Abstract:
An aircraft surveillance system that includes an uninterruptible aircraft information broadcasting system (such as an ADS-B system) for transmitting flight information from an aircraft, and one or more ground stations for receiving the broadcast flight information. The flight information preferably includes the aircraft&#39;s aircraft ID, current three-dimensional position, and intended path. Optional components according to the invention include a system and method for validating messages received from the aircraft, and an alternative system and method for determining the aircraft&#39;s position if a primary navigation aid, such as GPS, is not available. In one embodiment of the invention, the system includes a transmitter and an uninterruptible power supply that are disposed within a secure housing within the aircraft. This serves to prevent passengers and crew members from disabling the system.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application Serial No. 60/324,169, which was filed Sep. 21, 2001 and entitled “Uninterruptible ADS-B System for Aircraft Tracking”, and which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to aircraft surveillance systems, and more particularly to secure aircraft surveillance systems that are configured for alerting air traffic controllers if an aircraft departs substantially from an expected flight path or pattern of behavior. 
     BACKGROUND OF THE INVENTION 
     The terrorist hijacking and suicide crashes of four U.S. civilian airliners on Sep. 11, 2001, were tragic and unforeseen. The inability of the air traffic control and civil defense infrastructure to respond effectively to the situation as it developed was due, in part, to limitations of the current radar-based surveillance infrastructure. 
     Because of timing delays and accuracy limitations inherent in the design of current radar-based air traffic control systems, controllers are unable to determine the real-time intent of aircraft they are tracking. Moreover, current radar surveillance systems depend on transponder responses from the aircraft to operate at maximum efficiency. The September 11 terrorists disabled or turned off some aircraft transponders in the cockpit of the hijacked aircraft, making it difficult to track the aircraft. This, combined with the limitations of current surveillance radar systems, made it nearly impossible for controllers to realize that three of the aircraft had turned into collision courses with ground targets. 
     Thus, there is a need for an aircraft surveillance system that cannot be interrupted by unauthorized persons. There is a further need for an improved surveillance system that is configured to warn ground controllers when an aircraft acts outside of cleared parameters. Furthermore, there is a need for systems that alert ground controllers if someone in the aircraft is attempting to send a false signal to deceive the controllers into believing that the aircraft is following a planned course when in fact it has been diverted. In addition, there is a need for an improved surveillance system that allows pilots to quickly and reliably issue a distress signal upon realizing that their aircraft is in danger of being hijacked. 
     SUMMARY OF THE INVENTION 
     The present invention meets the objectives set forth above by providing an on-board broadcast surveillance system and method that cannot be interrupted and that provides three-dimensional position information to ground or airborne controllers via a data transmitter. Optionally, the broadcast information may include the Ownship aircraft&#39;s aircraft ID and intended path. Other optional components according to the invention include a system and method for validating messages received from the aircraft, and an alternative system and method for determining the aircraft&#39;s position if a primary navigation aid, such as the Global Positioning System (“GPS”), is not available. 
     In a preferred embodiment, the invention equips an aircraft with a small uninterruptible Universal Access Transceiver/Automatic Dependent Surveillance—Broadcast (“UAT/ADS-B”) transmitter (or other suitable transmitter) that is capable of broadcasting aircraft ID, three-dimensional position and aircraft intent. The device contains an internal GPS receiver to determine aircraft position and has an uninterruptible power supply that can not be disabled from the cockpit. The device, which is preferably located in the aircraft&#39;s equipment bay, automatically begins broadcasting aircraft position, ID and intent the moment the aircraft is powered up and continues to broadcast that information, in real-time, until the aircraft is on the ground and shut down. Via an optional external interface, the device may receive and broadcast flight plan information from the on-board flight management computer. Such information is then made available for comparison to originally filed ATC flight plan data. This allows air traffic control (“ATC”) authorities to detect any inappropriate changes to the aircraft&#39;s intended destination shortly after the changes are entered into the aircraft&#39;s flight management computer. 
     The invention may further provide a ground infrastructure that preferably comprises a plurality of receivers for receiving the information transmitted from the aircraft. Preferably, about 600 low-cost UAT ADS-B receivers would be located throughout the continental United States to provide full CONUS real-time surveillance. The system would be equipped with the capability of determining whether the equipped aircraft were flying within the “normal” parameters of its flight plan. Upon detecting a deviation from the flight plan, the system would provide controllers and civil defense authorities with an immediate alert. If an aircraft deviated from its flight plan, its ADS-B signal would continue to provide real-time position, altitude, heading and intent of the aircraft to air traffic control and civil defense authorities. Airborne controller stations may optionally replace some or all ground controller stations. 
