Abstract:
A method for controlling code power levels of individual codes of a composite quadrature phase shift keyed (QPSK) signal. The method may involve obtaining a sample of the composite QPSK signal; separating the individual codes from the composite QPSK signal; analyzing the individual codes against corresponding commanded code power settings for the individual codes; and determining a code power correction signal needed for each of the individual codes needed to adjust a code power for each of the individual codes to match their corresponding commanded code power settings.

Description:
FIELD 
     The present disclosure relates to power control systems and method used for controlling the downlink power of codes transmitted from one or more satellites, and more particularly to a system and method for more accurately determining power levels of the components of QPSK modulated signals being transmitted from one or more satellites in spite of temperature variations being experienced by electronic subsystems being used to transmit the signals. 
     BACKGROUND 
     The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
     Accurate downlink power control is highly important in a system that utilizes multiple satellites transmitting different information at the same frequency to one user. The user receiver that decodes information from one satellite at a time, when subjected to an ensemble of many signals at the same frequency, ultimately interprets the rest of all signals as background noise, thus reducing the energy-per-bit to noise ratio and increasing the probability of error. Also, maintaining accurate drive levels over a range of operating temperatures into a non-linear radio frequency (RF) power amplifier of a satellite transponder is highly desirable as it eliminates the phase (delay) uncertainty caused by the amplitude modulated/phase modulated (AM/PM) conversion factor of its non-linear devices. 
     It will also be appreciated that user range error (URE) is one highly important performance parameter of the GPS system. Maintaining accurate drive levels over temperature into a non-linear RF power amplifier of a satellite transponder eliminates the phase (delay) uncertainty caused by the AM/PM conversion factor of its non-linear devices. This means enhanced position fix accuracy for both commercial and military users. 
     Existing solutions for power measurements of composite QPSK (quadrature phase shift keyed) signals have been developed for receivers where the signals are not known in their entirety. These designs are generally highly complex since the QPSK signal has to be separated into its quadrature components, the codes for all constituents re-generated in the receiver, and finally the separated signals need to be multiplied by the proper codes before being filtered and submitted for sampling and power measurement. After typically thousands of samples, a mean power is obtained with its associated deviation or uncertainty. 
     For guaranteeing accurate power downlink power control, one specific previously used technique has involved monitoring at the ground stations the power settings of all components of the composite QPSK signal. More specifically, this involves demodulating and sampling the composite QPSK signal, recalculating the inter-related power settings of the components of the QPSK signal and uploading the new power settings. This technique requires more than one cycle due to the non-linear effects of the RF power amplifier and has a minimum inherent error of approximately 0.5 dB. Since the code power measurements have to be made from a distance of approximately 26,000 km and the atmosphere is an unstable propagation media due to its different layers at different temperatures and varying densities, the accuracy in the code power setting is generally quite limited (typically worse than 0.5 dB). 
     For the problem of minimizing user range error (URE), there presently is no satisfactory solution. The satellite transponder and its constituent subsystems will typically vary in temperature, which presents to the RF power amplifier small power fluctuations at its input. These small power fluctuations typically result ultimately in phase (i.e., delay) changes of the transmitted signal. This is because electromagnetic energy travels at the speed of approximately 30 cm per nanosecond. Thus, one nanosecond of delay uncertainty translates into a roughly 30 cm position fix error to the user receiver. 
     SUMMARY 
     In one aspect the present disclosure relates to a method for controlling code power levels of individual codes of a composite quadrature phase shift keyed (QPSK) signal. The method may comprise: obtaining a sample of the composite QPSK signal; separating the individual codes from the composite QPSK signal; analyzing the individual codes against corresponding commanded code power settings for the individual codes; and determining a code power correction signal needed for each of the individual codes needed to adjust a code power for each of the individual codes to match their corresponding commanded code power settings. 
     In another aspect the present disclosure relates to a method for controlling code power levels of four individual codes of a composite quadrature phase shift keyed (QPSK) signal being transmitted from a satellite. The method may comprise: obtaining a sample of the composite QPSK signal; separating the sample into first and second signals, the first signal including components of the composite QPSK signal that are ninety degrees out of phase with respect to a zero degree phase, and the second signal including components of the composite QPSK signal that are in phase with the zero degree phase; phase detecting the first and second signals to produce demodulated first and second signals; further processing the demodulated first and second signals to generate four sampled, peak signals corresponding to first, second third and fourth navigation data codes being generated by a navigation data unit being used with the satellite; analyzing the four sampled, peak signals in relation to corresponding commanded code power settings for the four navigation data codes; and determining a code power correction signal needed for each of the four navigation data codes needed to adjust a code power for each of the navigation data codes to match corresponding commanded code power settings for each of the navigation data codes. 
