Patent Publication Number: US-9418559-B2

Title: Method and system for determining height above ground using indirect information

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of, and claims priority benefit to, co-pending and commonly assigned U.S. patent application entitled “METHOD AND SYSTEM FOR DETERMINING HEIGHT ABOVE GROUND USING INDIRECT INFORMATION,” application Ser. No. 14/276,650, filed May 13, 2014, which claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application Ser. No. 61/823,233, filed May 14, 2013, and titled “METHOD OF DETERMINING GROUND LEVEL AND AIRCRAFT HEIGHT ABOVE TERRAIN USING INDIRECT INFORMATION.” Each of the above applications are herein incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     Airborne avionics systems may provide air traffic displays that are configured to display depictions of air traffic within the airspace surrounding the aircraft. In some systems, air traffic displays can display depictions of air traffic utilizing information received from the transponders of other aircraft. In this manner, air traffic displays can be furnished that provide flight crew members with a detailed, accurate and real-time depiction of air traffic in the vicinity of the aircraft. 
     SUMMARY 
     Techniques and systems are described that allow an air traffic display to suppress the display of on-ground traffic targets. In some implementations, an avionics system can use indirect information (e.g., traffic tracking data, database information, etc.) to determine whether an individual aircraft traffic target is to be presented to a pilot. The avionics system can determine whether an aircraft target is in proximity to the aircraft and/or whether the aircraft target is on the ground. When the aircraft target is on the ground, the avionics system can determine that an indication of the aircraft target should not be presented to the pilot. The aircraft&#39;s own height above ground may also be determined in this manner. 
     This Summary is provided solely as an introduction to subject matter that is fully described in the Detailed Description and Drawings. The Summary should not be considered to describe essential features nor be used to determine the scope of the Claims. Moreover, it is to be understood that both the foregoing Summary and the following Detailed Description are example and explanatory only and are not necessarily restrictive of the subject matter claimed. 
    
    
     
       DRAWINGS 
       The detailed description is described with reference to the accompanying figures. The use of the same reference numbers in different instances in the description and the figures may indicate similar or identical items. 
         FIG. 1  is a block diagram illustrating an environment that includes an integrated avionics system configured to provide an air traffic display in accordance with an example implementation of the present disclosure. 
         FIG. 2A  is a block diagram illustrating an avionics unit for an integrated avionics system, such as the integrated avionics system illustrated in  FIG. 1 , in accordance with an example implementation of the present disclosure. 
         FIG. 2B  is a block diagram illustrating an avionics unit configured in accordance with other example implementations of the present disclosure. 
         FIG. 3A  is a diagrammatic illustration of an air traffic display, where the air traffic display depicts air traffic targets within a monitored airspace. 
         FIG. 3B  is a diagrammatic illustration of an air traffic display where display of an air traffic target has been suppressed. 
         FIG. 4  is a flow diagram illustrating a method for selectively displaying air traffic targets on an air traffic display in accordance with an example implementation of the present disclosure. 
     
    
    
     The drawing figures do not limit the system to the specific implementations disclosed and described herein. The drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating elements of the system. 
     DETAILED DESCRIPTION 
     Overview 
     When an aircraft flies through complex, high-density traffic environments, such as near a busy airport, the depiction of air traffic by an air traffic display within the aircraft can become correspondingly busy due to the increased number of air traffic targets that are shown and/or tracked. The depiction of other aircraft (e.g., air traffic targets) can be a nuisance if the other aircraft are not relevant to the ownship aircraft. Generally, air traffic displays can show information for air traffic targets (e.g., heading, position, threat level, and so forth). Such information can include icons that represent the air traffic targets. 
     Aircraft with Mode C transponders will often (depending on transponder setting) respond to interrogations and broadcast altitude information even while on the ground and not engaged in flight activities. Conventional avionics systems alert pilots to the presence of these on-ground aircraft, which may be a nuisance. 
     Accordingly, techniques and systems are described that allow an avionics system of an aircraft to display aircraft targets while suppressing the display of on-ground air traffic targets. In one or more implementations, the techniques described herein may be implemented by an avionics unit, which may be part of an avionics system of the aircraft (e.g., an integrated avionics unit (IAU), one or more dedicated air traffic display units, a combination thereof, and so forth). 
     Display, suppression, and/or monitoring air traffic targets are employed for decreasing nuisance alerts. In some implementations, an avionics system can use indirect information (e.g., traffic tracking data, Mode S transponder data, database information, etc.) to determine the height above ground of an air traffic target. Indirect information may also be used to determine the height above ground for the ownship aircraft. The display of air traffic targets may be suppressed based on the determined height above ground of the air traffic targets. That is, if an air traffic target is on the ground, the avionics unit may suppress its display. 
