Method of facilitating the approach to a platform

Abstract

A method of facilitating the approach to a platform by an aircraft. The value of the required position integrity performance (RNP) is determined for a localization system. During an approach stage, this position integrity performance (RNP) is compared with a position integrity radius (HPL) resulting from the use of a GNSS localization system, and an alert is generated if the position integrity radius (HPL) is greater than the position integrity performance (RNP).

Claims

1. A method of facilitating the approach to a platform by an aircraft, comprising: a preparatory step (STP1) of constructing a database of platforms including for each platform at least one attribute defining the platform; a parameter-setting step (STP2) during which parameters are set for a distance referred to as the lateral distance (LOV); an approach flight stage (STP4) along an approach path comprising an initial approach fix (IAF), a final approach fix (FAF), and a decision point (MAP) that are present in a common vertical plane directed parallel to a course (CRS), the target platform to be reached being offset relative to the vertical plane by an orthogonal distance (D0) that is not less than the lateral distance (LOV); during the preparatory step (STP1), a step of storing, for each platform, an attribute relating to a positioning error (TPE) of the platform; prior to the approach flight stage commencing at the initial approach fix (IAF), a step of using an integrity-checking module to determine the value of a position integrity performance (RNP) required for a localization system of the aircraft, this position integrity performance (RNP) being equal to the minimum value between firstly a predetermined performance threshold and, secondly, the quotient of the difference of the lateral distance minus the positioning error for a target platform to be reached as divided by two according to the equation:
RNP=min[(LOVTPE)/2;k3] in which RNP represents the required position integrity performance, LOV represents the lateral distance, TPE represents the positioning error, k3 represents the predetermined threshold performance, and / represents the division sign; and during a monitoring step performed during the approach flight stage (STP4), the aircraft being provided with a GNSS localization system determining the radius of a circle in which the aircraft is located referred to as the position integrity radius (HPL); using the integrity-checking module to compare the position integrity performance (RNP) with the position integrity radius (HPL); and generating an alert if the position integrity radius (HPL) is greater than the position integrity performance (RNP).

2. A method according to claim 1, wherein the predetermined performance threshold has a value of 0.3 Nm.

3. A method according to claim 1, wherein the following steps are performed: during the parameter-setting step (STP2), a step of determining the lateral distance (LOV) by: automatically calculating the value of a minimum theoretical distance (COMPUTED LOV) that is equal to the sum of the positioning error (TPE) of the target platform, of a predetermined margin (HPL BUFFER), and of twice a maximum value between firstly the position integrity radius (HPL) and, secondly, a predicted radius provided by the GNSS localization system indicating the position integrity radius that will be reached at the end of a predetermined time period according to the equation:
COMPUTED LOV=2*max[HPL,HPL5]+HPLBUFFER+TPE in which COMPUTED LOV represents the minimum theoretical distance, TPE represents the positioning error, HPL BUFFER represents the margin, HPL represents the position integrity radius, HPL5 represents the predicted radius, and * represents the multiplication sign; and by comparing the minimum theoretical distance with distances stored in a list, the lateral distance being equal to the smallest stored distance that is greater than the minimum theoretical distance.

4. A method according to claim 1, wherein during the parameter-setting step (STP2), the lateral distance (LOV) is set at a stored predetermined value.

5. A method according to claim 1, wherein during the parameter-setting step (STP2), the lateral distance (LOV) is selected by a pilot from a list comprising a plurality of stored lateral distances.

6. A method according to claim 1, wherein since the path is prepared during a construction step (STP3), modification of the path is authorized up until a predetermined fixing point located upstream from the initial approach fix (IAF).

7. A method according to claim 1, wherein during a verification step (STP32) performed prior to the approach flight stage and following a path construction step (STP3) the following steps are performed: determining whether a lateral distance referred to as a current lateral distance that served to prepare the path is compatible with the position integrity radius calculated by the GNSS localization system of the aircraft by: automatically calculating the value of a minimum theoretical distance (COMPUTED LOV) that is equal to the sum of the positioning error (TPE) of the target platform, of a predetermined margin (HPL BUFFER), and of twice a maximum value between firstly the position integrity radius (HPL) and, secondly, a predicted radius provided by the GNSS localization system (20) indicating the position integrity radius that will be reached at the end of a predetermined time period according to the equation:
COMPUTED LOV=2*max[HPL,HPL5]+HPLBUFFER+TPE in which COMPUTED LOV represents the minimum theoretical distance, TPE represents the positioning error, HPL BUFFER represents the margin, HPL represents the position integrity radius, HPL5 represents the predicted radius, and * represents the multiplication sign; and by comparing the current lateral distance with the minimum theoretical distance; and when the current lateral distance is less than the minimum theoretical distance, and using a list comprising a plurality of stored distances, performing the following steps: abandoning the approach if the greatest distance of the stored distances is less than the minimum theoretical distance (COMPUTED LOV); or increasing the current lateral distance in order to obtain an amended lateral distance, this amended lateral distance being equal to the smallest stored distance in the list that is greater than the minimum theoretical distance, then the construction step is implemented again by replacing the current lateral distance by the amended lateral distance.

