Occultation detection load current monitoring system and monitoring method

Abstract

The invention provides an occultation detection load current monitoring system and method, and the system comprises a main and standby radio frequency transmission module, a data conversion module and an FPGA data preprocessing module which are respectively connected with corresponding power management modules through a current monitoring module, and are independently powered by each power management module; the current monitoring modules send monitored current data to the central control processing module, and the central control processing module stores the current data collected by each current monitoring module in real time and responds to the situation that the current data sampled by each channel exceeds a preset current value. And recording and displaying the abnormal data stored in each channel in real time and the position of the channel generating the abnormal data so as to control the power supply of each channel module. According to the invention, accurate current monitoring can be realized, and the reliability is high; meanwhile, the use space on the PCB is greatly reduced, the miniaturization design of the radio frequency module and the whole satellite load is facilitated, and the cost is obviously reduced.

Description

Technical Field

[0001] The present application belongs to the technical field of occultation detection, and in particular, relates to an occultation detection load current monitoring system and monitoring method. Background Art

[0002] Occultation detection technology is a high-precision and high-reliability real-time detection method for atmosphere and ionosphere information. In recent years, with the rapid development of commercial aerospace, this technology has played an increasingly important role in space environment monitoring, improving the real-time performance of weather forecasts, and enhancing the positioning accuracy of the Beidou navigation system. The basic principle of this detection technology is to use multiple satellite detection payloads distributed in low orbits to receive the navigation signals of the Global Navigation and Positioning System (GNSS) for processing and calculation, so as to obtain the required detection data; when the GNSS signal passes through the atmosphere and ionosphere, the important parameters of the GNSS signal itself, such as frequency, phase, amplitude, etc., will undergo relevant changes. These changes will carry the temperature and humidity information of the atmosphere under the path of the signal, as well as the electron density information of the ionosphere; the detection payload will actively receive the GNSS signal carrying key detection information, and invert it through the internal correlation algorithm of the payload to obtain important detection information such as real-time atmospheric temperature, humidity, air pressure and ionosphere electron density in the specified area.

[0003] For the occultation detection payload, its design mainly follows the principle of modular design, and each functional module completes the specified function according to the design positioning. Generally speaking, since each functional module is independently designed, tested, and works independently in the detection payload, it is required to monitor the overall working current of each module in real time to ensure the normal working state of each module and the overall payload. When the payload cannot work normally due to internal abnormalities or external interference, by monitoring the working current, it is possible to confirm in real time which specific functional module inside the payload has a problem.

[0004] Traditional current monitoring methods generally use current monitoring chips for current monitoring. This monitoring method often needs to be designed and used with necessary peripheral circuits, and the packaging of the monitoring chip is often SOIC packaging, which takes up more PCB area and does not have the advantage of miniaturization. When an abnormality occurs in the monitoring circuit, it is impossible to check the inside of the chip, and the chip can only be replaced. In addition, the current monitoring chips used with satellite payloads are often aerospace-grade chips, which do not have any cost advantages. Summary of the invention

[0005] In view of this, the present application aims to propose a load current monitoring system and monitoring method for occultation detection to solve at least one of the above problems.

[0006] To achieve the above purpose, the technical solution of this application is implemented as follows: In a first aspect, the present application provides a system for monitoring a load current of an occultation detection, including a main and standby radio frequency transmission module, a central control processing module, a data conversion module and an FPGA data preprocessing module; The main and standby radio frequency transmission modules, data conversion modules and FPGA data preprocessing modules are respectively connected to corresponding power management modules through current monitoring modules, and are independently powered by each power management module; The current monitoring module sends the monitored current data to the central control processing module, and the central control processing module stores the current data collected by each current monitoring module in real time. In response to the current data sampled by each channel exceeding the threshold range, the central control processing module records and displays the abnormal data stored in real time for each channel and the channel location where the abnormal data is generated, so as to control the power supply of each channel module.

[0007] Further, the central control processing module includes a central control processing unit, and a plurality of ADC sampling units, a plurality of registers and a special register connected to the central control processing unit; The plurality of ADC sampling units respectively collect current data from the RF transmission module, the data conversion module and the FPGA data preprocessing module, and store the current data in the corresponding registers after the sampling process is completed; The special register is configured to record abnormal current data stored in the register and the register position information where the abnormality occurs.

