Radar level gauge system and method for detecting submerged antenna condition

Abstract

Radar level gauge systems and methods to detect submerged antenna conditions are disclosed. A radar level gauge system includes a microwave antenna configured to radiate an electromagnetic transmit signal toward a surface of an aqueous substance and to receive a reflected signal resulting from a reflection of the transmit signal; a transceiver coupled to the microwave antenna and configured to: generate a transmit signal; providing the transmit signal to a microwave antenna; and receive a reflected signal; and processing circuitry configured to: form an echo representation indicative of received energy as a function of distance; determining, based on the echo representation, a surface for which the aqueous substance cannot be identified within a pre-configured maximum measurement distance from the microwave antenna; determining, based on the echo representation, that a received energy within a maximum range of the radar level gauge system is below a predetermined threshold energy; and determining that the microwave antenna is submerged in the aqueous substance.

Description

Technical Field

[0001] The invention relates to a radar level gauge system for measuring the distance to the surface of an aqueous substance in a level gauging installation and to a method for measuring the distance to the surface of an aqueous substance. Background Art

[0002] Radar level gauge systems are widely used in various facilities to determine the distance to the surface of a material. Different materials present different challenges. Materials with high relative dielectric constants, such as aqueous materials, generally provide relatively strong surface echoes, making it relatively easy to identify the surface echo.

[0003] However, if such an aqueous substance is spilled such that the antenna of the radar level gauge system is submerged, the echo representation formed by the radar level gauge system may easily be interpreted as indicating an empty condition. This may be particularly true if the radar level gauge system temporarily lacks power and is restarted while the antenna is submerged.

[0004] This may have undesirable consequences. For example, considering a wastewater pumping station, an erroneous determination of an empty condition may cause the pumps to be shut down when the wastewater pumping station has actually flooded.

[0005] It would be desirable to provide improved radar level gauging of aqueous materials, particularly with an improved ability to detect that a microwave antenna has been immersed in an aqueous material. Summary of the invention

[0006] In view of the above, a general object of the present invention is to provide improved radar level gauging of aqueous materials, particularly with an improved ability to detect that a microwave antenna has been immersed in an aqueous material.

[0007] According to aspects of the present invention, there is provided a radar level gauge system for measuring the distance to the surface of an aqueous material in a level metering facility, the radar level gauge system comprising: a microwave antenna configured to radiate an electromagnetic transmit signal toward the surface of the aqueous material and receive a reflected signal generated by reflection of the transmit signal; a transceiver coupled to the microwave antenna, the transceiver configured to: generate a transmit signal; provide the transmit signal to the microwave antenna; and receive the reflected signal; and a processing circuit system configured to: form an echo representation indicating received energy as a function of distance based on the reflected signal and the timing relationship between the transmit signal and the reflected signal; determine, based on the echo representation, that the surface of the aqueous material cannot be identified within a preconfigured maximum measurement distance of the level metering facility; determine, based on the echo representation, that the received energy within the maximum range of the radar level gauge system is lower than a predetermined threshold energy; and determine that the microwave antenna is immersed in the aqueous material.

[0008] It should be noted that in any level measurement installation using a radar level gauge system, the maximum range of the radar level gauge system is greater than the preconfigured maximum measurement distance. The maximum range of the radar level gauge system may depend on the specifications of the radar level gauge system, such as signal-to-noise ratio, timing accuracy, which measurement principle is implemented, etc.

[0009] When forming a level gauging facility, the preconfigured maximum measurement distance of the level gauging facility or another parameter indicating the preconfigured maximum measurement distance of the level gauging facility may be provided to the radar level gauge system as a configuration parameter at installation / initial configuration. The configuration parameter may be stored in a non-volatile memory included in the radar level gauge system, and in normal operation, only surface echo candidates within the maximum distance of a specific level guiding facility may be considered. This may provide improved performance and reduced energy consumption of the radar level gauge system.

[0010] The invention is based on the recognition that an evaluation of the flight time of a reflected signal indicating a longer distance than a preconfigured maximum measuring distance provides valuable information to enable a reliable determination of when a microwave antenna is immersed in an aqueous substance.

