|Publication number||USH2155 H1|
|Application number||US 10/057,864|
|Publication date||May 2, 2006|
|Filing date||Jan 28, 2002|
|Priority date||Jan 28, 2002|
|Publication number||057864, 10057864, US H2155 H1, US H2155H1, US-H1-H2155, USH2155 H1, USH2155H1|
|Inventors||James B. Y. Tsui, David M. Lin|
|Original Assignee||The United States Of America As Represented By The Secretary Of The Air Force|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (18), Referenced by (6), Classifications (13), Legal Events (1)|
|External Links: USPTO, USPTO Assignment, Espacenet|
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
where A represents the signal amplitude, f represents the signal carrier frequency and φrepresents the phase modulation impressed on the carrier (φ assumes values of +π and −π in representing biphase modulation). If a signal represented by equation 1 is processed by mathematical squaring the results are
In the final of the equation 2 three equalities the phase term, φ, has been eliminated through use of this squaring process. The carrier frequency of the received signal can be determined by way of fast Fourier transformation (FFT) processing of the squared signal representation; such Fourier transformation processing is a part of both the known signal processing and the present invention improvements.
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 104 and 108 in
According to a conventional approach to identifying the carrier frequency in data of the
The analog to digital converter at 204 samples the 21.25 megahertz signal at a rate of 5.0 megahertz as is discussed below herein and provides a digital output signal of 1.25 megahertz center frequency and 2.5 megahertz maximum bandwidth for input to the point-by-point data squaring circuit 208. The 2.5 megahertz bandwidth here corresponds to the bandwidth of the GPS C/A code signal. The point-by-point data squaring circuit 208 mechanizes the above-recited equations 1 and 2 and provides an output signal of doubled frequency together with a term of constant value i.e., a direct current signal component as described in equation 2 above. The 5 megahertz sampling scheme aliases the input signal to 1.25 megahertz thus, the digitized signal is centered at 1.25 MHz with a notch to notch bandwidth of 2.046 megahertz for a GPS signal.
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 204 is considered to be twice as large as that of most vehicle-related Doppler systems or ±10 KHz. Once the input signal is squared, the phase modulation component will be eliminated and the signal becomes a continuous wave with embedded Doppler as shown in Equation (2). The expected Doppler frequency modulation of this continuous wave signal is extended to ±20 kilohertz through the frequency doubling process.
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 212 and 214 in the
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 216 in the
The Fourier transformation of block 218 is performed at 4000 points of the averaged data from block 216 since the original 500,000 points of data have been reduced by a factor of 125 in the accomplished averaging, i.e., 500,000/125=4000. Moreover only 2000 of these 4000 frequencies bin points are represented in the
Therefore in using the invention the following steps are needed:
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.
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|U.S. Classification||375/333, 375/316, 375/354, 375/324, 375/322, 375/361|
|Cooperative Classification||H04L2027/0065, G01S19/29, H04L2027/0048, H04L27/2276|
|European Classification||G01S19/29, H04L27/227C1|
|Feb 25, 2002||AS||Assignment|
Owner name: AIR FORCE, GOVERNMENT OF THE UNITED STATES OF AMER
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:TSUI, JAMES B.Y.;LIN, DAVID M.;REEL/FRAME:012681/0824
Effective date: 20020102