Radar level meter

Abstract

The invention relates to a radar level meter (10) for determining a distance (D) from a surface (12) of a product (14) in a tank (16), the radar level meter comprising: a transceiver (18) configured to transmit an electromagnetic transmission signal (S) having a varying frequency to the surface via a signal propagation device (20) T ) and receive the electromagnetic return signal (S R ); a mixer (36) adapted to mix the transmitted electromagnetic transmit signal with the received electromagnetic return signal to provide a mixer output signal comprising information about an echo (56) from the surface and information about a further echo (58); and a processing circuit (22) configured to apply an algorithm to the mixer output signal, the algorithm providing a distance-to-level-rate matrix (54) in which the echo from the surface and the further echo are each determined in two dimensions, namely distance and level-rate, and the distance to the surface is determined based on the echo from the surface so determined.

Description

Technical Field

[0001] The invention relates to a radar level gauge and a method for determining the distance to the surface of a product in a tank. The invention also relates to a system, a computer program product, a computer readable storage medium and an electrical signal comprising such a radar level gauge. Background Art

[0002] Since radar level measurement was developed as a commercial product in the 1970s and 1980s, frequency modulated continuous wave (FMCW) has been the main measurement principle for high-precision applications. FMCW measurement involves transmitting a signal that is scanned over a frequency range of the order of several GHz into a tank. For example, the signal can be in the range of 24 GHz to 27 GHz or 9 GHz to 11 GHz. The transmitted signal is reflected by the surface of the product in the tank (or by any other impedance transformation), and an echo signal that has been delayed for a certain time returns to the level meter. The echo signal is mixed with the transmitted signal to generate a mixer (output) signal with a frequency equal to the frequency change of the transmitted signal that occurs during the time delay. Due to the linear sweep, this difference frequency, also called the intermediate frequency (IF), is proportional to the distance from the reflecting surface. The mixer signal is often referred to as the IF signal.

[0003] However, echoes that are close to each other may not be tracked because they are not resolved as two echoes. This may apply, for example, in the case where a moving surface passes a disturbance in a tank.

[0004] Additionally, for two echoes that are close to each other, there may be measurement errors due to interference, for example if the surface is close to the bottom of a tank. Summary of the invention

[0005] It is an object of the present invention to alleviate the above mentioned problems and to provide an improved radar level gauge and method.

[0006] According to a first aspect of the invention, this object and other objects are achieved by a radar level meter for determining the distance to the surface of a product in a tank, the radar level meter comprising: a transceiver configured to transmit an electromagnetic transmit signal having a varying frequency to the surface via a signal propagation device and to receive an electromagnetic return signal; a mixer adapted to mix the transmitted electromagnetic transmit signal with the received electromagnetic return signal to provide a mixer output signal comprising information about an echo of the surface and information about another echo; and a processing circuit configured to apply an algorithm to the mixer output signal, the algorithm providing a distance-to-level-rate matrix, wherein the echo of the surface and the another echo are each determined in two dimensions, namely distance and level-rate, and the distance to the surface is determined based on the echo of the surface so determined.

[0007] The invention is based on the understanding that by applying an algorithm, such as a so-called range Doppler algorithm, echoes that are close to each other in radar level measurement can be resolved (i.e. distinguished or separated), wherein in addition to the distance, the level rate of the two echoes is determined (e.g. mm / scan). For echoes of the same distance, this provides an increased measurement accuracy and for two echoes that are close to each other, an improved resolution.

[0008] The surface may be moving, wherein the other echo comes from an element in or of the tank that is stationary or is moving at a material level rate different from that of the moving surface, and wherein the processing circuitry applying the algorithm is configured to distinguish the echo of the moving surface and the echo of the element (as two separate echoes). Even if the element is stationary, in the case where the element is located below the moving surface, the element may be considered to be moving in the opposite direction compared to the surface, because electromagnetic signals have different speeds in different media. This makes it technically advantageous to apply the above algorithm to a radar level meter, because opposite directions resulting in opposite material level rates increase the separation of the echoes.

[0009] The element may be, for example, an interfering piece in the tank, wherein the processing circuit of the application algorithm is configured to distinguish the echo of the moving surface and the echo of the interfering piece when the moving surface passes through the interfering piece in the tank. In this way, an improved tracking capability can be achieved, because the surface can be distinguished and tracked when the surface passes through the interfering piece with different material level rates. Similarly, because the surface echo and the interfering piece echo are separated, the measurement accuracy when the surface is close to the interfering piece can be improved. The interfering piece may be, for example, a beam in the tank.

