JP2008145274A - Reliability indicating device in radio navigation system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a reliability indicating device in a radio navigation system, which can change, on the user side, the reliability level of position information broadcast by the radio navigation system to the reliability level required by each user. <P>SOLUTION: The radio navigation system includes a means for broadcasting information concerning an attainable reliability level as augmentation information. A user has a means for setting a reliability level required by the user himself to be changeable, and a means for determining whether the information concerning the attainable reliability level broadcast by the radio navigation system meets with the reliability level required by the user. If the attainable reliability level broadcast by the radio navigation system is higher than the reliability level required by the user, the user recognizes that the radio navigation system can be utilized. The user has a means for displaying the results of determining whether the required reliability level can be satisfied. Based on the determination results, it is displayed for the user whether the radio navigation system can be utilized. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a user receiver provided with a reliability indication device in a radio navigation system, and in particular, the reliability required for each user on the user side for the reliability level of position information broadcast by the radio navigation system. The present invention relates to a reliability indicating device in a radio navigation system that can be changed to a sex level.

  At present, in the conventional radio navigation system, there is no system that broadcasts information on the reliability of position information that can be obtained by the user in real time. Further, in a system using GPS, when an unexpected situation occurs in a GPS satellite, the system may output abnormal position information. Therefore, there are several systems to ensure the integrity required for civil aviation. There is a protection level method that broadcasts information on the reliability of information. This achieves integrity by giving a 99.99999% confidence limit (protection level PL) to the positioning error at any user position in the service area.

Protection level level systems include SBAS (Satellite-Based Augmentation System) and GBAS (Ground-Based Augmentation System), both of which are trusted by users. The sex level (integrity risk) is fixed at a fixed value (10 ⁻⁷ ) for the civil aviation industry.

  In this protection level method, since the reliability level on the user side is fixed to a constant value, the information broadcast from the system to the user is information on the reliability limit of the positioning error included in the position information. That is, the positional information actually obtained by the user from the system is information on reliability for determining and determining whether or not the user can use it for his own navigation (information on the standard deviation s of the positioning error, etc.). It is included.

  As shown in FIG. 5, the radio navigation system 60 broadcasts position information 61 and information 62 related to the standard deviation s of the positioning error as information related to reliability. These pieces of information are received by the user receiver 70 via the communication line 63. In the user receiver 70, the reliability level of the user is fixed to a constant value (step 71).

When the maximum allowable positioning error of the user receiver 70 is AL (Alert Limit) and the probability that the actual positioning error exceeds AL is the integrity risk 10 ⁻⁷ , the protection level PL is the radio navigation system 60. Is calculated from the information 62 on the standard deviation s of the positioning error broadcasted from (step 72). The actual positioning error must be expressed by the following equation (1).
Pr (actual positioning error> AL) <10 ⁻⁷ (1)

Since the protection level PL is expressed by the following equation (2), as long as PL <AL (step 73), the equation (1) is satisfied and the user receiver 70 can be used (step 74). ).
Pr (actual positioning error> PL) <10 ⁻⁷ (2)

  If PL> AL (step 75), the user receiver 70 becomes unavailable (step 76). In such a case, an alarm is issued to the user as necessary, and the user It is recognized that the information cannot be used by an alarm sound or the like (step 77).

  Note that, on the SBAS service provider side, it must be ensured that a protection level that satisfies the formula (2) can always be calculated for all users in the service area. That is, since it is unknown what navigation mode or maximum allowable positioning error AL of the user receiver is available for the user, the service provider side satisfies the equation (2) at any point. Is a necessary and sufficient condition for integrity. If Equation (2) does not hold, the integrity of this radio navigation system is broken regardless of the presence or absence of the user and the navigation mode, so that the navigation system cannot be used by the user.

Also, the reinforcement information broadcast by SBAS is designed to include integrity information necessary for the user receiver to calculate the protection level. When the protection level PL obtained by calculation is greater than AL determined by the navigation mode, the user receiver cannot use the navigation mode.
REAJ Magazine 2006 Vol. 28, no. 1 (Vol.149) JP 2000-206224 A

  In order to construct a navigation system that can be used safely, it is indispensable to ensure considerable reliability as position information that can be obtained by the user. If the reliability level required by the user is determined in advance, it is possible to cope with it by setting a procedure for calculating the protection level. However, when various users with different moving speeds and surrounding environments are targeted, the reliability level required varies depending on the user, so the protection level corresponding to the reliability level fixed at a fixed value alone is the highest. There was a problem that it was not possible to respond efficiently, only to meet the requirements.