8. A method according to claim 7, wherein implementation of the verification step is authorized so long as the aircraft is situated more than three minutes away from the initial approach fix (IAF).

9. A method according to claim 7, wherein implementation of the verification step is authorized so long as the aircraft is situated more than two minutes away from a turning point at which the aircraft must turn in order to reach the initial approach fix (IAF).

10. A method according to claim 7, wherein an alert is generated when an amended lateral distance is calculated.

11. A method according to claim 1, wherein: during the preparatory step (STP1), for at least one platform, attributes defining at least one target zone constituting a destination are stored, the at least one target zone including a geometrical center of the platform, the attributes for the geometrical center comprising geographical coordinates of the geometrical center, at least one height of the geometrical center, and the radius referred to as the obstacle radius (OR) of a circle centered on the geometrical center in which the platform lies; during the parameter-setting step (STP2), the parameters for the following information are set using a parameter-setting module: a target zone referred to as the selected target zone selected from the stored target zones; a course (CRS) to be followed in order to reach the target platform provided with the selected target zone; and a height parameter relative to a minimum decision altitude (MDA) of a decision point (MAP) for a final approach descent towards the target platform; and during a construction step (STP3) implemented by a navigation module, if the selected target zone is a geometrical center, the approach path is constructed by determining the positions of the initial approach fix (IAF), of the final approach fix (FAF), and of the decision point (MAP) in response to the information and to the attributes of the selected target zone, the initial approach fix (IAF) as well as the final approach fix (FAF) and the decision point (MAP) being present in the vertical plane directed parallel to the course (CRS), the geometrical center being offset relative to the vertical plane by the orthogonal distance (D0) equal to the sum of the lateral distance (LOV) plus the obstacle radius (OR).

12. A method according to claim 11, wherein: during the preparatory step (STP1), storing, for at least one platform, attributes that define at least one target zone of the landing zone type distinct from the geometrical center, the stored attributes including geographical coordinates of this landing zone and at least one height for the landing zone; and during the construction step (STP3) if the selected target zone is a landing zone, determining the positions of an initial approach fix (IAF), of a final approach fix (FAF), and of a decision point (MAP) in response to the information and to the attributes of the target zone, the initial approach fix (IAF) as well as the final approach fix (FAF) and the decision point (MAP) being present in the vertical plane (100) directed parallel to the course (CRS), and the landing zone being offset relative to the vertical plane by an orthogonal distance (D0) equal to the lateral distance (LOV).

13. A method according to claim 11, wherein during the parameter-setting step (STP2): either a pilot manually sets parameters for the minimum decision altitude (MDA); or the minimum decision altitude (MDA) is determined automatically by the navigation module, the minimum decision altitude (MDA) being the maximum value between firstly a threshold altitude and, secondly, the sum of the height of the selected target zone and of a minimum decision constant predetermined by the manufacturer.

14. A method according to claim 13, wherein the navigation module determines the arrival time at the target platform, the threshold altitude varying depending on the arrival time.

15. A method according to claim 11, wherein the decision point (MAP) is positioned by determining a longitudinal distance (D1) separating it from a tangent plane passing through the geographical coordinates of the selected target zone and orthogonal to the vertical plane in accordance with the following relationship:
D1=D0/[tan()] in which / represents the division sign, D0 represents the orthogonal distance, and represents an offset angle set by the manufacturer.

16. A method according to claim 11, wherein during the parameter-setting step (STP2), an offset side on the vertical plane is selected relative to the target platform.

17. A method according to claim 11, wherein the final approach fix (FAF) is positioned at a minimum safe altitude (MSA), the minimum safe altitude (MSA) being equal to the maximum value between firstly a threshold minimum safe altitude and, secondly, the sum of a height of a top of the target platform plus a minimum safe constant predetermined by the manufacturer.