     A system and method for validating messages received from the aircraft may be based on the ability of the UAT to perform range measurements based on signal propagation time, giving rise to several advantages. More specifically, the system can determine whether the aircraft&#39;s reported position (i.e., the position transmitted by the aircraft) is valid by comparing a range that corresponds to the target&#39;s reported position with a range derived from signal propagation times. As an additional validity check, or if GPS service is unavailable, the position of a particular aircraft can be determined by receiving at least a partial State Vector (identity plus altitude) at multiple controller stations, and then using time of reception differentials among a network of such stations to compute lines of position. 
     Additional anti-spoofing can be accomplished by validating the last received position information with a dead reckoning algorithm. When implementing this functionality, any jumps in speed and velocity outside a preset limit are rejected. 
     In the event that GPS position information becomes unavailable, a dead reckoning function can be used to estimate the aircraft&#39;s position. Inexpensive inputs such as heading from a magnetic flux gate, changes in velocity from a solid state accelerometer, and altitude from an optional self-contained barometer can also be used to enhance the dead reckoning calculation. As will be understood by one skilled in the art, this self-contained barometer is optional because the aircraft is preferably equipped with an altimeter that is not affected by any loss of GPS position information. 
     The invention may further include a panic button within the cockpit of the aircraft that, when activated, transmits a signal to an aircraft information broadcasting system indicating that the Ownship aircraft is in danger of being hijacked. The aircraft information broadcasting system then immediately transmits a signal that triggers an appropriate alarm at a ground station. This aspect of the system is advantageous because it allows pilots to issue a distress signal without having to enter a complex code. Also, in a preferred embodiment of the invention, the aircraft information broadcasting system is preferably tamper-proof and located outside the aircraft&#39;s cockpit. As a result, it is unlikely that a hijacker would be able to disable the aircraft information broadcasting system before the pilot transmits a distress signal to air traffic control. 
     More particularly, a system according to a preferred embodiment of the invention comprises a housing defining an interior portion and an aircraft information broadcasting system that is disposed within the interior portion of the housing. In this embodiment of the invention, the system is configured for broadcasting flight-related information that is related to the Ownship aircraft&#39;s current flight. The housing is configured for restricting access to the aircraft information broadcasting system while the aircraft is in flight. 
     In one embodiment of the invention, the system is configured for broadcasting the flight-related information in real time and in a substantially continuous manner while the aircraft is in flight. In a particular embodiment of the invention, the flight-related information includes one or more of the following: (1) the aircraft&#39;s current flight ID, (2) the current three-dimensional position of the aircraft, (3) the aircraft&#39;s current velocity, (4) waypoint information for the aircraft; and (5) the aircraft&#39;s current intent information. 
     In another embodiment of the invention, the system&#39;s housing comprises a robust material that substantially surrounds the aircraft information broadcasting system and that thereby restricts access to the aircraft information broadcasting system. In a particular embodiment of the invention, this housing is configured to be locked while the aircraft is in flight. In one embodiment of the invention, the housing and the aircraft information broadcasting system are configured for being disposed within a portion of the aircraft that is outside of the aircraft&#39;s cockpit while the aircraft is in flight. The aircraft information broadcasting system is preferably located within the aircraft&#39;s equipment bay. 
     In yet another embodiment of the invention, the system comprises an information reception and analysis station that is configured for comparing intent information that is broadcast from the aircraft&#39;s aircraft information broadcasting system with approved flight path information to determine whether the aircraft is being directed toward an improper destination. In this embodiment of the invention, the system is preferably configured for generating an alarm if the intent information differs from the approved flight path information by more than a predetermined amount. 
     In a further embodiment of the invention, the system comprises an information reception and analysis station that is configured to validate information received from the aircraft information broadcasting system using triangulation techniques. Similarly, the system is preferably configured for generating an alarm if one or more measured aspects of the aircraft&#39;s current flight differs from the expected or measured flight characteristics of the aircraft. 
