Abstract:
A transponder lock provides allows an aircraft transponder to continue transmitting an alerting code for the duration of an emergency. The transponder lock thereby enables the aircraft transponder to continue to provide crucial information to ground tracking stations. Installation of the transponder lock results in very little aircraft downtime because the transponder lock connects in-line with existing aircraft wiring.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Provisional Application Ser. Number 60/332,997, titled “Transponder Lock”, and filed Nov. 16, 2001, the disclosure of which is expressly incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to the field of avionics. In particular, this invention relates to a transponder lock that allows an aircraft transponder to continue transmitting an alerting code for the duration of an emergency. 
     BACKGROUND OF THE INVENTION 
     An enormous amount of air traffic, carrying thousands upon thousands of human lives, moves across the skies around the world every day. Coordinating the safe takeoff, flight, and landing of each aircraft requires a sophisticated air traffic control network. In the United States, for example, the airspace is divided into 21 zone or centers, and each zone is divided into sectors. Within the zones are 50-mile diameter portions of airspace called TRACON (Terminal Radar Approach CONtrol) airspaces within which lie individual airports with 5-mile radius airspace. 
     An aircraft provides critical information during flight to help the air traffic control network coordinate air traffic. In particular, once an aircraft takes off, the pilot activates an aircraft transponder. The transponder detects incoming radar signals and, in response, broadcasts an encoded transponder signal in the return direction. The transponder signal typically includes the ATC assigned 4-digit transponder code (aircraft flight number) and altitude. As a result, radar units operated by air traffic controllers are able to display a symbol representing the aircraft, and the radar and air traffic controller may follow the plane throughout its flight. 
     Without the information provided by the transponder, air traffic control has reduced ability to determine where, how high, or how fast an aircraft is flying. Not only does the lack of transponder information present an unacceptable risk to those on board the aircraft, it also presents an unacceptable risk to passengers on other aircraft nearby, and individuals and property on the ground. In the past, however, aircraft transponders were unduly susceptible to manipulation. 
     The Sep. 11, 2001 terrorist attacks on the World Trade Center and the Pentagon provide a grim example of the vulnerability of aircraft transponders. According to a CBS News report (retrievable as of Apr. 3, 2002, for example at http://www.kutv.com/now/story/0,1597,311657-412,00.shtml), each transponder on board the four hijacked aircraft was simply shut off. As a result, air traffic control was deprived of vital information concerning where the planes were headed. It is conceivable, had air traffic control known where the planes were heading and how high they were flying, that warnings, evacuations, or other safety steps could have been initiated and directed to suspected target areas. 
     Another problem is that aircraft idle time caused by grounding the plane to retrofit improved avionics results in an enormous cost to the airline operating the aircraft. Furthermore, the specifications that govern aircraft design are often so strict that retrofitting improved avionics cannot be accomplished without significant investment of time, money, and resources. 
     A need has long existed for a mechanism that provides continued transponder operation in the event of emergency situations. 
     SUMMARY OF THE INVENTION 
     The transponder lock allows an aircraft transponder to continue transmitting, for example, an alerting code or other information or emergency indicia for the duration of an emergency. The transponder lock connects quickly and cleanly in-line with existing aircraft wiring, thereby minimizing aircraft downtime arising from installation of the transponder lock. The absence of installation complexity permits, for example, installing the transponder lock at an airport gate, rather than requiring an airline to incur substantial time and expense penalties arising from flying each aircraft to a remote retrofitting hangar. 
     Other features, apparatus, methods, and advantages of the present invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the present invention, and be represented by the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a transponder lock inserted between side- 1  and side- 2  control panels and side- 1  and side- 2  transponders. 
     FIG. 2 shows a generalized circuit diagram of the transponder lock. 
     FIG. 3 shows a wiring harness for dual transponder installation. 
     FIG. 4 shows a wiring harness for connecting auxiliary functions to the transponder lock. 
     FIG. 5 shows detailed circuitry for implementing the control, detection, and signaling functionality of the transponder lock. 
     FIG. 6 shows a flow diagram for the operation of the control circuitry. 
     FIG. 7 shows a detailed circuit diagram for the power supply section of the transponder lock. 
     FIG. 8 shows a detailed connection diagram for placing the transponder lock in-circuit with the control panels and transponders. 
    
