Compensating for multipath errors, ionospheric time delay and noise errors in differential satellite positioning systems

• US 5,796,773 A
• Filed: 10/07/1996
• Issued: 08/18/1998
• Est. Priority Date: 03/22/1994
• Status: Expired due to Term

• Current Assignee: Trimble Navigation Limited (Trimble Inc.)
• Original Assignee: Trimble Navigation Limited (Trimble Inc.)
• Inventors: Sheynblat, Len
• Primary Examiner(s): Chin, Stephen
• Assistant Examiner(s): VO, DON NGUYEN
• Application Number:
  US08/722,436
• Time in Patent Office: 680 Days
• Field of Search: 375/346, 375/349, 375/200, 375/367, 342/357, 342/358, 342/356, 455/12.1, 364/443, 364/444, 364/449, 364/549
• US Class Current: 342/357.44
• CPC Class Codes: G01S 19/22 Multipath-related issues
• CPC Class Codes: G01S 19/32 Multimode operation in a single same satellite system, e.g. GPS L1/L2
• CPC Class Codes: G01S 19/41 Differential correction, e.g. DGPS [differential GPS]
• CPC Class Codes: H04B 1/7085 using a code tracking loop, e.g. a delay-locked loop

Abstract

Method and apparatus for reducing the contributions of ionospheric propagation time delay error, multipath signal error and receiver noise error from signals received in a Satellite Positioning System (SATPS) from one or more SATPS satellites by formation and appropriate filtering of differences of signals DD that are differences of SATPS signals received by the SATPS receiver/processor. A difference signal DD of code-phase-derived, ionosphere-free signals and carrier-phase-derived, ionosphere-free signals is formed, and this difference signal is passed through a first statistical processing filter with an associated time constant τ1 in the approximate range 50-500 sec, to produce smoothed or filtered first difference signal with the estimated receiver noise error reduced or removed. The difference signal DD is passed through second and third statistical processing filters having associated time constants τ2=5-20 sec and τ3=50-500 sec, and the difference of the output signals of the second and third statistical processing filters is formed and subtracted from the filtered first difference signal to form a new difference signal that is primarily due to multipath error. This new difference signal is subtracted from thge pseudorange measaurement values to produce a signal with reduced multipath error.

Claims

1. A method for reducing multipath signal error from signals received in a Satellite Positioning System (SATPS) from one or more SATPS satellites, the method comprising the steps of:

receiving SATPS signals, which have been transmitted from one or more SATPS satellites, numbered j=1, 2, . . . , at one or more SATPS reference stations, numbered i=1, 2, . . . , whose location coordinates are accurately known, where each SATPS signal includes at least one indicium that is specific to the satellite that transmitted that SATPS signal;

processing the SATPS signals at each SATPS reference station to produce pseudorange measurement values PR(t;

i;

j) and to produce carrier phase measurement values Φ

(t;

i;

j) that vary with time t, for a sequence of selected signal sampling times {t_n }_n ;

determining an ionospheric propagation time delay τ

_I (t;

i;

j) and a rate of change with respect to time of ionospheric propagation time delay τ

_I (t;

i;

j) for SATPS signal propagation through an ionosphere from satellite number j toward each SATPS reference station;

for a selected time t₀ and a subsequent sampling time t_n, forming a difference signal IDD(t_n ;

t₀ ;

i;

j)={PR(t_n ;

i;

j)-τ

_I (t_n ;

i;

j)}-{PR(t₀ ;

i;

j)-τ

_I (t₀ ;

i;

j)}-{Φ

(t_n ;

i;

j)+τ

_I (t_n ;

i;

j)}+{Φ

(t₀ ;

i;

j)+τ

_I (t₀ ;

i;

j)};

passing the difference signal IDD(t_n ;

t₀ ;

i;

j) through a first statistical processing filter having a first selected filter time constant τ

1 that lies in a range 5-20 seconds, to produce a first output signal IDD1(t_n ;

t₀ ;

i;

j);

passing the signal IDD(t_n ;

t₀ ;

i;

j) through a second statistical processing filter having a second selected filter time constant τ

2 that lies in a range 50-1000 seconds, to produce a second output signal IDD2(t_n ;

t₀ ;

i;

j); and

forming a difference signal IDD3(t_n ;

t₀ ;

i;

j)=IDD2(t_n ;

t₀ ;

i;

j)-IDD1(t_n ;

t₀ ;

i;

j) as a third output signal that contains primarily SATPS multipath signal error; and

forming a difference signal MPR(t_n ;

t₀ ;

i;

j)=PR(t_n ;

i;

j)-IDD3(t_n ;

t₀ ;

i;

j) as a pseudorange output signal that has reduced multipath signal error.

