chipping rate of the C/A code provides some protection, but the 10.23 MHz chipping rate of the P/Y code offers an additional 10 dB of J/S protection. If the jamming is known to be narrow band and to originate within the C/A code frequency band so as to deny enemy use of the C/A code signal component, then even more protection is available by notch filtering the center 2 MHz of the P/Y code input to the receiver. 
         [0012]    The ability to track low power GPS signals is important for a number of real-time applications, including cases where the GPS signal may be attenuated, jammed, or subject to interference. Previous approaches to these obstacles have included to varying degrees (i) signal processing to enhance sensitivity, (ii) controlled radiation pattern antennas (CRPAs) to thwart jamming, and (iii) control of the receiving environment, if possible, to ward off interference. Unfortunately, in many of these cases, only limited performance improvement is feasible due to practical constraints. 
         [0013]    In the case of (i) signal processing, the fundamental limit to increased performance is established by the data bit boundaries in the GPS message. The intrinsic GPS data broadcast rate is 50 bits per second. Ordinary receivers cannot integrate the signal across these 20 ms intervals. Extension of the integration interval would actually lead to a decrease in performance because the data bits will appear as random noise that averages to zero. Therefore, a general practical limit is 20 ms averaging. 
         [0014]    Some practitioners have tried to push this limit by squaring the GPS signal. However, squaring is an inefficient means of recovering information because the noise is mixed with itself, resulting with a significant baseband noise component superimposed over the squared signal at baseband. Other techniques have been devised that employ data stripping—the local application of limited a priori knowledge of the GPS bit sequence that takes advantage of a tendency in GPS to often repeat the same sequence multiple times—in order to remove the GPS data and obtain longer integration times. Unfortunately, this technique can often provide marginal results—especially in critical applications. The main shortcoming is that it completely falls apart when the GPS message changes, and this occurrence is frequent and unpredictable. 
         [0015]    Prior processing efforts have also taught away from the current invention of employing feed-forward data to enhance performance. For example, U.S. Pat. No. 6,133,874 teaches that, “Coherent integration beyond 20 milliseconds is normally inadvisable since the presence of a priori unknown 50 baud binary phase shift keyed data (the satellite data message) placed on top of the signal does not allow coherent processing gain beyond one data bit period, or 20 milliseconds.” Similarly, U.S. Pat. No. 5,664,734 explains, “If the carrier frequency and all data rates were known to great precision, and no data were present, then the signal-to-noise ratio could be greatly improved, and the data greatly reduced, by adding to one another successive frames. The presence of 50 baud data superimposed on the GPS signal still limits the coherent summation of PN frames beyond a period of 20 msec.” 
         [0016]    Null steering antennas (ii) reduce jamming by identifying the direction of origin of a jammer, then spatially notching out all signals in that direction. What remains is the unjammed GPS signals that come from other directions. With additional phased array electronics, it is also possible to create more tightly focused beams on individual GPS satellites, thereby increasing signal strength. While the jamming protection of CRPAs is excellent, such antennas are often heavy, bulky, and expensive. 
         [0017]    Last, controlling the receiving environment to minimize interference sources (iii) is often involved because it must be carried out under a regulatory regime. For example, ultrawideband (UWB) devices have already been shown to interfere with certain GPS devices on occasion. There is a general desire to have both of these devices coexist—the potential user base is fundamentally the same. In the best of all worlds, a regulatory environment will exist that will enable UWB to coexist with GPS and other incumbent bands. However, in spite of best efforts to create and conform to such a regulatory environment, there will always be deviations that create exceptions to proper performance. It is for these cases that the invention described herein is likely to be most useful, namely, as a “safety net” against unexpected interference. 
         [0018]    What is needed is a system that provides robust GPS performance under any of these adverse conditions with a simple solution that does not incur the associated penalties in terms of size, weight, power, and cost. 
