Abstract:
Instrument Landing System (“ILS”) localizer captures have historically been prone to performance degradations based upon erroneous estimates of the localizer deviation and deviation rate. The angular nature of the ILS beam and the relatively narrow trusted linear course guidance sector of the beam are the primary causes of many such issues. It is highly desirable to convert the angular guidance data into a rectilinear form when used by an autopilot and/or flight director systems, the latter of which allows for more consistent stability and performance with respect to distance to runway threshold. Systems and methods disclosed herein are for accurately converting angular localizer data into rectilinear localizer data, thus allowing for improved localizer capture performance and stability.

Description:
BACKGROUND 
       [0001]    Instrument Landing System (“ILS”) is a system supporting high precision guidance to an aircraft approaching and landing on a runway. The ILS typically includes transmitters and antenna arrays on the ground, receivers and antennas on the aircraft, and a display on the aircraft for the flight crew. Autopilots and/or flight directors may also be used on the aircraft. 
         [0002]    The portion of the ILS that provides lateral guidance is referred to as the localizer. Vertical guidance is provided via a glideslope portion. The localizer and glideslope portions provide an aircraft with an indication of its separation from a desired approach path, in the form of an angular error referred to as an angular ILS deviation. 
         [0003]    An aircraft preparing to perform a landing approach must fly a flight path that intersects the localizer. As the aircraft nears the desired approach path (i.e., the null of the localizer), it executes a turn to capture the null. This turn is typically executed in response to a steering input provided by a pilot following raw deviations on a display, in response to a steering input provided by a pilot following guidance received from a flight director system, or an autopilot system following guidance provided by the ILS. After a successful capture maneuver, the aircraft&#39;s flight path will be in line with the runway centerline. Ideally, the aircraft will perform a single turn to capture the localizer null, and will not fly through the null (overshoot) prior to completing its turn. The inherently angular nature of ILS deviations provides challenges when attempting to perform consistent localizer captures at varying distances from an ILS localizer transmitter. 
         [0004]    On aircraft equipped with flight director and/or autopilot systems, converting the angular ILS deviation into a rectilinear ILS deviation is beneficial such that consistent localizer capture guidance can be provided regardless of distance from the ILS localizer transmitter. On some aircraft, the distance estimate that is used for converting angular ILS deviations to rectilinear ILS deviations is prone to error. The distance estimate is typically formed using radio altitude and glideslope error. Terrain effects, varying runway lengths, unusual glideslope angles, or localizer captures attempted prior to receiving valid radio altitude and/or glideslope deviations can result in an inaccurate distance estimate. This inaccurate distance estimate can, in turn, provide inaccurate localizer deviation and deviation rate data to the localizer control laws, and result in degraded localizer capture performance characterized by undesirable roll and/or yaw attitude profiles along with additional overshoot during the capture maneuver. 
         [0005]    Moreover, the erroneous conversion factor used to convert an angular ILS deviation to a rectilinear ILS deviation manifests itself as a gain on the localizer deviation feedback loop in the control laws. This known source of inaccuracy requires control law gain reduction in the localizer deviation feedback loop, and the sacrifice of performance in favor of robustness. This known source of error also drives additional time and effort into the design and test of the control law, as the designer must show the control law is robust to a wide array of destination facility properties and approach geometries. 
         [0006]    In practice, large localizer overshoots are common. This is primarily due to the fact that the segment of the localizer beam that reliably provides an accurate indication of aircraft displacement is relatively narrow. This segment, referred to as the course guidance sector, may be only approximately +/−2 degrees of arc about the localizer null. If an aircraft does not begin its turn until it encounters this sector, it may have a smaller physical distance than required in which to complete its turn in order to avoid an overshoot. 
         [0007]    The propensity of an overshoot is exacerbated if the aircraft is intercepting the localizer with a large intercept angle, a high ground speed, or is close to the airfield (where the constant angular beam width corresponds to a smaller physical distance). Current aircraft systems do not begin the localizer capture maneuver until the linear part of the localizer beam is reached (i.e., the course guidance sector), and thus are prone to large overshoots. 
         [0008]    It is with respect to these and other considerations that the disclosure made herein is presented. 
       SUMMARY 
       [0009]    Concepts and technologies are described herein for consistent ILS localizer captures. In accordance with the concepts and technologies disclosed herein, a method for accurately converting angular localizer data into rectilinear localizer data is provided such that localizer deviation and deviation rate can be estimated with greater accuracy than current methods. 
         [0010]    According to one aspect, a method for guiding an aircraft during a final approach toward a runway includes receiving a geographic position of the aircraft, a geographic position of a runway threshold of the runway, a runway length of the runway, and a runway azimuth of the runway. The method further includes computing a distance from the aircraft to the runway threshold and a bearing to the runway threshold utilizing the geographic position of the aircraft and the geographic position of the runway threshold. The method further includes projecting the distance from the aircraft to the runway threshold into an along runway component and a cross runway component utilizing the runway azimuth of the runway, and computing a distance from the aircraft to a localizer transmitter by adding the runway length to the along runway component of the projection of the distance from the aircraft to the runway threshold. The method still further includes converting an angular localizer deviation into a rectilinear localizer deviation utilizing, in part, the distance from the aircraft to the localizer transmitter. 
         [0011]    It should be appreciated that the above-described subject matter may be implemented as a computer-controlled apparatus, a computer process, a computing system, or as an article of manufacture such as a computer-readable storage medium. In particular, the above-described subject matter may be implemented in one or more components of an avionics system. These and various other features will be apparent from a reading of the following Detailed Description and a review of the associated drawings. 
         [0012]    This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended that this Summary be used to limit the scope of the claimed subject matter. Furthermore, the claimed subject matter is not limited to implementations that solve any or all disadvantages noted in any part of this disclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a diagram illustrating an aerial view of an exemplary method for estimating distance from an aircraft to a localizer transmitter in an along runway axis to be used in an Instrument Landing System (“ILS”) localizer capture maneuver, according to an exemplary embodiment. 
           [0014]      FIG. 2  is a block diagram illustrating an avionics system in which methods for performing localizer captures may be implemented, according to an exemplary embodiment. 
           [0015]      FIG. 3  is a flow diagram illustrated various aspects of a method for estimating distance from an aircraft to a localizer transmitter in an along runway axis to be used in an ILS localizer capture maneuver, according to an exemplary embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    The following detailed description is directed to concepts and technologies for consistent localizer capture. According to the concepts and technologies described herein, information available on-board an aircraft regarding the aircraft&#39;s current position (e.g., latitude and longitude) and the destination runway&#39;s position (e.g., latitude and longitude) is utilized to determine a more accurate distance measurement for use in conversions of Instrument Landing System (“ILS”) deviations from inherently angular deviations to rectilinear deviations, thus improving localizer capture performance and stability. Prior to describing the accompanying drawings, several key details of current ILSs will be described to aid in distinguishing the novel concepts and technologies presented herein for consistent localizer capture. 
         [0017]    As described above, ILS guidance is inherently angular and is only provided for a narrow region referred to as the course guidance sector in which angular offset received by an aircraft is correctly reporting the aircraft&#39;s distance from a centerline of a destination runway. The centerline of the destination runway is also referred to as the ILS localizer null. The narrow course guidance sector limits the timing during which an aircraft can initiate or capture an ILS localizer. 
         [0018]    If the runway centerline is projected out to the intended point of localizer capture, and if the aircraft is approaching with a certain ground speed at a certain intercept angle toward a final approach course that the aircraft is trying to capture, a physical point exists at which the aircraft needs to initiate a turn in order to bank up to a certain bank angle (e.g., 30 degrees) and execute a one turn capture of the runway center line (i.e., the ILS localizer null) to obtain the final approach course without overshoot. Given the aforementioned requirements, a certain distance from the ILS localizer null exists where the aircraft needs to be located in order to initiate a proper turn to effectively (i.e., without overshoot) capture the ILS localizer null. 
         [0019]    For a wide array of localizer captures, the course guidance sector is not wide enough to allow the aircraft to initiate a localizer capture early enough to capture the localizer null in one turn. Typically, the narrow nature of the course guidance sector restricts the aircraft from performing a one turn capture and results in an overshoot of the runway centerline during a capture attempt. This aspect was a focus of U.S. Pat. No. 7,941,251 (“the &#39;251 patent”), which is incorporated herein by reference in its entirety. 
         [0020]    In the &#39;251 patent, flight management computer data is used to instruct an autopilot when to initiate a turn even if the aircraft is not located within the course guidance sector. This instruction to start turning in the correct direction is not based upon ILS guidance, but is based upon final approach course deviation data and utilizes a control law in the autopilot that turns the aircraft in the proper direction until the aircraft intercepts and enters the course guidance sector, at which point ILS guidance data is used to complete the capture. 
         [0021]    In some previous solutions for localizer captures, an autopilot of an aircraft receives localizer deviations in the form of an angular separation from a localizer null. This angular deviation is converted to a rectilinear deviation through the use of an estimated distance to a localizer transmitter. The control algorithms and localizer mode engage logic used for autopilot and flight director both utilize this calculated rectilinear deviation. 
         [0022]    The conversion of the localizer deviation into a rectilinear distance is currently performed in two stages. The first stage includes a conversion from difference in the depth of modulation (“DDM”) to degrees (or radians). The International Civil Aviation Organization (“ICAO”) specifies that the ILS localizer beam sensitivity shall be 0.00044 DDM/ft at the runway threshold. Based upon this specification, some aircraft (e.g., Boeing widebody airplanes), currently assume a standard runway length and can therefore apply a constant conversion factor to convert the deviation in DDM to a deviation in degrees (or radians). This conversion can be in error at any facility where the actual runway length does not match the assumed runway length used to arrive at the conversion constant. 
         [0023]    The second stage of localizer deviation conversion includes a conversion from degrees (or radians) to feet (or some other linear unit of distance) utilizing radio altitude and glideslope deviation data, such as are used on Boeing widebody aircraft. This approach to localizer capture relies upon several key assumptions:
       1) The terrain at an aircraft&#39;s current location is at the same elevation as the destination runway.   2) The glideslope transmitter is an assumed distance beyond the runway threshold.   3) The localizer transmitter is an assumed distance beyond the runway threshold.   4) The glideslope beam null is 3 degrees.   5) Valid and accurate radio altimeter and glideslope deviation data are available at the time of localizer capture.       
 