18. A method according to claim 11, wherein the final approach fix (FAF) is positioned at a final approach distance (DFAF) from the decision point (MAP) equal to: $\mathrm{DFAF}=k1+\frac{\mathrm{MSA}-\mathrm{MINI}}{\tan }$ in which DFAF represents the final approach distance, k1 represents an approach constant set by the manufacturer, MSA represents a minimum safe altitude at which the final approach fix is positioned, MINI represents the minimum decision altitude (MDA), and represents the angle of a flight descent segment connecting the final approach fix to a horizontal plane containing the decision point (MAP).

19. A method according to claim 17, wherein the initial approach fix (IAF) is positioned at the minimum safe altitude (MSA).

20. A method according to claim 11, wherein the initial approach fix (IAF) is positioned at an initial approach distance (DIAF) from the decision point (MAP) equal to: $\mathrm{DIAF}=k1+\frac{\mathrm{MSA}-\mathrm{MINI}}{\tan }+k2$ in which DIAF represents the initial approach distance, k1 represents an approach constant set by the manufacturer, k2 represents a variable determined by the manufacturer, MSA represents a minimum safe altitude at which the final approach fix is positioned, MINI represents the minimum decision altitude (MDA), and represents the angle of a flight descent segment connecting the final approach fix to a horizontal plane containing the decision point (MAP).

21. A method according to claim 11, wherein the approach path includes a leveling-off point (LPO) interposed between the final approach fix (FAF) and the decision point (MAP), the leveling-off point being placed at the same minimum decision altitude (MDA) as the decision point (MAP) and at a leveling-off distance (DLPO) from the decision point (MAP) equal to:
DLPO=k1 in which DLPO represents the leveling-off distance, and k1 represents an approach constant set by the manufacturer.

22. A method according to claim 1, wherein a vertical guidance module and a horizontal guidance module communicate with an autopilot system in order to guide the aircraft along the approach path prepared by the navigation module.

23. A method according to claim 1, wherein the method includes a guidance step of providing guidance during the calculated initial approach fix (IAF).

24. A method according to claim 1, wherein, the aircraft having a horizontal situation indicator provided with a scale comprising a plurality of markers, the horizontal situation indicator positioning on the scale a first symbol centered in the middle of the scale indicating the approach path and a second symbol indicating the difference in lateral position of the aircraft relative to the approach path to be followed, the difference between the first symbol and the adjacent second marker representing the value of the position integrity performance (RNP).

25. A method according to claim 1 further comprising: piloting the aircraft along the approach path to the target platform.

26. An aircraft comprising: an onboard database including for each of a plurality of platforms a plurality of attributes including at least one attribute defining the platform and an attribute relating to a positioning error (TPE) of the platform; a parameter-setting module in communication with the database, the parameter-setting module configured to set parameters for a distance referred to as the lateral distance (LOV); a navigation module in communication with the database and the parameter-setting module, the navigation module configured to prepare an approach path to a target platform to be reached, the approach path comprising an initial approach fix (IAF), a final approach fix (FAF), and a decision point (MAP) that are present in a common vertical plane directed parallel to a course (CRS), the target platform to be reached being offset relative to the vertical plane by an orthogonal distance (D0) that is not less than the lateral distance (LOV); an integrity-checking module configured to determine, prior to an approach flight stage of the aircraft commencing at the initial approach fix (IAF), the value of a position integrity performance (RNP) required for a localization system of the aircraft, this position integrity performance (RNP) being equal to the minimum value between firstly a predetermined performance threshold and, secondly, the quotient of the difference of the lateral distance minus the positioning error for a target platform to be reached as divided by two according to the equation:
RNP=min[(LOVTPE)/2;k3] in which RNP represents the required position integrity performance, LOV represents the lateral distance, TPE represents the positioning error, k3 represents the predetermined threshold performance, and / represents the division sign; a GNSS localization system configured to determine, during the approach flight stage of the aircraft, the radius of a circle in which the aircraft is located referred to as the position integrity radius (HPL); and the integrity-checking module further configured to compare the position integrity performance (RNP) with the position integrity radius (HPL) and generate an alert if the position integrity radius (HPL) is greater than the position integrity performance (RNP).