     In another aspect the present disclosure relates to a system for controlling code power levels of individual codes of a composite quadrature phase shift keyed (QPSK) signal being transmitted from a satellite. The system may comprise: a power divider for splitting a sampled portion of the composite QPSK signal into first and second signals of equal power; a first subsystem for phase detecting, demodulating and analyzing the first signal, and analyzing first and second navigation data codes being applied by a transmitter of the satellite in forming the composite QPSK signal, and generating first and second sampled, peak signals relating to peak power levels of the first and second navigation data codes; a second subsystem for phase detecting, demodulating and analyzing the second signal, and analyzing and third and fourth navigation data codes being applied by the transmitter to form the composite QPSK signal, and generating third and fourth sampled, peak signals relating to peak power levels of the third and fourth navigation data codes; and a processor for further analyzing the first, second, third and fourth peak sampled navigation data codes and commanded navigation data codes, and generating power correction signals needed to adjust a power level of each one of the first, second, third and fourth navigation data codes being used to form the composite QPSK signal. 
     Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
         FIG. 1  is a block diagram illustration of a prior art, composite QPSK modulator system used to adjust the power levels of four navigation codes being used to form a composite QPSK signal that is transmitted from a transmitter of a satellite; 
         FIG. 2  is a block diagram of one embodiment of a system in accordance with the present disclosure for analyzing and determining code power correction signals that need to be applied to the transmitter; 
         FIGS. 3A and 3B  illustrate the formulas used by the processor of the system to determine the needed code power corrections signals; 
         FIG. 4  is a simulation block diagram illustrating the inaccurate code power that results without the use of the system and method of  FIG. 2 ; 
         FIG. 5  shows the formulas of  FIG. 3B  being used to determine the code power correction signals that need to be applied to attenuators  1  and  2  in  FIG. 1  which are also shown in the simulation block diagram of  FIG. 4 ; 
         FIG. 6  is a simulation block diagram illustrating how the measured, scaled code powers for codes  1  and  2  have been corrected to be essentially equal to their respective uploaded code power settings; and 
         FIG. 7  is a flowchart illustrating major operations performed in detecting and adjusting the individual code powers for the four navigation data codes that are used to make up the composite QPSK signal. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. 
     Referring to  FIG. 1 , a prior art transmitter  10  is shown for generating a composite quadrature phase shift keyed (QPSK) signal from a satellite  12 . The transmitter typically forms a portion of a transponder that is carried by the satellite. The satellite in this example is a positioning satellite of the GPS IIF system, although it will be appreciated that the teachings of the present disclosure could be applied to any satellite transponder where it is desired to achieve a high level of control and accuracy over the downlink power used for the downlink signals transmitted from a transponder of a satellite. In this example the composite QPSK signal is received by a user receiver and used to determine a geographic location of the user receiver. However, due to temperature variations experienced by the transponder, fluctuations may exist at the input to its radio frequency (RF) amplifier that ultimately cause phase delay changes in the transmitted composite QPSK signal. 
     In  FIG. 1  the composite QPSK signal is generated by the prior art transmitter  10  by initially receiving a carrier signal from a frequency synthesizer at the input of a power divider  14 . The power divider  14  splits the carrier signal into two signals that are transmitted from outputs  16  and  18 , where the two signals have the same power. A second power divider  20  receives the carrier signal at its input  22  and further splits the power such that all components of the carrier signal that are at a zero degree phase angle are applied to output  24  thereof, and all components of the carrier signal that are 90 degrees out of phase from the zero phase are applied to output  26 . The signal from output  26  is then applied to a binary phase shift keyed (BPSK) Q channel modulator  28  that modulates the component of the carrier signal in accordance with information from navigation data code d1(t) from a navigation data unit (NDU), which is not shown in  FIG. 1 . Similarly, the component of the carrier signal at output  24  is applied to a BPSK I channel modulator  30  that also receives code d3(t) from the NDU. The Q channel modulator  28  outputs a modulated carrier signal to a first variable attenuator  32  that is controlled by an external controller (not shown). The first variable attenuator  32  outputs a signal represented by the designation “A1pk”, which represents an attenuated version of the peak output from the Q channel modulator  28 , to an input  34  of a power combiner  36 . Similarly the I channel modulator  30  generates an output to a third attenuator  38 , which in turn generates an attenuated signal designated by the notation “A3pk”, which is the attenuated version of the peak output from the I channel modulator  30 . The attenuated signal A3pk signal is likewise applied to an input  38  of the signal combiner  36 . The output of the signal combiner  36  represents a partial formation of the QPSK signal containing the carrier being modulated with code  1  on the Q channel and the carrier being modulated with code  3  on the I channel. 