In addition, the current state of users who require navigation has many users who do not require a very high reliability level. Therefore, when there are many users outside the aviation industry, it is necessary to lower the reliability level or make it variable. For example, when a reliability level of 10 ⁻⁷ is required, there is WAAS (Wide Area Augmentation System) developed by the United States as a measure corresponding to Japanese SBAS. The level is as large as several tens of meters. However, when the reliability level may be 10 ⁻⁴ , the protection level may be 10 m or less. Therefore, there is a need for a method that can calculate the protection level according to the reliability level required by the user.

Even in the systems (SBAS and GBAS) that are currently in practical use, the reliability level is fixed (10 ⁻⁷ ) for the civil aviation industry. In this way, if an unnecessarily high reliability level is set, there is a problem that the operating rate is lowered and the system becomes impractical for the user. Furthermore, if the reliability level is set unnecessarily high, there is a problem that the cost of the entire system increases.

The invention according to claim 1, the radio navigation system comprises means for broadcasting as reinforcing information information on the probability P ₁ can not detect abnormality, the user, the probability maximum AL or more positioning error of the positioning error occurs P Means for setting ₂ , a means for setting the reliability level required by the user to be changeable, a means for calculating a constant K from the set probability P _2, and a standard deviation s broadcasted by the radio navigation system together with position information Means for calculating a protection level PL from information and a constant K, and means for judging whether or not PL <AL is satisfied, and relates to position information broadcasted by the radio navigation system when PL <AL. The reliability limit of the positioning error is obtained.

  According to a second aspect of the present invention, in the first aspect of the invention, the user includes means for displaying a determination result regarding PL <AL, and based on the determination result, whether the radio navigation system is usable or not usable. Whether it is possible is displayed to the user.

  According to a third aspect of the present invention, the radio navigation system includes means for broadcasting information on the achievable reliability level as reinforcement information, and the user sets the reliability level required by the user to be changeable, Means for determining whether the information about the achievable reliability level broadcast by the radio navigation system satisfies the reliability level required by the user, and the achievable reliability being broadcast by the radio navigation system When the level is greater than the reliability level required by the user, the user recognizes that the radio navigation system is available.

  According to a fourth aspect of the present invention, in the third aspect of the present invention, the user includes means for displaying a determination result as to whether or not the requested reliability level is satisfied, and based on the determination result, the radio navigation system is provided. Is displayed to the user as to whether it is available or unavailable.

  Since the invention according to claim 1 is configured as described above, in order to enable the wireless navigation system to cope with users of various moving speeds and surrounding environments, a positioning error according to the reliability level required by the user is provided. The confidence limit of can be calculated. By comparing this calculated confidence limit with the allowable range of positioning error required by the user, each user can determine whether or not the radio navigation system can be used, thereby realizing a safe radio navigation system. can do.

In the case of steady state, for the probability P ₂ of the maximum value AL more excessive positioning error of the positioning error of the user occurs with are determined by the user based on the confidence level R the user needs, constant K required for the calculation of the protection level PL is determined by their P _2, which can each user set in accordance with the reliability level in need, it is possible to suppress the constant K to a minimum . In other words, the protection level PL (= K * s) can be minimized and a high operating rate can be maintained.

  Since the invention according to claim 2 is configured as described above, in addition to the effect of the invention according to claim 1, the user can further indicate whether or not the navigation mode set by the user can be used by display means. Can be judged.

  Since the invention according to claim 3 is as described above, the reliability level of the user, which has been conventionally fixed at a constant value, can be changed according to the user's request.

  Since the invention according to claim 4 is as described above, in addition to the effect of the invention according to claim 3, the user further determines whether or not the navigation mode can be used at the reliability level set by the user. It can be determined by the display means.

  The radio navigation system includes means for broadcasting information on the achievable reliability level as reinforcement information, and the user can set the reliability level required by the user to be changeable and the achievable broadcast by the radio navigation system. Means for determining whether or not the information regarding the reliability level satisfies the reliability level required by the user, and the achievable reliability level broadcasted by the radio navigation system is the reliability required by the user. When greater than the sex level, the user can recognize that the radio navigation system is available.