     Example Implementations 
       FIG. 1  illustrates an environment in an example implementation that includes an integrated avionics system  100  configured to provide an air traffic display and/or alerts in accordance with the techniques of the present disclosure. In some configurations, the avionics system  100  may comprise a single device such as avionics unit  110  and does not comprise a portion of an integrated avionics system. In other configurations, as illustrated in  FIG. 2B , the avionics system  100  may comprise avionics unit  110  and a traffic advisory system (TAS)  115 . In one configuration, avionics unit  110  is a Garmin® GPS/NAV/COM such as a GTN 650/750 and TAS  115  is a Garmin@ GTS 800 TAS, Garmin@ GTS 850 TCAS I, Garmin@ GTS 8000 TCAS II, or the like. 
     Avionics system  100  may comprise an integrated flight deck as illustrated in  FIGS. 1 and 2A , and include one or more primary flight displays (PFDs)  102 , and/or one or more multifunction displays (MFDs)  104 . For instance, in the specific implementation illustrated in  FIG. 1 , the avionics system  100  may be configured for use in an aircraft  150  that is flown by a flight crew having two pilots (e.g., a pilot and a copilot). In this implementation, the integrated avionics system  100  may include a first PFD  102 ( 1 ), a second PFD  102 ( 2 ), and an MFD  104  that are mounted in the aircraft&#39;s instrument panel. As shown, the MFD  104  is mounted generally in the center of the instrument panel so that it may be accessed by either pilot (e.g., by either the pilot or the copilot). In an example implementation, the first PFD  102 ( 1 ) is mounted in the instrument panel generally to the left of the MFD  104  for viewing and access by the pilot. Similarly, the second PFD  102 ( 2 ) is mounted in the instrument panel generally to the right of the MFD  104  for viewing and access by the aircraft&#39;s copilot or other crew member or passenger. 
     The PFDs  102  may be configured to display primary flight information, such as aircraft attitude, altitude, heading, vertical speed, and so forth. In implementations, the PFDs  102  may display primary flight information via a graphical representation of basic flight instruments such as an attitude indicator, an airspeed indicator, an altimeter, a heading indicator, a course deviation indicator, and so forth. The PFDs  102  may also display other information providing situational awareness to the pilot such as terrain information and ground proximity warning information. 
     As shown in  FIG. 1 , primary flight information may be generated by one or more flight sensor data sources including, for example, one or more attitude, heading, angular rate, and/or acceleration information sources such as attitude and heading reference systems (AHRSs)  106 , one or more air data information sources such as air data computers (ADCs)  108 , and/or one or more angle of attack information sources. For instance, in one implementation, the AHRSs  106  may be configured to provide information such as attitude, rate of turn, slip and skid; while the ADCs  108  may be configured to provide information including airspeed, altitude, vertical speed, and outside air temperature. Other configurations are possible. 
     One or more avionics units  110  (e.g., a single integrated avionics unit (IAU) is illustrated) may aggregate the primary flight information from the AHRSs  106  and ADCs  108  and provide the information to the PFDs  102  via an avionics data bus  112 . The avionics unit  110  may also function as a combined communications and navigation radio. For example, as shown in  FIG. 2 , the avionics unit  110  may include a two-way Very High Frequency (VHF) communications transceiver  202 , a VHF navigation receiver with glide slope  204 , a global navigation satellite system (GNSS) receiver, such as a global positioning system (GPS) receiver  206  or the like, an avionics data bus interface  208 , a processor  210 , a memory  212  including a traffic display module  214 , and so forth. 
     In configurations where the avionics unit  110  is not part of an integrated avionics system and instead provides stand-alone navigation and/or communication functionality, the avionics unit  110  may include and/or be coupled with only GPS  206 , processor  210 , memory  212 , traffic display module  214 , and display. In other stand-alone configurations, avionics unit  110  may include a display, VHF transceiver  202 , VHF navigation receiver  204 , processor  210 , GPS  206 , memory  212 , and module  214 . In some stand-alone configuration bus interface  208  communicates with TAS  115 . 
     The processor  210  provides processing functionality for the avionics unit  110  and may include any number of processors, micro-controllers, or other processing systems and resident or external memory for storing data and other information accessed or generated by the avionics unit  110 . The processor  210  may execute one or more software programs which implement techniques described herein. The processor  210  is not limited by the materials from which it is formed or the processing mechanisms employed therein and, as such, may be implemented via semiconductor(s) and/or transistors (e.g., electronic integrated circuits (ICs)), and so forth. 