         [0029]    If any of the aforementioned assumptions are not satisfied during a localizer capture, the localizer deviation and the deviation rate provided to the control laws will be in error. The vast majority of the time at least one of these assumptions is not satisfied. Thus, the capture logic and the control law algorithm for capturing and tracking the localizer null must be robust to account for significant errors in the location deviation and deviation rate. 
         [0030]    In other previous solutions, an autopilot of an aircraft utilizes angular ILS deviations directly without a conversion to rectilinear. These designs have their own performance issues, which are outside the scope of the disclosure made herein. 
         [0031]    The concepts and technologies presented herein for consistent localizer capture also aid in mitigating the aforementioned issues and, more fundamentally, aid with the tracking and capture of the ILS localizer once the aircraft has entered the course guidance sector. The tracking and capture of the ILS localizer has historically been poor based upon the lack of an accurate way to convert an inherently angular ILS deviation into a rectilinear ILS deviation. Moreover, even when the aircraft is in the course guidance sector and the angular ILS deviation is accurate, it is beneficial to convert the angular ILS deviation into a rectilinear ILS deviation in order to provide a consistent capture performance that is decoupled from the aircraft&#39;s distance to the destination runway threshold. 
         [0032]    In the following detailed description, references are made to the accompanying drawings that form a part hereof, and in which are shown by way of illustration specific embodiments or examples. Referring now to the drawings, in which like numerals represent like elements throughout the several figures, aspects of a computing system, computer-readable storage medium, and computer-implemented methodology for consistent ILS localizer captures will be presented. 
         [0033]    Turning now to  FIG. 1 , an aerial view  100  illustrating a method for estimating distance from an aircraft  102  to an ILS localizer transmitter  104  located at a destination runway  106  for use in ILS localizer capture maneuvers will be described, according to an exemplary embodiment. In some embodiments, the ILS localizer transmitter  104  is part of a ground component of the localizer portion of an ILS that includes one or more transmitters and one or more antenna arrays (not shown) for transmitting guidance signals to the aircraft  102 . The ILS localizer transmitter  104  emanates a course guidance sector, which includes an ILS localizer null  108  extending from the ILS localizer transmitter  104  along the centerline of the destination runway  106  and beyond. The course guidance sector provides a small trusted region in which angular deviation received by the aircraft  102  is correctly reporting the aircraft&#39;s  102  distance from the centerline of the destination runway  106 . 
         [0034]    Typically, the course guidance sector may be visualized as an area defined by an arc emanating from the ILS localizer transmitter  104  and enclosed by ILS localizer boundaries extending therefrom. For ease of illustration and so as not to obscure various aspects of the disclosed method, the ILS course guidance sector and the ILS localizer boundaries are not illustrated. The illustrated method is focused on tracking and capturing the ILS localizer null  108  during a capture maneuver executed within the course guidance sector. It should be understood, however, that aspects of the illustrated method may be applicable to tracking and capturing the ILS localizer null  108  during a capture maneuver initiated outside of the course guidance sector for a given scenario. 
         [0035]    The aerial view  100  also illustrates a distance  110  to a runway threshold  112  that is calculated utilizing an algorithm of a flight management function (“FMF”) of the aircraft  102 , as will be described in greater detail below. This calculation is based upon the geographic position (e.g., in terms of latitude and longitude) of the aircraft  102  and the geographic position (e.g., in terms of latitude and longitude) of the runway threshold  112  as obtained via a navigation system of the aircraft  102 , as will also be described in greater detail below. A bearing  114  to the runway threshold  112  is also calculated utilizing an algorithm of the FMF. 
         [0036]    In some embodiments, in order to calculate a distance  116  from the aircraft  102  to the ILS localizer transmitter  104 , a runway length (not shown) and a runway azimuth  118  are needed. In some embodiments, the actual runway length and the runway azimuth  118  are also be obtained from a navigation system of the aircraft  102 . If a runway length for the runway  106  is not available, in some embodiments, a standard runway length or an estimated runway length can be used. In some embodiments, the runway azimuth  118  may also be estimated. Estimations of the runway azimuth  118  and/or the runway length may result in decreased accuracy of the distance  116  from the aircraft  102  to the ILS localizer transmitter  104 , but may, in certain scenarios, be beneficial or of greater accuracy than estimates determined using previous methodologies. 
         [0037]    Utilizing the distance  110  to the runway threshold  112 , the bearing  114  to the runway threshold  112 , the runway azimuth  118 , and the runway length, the distance  116  from the aircraft  102  to the ILS localizer transmitter  104  is calculated according to the below equation: 
         [0000]      dist_to_loc=dist_to_rwy_thd*cos(brg_to_thd−rwy_azmth)+rwy_len.
 