[0008] Further, the current monitoring module is composed of a monitoring resistor, an operational amplifier and a first switch tube, wherein one end of the monitoring resistor is connected to the power management module, and the other end is connected to the main and standby RF transmission module, or the data conversion module, or the FPGA data preprocessing module at the back end, the fourth pin of the operational amplifier is connected to one end of the monitoring resistor close to the power management module through the first resistor, and the third pin of the operational amplifier is connected to the other end of the monitoring resistor through the second resistor; The first end of the first switch tube is connected to the first end of the operational amplifier, the second end of the first switch tube is connected to one of the ADC sampling units through a third resistor, the third end of the first switch tube is connected to the fourth pin of the operational amplifier, one end of the third resistor is also grounded through a fourth resistor, and the other end of the third resistor is also grounded through the second switch tube.

[0009] Furthermore, the first switch tube includes a MOS tube, which is a PMOS tube. The gate of the MOS tube is connected to the first pin of the operational amplifier, the source of the MOS tube is respectively connected to the third pin of the operational amplifier and one end of the first resistor, and the drain of the MOS tube is connected to the third resistor.

[0010] Furthermore, the second switch tube is a voltage regulator diode, the anode of the voltage regulator diode is grounded, and the cathode of the voltage regulator diode is connected to the line between the third resistor and the ADC sampling unit.

[0011] Furthermore, the model of the monitoring resistor is WSL2010R0500FEA, and its resistance is 0.05Ω.

[0012] Furthermore, the operational amplifier is a LM324 chip from TI, supporting a power supply voltage of +3V to +30V.

[0013] Furthermore, the active and standby RF transmission modules are connected to a receiving antenna via a preamplifier for receiving external occultation detection signals, and the preamplifier is powered by a RF transmission cable through the active and standby RF transmission modules at the rear end.

[0014] In a second aspect, based on the same inventive concept, the present application further provides a method for monitoring a load current of an occultation detection, which is used in the occultation detection load current monitoring system as described in the first aspect, comprising: The current monitoring module collects current data through a preset sampling strategy and makes a judgment based on the collected current data; In response to the collected current data being normal data, the current data is written into a register and displayed in real time by a host computer; In response to the collected current data exceeding the threshold range, it is determined to be abnormal data, the abnormal data is captured, the channel generating the abnormal data is marked, the abnormal data and the channel information are packaged and written into a special register, and all multi-channel power supplies are cut off.

[0015] Furthermore, the sampling strategy includes continuously collecting multiple current data at equal time intervals per second, accumulating the current data and taking an average value, and making an abnormality determination based on the average value.

[0016] Compared with the prior art, the occultation detection load current monitoring system and monitoring method described in this application have the following beneficial effects: The occultation detection payload current monitoring system and monitoring method described in the present application can realize accurate current monitoring and have high reliability, avoiding the high cost and peripheral circuit design problems caused by the use of monitoring chips; and can greatly reduce the usage space of PCB circuit boards, facilitating the miniaturization design of RF modules and the overall satellite payload. At the same time, it has obvious advantages in cost, reliability and PCB circuit board usage area. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The drawings constituting a part of the present application are used to provide a further understanding of the present application. The illustrative embodiments and descriptions of the present application are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings: Figure 1 This is an overall block diagram of a load current monitoring system for occultation detection described in an embodiment of the present application; Figure 2 This is a structural block diagram of the central control processing module described in this application example; Figure 3 This is a circuit diagram of the current monitoring module described in the example of this application; Figure 4 This is a flow chart of a method for monitoring load current in occultation detection described in an example of this application. DETAILED DESCRIPTION

[0018] In order to make the objectives, technical solutions and advantages of the present application more clearly understood, the present application is further described in detail below in combination with specific embodiments and with reference to the accompanying drawings.