2. The method of claim 1, further comprising the steps of:

transmitting said time delay τ

_I (t;

i;

j) and said rate of change with respect to time of time delay τ

_I (t;

i;

j) to a selected SATPS station, numbered i'"'"';

passing said output signal MPR(t_n ;

t₀ ;

i;

j) through a third statistical processing filter, having a selected filter time constant τ

0 that lies in a range 50-1000 seconds, to form a filtered output signal FMPR(t_n ;

t₀ ;

i;

j);

transmitting the filtered output signal FMPR(t_n ;

t₀ ;

i;

j) to the selected SATPS station;

forming a signal PR(t_n ;

i'"'"';

j)-τ

_I (t_n ;

i;

j) at the selected SATPS station and passing this signal through a fourth statistical processing filter having a selected time constant τ

00 that lies in a range 20-1000 seconds, to produce a filtered output signal F{PR(t_n ;

i'"'"';

j)-τ

_I (t_n ;

i;

j)}; and

forming the sum F{PR(t_n ;

i'"'"';

j)-τ

_I (t_n ;

i;

j)}+FMPR(t_n ;

t₀ ;

i;

j), as a pseudorange signal that is corrected for multipath signal errors and for ionospheric time delay errors and ionospheric propagation rate of change of time delay errors.

3. The method of claim 2, further comprising the steps of:

forming a difference signal MΦ

(t_n ;

t₀ ;

i;

j)=Φ

(t_n ;

i;

j)+τ

_I (t_n ;

i;

j)-IDD3(t_n ;

t₀ ;

i;

j) as a carrier phase output signal;

passing the output signal MΦ

(t_n ;

t₀ ;

i;

j) through said third statistical processing filter to produce a filtered output signal FMΦ

(t_n ;

t₀ ;

i;

j);

transmitting the filtered output signal FMΦ

(t_n ;

t.sub.) ;

i;

j) to said selected SATPS station;

forming a signal Φ

(t_n ;

i;

j)+τ

_I (t_n ;

i;

j) at said selected SATPS station and passing this signal through said fourth statistical processing filter to produce a filtered output signal F{Φ

(t_n ;

i;

j)+τ

_I (t_n ;

i;

j)}; and

forming a sum F{Φ

(t_n ;

i;

j)+τ

_I (t_n ;

i;

j)}+FMΦ

(t_n ;

t₀ ;

i;

j), as a carrier phase output signal that is corrected for ionospheric propagation time delay errors.

4. The method of claim 1, further comprising the step of choosing at least one of said first and second statistical processing filters from a group consisting of a Morrison filter, a Hatch filter and a Kalman filter.

5. The method of claim 1, further comprising the step of choosing said time constant τ

1 to be in a range 6-10 seconds.

6. The method of claim 1, further comprising the step of choosing said time constant τ

2 to be in range 600-1000 seconds.

7. The method of claim 1, further comprising the steps of:

forming a difference signal IDD(t_n ;

t_n-1 ;

i;

j)={PR(t_n ;

i;

j)-τ

_I (t_n ;

i;

j)}-{PR(t_n-1 ;

i;

j)-τ

_I (t_n-1 ;

i;

j)}-{Φ

(t_n ;

i;

j)+τ

_I (t_n ;

i;

j)}+{Φ

(t_n-1 ;

i;

j)+τ

_I (t_n-1 ;

i;

j)};

passing the signal IDD(t_n ;

t_n-1 ;

i;

j) through said first and said second statistical processing filters, to produce output signal IDD1(t_n ;

t_n-1 ;

i;

j) and IDD2(t_n ;

t_n-1 ;

i;

j);

forming a difference signal IDD3(t_n ;

t_n-1 ;

i;

j)=IDD2(t_n ;

t_n-1 ;

i;

j)-IDD1(t_n ;

t_n-1 ;

ij;

);

passing the signal IDD(t_n ;

t_n-1 ;

i;

j) through a third statistical processing filter having a third selected filter time constant τ