27. A method of facilitating the approach to a platform by an aircraft, comprising: a preparatory step (STP1) of constructing a database of platforms including for each platform at least one attribute defining the platform; a parameter-setting step (STP2) during which parameters are set for a distance referred to as the lateral distance (LOV); an approach flight stage (STP4) along an approach path comprising an initial approach fix (IAF), a final approach fix (FAF), and a decision point (MAP) that are present in a common vertical plane directed parallel to a course (CRS), the target platform to be reached being offset relative to the vertical plane by an orthogonal distance (D0) that is not less than the lateral distance (LOV); during the preparatory step (STP1), a step of storing, for each platform, an attribute relating to a positioning error (TPE) of the platform; prior to the approach flight stage commencing at the initial approach fix (IAF), a step of using an integrity-checking calculator to determine the value of a position integrity performance (RNP) required for a localization system of the aircraft, this position integrity performance (RNP) being equal to the minimum value between firstly a predetermined performance threshold and, secondly, the quotient of the difference of the lateral distance minus the positioning error for a target platform to be reached as divided by two according to the equation:
RNP=min[(LOVTPE)/2;k3] in which RNP represents the required position integrity performance, LOV represents the lateral distance, TPE represents the positioning error, k3 represents the predetermined threshold performance, and / represents the division sign; and during a monitoring step performed during the approach flight stage (STP4), the aircraft being provided with a GNSS localization system determining the radius of a circle in which the aircraft is located referred to as the position integrity radius (HPL); using the integrity-checking calculator to compare the position integrity performance (RNP) with the position integrity radius (HPL); and generating an alert if the position integrity radius (HPL) is greater than the position integrity performance (RNP).

28. A method according to claim 27 further comprising: piloting the aircraft along the approach path to the target platform.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) The invention and its advantages appear in greater detail in the context of the description below with examples given by way of illustration and with reference to the accompanying figures, in which:

(2) FIG. 1 is a diagram presenting an aircraft implementing the method of the invention;

(3) FIG. 2 is a diagram presenting a navigation computer of that aircraft;

(4) FIG. 3 is a diagram presenting the method of the invention;

(5) FIGS. 4 to 9 are diagrams explaining a parameter-setting step; and

(6) FIG. 10 is a diagram presenting the constructed approach path; and

(7) FIGS. 11 and 12 are diagrams explaining the verification step and the integrity-checking step.

(8) Elements that are present in more than one of the figures are given the same references in each of them.

DETAILED DESCRIPTION OF THE INVENTION

(9) FIG. 1 presents an aircraft 1, and in particular a rotary wing aircraft that is suitable for landing on a platform at sea.

(10) This aircraft 1 is provided with a navigation assistance system 7 including an onboard database 5 communicating with a parameter-setting module 10.

(11) By way of example, the parameter-setting module 10 includes a visual display unit, data entry means that are operable by a crew, a memory, and a computing member of the processor type, e.g. executing stored instructions.

(12) In addition, the navigation assistance system includes a navigation module 15 that is suitable for being connected both to the database 5 and to the parameter-setting module 10. As a function of the elements transmitted by the database 5 as well as by the parameter-setting module 10 and possibly by stored elements, the navigation module 15 automatically prepares an approach path.

(13) By way of example, the navigation module 15 includes a visual display unit, data entry means that are operable by a crew, a memory, and a computing member of the processor type, e.g. executing stored instructions.

(14) The navigation module 15 may then communicate with a vertical guidance module 25 and/or with a horizontal guidance module 30 that generate setpoints transmitted to an autopilot system 35. The autopilot system thus acts on control means in order to control the aircraft. Reference may be made to the literature in order to obtain information about such an autopilot system or such guidance modules.

(15) Furthermore, each guidance module may communicate with a GNSS localization system 20. This GNSS localization system 20 may be a conventional system or a hybrid system with an inertial unit.

(16) The navigation module may also communicate with the instruments and controls, possibly via the guidance modules. Thus, data about the actions to be performed in order to follow the prepared path may be displayed on the instruments onboard the aircraft.

(17) In particular, the system 7 may comprise a horizontal situation indicator 40 connected to the GNSS localization system 20 in order to display the path to be followed and a representation of the aircraft relative to this path.

(18) In addition, the system may comprise a verification module 17 for verifying that the prepared path is compatible with the position integrity calculated by the GNSS localization system 20. By way of example, the verification module 17 includes a visual display unit, data entry means that are operable by a crew, a memory, and a computing member of the processor type, e.g. executing stored instructions.

(19) Furthermore, the system may comprise an integrity-checking module 16 for verifying during an approach flight stage that the path being followed is compatible with the current position integrity calculated by the GNSS localization system 20. By way of example, the integrity-checking module 16 includes a visual display unit, data entry means that are operable by a crew, a memory, and a computing member of the processor type, e.g. executing stored instructions.

(20) By way of example, at least one of the parameter-setting module 10, the navigation module 15, the integrity-checking module 16, and the verification module 17 forms part of a navigation computer (flight management system (FMS)).