[0019] It should be noted that, unless otherwise defined, the technical terms or scientific terms used in the embodiments of the present application should have the usual meanings understood by people with ordinary skills in the field to which the present application belongs. The words "first", "second" and similar words used in the embodiments of the present application do not indicate any order, quantity or importance, but are only used to distinguish different components. "Include" or "comprises" and other similar words mean that the elements or objects appearing before the word include the elements or objects listed after the word and their equivalents, without excluding other elements or objects. "Connect" or "connected" and other similar words are not limited to physical or mechanical connections, but can be Including electrical connections, whether direct or indirect. "Up", "down", "left", "right", etc. are only used to indicate relative position relationships. When the absolute position of the object being described changes, the relative position relationship may also change accordingly.

[0020] This embodiment provides a load current monitoring system for occultation detection, including a main and standby radio frequency transmission module, a central control processing module, a data conversion module and an FPGA data preprocessing module; The main and standby RF transmission modules, data conversion modules and FPGA data preprocessing modules are respectively connected to the corresponding power management modules through the current monitoring modules, and are independently powered by each power management module; The current monitoring module sends the monitored current data to the central control processing module, which stores the current data collected by each current monitoring module in real time. In response to the current data sampled by each channel exceeding the threshold range, the central control processing module records and displays the abnormal data stored in real time by each channel and the channel location that generates the abnormal data, so as to control the power supply of each channel module.

[0021] In view of the special application scenarios of satellite payloads and the requirements for reliability and accuracy of current monitoring of each module of the payload, this application designs a current monitoring unit module to achieve accurate monitoring of the current of each module of the payload; after obtaining the current monitoring data, the control logic is designed to send the data to the ADC sampling unit of the central control processing module, and after internal processing, it is displayed in real time by the host computer, which is used to monitor the overall working status of each module of the payload in real time to ensure the stable operation of the overall occultation detection payload.

[0022] When the collected monitoring current data is abnormal, the central control processing module will first store the abnormal value in a special register according to the pre-set control logic, and then cut off the power supply of each module to ensure the safety of the detection payload, ensure the normal operation of the overall detection payload, and achieve high reliability; compared with the traditional method of using current monitoring chips for current monitoring, the present application designs circuit connection relationships and control logic, and only uses a single military-grade operational amplifier, 5 military-grade resistors, and 1 military-grade voltage regulator diode to achieve accurate current monitoring with high reliability; avoids the high cost and peripheral circuit design problems brought about by the use of monitoring chips, and can greatly reduce the usage space on the PCB, which is convenient for the miniaturization design of the RF module and the overall satellite payload, and has obvious advantages in cost, reliability and PCB usage area. The system described in the present application has been actually applied in a certain occultation detection project.

[0023] The system described in this application can realize accurate monitoring of the working current of each functional module of the occultation payload, and the complete technical solution adopted is as follows: as follows Figure 1 As shown, this is the overall structural block diagram of the present application. The specific working principle is as follows: the main functional modules of the occultation detection payload can be divided into a radio frequency receiving and transmission module, an FPGA data preprocessing module, a data conversion module and a central control processing unit; each functional module adopts an independent power supply method to ensure that when an abnormality occurs, the abnormal position will not affect other modules due to the independent power supply of each functional module.

[0024] Among them, the RF receiving and transmission module mainly includes a receiving antenna, a preamplifier and a main and standby dual RF transmission module. The function of the receiving antenna is to receive external occultation detection data and transmit it to the back-end preamplifier. The preamplifier performs pre-selection filtering and first-stage amplification processing on the occultation detection signal collected by the antenna. After the processing is completed, the data is transmitted to the back-end RF transmission module for further processing. The power supply of the preamplifier adopts an RF transmission cable to feed the RF transmission module at the back end, which simplifies the power supply design of the preamplifier and the use area of ​​the PCB board.

[0025] After receiving the occultation detection data, the radio frequency transmission module performs secondary filtering and secondary amplification processing on the detection data, and performs frequency reduction processing and analog-to-digital conversion processing on the detection data. After the processing is completed, the detection data is converted into digital intermediate frequency data, and then the digital intermediate frequency data is transmitted to the FPGA data preprocessing module for preprocessing (including outsourcing protocol parsing of the received original occultation detection data, binary data decoding, data splitting, and removing invalid signals and correcting various errors); after the preprocessing is completed, the detection data is transmitted to the back-end central control processing unit through the data conversion module for deep processing (including calculating the received preprocessed data step by step through the mature occultation data inversion algorithm in the field and obtaining the basic occultation inversion product, and the steps mainly include Doppler frequency shift calculation, bending angle calculation, curvature center calculation, and Abel inversion refractive index calculation and other main processes). After the solution is completed, the data is packaged and sent to the host computer for data display. It should be noted that the step-by-step solution process described in this embodiment is a conventional technical means well known to those skilled in the art, and the content of this step is not the invention point of this application, so it will not be further described here.