     An additional embodiment of the invention comprises a method of broadcasting information related to an aircraft. This method comprises the steps of: (1) providing a housing that defines an interior portion and that is configured for restricting access to the housing&#39;s interior portion while the aircraft is in flight; (2) providing an aircraft information broadcasting system that is configured for broadcasting flight-related information from the aircraft; (3) positioning the aircraft information broadcasting system within the interior portion of the housing; and (4) using the aircraft information broadcasting system to broadcast the flight-related information from the aircraft while the aircraft information broadcasting system is disposed within the interior portion of the housing and while the housing is restricting access to the interior portion. 
     Yet another embodiment of the invention comprises a method of tracking an aircraft. This method comprises the step of providing an aircraft with an information broadcasting system that is configured for broadcasting information related to a current flight of the aircraft in a substantially uninterruptible manner while the aircraft is in flight. 
     The invention described above is expressed in terms of systems and methods that are configured for executing, or that include, various steps. However, it will be understood by one skilled in the art that the invention also includes systems that are configured to perform, and computer-readable media that include computer-executable instructions for executing, the steps included within the above-described methods. Similarly, one skilled in the art will understand that the invention also includes methods that include, and computer-readable media that include computer-executable instructions for executing, the steps executed by the above-described systems. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein: 
     FIG. 1 is a block diagram of an aircraft information broadcasting system according to a preferred embodiment of the present invention. This figure depicts the interface between the aircraft information broadcasting system and various external sources of information. 
     FIG. 2 is a block diagram of an aircraft surveillance system according to a preferred embodiment of the invention. 
     FIGS. 3A and 3B depict a flowchart that generally illustrates a surveillance module according to the current invention. 
     FIG. 4 depicts a flowchart that generally illustrates an aircraft information analysis module according to the current invention. 
     FIG. 5 is a block diagram of an aircraft information broadcasting system according to an alternative embodiment of the present invention. Like FIG. 1, this figure depicts the interface between the aircraft information broadcasting system and various external sources of information. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. 
     Overview 
     The present invention provides an aircraft surveillance system that allows air traffic control and civil defense to track aircraft. In one embodiment of the invention, the system continuously transmits aircraft flight information to a computer (e.g., a ground-based “ground computer”) that compares current flight information with approved clearances. In a preferred embodiment of the invention, the system transmits this flight information in ADS-B format based on GPS position information. If the flight characteristics of the Ownship aircraft are outside of approved clearances, the system issues an alert to air traffic control and civil defense authorities, which can then act quickly to prevent the aircraft from being used for destructive purposes. As discussed in more detail below, one embodiment of this system is tamper-proof and is positioned in the equipment bay of the aircraft so that potential hijackers can not shut down or otherwise hinder the operation of the aircraft surveillance system. The system also preferably includes anti-spoofing functionality, and the capability to function even when GPS service is unavailable. The structure and operation of the system are discussed in greater detail below. 
     General Structure of the Aircraft Surveillance System 
     In one embodiment of invention, an Ownship Aircraft  95 , shown in FIG. 2, is outfitted with a small, robust aircraft information broadcasting system  10  that is configured for continuously transmitting aircraft flight information, preferably in real-time, while the aircraft is in flight. As shown in FIG. 1, this aircraft information broadcasting system  10  preferably comprises a power source  20 , an external interface  30 , an internal WAAS GPS Engine  40 , a UAT Transmitter  50 , and an ADS-B processor  5 . These components are preferably housed within a sturdy, tamperproof housing  12  (such as a steel or strong plastic housing) that may be locked during flight to prevent hijackers from tampering with the aircraft information broadcasting system  10 . In addition, as shown in FIG. 2, the aircraft information broadcasting system  10  is preferably stored within the equipment bay  15  of the Ownship aircraft  95  so that the aircraft information broadcasting system  10  may not easily be accessed during flight. The structure and functionality of the power source  20 , external interface  30 , internal GPS Engine  40 , UAT Transmitter  50 , and ADS-B Processor  5  are discussed in greater detail below. 
     The Power Source 
