Abstract:
A method for generating a global positioning signal from a space based craft including the steps of generating a plurality of binary modulating signals using a waveform generator; separately modulating an in-phase component of a desired carrier of the global positioning signal to be generated with at least a first binary, modulating signal selected from the plurality to generate at least one corresponding in-phase modulated signal component; separately modulating a quadrature component of the carrier with at least a second binary modulating signal selected from the plurality to generate at least one corresponding quadrature modulated signal component; and, combining the at least one in-phase modulated signal component and the at least one quadrature modulated signal component to generate the global positioning signal; wherein the global positioning signal has a constant envelope.

Description:
FIELD OF INVENTION 
     This invention relates to Global Positioning Systems (G.P.S.&#39;s) and more particularly to a programmable waveform generator therefore. 
     BACKGROUND OF INVENTION 
     As is well known in the art, the Global Positioning System (G.P.S.) includes 24 satellites orbiting the Earth in six orbit planes, each being inclined with respect to equatorial plane by 55° and longitudinally offset from each other by 60°. It should be recognized that the present method is equally applicable for other positioning systems as well, such as GLONASS for example, wherein 24 satellites (21 operational and 3 spares) are inclined at 65° angles and offset by 120°. 
     In G.P.S. each satellite broadcasts carrier signals on the same frequencies. Navigational data is spread by a clear acquisition (C/A) code and modulated on the L 1  (1575.42 MHZ) in-phase channel. The navigational data is further spread by a P(Y) code and modulated on both the L 1  quadrature and L 2  (1227.60 MHZ) in-phase channels. 
     As is well known in the art, by measuring delay in the signals transmitted from several of these satellites, it is possible to triangulate ones position and further to correct for ionospheric induced, tropospheric induced and ephemeral/clock based errors, for example. 
     As both military and civilian requirements for G.P.S. rapidly change, a condition which will likely continue well into the 21st century, operational modifications will become increasingly necessary. In fact, requirements for the G.P.S. system have changed since the contract for the next generation satellites (Block IIF) was awarded. Many of these changing requirements revolve around the waveform structure and the desire to evolve it. 
     The lead time from award of contract to delivery (often 5-7 years) and from delivery to launch (often an additional 5-7 years) and then to eventual obsolescence (often greater than an additional 7.5 years) is such that once a signal structure is selected for a new block of satellites, to satisfy a current set of requirements, the navigation community is locked into that signal structure for several decades. Accordingly, there is a need for a flexible waveform generator aboard the G.P.S. satellites that can be reprogrammed in orbit, to facilitate continued evolution of the navigation system to meet changing operational requirements. 
     Conventional waveform generator architectures, wherein the entire composite signaling waveform is generated at baseband and then upconverted to the broadcast radio frequency (Rf), have been utilized in various types of communications systems (i.e. software radios) (See FIG.  1 ). 
     Although such an architecture can generally be used for communications systems, such an approach is not suitable for broadcast of the G.P.S. navigation signals from space. One limitation is the availability of space-qualified components that are fast enough to perform the required processing. This limitation results in an intermediate frequency that is too low to preclude harmonic interference in the resultant modulated signal. Another limitation is that any time jitter on the D/A converter results in additional phase noise on the composite output signal, which can violate the very stringent G.P.S. requirements. A third limitation is that bandpass filters are required for every mixer stage in the upconverter. When phase modulated signals are passed through these filters, they generate amplitude variations (“ringing”) at every phase transition. These amplitude variations are problematic regarding satellite efficiency. Applying non-constant envelope signals to saturated amplifiers results in signal distortions that can impact navigation accuracy. 
     Of course, the use of saturated amplifiers on satellites is desirable due to the increased efficiency characteristics thereof. 