     The memory  212  is an example of computer-readable media that provides storage functionality to store various data associated with the operation of the avionics unit  110 , such as the software programs and code segments mentioned above, or other data to instruct the processor  210  and other elements of the avionics unit  110  to perform the functionality described herein. Although a single memory  212  is shown, a wide variety of types and combinations of memory may be employed. The memory  212  may be integral with the processor  210 , stand-alone memory, or a combination of both. The memory  212  may include, for example, removable and non-removable memory elements, such as random-access memory (RAM), read-only memory (ROM), flash memory (e.g., a secure digital (SD) card, a mini-SD card, and/or a micro-SD card), magnetic memory, optical memory, universal serial bus (USB) memory, and so forth. 
     The avionics data bus interface  208  furnishes functionality to enable the avionics unit  110  to communicate with one or more avionics data buses, such as the avionics data bus  112 . In various implementations, the avionics data bus interface  208  may include a variety of components, such as processors, memory, encoders, decoders, and so forth, and any associated software employed by these components (e.g., drivers, configuration software, etc.). 
     As shown in  FIG. 1 , the integrated avionics unit  110  may be paired with one or more PFDs  102 , which may function as a controlling unit for the integrated avionics unit  110 . In implementations, the avionics data bus  112  may comprise a high speed data bus (HSDB), such as data bus complying with ARINC  429  data bus standard promulgated by the Airlines Electronic Engineering Committee (AEEC), a MIL-STD-1553 compliant data bus, and so forth. 
     The MFD  104  displays information describing operation of the aircraft  150 , such as navigation routes, moving maps, engine gauges, weather radar, ground proximity warning system (GPWS) warnings, traffic collision avoidance system (TCAS) warnings, airport information, and so forth, that are received from a variety of aircraft systems via the avionics data bus  112 . 
     In implementations, the integrated avionics system  100  employs redundant sources of primary flight information to assure the availability of the information to the pilot, and to allow for cross-checking of the sources of the information. For example, the integrated avionics system  100  illustrated in  FIG. 1  employs two PFDs  102  that receive primary flight information from redundant AHRSs  106  and ADCs  108  via the avionics unit  110 . The integrated avionics system  100  is configured so that the first PFD  102 ( 1 ) receives a first set of primary flight information aggregated by the avionics unit  110  from a first AHRS  106 ( 1 ) and ADC  108 ( 1 ). Similarly, the second PFD  102 ( 2 ) receives a second set of primary flight information aggregated by the avionics unit  110  from a second AHRS  106 ( 2 ) and ADC  108 ( 2 ). Additionally, although a single avionics unit  110  and a single avionics data bus  112  are illustrated in  FIG. 1 , it is contemplated that redundant IAU&#39;s and/or redundant data buses may be employed for communication between the various components of the integrated avionics system  100 . 
     In implementations, primary flight information provided by either the first AHRS  106 ( 1 ) and ADC  108 ( 1 ) or the second AHRS  106 ( 2 ) and ADC  108 ( 2 ) may be displayed on either PFD  102 ( 1 ) or  102 ( 2 ), or on the MFD  104  upon determining that the primary flight information received from either AHRS  106  and ADC  108  is in error or unavailable. One or both of the PFDs  102  may also be configured to display information shown on the MFD  104  (e.g., engine gauges and navigational information), such as in the event of a failure of the MFD  104 . 
     The first PFD  102 ( 1 ), the second PFD  102 ( 2 ), and/or the MFD  104  may receive additional data aggregated by the avionics unit  110  from one or more of a plurality of systems communicatively coupled with the avionics unit  110 . For example, the avionics unit  110  may be communicatively coupled with TAS  115 . TAS  115  may include functionality such as: an Automatic Dependent Surveillance-Broadcast (ADS-B) system  114 , Traffic Collision Avoidance System (TCAS)  116  (which can include TCAS, TCAS II, TCAS III, TCAS IV, etc.), a Traffic Information Services-Broadcast (TIS-B) system  118 , a Mode C/A transponder, a Mode S transponder, and/or other receiver of traffic broadcasts from airborne and ground sources. In some configurations, TAS  115  may comprise a portion of an integrated avionics system. In other configurations, TAS  115  may be stand-alone unit operable for communication with avionics unit  110  as shown in  FIG. 2B . 