         [0000]    In the above equation, dist_to_loc is the distance  116 , dist_to_rwy_thd is the distance  110 , brg_to_thd is the bearing  114 , rwy_azmth is the runway azimuth  118 , and rwy_len is the runway length. Moreover, in the above equation, the product of the distance  110  and the cosine of the difference between the bearing  114  and the runway azimuth  118  provides an along runway distancecomponent  120  of the distance  110  in the along runway axis. 
         [0038]    In some embodiments, a cross runway distance component  122  of the distance  110  is calculated. Moreover, in some embodiments, the cross runway distance component  122  is utilized as a trigger, in accordance with embodiments disclosed in the &#39;251 patent, for initiating a capture maneuver outside of a course guidance sector. 
         [0039]    The above-described method for computing the distance  116  from the aircraft  102  to the ILS localizer transmitter  104  in the along runway axis creates an accurate distance estimate that does not rely on any of the aforementioned assumptions that are currently used, and that often cause distance estimates from an aircraft to an ILS localizer transmitter to be grossly in error. Moreover, although this method injects an opportunity for error in the form of an unknown distance between the far end of the destination runway  106  and the actual location of the ILS localizer transmitter  104 , this error is negligible as compared to the errors introduced by relying upon the aforementioned assumptions for calculating such distances. 
         [0040]    In some embodiments, the aforementioned error in the form of an unknown distance between the far end of the destination runway  106  and the actual location of the ILS localizer transmitter  104  is eliminated by storing the geographic position of the ILS localizer transmitter  104  in the navigation database of the aircraft  102 . In these embodiments, the need for a runway length value and an assumption regarding the distance between the end of the destination runway  106  and the ILS localizer transmitter  104  is negated. Moreover, in these embodiments, a distance from the runway threshold  112  to the localizer transmitter  104  is computed utilizing the respective geographic positions of the runway threshold  112  and the localizer transmitter  104 . The distance from the runway threshold  112  to the localizer transmitter  104  then replaces the rwy_len in the above equation, as shown in the below modified equation: 
         [0000]      dist_to_loc=dist_to_rwy_thd*cos(brg_to_thd−rwy_azmth)+dist_from_rwy_thd_loc
 