[0026] After the primary power supply is supplied, it first passes through the independent power management module of each functional module, then passes through the current monitoring module, and finally is given to each functional module at the back end. After normal operation, the current monitoring module starts to collect the stable working current and send it to the central control processing module.

[0027] Among them, the main backup RF transmission module is powered by the power management module 1, and is monitored by the current monitoring module 1. The current data collected is the monitoring data 1. After the collection is completed, it is sent to the ADC sampling unit 1 of the central control processing module.

[0028] The data conversion module is powered by the power management module 2, and is monitored by the current monitoring module 2. The collected current data is the monitoring data 2. After the collection is completed, it is sent to the ADC sampling unit 2 of the central control processing module.

[0029] The FPGA data preprocessing module is powered by the power management module three and is monitored by the current monitoring module three. The collected current data is the monitoring data three. After the collection is completed, it is sent to the ADC sampling unit three of the central control processing module.

[0030] After receiving the sampled data, the central control processing module performs internal processing and sends it to the host computer for real-time display.

[0031] like Figure 2 As shown, the current value of the RF module is sampled by the ADC sampling unit 1. After the sampling process is completed, it is stored in the register 1 for real-time storage. The sampled data will be updated in real time. After the update, the current data of the original register will be overwritten.

[0032] The current value of the data conversion module is sampled by ADC sampling unit 2. After the sampling process is completed, it is stored in register 2 for real-time storage. The sampled data will be updated in real time. After the update, the current data of the original register will be overwritten.

[0033] The current value of the FPGA data preprocessing module is sampled by ADC sampling unit three. After the sampling process is completed, it is stored in register three for real-time storage. The sampled data will be updated in real time. After the update, the current data of the original register will be overwritten.

[0034] When an abnormality occurs, that is, the sampled current data exceeds the threshold range (i.e., the normal current value), the central control processing unit will send the abnormal data stored in the register in real time to the special register, record the abnormal current sampling data, and send the register location information that caused the abnormality to the special register for subsequent troubleshooting. After that, the original register will continue to work and refresh regularly.

[0035] like Figure 3 As shown in the figure, the hardware connection design of the current monitoring module of this application is described in detail by taking the main backup radio frequency transmission module as an example: VCC_RF is the power supply voltage output from the power management module to the main backup RF transmission module. After passing through the monitoring resistor Rsense, it is given to the back-end main backup RF transmission module. The current monitoring module is based on the operational amplifier U1. The operational amplifier model is TI's LM324 chip, which supports a power supply voltage of +3V~+30V; the two input pins of the operational amplifier U1 are connected to the two ends of the monitoring resistor Rsense through two 0.1K resistors; using the virtual short-circuit characteristics of the operational amplifier, it can be seen that the third pin of the operational amplifier U1 and the fourth pin of the operational amplifier U1 have the same potential. Using the virtual open circuit characteristics of the operational amplifier, the current flowing through the two ends of the resistor R2 is zero. Therefore, the potentials at both ends of the resistor R2 are equal, so it can be concluded that the voltage difference across the monitoring resistor Rsense is equal to the voltage difference across the resistor R1, that is, UR1 =U Rsense ; In normal operation, the negative input pin of the operational amplifier is connected to the MOS tube for negative feedback, so that it works in the linear amplification area; the MOS tube model is IRF5210S. In normal operation, the PMOS tube works in the constant current area. The PMOS tube is turned on, the drain D and the source S have the same potential and the same current. At this time, the voltage of the PMOS tube drain D pin is Vout=I*Rout, I is the current flowing through the PMOS tube, and the current of the PMOS tube is the same as the current flowing through the resistor R1, so Vout=(U R1 / R1)*Rout; And U R1 =U Rsense , then U Rsense =(Vout / Rout)*R1; get U Rsense After that, if the resistance value of Rsense is known, Isense can be obtained, that is, Isense=[(Vout / Rout)*R1] / Rsense; For example, when the monitoring voltage is 2.5V and the monitoring resistance Rsense is 0.05Ω, the actual current used by the RF board can be obtained as 0.5A through the above current calculation formula.