  Further, the user is provided with means for displaying a determination result as to whether or not the requested reliability level is satisfied, and based on the determination result, the user can determine whether the radio navigation system is usable or not. To display.

  In the conventional protection level method, the matters described in (1) and (2) below are the basic principles. That is, (1) Various types of monitors are provided to detect abnormal situations such as communication line and processor faults, navigation message abnormalities, ionospheric storms, etc. When these abnormal situations occur, Limited. (2) In a steady state that is not an abnormal situation, a normal distribution is assumed for the positioning error.

  Of these, the steady state (2) has no problem since the protection level can be calculated by changing the reliability level (arbitrarily) on the user side. The problem is when the abnormal situation described in (1) occurs. In consideration of this abnormal situation, a reliability level higher than a predetermined reliability level cannot be obtained. In other words, the present situation is that the reinforcement information is generated in accordance with the highest reliability level among the reliability levels required by various users.

  Accordingly, the inventor assumes a protection level method in a radio navigation system, and in order to make a predetermined reliability level variable when the abnormal situation described in (1) occurs, the radio navigation system Pay attention to the fact that if the information on the reliability level is broadcast as the reinforcement information, the reliability level required on the user side can be made variable by combining (1) and (2). Therefore, a basic principle according to the present invention will be described, and then a reliability indicating device capable of displaying whether or not a navigation mode required by the user can be used will be described using this principle.

First, the basic principle of the present invention will be described with reference to FIGS. FIG. 1 is a conceptual diagram of a system according to the present invention, and FIG.
In FIG. 1 to FIG. 2, positioning signals broadcast by the navigation satellite 1 are received by the receiving stations 2, 3, and 4, collected via the communication line 5, and then collected on the abnormality monitor 6. When the positioning signal cannot be received, when trouble occurs in the communication line 5, or when an unexpected abnormal situation occurs, the abnormal monitor 6 detects the abnormal situation.

  In such a case, the navigation satellite 1 or the entire radio navigation system cannot be used. As described above, when an abnormality is detected from the navigation satellite 1 to the abnormality monitor 6, an abnormal situation has occurred in the radio navigation system 40.

Therefore, even though the abnormality monitor 6 some abnormality has occurred in the radio navigation system 40, and P ₁ the probability that can not detect that an abnormal situation. Radionavigation system 40 broadcasts information 43 on the probability P ₁ that can not be detected with this abnormal situation as a reinforcing information.

On the other hand, when an abnormal situation has not occurred, that is, in a steady state, the positioning error has a normal distribution. When the standard deviation of the positioning error broadcast together with the positioning signal from the navigation satellite 1 is estimated as s, the probability that an excessive positioning error exceeding the maximum positioning error AL of the user receiver 8 will be P ₂ . The following formula (3) is established.
Pr (actual positioning error> T | K * s <T) = P ₂ (3)

From equation (3), a constant K that can be said to be “the probability that the actual positioning error exceeds T when K * s <T is satisfied is P ₂ ” is the positioning error of the user receiver 8. it can be determined by the probability P ₂ of the maximum value AL more excessive positioning error.

Therefore, in the protection level mode, in the formula (3), if the maximum value AL of the positioning error allowed the T, i.e., if T = AL, the probability that a positioning error exceeds an allowable range occurs P ₂ becomes , K * s is the protection level PL. Thus, the protection level mode, K * s (= PL) < If AL is satisfied, the real positioning error, probability of exceeding the AL becomes _{P 2} less.

Therefore, in the user (user receiver 8) side, by selecting the probability P ₂ of the maximum value AL more excessive positioning error of the positioning error of the user receiver 8 in accordance with the risk acceptable results, it requires trust A sex level can be secured. When K * s (= PL) <AL is not satisfied, it is determined that the radio navigation system 40 cannot be used.

Here, when the reliability level required by the user (user receiver 8) is R, the following equation (4) needs to be satisfied.
P ₁ + P ₂ <R (4)

From this equation (4), the probability P ₁ is determined not detect an abnormal situation, so that the lower limit Rmin of reliability level R users demand is determined.