         [0041]    In some embodiments, this method is used for front course ILS localizer approaches that are coupled with an ILS glideslope, or for ILS Backcourse (“BCRS”) approaches. Current methods, however, can perform particularly poorly when no glideslope or glidepath reference is available, such as is often the case for BCRS approaches. 
         [0042]    Utilizing the distance  116  from the aircraft  102  to the ILS localizer transmitter  104  calculated according to the above-described methods, an accurate conversion of angular ILS deviations to rectilinear ILS deviations can be made. This enables modifications to existing localizer capture logic and control laws to mitigate performance problems that plague ILS localizer captures today. 
         [0043]    In some embodiments, the aircraft  102  is a commercial passenger aircraft including, for example, the 737, 747, 757, 767, 777, 787 models commercially-available from The Boeing Company of Chicago, Ill. The concepts and technologies disclosed herein may also be applied for use in, manufacture of, and/or assembly of other aircraft including, but not limited to, other commercial aircraft, civilian aircraft, military aircraft, passenger aircraft, cargo aircraft, fixed wing aircraft, rotary aircraft, hybrid fixed wing and rotary aircraft, unmanned aircraft, and manned aircraft. 
         [0044]    Turning now to  FIG. 2 , a system diagram illustrating an exemplary avionics system  200  of an aircraft in which methods for estimating a distance from the aircraft to an ILS localizer transmitter located at a destination runway for use in ILS capture maneuvers will be described, according to an exemplary embodiment. To aid in facilitating the following description, the avionics system  200  is described with further reference to  FIG. 1 . In particular, the avionics system  200  is described as being included in the aircraft  102  executing a landing approach towards the destination runway  106 . 
         [0045]    The avionics system  200  includes an ILS sensor  202 , an autopilot  204  that includes a deviation calculator  206 , a navigation system  208  that includes a location component (“LC”)  210 , a flight management function (“FMF”)  212  and a navigation database  214 , a flight director  216 , an announcer  218 , and a capture maneuver computer  220 . In some embodiments, methods for performing localizer capture maneuvers may be implemented in the capture maneuver computer  220 . 
         [0046]    The ILS sensor  202  is configured to receive guidance signals from one or more ILS localizer transmitters, such as the ILS localizer transmitter  104 , via one or more receivers. For each landing approach of the aircraft  102 , the ILS sensor  202  processes received guidance signals and acquires one or more angular separations of the aircraft  102  from the ILS localizer null  108  ( FIG. 1 ). 
         [0047]    The autopilot  204  is configured to receive an ILS localizer null that the pilot of the aircraft  102  wants to capture, and to capture that ILS localizer null without further human intervention. In some embodiments, the autopilot  204  is configured to utilize the above equation to calculate the distance  116  to the ILS localizer transmitter  104 . In some embodiments, the autopilot  204  is configured to receive angular ILS deviations during the landing approach of the aircraft  102  towards the destination runway  106 . In some embodiments, the deviation calculator  206  converts the angular ILS deviations to rectilinear ILS deviations utilizing, in part, the distance  116  to the ILS localizer transmitter  104  calculated by the autopilot  204  in accordance with the above equation. 
         [0048]    In some embodiments, the deviation calculator  206  is implemented as one or more software algorithms embodied within computer-executable instructions that are executable by the autopilot  204 . In other embodiments, the deviation calculator  206  is implemented in a similar manner in one or more other avionic components that are capable of receiving, processing, and storing data. 
         [0049]    The navigation system  208  is used to provide, in various combinations according to various embodiments described above, the geographic position (e.g., latitude and longitude) of the aircraft  102  during flight, the geographic position of the runway threshold  112 , the runway azimuth  118 , the runway length, and the actual location of the ILS localizer transmitter  104 . The geographic position of the aircraft  102  may be provided utilizing the LC  210  embodied as an Inertial Reference System (“IRS”), an Attitude Heading and Reference System (“AHRS”), a Global Positioning System (“GPS”), some combination thereof, or the like. The geographic position of the runway threshold  112 , the runway azimuth  118 , the runway length, and the actual location of the ILS localizer transmitter  104  are stored in the navigation database  214  in some combination, in accordance with the various embodiments described above. This information may be updated as needed for a particular destination. 
         [0050]    The navigation system  208  is also equipped with the FMF  212 . The FMF  212  includes one or more algorithms that are used to calculate the distance  110  to the runway threshold  112  and the bearing  114  to the runway threshold  112 , as described above. 
         [0051]    The flight director  216  is configured to compute and display the proper path for the aircraft  102  to one or more pilots during a specific flight. In some embodiments, the flight director  216  includes a flight director indicator (“FDI”), a horizontal situation indicator (“HIS”), a mode selector, and/or a flight director computer (all not shown). Moreover, in some embodiments, the FDI includes a display for presenting one or more symbols, indicators, or other information including, an attitude indicator, a fixed aircraft symbol, pitch and bank command bars, a glide slope indicator, a localizer deviation indicator, and/or the like. In some embodiments, the flight director  216  provides a pilot with steering commands necessary to obtain and hold a desired course. These steering commands may be, for example, those necessary to perform capture maneuvers. In some embodiments, the flight director  216  provides steering commands that account for a rectilinear ILS deviation calculated by the autopilot. 
         [0052]    The announcer  218 , in some embodiments, includes one or more of a speaker, a buzzer, or other type of warning or noise-generating device. The announcer  218  may be activated by the exemplary avionics system  200  to provide audio warnings and messages to a flight crew. In some embodiments, the announcer  218  is activated by the capture maneuver computer  220 . 
         [0053]    The autopilot  204 , the navigation system  210 , the flight director  216 , and the announcer  218  are each configured to communicate with the capture maneuver computer  220 . Although exemplary directions of communication between these components of the avionics system  200  are illustrated, such directions are not intended to be limiting in any way. Moreover, the various components of the avionics system  200  may communicate with one another for various purposes should the need arise for a particular implementation. 