     The signal at output  18  of power divider  14  is also applied to a power divider  20   a  which as power divider  20 , also has one output at zero degrees phase shift with respect to the input and the second output at a 90 degree phase shift with respect to the input forming an I channel and a Q channel respectively. Components in the upper half of the drawing in common with those in the lower part of the drawing have been designated with the same reference numbers, but that also include a prime “′” symbol. The only difference being that code d 4 (t) is applied to the I channel modulator  30   a  and code d 2 (t) is applied to the Q channel modulator. The output from the signal combiner  36   a  represents the other partial formation of the QPSK signal containing the carrier being modulated with code  2  on the Q channel and the carrier being modulated with code  4  on the I channel. Outputs from combiners  36  and  36   a  are routed to a final combiner  40  where the complete composite QPSK signal is finally formed containing d1pk=A1pk+A2pk and d2pk=A1pk−A2pk on the Q channel and d3pk=A3pk+A4pk, and d4pk=A3pk−A4pk in the I channel. 
     From  FIG. 1  it will be apparent that changing just the level of attenuator  32  or  32   a  will affect both d1pk and d2pk, since A1pk is used in forming both d1pk and d2pk. Similarly, changing either of one of attenuators  38  and  38   a  will have the affect of altering the value of both d3pk and d4pk. Thus, changing the power level of the modulated signal associated with any one of the four navigation codes will affect the power level of the modulated signal associated with the other codes. The system and method of the present disclosure provides a unique approach to solving the complex problem of adjusting the code powers for each of the four navigation data codes used to make up the composite QPSK signal. 
     Referring now to  FIG. 2 , a system  100  in accordance with one embodiment of the present disclosure is provided for adjusting the code powers of each one of the four navigation data codes d 1 (t), d 2 (t), d 3 (t) and d 4 (t) used in constructing the composite QPSK signal. The system  100  initially uses a sample of the composite QPSK signal that is output from the transmitter and feeds the sample to an input  102  of a power divider  104 . The power divider  104  generates two signals at its outputs  106  and  108  that are of equal power and applies one of the two signals to a phase detector  110  of a first subsystem  112 . The other one of the signals from output  108  is applied to a phase detector  110   a  of a second subsystem  112   a . Since subsystems  112  and  112   a  are functionally identical, only the operation of subsystem  112  will be provided, with it being understood that the components of subsystem  112   a  operate in identical fashion to those of subsystem  112 . Furthermore, the components of subsystem  112   a  in common with those of subsystem  112  have been designated with the same reference numbers used for subsystem  112  but also include a prime “′” symbol. Subsystem  112  may be termed the “Q channel” and subsystem  112   a  may be termed the “I channel”. 
     A power divider PD 1  is used to receive the carrier signal from a local oscillator and to split it into two signal components of equal power, with those components that are at zero degrees phase being applied to the phase detector  110   a  of subsystem  112   a  and those that are 90 degrees from zero phase being applied to the phase detector  110 . The signal at the output of the phase detector  110  is routed to a low pass filter  114 . The signal components of the demodulated signal output from the phase detector  110  will correspond to the codes on the Q-channel and those at the output of phase detector  110   a  to the codes on the I-Channel. The low pass filter  114  removes the residual carrier from the demodulated signal and generates a filtered signal to an input of a peak detector  116 . The output from the peak detector  116  represents the peak of the RF signal on the Q-channel applied to the power combiner  104 . The output from peak detector  116   a  represents the peak of the RF signal on the I-channel, which is also applied to the power combiner  104 . 