     One or more of the displays PFD  102 ( 1 ), PFD  102 ( 2 ), MFD  104  of the integrated avionics system  100  may be one of: a liquid crystal diode (LCD) display, a thin film transistor (TFT) LCD display, a light emitting polymer (LEP) or polymer light emitting diode (PLED) display, a cathode ray tube (CRT) display, and so forth, capable of displaying text and graphical information. Further, one or more of the displays PFD  102 ( 1 ), PFD  102 ( 2 ), MFD  104  may be backlit via a backlight such that it may be viewed in the dark or other low-light environments. 
     The integrated avionics system  100  may include a controller  120  which communicates with the avionics data bus  112 . The controller  120  may provide a user interface (e.g., a touch interface) for the pilot for controlling the functions of one or more of the displays PFD  102 ( 1 ), PFD  102 ( 2 ), MFD  104  and for entering navigational data into the system  100 . The avionics unit  110  may be configured for aggregating data and/or operating in an operating mode selected from a plurality of user-selectable operating modes based upon inputs provided via the controller  120 . 
     In some implementations, the controller  120  may include a touch interface configured as a touch screen (e.g., a touch panel overlaying a display) that can detect a touch input within the area of the display for entry of information and commands. In implementations, the touch screen may employ a variety of technologies for detecting touch inputs. For example, the touch screen may employ infrared optical imaging technologies, resistive technologies, capacitive technologies, surface acoustic wave technologies, and so forth. In implementations, buttons, keypads, knobs and so forth, may be used for entry of data and commands instead of, or in addition to, a touch screen. 
     As should be appreciated, the forgoing description of the system  100  is exemplary only and embodiments of the present invention may be employed in any avionics configuration. For example, in some configurations, avionics unit  110  may be configured as a stand-alone avionics unit such as a Garmin® GTN 650/750, GNS 430/530, and the like. The avionics unit  110  may be configured to couple with TAS  115  such as a Garmin® GTS 800 TAS or the like to receive traffic information therefrom. Embodiments of the invention may be implemented in the avionics unit  110 , TAS  115 , or components of the avionics system  100 , combinations thereof, and the like. 
     The avionics unit  110  may be configured to generate an air traffic display based upon the data that it receives and aggregates from the TAS  115 , such as the transponder, ADS-B system  114 , and/or the TCAS  116 . For example, the avionics unit  110  is illustrated as including a traffic display module  214 , which is storable in memory  212  and executable by the processor  210 . The traffic display module  214  is representative of mode of operation selection and control functionality to access the received data (e.g., air traffic data) and generate an air traffic display based upon the received and aggregated data. The generated air traffic display may then be provided to and displayed by one or more of the display device(s). In some configurations, traffic display module  214  may be fully or partially implemented by the TAS  115  and/or other traffic system. 
     In an implementation, an avionics unit  110  can display and/or indicate at least one air traffic target (e.g., an aircraft being tracked or monitored) and/or suppress the display of at least one air traffic target that is determined to be on ground (e.g., landed, taxing, parked at an airport, etc). An example of a displayed, generated air traffic display (e.g., a screenshot of the air traffic display) is shown in  FIG. 3A . The air traffic display can provide graphical depictions of air traffic that is located proximal to the aircraft in which the avionics unit  110  is implemented (e.g., in a three-dimensional vicinity surrounding the aircraft, which may be pre-determined or selectable by a flight crew member). For instance, in  FIG. 3A , an air traffic display  300  provides a graphical (e.g., iconic) representation of the aircraft  150  (e.g., the ownship aircraft) as a fixed central reference or focal point, while also showing graphical and/or iconic representations of other aircraft (e.g., air traffic targets)  302 ,  303 ,  304  located within airspace  306  (e.g., airspace surrounding the aircraft  150 ). The example display of  FIG. 3A  includes example Mode C targets  302 ,  303  and Mode S target  304 . As described below, Mode S target  304  is on-ground in the example of  FIGS. 3A and 3B  and would not be presented to the pilot in either example and therefore is illustrated in shaded line for discussion purposes. 
     In the implementations shown, the monitored airspace  306  covers up to a two (2) nautical mile radius around the aircraft  150 . However, a larger or smaller monitoring area (e.g., the area covered by the monitored airspace) may be selected to monitor a larger or smaller area as desired. Further, the air traffic display  300  can provide boundary markers (e.g., concentric rings  308 ,  310 ) for demarcating sub-zones within the monitored airspace. For instance, in  FIG. 3 , concentric rings  308 ,  310  are provided to demarcate a one (1) nautical mile radius and a two (2) nautical mile radius, respectively, around the aircraft  150 . 