[0010] In another example, the element is the bottom of the tank. Here, the measurement accuracy can be improved in the case of a (moving) surface close to the (stationary) bottom.

[0011] The electromagnetic emission signal may comprise a plurality of frequency sweeps. The electromagnetic emission signal may for example be sawtooth shaped (up-chirp). The electromagnetic emission signal may for example be in the range of 24 GHz to 27 GHz or 9 GHz to 11 GHz.

[0012] There may be a delay after each of the multiple frequency scans. This can improve resolution without having to increase the number of frequency scans per unit time. The latter can save processing power, which is particularly advantageous in radar level meters that typically have a limited current supply (e.g. 4mA to 20mA). It can also require less memory. The delay can, for example, be in the range of 0.5s to 1.5s, for example a delay of 1s.

[0013] The processing circuit can be configured to apply an algorithm to the mixer output signal by the following operations: for each frequency scan of at least some of the multiple frequency scans of the electromagnetic transmission signal, a first fast Fourier transform is performed on the mixer output signal; the resulting spectrum is stored in a distance time matrix; and a second fast Fourier transform is performed for each distance in the distance time matrix, thereby giving a distance material level rate matrix, wherein the echo of the surface and the other echo are each determined in two dimensions. The distance material level rate matrix can be based on 32 scans, for example. In addition, each distance can be a distance bin. The bin can be, for example, 25 mm. A 2D FFT can be performed once instead of two (1D) FFTs. In addition, a DFT (discrete Fourier transform) or a Goertzel algorithm can be used instead of FFT, for example.

[0014] As indicated above, the algorithm may be a Range Doppler algorithm.

[0015] The radar level meter may be a frequency modulated continuous wave radar level meter (non-contact or GWR (guided wave radar)).

[0016] According to a second aspect of the present invention, there is provided a system comprising: a storage tank and the radar level meter according to the first aspect.

[0017] According to a third aspect of the present invention, there is provided a method for determining the distance from the surface of a product in a storage tank, the method comprising: transmitting an electromagnetic transmission signal having a varying frequency to the surface; receiving an electromagnetic return signal; mixing the transmitted electromagnetic transmission signal with the received electromagnetic return signal to provide an output signal comprising information about an echo of the surface and information about another echo; applying an algorithm to the output signal, the algorithm providing a distance-to-level-rate matrix, wherein the echo of the surface and the another echo are each determined in two dimensions, namely, distance and level rate; and determining the distance from the surface based on the echo of the surface so determined. This aspect may present the same or similar features and technical effects as the first aspect and / or the second aspect, and vice versa.

[0018] The surface may be moving, wherein the further echo is from an element in or of the tank which is stationary or moving at a different level rate than the moving surface, and wherein applying the algorithm comprises resolving the echo of the moving surface from the echo of the element.

[0019] The element may be a disturbing object in the tank, wherein applying the algorithm comprises distinguishing an echo of the moving surface from an echo of the disturbing object when the moving surface passes through the disturbing object in the tank.

[0020] The element may be the bottom of the tank.

[0021] The electromagnetic emission signal may include a plurality of frequency sweeps.

[0022] There may be a delay after each frequency scan of the plurality of frequency scans.

[0023] Applying the algorithm to the output signal may include: performing a first fast Fourier transform on the output signal for each of at least some of the multiple frequency scans of the electromagnetic transmit signal; storing the resulting spectrum in a distance-time matrix; and performing a second fast Fourier transform on each distance in the distance-time matrix to give a distance-level rate matrix, wherein the echo of the surface and the further echo are each determined in two dimensions.

[0024] The algorithm may be a range Doppler algorithm.

[0025] According to a fourth aspect of the present invention, there is provided a computer program product comprising a computer program code for performing the following steps when executed on a computer: applying an algorithm to an output signal provided by mixing an electromagnetic transmission signal emitted to the surface of a product in a storage tank with a received electromagnetic return signal, wherein the output signal comprises information about an echo of the surface and information about another echo, wherein the algorithm provides a distance level rate matrix, wherein the echo of the surface and the other echo are each determined in two dimensions, namely, distance and level rate; and determining the distance to the surface based on the echo of the surface so determined. This aspect may present the same or similar features and technical effects as one or more of the first to third aspects, and vice versa. The computer program product may be added to an existing radar level meter as a software upgrade. The computer program product may be implemented on a computer-readable storage medium.

