Weak signal acquisition

• US 20070085736A1
• Filed: 05/23/2006
• Published: 04/19/2007
• Est. Priority Date: 10/14/2005
• Status: Active Grant

• Current Assignee: Accord Ideation Private Limited
• Original Assignee: Accord Software & Systems Pvt. Ltd.
• Inventors: Ray, Jayanta, Channarayapatna, Nagaraj, Deshpande, Sameet
• Granted Patent: US 7,750,843 B2
• Time in Patent Office: Days
• US Class Current: 342/357.590
• 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 19/24 Acquisition or tracking or demodulation of signals transmitted by the system synchronisation aspects of direct sequence spread spectrum modulation H04B1/7073

Abstract

A method for acquisition of a weak signal from a satellite in the presence of a strong interfering signal from another satellite is disclosed. The method encompasses identifying the auto-correlation peak due to the weak satellite signal from the cross correlation peaks due to the strong satellite signal. This invention presents a method and apparatus of acquiring a weak satellite signal in the presence of a strong interfering satellite signal in a receiver by two techniques, namely, the millisecond boundary correlation histogram method, and the frequency response correlation histogram method. Both the techniques distinguish between the correlation characteristics for auto-correlation and cross-correlation. The apparatus presented in the invention implements the methods of weak satellite signal acquisition in presence of a strong interfering satellite signal in a pseudorandom noise (PRN) receiver.

Claims

1. A method of acquisition of a weak satellite signal from a satellite, in the presence of an interfering strong satellite signal from another satellite in a global positioning system receiver, comprising the steps of:

identifying said strong interfering satellite signal that causes cross-correlation;

determining the millisecond boundary correlation histogram for said incoming strong interfering satellite signal, wherein the millisecond boundary correlation histogram is derived by determining and integrating correlation values starting from different millisecond boundaries for a navigation data bit;

correlating and integrating the locally generated code representing said weak satellite signal with the incoming signal at different chip delays for an extended period of time, thereby identifying the chip delays at which the correlation peaks occur;

generating millisecond boundary correlation histograms for navigation data bit at each of said identified chip delays at which correlation peaks occur; and

identifying the unique histogram amongst the multiple correlation histograms.

4. The method of claim 1, wherein the correlation involves the accumulation of correlation values for an extended period of time, in the order of several seconds, by continuous integration without discarding the prior data, for the search of progressively weaker signals.

5. The method of claim 1, wherein the step of generating millisecond boundary correlation histograms, occurs in a simultaneous chip frequency time multiplexed search.

6. The method of claim 1, wherein the correlation histogram is drawn for all the correlation peaks by integrating the correlation values for the extended integration period, and by delaying the start of integration by 1 millisecond each time until the end of the navigation data bit period.

2. A method of acquisition of a weak satellite signal from a satellite, in the presence of an undesirable strong interfering satellite signal from another satellite in a global positioning system receiver, comprising the steps of:

identifying said strong satellite signal that causes cross correlation;

determining the frequency response correlation histogram of the strong satellite signal with a high frequency resolution around the tracking frequency, wherein said frequency response correlation histogram is derived by integrating over time the correlation values as a result of changing the local carrier at small steps around the tracking frequency;

correlating and integrating the locally generated code representing the weak satellite signal with the incoming satellite signal at different chip delays for an extended period of time, thereby identifying the chip delays at which the autocorrelation and cross correlation peaks occur;

generating frequency response correlation histograms at each of said identified chip delays at which correlation peaks occur; and

identifying the unique histogram amongst the multiple frequency response correlation histograms.

7. The method of claim 2, wherein the correlation involves the accumulation of correlation values for an extended period of time, in the order of several seconds, utilizing prior correlation data and allowing accumulation of correlation values until the correlation peaks of the correlation histogram stand out from the noise floor.

8. The method of claim 2, wherein the step of generating frequency response correlation histograms occurs in a simultaneous chip frequency time multiplexed search.

9. The method of claim 2, wherein the frequency response correlation histogram is drawn for all the correlation peaks by integrating the correlation values for the extended integration period, and by changing the local carrier at small steps around the tracking frequency.

3. A method of acquisition of a weak satellite signal from a satellite, in the presence of an undesirable strong interfering satellite signal from another satellite in a global positioning system receiver, comprising the steps of:

performing the frequency response correlation histogram method and the millisecond boundary correlation histogram method successively and iteratively until the weak satellite signal peak is successfully identified, wherein the step of performing the frequency response correlation histogram method further comprises the steps of;

identifying said strong satellite signal that causes cross correlation;

determining the frequency response correlation histogram of the strong satellite signal with a high frequency resolution around the tracking frequency, wherein said frequency response correlation histogram is derived by integrating over time the correlation values as a result of changing the local carrier at small steps around the tracking frequency;

correlating and integrating the locally generated code representing the weak satellite signal with the incoming satellite signal, at different chip delays for an extended period of time, and thereby identifying the chip delays at which the autocorrelation and cross correlation peaks occur;

generating frequency response correlation histograms at each of said identified chip delays at which correlation peaks occur; and

identifying the unique histogram amongst the multiple frequency response correlation histograms.

and wherein the step of performing the millisecond boundary correlation histogram method further comprises the steps of;

identifying said strong interfering satellite signal that causes cross correlation;

determining the millisecond boundary histogram for said incoming strong interfering satellite signal, wherein the millisecond boundary correlation histogram is derived by determining and integrating correlation values starting from different millisecond boundaries for a navigation data bit;

correlating and integrating the locally generated code representing said weak satellite signal with the incoming signal, at different chip delays for an extended period of time, and thereby identifying the chip delays at which the correlation peaks occur;

generating millisecond boundary correlation histograms for navigation data bit at each of said identified chip delays at which correlation peaks occur; and

identifying the unique histogram amongst the multiple correlation histograms.

10. A global positioning system receiver for detecting a desired weak satellite signal in the presence of a strong interfering satellite signal, comprising:

a radio frequency down converter;

a navigation processor;

a communication controller that receives information regarding weak signal satellite identification, Doppler frequency search range and chip delay search range from said navigation processor;

a chip-frequency-time multiplexed correlator array that is operated in a time multiplexed manner for different sets of frequency range and code delay range, simultaneously generating the correlation values by continuous integration and peak examination, without discarding the prior data, at desired chip and frequency ranges for the strong interfering signal and weak desired signal, and thereafter creating the millisecond boundary correlation histogram and frequency response correlation histogram, wherein the millisecond boundary correlation histogram is derived by determining and integrating correlation values starting from different millisecond boundaries for a navigation data bit, and wherein said frequency response correlation histogram is derived by integrating over time the correlation values as a result of changing the local carrier at small steps around the tracking frequency; and

a chip frequency time multiplexed controller that programs said chip, frequency and time multiplexed correlator array.

11. The global positioning system receiver of claim 10, wherein the array of blocks within the chip frequency time multiplexed correlator array is shared between chip delay searches and Doppler frequency searches.

12. The global positioning system receiver of claim 11, wherein in successive modes of operation, said blocks are programmable and generate Doppler frequencies at different offsets and generate simultaneous chip delays, thereby enabling search in different frequency steps and chip delays simultaneously.