(21) With reference to FIG. 2, the navigation computer FMS comprises a casing carrying a screen 18 and multiple push buttons 6. Said casing housing a calculation unit 2 executing instructions stored in a storage unit 3.

(22) Under such circumstances, this storage unit 3 may comprise at least one memory storing instructions, e.g. in the form of segments of code.

(23) Segments of code associated with the calculation unit may thus represent respectively the parameter-setting module 10, the navigation module 15, the integrity-checking module 16, and the approach monitoring module 17.

(24) In addition, the database 5 may be stored in a memory of the storage unit 3.

(25) Such a navigation computer FMS is thus in communication via multiple interfaces with the GNSS localization system 20, the horizontal situation indicator 40, the vertical guidance module 25, and/or the horizontal guidance module 30.

(26) FIG. 3 presents various steps of a method of the invention implemented by a navigation assistance system 7.

(27) During a preparatory step STP1, a user prepares the database 5. This user prepares a list of platforms likely to constitute a destination for the user's aircraft.

(28) Under such circumstances, the user stores a plurality of attributes in the database 5 for each platform selected. These attributes may include an identifier ID for the platform.

(29) In addition and with reference to FIG. 7, these attributes define at least one target zone per platform.

(30) The database 5 need not be provided with a platform identifier. Under such circumstances and by way of example, the identifier of the target zones may, comprise a reference with the name of the corresponding platform.

(31) In particular, these attributes define a target zone marking a geometrical center 50 of the platform 46. Thus, the attributes include the geographical coordinates of said geometrical center 50, a height for said geometrical center, and the radius referred to as the obstacle radius OR of a circle centered on the geometrical center 50 in which the platform lies.

(32) In addition, these attributes may define at least one target zone marking a landing zone 51. Thus, the attributes include the geographical coordinates of said landing zone and a height of said landing zone.

(33) Furthermore, these attributes may comprise a maximum height of the platform, i.e. the height of the top of the platform.

(34) In addition, each target zone of a platform may be associated with an attribute qualifying a positioning error of the platform.

(35) With reference to FIG. 9, a platform 46 may be a so-called fixed platform 47 resting on the sea bottom 200.

(36) Under such circumstances, the freedom of movement of the fixed platform 47 around the geographical coordinates of the stored target zones is small. The positioning error TPE of such a fixed platform is thus relatively small.

(37) However, a platform 46 may be a so-called mobile platform 48 anchored on the sea bottom 200. Under such circumstances, the freedom of the mobile platform 48 to move around the geographical coordinates of the stored target zones is considerable. The positioning error TPE of such a mobile platform is thus relatively large.

(38) Consequently, the database associates each target zone with multiple attributes, these attributes including information specific to the corresponding platform and/or specific to the target zone.

(39) With reference to FIG. 3, the crew of an aircraft may implement a parameter-setting step STP2 in order to set parameters for the approach to a platform.

(40) To this end, a plurality of parameters are set or are settable during this parameter-setting step STP2.

(41) Thus, the crew may select the target platform on which the aircraft is to land.

(42) With reference to FIG. 4, the parameter-setting module 10 may present the list of platforms P1, P2, P3 stored in the database 5. The crew may, for example, select the platform P1 using the navigation computer FMS.

(43) With reference to FIG. 5, and optionally, the parameter-setting module 10 may for example present various types of available approaches. It should be observed that the database may possibly include an attribute for stored platforms making reference to these types of approach.

(44) This approach list may in particular include an approach of the airborne radar approach (ARA) type, or also an approach of the DELTA30 Offshore standard procedure (OSAP) type, or an offset approach of the OFFSET type of the invention.

(45) With reference to FIG. 6, the parameter-setting module 10 enables the crew of the aircraft to set other parameters. By way of example, the parameter-setting module 10 may present values by default for the parameter settings, it may also enable a user to select values from a list, or it may even execute instructions for calculating said information as a function of additional data.

(46) During this parameter-setting step, the system 7 makes it possible to set parameters for the target zone of the platform to be reached, represented by the term TARGET in the example of FIG. 6. The crew then selects a target zone from the list of target zones associated with the selected target platform.

(47) In a variant, an operator may directly select a target zone, without firstly determining a target platform to be reached.

(48) In addition, the system 7 makes it possible to set parameters for a minimum decision altitude MDA at which the decision point MAP is located. The minimum decision altitude MDA can refer to an altitude determined with a radioaltimeter or a height determined with an altimeter settled to the QNH.