[0036] The current monitoring resistor model of this application is WSL2010R0500FEA, the package is 2010, the rated current capacity is 2A, which meets the first-level derating requirements. The reason for choosing 0.05Ω is that by choosing a small-resistance monitoring resistor, the extra power consumption of the monitoring resistor can be reduced when the RF working current is large, and the power consumption and heat generation of the overall load can be reduced. According to the above example, when the working current of the RF board is 0.5A, the actual power consumption of the monitoring resistor is only 0.0125W, which will not affect the power usage of the load unit, nor will it affect the thermal characteristics of the load unit.

[0037] In addition, when the central control processing unit performs AD conversion on the monitoring data, the monitoring signal passes through the external input resistor RL and the internal switch resistor (R ADC +RL), the internal sample and hold capacitor C ADC Under normal circumstances, sampling can only ensure accurate sampling after the voltage on the sampling capacitor is charged to the same level as the external signal to be measured. ADC The effective charging is controlled by the sum of the external resistance and the internal switch resistance. Because the sampling and holding capacitor is fixed, the external impedance configuration will be related to the charging time of the sampling and holding capacitor. The sampling time needs to be greater than the charging time to ensure that the sampling error is as small as possible.

[0038] In this embodiment, the external input resistance RL is set to 4.99K, and the actual test meets the normal sampling requirements. After the ADC_OUT data is output, a voltage stabilizing diode is added, the model is 1N5819, and the voltage stabilizing value is 3.3V. The purpose of setting this diode is to prevent the central control processing unit from being damaged when a large voltage is output. When the output voltage exceeds 3.3V, the voltage stabilizing diode starts working to ensure that the large voltage will not be transmitted to the ADC pin at the back end, thereby ensuring the safe operation of the central control processing unit. The current monitoring module of this application is compared with the current monitoring chip data through software simulation, and the monitoring data is consistent, achieving the purpose of accurately monitoring the load current.

[0039] Based on the same inventive concept, corresponding to any of the above-mentioned embodiment systems, an embodiment of the present application further provides a method for monitoring load current of occultation detection, specifically comprising: The current monitoring module collects current data through a preset sampling strategy and makes judgments based on the collected current data; In response to the collected current data being normal data, the current data is written into a register and displayed in real time through a host computer; In response to the collected current data exceeding the threshold range, it is determined to be abnormal data, the abnormal data is captured, the channel generating the abnormal data is marked, the abnormal data and the channel information are packaged and written into a special register, and all multi-channel power supplies are cut off.

[0040] Specifically, in this embodiment, Figure 4 As shown in the figure, after the load is working stably, the current monitoring module starts to collect data. The collection strategy is to collect data continuously for 5 times at equal time intervals per second. After the collection is completed, the data is accumulated and averaged to ensure the stability and reliability of the current data. The collected current data is then sent to the central control processing unit. After receiving the data, the central control processing unit will judge the sampled data. When the sampled data is normal, the central control processing unit will write the collected data into the normal register, and then display it in real time through the host computer.

[0041] When the current data exceeds the threshold range and is judged to be abnormal by the central control processing unit, the central control processing unit will immediately capture the abnormal data, mark the channel that generates the abnormal data, and then package the abnormal current data and channel information and write them into a special register. The abnormal data and channel information will then be sent to the host computer for real-time display, and all three power supplies will be cut off. After cutting off, the current monitoring data is theoretically 0, and then the data will be sent to the host computer for real-time display.

[0042] It should be noted here that due to the different actual working conditions of the three monitoring channels, the collected current data are different, so the current threshold ranges set for the three channels are also different. In addition, the threshold range set in this embodiment can be flexibly adjusted according to actual conditions, and no specific limitation is made here.

[0043] This control logic realizes the precise monitoring of the current data of each working module of the occultation detection payload, and provides preventive and positioning measures when anomalies occur. It greatly facilitates the tracing of subsequent problems and accelerates the resolution of abnormal problems. It has high innovation and application value.