[0026] According to a fifth aspect of the present invention, there is provided a computer readable storage medium comprising the computer program product according to the fourth aspect.

[0027] According to a sixth aspect of the present invention, there is provided an electrical signal embodied on a carrier wave and propagating on a dielectric medium, the electrical signal comprising a computer program product according to the fourth aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The present invention will be described in more detail with reference to the accompanying drawings which show currently preferred embodiments of the invention.

[0029] Figure 1 is a schematic cross-sectional view of a radar level meter suitable for implementing the present invention.

[0030] Figure 2 According to an embodiment of the present invention Figure 1Schematic block diagram of a transceiver in a radar level meter.

[0031] Figure 3a An electromagnetic transmission signal, an electromagnetic return signal, a distance-time matrix, and a distance-level-rate matrix according to an embodiment of the present invention are shown.

[0032] Figure 3b An alternative electromagnetic transmit signal is shown.

[0033] Figure 4a A distance level rate matrix according to an embodiment of the present invention is shown.

[0034] Figure 4b It is shown Figure 4a 3D representation of the distance-to-level velocity matrix as well as the amplitude.

[0035] Figure 4c A spectrum according to the prior art is shown.

[0036] Figure 5a is a graph of distance versus time according to an embodiment of the present invention.

[0037] Figure 5b is a graph of distance versus time according to the prior art.

[0038] Figure 6 is a flow chart of a method according to an embodiment of the present invention. DETAILED DESCRIPTION

[0039] Figure 1 Schematically shown is a radar level gauge 10 according to an embodiment of the invention for determining a distance D from a reference position to a surface 12 of a product 14 in a tank 16. By determining the distance D, the filling level L can be deduced.

[0040] The radar level gauge 10 is a FMCW (Frequency Modulated Continuous Wave) radar level gauge. In particular, the radar level gauge 10 shown is a non-contact FMCW radar level gauge. The radar level gauge 10 and the storage tank 16 may form part of a system 100 .

[0041] exist Figure 1 The product 14 that partially fills the tank 16 may be a liquid, a liquid gas, or even a solid, such as granules or plastic pellets. It should be noted that although the tank 16 is described herein as including a single product 14, the distance to any material interface present in the tank 16 may be determined in a similar manner.

[0042] The radar level gauge 10 may be arranged at the top of the tank 16. The radar level gauge 10 is usually stationary. The radar level gauge 10 is usually aimed in one direction (downward). The radar level gauge 10 includes a transceiver 18, which is configured to transmit an electromagnetic transmission signal S with a varying frequency to the surface 12 via a signal propagation device of the radar level gauge 10. T and receives the electromagnetic return signal S returned from the surface 12 R Usually, the electromagnetic emission signal S T and electromagnetic return signal S R Traveling perpendicular to the surface 12 .

[0043] Here, the signal propagation device is an antenna 20 connected to the transceiver 18. The antenna 20 may be connected to the transceiver 18 via a waveguide. It is noted that the antenna 20 may alternatively be directly connected to the transceiver 18, or connected to the transceiver 18 via a suitable signal medium such as a coaxial cable. The antenna 20 may be a free propagation antenna for radiating and capturing electromagnetic signals. The antenna 20 may be, for example, a conical antenna, a rod antenna, or a parabolic antenna. Alternatively, the signal propagation device may be a detector (GWR).

[0044] The radar level gauge 10 further comprises a processing circuit 22 connected to the transceiver 18. The processing circuit 22 may generally be configured to control the transceiver 18 and to determine or deduce the filling level L based on signals from the transceiver 18. The processing circuit 22 may be, for example, an ARM Cortex M4 processor.

[0045] The processing circuit 22 may also be connected to a memory 24 of the radar level gauge 10 which may store any software (computer program product) 26 required for the operation of the radar level gauge 10. The memory 24 may also provide RAM for use during operation.

[0046] The processing circuit 22 can also be connected to an external communication line 28 for analog and / or digital communication via an interface 30 of the radar level meter 10. For example, the communication between the interface 30 and an external control station (not shown) can be provided by a two-wire interface, which has the following combined functions: sending the measurement results to the control station and receiving power for the operation of the radar level meter 10. Such a two-wire interface can provide more or less constant power, and the measurement results can be superimposed on the power voltage using a digital protocol such as Fieldbus Foundation or HART. Alternatively, the current in the external communication line 28 is adjusted according to the main measurement results. An example of such an interface is a 4mA to 20mA industrial loop, where the current is adjusted between 4mA and 20mA according to the measurement results. Alternatively, the radar level meter 10 can use, for example, a wireless HART protocol to communicate wirelessly with an external control station, and use a local power supply (not shown) with a battery or other device for extracting energy to operate autonomously.