(49) In a first variant, the crew specifies that the minimum decision altitude MDA is to be set manually, and sets such a parameter. For safety reasons, the manufacturer may impose a minimum decision altitude. By way of example, the manufacturer may decide that the minimum decision altitude MDA should not be less than 300 ft, or not less than the sum of the height of the selected target zone plus a security margin. This margin may be of the order of 50 ft.

(50) In a second variant, the crew specifies that the parameters for the minimum decision altitude MDA should be set automatically.

(51) Under such circumstances, the minimum decision altitude MDA is either a threshold altitude, or the sum of the height HDECK of the selected target zone plus a minimum decision constant that is predetermined by the manufacturer. The higher of these values is then used.

(52) The threshold altitude may depend on visibility conditions, and in particular on whether landing takes place by day or by night. Thus, the threshold altitude may be 200 ft for landing by day and 300 ft for landing by night.

(53) Consequently, if necessary, the navigation module 15 may perform preprocessing in order to estimate the arrival time at the target platform provided with the selected target zone so as to determine whether landing will take place by day or by night.

(54) In the example presented, and where necessary, the crew may select an offset side SIDE in order to determine where the platform is located relative to the vertical plane containing the decision point MAP.

(55) Said decision point corresponds to a point at which the crew must decide either to fly towards the platform in order to land, or to implement an alternative procedure if landing proves impossible.

(56) The parameter-setting module 10 makes it possible, in particular during the parameter-setting step STP2, to set parameters for a course CRS to be followed in order to reach the selected target zone.

(57) The course parameters may be set manually by the crew.

(58) However, and with reference to FIG. 8, the course CRS may be determined automatically by the system 7.

(59) The system 7 may comprise a meteorological device connected to the navigation computer in order to transmit to it the wind direction V1, V2 at the target platform.

(60) Said wind direction parameter may also be set by the crew.

(61) Under such circumstances, the navigation computer consults the database in order to determine whether the selected target zone is associated with an angular sector approach 400.

(62) Otherwise, the set course is then selected so as to be into the wind.

(63) Conversely, the navigation computer determines if a course into the wind is contained within the angular sector 400 approach. If so, the set course is then selected so as to be into the wind. Otherwise, the set course is then selected to be directed along the nearest limit 401, 402 of the angular sector 400.

(64) In the first example shown, the first wind direction V1 makes it possible for the first course CRS1 to be into the wind.

(65) However, the second wind direction V2 leads to placing the second course CRS2 along a limit 401 of the angular sector 400.

(66) Furthermore, and with reference to FIG. 3, during the parameter-setting step, the system 7 sets a parameter for a lateral distance LOV.

(67) In a first variant, the lateral distance parameter LOV is set by a crew. Thus, said crew selects a lateral distance LOV, e.g. based on a list comprising a plurality of stored lateral distances.

(68) In a second variant, the parameter-setting module 10 sets the lateral distance at a stored predetermined value.

(69) In a third variant, the parameter-setting module 10 calculates the lateral distance.

(70) Consequently, the parameter-setting module 10 calculates a theoretical minimum distance COMPUTED LOV by applying the following first relationship:
COMPUTED LOV=2*max[HPL,HPL5]+HPLBUFFER+TPE
in which COMPUTED LOV represents said minimum theoretical distance, TPE represents said positioning error of the selected target platform, HPL BUFFER represents a stored constant margin, HPL represents a current position integrity radius, HPL5 represents a predicted radius, and * represents the multiplication sign.

(71) The position integrity radius referred to as current represents the position integrity radius determined at the current instant at which the minimum theoretical distance is calculated. Conversely, the predicted radius represents the position integrity radius that will be reached at the end of a predetermined duration.

(72) The position integrity radius HPL (Horizontal Protection Limit) and the predicted radius HPL5 are data that is usually determined by a GNSS localization system. This data is therefore transmitted to the parameter-setting module 10 via said GNSS localization system.

(73) Under such circumstances, the parameter-setting module 10 compares the calculated minimum theoretical distance with distances stored in a list.

(74) The parameter-setting module 10 thus considers that the value of the lateral distance is equal to the value of the shortest stored distance that is greater than said minimum theoretical distance.

(75) By way of example, assume that the calculated minimum theoretical distance has a value of 0.38 Nm. Furthermore, the list of distances possibly includes the following distances: 0.25, 0.3, 0.35, 0.4, 0.5, 0.6, 0.75. Under such circumstances, the parameter-setting module 10 automatically sets the lateral distance parameter by assigning it the value of 0.4 Nm.

(76) At the end of the parameter-setting step, the system 7 prepares an approach path by using the attributes and the parameters that were set.

