Processing of tank signal in radar level gauge system

1. A method for processing a reflected microwave signal used to determine at least one process variable of a content in a tank, wherein said reflected signal has been generated by a radar level gauge system arranged to transmit microwaves towards the material in the tank during consecutive measurement cycles, and receive a reflection of said microwave signal as a tank signal comprising consecutive tank signal portions each corresponding to a measurement cycle, said method comprising:

processing each portion of the received tank signal in a plurality of processes, each process being adapted to determine a process variable in a specific region of the tank, each specific region corresponding to a separate predefined propagation distance range.

2. The method according to claim 1 , wherein said processes are performed in parallel.

3. The method according to claim 1 , wherein the received tank signal is a frequency domain tank signal, and wherein a frequency spectrum is determined by Fourier transforming said tank signal.

4. The method according to claim 1 , wherein at least one of said processes includes subtracting a compensation signal from the tank signal, said compensation signal including background information about the specific region.

5. The method according to claim 4 , wherein said compensation signal CS n (t) is formed by low-pass filtering a tank signal received when no surface reflection occurred inside the specific region.

6. The method according to claim 5 , wherein an updated compensation signal CS n *(t) is formed by combining said compensation signal CS n (t) with a previous compensation signal CS n−1 (t), according to

CS n *( t )= a CS n ( t )+(1 −a )CS n−1 ( t ),

where a is a weight factor between zero and 1.

7. The method according to claim 1 , wherein at least one of said processes is a near zone detection process, adapted to detect a surface reflection in a near zone of the tank.

8. The method according to claim 7 , wherein said near zone detection process comprises:

for each portion of the received tank signal, subtracting a compensation signal CS n (t) based on a near zone signature of the tank from said received tank signal, and detecting any presence of a peak with an amplitude greater than a predetermined threshold in the near zone,

monitoring the number of measurement cycles in which a peak occurs, and

identifying a surface reflection based on said peak.

9. The method according to claim 8 , wherein said step of monitoring includes incrementing a counter for every measurement cycle in which a peak is detected, and decrementing said counter for every measurement cycle in which no peak is detected.

10. The method according to claim 8 , wherein said peak is detected in a frequency spectrum determined by Fourier transforming said subtraction result.

11. The method according to claim 1 , wherein said tank signal is a time domain reflectometry (TDR) signal.

12. A radar level gauge system used for determining a process variable of a content in a tank, comprising:

a microwave transmitter for generating a microwave signal,

means for guiding said microwave signal into the tank, and allowing it to propagate towards the content in the tank,

a microwave receiver for receiving a reflection of said microwave signal from the tank as a tank signal, comprising consecutive tank signal portions each corresponding to a measurement cycle,

processing circuitry arranged to determine said process variable based on each portion of said tank signal,

said processing circuitry being adapted to perform a plurality of processes on each portion of said tank signal, each process being adapted to determine a process variable in a specific region of the tank, each specific region corresponding to a separate predefined propagation distance range.

13. The radar level gauge according to claim 12 , wherein said microwave signal is a frequency domain signal.

14. The radar level gauge according to claim 12 , wherein said tank signal is a time domain reflectometry (TDR) signal.

15. The radar level gauge according to claim 12 , wherein said signal guiding and propagating means include a free radiating antenna arranged inside the tank.

16. The radar level gauge according to claim 12 , wherein said signal guiding and propagating means include a probe extending into the tank.

17. The radar level gauge according to claim 12 , wherein at least one of said processes includes means for subtracting a compensation signal from the tank signal, said compensation signal including background information about the specific region.

18. The radar level gauge according to claim 12 , wherein at least one of said processes is a near zone detection process, adapted to detect a surface reflection in a near zone of the tank.

19. The radar level gauge according to claim 18 , wherein said near zone detection process comprises:

means for subtracting a compensation signal CS n (t) based on a near zone signature of the tank from said received tank signal,

means for detecting any presence of a peak with an amplitude greater than a predetermined threshold in the near zone,

means for monitoring the number of measurement cycles in which a peak occurs, and

means for identifying a surface reflection based on said peak.

20. The radar level gauge according to claim 19 , wherein said monitoring means includes a counter arranged to be incremented for every measurement cycle in which a peak is detected, and decremented for every measurement cycle in which no peak is detected.

21. The radar level gauge according to claim 19 , wherein said peak detection means are arranged to form a frequency spectrum by Fourier transforming a result from said subtraction means.