[0047] Further references Figure 2 , the transceiver 18 may include a microwave source 32. The microwave source 32 may generate an electromagnetic transmission signal S T . Electromagnetic emission signal S T For example, it can be in the range of 24 GHz to 27 GHz or 9 GHz to 11 GHz. For GWR, the electromagnetic emission signal S T The microwave source 32 may be in the range of 1 GHz to 2 GHz. The microwave source 32 may be controlled by a timer or timing circuit (not shown) of the processing circuit 22. The microwave source 32 is connected to the antenna 20 via a power divider 34 and is also connected to a mixer 36 (of the transceiver 18) of the radar level meter 10. The power divider 34 is arranged to divide the electromagnetic return signal S from the antenna 20 into R is connected to the mixer 36 so that the mixer 36 can generate the electromagnetic emission signal S from the microwave source 32. T With the electromagnetic return signal S R Mixing is performed and a (mixer) output signal is provided, which is also referred to as an intermediate frequency signal. The mixer 36 may also be connected to a band pass filter 38 , which in turn may be connected to an amplifier 40 .

[0048] While the elements of the transceiver 18 are typically implemented in hardware and form part of an integrated unit often referred to as a microwave unit, at least some portions of the processing circuitry 22 are typically implemented by software modules executed by an embedded processor. The present invention is not limited to this particular implementation, and any implementation suitable for implementing the functionality described herein is contemplated.

[0049] The output signal of mixer 36 can not only include the information about the echo of surface 12, but also include the information about another echo.Another echo can come from an element, such as an interference piece 46 in storage tank 16 or the bottom 48 of storage tank 16. However, as pointed out in the background technology part of the application, using existing solutions, it may be impossible to track the echoes that are close to each other because they are not resolved as two echoes. This can be, for example, applicable to the following situation: surface 12 is moving up / down and passes through interference piece 46 or moving surface 12 close to the bottom 48 of storage tank 16.

[0050] To this end, the processing circuit 22 can be configured to apply an algorithm, typically a range Doppler algorithm, to the mixer output signal to determine the above echoes for two dimensions (distance and level velocity) and to determine the distance D from the surface 12 based on the echoes of the surface 12 so determined. The dimension distance in this context corresponds to the distance D (value).

[0051] Further references Figure 3a to Figure 3b , electromagnetic emission signal S T A plurality of identical frequency sweeps 50 may be included. The electromagnetic transmission signal may be, for example, sawtooth-shaped (positive linear frequency modulation pulse). Each frequency sweep 50 may, for example, have a length of 8 ms and a bandwidth of 3 GHz. Figure 3a , the frequency sweeps 50 occur immediately after each other. Alternatively, as Figure 3b As shown, there may be a delay T after each frequency scan 50 delay Delay T delay It can be 1 s, for example. Delay T delay The resolution can be improved so that the radar level meter 10 can resolve echoes that are closely spaced in the level rate because the delay T delay The total time T that the radar level meter 10 looks at the surface 12 and the element 46 is extended f .

[0052] When applying the range Doppler algorithm to the mixer output signal, the processing circuit 22 may: for the electromagnetic transmission signal S T Each of the plurality of frequency scans 50 performs a first fast Fourier transform FFT on the mixer output signal. 1 Each FFT 1 A spectrum is generated. The processing circuit 22 stores the resulting continuous spectrum in a distance time matrix 51, where each distance may be, for example, a distance bin 52 of 25 mm. The "distance" here is the distance D, which may alternatively be referred to as a "range". In addition, the processing circuit 22 performs a second fast Fourier transform FFT on each distance (distance bin 52) in the distance time matrix 51. 2, thus giving a distance-level-rate matrix 54, in which the surface echo 56 and the aforementioned further echo 58 are each determined according to the above two dimensions (distance and level-rate), see also Figure 4a to Figure 4b Alternatively, the distance level rate matrix 54 may be referred to as a two-dimensional spectrum.

