Digitally synthesized phased antenna for multibeam global positioning

• US 6,828,935 B1
• Filed: 07/19/2002
• Issued: 12/07/2004
• Est. Priority Date: 07/19/2002
• Status: Active Grant

• Current Assignee: US Government As Represented By The Director National Security Agency
• Original Assignee: United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration
• Inventors: Dunn, Charles E., Young, Lawrence E.
• Primary Examiner(s): Issing, Gregory C.
• Application Number:
  US10/176,761
• Time in Patent Office: 872 Days
• Field of Search: 342/380, 342/383, 342/375, 342/372, 342/357.12, 375/150
• US Class Current: 342/375
• CPC Class Codes: G01S 19/21 Interference related issues ; Issues related to cross-correlation, spoofing or other methods of denial of service
• CPC Class Codes: G01S 19/30 code related G01S19/246 takes precedence
• CPC Class Codes: G01S 19/36 relating to the receiver frond end
• CPC Class Codes: G01S 19/37 Hardware or software details of the signal processing chain
• CPC Class Codes: H01Q 3/26 varying the relative phase or relative amplitude of energisation between two or more active radiating elements; varying the distribution of energy across a radiating aperture H01Q3/12, H01Q3/22, H01Q3/24 take precedence

Abstract

In a system according to the proposed technique (see figure), the signal received by each element of the array antenna would be subjected to downconversion, and spread-spectrum demodulation and correlation as necessary; this processing would be performed separately from, and simultaneously with, similar processing of signals received by the other antenna elements. For the GPS implementation, following downconversion to baseband, the signals would be digitized, and all subsequent processing would be digital. In the digital process, residual carriers would be removed and each signal would be correlated with a locally generated model pseudo random-noise code, all following normal GPS procedure. As part of this procedure, accumulated values would be added in software and the resulting signals would be phase-shifted in software by the amounts necessary to synthesize the desired antenna directional gain pattern of peaks and nulls. The principal advantage of this technique over the conventional radio-frequency-combining technique is that the parallel digital baseband processing of the signals from the various antenna elements would be a relatively inexpensive and flexible means for exploiting the inherent multiple-peak/multiple-null aiming capability of a phased-array antenna. In the original intended GPS application, the peaks and nulls could be directed independently for each GPS signal being tracked by the GPS receiver. This will improve the SNR simultaneously for each GPS signal being tracked while steering multiple nulls toward sources of interference. The technique could also be applied to other code-division multiple-access communication systems.

Claims

1. A global positioning system (GPS) receiver for use with plural satellites that transmit RF signals distinguished from one another by unique pseudo-random spread spectrum codes, comprising:

an array of plural antennas having a spacing D between adjacent antennas of the array, and providing respective received RF signals;

a set of parallel digital signal processing channels for processing said respective RF signals, each one of said parallel digital signal processing channels comprising;

an analog-to-digital converter for converting the respective received RF signal to a respective digital signal;

a local pseudo-random code generator for generating a pseudo-random code corresponding to a selected one of said satellites;

a first correlator for computing a first correlation function between the pseudo-random code from said local pseudo-random code generator and said digital signal;

means for imposing a phase delay corresponding to a phase angle Θ

in the digital signal flow through said one digital signal processing channel, the phase angle Θ

being different in each of said channels and the difference between phase angles of channels corresponding to adjacent ones of said plural antennas being such as to align an antenna beam direction of said array toward said selected satellite corresponding to said pseudo-random code; and

a first summing node for summing the first correlation functions produced by all of said parallel digital processing channels.

2. The apparatus of claim 1 wherein said means for imposing a phase delay comprises:

means for digitally multiplying said correlation function by a phase factor e^1Θ

.

3. The apparatus of claim 1 wherein:

each one of said parallel digital signal processing channels further comprises;

a quadrature generator for producing a digital sine signal and a digital cosine signal, and

multipliers for multiplying said digital signal by, respectively, said digital cosine signal and by said digital sine signal so as to convert said digital signal to in-phase and quadrature digital signal components; and

said means for imposing a phase delay comprises means connected to said quadrature generator for shifting said digital sine and cosine signals by an amount corresponding to said phase angle Θ

.

4. The apparatus of claim 1 wherein said selected satellite is at an azimuthal angle Φ

relative to a plane of said array of antennas, and wherein said difference between phase angles is δ

Θ

=D sin Φ

.

5. The apparatus of claim 1 further comprising a feedback loop between said correlation function and said means for imposing a phase delay for correcting said phase angle so as to optimize said correlation function.