[0011] Such a flight time indicating a longer distance than the preconfigured maximum measuring distance from the microwave antenna may be caused, for example, by the reflected signal being reflected multiple times at the walls of the fixture and / or tank, etc. When empty conditions exist, multiple reflections may occur, resulting in a relatively weak reflected signal with a flight time indicating a longer distance than the maximum measuring distance. However, when the microwave antenna is immersed in an aqueous substance, such reflections do not occur, since the transmitted signal can only propagate a short distance through the aqueous substance.

[0012] Thus, in particular, the evaluation of the echo representation within the maximum range of the radar level gauge system, including the evaluation of the flight time of the reflected signal indicating a longer distance than the preconfigured maximum measuring distance, helps to distinguish between a situation where the microwave antenna is immersed in the aqueous substance and an empty condition or a general low level condition. For example, an empty / low level condition can be set at a level where there is a risk of dry running of the installed pump. This in turn provides for improved operation of water and wastewater facilities where there is a risk of temporary overflow, for example.

[0013] In an embodiment of the radar level gauge system according to the invention, the processing circuit system may be configured to determine that the surface of the aqueous substance cannot be identified within the maximum measurement distance by determining that the echo representation fails to meet a predetermined surface echo detection criterion within the maximum measurement distance from the microwave antenna. Advantageously, the surface echo detection criterion may include detecting a peak in the echo representation within the maximum measurement distance from the microwave antenna, the amplitude of the peak being higher than a predetermined first echo detection threshold amplitude. When a peak with an amplitude higher than the first echo detection threshold amplitude is detected in the echo representation within the time-of-flight range of the reflected signal indicating the distance within the maximum measurement distance from the microwave antenna, for the level metering facility, the distance to the surface of the aqueous substance may be determined based on the time-of-flight of the peak. When no such peak is detected within the time-of-flight range of the reflected signal indicating the distance within the maximum measurement distance from the microwave antenna, it may be concluded that the surface of the aqueous substance cannot be identified within the preconfigured maximum measurement distance of the level metering facility.

[0014] In addition, in an embodiment of the radar level gauge system according to the present invention, the processing circuit system can be configured to determine that the received energy within the maximum range of the radar level gauge system is lower than a predetermined threshold energy based on the failure to detect the following peak in the echo representation within the maximum range of the radar level gauge system, the amplitude of which is higher than a predetermined second echo detection threshold amplitude that is lower than the first echo detection threshold amplitude.

[0015] The processing circuit system of the radar level gauge system can be configured to: when it has been concluded that the surface of the aqueous substance cannot be identified within the maximum measurement distance from the microwave antenna, additionally evaluate the echo representation within at least a portion of the echo representation indicating a distance longer than the maximum measurement distance from the microwave antenna. For at least this portion of the echo representation, the processing circuit system can be configured to evaluate the amplitude of the echo representation and / or integrate the echo representation to evaluate the total amount of microwave energy received, wherein the flight time indicates a distance longer than the preconfigured maximum measurement distance of the level metering facility. Alternatively or in combination, an evaluation can be performed on the amplitude and / or cumulative energy of the echo representation within the entire maximum range of the radar level gauge system, and a conclusion can be drawn from such an evaluation as to whether the microwave antenna is immersed in the aqueous substance.

[0016] In various embodiments, the radar level gauge system may be of the so-called FMCW (frequency modulated continuous wave) type. In these embodiments, the transceiver may be configured to: generate a transmit signal in the form of a measurement scan; and form an intermediate frequency signal indicating the frequency difference between the reflected signal and the transmit signal as a function of time; the radar level gauge system may include a sampler coupled to the transceiver and configured to sample the intermediate frequency signal using a predetermined number of samples during the measurement scan; and the processing circuit system may be coupled to the sampler and configured to form an echo representation based on the samples. In these embodiments, the maximum range of the radar level gauge system may be related to the bandwidth and the predetermined number of samples, which is itself well known to those skilled in the art or ordinary technicians.

[0017] Furthermore, a radar level gauge system according to an embodiment of the present invention may be included in a level metering facility, which further includes a tank having a tank bottom and a tank wall, wherein the radar level gauge system is arranged such that a microwave antenna of the radar level gauge system is vertically spaced apart from the tank bottom.