     To prevent hijackers from disabling the aircraft information broadcasting system  10  during flight, the aircraft information broadcasting system  10  is preferably equipped with a backup power source  20  that is independent of the aircraft information broadcasting system&#39;s  10  primary power source (which is not shown and which may be, for example, the aircraft&#39;s power bus). In a preferred embodiment of the invention, this backup power source  20  is a rechargeable battery that is automatically charged by the aircraft&#39;s primary power source. This allows the aircraft information broadcasting system  10  to operate without interruption even if power is no longer available from the aircraft information broadcasting system&#39;s primary power source. 
     The External Interface 
     As may be understood from FIG. 1, the aircraft information broadcasting system  10  is equipped with an external interface  30  for receiving information from various optional external navigational components within the Ownship aircraft. For example, in one embodiment of the invention, the external interface  30  is connected to the Ownship aircraft&#39;s altimeter  60 , flight management computer  70 , and control panel  80 . In a particular embodiment of the invention, the aircraft&#39;s altimeter  60 , flight management computer  70 , and control panel  80  are all located in the Ownship aircraft&#39;s cockpit  90 , and the external interface  30  is located outside of the aircraft&#39;s cockpit  90 . For example, in one embodiment of the invention, the external interface  30  is located in a sturdy, tamperproof housing  12  within the Ownship aircraft&#39;s equipment bay  15 . As is discussed in greater detail below, the external interface  30  is configured to receive information regarding the Ownship aircraft&#39;s altitude from the altimeter  60 , information regarding the Ownship&#39;s aircraft&#39;s flight path and flight plan from the Flight Management Computer  70 , and the Ownship aircraft&#39;s current flight ID and UAT configuration from the control panel  80 . 
     GPS Engine 
     The aircraft information broadcasting system  10  further includes an internal GPS Engine  40 , such as a Wide Area Augmentation System (WAAS) GPS Engine. This allows the aircraft information broadcasting system  10  to determine the latitude and longitude of the Ownship aircraft. In the alternative, the GPS Engine can be used to determine the altitude and three-dimensional position of the Ownship aircraft independently of other instrumentation within the Ownship aircraft. 
     Transmitter 
     As may be understood from FIG. 1, the aircraft information broadcasting system  10  includes a transmitter, such as a UAT Transmitter  50 , for transmitting ADS-B information from the aircraft information broadcasting system  10  to various ground stations  100  or aircraft. Such information may include, for example, the Ownship aircraft&#39;s location information, altitude information, velocity information, waypoint information and intended course information. 
     ADS-B Processor 
     The ADS-B Processor (e.g., a LDPU Series ADS-B link and display processing unit from UPS Aviation Technologies) includes computer hardware and software, along with interfaces to the other components of the aircraft information broadcasting system  10 . The ADS-B Processor implements necessary communication and control tasks that are performed by the ADS-B system. 
     Additional Components 
     In addition to the aircraft information broadcasting system described above, the Aircraft Surveillance System preferably comprises at least one ground station (GBT)  100  for receiving ADS-B signals from the aircraft information broadcasting system  10 . As may be understood from FIG. 2, each ground station  100  is preferably connected to a ground computer  102 , which is connected to the Air Traffic Control (ATC) computer network  104 . Accordingly, the ground computer  102  is configured to receive ADS-B information from the Ownship aircraft via the GBT  100 . As discussed in greater detail below, the ground computer  102  is further configured for receiving the Ownship&#39;s ATC approved clearances from the ATC network  104  and for automatically determining whether the Ownship aircraft has diverged substantially from the ATC approved clearances. For the purposes of this application, an “information reception and analysis station” may comprise at least one ground station  100  and at least one ground computer  102 . 
     Applicable Technology 
     Technology for use in implementing the aircraft information broadcasting system  10  is available from multiple vendors including UPS Aviation Technologies, Inc. of Salem, Oreg., namely: (1) ADS-B link and display processing unit (LDPU) Model AT9000 (2) ADS-B ground based transceiver Model GBT2000; and (3) Model UAT Datalink. Similarly, various components for use in this system are described generally in the following U.S. patents and patent applications, which are incorporated herein by reference: (1) U.S. Pat. No. 5,493,309 to Bjornholt, which issued on Feb. 20, 1991; and (2) published U.S. Patent Application 20020032528 to Lai, which was filed on Mar. 14, 2002. 
     Functionality of the Aircraft Surveillance System 