     As mentioned above, the avionics unit  110  may be configured to aggregate data and/or operate in an operating mode (e.g., display mode) selected from a plurality of operator-selectable operating modes based upon inputs provided via the controller  120 . For example, the avionics unit  110  may be placed in one operating mode via the provided input(s), in which the avionics unit  110  aggregates data and provides an air traffic display comprised of a software-generated depiction to the display device(s) (e.g., PFD  102 ( 1 ), PFD  102 ( 2 ), or MFD  104 , avionics unit  110 ) which, when displayed, depicts selected air traffic target(s) (e.g., as icons representing aircraft  302 ,  304 ) on the display. 
     Generally, any of the functions described herein can be implemented using software, firmware, hardware (e.g., fixed logic circuitry), manual processing, or a combination of these implementations. The terms “module” and “functionality” as used herein generally represent software, firmware, hardware, or a combination thereof. The communication between modules in the integrated avionics system  100  of  FIG. 1  and/or the avionics unit  110  of  FIG. 2  can be wired, wireless, or some combination thereof. In the case of a software implementation, for instance, the module represents executable instructions that perform specified tasks when executed on a processor, such as the processor  210  of the avionics unit  110  shown in  FIG. 2 . The program code can be stored in one or more storage media, an example of which is the memory  212  associated with the avionics unit  110  of  FIG. 2 . While an avionics system  100  is described herein, by way of example, it is contemplated that the functions described herein can also be implemented in one or more independent (stand-alone) avionics units or systems implemented within an aircraft, such as an aircraft that does not include an integrated avionics system. 
     Example Procedures 
     The following discussion describes procedures that allow an air traffic display of an aircraft to suppress display and/or alerts pertaining to air traffic targets that are on-ground. Aspects of the procedures may be implemented in hardware, firmware, or software, or a combination thereof. The procedure is shown as a set of blocks that specify operations performed by one or more devices and are not necessarily limited to the orders shown for performing the operations by the respective blocks. In portions of the following discussion, reference will be made to the integrated avionics system  100  of  FIG. 1 , the avionics unit  110  of  FIGS. 2 a  and 2 b   , and the air traffic display  300  of  FIGS. 3A and 3B . 
       FIG. 4  illustrates a procedure, in an example implementation, in which an avionics system  100  and/or avionics unit  110  implemented on an aircraft may selectively suppress and/or display an aircraft target that is on-ground. 
     As illustrated in Block  400 , the system  100  may receive Mode C transponder data corresponding to an air traffic target. The terms “aircraft target” and/or “air traffic target” as used herein can include both aircraft and/or air traffic targets in the airspace surrounding the aircraft  150  and/or on the ground, such as at an airport. For example, with reference to  FIGS. 1 and 2 , the avionics unit  110  within the ownship aircraft  150  may receive data from the TAS  115  (e.g., transponder, ADS-B system  114 , the TCAS  116 , TIS-B  118 , etc.) which are communicatively coupled with the avionics unit  110 . In the example of  FIG. 3A , targets  302  and  303  represent Mode C air traffic targets. System  100 , such as avionics unit  110  and/or TAS  115 , may determine (or otherwise estimate) the relative location of the Mode C air traffic target in a generally conventional manner utilizing the Mode C transponder data corresponding to the Mode C air traffic target. Thus, in the example of  FIG. 3A , the relative location of targets  302 ,  303  with respect to the ownship aircraft  150  have been determined to provide the traffic display. 
     The received Mode C transponder data is generally conventional transponder data indicating the pressure altitude of the air traffic target. In configurations, the Mode C transponder data may include Mode 3/A data to also provide a transponder code corresponding to the air traffic target. Mode C transponders and associated data have long been used, and in some cases mandated, by aircraft within the United States and elsewhere. However, conventional Mode C data does not include an indication of aircraft on ground status. That is, aircraft on the ground at an airport may still respond to interrogations and broadcast (squawk) Mode C data even though the aircraft is not engaged in flight operations. These broadcasts may be of little use to a pilot flying near the airport and clutter the pilot&#39;s air traffic display. And, because the Mode C data lacks an on-ground indication, conventional traffic systems must use a radar altimeter to suppress Mode C traffic alerts. For example, a target aircraft on the ground at an airport with an elevation of 1000 ft MSL might broadcast Mode C data indicating a pressure altitude of 1000 ft (depending on atmospheric pressure). This “1000 ft” air traffic target might be displayed on the air traffic display of a pilot flying nearby the airport at 2000 ft MSL—even though the “1000 ft” air traffic target is on the ground. For instance, in the example of  FIG. 3A , air traffic target  303  could represent an on-ground target unless it is suppressed as discussed below. Further, broadcasts by ground targets may reflect off of buildings and other structures nearby airports to artificially multiply the number of on-ground targets. 