In the conventional method, the reliability level R requested by the user is determined to be a constant value in advance, and the radio navigation system has been designed so as to obtain a probability P ₁ that cannot be detected as an abnormal situation that can satisfy the reliability level R.

Therefore, in the present invention, as will be described later, the wireless navigation system 40 broadcasts information 43 regarding the probability P _{1 at} which an abnormal situation cannot be detected to the user, thereby providing information 41 regarding the achievable reliability level Rmin to each user. Is to be able to know. Instead of broadcasting the information 43 regarding the probability P ₁ that the abnormal situation cannot be detected to the user, the wireless navigation system 40 broadcasts the information 43 regarding the reliability level that can be achieved by the wireless navigation system 40 ′ to the user as reinforcement information. This will be described in the second embodiment.

In the first embodiment of the present invention, firstly, the radio navigation system 40, information 43 about the probability P ₁ that can not detect the abnormal state in the case being broadcast will be described with reference to FIGS.

Radionavigation system 40, as reinforcing information, broadcasts the information 43 about the probability P ₁ can not detect abnormality. On the other hand, on the user (user receiver 8) side, when an abnormal situation has not occurred, that is, in a steady state, the probability that an excessive positioning error exceeding the maximum positioning error AL of the user receiver 8 will occur. P ₂ can be set on the user side in accordance with the reliability level R required by the user receiver 8.

Furthermore, the constant K required for the calculation of the protection level PL is a value determined by the probability P _{2 of} occurrence of an excessive positioning error greater than or equal to the maximum positioning error AL of the user receiver 8. Therefore, the probability P ₂ and the constant K are configured to be set and calculated on the user side in accordance with the reliability level R required by the user receiver 8.

  Next, a first embodiment of the present invention will be specifically described with reference to FIGS. FIG. 3 is a configuration diagram of the user receiver 8 provided with the reliability instruction device 50.

As shown in FIGS. 1 to 3, a radio navigation system adopting the protection level method needs to broadcast information 42 relating to the standard deviation s necessary for calculating the protection level PL to the user together with the position information. Therefore, information 43 regarding the probability P ₁ or information 41 regarding the reliability level that cannot be detected as an abnormal situation can be broadcast as reinforcement information. These pieces of information and other information necessary for calculating the protection level PL are all generated by the data generation unit 7 (FIG. 2).

As shown in FIGS. 3 and 1, the user receiver 8 has a configuration in which a navigation information receiver 12 is added to a GPS receiver 9 that receives a normal positioning signal (GPS signal). The navigation information receiving unit 12 is navigation information specially broadcast from the wireless navigation system 40 (information 43 on the probability P _{1 that} cannot be detected as an abnormal situation 43 or information 41 on the achievable reliability level described in the second embodiment) Is for receiving.

The GPS receiver 9 receives a positioning signal for distance measurement broadcast from a GPS satellite by the GPS receiving antenna 10. Reference numeral 11 denotes a position calculation unit that calculates a user position. The navigation information receiving unit 12 receives navigation information (information 43 on the probability P _{1 that} cannot be detected as an abnormal situation) broadcasted as reinforcement information from the radio navigation system 40 by the navigation information receiving antenna 13 that receives the navigation information. Receive. 14 is a constant K calculation unit, 15 is a protection level calculation unit, and 16 is a PL <AL determination unit, which determines whether or not the obtained protection level PL is smaller than the maximum allowable positioning error AL. The

A navigation information display unit 17 displays the user position obtained by the position calculation unit 11 on the display unit 18. The navigation information display unit 17 further uses a mode switch 19 for setting a navigation mode, a first lamp 20 that lights when the navigation mode selected by the mode switch 19 is available, and a navigation mode selected by the mode switch 19. A second lamp 21 that is turned on when impossible is arranged. Reference numeral 22 denotes a P ₂ , R setting unit that sets a probability P _{2 of} occurrence of a positioning error equal to or greater than the maximum navigation positioning error AL and a reliability level R requested by the user when the mode switch 19 is operated to select a navigation mode. The The two antennas 10 and 13 are shared if both information signals can be received by one antenna.

Next, the operation will be described with reference to FIGS.
Since the information 42 on the standard deviation s of the positioning error is broadcast together with the positioning signal from the navigation satellite 1, if T is the maximum allowable positioning error AL from equation (3), as described above. , T = AL. Therefore, when K * s <T (that is, PL <AL) is satisfied, the constant K is determined by the probability P ₂ that an excessive positioning error exceeding the maximum positioning error AL of the user receiver 8 occurs. Can do.