     In a preferred embodiment of the invention, the aircraft surveillance system continuously executes a surveillance module  200  while the Ownship aircraft  95  is airborne. As may be understood from FIG. 3, when executing the surveillance module  200 , the system preferably first transmits, at step  210 , intended course and related information (sometimes referred to as “intent”) from the Ownship aircraft&#39;s flight management computer  70  to the aircraft information broadcasting system  10 . Next, at step  220 , the system optionally transmits UAT Control Information from the Control Panel  80  to the aircraft information broadcasting system  10 . The system then proceeds to Step  230 , where it transmits the altitude of the Ownship aircraft  95  from the altimeter  60  to the aircraft information broadcasting system  10 . The system then advances to Step  240  where it uses the internal GPS engine  40  to determine the location, velocity, and (optionally) the altitude of the Ownship aircraft  95 . As will be understood by one of ordinary skill in the art, while steps  210 - 240  above are described as occurring in a particular sequence, these steps may be executed in any other order or at the same time. Furthermore, one or more of Steps  210 - 230  may optionally be excluded from the surveillance module  200 . This may be done, for example, in an embodiment of the invention in which the aircraft information broadcasting system  10  is autonomous in that it does not receive information from other aircraft control systems such as an altimeter  60 , flight management computer  70 , or control panel  80 . 
     After completing steps  210 - 240 , the system proceeds to step  250  where it uses the UAT transmitter  50  to transmit the Ownship aircraft&#39;s altitude information, location information, velocity information waypoint information and intended course information from the aircraft information broadcasting system  10  to the GBT  100 . The system then transmits, at Step  260 , the altitude information, location information, velocity information, waypoint information and intended course information from the GBT  100  to the Ground Computer  102 . The system then advances to Step  270  where it uses the ground computer  102  to execute the Aircraft Information Analysis Module  300 . The system then repeats Steps  210  through  270 . In one embodiment of the invention, the system repeats Steps  210  through  270  substantially continuously while the Ownship aircraft  95  is in flight. 
     It should be understood that other embodiments of the invention may be configured to transmit greater or fewer items of information from the aircraft information broadcasting system  10  to the GBT  100  than detailed above. For example, in one embodiment of the invention, the system is only configured to transmit altitude information, location information, velocity information, and waypoint information from the GBT  100  to the Ground Computer  102 . 
     As may be understood from FIG. 4, when executing the aircraft information analysis module  300 , the system first executes step  310  where the ground computer  102  receives, from the ATC Computer Network  104 , the ATC approved clearances that correspond to the Ownship aircraft&#39;s flight ID. Next, at step  320 , the ground computer  102  compares the altitude, location, velocity, waypoint and intended course information that was received from the GBT  100  with the ATC approved clearances that correspond to the Ownship aircraft&#39;s flight ID. At step  330 , the ground computer  102  then determines whether the altitude, location, velocity, waypoint, and intended course information that was received from the GBT  100  are each within a predetermined safety margin of the ATC approved clearances for the Ownship&#39;s current flight. If so, the system proceeds to Step  350  where it completes execution of the Aircraft Information Analysis Module  300 . If not, the system proceeds to Step  340  where it issues an alarm indicating that the information transmitted from the Ownship aircraft does not comport with approved clearances. For example, the system may issue an alarm by sending an electronic message from the ground computer  102  to an air traffic controller or to a civil defense authority. Military aircraft can be equipped with a monitor that displays information that has been transmitted from the aircraft information broadcasting system  10  or the GBT  100 . This information can help the pilots of the military aircraft track the Ownship aircraft  95 . 
     Returning for a moment to Step  330 , the safety margin for the various parameters received by the GBT is preferably chosen to minimize the occurrence of false alarms, and also to assure that the system detects any improper divergence from the ATC approved clearances early enough to take proper action. The system is preferably configured to allow a user to specify a different safety margin for each parameter. For example, the user may specify that if the Ownship aircraft&#39;s actual altitude is not within 5% of the approved altitude, the system should issue an alarm. Similarly, the user may specify that if the Ownship aircraft&#39;s actual velocity is not within 4% of the approved velocity, the system should issue an alarm. By the same token, the user may specify that if the intended course of the aircraft is substantially different from the ATC approved clearances, the system should issue an alarm. For example, the system is preferably configured to issue an alarm in response to the intended course of the aircraft and the ATC approved clearances differing by a pre-determined margin. In one embodiment of the invention, the system is configured to issue an alarm in response to the intended course of the aircraft differing from the aircraft&#39;s approved course by more than a predetermined number of nautical miles (e.g., 5 nautical miles). 