     Referring now to FIG. 1, therein is illustrated a conventional waveform generator  10  using the software radio method. Therein, a baseband composite signal is generated  12  and converted to an analog signal using digital to analog converters  14 . The frequency of the analog signal is then up-converted to the desired L-band  18 . As set forth though, due to the speed capabilities of current space qualified components (FPGA&#39;s and D/A&#39;s for example), interference can be caused by mixer harmonics when the first If frequency is too low in relation to signal bandwidth. Further, ringing of the bandpass filters in the mixer stages can also result in a non-constant envelope which results in further errors and reduced amplifier efficiency. 
     One of the requirements that has changed since the awarding of the G.P.S. Block IIF contract is the need for an additional military signal on the same L Band carrier as the C/A and P(Y) signals. Although several techniques are presently under consideration for combining the 3 signals, such as majority voting and hard limiting, the present invention utilizes the Interplex Modulation technique. Interplex Modulation is a very efficient technique for combining three or more signals to generate a constant envelope composite signal with minimal combining losses. FIG. 3 illustrates the conventional method for implementing Interplex Modulation and, as depicted therein, it requires the use of a linear modulator. A constant envelope composite signal is highly desirable so that a highly efficient saturated power amplifier can be utilized. 
     Referring now to FIG. 2, an alternative approach  20  would be to relocate the digital-to-analog interface  14  as illustrated therein. The first step would be to generate a baseband modulating signal  22  and convert that from digital to analog format  14 . By converting at this step, a lower speed digital-to-analog converter  14  can be utilized because the modulating signal has less bandwidth than the composite signal. The analog signal would then be fed into a linear modulator  24  wherein it modulates the If frequency from a synthesizer  26  and up-converted  16  thereafter. 
     However, as will be evident to one skilled in the art, such an approach could still lead to ringing of the bandpass filters in the frequency up-converter  15  which in turn represents a significant risk in the development, certification and space use of such a linear modulator. Accordingly, not all of mentioned drawbacks would be overcome by such a system and method. Therefore, it is also desirable to avoid the inclusion of linear modulators. 
     FIG. 4 illustrates an interplex modulation technique  42  utilizing a combination of linear  44  and biphase  46  modulators. The output of this modulator  42  is able to produce a signal  48 , however, no IM term is generated. Accordingly, the envelope of the output signal  48  is not constant and hence undesirable. 
     Referring now to FIG. 5, therein is illustrated a second interplex modulation technique  50  utilizing only biphase modulators  52 . Again though, as illustrated for FIG. 4, the envelope of the resultant output signal  54  again is not sufficiently constant. 
     Referring now to FIG. 6, therein is illustrated a third interplex modulation technique  60  with a linear phase modulator  62  with the S 3  channel in phase with S 1  channel. However, as illustrated therein, a linear phase modulator  62  is required, therefore, such a configuration fails to remedy all of the aforementioned shortcomings. 
     Finally, referring to FIG. 7, therein is illustrated a fourth interplex modulation technique  70  utilizing a linear phase modulator  72  with the S 3  channel in phase with the S 2  channel. Again though, please note the inclusion of a linear phase modulator  72 , as illustrated for FIG.  6 . 
     Accordingly, each of these interplex modulation techniques ( 40 ,  42 ,  50 ,  60 ,  70 ) fails to remedy all of the aforementioned shortcomings of the prior art and hence satisfy each of the objects of the present invention. 
     SUMMARY OF INVENTION 
     A method for generating a global positioning signal from a space based craft including the steps of generating a plurality of binary modulating signals using a waveform generator; separately modulating an in-phase component of a desired carrier of the global positioning signal to be generated with at least a first binary, modulating signal selected from the plurality to generate at least one corresponding in-phase modulated signal component; separately modulating a quadrature component of the carrier with at least a second binary modulating signal selected from the plurality to generate at least one corresponding quadrature modulated signal component; and, combining the at least one in-phase modulated signal component and the at least one quadrature modulated signal component to generate the global positioning signal; wherein the global positioning signal has a constant envelope. 
    
