US H2155 H1 Abstract An improved apparatus and method for determining the carrier frequency in a biphase coded signal such as the course acquisition code signal in a global position sensing system. The described system may also be used for other purposes. The described system is based on use of the conventional data signal squaring or frequency doubling step to remove biphase coding but performs a series of frequency reducing steps prior to applying the Fourier transformation sequence. The frequency reducing steps include heterodyne mixing and signal averaging. These frequency-reducing steps diminish the speed and capacity requirements imposed on the Fourier transformation sequence and thereby decrease the cost and complexity of the overall system.
Claims(17) 1. A method of determining carrier frequency of a biphase code-modulated radio frequency input signal, said method comprising the steps of:
converting a sample of said biphase code-modulated radio frequency input signal from an analog signal format to a first sequence of digital signals;
generating a second sequence of signals from said first sequence of digital signals by performing a point by point squaring of said first sequence digital signals;
removing a direct current component from said squared first sequence, second sequence, signals to form a third sequence of signals;
mixing a local oscillator signal with said third sequence signals to form a frequency down converted sequence of real signal and imaginary signal complex value pairs;
averaging selected length groupings of said real signal and said imaginary signal complex value pairs to form lowered frequency representations of said real signal sequence and said imaginary signal sequence;
combining said lowered frequency real signal sequence and said lowered frequency imaginary signal sequence to form a composite lowered frequency representation of said biphase code-modulated input signal; and
identifying included frequency components of said biphase-code-modulated input signal by performing a Fourier transformation on said composite lowered frequency representation of said frequency down converted signals.
2. The method of determining carrier frequency of a biphase code-modulated radio frequency input signal of
3. The method of determining carrier frequency of a biphase code-modulated radio frequency input signal of
4. The method of determining carrier frequency of a biphase code-modulated radio frequency input signal of
5. The method of determining carrier frequency of a biphase code-modulated radio frequency input signal of
6. The method of determining carrier frequency of a biphase code-modulated radio frequency input signal of
7. The method of determining carrier frequency of a biphase code-modulated radio frequency input signal of
8. The method of determining carrier frequency of a biphase code-modulated radio frequency input signal of
9. The method of determining carrier frequency of a biphase code-modulated radio frequency input signal of
10. Biphase code-modulated radio frequency input signal carrier frequency determination apparatus, said apparatus comprising:
analog to digital converter means for converting a sample of said biphase code-modulated radio frequency input signal to a first sequence of digital signals;
multiplication means for generating a second sequence of signals from said first sequence of digital signals by performing a point by point squaring of said first sequence digital signals;
average value-subtraction means for removing a direct current component from said second sequence signals to form third sequence signals;
local oscillator and heterodyne mixer means for adding a local oscillator circuit output signal to said third sequence signals to form a frequency down converted sequence of complex value pairs;
means for averaging selected length groupings of said real signal and said imaginary signal complex value pairs to form lowered frequency representations of said real signal sequence and said imaginary signal sequence;
means for combining said lowered frequency real signal sequence and said lowered frequency imaginary signal sequence to form a composite lowered frequency signal sequence representation of said biphase code-modulated input signal; and
Fourier transformation means for identifying included frequency components of said biphase-coded input signal in said composite lowered frequency representation of said frequency down converted signals.
11. The biphase code-modulated radio frequency input signal carrier frequency determination apparatus of
12. The biphase code-modulated input signal carrier frequency determination apparatus of
13. The biphase code-modulated radio frequency input signal carrier frequency determination apparatus of
14. The biphase code-modulated radio frequency input signal carrier frequency determination apparatus of
15. The biphase code-modulated radio input signal carrier frequency determination apparatus of
16. The biphase code-modulated radio input signal carrier frequency determination apparatus of
17. The method of determining carrier frequency of a digitally converted, mathematically squared, frequency doubled, dc component-removed, representation of an analog biphase modulated radio frequency input signal, said method comprising the steps of:
converting said squared, frequency doubled, dc component-removed, representation of a biphase modulated radio frequency input signal to a first signal of lowered frequency composition in a heterodyning mixer process;
said first signal of lowered carrier frequency having recombined complex real and imaginary signal components generated during said converting step;
generating a succession of representative average magnitudes of said recombination signal of lowered carrier frequency over selected time intervals;
said succession of representative average magnitudes comprising a second signal of additionally lowered frequency content representing said radio frequency input signal; and
determining a frequency content of said second signal of additionally lowered frequency content using Fourier transformation processing.