         [0054]    As further shown in  FIG. 2 , the capture maneuver computer  220  has processing capabilities and memory suitable to store and execute computer-executable instructions. In the illustrated embodiment, the capture maneuver computer  220  includes one or more processors  222  and a memory  224 . The memory  224  may include volatile and nonvolatile memory, removable and non-removable media implemented in any method or technology for storage of information, such as computer-readable instructions, data structures, program modules or other data. Such memory includes, but is not limited to, random access memory (“RAM”), read-only memory (“ROM”), electrically erasable programmable read-only memory (“EEPROM”), flash memory or other memory technology, compact disc, read-only memory (“CD-ROM”), digital versatile disks (“DVD”) or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, redundant array of independent disks (“RAID”) storage systems, or any other medium which can be used to store the desired information and which can be accessed by a computer system. 
         [0055]    Software program modules which enable the capture maneuver computer  220  to perform various functions may be stored in the memory  224 . In the illustrated embodiment, the memory  224  includes an autopilot interface module  226 , a flight management function interface module  228 , a database interface module  230 , a capture maneuver module  232 , a flight director interface module  234 , an alert interface module  236 , and a capture maneuver computer database  238 . These modules may be implemented as computer-executable instructions that are executed by the one or more processors  222  to perform the functions described below. 
         [0056]    The autopilot interface module  226  is configured to communicate with the autopilot  204 . The communication may be established over an electrical connection, an optical connection, and/or the like. In some embodiments, the autopilot interface module  226  is configured to receive one or more angular deviations, or alternatively, estimated rectilinear deviations, from the autopilot  204 . As described above, the estimated rectilinear deviations are calculated from the angular deviations received by the autopilot  204 . In some embodiments, the autopilot interface module  226  is also configured to enable the autopilot  204  to perform capture maneuvers under the direction of the capture maneuver computer  220 , as described below. 
         [0057]    The FMF interface module  228  is configured to communicate with the FMF  212  of the navigation system  208 . The communication may be established over an electrical connection, an optical connection, and/or the like. In some embodiments, the FMF interface module  228  is configured to receive the calculated FAC deviations from the FMF  212 . 
         [0058]    The database interface module  230  enables the reading of data from and writing of data to the capture maneuver computer database  238 . In some embodiments, the database interface module  230  is activated by one or more of the other modules illustrated in the memory  224 , as further described below. In some embodiments, the capture maneuver computer database  238  contains data that may be necessary to initiate a standard capture maneuver based on ground track angle, ground speed, and runway heading. 
         [0059]    In some embodiments, the capture maneuver module  232  is configured to command the autopilot  204  to automatically perform localizer capture maneuvers. Alternatively, in other embodiments, the capture maneuver module  232  is configured to provide information to the flight director  216 . This information may enable a pilot to manually perform localizer capture maneuvers. 
         [0060]    The flight director interface module  234  facilitates the communication between the flight director  216  and the capture maneuver module  232 . For example, the flight director interface module  234  may enable the flight director  216  to provide a pilot with the necessary steering commands to complete a capture maneuver. 
         [0061]    The alert interface module  236 , in some embodiments, is directed by the capture maneuver module  232  to cause the announcer  218  to alert a flight crew that a particular flight mode is active. In some embodiments, the alert interface module  236  causes the announcer  218  to inform the flight crew of the initialization and the termination of a particular flight mode. In some embodiments, the alert interface module  236  causes the announcer  218  to be activated for the duration that the particular flight mode is active. The alert interface module  236  activates the announcer  218  to convey information by representative tones, human speech, and/or computer speech. 
         [0062]    In some embodiments, the capture maneuver computer  220  uses the alert interface module  236  to cause one or more cockpit displays (not shown) to visually indicate that a particular flight mode is active. For example, in one implementation, a primary flight display (“PFD”) is activated by the capture maneuver computer  220  to present symbols and/or characters during the activation of the particular flight mode. In another implementation, one or more push button lights on the mode control panel (“MCP”) are illuminated during the particular flight mode activation to provide visual alert. 
         [0063]    It should be appreciated that the illustrated avionics system  200  is only one example of a suitable operating environment and is not intended to suggest any limitation as to the scope of use or functionality of the invention defined by the appended claims. Other avionic environments and/or configurations may be suitable for use with the concepts and technologies disclosed herein. For example, the exemplary capture maneuver computer  220  may be provided as a part of a flight management computer (“FMC”). In other exemplary examples, one or more of the modules  226 - 236  is directly implemented on the FMC, the autopilot  204 , or any other suitable avionic component of a flight management system (“FMS”), navigation system, or any avionic system present in an aircraft that is capable of receiving, processing, and storing data. 
         [0064]    Encoding the software program modules presented herein may transform the physical structure of the computer-readable media, such as the memory  224 , presented herein. The specific transformation of physical structure may depend on various factors, in different implementations of this description. Examples of such factors may include, but are not limited to, the technology used to implement the computer-readable media, whether the computer-readable media is characterized as primary or secondary storage, and the like. For example, if the computer-readable media is implemented as semiconductor-based memory, the software disclosed herein may be encoded on the computer-readable media by transforming the physical state of the semiconductor memory. For example, the software may transform the state of transistors, capacitors, or other discrete circuit elements constituting the semiconductor memory. The software also may transform the physical state of such components in order to store data thereupon. 
         [0065]    As another example, the computer-readable media disclosed herein may be implemented using magnetic or optical technology. In such implementations, the software presented herein may transform the physical state of magnetic or optical media, when the software is encoded therein. These transformations may include altering the magnetic characteristics of particular locations within given magnetic media. These transformations also may include altering the physical features or characteristics of particular locations within given optical media, to change the optical characteristics of those locations. Other transformations of physical media are possible without departing from the scope and spirit of the present description, with the foregoing examples provided only to facilitate this discussion. 