[0018] According to a second aspect of the present invention, there is provided a method for measuring the distance to the surface of an aqueous substance using a radar level gauge system, comprising: generating an electromagnetic transmission signal by a transceiver of the radar level gauge system; providing the transmission signal to a microwave antenna of the radar level gauge system; radiating the transmission signal by the microwave antenna substantially vertically toward the surface of the aqueous substance; receiving a reflection signal generated by reflection of the transmission signal by the transceiver via the microwave antenna; forming an echo representation indicating received energy as a function of distance based on the reflection signal and a timing relationship between the transmission signal and the reflection signal by a processing circuit system of the radar level gauge system coupled to the transceiver; determining by the processing circuit system based on the echo representation that the surface of the aqueous substance cannot be identified within a preconfigured maximum measurement distance from the microwave antenna; determining by the processing circuit system based on the echo representation that the received energy at a distance longer than the preconfigured maximum measurement distance is lower than a predetermined threshold energy; and determining by the processing circuit system that the microwave antenna is immersed in the aqueous substance.

[0019] In summary, aspects of the present invention therefore relate to a radar level gauge system, comprising: a microwave antenna, which is configured to radiate an electromagnetic transmit signal toward the surface of an aqueous material and receive a reflected signal generated by reflection of the transmit signal; a transceiver, which is coupled to the microwave antenna and is configured to: generate a transmit signal; provide the transmit signal to the microwave antenna; and receive the reflected signal; and a processing circuit system, which is configured to: form an echo representation indicating received energy as a function of distance; determine based on the echo representation that the surface of the aqueous material cannot be identified within a preconfigured maximum measurement distance from the microwave antenna; determine based on the echo representation that the received energy within the maximum range of the radar level gauge system is lower than a predetermined threshold energy; and determine that the microwave antenna is immersed in the aqueous material. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] These and other aspects of the present invention will now be described in more detail with reference to the accompanying drawings, in which:

[0021] Figure 1 Schematically illustrates an example level gauging installation comprising a radar level gauge system according to an embodiment of the present invention;

[0022] Figure 2 is a schematic diagram of a radar level gauge system according to an embodiment of the present invention;

[0023] Figure 3 is a flowchart illustrating a method according to an example embodiment of the present invention; and

[0024] FIG. 4A to FIG. 4C is a diagram showing exemplary echo representations of three different states of a level gauging facility. DETAILED DESCRIPTION

[0025] In the detailed description below, example embodiments of the present invention are described primarily with reference to a level metering facility, wherein the aqueous substance is contained in a tank, and the maximum measurement distance of the level metering facility is set to correspond to the distance between the microwave antenna and the bottom of the tank. It should be noted that this should in no way be considered as limiting the scope of the present invention as defined by the independent claims. For example, the aqueous substance need not be contained in a tank, but may be supported by another natural or artificial structure, including but not limited to a canal, a riverbed, a reservoir, etc. In addition, it should be noted that the preconfigured maximum measurement distance of a particular level metering facility may be related to the distance between the microwave antenna and the structure supporting the aqueous substance, or may be unrelated to the distance between the microwave antenna and the structure supporting the aqueous substance. For example, a user configuring a particular level metering facility may enter a "zero level" that defines the minimum reference level of the level metering facility, which may be at a level higher or lower than the bottom of the structure supporting the aqueous substance.

[0026] Figure 1 An example level gauging installation 1 is schematically shown, comprising a tank 3 having a bottom 5 and a wall 7, and a radar level gauge system 9 according to an embodiment of the invention. The radar level gauge system 9 comprises a microwave antenna 11 and a measuring unit 13, which will be described in more detail further below. The radar level gauge system 9 is arranged such that the microwave antenna 11 is vertically spaced from the tank bottom 5 by a preconfigured maximum measuring distance D of the level gauging installation. max .

[0027] exist Figure 1 In the exemplary illustration of , the level metering facility 1 is a pumping station involved in conveying wastewater to a wastewater treatment plant, so that the aqueous substance 15 metered by the level meter system 9 is wastewater. Therefore, the level metering facility 1 comprises a wastewater inlet 17 and a pump 19 for pumping wastewater to a higher level through an outlet pipe 21.

[0028] Figure 1 The radar level gauge system 9 in the tank operates by radiating an electromagnetic transmission signal S toward the surface 23 of the aqueous substance 15 in the tank. T , receiving the reflected signal S generated by the reflection at the surface 23 R , and based on the reflected signal S R With the transmission signal S T The time relationship between the microwave antenna 11 of the radar level gauge system 9 and the surface 23 is determined. Of course, if necessary, the liquid level of the substance 15 in the tank 3 can be determined based on the distance D between the microwave antenna 11 and the preconfigured zero level of the liquid level measurement facility 1. max The preconfigured zero level of the level measuring device 1 is derived from the determined distance and is indicated here as the bottom 5 of the tank 3 .