    
     DETAILED DESCRIPTION 
     Turning to FIG. 1, that figure presents a transponder lock  100  in place between a side- 1  control panel  102 , a side- 2  control panel  104 , a side- 1  transponder  106 , and a side- 2  transponder  108 . As will be explained in more detail below, a connection harness  110  allows the transponder lock  100  to connect in place quickly and without substantial rewiring of the aircraft. In addition, a wiring harness  112  may be optionally included to provided direct hijack switch inputs, auxiliary power, and a hijack mode output, as described in more detail below. 
     The side- 1  control panel  102  supports the control interface for the side- 1  transponder  106 , while the side- 2  control panel  104  supports the control interface for the side- 2  transponder  108 . The control interfaces are conventional interfaces and typically include a display and the inputs needed to operate a transponder. The inputs include, as examples, an input for channeling a new code for transponder, as well as an operational mode switch that includes a standby and an on setting. Typically, only one transponder is operational at a given time, although the invention may be configured to work with multiple transponders if desired. 
     Turning next to FIG. 2, that figure shows an exemplary implementation of the transponder lock  100 . The transponder lock  100  includes a side- 1  control panel connector  202  (that leads to the control interface for the side- 1  transponder), a wiring harness connector  204  (that leads to a connection harness explained below), and an aircraft wiring connector  206  (which leads to the side- 1  transponder itself). The transponder lock  100  also includes control circuitry  208 , a primary power supply  210 , a secondary power supply  212 , and a backup power supply selector  214 . A primary power input  236  and a secondary power input  238  are shown in FIG. 2 as well. 
     The transponder lock  100 , as shown, also includes a side- 1  transponder selector  216  and a side- 2  transponder selector  218 . A side- 1  auxiliary selector  220  and a side- 2  auxiliary selector  222  are also provided. 
     As shown in FIG. 2, the side- 1  control panel connector  202  passes many control panel signals unmodified to the aircraft wiring connector  206 . However, the control panel connectors  202  (and  302 ), through the wiring harness connector  204 , also provide certain inputs that the transponder lock  100  passively monitors but manipulates when the transponder lock  100  becomes active. In particular, the side- 1  control panel connector  202  supplies a side- 1  transponder code input  224 , a side- 1  antenna transfer input  226 , and a side- 1  standby input  228 . Similarly, the wiring harness connector  204  supplies a side- 2  transponder code input  230 , a side- 2  antenna transfer input  232 , and a side- 2  standby input  234 . 
     The control circuitry  208  provides a transponder alerting output  240 , a transponder override output  242 , and a backup power supply enable  244 . As will be explained in more detail below with regard to FIG. 5, the control circuitry  208  functionally includes a transponder code detector coupled to one of the transponder code inputs  224 ,  230  (in this case, the transponder code inputs  224 ) and the transponder override output  242 . The control circuitry  208  further functionally includes a transponder signal generator coupled to the transponder alerting output  240  to generate a transponder code independently of the control panel interfaces to the transponders. 
     The transponder code inputs  224 ,  230  are preferably ARINC-429 compatible inputs. The transponder code inputs  224 ,  230  are thus differential inputs and connect to the control circuitry  208  through the ARINC-429 receiver  246 . In the implementation illustrated herein, only the side- 1  transponder control input  224  is connected to the control circuitry  208 . The side- 2  input  230  control code information is assumed to be identical by control panel design. The transponder alerting output  240  connects back into the aircraft wiring through the ARINC-429 transmitter  248 . 
     FIG. 3 shows the connection harness  110  for the side- 1  and side- 2  transponder installation shown in FIG.  2 . The connection harness  110  includes a side- 2  control panel connector  302  (which leads to the control interface for the side- 2  transponder), a wiring harness connector  304  (which connects to the wiring harness connector  204 ), and an aircraft wiring connector  306  (which leads to the side- 2  transponder itself). 
     As shown in FIG. 3, many of the connections pass uninterrupted via the control panel connector  302  to the aircraft wiring connector  306 . As shown above with regard to FIG. 2, however, the side- 2  transponder code input  308 , the side- 2  antenna transfer input  310 , and the side- 2  standby input  312  connect first to the transponder lock  100  through the wiring harness connector  304 , then back to the side- 2  transponder through the aircraft wiring connector  306 . Thus, the transponder lock  100  may, under the conditions described above, manipulate the inputs  308 - 312 . 
     Note also that the wiring harness connector  304  and the aircraft wiring connector  306  provide for auxiliary connections. The auxiliary connections include a secure power input  314 , hijack switch inputs  316  and  318 , and a hijack indicator output  320 . 
     FIG. 4 shows a wiring harness  112  for connecting the auxiliary functions to the transponder lock  100  through the aircraft wiring connector  306 . The signals shown may also be connected directly to the wring harness connector  204  in installations that have only one transponder. In particular, the wiring harness  112  allocates pins for a secure power connection  402 , hijack switches  404  and  406 , and for a hijack indicator output  408 . 
     The secure power connection  402  connects to a secondary power input (e.g., a 115 VAC 400 Hz power source) for the transponder lock  100 . The hijack switches  404  and  406  provide inputs (e.g., discretely located in the cockpit) that allow personnel to signal an emergency situation without having to channel in a code into the transponder. To that end, the transponder lock  100  monitors the hijack switch inputs  316  and  318  to determine whether one of the hijack switches  404  or  406  has been depressed. If so, the transponder lock  100  immediately enters override mode, rather than waiting to detect a selected transponder lock code. 
     Referring back to FIG. 1, the transponder lock  100 , using the connection harness  110  and the auxiliary wiring harness  112  connects inline between the control panels and the aircraft wiring. No rewiring of the aircraft is required, and the installation can be performed quickly. 
     Note that the transponder lock  100  may be used in single sided installations as well. In other words, the transponder lock  100  may connect between a single control panel and a single transponder. In a single sided configuration, the selectors  218  and  222  may be eliminated. If the auxiliary inputs (e.g., the hijack switch inputs  250  and  252  and the backup power input  238 ) are not used (or they are provided using a different connector), the wiring harness connector  204  (and connection harness  110 ) may be eliminated also. 
     In the single sided implementation, the transponder lock  100  functions in the same manner as described above. Note, however, that the transponder lock  100  does not need to determine with which of the multiple transponders to communicate. Rather, the transponder lock  100  provides transponder codes for the single transponder, and holds the transponder standby signal for that transponder in an unasserted state. 
     In one implementation, the side- 1  control panel connector  202  is a M83723/75R16247 circular connector with pin assignments shown below in Table 1. 
     