(77) During a construction step STP3, the navigation module prepares an approach path to be followed in order to reach the selected target zone as a function of the attributes present in the database 5, and of parameters that were set during the parameter-setting step STP2.

(78) FIG. 10 presents the path prepared by the navigation module. More precisely, the bottom portion of FIG. 10 presents a side view in elevation of the path prepared for reaching a selected target zone of a platform 46. The top portion of FIG. 10 presents a plan view of the prepared path.

(79) The navigation module determines in particular the position of an initial approach fix IAF, of a final approach fix FAF, and of a decision point MAP as a function of said information and said attributes.

(80) Possibly, a leveling-off point LPO is also determined.

(81) The initial approach fix IAF, the final approach fix FAF, the decision point MAP, and possibly the leveling-off point LPO are arranged in a common vertical plane 100. Said vertical plane 100 is parallel to the course CRS as set.

(82) However, the vertical plane 100 is offset laterally by an orthogonal distance D0 relative to the platform, and in particular to the selected target zone. Thus, said vertical plane 100 is not directed onto the target platform.

(83) Whether the offset is to be to the left or to the right of the vertical plane relative to the platform is a setting that might be set during the parameter-setting step STP2.

(84) If the selected target zone is the geometrical center of the platform, the orthogonal distance D0 is equal to the sum of the previously set lateral distance LOV plus the obstacle radius OR associated with the selected target zone.

(85) If the selected target zone is a landing zone 51, the orthogonal distance D0 is equal to the previously set lateral distance LOV.

(86) The decision point MAP and possibly the leveling-off point LPO are placed at a height equal to the minimum decision altitude MDA.

(87) Furthermore, the navigation module determines the position of the decision point MAP by determining a longitudinal distance D1 separating it from a tangent plane 101 passing through the geographical coordinates of the selected target zone and orthogonal to the vertical plane 100 in accordance with the following relationship:
D1=D0/[tan()]
in which / represents the division sign, D0 represents said orthogonal distance, and represents an offset angle set by the manufacturer.

(88) The offset angle may have a value of 30 degrees.

(89) With reference to FIG. 8, the decision point is further from or closer to a landing zone as a function of the target zone selection.

(90) By way of example, if the crew has selected the geometrical center as the target, the landing zone 52 located to the left of the geometrical center is relatively far away from the decision point MAP. Conversely, the landing zone 52 located to the right of the geometrical center is relatively close to the decision point MAP.

(91) Under such circumstances, during the parameter-setting step, the system may display the informations of FIG. 8 in a representation on a screen. Under such circumstances, a crew may select the target zone that is the best suited to its needs.

(92) With reference to FIG. 10, if the approach path includes a leveling-off point LPO, the leveling-off point is placed at a leveling-off distance DLPO equal to:
DLPO=k1
in which DLPO represents the leveling-off distance, and k1 represents an approach constant set by the manufacturer.

(93) In addition, the final approach fix FAF and the initial approach fix IAF are arranged at a minimum safe altitude MSA.

(94) Said minimum safe altitude MSA is either equal to a threshold minimum safe altitude HMSS, or equal to the sum of a stored height HPFM of top of the platform to be reached plus a minimum safe constant CMIN predetermined by the manufacturer. The higher value of the two values thus obtained is then used, i.e.:
MSA=max(HMSS,CMIN+HPFM)

(95) The threshold minimum safe altitude HMSS may have a value of 1000 ft. In addition, the minimum safe constant CMIN may have a value of 500 ft.

(96) The final approach fix FAF is thus positioned at a final approach distance DFAF from the decision point MAP equal to:

(97) $\mathrm{DFAF}=k1+\frac{\mathrm{MSA}-\mathrm{MINI}}{\tan }$
in which DFAF represents the final approach distance, k1 represents an approach constant set by the manufacturer, MSA represents a minimum safe altitude at which the final approach fix is positioned, MINI represents said minimum decision altitude MDA, and represents the angle of a flight descent segment connecting the final approach fix to a horizontal plane containing said decision point MAP.

(98) The initial approach fix IAF is thus at an initial approach distance DIAF from the decision point MAP equal to:

(99) $\mathrm{DIAF}=k1+\frac{\mathrm{MSA}-\mathrm{MINI}}{\tan }+k2$
in which DIAF represents the initial approach distance, k1 represents an approach constant set by the manufacturer, k2 represents a variable determined by the manufacturer, MSA represents a minimum safe altitude at which the final approach fix is positioned, MINI represents said minimum decision altitude MDA, and represents the angle of a flight descent segment connecting the final approach fix to a horizontal plane containing said decision point MAP.