         [0066]    In light of the above, it should be appreciated that many types of physical transformations take place in the avionics system  200  in order to store and execute the software components presented herein. It also should be appreciated that the avionics system  200  may include other types of computing devices, including hand-held computers, embedded computer systems, personal digital assistants, and other types of computing devices known to those skilled in the art. It is also contemplated that the avionics system  200  may not include all of the components shown in  FIG. 2 , may include other components that are not explicitly shown in  FIG. 2 , or may utilize an architecture completely different than that shown in  FIG. 2 . 
         [0067]    Turning now to  FIG. 3 , aspects of a method  300  for estimating a distance from an aircraft to an ILS localizer transmitter will be described in detail, according to an exemplary embodiment. To aid in facilitating the following description, the method  300  is described with further reference to  FIGS. 1 and 2 . 
         [0068]    It should be understood that the operations of the method  300  are not necessarily presented in any particular order and that performance of some or all of the operations in an alternative order(s) is possible and is contemplated. The operations have been presented in the demonstrated order for ease of description and illustration. Operations may be added, omitted, and/or performed simultaneously, without departing from the scope of the appended claims. 
         [0069]    It also should be understood that the methods disclosed herein can be ended at any time and need not be performed in its entirety. Some or all operations of the method  300 , and/or substantially equivalent operations, can be performed by execution of computer-readable instructions included on a computer-storage media, as defined above. The term “computer-readable instructions,” and variants thereof, as used in the description and claims, is used expansively hereinto include routines, applications, application modules, program modules, programs, components, data structures, algorithms, and the like. Computer-readable instructions can be implemented on various system configurations, including single-processor or multiprocessor systems, minicomputers, mainframe computers, personal computers, hand-held computing devices, microprocessor-based, programmable consumer electronics, combinations thereof, and the like. 
         [0070]    Thus, it should be appreciated that the logical operations described herein are implemented (1) as a sequence of computer implemented acts or program modules running on a computing system and/or (2) as interconnected machine logic circuits or circuit modules within the computing system. The implementation is a matter of choice dependent on the performance and other requirements of the computing system. Accordingly, the logical operations described herein are referred to variously as states, operations, structural devices, acts, or modules. These operations, structural devices, acts, and modules may be implemented in software, in firmware, in special purpose digital logic, and any combination thereof. 
         [0071]    For purposes of illustrating and describing the concepts and technologies disclosed herein, the method  300  disclosed herein is described as being performed by the avionics system  200  or some portion thereof. It should be understood that the avionics system  200 , the components thereof, as well as additional and/or alternative systems, devices, or components thereof, can provide the functionality described herein via execution of one or more application programs including, but not limited to, application programs embodied at least in part by the program modules, calculators, and/or functions described above. Furthermore, it should be understood that the functionality of the avionics system  200  can be provided by any number of systems or devices, and is not limited to the avionics system  200  illustrated in  FIG. 2 . Thus, the illustrated embodiment is exemplary, and should not be viewed as being limiting in any way. 
         [0072]    The method  300  begins at operation  302 , wherein the FMF  212  receives the geographic position (e.g., latitude, longitude) of the runway threshold  112  from the navigation database  214 . From operation  302 , the method  300  proceeds to operation  304 , wherein the FMF  212  receives the geographic position (e.g., latitude, longitude) of the aircraft  102  from the navigation system  208 . In some embodiments, the geographic position of the aircraft  102  is acquired via the LC  210  and then stored at least temporarily in the navigation database  214 . Alternatively, in some embodiments, the geographic position of the aircraft  102  is received directly from the LC  210 , thus bypassing the at least temporary storage in the navigation database  214 . 
         [0073]    From operation  304 , the method  300  proceeds to operation  306 , wherein the capture maneuver computer  220  receives a length of the destination runway  106  and the runway azimuth  118  from the navigation database  214 . As disclosed above, the length of the destination runway  106  may be an actual length of the destination runway  106  or a standard runway length. For example, the standard runway length may be standard to the destination airfield of which the destination runway  106  is a part, or based upon some other criteria. Moreover, in some embodiments, the need for the runway length is negated if replaced by an actual geographic position of the ILS localizer transmitter  104 . 
         [0074]    From operation  306 , the method  300  proceeds to operation  308 , wherein the FMF  212  computes the distance  110  to the runway threshold  112  and the bearing  114  to the runway threshold  112  utilizing the aircraft  102  latitude and longitude and the runway threshold  112  latitude and longitude received from the navigation system  208 . 
         [0075]    From operation  308 , the method  300  proceeds to operation  310 , wherein the FMF  212  projects the distance  110  to the runway threshold  112  computed in operation  308  into an along runway component (e.g., the along runway distance component  120 ) and a cross runway component (e.g., the cross runway distance component  122 ) utilizing the runway azimuth  118 . The product of the distance  112  and the cosine of the difference between the bearing  116  and the runway azimuth  118  provides the along runway distance component, illustrated in  FIG. 1  as the projection  120  of the distance  110 . 
         [0076]    From operation  310 , the method  300  proceeds to operation  312 , wherein the FMF  212  adds the runway length received in operation  306  to the along runway component of the projection of the distance  110 , thereby creating an accurate estimate of the distance  116  from the aircraft  102  to the ILS localizer transmitter  104 . The distance  116  is thus provided by the following equation, duplicated from above: 
         [0000]      dist_to_loc=dist_to_rwy_thd*cos(brg_to_thd−rwy_azmth)+rwy_len.
 