Description The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty. In order to acquire and process a biphase-coded signal such as the NAVSTAR/global position system (GPS) Coarse/Acquisition or Clear/Acquisition [C/A] code signal one needs to find the carrier frequency and the initial phase in the suppressed carrier signal received from a GPS satellite. The purpose of the acquisition system usually employed in a GPS receiver is to accomplish just these carrier frequency and initial phase determination functions in a signal, which actually has no carrier presence. Such an acquisition system in fact needs to perform a two-dimensional searching, i.e., searching in time and searching in frequency. This operation is time consuming; however, if one of the quantities is identified the other can be obtained rather easily because the search then becomes one-dimensional in nature. A known signal processing method to determine the carrier frequency of a biphase-coded signal includes the step of squaring the frequency representation in order to remove the appended biphase code from the signal. Such a squaring operation in fact both doubles the frequency involved and eliminates the phase modulation component of the signal. In order to demonstrate this action a biphase coded signal, s(f), may be expressed mathematically in terms of
In a real world environment with noise-inclusive signals this squaring process has the effect of increasing the noise component in the processed signal, especially under conditions where the noise is of greater magnitude than the signal and the bandwidth is relatively large. Therefore in order to find a signal a long record of data is often used. To perform Fourier transformation on a long data record is however complicated and time consuming. The present invention avoids these difficulties with a reduced Fourier transformation requirement. The present invention provides a simplified frequency doubling-based carrier frequency determination for a biphase coded signal. It is therefore an object of the present invention to provide an alternate biphase coded signal carrier frequency determination arrangement. It is another object of the invention to provide carrier frequency determination arrangement that is based on the data squaring or frequency doubling principle. It is another object of the invention to provide a carrier frequency determination arrangement in which the addition of several steps results in a simplification of the overall frequency determination process. It is another object of the invention to provide a carrier frequency determination arrangement in which frequency reducing steps enable a simplified computation. It is another object of the invention to provide a carrier frequency determination arrangement which may be practiced in either the off-line or real-time operating modes. It is another object of the invention to provide a carrier frequency determination arrangement which may be embodided in the form of either a hardware or a software algorithm. It is another object of the invention to provide a frequency determination arrangement usable to advantage in decoding a global position system signal component. It is another object of the invention to provide a frequency determination arrangement usable to advantage in decoding a plurality of global position system signal components. It is another object of the invention to provide a frequency determination arrangement usable to advantage in determining the received course acquisition code component of a global position system signal. It is another object of the invention to simplify the frequency doubling based calculation scheme for biphase-coded signals. These and other objects of the invention will become apparent as the description of the representative embodiments proceeds. These and other objects of the invention are achieved by the method of determining frequency content of a biphase code-modulated radio frequency input signal, said method comprising the steps of: converting a sample of said biphase code-modulated radio frequency input signal from an analog signal format to a first sequence of digital signals; generating a second sequence of signals from said first sequence of digital signals of performing a point by point squaring of said first sequence digital signals; removing a direct current component from said squared first sequence, second sequence, signals to form a third sequence of signals; mixing a local oscillator signal with said third sequence signals to form a frequency down converted sequence of real signal and imaginary signal complex value pairs; averaging selected length groupings of said real signal and said imaginary signal complex value pairs to form lowered frequency representations of said real signal sequence and said imaginary signal sequence; combining said lowered frequency real signal sequence and said lowered frequency imaginary signal sequence to form a composite lowered frequency representation of said biphase code-modulated input signal; identifying included carrier frequency components of said biphase-code-modulated input signal by performing a Fourier transformation on said composite lowered frequency representation of said frequency down converted signals. The accompanying drawings incorporated in and forming a part of the specification, illustrates several aspects of the present invention and together with the description serve to explain the principles of the invention. In the drawings: In the present invention the known data squaring process for determination of carrier frequency in a received biphase-modulated signal is improved-upon by the addition of processing steps enabling a reduction in the complexity and cost of subsequently-needed processing steps including a Fourier transformation step. The invention also provides an averaging removal of signal noise components. The underlying principle of the invention is to change a processing-doubled frequency into a low frequency and through signal averaging reducing the total number of data points to be processed. A GPS signal may be used as an exemplary input signal to illustrate the operation of the invention. For this purpose it may be noted that the L1 band of the GPS signal is located at a carrier frequency of 1575.42 megahertz. The C/A code signal is biphase modulated onto this carrier by a frequency of 1.023 megahertz, therefore, the bandwidth of the modulated signal is 2.046 megahertz. In a biphase modulated signal the modulation can shift the phase of the carrier by either of two different values, a shift forward by π radians or one hundred eighty degrees and a shift backward by π radians or one hundred eighty degrees. The phase perturbations at According to a conventional approach to identifying the carrier frequency in data of the In the The analog to digital converter at In view of an interest in processing aircraft-related signals in an utilizing the present invention the expected Doppler frequency embedded in the carrier signal to the analog-to-digital converter In the In the This 2.5 megahertz center frequency and 5 megahertz bandwidth signal may be further reduced in frequency in order to make the Fourier transformation operation easier to perform. Such frequency change may be accomplished by way of the second heterodyne mixer and local oscillator arrangements shown at Averaging of 125 data points to obtain one representative data point may of course be accomplished by way of numerical processing also performed in either software or hardware form, such processing is represented at The Fourier transformation of block Therefore in using the invention the following steps are needed: -
- A. Square the input data to obtain s
^{2 } - B. Generate a complex ratio frequency (RF) data as
*rf=e*^{j2πf}^{ 0 }^{t }(3) - with 500,000 points, where f
_{0}=1.25 MHz. - C. Multiply s
^{2 }and rf to convert the input to a complex data at baseband - D. Average 125 points to obtain a new set of data. The total data points are 4,000 (500,000/125).
- E. Perform 4000 point FFT to obtain the desired frequencies.
- A. Square the input data to obtain s
Even though with use of the present invention one needs to perform three additional steps, the steps B. C and D recited above, the overall calculation is much simpler than with the presently used process involving performance of a 500,000 point Fourier transformation. The foregoing description of the preferred embodiment has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiment was chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the inventions in various embodiments and with various modifications as are suited to the particular scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally and equitably entitled. Patent Citations
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