[0053] Thus, for example, in the case where the surface 12 is moving (typically upward or downward) and, for example, the interferer 46 is stationary (or has a different level velocity than the moving surface 12), the processing circuitry 22 applying the range Doppler algorithm distinguishes the moving surface echo 56 from the interferer echo 58. Figure 4a to Figure 4b , the two echoes are at the same distance of approximately 10 meters, but they are resolved as two separate echoes 56 and 58. Therefore, processing circuitry 22 can determine distance D from surface 12 based on echo 56. Figure 4a to Figure 4b FIG. 5 shows a specific frequency scan 50 among a plurality of frequency scans 50, and FIG. Figure 4a to Figure 4b The distance level rate matrix 54 shown may be based on, for example, 32 ( Figure 3b N) the aforementioned frequency scan 50. In addition Figure 4a to Figure 4b In , the echo 56 of the moving surface has a material level rate of 1 mm / scan, and the echo 58 of the interference piece has a material level rate of -0.3 mm / scan. For comparison, Figure 4c A similar situation is shown but without the use of the range Doppler algorithm, so that the echoes 56, 58 are not resolved (separated).

[0054] exist Figure 5a In the particular example shown, the surface 12 moves as the product 14 is removed from the tank 16. The echoes of the surface 12 at different frequency sweeps determined by the processing circuit 22 are plotted and represented by the solid line 60, and the echoes of the interference element 46 (stationary) at different frequency sweeps determined by the processing circuit 22 are plotted and represented by the dashed line 62. Figure 5a As shown, the processing circuit 22 applying the range Doppler algorithm can also distinguish the echo of the moving surface from the echo of the interference element when the moving surface 12 passes through the interference element 46. On the other hand, if the range Doppler algorithm is not used, the echo of the surface may not be distinguished, and the determination of the distance to the surface may be lost when the moving surface passes through the interference element, such as Figure 5b As shown, the echo of the surface is represented by + and the echo of the interfering element is represented by x.

[0055] For example, Figure 5a It can also be noted that even though the disruptor 46 is stationary, in the case where the disruptor 46 is located below the moving surface 12 and is therefore immersed in the product 14, the disruptor "moves" in the opposite direction compared to the surface 12. This is achieved by Figure 5aThe dotted line 62 in FIG. 6 is shown by the inclined portion to the left of the intersection with the solid line 60. This is also shown by Figure 4a to Figure 4b The negative material level velocity of the interference echo 58 in FIG. 4 is shown. The reason for this perceived movement of the interference element 46 is that the signal S T and S R The speed depends on the medium.

[0056] Figure 6 is a flow chart of a method according to an embodiment of the present invention. The method may be performed by the radar level gauge 10. That is, the steps of the method may be performed during operation of the radar level gauge 10. The method comprises: transmitting (S1) an electromagnetic transmission signal S having a varying frequency to the surface 12 T ; Receive (S2) electromagnetic return signal S R ; The electromagnetic emission signal S T The received electromagnetic return signal S R Mixing (S3) is performed to provide an output signal including information about the echo 56 of the surface and information about the further echo 58; applying (S4) an algorithm to the output signal, the algorithm providing a distance-level-rate matrix 54, wherein the echo 56 of the surface and the further echo 58 are each determined in two dimensions, namely, distance and level-rate; and determining (S5) a distance D from the surface 12 based on the echo 56 of the surface 12 thus determined. Applying the algorithm to the output signal may include: for the electromagnetic transmission signal S T Each frequency scan 50 in the plurality of frequency scans 50 performs (S4) on the output signal 1 ) First Fast Fourier Transform FFT 1 ; Store the obtained continuous spectrum (S4 2 ) in the distance time matrix 51; and for each distance 52 in the distance time matrix 51, (S4 3 ) Second Fast Fourier Transform FFT 2 , which gives a distance-to-level velocity matrix 54 in which the surface echo 56 and the further echo 58 are each determined in the two dimensions mentioned above.

[0057] Those skilled in the art realize that the present invention is by no means limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims.

[0058] For example, more than two echoes may be resolved, such as an echo of the surface 12 , an echo of the interferer 46 , and an echo of the bottom 48 .

[0059] In addition, instead of Figure 3a Instead of two (1D) FFTs in , one 2D FFT can be performed. In addition, DFT (Discrete Fourier Transform) or Goertzel algorithm can be used instead of FFT, for example.

[0060] Alternatively, instead of adding a delay T delay , by adding more frequency sweeps to the electromagnetic emission signal S T To expand the total time T f , where the frequency scans occur immediately after each other, e.g. using 64 frequency scans instead of 32, and then performing the FFT only for some of the frequency scans, e.g. every other frequency scan 1 .

