Near zone detection in radar level gauge system

1. A method for processing a time domain reflectometry (TDR) signal used to determine at least one process variable for a material in a tank, wherein said TDR signal has been generated by a radar level gauge system comprising a pulse generator, a probe extending into the tank for guiding the pulse, and a receiver for receiving said TDR signal, and wherein the TDR signal comprises at least a surface reflection pulse caused by an interface between different materials in the tank, and an interfering pulse caused by a transition between the pulse generator and the probe, said method comprising:

obtaining a first sampled TDR signal with an amplification such that the interfering pulse is unsaturated,

determining a compensation pulse signal using a previously stored pulse shape and a current pulse position,

subtracting said compensation pulse signal from the first sampled TDR signal,

determining if a surface reflection occurs in a zone where said interfering pulse has a significant interfering effect on any surface reflection pulse occurring in this zone, and

if no surface reflection occurs in this zone, updating the current pulse position.

2. The method of claim 1 , wherein said zone is defined to begin where the interfering pulse has declined below a given percentage of its maximum amplitude.

3. The method of claim 2 , wherein said percentage is less than 5%.

4. The method of claim 1 , wherein a signal resulting from the subtraction is used to determine said process variable.

5. The method of claim 1 , wherein the step of determining said compensation pulse signal further comprises scaling said pulse shape using a detected pulse amplitude.

6. The method of claim 5 , further comprising:

for each first sampled TDR signal, detecting the amplitude of the interfering pulse, determining if the detected amplitude is greater than a stored value for the pulse amplitude, and if so, replacing said stored value with said detected pulse amplitude.

7. The method of claim 1 , wherein said pulse position comprises one surface reflection independent component, and one surface reflection dependent component.

8. The method of claim 7 , wherein said surface reflection independent component is updated for each first sampled TDR signal.

9. The method of claim 7 , wherein said surface reflection dependent component is updated only if no surface reflection is present in said area.

10. The method of claim 7 , wherein said surface reflection independent component is defined as an intersection of the first sampled TDR signal with a threshold value.

11. The method of claim 10 , wherein said threshold value is in the range 20–40% of the maximum pulse amplitude.

12. The method of claim 10 , wherein said surface reflection dependent component is defined as a distance between said intersection and an extreme point of said interfering pulse.

13. The method of claim 1 , further comprising:

if no surface reflection is present in said zone, obtaining a second sampled TDR signal amplified in relation to said first TDR signal,

determining a compensation pulse signal using said stored pulse shape, said current maximum pulse amplitude and the updated current pulse position, and

subtracting said compensating pulse signal from the second sampled TDR signal.

14. The method of claim 13 , wherein a signal resulting from the subtraction is used to determine said process variable.

15. The method of claim 13 , wherein said second sampled TDR signal comprises a larger range than said first sampled TDR signal.

16. The method of claim 1 , further comprising:

if a surface echo is present in said zone, obtaining a second sampled TDR signal comprising a larger range than said first sampled TDR signal, and

subtracting said compensating pulse signal from the second sampled TDR signal.

17. The method of claim 16 , wherein a signal resulting from the subtraction is used to determine said process variable.

18. The moethod of claim 1 , where the process variable is a level of an interface between two materials in the tank.

19. A device for processing a time domain reflectometry (TDR) signal used to determine at least one process variable for a material in a tank, comprising:

means for generating a compensation pulse signal using a previously stored pulse shape, a current maximum pulse amplitude and a current pulse position,

means for subtracting said compensation pulse signal from a first sampled TDR signal,

means for determining if a surface reflection occurs in a zone where said interfering pulse will have a significant interfering effect on the surface reflection pulse, and

means for updating the current pulse position if no surface reflection occurs in this area.

20. A radar level gauge system, comprising:

a pulse generator, a probe extending into a tank for guiding the signal,

a sampling device for acquiring a sampled time domain reflectometry (TDR) signal, and

a TDR signal processing device for determining at least one process variable for a material in the tank,

said TDR signal comprising at least a surface reflection pulse caused by an interface between different materials in the tank, and an interfering pulse caused by the transition between the signal generator and the probe,

wherein said processing device includes:

means for generating a compensation signal using a previously stored pulse shape, a current maximum pulse amplitude and a current pulse position,

means for subtracting said compensation pulse signal from a first sampled TDR signal,

means for determining if a surface reflection occurs in a zone where said interfering pulse will have a significant interfering effect on the surface reflection pulse, and

means for updating the current pulse position if no surface reflection occurs in this area.