3 that lies in a range 50-1000 sec, to produce a fourth output signal IDDR(t_n ;

t_n-1 ;

i;

j);

forming a difference signal IDD6(t_n ;

t_n-1 ;

i;

j)=IDD(t_n ;

t_n-1 ;

i;

j)-IDD3(t_n ;

t_n-1 ;

i;

j) as an output signal, which corresponds to an SATPS signal with reduced SATPS multipath signal error and reduced ionospheric propagation time delay error; and

passing the output signal IDD6(t_n ;

t_n-1 ;

i;

j) through a fourth statistical processing filter having a fifth selected filter time constant τ

5 that lies in a range 50-1000 sec, to produce an output signal IDD7(t_n ;

t_n-1 ;

i;

j), which corresponds to an SATPS signal with reduced SATPS receiver noise error and reduced ionospheric propagation time delay error.

8. The method of claim 7, further comprising the step of choosing said time constant τ

3 to be in a range 300-1000 seconds.

9. The method of claim 7, further comprising the step of choosing said time constant τ

4 to be in a range 300-1000 seconds.

10. The method of claim 7, further comprising the step of choosing at least one of said first and second statistical processing filters to be a notch filter having a selected filter time constant f_C =1/τ

_C that lies in a range 0.001-0.2 Hertz.

11. The method of claim 7, further comprising the step of choosing at least one of said first, second, third and fourth statistical processing filters from a group consisting of a Morrison filter, a Hatch filter and a Kalman filter.

12. The method of claim 7, further comprising the step of choosing at least one of said first, second, third and fourth statistical processing filters to be a positive, monotonic, finite width filter.

13. The method of claim 7, further comprising the steps of:

forming a squared signal IDD3(t_n ;

t_n-1 ;

i;

j)² from the squared signal IDD3(t_n ;

t_n-1 ;

i;

j)² and passing this squared signal through a fifth statistical processing filter having a fifth selected filter time constant τ

4 that lies in a range 50-1000 seconds, to produce an output signal IDD5(t_n ;

t_n-1 ;

i;

j); and

forming a combined signal {IDD5t_n ;

t_n-1 ;

i,j)+IDD3(t_n ;

t₀ ;

i,j)² }^1/2 as a User Differential Range Error Signal.

14. The method of claim 7, further comprising the steps of:

forming a squared signal IDD6(t_n ;

t_n-1 ;

i;

j)² from said signal IDD6(t_n ;

t_n-1 ;

i;

j) and passing squared signal IDD6(t_n ;

t_n-1 ;

i;

j)² through a sixth statistical processing filter having a fifth selected filter time constant τ

6 that lies in a range 50-1000 seconds, to produce an output signal IDD8(t_n ;

t_n-1 ;

i;

j); and

forming a combined signal {IDD8(t_n ;

t_n-1 ;

i,j)+IDD3(t_n ;

t_n-1 ;

i,j)² }^1/2 as a User Differential Range Error signal.

15. The method of claim 7, further comprising the step of forming a signal IDD(t_n ;

t_n-1 ;

i;

j)² and passing this signal through a seventh statistical processing filter having a seventh selected filter time constant τ

7 that lies in a range 50-1000 seconds, to produce an output signal IDD9(t_n ;

t_n-1 ;

i;

j), as a measure of variance of said signal IDD(t_n ;

t₀ ;

i;

j).

20. The method of claim 11, further comprising the steps of:

forming a thirteenth signal that is the square of said eleventh signal, and passing the thirteenth signal through a fifth statistical processing filter having a fifth selected filter time constant τ

4 that lies in a range 50-1000 seconds, to produce a filtered thirteenth signal;

forming a sum of the filtered thirteenth signal and a square of said eleventh signal to form a first combined signal that is a measure of a User Differential Range Error signal.

16. A method for reducing multipath signal error from signals received in a Satellite Positioning System (SATPS) from one or more SATPS satellites, the method comprising the steps of:

receiving SATPS signals from one or more SATPS signal sources at one or more SATPS reference stations, whose location coordinates are accurately known, where each SATPS signal includes at least one indicium that is specific to the signal source that transmitted that SATPS signal;

processing the SATPS signals at each SATPS reference station to produce pseudorange measurement values and carrier phase measurement values that vary with time t, for a sequence of selected signal sampling times;

determining an ionospheric propagation time delay and a rate of change with respect to time of ionospheric propagation time delay for SATPS signal propagation through an ionosphere from satellite number j toward each SATPS reference station;

forming first and second signals that are differences between the pseudorange measurement value and the ionospheric propagation time delay for a selected time and for a subsequent sampling time, respectively;

forming third and fourth signals that are sums of the carrier phase measurement value and the rate of change of the ionospheric propagation time delay for a selected time and for a subsequent sampling time, respectively;

forming a fifth signal that is a sum of the second and third signals minus a sum of the first and fourth signals, and passing the fifth signal through a first statistical processing filter having a first selected filter time constant τ