       SUMMARY 
       [0019]    In accordance with preferred embodiments of the present invention, data from GPS satellites within the field of view of a ground station are retransmitted to LEO satellites, such as Iridium satellites, and cross-linked if necessary before being transmitted to a user. The user is then able to combine the fed-forward data with data received directly from GPS satellites in order to resolve errors due to interference or jamming. Iridium and data aiding thus provides a means for extending GPS performance under a variety of data-impaired conditions because it can provide certain aiding information over its data link in real time. 
         [0020]    Applications of the present invention include (but are not limited to) ultra-wide-band interference protection, anti-jam protection, and enhanced ability to use GPS indoors. 
         [0021]    Ultra-Wide-Band (UWB) Interference Protection. Many consumer and business local networks are beginning to employ UWB for wireless communication. Because of its likely popularity among business travelers, UWB is also a candidate for air travel such as Boeing&#39;s Connexions service. Unfortunately, UWB also happens to be an interference risk to GPS since it is partially overlaid on the GPS frequencies. The GPS and aviation communities are making every effort through the FCC to create a regulatory guardband between GPS and UWB so that the two can coexist. But even with a regulatory guardband, it is possible that incidental UWB emissions onboard an aircraft could interfere with onboard GPS navigation equipment. Iridium data supplied in real time to the aircraft anywhere in the world would provide enough information to allow for continued tracking of the GPS signal—even in the presence of UWB interference. 
         [0022]    Antijam Protection. Jamming in battlefield environments is also a key consideration. A stand-alone Y-code GPS receiver can typically withstand a Jamming to Signal (J/S) ratio up to 50 dB before it will lose lock. Iridium data supplied to a user receiver in real time could provide the user with significant enhancements in antijam performance over a stand-alone receiver. 
         [0023]    Indoor Operations. The strong Iridium signal could be used to transmit aiding information in real time to a GPS receiver inside a building that can only access attenuated signals. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings. 
           [0025]      FIG. 1  is a representative view of an aircraft equipped with a GPS receiver and subject to interference; 
           [0026]      FIG. 2  is a representative view of a system in accordance with the present invention; 
           [0027]      FIG. 3  is an illustration of the timing of real-time and feed-forward GPS signals received by a user; 
           [0028]      FIG. 4  is a block diagram of a LEO and GPS receiver; 
           [0029]      FIG. 5  is a block diagram of a preferred GPS or LEO receiver; 
           [0030]      FIG. 6  is a block diagram of the preferred signal processing for a GPS half channel; 
           [0031]      FIG. 7  is a block diagram for a GPS receiver for processing a single channel; 
           [0032]      FIG. 8  is a block diagram of a GPS carrier tracking loop estimator model; 
           [0033]      FIG. 9  is a flow diagram of a preferred method in accordance with the present invention; and 
           [0034]      FIG. 10  is a flow diagram of a preferred method in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0035]      FIG. 1  shows an example of a basic situation that could produce interference in a commercial passenger jet  10  equipped with a GPS antenna  12  and GPS receiver  13 . The GPS antenna  12  is capable of receiving GPS signals  16  and LEO satellite signals  17  from those satellites within its field of view. If a passenger operates a portable electronic device  12  such as a notebook computer, it is possible that this device could emit interference  15  in the GPS band. Through a number of pathways, this interference can reach the GPS antenna on board the aircraft that is being used for navigation. Depending on its strength, the interference could render the GPS receiver useless, possibly jeopardizing the aircraft and its occupants. 
         [0036]    The system described herein is directed towards maintaining carrier and pseudorange lock in the presence of interference, jamming, or reduced signal strength. Typical parameters required to acquire a GPS satellite include knowledge of its pseudorandom noise code phase, Doppler shift, and user clock offset. In the absence of data modulation on the GPS signal, for a given signal strength and uncertainty in acquisition parameters, there is a standard tradeoff between time to acquire and sensitivity. Since one purpose of this invention is to enhance receiver sensitivity, it does not address the acquisition of a signal except to assume that a receiver has already successfully locked onto a signal. Co-pending patent application Ser. No. 10/720,736 (which is hereby incorporated by reference) shows how such acquisition can be carried out under more demanding conditions when an additional 10 dB of processing gain is desirable. The present invention shows how a receiver may continue to track this signal when conditions drive it to be hardly detectable. 
