Multipath mitigation in positioning systems
The location of a receiver is determined by receiving respective ranging signals from each of a plurality of transmitters at known locations. The ranging signals are cross-correlated with respective model signals to provide respective cross-correlation functions. For cross correlation functions that are determined to include multipath noise, the multipath noise is estimated and removed. Respective delays of the cross-correlation functions are estimated and the location of the receiver is inferred from the delays.
1. A method of determining a location of a receiver, comprising the steps of:
(a) using the receiver, receiving, from each of a plurality of transmitters at known locations, a respective ranging signal; and
(b) for each said ranging signal:
(i) cross-correlating said each ranging signal with a respective model signal thereby obtaining a respective cross-correlation function,
(ii) determining whether said respective cross-correlation function includes multipath noise, by steps including: for each said ranging signal:
(A) generating a template function from said respective cross-correlation function,
(B) computing a quality measure for said respective cross-correlation function by comparing said template function to said respective cross-correlation function, and
(C) comparing said quality measure to a predetermined threshold,
and
(iii) if said respective cross-correlation function includes multipath noise:
(A) estimating said multipath noise, and
(B) removing said multipath noise from said respective cross-correlation function.
2. The method of claim 1 , wherein, for each said ranging signal, said model signal is a respective ranging code of said respective transmitter.
3. The method of claim 1 , wherein said template function has a predetermined shape and wherein said template function is generating by fitting said shape to said respective cross-correlation function.
4. The method of claim 3 , wherein said shape is triangular.
5. The method of claim 1 , wherein said estimating of said multipath noise is a least squares estimating.
6. The method of claim 1 , wherein said multipath noise is removed from said respective cross-correlation function by subtracting said multipath noise from said respective cross-correlation function.
7. The method of claim 1 , further comprising the step of:
(c) for each said ranging signal, estimating a respective delay of said respective cross-correlation function.
8. The method of claim 7 , wherein, for each said ranging signal, said estimating of said respective delay is effected by steps including fitting a predetermined functional form to said respective cross-correlation function.
9. The method of claim 8 , wherein said functional form is parabolic.
10. The method of claim 7 , further comprising the step of:
(d) inferring the location of the receiver from said delays.
11. A device for determining a location at which a plurality of ranging signals are received from a plurality of transmitters at known respective locations, comprising:
(a) a receiver for receiving the ranging signals;
(b) at least one cross-correlator for cross-correlating each ranging signal with a respective model signal, thereby obtaining, for each ranging signal, a corresponding cross-correlation function; and
(c) a multipath mitigation unit for, for each cross-correlation function:
(i) determining whether said each cross-correlation function includes multipath noise, by steps including: for each said ranging signal:
(A) generating a template function from said respective cross-correlation function,
(B) computing a quality measure for said respective cross-correlation function by comparing said template function to said respective cross-correlation function, and
(C) comparing said quality measure to a predetermined threshold,
and
(ii) if said each cross-correlation function includes multipath noise:
(A) estimating said multipath noise, and
(B) removing said multipath noise from said corresponding cross-correlation function.
12. The device of claim 11 , further comprising:
(d) a delay estimation unit for estimating a respective delay of each said cross-correlation function after said multipath noise has been removed from said cross-correlation functions that include said multipath noise.
13. The device of claim 12 , further comprising:
(e) a navigation unit for inferring the location from said delays.
14. A non-transitory computer-readable storage medium having embodied thereon computer-readable code for mitigating multipath noise in a cross-correlation function obtained by cross-correlating a ranging signal with a model signal, the computer-readable code comprising:
(a) program code for determining whether the cross-correlation function includes multipath noise, by steps including: for each said ranging signal:
(i) generating a template function from said respective cross-correlation function,
(ii) computing a quality measure for said respective cross-correlation function by comparing said template function to said respective cross-correlation function, and
(iii) comparing said quality measure to a predetermined threshold; and
(b) program code for, if the cross-correlation function includes multipath noise:
(i) estimating said multipath noise; and
(ii) removing said multipath noise from the cross-correlation function.
15. A method of determining a location of a receiver, comprising the steps of:
(a) using the receiver, receiving, from each of a plurality of transmitters at known locations, a respective ranging signal; and
(b) for each said ranging signal:
(i) cross-correlating said each ranging signal with a respective model signal thereby obtaining a respective cross-correlation function,
(ii) explicitly determining whether said respective cross-correlation function includes multipath noise, and
(iii) only if, according to said determining, said respective cross-correlation function includes multipath noise:
(A) estimating said multipath noise, and
(B) removing said multipath noise from said respective cross-correlation function.
16. A device for determining a location at which a plurality of ranging signals are received from a plurality of transmitters at known respective locations, comprising:
(a) a receiver for receiving the ranging signals;
(b) at least one cross-correlator for cross-correlating each ranging signal with a respective model signal, thereby obtaining, for each ranging signal, a corresponding cross-correlation function; and
(c) a multipath mitigation unit for, for each cross-correlation function:
(i) explicitly determining whether said each cross-correlation function includes multipath noise, and
(ii) only if, according to said determining, said each cross-correlation function includes multipath noise:
(A) estimating said multipath noise, and
(B) removing said multipath noise from said corresponding cross-correlation function.
17. A non-transitory computer-readable storage medium having embodied thereon computer-readable code for mitigating multipath noise in a cross-correlation function obtained by cross-correlating a ranging signal with a model signal, the computer-readable code comprising:
(a) program code for explicitly determining whether the cross-correlation function includes multipath noise; and
(b) program code for, only if, according to said determining, the cross-correlation function includes multipath noise:
(i) estimating said multipath noise; and
(ii) removing said multipath noise from the cross-correlation function.