Therefore, in the navigation information display section 17 of the user receiver 8, when the navigation mode is selected by operating the mode switch 19, the reliability level R requested by the user and the allowable positioning error are selected in the P ₂ and R setting section 22. Is set to the maximum value AL (that is, the probability P _{2 of} occurrence of an excessive positioning error) (step 30).

Next, the information 43 regarding the probability P ₁ and the information 42 regarding the standard deviation s that cannot be detected as an abnormal situation broadcast as reinforcement information from the radio navigation system 40 are received, and the set probability P set in the constant K calculation unit 14. _A constant K is calculated from ₂ (step 33).

Furthermore, the information 43 on the probability P ₁ and the information 42 on the standard deviation s that cannot be detected as an abnormal situation broadcast from the radio navigation system 40 are received, and the protection level calculation unit 15 receives the constant K calculated. K * s is calculated by multiplying the standard deviation s derived from the information 42 on the standard deviation s (step 34). This K * s becomes the protection level PL.

  Further, based on the equation (3), the PL <AL determination unit 16 compares the protection level K * s (= PL) with the maximum value AL of the positioning error of the user receiver 8, and K * s <AL ( That is, it is determined whether or not PL <AL) (step 35). If K * s <AL, it is displayed that the navigation mode selected by the mode switch 19 is available (step 32). Specifically, the first lamp 20 is turned on. If K * s <AL is not satisfied, the radio navigation system 10 determines that the radio navigation system 10 cannot be used (step 35), that is, displays that the navigation mode selected by the mode switch 19 is not available (step). 32). Specifically, the second lamp 21 is turned on. Therefore, when PL <AL, the user can grasp the reliability limit of the positioning error regarding the position information broadcast by the radio navigation system.

  Note that the position calculation unit 11 calculates and displays the user position on the display unit 18 of the navigation information display unit 17 based on the positioning signal received by the GPS reception unit 9.

Thus, in this invention, when the steady state, the probability P ₂ of the maximum value AL more excessive positioning error of the positioning error of the user occurs, the user side based on the reliability level R the user needs Has been decided. Further, since the constant K required for the calculation of the protection level PL is determined by P ₂ , the constant K can be minimized by setting it according to the reliability level required by each user. I can do it. In other words, the protection level PL (= K * s) can be minimized and a high operating rate can be maintained. Further, the user can determine from the lighting state of the lamp whether or not the navigation mode set by the user can be used.

In Example 1, had been broadcast information 43 on the probability P ₁ that the radio navigation system 40 can not detect the abnormal state as a reinforcing information, instead of broadcasting the probability P _1, the radio navigation system 40 ', can be achieved Information 41 regarding the reliability level may be broadcast to the user. In the second embodiment of the present invention, a case where the radio navigation system 40 ′ is broadcasted with the information 41 relating to the achievable reliability level as reinforcement information will be described with reference to FIG. 1, FIG. 2, and FIG. In addition, the same thing as Example 1 uses the same name and the same number, and abbreviate | omits the description. FIG. 4 is a configuration diagram of a user receiver 8 ′ provided with a reliability instruction device 50 ′.

  As shown in FIGS. 1 and 4, the radio navigation system 40 ′ is newly achieved in addition to the information 42 related to the standard deviation s necessary for the calculation of the protection level PL broadcasted by the conventional protection level method. Information 41 regarding possible reliability level Rmin is broadcast as reinforcement information.

In addition, as described in the first embodiment, the radio navigation system uses the information 41 related to the reliability level Rmin that can be achieved in addition to the information 43 related to the probability P ₁ that the radio navigation system 40 cannot detect an abnormal situation as reinforcement information. You may broadcast. When both types of information are broadcast as reinforcement information, the reliability limit of positioning error can be calculated according to the reliability level required by the user, so it must be compatible with users moving at various moving speeds and users with different surrounding environments. Can do. Alternatively, instead of the information 43 related to the probability P _{1 in} which an abnormal situation cannot be detected, only the information 41 related to the achievable reliability level Rmin may be broadcast as reinforcement information.