       
         
               
               
             
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Pin # 
                 Function 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 Panel Light High 
               
               
                 2 
                 Panel Light Low 
               
               
                 3 
                 115 V AC High 
               
               
                 4 
                 115 V AC Low 
               
               
                 5 
                 Antenna Transfer 1 (IN) 
               
               
                 6 
                 DC Ground 
               
               
                 7 
                 Standby/On 1 (IN) 
               
               
                 8 
                 Chassis Ground 
               
               
                 9 
                 Remote Test 
               
               
                 10 
                 Valid Output 
               
               
                 11 
                 (ARINC 429 IN A) side-1 transponder code 
               
               
                 12 
                 (ARINC 429 IN B) side-1 transponder code 
               
               
                 13 
                 (5VAC Monitor Hot) 
               
               
                 14 
                 (5VAC Monitor Cold) 
               
               
                 15 
                 Air/Ground Switch 
               
               
                 16 
                 Altitude Air Select 
               
               
                 17 
                 Altitude Compare ON/OFF 
               
               
                 18 
                 Monitor Light Power 
               
               
                 19 
                 Altitude Compare Fail #1 
               
               
                 20 
                 Transponder Fail #1 
               
               
                 21 
                 Display Test 
               
               
                 22 
                 429 1 A (IN) 
               
               
                 23 
                 429 1 B (IN) 
               
               
                 24 
                 Air/Ground 
               
               
                   
               
             
          
         
       
     
     Similarly, the aircraft wiring connector  206  may be a M83723/72R16247 circular connector with a pin assignment shown below in Table 2. 
     