[0044] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein by equivalents. These modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention, and they should all be included in the scope of the claims and specification of the present invention.

[0045] The embodiments of the present application are intended to cover all such substitutions, modifications and variations that fall within the broad scope of the appended claims. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the embodiments of the present application should be included in the scope of protection of the present application.

Claims

1. A load current monitoring system for occultation detection, characterized in that: It includes main and standby radio frequency transmission modules, central control processing module, data conversion module and FPGA data preprocessing module; The main and standby radio frequency transmission modules, data conversion modules and FPGA data preprocessing modules are respectively connected to corresponding power management modules through current monitoring modules, and are independently powered by each power management module; The current monitoring module sends the monitored current data to the central control processing module, and the central control processing module stores the current data collected by each current monitoring module in real time. In response to the current data sampled by each channel exceeding the threshold range, the central control processing module records and displays the abnormal data stored in real time for each channel and the channel location where the abnormal data is generated, so as to control the power supply of each channel module.

2. The system according to claim 1, characterized in that: The central control processing module includes a central control processing unit, and a plurality of ADC sampling units, a plurality of registers and a special register connected to the central control processing unit; The plurality of ADC sampling units respectively collect current data from the RF transmission module, the data conversion module and the FPGA data preprocessing module, and store the current data in the corresponding registers after the sampling process is completed; The special register is configured to record abnormal current data stored in the register and the register position information where the abnormality occurs.

3. The system according to claim 2, characterized in that: The current monitoring module is composed of a monitoring resistor, an operational amplifier and a first switch tube, wherein one end of the monitoring resistor is connected to the power management module, and the other end is connected to the main and standby RF transmission module, or the data conversion module, or the FPGA data preprocessing module at the back end, the fourth pin of the operational amplifier is connected to one end of the monitoring resistor close to the power management module through the first resistor, and the third pin of the operational amplifier is connected to the other end of the monitoring resistor through the second resistor; The first end of the first switch tube is connected to the first end of the operational amplifier, the second end of the first switch tube is connected to one of the ADC sampling units through a third resistor, the third end of the first switch tube is connected to the fourth pin of the operational amplifier, one end of the third resistor is also grounded through a fourth resistor, and the other end of the third resistor is also grounded through the second switch tube.

4. The system according to claim 3, characterized in that: The first switch tube includes a MOS tube, which is a PMOS tube. The gate of the MOS tube is connected to the first pin of the operational amplifier, the source of the MOS tube is respectively connected to the third pin of the operational amplifier and one end of the first resistor, and the drain of the MOS tube is connected to the third resistor.

5. The system according to claim 3, characterized in that: The second switch tube is a voltage regulator diode, the anode of the voltage regulator diode is grounded, and the cathode of the voltage regulator diode is connected to the line between the third resistor and the ADC sampling unit.

6. The system according to claim 3, characterized in that: The model of the monitoring resistor is WSL2010R0500FEA, and its resistance is 0.05Ω.

7. The system according to claim 3, characterized in that: The operational amplifier is a LM324 chip from TI, and supports a power supply voltage of +3V to +30V.

8. The system according to claim 1, characterized in that : The main and standby radio frequency transmission modules are connected to the receiving antenna through a preamplifier to receive external occultation detection signals. The preamplifier is powered by a radio frequency transmission cable and fed through the main and standby radio frequency transmission modules at the rear end.

9. A method for monitoring load current of occultation detection, used in the occultation detection load current monitoring system according to any one of claims 1 to 8, characterized in that: include: The current monitoring module collects current data through a preset sampling strategy and makes a judgment based on the collected current data; In response to the collected current data being normal data, the current data is written into a register and displayed in real time by a host computer; In response to the collected current data exceeding the threshold range, it is determined to be abnormal data, the abnormal data is captured, the channel generating the abnormal data is marked, the abnormal data and the channel information are packaged and written into a special register, and all multi-channel power supplies are cut off.

10. The method according to claim 9, characterized in that: The sampling strategy includes continuously collecting multiple current data at equal time intervals per second, accumulating the current data and taking an average value, and making an abnormality determination based on the average value.

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