         [0000]    In the above equation, dist_to_loc is the distance  116 , dist_to_rwy_thd is the distance  110 , brg_to_thd is the bearing  114 , rwy_azmth is the runway azimuth  118 , and rwy_len is the runway length. 
         [0077]    From operation  312 , the method  300  proceeds to operation  314 , wherein the FMF  112  provides the distance  116  from the aircraft  102  to the ILS localizer transmitter  104  is provided to the capture maneuver computer  220 . The computer maneuver computer  220 , at operation  316 , provides the distance  116  to the autopilot  204 . Alternatively, in some embodiments, the FMF  212  provides the distance  116  directly to the autopilot  204 . 
         [0078]    From operation  316 , the method  300  proceeds to operation  318 , wherein the deviation calculator  206  of the autopilot  204  converts one or more angular localizer deviations into one or more rectilinear localizer deviations utilizing, in part, the distance  116  created in operation  312 . The autopilot  204 , at operation  320 , outputs the one or more rectilinear localizer deviations to the capture maneuver computer  220  for use in capturing the localizer null. For example, the capture maneuver computer  220  may then provide the one or more rectilinear deviations to the flight director  216 , which then instructs the pilot to steer the aircraft  102  to capture the localizer null. Alternatively, in some embodiments, the rectilinear deviations are not output and are utilized by the autopilot  204  to aid in automatically flying the aircraft  102  to capture the localizer null and continue the landing approach towards the destination runway  106 . Further communications between the capture maneuver computer  220  and the autopilot  204  may be used to implement these embodiments. 
         [0079]    From operation  320 , the method  300  proceeds to operation  322 . The method  300  ends at operation  322 . 
         [0080]    In some embodiments, the actual runway length obtained from the navigation database  214  is used to perform the DDM to degree (or radian) conversion based upon the standard DDM/ft conversation at the runway threshold as specified by the ICAO (i.e., 0.00044 DDM/ft). In these embodiments, a small angle approximation is used and the DDM to degree scale factor is computed as follows: 
         [0000]    
       