     The output of the peak detector  116  is applied to two sample and hold circuits  118  and  120 . An exclusive OR-gate  122  is used to receive the navigation codes d 1 (t) and d 2 (t) on its inputs, and its output is coupled to each of the sample and hold circuits  118  and  120 . An inverter  124  is interposed between the output of the exclusive OR-gate  124  and the sample and hold circuit  118 . When the d 1 (t) and d 2 (t) codes are different the exclusive OR-gate  122  outputs a logic “1” level signal. This turns on the sample and hold circuit  120 , and turns off sample and hold circuit  118  because of inverter  118 . When the d 1 (t) and d 2 (t) codes are the same, the output of the exclusive OR-gate is a logic 0 level, which turns on sample and hold circuit  118  and turns off sample and hold circuit  120 . The d 1 (t) and d 2 (t) codes are received from the NDU unit (not shown). 
     When sample and hold circuit  118  is active it captures the peak of the sampled d 1 (t) signal, which can be designated as “d1spk”. This signal can be viewed as being equal to the sampled sum (A1spk+A2spk) of the peak outputs from attenuator  1  (labeled  32  in  FIG. 1 ) and attenuator  2  (labeled  32   a  in  FIG. 1 ). The signal d1spk is then passed through a low pass filter  126  and then through an analog-to-digital (A/D) converter  128 . The output from the A/D converter  128  is a digital representation of the d1spk signal. Similar operations occur at sample and hold circuit  120  which generates an output that is the sampled peak of the d 2 (t) navigation code, and which can be labeled as “d2spk”. The value of d2spk will also be equal to A1spk−A2spk, which is the difference between the sampled, peak outputs from attenuators  1  and  2  (components  32  and  38 ) in  FIG. 1 . This signal is input to a low pass filter  130  and then A/D converted by A/D converter  132 . Thus, the output of the A/D converter  132  represents a digital value for d2spk. 
     The I channel subsystem  112   a  operates identically to the Q channel subsystem  112  described above but rather operates on the in-phase signal components of the sampled composite QPSK signal. Thus, the exclusive OR-gate  122   a  receives codes d 3 (t) and d 4 (t) on its inputs to control the on/off operation of the sample and hold circuits  118   a  and  120   a . Sample and hold circuit  118   a  generates the sampled, peak signal for code d 3 (t), which is labeled as d3spk in  FIG. 2 . The value d3spk is equal to A3spk+A4spk, which are the sampled, attenuated outputs from attenuators  3  and  4  (components  38   a  and  32   a  respectively in  FIG. 1 ). Similarly, the output of sample and hold circuit  120   a  is the sampled, peak of code d 4 (t), which is labeled in  FIG. 2  as d4spk, which is equal to A3spk−A4spk. Thus, the outputs d1spk, d2spk, d3spk and d4spk represent the actual peak, sampled values for each of the navigation codes d 1 (t), d 2 (t), d 3 (t) and d 4 (t). These values are obtained essentially in real time by the system  100 . 
     A processor  140 , which may be a microprocessor, is used to receive the outputs from each of the A/D converters  128 . The processor  140  also receives the uploaded (i.e., “commanded”) d1, d2, d3 and d4 code power levels that need to be used by the transmitter  10  ( FIG. 1 ) to form the composite QPSK signal. The processor  140  calculates the power level correction that needs to be applied to each of attenuators  1 - 4  (components  32 ,  32   a ,  38  and  38   a ) of the transmitter  10  to bring the actual power levels for each of the d 1 (t), d 2 (t), d 3 (t) and d 4 (t) codes into conformity with their uploaded (i.e., commanded) code power levels. The formulas used for calculating the corrections signals for attenuator  1  and attenuator  2  are labeled as Equations 1 and 2, respectively in  FIG. 3B .  FIG. 3A  shows the derivation for Equations 1 and 2. The equations used for the corrections needing to be applied to attenuators  3  and  4  are the same as Equations 1 and 2, respectively. 
     Referring briefly to  FIGS. 4 ,  5  and  6 , one example is shown using a mathematical computer simulation model  200  generated with the VISSIM/COMM™ computer simulation program available from Visual Solutions, Inc. of Westford, Mass. The  FIGS. 4 and 6  include mathematical operations that correspond to the functions of certain components of the system  100 , and those operations have been designated by using the reference numbers used to describe the system  100 , but designated with a prime “′” symbol, so that the reader is better able to correlate the mathematical operations that the simulation model  200  implements to the operations and components of the actual system  100 . Boxes  117  in  FIGS. 4 and 6  represent mathematical algorithms that are used to convert the peak signal to dBm values for codes  1  and  2 . The boxes that include the terminology “L=31 PN” and “L=63 PN” convey that the lengths of the pseudorandom codes used are 31 bits and 63 bits respectively. It will also be noted that the codes have different frequencies of 1 Hz and 2 Hz. So in this simulation example, the two codes being operated on are different in both length and frequency, which adds to the complexity of the simulation. 