1 that lies in a range 5-20 seconds, to produce a first output signal;

passing the fifth signal through a second statistical processing filter having a second selected filter time constant τ

2 that lies in a range 50-1000 seconds, to produce a second output signal;

forming a third output signal that is a difference between the first output signal and the second output signal; and

forming a fourth output signal that is a difference between the pseudorange measurement value at the subsequent sampling time and the third output signal.

17. The method of claim 16, further comprising the steps of:

transmitting said ionospheric propagation time delay and said rate of change with respect to time of ionospheric propagation time delay to a selected SATPS station;

passing said fourth output signal through a third statistical processing filter, having a selected filter time constant τ

0 that lies in a range 50-1000 seconds, to form a filtered fourth output signal;

transmitting the filtered fourth output signal to the selected SATPS station;

forming said second signal at the selected SATPS station and passing this signal through a fourth statistical processing filter having a selected time constant τ

00 that lies in a range 20-1000 seconds, to produce a filtered second signal; and

forming a fifth output signal as a sum of the filtered fourth output signal and the filtered second signal, as a pseudorange signal that is corrected for multipath signal errors and for ionospheric time delay errors and ionospheric propagation rate of change of time delay errors.

18. The method of claim 17, further comprising the steps of:

forming a sixth output signal that is a difference between said fourth signal and said third output signal;

passing the sixth output signal through said third statistical processing filter to produce a filtered sixth output signal;

transmitting the filtered sixth output signal to said selected SATPS station;

forming said fourth signal at said selected SATPS station and passing this signal through said fourth statistical processing filter to produce a filtered fourth signal; and

forming a sum of the filtered sixth output signal and the filtered fourth signal, as a carrier phase output signal that is corrected for ionospheric propagation time delay errors.

19. The method of claim 16, further comprising the steps of:

forming sixth and seventh signals that are differences between the pseudorange measurement value and the ionospheric propagation time delay for a selected sampling time and for a preceding sampling time, respectively;

forming eighth and ninth signals that are sums of the carrier phase measurement value and the rate of change of the ionospheric propagation time delay for the selected sampling time and for the preceding sampling time, respectively;

forming a tenth signal that is a sum of the seventh and eighth signals minus a sum of the sixth and ninth signals, and passing the tenth signal through said first statistical processing filter to produce a first filtered tenth signal;

passing the tenth signal through said second statistical processing filter to produce a second filtered tenth signal;

forming an eleventh signal that is a difference between the first filtered tenth signal and the second filtered tenth signal;

passing the tenth signal through a third statistical processing filter having a third selected filter time constant τ

3 that lies in a range 50-1000 seconds, to produce a third filtered tenth signal;

forming a twelfth signal that is a difference between the tenth signal and the eleventh signal, as an output signal that corresponds to an SATPS signal with reduced SATPS multipath signal error and reduced ionospheric propagation time delay error; and

passing the twelfth signal through a fourth statistical processing filter having a fifth selected filter time constant τ

5 that lies in a range 50-1000 seconds, to produce an filtered twelfth signal that corresponds to an SATPS signal with reduced SATPS receiver noise error and reduced ionospheric propagation time delay error.

21. The method of claim 19, further comprising the steps of:

forming a fourteenth signal that is the square of said twelfth signal, and passing the fourteenth signal through a sixth statistical processing filter having a sixth selected filter time constant τ

6 that lies in a range 50-1000 seconds, to produce a filtered fourteenth signal;

forming a sum of the filtered fourteenth signal and a square of said twelfth signal to form a second combined signal that is a measure of a User Differential Range Error signal.

22. The method of claim 19, further comprising the step of forming a square of said tenth signal and passing this signal through a seventh statistical processing filter having a seventh selected filter time constant τ

7 that lies in a range 50-1000 seconds, to produce a fifteenth signal that is a measure of the variance of said tenth signal.