[0029] In such Figure 1 In the application of the level measurement facility 1 in the embodiment of the present invention, the determined distance can be used, for example, to monitor the status of the pumping station and to control the operation of the pump 19. In order to protect the pump 19, for example, if it is determined that the tank 3 is empty, the pump 19 can be controlled to stop. For example, in connection with an overflow event, the tank 3 may be overfilled, so that the microwave antenna 11 is immersed in the aqueous substance 15. In the overfilled situation, it would be highly undesirable to control the pump 19 to stop and / or report to the supervision system that the tank 3 is empty. The embodiment of the radar level gauge system 9 according to the present invention realizes an improved ability to distinguish between an overfilled situation and a situation with an empty tank 3.

[0030] Figure 2 is a schematic diagram of a radar level gauge system 9 according to an example of the present invention. Figure 2 , and in addition to the above reference Figure 1Apart from what is briefly mentioned, the radar level gauge system 9 includes a transceiver 25 , a processing circuit system 27 , a communication interface 29 , and optionally a non-volatile memory 31 for storing configuration parameters of the radar level gauge system 9 .

[0031] The microwave antenna 11 , which may be, for example, a dielectric lens antenna, is configured to radiate an electromagnetic transmission signal S toward the surface 23 of the aqueous material 15 . T , and is configured to receive a signal transmitted by S T The reflection signal S generated by the reflection R The transceiver 25 is coupled to the microwave antenna 11 and is configured to generate a transmission signal S T , the signal S will be transmitted T Provided to the microwave antenna 11 and receive the reflected signal S R The processing circuit system 27 is configured to calculate the reflected signal S R And the transmission signal S T With the reflected signal S R The processing circuit system 27 is configured to use the echo representation to determine the distance to the surface 23 of the aqueous material 15, and is configured to externally provide an indication of the distance from the radar level gauge system 9, as schematically indicated by the communication interface 29.

[0032] When the processing circuit system 27 determines that the surface 23 of the aqueous material 15 cannot be identified based on the echo representation, the processing circuit system 27 can be configured to evaluate the reflected signal S at the maximum range of the radar level gauge system 9. R When the received energy is below a predetermined threshold energy, the processing circuit system 27 may be configured to determine that the microwave antenna 11 is immersed in the aqueous substance 15. An indication that the microwave antenna 11 is immersed may be provided external to the radar level gauge system 9 via the communication interface 29. The received energy may be evaluated for a portion of the maximum range and / or for the entire maximum range.

[0033] Figure 3 is a flow chart illustrating a method according to an example embodiment of the invention, which will be further described with reference to other figures as indicated or implied below.

[0034] Electromagnetic emission signal S T The transmission signal S is generated 100 by the transceiver 25 of the radar level gauge system 9. T The microwave antenna 11 of the radar level gauge system 9 is provided 101 by the transceiver 25. The transmission signal S T The microwave antenna 11 radiates 102 toward the surface 23 of the aqueous substance 15 in the tank 3. The reflected signal S RThe reflected signal S is received by the transceiver 25 via the microwave antenna 11 103. R and the transmitted signal S T The timing relationship between the reflected signals is then formed 104 by the processing circuitry 27. The echo representation is indicative of the received energy as a function of the distance travelled by the reflected signal and may be formed using techniques known to those skilled in the art of radar level gauging.

[0035] When a representation of the echo, sometimes referred to as an echo curve, has been formed, it is evaluated by the processing circuitry 27 of the radar level gauge system 9. Reference will now be made additionally to FIG. 4A to FIG. 4C The evaluation of three different echo representations resulting from different situations in a level gauging installation 1 is described.