  The user receiver 40 provided with the reliability instruction device 21 receives these information via the communication line 5. In the user receiver 8 ′, assuming that the reliability level requested by the user is R (step 30), the navigation mode is selected by operating the mode switch 19 as in the first embodiment. A reliability level R required by the receiver 8 ′ is set.

  Next, when the user receiver 8 ′ receives the information 41 regarding the achievable reliability level Rmin broadcasted as the reinforcement information from the radio navigation system 40 ′, the determination unit 24 regarding R receives the information 41 from the radio navigation system 8 ′. If it is determined that the information 41 regarding the achievable reliability level Rmin exceeds the reliability level R requested by the user (step 31), the radio navigation system 8 ′ can be used. Is displayed (step 32). That is, the navigation mode selected by the mode switch 19 is displayed as available (step 32), and the first lamp 20 is turned on.

  Further, if the information 41 regarding the achievable reliability level Rmin received from the radio navigation system 40 ′ is below the reliability level R requested by the user in the determination unit 24 regarding R, it is determined that the information is not satisfied. (Step 31), the radio navigation system 40 'is displayed as being unavailable (Step 32), and the second lamp 21 is turned on. Thus, the user can determine from the lighting state of the lamp whether or not the navigation mode set by the user can be used. The means for displaying the determination result as to whether or not the navigation mode can be used is not limited to the above-described embodiment, and other means such as a blinking lamp, a warning sound, etc. Also good.

  It is possible to build a highly reliable navigation system for users who move at various moving speeds, users with different surrounding environments, and the like. Various users envisioned in the quasi-zenith satellite system are available.

1 shows an embodiment of the present invention and is a system conceptual diagram of the present invention. FIG. It is a screen block diagram of the display screen of a flight plan display apparatus. 1, showing an embodiment of the present invention, is a block diagram showing an overall configuration. 1, showing an embodiment of the present invention, is a configuration diagram of a user receiver 8 provided with a reliability indicating device 50. FIG. FIG. 6 is a block diagram of a user receiver 8 ′ including a reliability indicating device 50 ′ according to an embodiment of the present invention. It is a system conceptual diagram which shows a prior art example.

Explanation of symbols

8, 8 ′ user receiver 14 constant K calculation unit 15 protection level calculation unit 16 PL <AL determination unit 17 navigation information display unit 19 mode switch 22 P ₂ , R setting unit 23 R setting unit 24 R determination unit 40, 40 'radio navigation system 41 information on achievable reliability level 42 information on standard deviation s 43 information on P ₁ 50, 50' reliability indication device AL maximum allowable positioning error of user receiver K constant PL protection Level (positioning error reliability limit)
P ₁ Probability that the radio navigation system cannot detect an abnormal situation P ₂ Probability of an excessive positioning error exceeding the maximum positioning error AL of the user receiver R Reliability level required by the user receiver

Claims (4)

The radio navigation system includes means for broadcasting information on the probability P _{1 in} which an abnormal situation cannot be detected as reinforcement information,
The user, means for setting the probability P ₂ of the maximum value AL or more positioning error of the positioning error occurs, means for setting a changeable reliability levels requested by the user, the constant K from the probability P ₂ set Means for calculating, means for calculating a protection level PL from information on the standard deviation s broadcasted together with position information by the radio navigation system and the constant K, means for determining whether PL <AL is satisfied, With
A reliability indication device in a radio navigation system, wherein when PL <AL, a reliability limit of a positioning error related to position information broadcasted by the radio navigation system is obtained. The user includes means for displaying a determination result regarding the PL <AL,
The reliability indication device in the radio navigation system according to claim 1, wherein whether or not the radio navigation system is usable is displayed to the user based on the determination result. The radio navigation system comprises means for broadcasting information on the achievable reliability level as reinforcement information,
The user sets whether or not the reliability level required by the user can be changed and whether the information on the achievable reliability level broadcast by the radio navigation system satisfies the reliability level required by the user. A means for judging,
When the achievable reliability level broadcast by the radio navigation system is greater than the reliability level required by the user, the user recognizes that the radio navigation system is available. Reliability indication device for wireless navigation system. The user is provided with means for displaying a determination result as to whether or not the requested reliability level is satisfied,
The reliability indication device in the radio navigation system according to claim 3, wherein whether or not the radio navigation system is usable is displayed to a user based on the determination result.

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