(100) This path may also include a turning point TP located upstream from the initial approach fix IAF.

(101) When the path is prepared, the method may allow said path to be modified so long as a predetermined fixing point situated upstream from the initial approach fix IAF has not been reached.

(102) By way of example, the path may be permanently frozen one minute before reaching the turning point TP.

(103) Thus, during a modification step STP31, the crew can modify the path.

(104) Furthermore, a verification step STP32 may also be undertaken.

(105) By way of example, this verification step may be performed once, e.g. while the aircraft is situated more than three minutes away from the initial approach fix IAF or while the aircraft is situated more than two minutes away from the turning point TP.

(106) During this verification step, the verification module 17 determines whether the lateral distance referred to as the current lateral distance that was used to prepare the path is compatible with the position integrity provided by the GNSS localization system 20.

(107) The verification module 17 then calculates the minimum theoretical distance COMPUTED LOV in accordance with the above-described first relationship by acting on the GNSS localization system 20.

(108) The verification module performs this calculation, even if the calculation was performed during the preparation step STP2. The position integrity provided by the GNSS localization system 20 might have become degraded since performing the preparation step STP2.

(109) Under such circumstances, the verification module 17 compares the current lateral distance with the minimum theoretical distance.

(110) When said current lateral distance is less than said minimum theoretical distance, two alternatives can be envisaged.

(111) Thus, if the greatest stored distance in the above-described list is less than the calculated minimum theoretical distance, then the system 7 warns the crew to abandon the approach by means of an alert of the audible or visual type.

(112) Otherwise, and with reference to FIG. 11, the verification module increases the current lateral distance LOV1 in order to obtain an amended lateral distance LOV2. This amended lateral distance LOV2 is equal to the value of the smallest stored distance in said list that is greater than said minimum theoretical distance.

(113) The construction step STP3 is thus implemented once more by replacing said current lateral distance by said amended lateral distance.

(114) With reference to FIG. 12, the system prepares a new path TRAJ2, by replacing the initial path TRAJ1.

(115) The system 7 may also generate an alert when an amended lateral distance is calculated.

(116) When the path is frozen, the navigation module may require the aircraft to be guided towards the initial approach fix IAF. By way of example, the initial approach fix IAF may be reached by means of a turn passing through a turning point TP.

(117) Under such circumstances, the prepared approach path is followed during an approach flight stage STP4:

(118) either by manually controlling the aircraft operated entirely by the crew, possibly by means of flying assistance data displayed on at least one flight instrument 40;

(119) or by manually controlling the aircraft operated by the crew together with automatic control operated by the autopilot system, the crew managing the vertical flight of the aircraft and the autopilot system managing the horizontal flight of the aircraft, for example;

(120) or by automatic control operated by the autopilot system.

(121) The crew may then select to stop following the approach path, or may even request a modification of the approach path, if necessary.

(122) With reference to FIG. 10, the aircraft is directed towards the decision point MAP. When the decision point is reached, if the crew can see the platform and if landing is possible, the crew follows arrow F1 in order to fly towards the platform. Otherwise, the crew implements a conventional procedure aiming to fly the aircraft along arrow F2 towards an auxiliary path.

(123) During the approach flight stage commencing at the initial approach fix IAF, the method implements an integrity-checking stage.

(124) With reference to FIG. 11, prior to the approach flight stage and once the path has been frozen, an integrity-checking module 16 determines the value of the required position integrity performance RNP for the localization system during the approach, in accordance with the following second relationship:
RNP=min[(LOVTPE)/2;k3]
in which RNP represents the required position integrity performance, LOV represents the lateral distance that gave rise to the path being followed, TPE represents the error in positioning of the target platform, and k3 represents a threshold integrity performance, e.g. of the order of de 0.3 Nm, and / represents the division sign.

(125) During a monitoring step implemented during the approach flight stage, the integrity-checking module compares the position integrity performance RNP with the current position integrity radius HPL transmitted by the GNSS localization system 20.

(126) The integrity-checking module generates an alert if the current position integrity radius HPL is greater than the position integrity performance RNP so that the crew abandons the approach.

(127) Furthermore, FIG. 11 shows a horizontal situation indicator 40.

(128) This horizontal situation indicator 40 is provided with a scale comprising a plurality of markers 430. Under such circumstances, the horizontal situation indicator 40 positions a first symbol 420 on the scale indicating the approach path and a second symbol 410 on the scale indicating the difference in lateral position of the aircraft relative to the path to be followed.