     Block  400  may include receiving Mode C transponder data from a plurality of aircraft. In the example of  FIG. 3A , targets  302 ,  303  represent Mode C targets. For instance, TAS  115  may receive Mode C transponder data from all Mode C target aircraft within its reception (monitor) range. Further, Block  400  may include receiving other types of data from Mode C target aircraft, including Mode A, Mode S, ADS-B, and other information. 
     In Block  402 , system  100  may receive traffic tracking data for a second target aircraft. For example, with reference to  FIG. 2B , the avionics unit  110  within the ownship aircraft  150  may receive data from the TAS  115  (e.g., transponder, ADS-B system  114 , the TCAS  116 , TIS-B  118 , etc.) which are communicatively coupled with the avionics unit  110 . In the example of  FIG. 3A , target  304  represents the second target aircraft. The traffic tracking data includes pressure altitude information and an on-ground indication for the second target aircraft. 
     In configurations, the traffic tracking data may include Mode S transponder data, ADS-B data, other target and traffic data, and/or combinations thereof. In configurations where the traffic tracking data includes Mode S, ADS-B, or related traffic data, the traffic tracking data includes one or more on-ground bits that indicate whether the second target aircraft is on ground. The traffic tracking data may additionally include location information for the second target aircraft, such as GPS-derived location information for the second target aircraft. 
     The traffic tracking data may be received by the system  100  directly from the second target aircraft, such as in situations where the second target aircraft is broadcasting Mode S, ADS-B, or other related data. The traffic tracking data may also be received by the system  100  from sources other than the second target aircraft, such as from ADS-B ground stations, air-to-ground datalinks, or other ground and air-based information sources that broadcast or otherwise transmit information regarding the location and status of air traffic targets. 
     Block  402  may include receiving traffic tracking data corresponding to a plurality of aircraft. For instance, TAS  115  may receive Mode S transponder data from all Mode S target aircraft within its reception (monitor) range. Further, Block  402  may include receiving other types of data regarding Mode S target aircraft, including Mode A, Mode S, ADS-B, and other information. 
     In embodiments, the system  100 , such as avionics unit  110 , may create a database of “on ground” altitudes based on received Mode S and/or other traffic tracking data. For example, the avionics unit  110  can store within memory  212  a table of on-ground locations (e.g., corresponding to GPS positions reported in Mode S broadcasts) and corresponding pressure altitudes (e.g., as reported in the Mode S broadcasts). Although pressure altitudes will likely vary over time due to changes in atmospheric conditions, the database stored within the memory  212  may be employed to check height above ground determinations made in Block  406  and cartographic data retrieved in Block  404 . 
     In some configurations, the second target aircraft may be the ownship aircraft  150 . Thus, the avionics unit  110  can use its own traffic tracking data, such as Mode S data and on-ground indication, to determine the difference between pressure altitude and height above ground (e.g., zero when the on-ground indication reports that the aircraft  150  is on the ground). This information may be used, as described below, to determine the height above ground of other aircraft such as targets  302 ,  303 . This information may also be used to populate the database described in the preceding paragraph. 
     In Block  404 , system  100  may access cartographic data. For example, the avionics unit  110  can receive location information from an airport database, for example in memory  212  of avionics unit  110 . In this embodiment, the airport database may include airport information, such as elevation, GPS position, and/or airport identification information. In a similar embodiment, the avionics unit  110  can receive location information from a terrain database stored in memory  212  of avionics unit  110 . The terrain database may additionally or alternatively be stored by other components of the system  100  (e.g., other line-replaceable units (LRUs) such as PFD  102 , etc.) and accessed by the avionics unit  110  through data bus  112 . In some configurations, the terrain database may be configured as a stand-alone database independent of the avionics unit  110 , PFD  102 , MFD  104 , TAS  115 , and other system  100  components. In such configurations, the terrain database may be accessed by avionics unit  110 , TAS  115 , and/or other system components through avionics data bus  112 . 
     In this embodiment, the terrain database can include information pertaining to the terrain and elevation of the terrain. For example, the terrain database may include a digital elevation model (DEM) such those utilized in a terrain awareness and warning system, synthetic vision display, etc. 