         [0037]      FIG. 2  shows the system architecture in a preferred embodiment of the invention. Reference GPS receivers  50  are placed on the Earth away from the region of degraded GPS signal conditions of interference, jamming, and attenuation. In a single application, only one or a plurality of such receivers  50  may be employed. These reference GPS reference receivers track the 50 bps GPS ranging signal data stream  32  for each satellite in view. A collocated Iridium ground transmitter telemeters a signal  34  containing this received GPS bit stream in real time into the Iridium system via the nearest satellite  40 . The bit stream is encoded into an Iridium broadcast channel, an example of which is the Iridium “visit message.” This broadcast message is promulgated throughout the Iridium network via its crosslinks  42 ,  44 . Iridium broadcasts the 50 bps real time data stream for each GPS satellite from each satellite in the Iridium constellation, as indicated in  FIG. 2  by a broadcast signal  46  being transmitted from an ultimate Iridium LEO satellite  40 . A user receiver  60  in a degraded environment with interference, jamming, or attenuation then has access to these GPS data bits. The user receiver may then apply these GPS data bits to the signal processing of its own real time GPS measurements. 
         [0038]      FIG. 3  conceptually illustrates a 50 bps data stream  80  from a GPS satellite  30   a , received in real time at a user (see  FIG. 2 ). The entire GPS navigation message is 25 frames in length, with each frame of the GPS message further divided into 5 subframes that are six seconds, or 300 bits, in length. The feed forward data stream  82  from the same GPS satellite  30   a  contains the identical data, but because of propagation delays through the Iridium system the feed forward stream  82  arrives at the user  60  later than the real time stream  80 . As indicated in  FIG. 3A , the feed forward stream is delayed by a fraction of a single frame. The actual delay will vary, depending on user location with respect to the GPS satellite and ground station. As part of the present invention, the feed forward data is then retrieved from the Iridium data stream and time-aligned with the GPS real time data so that it can be used to enhance the real-time data. 
         [0039]      FIG. 4  shows a preferred receiver  100  that receives and demodulates both GPS and Iridium satellite signals. Note that the invention should not be limited to Iridium or any other particular satellite systems. Rather, the system will work equally well by using a receiver for any other satellite system in which GPS data can be retransmitted and fed forward to a user. 
         [0040]    The receiver  100  includes a dual band antenna  102 , band pass filter  104 , and preamp  106 . The filter and preamp permit both the L1 and L2 GPS signals to pass along with the Iridium signal, which is close to the L1 signal frequency (1575+/−10.23 MHz for GPS; 1621+/−5 MHz for Iridium). The filtered and amplified signal is passed to a GPS receiver  110  and an Iridium (or other LEO) receiver  112 . The Iridium and GPS receivers are tied together with a common clock  108 , which is preferably a temperature compensated crystal oscillator. 
         [0041]    The GPS receiver  100  produces position fixes as an output, in a manner as discussed above. The GPS receiver also uses GPS aiding data forwarded to it from the Iridium receiver  112  in order to assist in resolving attenuated or missing data at the GPS receiver under interference or jamming conditions. Optionally, the receiver also includes an inertial unit (IRU)  114  in communication with the GPS receiver. The IRU  114  provides inertial motion information to the GPS receiver  110  to enable the GPS receiver  110  to provide accurate and updated position fixes during times of limited data or no data from satellites. Depending on the quality of the IRU, the position fixes from the GPS receiver may continue to be accurate for fairly long periods of time without GPS data following an initial accurate position fix. 
         [0042]    In the preferred embodiment, the satellite signals can be demodulated with a software receiver as shown in  FIG. 5 . Alternatively, existing Iridium receivers can be obtained and used for this purpose. Nonetheless, the preferred form of this invention employs software for much of the Iridium receiver  112  so as to make it both low cost and easy to maintain and upgrade. 