         
           
             
               
                 ( 
                 
                   57.3 
                    
                   
                     ( 
                     
                       deg 
                       rad 
                     
                     ) 
                   
                 
                 ) 
               
               
                 [ 
                 
                   
                     
                       
                         
                           ( 
                           
                             0.00044 
                              
                             
                               ( 
                               
                                 DDM 
                                 ft 
                               
                               ) 
                             
                           
                           ) 
                         
                         * 
                         
                           ( 
                           
                             
                               rwy_len 
                                
                               
                                 ( 
                                 ft 
                                 ) 
                               
                             
                             + 
                           
                         
                       
                     
                   
                   
                     
                       
                         assumed_dist 
                          
                         _from 
                          
                         _far 
                          
                         _end 
                          
                         _of 
                          
                         _rwy 
                          
                         _to 
                          
                         _loc 
                          
                         
                           ( 
                           ft 
                           ) 
                         
                       
                     
                   
                 
                 ] 
               
             
             , 
           
         
       
     
         [0000]    wherein rwy_len is the actual runway length as obtained from the navigation database  214  and assumed_dist_from_far_end_of_rwy_to_loc is the assumed distance from the far end of the destination runway  106  (i.e., the end of the destination runway  106  closest to the localizer transmitter  104 ) to the localizer transmitter  104 . 
         [0081]    Based on the foregoing, it should be appreciated that technologies for consistent localizer capture have been disclosed herein. Although the subject matter presented herein may have been described at times in language specific to computer structural features, methodological and transformative acts, specific computing machinery, and computer readable media, it is to be understood that the invention defined in the appended claims is not necessarily limited to the specific features, acts, or media described herein. Rather, the specific features, acts and mediums are disclosed as example forms of implementing the claims. 
         [0082]    The subject matter described above is provided by way of illustration only and should not be construed as limiting. Various modifications and changes may be made to the subject matter described herein without following the example embodiments and applications illustrated and described, and without departing from the true spirit and scope of the present invention, which is set forth in the following claims.