     In Block  406 , system  100  may determine height above ground for the first target aircraft, such as target aircraft  302  in the example of  FIG. 3A . As discussed above in Block  400 , the Mode C transponder data for the first target aircraft indicates the pressure altitude of the first target aircraft but lacks an on-ground indication. That is, the pressure altitude for the first target aircraft, by itself, does not provide an indication as to whether the first target aircraft is on the ground. Pressure altitude is unrelated to height above ground when the ground under the first target aircraft is not at sea level. Further, even in the situation where the first target aircraft is taxing at an airport at sea level, variations from standard temperature and pressure will cause the reported pressure altitude for the first target aircraft to vary from the target aircraft&#39;s height above the ground. In short, pressure altitude for an on-ground aircraft will rarely be zero. 
     In configurations, the height above ground for the first target aircraft is determined by the difference between the pressure altitudes for the first and second target aircraft when the on-ground indication for the second target aircraft indicates that the second target aircraft is on the ground. To aid in this comparison, the location (relative or geographic) of the second target aircraft may be compared to the location (relative or geographic) of the first target aircraft and the height above ground for the first target aircraft may be determined using the difference between the pressure altitudes of the first and second target aircraft when the first target aircraft is in proximity to the second target aircraft and the on-ground indication indicates that the second target aircraft is on the ground. 
     Referring to the example of  FIG. 3A , system  100  may determine the height above ground for targets  302  and  303  using the traffic tracking data for target  304 . Locations of each of the targets  302 ,  303 ,  304  may be determined by the system  100  utilizing the received Mode C data for targets  302 ,  303 , the traffic tracking data for target  304 , and/or other methods. The pressure altitude for each of the targets  302 ,  303 , and  304  is likewise known from the received Mode C data and traffic tracking data. If the traffic tracking data for target  304  indicates that target  304  is on ground, then the pressure altitude for target  304  will roughly equate to the height above ground for target  304 . 
     If target  304  is in proximity to targets  302 ,  303 , and target  304  is on ground, then the difference between the pressure altitudes for each of targets  302 , and  304  and the pressure altitude for target  304  may be used to determine the height above ground for targets  302 ,  303 . For instance, if target  304  reports a pressure altitude of 1080 ft. and is on-ground, target  302  reports a pressure altitude of 7000 ft., target  303  reports a pressure altitude of 1090 ft., and each of the targets  302 - 304  are in proximity to each other, then the height above ground for target  302  may be determined to be 5920 ft. and the height above ground for target  303  may be determined to be 10 ft. 
     In configurations where the system  100  has received data for a plurality of Mode C targets and a plurality of Mode S (or other traffic tracking data targets), then the above functionality may be utilized to determine the height above ground for one or more of the Mode C targets. Thus, for example, system  100  may receive Mode C transponder data from a first plurality of target aircraft, receive traffic tracking data from a second plurality of target aircraft, and determine height above ground for each of the first plurality of target aircraft using the received traffic tracking data from one or more of the second plurality of target aircraft. 
     In some implementations, the threshold for proximity between targets (e.g., targets  302  and  304 ) required to perform the above pressure altitude comparison may be a fixed distance, such as 0.25 nm, 0.5 nm, 1 nm, 5 nm, etc. In other implementations, the threshold may be dynamic and based on the flight profile and/or flight track of the targets. Thus, for example, moving targets or targets with a changing heading (or bearing) may be required to be in closer proximity (e.g., less than 0.25 nm) to enable the pressure altitude comparison described above. 
     Further, the cartographic data such as terrain elevation and airport elevation information accessed in Block  404  may be utilized to determine the necessary proximity between the targets to provide the pressure altitude comparison and/or the height above ground for the targets. For example, if the accessed cartographic data indicates that the terrain surrounding the targets does not substantially vary in elevation, then a greater threshold may be used. Similarly, if the elevation of the terrain underlying the targets does substantially vary, then a lesser threshold may be used. Additionally or alternatively, accessed airport data may be utilized to determine the proximity threshold. For example, if the geographic location of the Mode S target indicates that it is in proximity to an airport represented by the airport data, then a greater proximity threshold may be employed. Similarly, if the geographic location of the Mode S target indicates that it is not in proximity to an airport represented in the airport data, then a greater (or even infinite) threshold may be used. 