     The simulation model  200  shows how the present system and method corrects code power errors for just the Q channel subsystem  112  of  FIG. 2  (i.e., handling just the d 1 (t) and d 2 (t) codes). It will be appreciated that the I channel subsystem  112   a  operates in identical fashion on the d 3 (t) and d 4 (t) codes. In  FIG. 4 , attenuators  1  and  2  (components  32  and  32   a  in  FIG. 1 ) are set for 1.8 dB and 15.7 dB respectively, as indicated at the left side of the Figure. The uploaded code  1  has a scaled power of 7 dBm and uploaded code  2  has a scaled power of 4 dBm, as indicated at the far right side of the Figure. The measured, scaled power of code  1  (in dBm) is 6.77631, which is significantly off from the uploaded scaled code power of 7 dBm for code  1  in  FIG. 4 . Similarly, the measured (i.e., actual) scaled power of 3.22124 dBm for code  2  is significantly off from the uploaded code power of 4 dBm for code  2 . 
       FIG. 5  illustrates the calculations that the processor  140  of the system  100  performs, using the just-mentioned uploaded and measured scaled power measurements for codes  1  and  2 , to generate the needed code power corrections signals. In  FIG. 5 , the correction signal needed to be applied to attenuator  1  (component  32  in  FIG. 2 ) to bring the measured, scaled power for code  1  into close conformity with the 7 dBm uploaded code power value is represented by ΔA tt1 , which in this example produces a value of −0.457 dBm. This is the change that has to be applied to attenuator  1  to bring the measured, scaled code power (6.77631 dBm in this example) up to the uploaded, scaled code power of 7 dbm. Similarly, ΔA tt2  represents the code power level change that has to be applied to attenuator  2  (component  32   a  in  FIG. 4 ) to bring the measured, scaled code power for code  2  up from 3.22124 to the uploaded code power value of 4 dBm for code  2 . Referring to  FIG. 6 , applying the ΔA tt1  and ΔA tt2  values of −0.457 dB for code  1  and 0.833 dB for code  2 , respectively, shows a new measured, scaled code power of 7.02962 dBm for code  1  and a new measured, scaled power for code  2  of 3.97621 dB. These values are a significant improvement over the uncorrected, measured scaled code power values produced in  FIG. 4  and cause the measured, scaled code powers to virtually exactly match the uploaded code powers. It is to be noted that both changes should be applied simultaneously to both attenuators to obtain the desired power on both codes since each attenuator interacts with the power of both codes. In other words, these power settings are not independent of each other. 
     Referring to  FIG. 7 , a flowchart  300  is illustrated that summarizes major operations of the system  100 . At operation  302  a sample of the composite QPSK modulated signal emanating from the satellite transponder output is obtained. At operation  304  the four code components (d 1 (t), d 2 (t), d 3 (t) and d 4 (t)) of the composite QPSK signal are separated using demodulators  110  and  110   a  in  FIG. 2 . At operation  306  each of the four codes are digitally sampled and their individual code powers are measured using the processor  140 . At operation  308 , the processor  140  compares the measured code powers to their corresponding uploaded code power settings. At operation  310  the processor  140  uses the equations shown in  FIG. 3  to generate the code power correction signals needed to drive the measured code power errors essentially to zero. At operation  312  the code power correction signals are applied to the four attenuators (components  32 ,  32   a ,  38  and  38   a ) in  FIG. 2 . 
     The system and method of the present disclosure takes advantage of the fact that, at the output of the transmitter  10 , the composite QPSK signal is known. Put differently, the transmitter  10  knows exactly what signals it is sending and when. This allows the implementation of a relatively inexpensive system to demodulate and measure the code power levels of each of the four navigation data codes being used to form the composite QPSK signal. The system  100  of the present disclosure provides the significant advantage of operating in real time (i.e., essentially instantaneously) to monitor the composite QPSK signal, sample it, demodulate it, measure the sampled power levels of the four independent codes making up the composite QPSK signal, and to determine the needed correction values for each of the codes. The system  100  is relatively compact and relatively inexpensive to construct. 
     While various embodiments have been described, those skilled in the art will recognize modifications or variations which might be made without departing from the present disclosure. The examples illustrate the various embodiments and are not intended to limit the present disclosure. Therefore, the description and claims should be interpreted liberally with only such limitation as is necessary in view of the pertinent prior art.