     A tracking file built in the ground computer  102  or the ATC network  104  can be used to predict a probable subsequent position and velocity for the aircraft. This predicted subsequent position and velocity may be compared to the position and velocity information received from the aircraft. If the predicted position and/or velocity differ from the received position and velocity information by more than a specified amount, the system preferably issues an alarm in the manner discussed above, or in any other suitable manner known in the art. 
     It should be understood that the modules executed by the ground computer  102  may alternatively be programmed into a computer within the ATC network  104 . 
     Anti-Spoofing Features 
     In view of the fact that terrorists are becoming increasingly sophisticated in their methods of hijacking aircraft, it is useful to provide a system with anti-spoofing features. Such anti-spoofing features may be used, for example, to detect the presence of a false ADS-B signal that hijackers might transmit from a hijacked aircraft to deceive the system into perceiving that the aircraft is flying according to its approved flight plan when it is actually flying off course. One such anti-spoofing feature is based on the ability of ground-based systems to use well-known techniques based on signal propagation times to derive the distance between a particular ground station  100  and the UAT/ADS-B transmitter  50  within the aircraft information broadcasting system  10 . The system may then compare this calculated “range value” with a range value that corresponds to the distance from the ground station to the Ownship&#39;s reported position. If these two range values do not agree within a predetermined safety factor, the system issues an appropriate alarm. This alarm may be used to alert the authorities that an false ADS-B signal is being transmitted from the aircraft. 
     As an additional validity check, the actual position of the Ownship aircraft  95  can be determined using well-known triangulation techniques, provided that multiple ground stations  100  receive at least a partial State Vector (identity plus altitude) of the Ownship aircraft  95 . The Ownship aircraft&#39;s triangulated position can then be compared with the position that is transmitted from the Ownship aircraft  95  to a ground station  100  via the UAT transmitter. If these two position values do not agree within a predetermined safety factor, the system issues an appropriate alarm. This alarm may be used to alert the proper authorities that a false ADS-B signal is being transmitted from the aircraft. 
     As a further validity check, the actual position of the Ownship aircraft  95  can be calculated using well-known dead reckoning algorithms. This calculated position is then compared with the position that is transmitted from the Ownship aircraft  95  to a ground station  100  via the UAT transmitter. If these two position values do not agree within a predetermined safety factor, the system issues an appropriate alarm. This alarm may be used to alert the proper authorities that a false ADS-B signal is being transmitted from the aircraft. 
     Position Loss Considerations 
     In the event that the internal GPS engine  40  becomes unable to generate position information, the position of the Ownship aircraft  95  can be determined using well-known triangulation techniques, provided that multiple ground stations  100  receive at least a partial State Vector (identity plus altitude) of the Ownship aircraft  95 . Alternatively, the system may use well-known dead-reckoning techniques to estimate the position of the Ownship aircraft  95 . As shown in FIG. 5, the aircraft information broadcasting system  10  may include inexpensive measurement devices such as a magnetic flux gate  410 , a solid state accelerometer  420 , and a self-contained barometer  430  to assist in making such dead-reckoning calculations. These dead-reckoning calculations may also be used in Step  330  above to determine whether various system parameters are within pre-determined safety margins of the ATC approved clearances for the flight. 
     Panic Button 
     Prior art systems allow pilots to enter a particular code within the aircraft navigation system to indicate that the aircraft is in danger of being hijacked. However, hijackers are often able to stop pilots from entering the code, or disable this functionality before an alert is sent to air traffic control. To solve this problem, the present system preferably includes a panic button within the cockpit of the aircraft that transmits a signal to the aircraft information broadcasting system  10  indicating that the Ownship aircraft  95  is in danger of being hijacked. The aircraft information broadcasting system  10  then immediately transmits a signal that triggers an appropriate alarm at the ground station  100 . This aspect of the system is advantageous because it allows pilots to issue a distress signal without having to enter any sort of code, and because, due to the fact that the aircraft information broadcasting system is tamper-proof and preferably located outside of the aircraft&#39;s cockpit  90 , it is unlikely that a hijacker would be able to disable the aircraft information broadcasting system before the pilot transmits a distress signal to air traffic control. 
     Ground Station Technical Considerations 
     The number of ground stations  100  needed for proper surveillance coverage may vary based upon the desired low altitude threshold for coverage. This is because line-of-site communication is required between the aircraft information broadcasting system  10  and the ground stations  100  that are receiving signals from the Surveillance Unit  10 . The following chart gives some typical values for range vs. altitude. 
     