     Further, the pressure altitudes of the targets  302 ,  303  may be compared to known airport elevations corresponding to the location of the targets  302 ,  303  to roughly determine height above ground for the targets  302 ,  303 . Thus, for example, avionics unit  110  may be equipped with GPS or other location-determining components to determine its geographic location. The geographic location of Mode C targets  302 ,  303  may be determined using the known geographic location of the ownship aircraft  150  and the relative position between the ownship aircraft  150  and the targets  302 ,  303 . Once the geographic locations of the targets  302 ,  303  is known, the target locations may be compared with airport elevation data and/or terrain data (e.g., DEM) stored within the memory of the avionics unit  110  to determine ground elevation. The ground elevation corresponding to each target  302 ,  303  may be compared with the pressure altitude for each target  302 ,  303  to determine the height above ground for each target even in the absence of traffic tracking data for other aircraft. 
     In Block  408 , the height above ground for the ownship aircraft  150  may be determined. For example, if the aircraft  150  is in proximity to an air target having traffic tracking data, such as a Mode S aircraft like target  304  in the example of  FIG. 3A , then the pressure altitudes of the ownship aircraft  150  and the target may be compared to determine the height above ground of the ownship aircraft. In the example of  FIG. 3A  where target  304  reports on-ground and a pressure altitude of 1080 ft., and ownship aircraft  150  has a pressure altitude of 5000 ft., the height above ground for the ownship aircraft may be determined to be 3920 ft. 
     The proximity threshold utilized by the system  100  when providing the above comparison may be similar or the same to the threshold used when determining the height above ground for other targets such as targets  302 ,  303  of  FIG. 3A . Thus, fixed thresholds may be used (0.25 nm, 0.5 nm, 1 nm, etc.) as well as dynamic thresholds based on the flight profile and/or track of the ownship aircraft  150  and/or the accessed cartographic data. 
     The height above ground for the ownship aircraft  150  may be utilized for various purposes including traffic display and suppression. It may also be utilized for other purposes, such as to provide height above ground functionality for aircraft that lack radar altimeters, terrain awareness warning systems (TAWS), or other conventional methods for determining aircraft height above ground. Height above ground for the ownship aircraft  150  may also be used to declutter displayed targets, silence or suppress visual and aural alerts, and/or change the timing of alerts based on the proximity of the aircraft  150  to the ground. For example, aural alerts may be generated quicker if the aircraft  150  is near the ground while more geographically remote targets may be presented if the aircraft  150  is at altitude. 
     In Block  410 , the system  100  may suppress the display of the various air traffic targets. For instance, the system  100  may determine whether to display and/or suppress targets  302 - 304  on avionics display  300  using computer processor  210  and/or memory  212 . Conventional suppression and filtering methods may be used, such as distance and altitude, to control the appearance and display of air traffic targets. Additionally or alternatively, the height above ground determination made in Block  406  may be utilized to suppress the display of air traffic targets to reduce nuisance traffic alerts and display. In some embodiments, an audible alert may be suppressed by the avionics unit  110  instead of or in addition to a suppressed visual display. 
     Referring to the examples of  FIG. 3A  and  FIG. 3B , in Block  406 , the height above ground for target  302  was determined to be 5920 ft. and the height above ground for target  303  was determined to be 10 ft. Because target  303  is in proximity to the ground (e.g., 10 ft), its display is suppressed and not presented on the example display of  FIG. 3B . Because target  302  is not in proximity to the ground (e.g., 5920 ft.), its display is not suppressed and it is presented on the example display of  FIG. 3B . Note that target  304  may not be presented on either display ( FIG. 3A, 3B ) as its Mode S broadcast indicates that it is on the ground and therefore the height above ground comparison need not be computed to suppress its display. 
     The distance above ground threshold required to suppress the display of an air traffic target may be fixed or dynamic. Thus, for example, target aircraft with a determined height above ground less than 100 feet, 50 feet, 25 feet, or 10 feet may be determined to be “on ground” and therefore suppressed. Due to variances in barometric pressure and altimeter functionality and settings, two aircraft are unlikely to have mathematically identical pressure altitudes even if they are both parked next to each other on the ground. Dynamic thresholds may additionally be employed based on the flight profile and/or track of the targets and/or cartographic data accessed by the system  100 . For example, terrain variances underlying the targets and the ownship aircraft  150  may lessen the threshold used to determine when targets are suppressed (e.g., require a height above ground closer or near to zero). 
     CONCLUSION 
     Although the avionics system  100  and avionics unit  110  have been described with reference to example implementations illustrated in the attached drawing figures, it is noted that equivalents may be employed and substitutions made herein without departing from the scope of the disclosure as recited in the claims. Further, the integrated avionics system  100  and its components as illustrated and described herein are merely examples of a system and components that may be used to implement the present disclosure and may be replaced with other devices and components without departing from the scope of the present disclosure.