[0036] Figure 4A A first echo representation 33 is shown which shows the amplitude A of the microwave energy received by the transceiver 25 as a function of the distance d from the microwave antenna 11. The preconfigured maximum measurement distance is represented by D in the echo representation 33. max As mentioned further above, the maximum distance D max Typically, the radar level gauge system 9 may be provided in conjunction with its initial configuration in the level gauging facility 1. As further mentioned above, the maximum range of the radar level gauge system 9 is determined by Figure 4A The maximum range (Maxrange) indicates the maximum distance D of the level measurement facility 1. max Longer. When evaluating Figure 4A The echo in represents 33 hours, such as Figure 4A As schematically indicated in FIG. 1 , the processing circuit system 27 may set the maximum distance D max The echo representation 33 within is compared to a predetermined first threshold amplitude TH1 and a conclusion is drawn that a surface echo 35 has been identified. The processing circuit system 27 can use the surface echo 35 to determine the distance D to the surface 23 of the aqueous material 15. S , and provide an indication thereof externally.

[0037] Figure 4B A second echo is shown 37. When evaluating Figure 4B The echo in 37 is as above for Figure 4A For the purpose of identifying and locating the surface 23 as indicated in the echo representation 33, the processing circuit system 27 may determine 105 the preconfigured maximum measurement distance D from the microwave antenna 11. max The surface 23 of the aqueous substance 15 cannot be identified within the range. This determination can be performed by: setting the maximum distance D max The echo representation 37 within the range is compared with the first threshold amplitude TH1, and it is found that the amplitude of the echo representation 37 is within the maximum distance D maxIt is concluded that any position within is below the first threshold amplitude TH1.

[0038] When the maximum distance D of the level measuring facility 1 has been determined max When the surface 23 of the aqueous material 15 cannot be identified within the maximum distance, the processing circuit system 27 continues to evaluate the echo representation 37 at a distance greater than the maximum distance. For example, the processing circuit system 27 may compare the echo representation 37 with a predetermined second echo detection threshold TH2. Figure 4B In the schematically illustrated measurement situation, an echo peak 39 having an amplitude above the second echo detection threshold TH2 is identified by the processing circuit system 27, indicating that the distance traveled by the reflected signal is greater than the maximum distance D of the level gauging device 1. max .

[0039] From this finding, the processing circuitry 27 may conclude that microwave energy can travel long distances within the tank 3 and therefore the tank 3 is likely empty. The processing circuitry 27 may proceed to provide an indication of this finding external to the radar level gauge system 9 via the communications interface 29 .

[0040] Figure 4C A third echo representation 41 is shown. Figure 4B The processing circuit system 27 can determine 105 the echo representation 37 at a preconfigured maximum measurement distance D from the microwave antenna 11. max The surface 23 of the aqueous substance 15 cannot be identified.

[0041] When the maximum distance D of the level measuring facility 1 has been determined max When the surface 23 of the aqueous material 15 cannot be identified within the maximum distance, the processing circuit system 27 continues to evaluate the echo representation 41 at a distance greater than the maximum distance. For example, the processing circuit system 27 may compare the echo representation 37 with a predetermined second echo detection threshold TH2. Figure 4C In the schematically shown measurement situation, the echo representation 41 has a maximum measured distance D from the microwave antenna 11. max For example, based on this finding, the processing circuit system 27 determines 106 that for all distances greater than the maximum measured distance D from the microwave antenna 11, the second echo detection threshold amplitude TH2 is not reached. max The received energy at the longer distance is below the predetermined threshold energy, and it is determined 107 that the microwave antenna 11 is most likely immersed in the aqueous substance.

[0042] Those skilled in the art realize that the present invention is by no means limited to the preferred embodiments described above. Instead, many modifications and variations are possible within the scope of the appended claims.

Claims

1. A radar level gauge system, the radar level gauge system is used to measure the distance to the surface of a water-based substance in a liquid level measurement facility, the radar level gauge system comprising: a microwave antenna configured to: radiate an electromagnetic transmission signal toward a surface of the aqueous substance, and receive a reflection signal generated by reflection of the transmission signal; A transceiver coupled to the microwave antenna, the transceiver being configured to: generating the transmission signal; providing the transmission signal to the microwave antenna; and receiving the reflected signal; and processing circuitry configured to: forming an echo representation indicative of received energy as a function of distance based on the reflected signal and a timing relationship between the transmitted signal and the reflected signal; determining, based on the echo representation, that a surface of the aqueous material cannot be identified within a preconfigured maximum measurement distance of the level gauging facility; determining based on the echo representation that a received energy within a maximum range of the radar level gauge system is below a predetermined threshold energy; and Make sure that the microwave antenna is immersed in the aqueous substance. 2 . The radar level gauge system of claim 1 , the processing circuitry being configured to provide an indication that the microwave antenna is submerged. 3 . The radar level gauge system according to claim 1 , wherein the processing circuit system is configured to receive a configuration parameter indicating a maximum measurement distance of the level gauging facility. 4 . 4 . The radar level gauge system of claim 3 , the processing circuit system being configured to store the configuration parameters in a non-volatile memory included in the radar level gauge system. 5 .