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Altitude (AGL, 
                   
               
               
                   
                 feet) 
                 Range (nmi) 
               
               
                   
                   
               
             
             
               
                   
                  1,000 
                  40 
               
               
                   
                  5,000 
                  85 
               
               
                   
                 10,000 
                 120 
               
               
                   
                 15,000 
                 150 
               
               
                   
                   
               
             
          
         
       
     
     Based on the above chart, if coverage down to 1,000 feet above ground level (AGL) is desired, a ground station  100  would need to be installed every 40 to 80 miles, depending on how much overlap is desired to support the anti-spoofing and position-loss functionality described above. Assuming that every ground station  100  can cover a 40 nmi radius area on average, each base station site can provide surveillance over a 5,000 square mile area. With approximately 3.0 million square miles in the continental United States, approximately 600 ground station sites would be needed to cover the entire continental United States. 
     One embodiment of the invention comprises a hexagonal “cellular” grid of ground stations  100 , spaced 100 nmi apart. This is estimated to give coverage down to 3,000 feet AGL. In this embodiment of the invention, each ground station  100  provides coverage over a 70 nmi radius, or 15,000 square miles. As will be understood by one skilled in the relevant technical field, the exact configuration of these ground stations  100  is determined taking into account various terrain-specific factors such as terrain blockages. 
     It should be understood that controller functions, the GBT  100 , ground computer  102 , and computers within the ATC computer network  104  may alternatively be airborne. For example, these components may be located within an AWACS aircraft rather than on the ground. 
     CONCLUSION 
     Many modifications and other embodiments of the invention will come to mind to one skilled in the art to which this invention pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. For example, it should be understood that, while the system is described as being to configured to transmit the Ownship aircraft&#39;s flight-related information in ADS-B format, the system may be configured to transmit this information in any other format known in the art. Accordingly, it should be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.