    
     BRIEF DESCRIPTION OF FIGURES 
     FIG. 1 illustrates a conventional software radio approach. 
     FIG. 2 illustrates a modified software radio approach. 
     FIG. 3 illustrates an interplex modulation method. 
     FIGS. 4-7 illustrate alternative interplex modulation method. 
     FIG. 8 illustrates the method according to the present invention. 
     FIG. 9 illustrates present G.P.S. IIR implementation of the P(Y) and CA codes. 
     FIG. 10 illustrates a preferred embodiment of the implementation of the present invention. 
     FIG. 11 illustrates the preferred embodiment of FIG. 10 adapted to be interfaced with conventional G.P.S. satellites. 
     FIG. 12 illustrates the amplitude variations in the composite signals resultant from the phase mismatch and the variable attenuators. 
    
    
     DETAILED DESCRIPTION OF INVENTION 
     The present invention utilizes a unique architecture wherein modulating signals are generated using a waveform generator and are then used to directly modulate an L-Band Rf carrier. This approach eliminates all of the limitations of these previous approaches. The modulating signal has a much lower frequency content than the modulated If signal, and the processing required to generate the modulating signal is well within the capabilities of space-qualified processors readily available today. In this new architecture, the modulating signals from the waveform generator are clocked binary signals that are sent directly to the modulators, thereby eliminating the D/A converter and any associated jitter and phase noise. Further, because there is no upconversion of the modulated signal required, there is no amplitude variation introduced by bandpass filters. 
     The operation of this invention depends upon the interplex modulation technique, which has been documented, for example in Ananda, M., P. Munjal, B. Siegel, R. Sung and K. T. Woo, “Proposed GPS Integrity and Navigation Payload on DSCS (U)”, 1993 IEEE Military Communications Conference, Boston, Mass., October, 1993, the disclosure of which is hereby incorporated by reference. 
     Referring now to FIG. 3, therein is illustrated a standard interplex modulation approach  30  for three signals, S 1 , S 2  and S 3 . Interplex modulation allows combining of multiple signals into a phase modulated composite signal that maintains a constant envelope, thereby allowing the use of saturated power amplifiers without signal distortion. For the case of these three component signals S 1 , S 2  and S 3 , the interplex modulator  30  generates a composite signal  32  that includes the three desired components ( 34 ,  36 ,  38 ) plus an unwanted intermodulation product ( 40 ). These products as illustrated in FIG. 3 are: 
     
       
         cos(β1) cos(β2) s 1  sin(ω1t),  ( 34 ) 
       
     
     
       
         +sin(β1) cos(β2) s 2  cos(ω1t)  ( 36 ) 
       
     
     
       
         +cos(β1) sin(β2) s 3  cos(ω1t)  ( 38 ) 
       
     
     
       
         −sin(β1) sin(β2) s 1  s 2  s 3  sin(ω1t).  ( 40 ) 
       