6. The apparatus of claim 1 further comprising:

a second correlator for computing a second correlation function between a delayed version of the pseudo-random code from said local pseudo-random code generator and said digital signal;

a second summing node for summing the second correlation functions produced by all of said parallel digital processing channels;

a global positioning processor for processing the sums of said first and second correlation functions so as to determine a phase difference between said received signal and said local pseudo-random code generator.

7. The apparatus of claim 1 further comprising a satellite acquisition circuit for providing said phase difference δ

Θ

corresponding to said selected satellite based upon a known azimuthal angle of said selected satellite.

8. The apparatus of claim 1 further comprising a satellite acquisition circuit for slewing said phase difference δ

Θ

as to maximize said correlation function with the pseudo-random code of the selected satellite.

9. The apparatus of claim 1 further comprising a local oscillator for down-converting said RF signal to an IF signal, wherein said IF signal is the signal sampled by said analog-to-digital converter.

10. The apparatus of claim 1 further comprising a filter connected between said antenna and said analog-to-digital converter and having a bandwidth corresponding to the frequency deviation occasioned by said pseudo-random code, said analog-to-digital converter having a sampling frequency corresponding to a sub-harmonic sampling frequency of the filtered RF signal.

11. The apparatus of claim 1 wherein said summing circuit sums over plural sample times of said analog-to-digital converter.

12. A method of processing received global positioning system signals from plural satellites that transmit RF signals distinguished from one another by unique pseudo-random spread spectrum codes, said method comprising:

providing an array of plural antennas having a spacing D between adjacent antennas of the array and that capture respective received RF signals;

providing a set of parallel digital signal processing channels for processing said respective RF signals, and within each one of said parallel digital signal processing channels performing the following steps;

converting the respective received RF signal to a respective digital signal at a predetermine sampling frequency;

locally generating a pseudo-random code corresponding to a selected one of said satellites;

computing a first correlation function between the pseudo-random code from said local pseudo-random code generator and said digital signal;

imposing a phase delay corresponding to a phase angle Θ

in the digital signal flow through said one digital signal processing channel, the phase angle Θ

being different in each of said channels and the difference between phase angles of channels corresponding to adjacent ones of said plural antennas being such as to align an antenna beam direction of said array toward said selected satellite corresponding to said pseudo-random code; and

summing the first correlation functions produced by all of said parallel digital processing channels.

13. The method of claim 12 wherein the step of imposing a phase delay comprises:

digitally multiplying said correlation function by a phase factor e^iΘ

.

14. The method of claim 12 wherein:

the steps for digitally processing in each one of said parallel digital signal processing channels further comprise;

producing a digital sine signal and a digital cosine signal, and

multiplying said digital signal by, respectively, said digital cosine signal and by said digital sine signal so as to convert said digital signal to in-phase and quadrature digital signal components; and

wherein the step of imposing a phase delay comprises shifting said digital sine and cosine signals by an amount corresponding to said phase angle Θ

.

15. The method of claim 12 wherein said selected satellite is at an azimuthal angle Φ

relative to a plane of said array of antennas, and wherein said difference between phase angles is δ

Θ

=D sin Φ

.

16. The method of claim 12 further comprising providing a feedback loop between said correlation function and the step of imposing a phase delay and correcting said phase angle so as to optimize said correlation function.

17. The method of claim 12 further comprising:

computing a second correlation function between a delayed version of the pseudo-random code from said local pseudo-random code generator and said digital signal;

summing the second correlation functions produced by all of said parallel digital processing channels;

processing the sums of said first and second correlation functions so as to determine a phase difference between said received signal and said local pseudo-random code generator.

18. The method of claim 12 further comprising providing said phase difference δ

Θ

corresponding to said selected satellite based upon a known azimuthal angle of said selected satellite.

19. The method of claim 12 further comprising slewing said phase difference δ

Θ

so as to maximize said correlation function with the pseudo-random code of the selected satellite.

20. The method of claim 12 further comprising down-converting said RF signal to an IF signal, wherein said IF signal is the signal sampled by said analog-to-digital converter.

21. The method of claim 12 further comprising filtering the RF signal prior to the step of converting it to a digital signal, the filtering being performed with a bandwidth corresponding to the frequency deviation occasioned by said pseudo-random code, said sampling frequency corresponding to a sub-harmonic sampling frequency of the filtered RF signal.

22. The method of claim 12 wherein the step of summing is carried out over plural cycles of said sampling frequency.