UNBIASED CODE PHASE DISCRIMINATOR

• US 20080205494A1
• Filed: 02/26/2008
• Published: 08/28/2008
• Est. Priority Date: 02/27/2007
• Status: Active Grant

• Current Assignee: Hemisphere GNSS(USA) Incorporated
• Original Assignee: Hemisphere GPS LLC (Kubota Corporation)
• Inventors: Whitehead, Michael L., Miller, Steve R.
• Granted Patent: US 8,000,381 B2
• Time in Patent Office: Days
• US Class Current: 375/150
• CPC Class Codes: G01S 19/29 carrier including Doppler, related G01S19/246 takes precedence
• CPC Class Codes: H04B 1/7085 using a code tracking loop, e.g. a delay-locked loop
• CPC Class Codes: H04B 2201/70715 with application-specific features

Abstract

A feedback control law steers a reference phase that tracks the phase of a received code sequence. The reference phase clocks a track-reference signal consisting of a series of correlation kernels, over which data is extracted and then summed in various combinations. The correlation kernels are designed in such a manner that errors caused by multipath are eliminated or substantially reduced. Furthermore, the areas of the correlation kernels are balanced across level-transitions of a code and non-transitions to eliminate phase biases when tracking specific satellites. Extra care must be taken to balance the correlation kernels in this manner due to a little known aspect of GPS C/A codes. Specifically, not all C/A codes have the same ratio of level-transitions to non-transitions as has been assumed in prior art.

Claims

1. A method of determining the phase of a code signal where the code C consists of a known sequence of positive and negative symbols of average width T, which method comprises the following steps:

generating a first correlation that corresponds to non-transitions of the code signal;

generating a second correlation that corresponds to transitions of the code signal; and

producing a correlative phase discriminator that utilizes the first and second correlations.

2. A method of determining the phase of a code portion of a signal where the code C consists of a known sequence of positive and negative symbols of average width T with the method comprising the following steps:

generating a first track reference signal S1 consisting of both zero and non-zero segments such that when S1 and C are aligned in phase, all non-zero segments of S1 occur within a width T/2 of the symbol boundaries of C that exhibit level transitions, either positive-to-negative or negative-to-positive; and

providing the signal S1 with the further properties that;

each non-zero segment of the product of C and S1, when phase aligned, has a right-hand portion and a left-hand portion with the right-hand portion occurring later in time than the left-hand portion and the division between right and left coinciding with a symbol boundary of C;

the mass-balance sum of all right-hand portions is zero; and

the mass balance sum of all left-hand portions exhibits a non-zero value B1.

generating a second track reference signal S2 consisting of both zero and non-zero segments such that when S2 and C are aligned in phase, all non-zero segments of S2 occur within a width T/2 of the symbol boundaries of C that exhibit no level transitions, with the signal S2 having the further property that;

the mass-balance sum of the product of C and S2, when phase aligned, has a non zero value B2;

performing a first correlation by accumulating of the product of D and S1;

performing a second correlation by accumulating of the product of D and S2;

determining at least one scale factor based on the values of B1 and B2;

multiplying at least one of the said correlations by at least one said scale factor;

forming an error discriminator by adding together the first and second correlation after at least one is multiplied by the said scale factor; and

using the error signal, control the phase of S1 and S2 so that the error discriminator is driven to zero.

3. The method of claim 2 where the scale factor is based on the number of symbol boundaries of C that exhibit level transitions and on the number of symbol boundaries of C that do not exhibit level transitions.

4. The method of claim 2 where the signal S1 and the signal S2 are combined into one composite signal S where at least two correlations are formed by accumulating the product of S and D, with at least one correlation corresponding to symbol boundaries of C with level transitions and with at least one correlation corresponding to symbol boundaries of C with no level transitions.

5. The method of claim 2 where S1 and S2 are further broken down into sub-signals, that when all sub-signals are added together produce the sum of S1 and S1

6. A method of providing dead-zone compensation to a correlative phase discriminator by making use of a shifted Early-Minus-Late correlation.

7. A shifted Early-Minus-Late correlator employed in a GNSS receiver system where said shifted Early-Minus-Late correlator is constructed as a Summed Correlation by summing or differencing an Early-Minus-Late Correlator (EML) with a Prompt correlator where either:

the Prompt correlator is first multiplied by a scale factor;

or the Early-Minus-Late Correlator is first multiplied by a scale factor;

the Summed Correlation is zeroed over either the positive portion of the resulting Summed Correlation or the negative portion,

said shifted Early-Minus-Late correlator is employed in a code phase tracking loop.

8. The method of claim 7 where the shifted Early-Minus-Late correlator is zero for code phase errors that are near to zero.

9. The method of claim 8 where the shifted Early-Minus-Late correlator is zero for positive code phase errors.

10. The method of claim 8 where the shifted Early-Minus-Late correlator is zero for negative code phase errors.

11. The method of claim 7 where:

the shifted Early-Minus-Late correlator is employed in a code phase tracking loop for the purpose of preventing the track loop from losing lock;

the track loop employs a separate correlator for tracking code phase errors; and

said separate correlator is designed to aid in the tracking of code phase error while not being responsive to at least some forms of multipath.

12. The method of claim 7 where:

the shifted Early-Minus-Late correlator is employed in a code phase tracking loop for the purpose of aiding in the initial acquisition of code phase;

the said track loop employs a separate correlator for tracking code phase errors; and

said separate correlator is designed to aid in the tracking of code phase error while not being responsive to at least some forms of multipath.