Claims

1. A radar level meter for determining the distance to a surface of a product in a tank, the radar level meter comprising: a transceiver configured to transmit an electromagnetic transmit signal having a varying frequency to the surface via the signal propagation device and to receive an electromagnetic return signal; a mixer adapted to mix the transmitted electromagnetic transmit signal with the received electromagnetic return signal to provide a mixer output signal comprising information about the echo of the surface and information about the further echo; as well as A processing circuit is configured to apply an algorithm to the mixer output signal, the algorithm providing a distance-to-level-velocity matrix, wherein the echo of the surface and the further echo are each determined in two dimensions, distance and level-velocity, and the distance to the surface is determined based on the echo of the surface so determined.

2. The radar level meter according to claim 1, wherein: The surface is moving, wherein the other echo comes from an element in or of the tank, which element is stationary or is moving at a different material level rate than the moving surface, and wherein the processing circuit applying the algorithm is configured to distinguish the echo of the moving surface from the echo of the element.

3. The radar level meter according to claim 2, wherein: The element is a disturber in the tank, and wherein the processing circuit applying the algorithm is configured to distinguish an echo of the moving surface from an echo of the disturber in the tank if the moving surface passes through the disturber.

4. The radar level meter according to claim 2, wherein: The element is the bottom of the tank.

5. The radar level meter according to claim 1 or 2, wherein: The electromagnetic transmission signal includes a plurality of frequency sweeps.

6. The radar level meter according to claim 5, wherein: There is a delay after each frequency scan in the plurality of frequency scans.

7. The radar level meter according to claim 5, wherein: The processing circuit is configured to apply the algorithm to the mixer output signal by: for each of at least some of the plurality of frequency sweeps of the electromagnetic transmit signal, performing a first fast Fourier transform on the mixer output signal; The resulting spectrum is stored in a range-time matrix; as well as A second Fast Fourier Transform is performed for each distance in the range-time matrix, thereby giving the range-level-rate matrix, wherein the echo of the surface and the further echo are each determined in the two dimensions.

8. The radar level meter according to claim 1 or 2, wherein: The algorithm is a Range Doppler algorithm.

9. The radar level meter according to claim 1 or 2, wherein: The radar level meter is a frequency modulated continuous wave radar level meter.

10. A system comprising: Storage tanks; as well as The radar level meter according to claim 1.

11. A method of determining a distance from a surface of a product in a tank, the method comprising: emitting an electromagnetic transmission signal having a varying frequency toward the surface; receiving an electromagnetic return signal; mixing the transmitted electromagnetic transmit signal with the received electromagnetic return signal to provide an output signal including information about the echo from the surface and information about another echo; applying an algorithm to the output signal, the algorithm providing a distance-level-velocity matrix in which the echo of the surface and the further echo are each determined in two dimensions, distance and level-velocity; and The distance to the surface is determined based on the echo of the surface determined in this way.

12. The method according to claim 11, wherein: The surface is moving, wherein the further echo is from an element in or of the tank, the element being stationary or moving at a different level rate than the moving surface, and wherein applying the algorithm comprises resolving the echo of the moving surface from the echo of the element.

13. The method according to claim 12, wherein: The element is a disturber in the tank, and wherein applying the algorithm comprises resolving an echo of the moving surface from an echo of the disturber in a case where the moving surface passes through the disturber in the tank.

14. The method according to claim 12, wherein: The element is the bottom of the tank.

15. The method according to claim 11 or 12, wherein: The electromagnetic transmission signal includes a plurality of frequency sweeps.

16. The method according to claim 15, wherein: There is a delay after each frequency scan in the plurality of frequency scans.

17. The method according to claim 15, wherein: Applying the algorithm to the output signal comprises: for each of at least some of the plurality of frequency sweeps of the electromagnetic transmit signal, performing a first fast Fourier transform on the output signal; storing the resulting spectrum in a range-time matrix; and A second Fast Fourier Transform is performed for each distance in the range-time matrix, thereby giving the range-level-rate matrix, wherein the echo of the surface and the further echo are each determined in the two dimensions.

18. The method according to claim 11 or 12, wherein: The algorithm is a Range Doppler algorithm.

19. A computer program product comprising computer program code for performing the following steps when executed on a computer: applying an algorithm to an output signal provided by mixing an electromagnetic transmit signal transmitted toward a surface of the product in the tank with a received electromagnetic return signal, wherein The output signal comprises information about the echo of the surface and information about a further echo, wherein the algorithm provides a distance-level-velocity matrix, wherein the echo of the surface and the further echo are each determined in two dimensions, distance and level-velocity; and The distance to the surface is determined based on the echo of the surface determined in this way.

Images