         [0043]    The front end of the receiver includes a bandpass filter  120 . The filter  120  has a 10 MHz passband at 1,621 MHz with a steep cutoff, therefore requiring it to be a high Q filter. New technologies allow this sharp filtering to be done at this frequency. The sharp filtering rejects out-of-band interference, such as nearby cellular telephones. An example of the electrical components suitable for this purpose is the specialized RF chip pair contained in IBM GPS receiver IBM43GAENGP0001. This receiver contains an integrated SiGe chip which carries out the bandpass, automatic gain control (AGC), and direct RF sampling functions using 2-bit quantization. Schematically, these functions are illustrated in  FIG. 5  as the signal travels from the bandpass filter  120 , then is separated into inphase and quadrature components via the hybrid coupler  122 . The component signals are passed to inphase and quadrature A/D converters  124 ,  126 . One implementation would be to translate the equivalent design up by 46 MHz to the Iridium band. However, it is not critical that direct downconversion be used. It is also possible to employ a traditional quadrature downconversion scheme. 
         [0044]    The A/D converters  124 ,  126  sample the incoming inphase and quadrature signals at 40 MHz. The output is fed directly into a digital signal processor (DSP)  130  (preferably a TMS 320 DSP) which can then process sections of the data to lock onto the Iridium signal, calculate signal phasing and phase rate, and extract the broadcast bits containing the GPS data fed forward by the Iridium satellite. The GPS data bits from the Iridium broadcast message are demodulated in accordance with the Iridium specification. A memory  132  is also provided to store data and computer programming instructions for processing by the DSP  130 . The memory  132  may take any of a variety of forms, such as EEPROM chips or other electromagnetic or optical storage devices. 
         [0045]      FIG. 6  shows how the GPS tracking is implemented for a half channel. The raw, real time digital GPS data measurements  200  are input and mixed at a first mixer  202  with a specific version of the GPS Code provided by a code generator  204 , operating on feed forward data  201  obtained from the Iridium satellite. In the preferred embodiment, the code is set to either punctual or early minus late. The use of punctual or early minus late partially determines whether the half channel will accumulate code or carrier errors for the tracking loops. Two identical half channels make up a full channel, as will be described in greater detail below. 
         [0046]    The output of the code mixer is mixed again at a second mixer  206  with an internally synthesized version of the GPS carrier generated by the carrier tracking loop of the receiver at a synthesizer  208 . An accumulator  210  builds up one millisecond samples of the incoming channel whose boundaries align with the received signal epoch. This integration boundary is chosen to be general enough to handle short overall integration times, such as one millisecond, but to also accommodate longer integration times. According to the GPS signal specification [ICD-200], data bit boundaries will also occur on 20 ms boundaries aligned with the received signal epochs. Therefore, the one millisecond epoch boundary can never cut across a data bit. 
         [0047]    For high dynamic applications, an inertial-grade Inertial Reference Unit (IRU)  114  is coupled to the system at a third mixer  212 . The inertial is shown in dotted lines because such high performance may not be necessary in all applications. In less demanding applications, a MEMS inertial unit may be sufficient. The IRU senses inertial vector translation, x I , and attitude motion, represented by the 3×3 attitude rotation matrix A, of the user platform. With prior knowledge of the antenna mounting lever arm, b, with respect to the body frame of the user platform, it is possible to use the inertial to project the antenna motion into the line of sight of the satellite, ŝ, to yield a complex, real-time phase correction for each channel. The net effect is to subtract out short-term user motion and enable long integration times on the GPS signal. 
         [0048]    The Iridium receiver demodulates the feedforward GPS data bits for all the GPS satellites in view via the Iridium system. A decommutator  214  in the user receiver sorts out the incoming bit stream within the Iridium data frame and positions the reference GPS bits in proper phasing for each channel. To keep track of delays, the GPS data is time tagged according to the GPS time at which it was broadcast from the GPS satellite. In general, these GPS data bits are subject to a latency of around 80 ms as they propagate through space from the GPS satellite to the ground reference receiver. An additional latency of order 10-90 ms from the time they are received at the ground reference station will accrue from transmission to the user from the ground station due to propagation delay through the Iridium system. 