5. The radar level gauge system according to claim 1 or 2, wherein the processing circuit system is configured to determine that the surface of the aqueous substance cannot be identified within the maximum measurement distance by determining that the echo indicates that a predetermined surface echo detection standard is not met within the maximum measurement distance. 6 . The radar level gauge system of claim 5 , wherein the surface echo detection criterion comprises detecting a peak in the echo representation within the maximum measurement distance, the peak having an amplitude higher than a predetermined first echo detection threshold amplitude.

7. According to the radar level gauge system of claim 6, the processing circuit system is configured to determine that the received energy within the maximum range of the radar level gauge system is lower than the predetermined threshold energy based on the failure to detect the following peak in the echo representation within the maximum range of the radar level gauge system, the amplitude of which peak is higher than a predetermined second echo detection threshold amplitude that is lower than the first echo detection threshold amplitude.

8. The radar level gauge system according to claim 1 or 2, The transceiver is configured to: generating a transmit signal in the form of a measurement scan; and forming an intermediate frequency signal indicating a frequency difference between the reflected signal and the transmitted signal as a function of time; The radar level gauge system includes a sampler coupled to the transceiver and configured to sample the intermediate frequency signal with a predetermined number of samples during a measurement scan; and The processing circuitry is coupled to the sampler and is configured to form the echo representation based on samples.

9. A liquid level measurement facility, comprising the radar level gauge system according to claim 1 or 2, the radar level gauge system being arranged such that a microwave antenna of the radar level gauge system is vertically spaced apart from a surface of an aqueous substance.

10. A method of measuring the distance to the surface of an aqueous substance using a radar level gauge system, the method comprising: The transceiver of the radar level gauge system generates an electromagnetic transmission signal; providing the transmission signal to a microwave antenna of the radar level gauge system; radiating the transmission signal from the microwave antenna substantially vertically toward the surface of the aqueous material; receiving, by the transceiver via the microwave antenna, a reflected signal generated by reflection of the transmitted signal; forming, by processing circuitry of the radar level gauge system coupled to the transceiver, an echo representation indicative of received energy as a function of distance based on the reflected signal and a timing relationship between the transmitted signal and the reflected signal; determining, by the processing circuitry based on the echo representation, that a surface of the aqueous material cannot be identified within a preconfigured maximum measurement distance from the microwave antenna; determining, by the processing circuitry based on the echo representation, that received energy at a distance longer than a maximum measured distance from the microwave antenna is below a predetermined threshold energy; as well as A determination is made by the processing circuitry that the microwave antenna is immersed in the aqueous substance.

11. The method according to claim 10, comprising: An indication is provided that the microwave antenna is submerged.

12. The method according to claim 10 or 11, comprising: A determination is made by the processing circuitry based on the echo representation that a received energy within a maximum range of the radar level gauge system is below a predetermined threshold energy.

13. The method according to claim 10 or 11, comprising: A configuration parameter indicative of a maximum measured distance from the microwave antenna is received by the processing circuitry.

14. The method according to claim 10 or 11, wherein: Determining that the surface of the aqueous substance cannot be identified within the maximum measurement distance from the microwave antenna includes: comparing echo representations of all distances within a maximum measured distance from the microwave antenna to a predetermined first echo detection threshold magnitude; and It is determined that echoes at all distances within a maximum measured distance from the microwave antenna indicate a failure to reach the first echo detection threshold amplitude.

15. The method according to claim 14, wherein: Determining that the received energy at a distance longer than a maximum measured distance from the microwave antenna is below a predetermined threshold energy includes: comparing all echo representations of the echo representation having distances longer than a maximum measured distance from the microwave antenna to a predetermined second echo detection threshold amplitude lower than the first echo detection threshold amplitude; and It is determined that all echo representations of the echo representations having distances longer than the maximum measured distance fail to reach the second echo detection threshold magnitude.

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