     
     The first three terms correspond to the desired signal terms S 1 , S 2  and S 3  respectively; the fourth term ( 40 ) is an intermodulation (IM) product, which is an undesired term generated by the modulator. Further, the resulting modulator output signal  32  has a constant envelope, thus, a saturated amplifier can be used to transmit this signal on the satellite without backoff. In other words, although the intermodulation product consumes some of the available power, it serves to keep the amplitude of the composite signal envelope constant, which in turn facilitates the use of saturated amplifiers. 
     This invention incorporates a unique implementation of an interplex modulator  80 , wherein the three signal components (S 1 , S 2 , S 3 ) and the required intermodulation product (IM) are all generated individually in the waveform generator  82  and are sent to the modulators as binary signals  86  (See FIG.  8 ). Each of these binary signals  86  is fed to a BPSK modulator  84  which modulates either the in-phase  88  or quadrature phase  90  component of the desired L-Band carrier (i.e. L 1  or L 2 ). The outputs  92  of the BPSK modulators  84  are scaled, using variable attenuators  94 , to achieve the desired relative power ratios between the various signal components  96 . 
     Reprogrammability of the waveform generator  82  allows modification of all of the critical signaling parameters, such as chipping rates, code types, fixed carrier offset, hopping sequence for hopped carrier offset, and relative power ratios. 
     Referring now also to FIG. 9, therein is illustrated current G.P.S. IIR implementation  100  of the P(Y) and C/A codes. Therein, the P(Y) code is implemented via a BPSK modulator  102  with sin(ωt) and the C/A code is implemented via a second BPSK modulator  104  with cos(ωt). Appropriate pads ( 106 ,  108 ) are included in both the P(Y) and C/A signal paths to effect the correct power ratio. The signals are finally mixed together to form the desired signal. Accordingly implementation of a preferred form of the present invention, as illustrated in FIG. 8, can be accomplished using currently available space certified equipment. 
     More particularly, preferably the waveform generator  82  generates binary modulating signals (S 1 , S 2  and S 3 ) including the intermodulation term (S 1  S 2  S 3 ) as has been discussed. The first signal S 1  is fed to a BPSK modulator  84  substantially identical to that of FIG. 9 along with sin(ωt). The second signal S 2  is fed to a BPSK modulator substantially identical to that of FIG. 9 along with cos(ωt). The third signal S 3  is fed to a BPSK modulator  84  substantially identical to that of FIG. 9 along with cos(ωt). And, the fourth signal S 1  S 2  S 3  is fed to a BPSK modulator  84  substantially identical to that of FIG. 9 along with sin(ωt). Accordingly, the three signals (S 1 , S 2  and S 3 ) and intermodulation signal (s 1  s 2  s 3 ) modulate the L-band carrier desired. Each of these modulated signals can be variably attenuated to the desired level prior to mixing to form the final signal to be transmitted using variable attenuator  94 . Accordingly, all of the prior art problems heretofore discussed can be overcome. 
     Referring now to FIG. 10, in a preferred embodiment, implementation of this method includes a Field Programmable Gate Array (FPGA)  110  which receives data and programming instructions from the existing MDU  112 . Alternatively this invention can be implemented utilizing an Application Specific Integrated Circuit (ASIC). Preferably the FPGA  110  further includes a cryptography device  114  (See FIG. 11 also) to enable encrypted communication therewith and encryption of the signals transmitted therefrom. In this FPGA  110 , code generators (3-4)  116  are used to generate various codes to be combined with various data streams. The code generator  116  preferably includes of a total of 6-8 shift registers ( 10 - 14  stage), selectable feedback taps and can be clocked up to 10 MHZ. Further a code storage device  120  provides up to two 3-4 K length codes which can be used for codes that cannot be generated with shift registers. 
     The data stream generators  118  (3-4) combine code with data for 3-4 signals some of which can eventually be combined with the offset carrier provided by synthesis  122 . The FPGA  110  thus provides 6 bit digital words at a very low rate to the variable attenuators  94  and binary signals (±1) up to 10 MHZ to the BPSK modulators  84 . 
     FIG. 11 illustrates the preferred embodiment of FIG. 10 adapted to the specific application of retrofitting the existing G.P.S. Block IIR satellites, wherein a processor already exists in the Mission Data Unit (MDU). The interface  124  of the MDU  112  and interface  126  of the waveform generator  82  cooperate to seamlessly integrate the MDU  112  and waveform generator  82  into a single functional unit. The MDU will continue to provide CV management functions such as uploading, long-term storage, key derivation, WG key encryption and periodic WG updates. The waveform generator will assume the responsibility for receiving periodic updates, WG key decryption and short-term local storage including the use of cipher codes, baseline Y coders, advance Y coders and LM coders. 
     Referring now to FIG. 12, therein is illustrated amplitude variations in the composite signal resultant from the phase mismatch in the variable attenuators. Waveform  12 A illustrates the envelope with no phase errors induced, waveform  12 B illustrates the envelope with ±3° phase errors induced (resulting in a 0.6 dB amplitude variation), and waveform  12 C illustrates the envelope with ±1.5° phase errors induced (resulting in a 0.3 dB amplitude variation). When comparing these results to typical specifications (such as ICD-200) 100 mrad of phase noise is acceptable, however the present invention results in only ≈30 mrad, which is well within specified limitations. Additionally, because modulation is performed in the same way as on the IIR, synthesizer contribution is the same. Further, because the binary signals from the FPGA are reclocked with the MDU reference just before output, they have the same signal quality as current modulation techniques from the MDU. Also, measured AM to PM distortion for IIR HPA&#39;s ≈4°/dB, resulting in 1.2° (21 mrad) for the selected attenuator ports. Therefore, the total expected phase noise=RSS of 30 mrad and 21 mrad=37 mrad, which is well within acceptable limits. 
     One should also recognize correlation loss, specified out around 0.6 dB, using the present invention is 0.1 dB. Further, contribution due to attenuator phase mismatch, when supplying a centered C/A code and P(Y) code, as is currently the case on IIR L 1 , is &lt;0.1 dB. Therefore, the total expect correlation loss attributable to the present system is &lt;0.2 dB, which is well within the 0.6 dB limit specified. 
     Finally, one should also be aware that no additional group delay uncertainty, differential group delay, resultant user range error, or spurious transmissions are expected to result over current methods. 
     Having described the current embodiment of this invention, it is evident that other embodiments incorporating these concepts may be used. Accordingly, although the invention has been described and pictured in a preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the detail of construction in combination and arrangement of parts may be made without departing from the spirit and scope of the invention as here and after claimed. It is intended that the patent shall cover by suitable expression in the appended claims, the whatever features of patentable novelty exist in the invention disclosed.