         [0049]    To ensure that the real-time GPS data bits are appropriately delayed and aligned in time with the decommutated feedforward data bits produced at a decommutator  214 , a first-in-first-out (FIFO) synchronization buffer  216  is employed, receiving and buffering data from the accumulator (optionally mixed with the IRU data). The GPS time tags on each data stream are used to ensure that the same millisecond epochs of data are mixed together. The FIFO  216  holds each batch of millisecond measurements from the accumulators until they are exactly time aligned with the incoming data bits from Iridium. Then, the Iridium data bit stream is used to wipe off the 50 bps data modulation from the incoming GPS signal at a fourth mixer  218 . The result is a true coherent GPS carrier phase detector, at accumulataor  220 , extendible from 1 ms to longer integration times that are limited by the quality of the inertial unit—potentially 10 seconds for an inertial grade unit. 
         [0050]    Each channel is formed by two half channels. As shown in  FIG. 7 , the output for each half channel may be interpreted as follows: if the half channel code generator is commanded to synthesize an early minus late code sequence  204   b , then the real component  232  of the output of that half channel  230  will correspond to the code phase tracking error for that satellite. If the half channel code generator is commanded to synthesize a punctual code sequence  204   b , then the output  242  of that half channel  240  will correspond to the carrier phase error. 
         [0051]    The resulting carrier phase tracking error is fed into the estimator  250  shown in  FIG. 8 . This general-purpose estimator is capable of estimating the carrier phase and any number of its time derivatives upon receipt of the latest phase tracking error measurement, □□. In the preferred embodiment, a third order tracking loop is implemented—mostly to take into account satellite orbital motion. Therefore, there are three states, □, to the tracking loop estimator/synthesizer: phase, phase velocity, and phase acceleration. The following matrix dynamic model, □, is employed to model the carrier phase synthesizer in its progress from the a posteriori estimate at one epoch to the a priori estimate at the next: 
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         [0052]    The sole observable, as encoded in the observability matrix H=[1 0 0], is the measured tracking error, 
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         [0053]    Using optimal estimation theory, an optimal gain, L, may be chosen to yield a measurement update equations as follows from the tracking error: 
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         [0054]    Each epoch this correction vector is sent to the carrier phase synthesizer to adjust its advance. The end result is an effectively optimal carrier phase tracking loop. Care must be taken to ensure that the latency from Iridium does not amount to a significant portion of the sample interval. If this must be the case, then it may be advisable to implement a dynamic model that takes into account the delay. Also, in the preferred embodiment, the carrier phase synthesizer effectively provides third order functionality from one integration interval to the next. The reason for this is to cover those cases where the integration interval becomes significant with respect to one second. 
         [0055]    The method for implementing the present invention is further described with references to  FIGS. 9 and 10 . In a first step  300 , a monitoring station, such as a ground monitoring and control station  50  (see  FIG. 2 ) receives GPS signals from those GPS satellites in its field of view. The monitoring station modulates those signals in accordance with the Iridium satellite specification and transmits the received and modulated GPS data to an Iridium satellite in a second step  302 . It should be understood that other LEO satellites other than Iridium may be used. 
         [0056]    Once received by an initial Iridium satellite, at a next step  304  the GPS feed-forward data is cross-linked to other Iridium satellites. Any number of cross-linkings of the feed forward data may occur, including no cross-links at all, consistent with this invention. Ultimately, the feed forward data is broadcast toward the earth  306  for receipt by any number of users, who receive the feed forward data at a final step  308 . 
         [0057]    With reference to  FIG. 10 , the user receives real time GPS signals and feed forward GPS data via the Iridium satellite, at step  310 . The user receiver processes both data streams, extracting the feed forward GPS data at a step  312  and, generally in parallel, processing the real time GPS data and buffering it at a step  314  in order to synchronize it with the feed forward data. The feed forward data is then used to supplement the real time data  316 , enabling the system to obtain and use accurate GPS data at the user even under jamming or interference conditions. Finally, the receiver produces position fixes  318  from the processed GPS data, optionally aided by an inertial reference unit. 
         [0058]    While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.