       
         
               
               
             
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Pin # 
                 Function 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 Panel Light High 
               
               
                 2 
                 Panel Light Low 
               
               
                 3 
                 115 V AC High 
               
               
                 4 
                 115 V AC Low 
               
               
                 5 
                 Antenna Transfer 1 (OUT) 
               
               
                 6 
                 DC Ground 
               
               
                 7 
                 Standby/On 1 (OUT) 
               
               
                 8 
                 Chassis Ground 
               
               
                 9 
                 Remote Test 
               
               
                 10 
                 Valid Output 
               
               
                 11 
                 (ARINC 429 IN A) 
               
               
                 12 
                 (ARINC 429 IN B) 
               
               
                 13 
                 (5VAC Monitor Hot) 
               
               
                 14 
                 (5VAC Monitor Cold) 
               
               
                 15 
                 Air/Ground Switch 
               
               
                 16 
                 Altitude Air Select 
               
               
                 17 
                 Altitude Compare ON/OFF 
               
               
                 18 
                 Monitor Light Power 
               
               
                 19 
                 Altitude Compare Fail #1 
               
               
                 20 
                 Transponder Fail #1 
               
               
                 21 
                 Display Test 
               
               
                 22 
                 429 1 A (OUT) 
               
               
                 23 
                 429 1 B (OUT) 
               
               
                 24 
                 Air/Ground 
               
               
                   
               
             
          
         
       
     
     In one implementation, the side- 2  control panel connector  302  is a M83723/75R16248 circular connector with pin assignments shown below in Table 3. 
     
       
         
               
               
             
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 Pin # 
                 Function 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 Not Connected 
               
               
                 2 
                 Not Connected 
               
               
                 3 
                 115 V AC High 
               
               
                 4 
                 115 V AC Low 
               
               
                 5 
                 Antenna Transfer 2 (IN) 
               
               
                 6 
                 DC Ground 
               
               
                 7 
                 Standby/On 2 (IN) 
               
               
                 8 
                 Chassis Ground 
               
               
                 9 
                 Not Connected 
               
               
                 10 
                 Valid Output 
               
               
                 11 
                 (ARINC 429 IN A) side-2 transponder code 
               
               
                 12 
                 (ARINC 429 IN B) side-2 transponder code 
               
               
                 13 
                 Not Connected 
               
               
                 14 
                 Not Connected 
               
               
                 15 
                 Air/Ground Switch 
               
               
                 16 
                 Altitude Air Select 
               
               
                 17 
                 Altitude Compare ON/OFF 
               
               
                 18 
                 Monitor Light Power 
               
               
                 19 
                 Altitude Compare Fail #2 
               
               
                 20 
                 Transponder Fail #2 
               
               
                 21 
                 Display Test 
               
               
                 22 
                 429 2 A (IN) 
               
               
                 23 
                 429 2 B (IN) 
               
               
                 24 
                 Air/Ground 
               
               
                   
               
             
          
         
       
     
     The aircraft wiring connector  306  may be a M83723/72R16248 circular connector with a pin assignment shown below in Table 4. 
     
       
         
               
               
             
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                 Pin # 
                 Function 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 Secured Power 115VAC 
               
               
                 2 
                 Hijack Switch 1 
               
               
                 3 
                 115 V AC High 
               
               
                 4 
                 115 V AC Low 
               
               
                 5 
                 Antenna Transfer 2 (OUT) 
               
               
                 6 
                 DC Ground 
               
               
                 7 
                 Standby/On 2 (OUT) 
               
               
                 8 
                 Chassis Ground 
               
               
                 9 
                 Hijack Switch 2 
               
               
                 10 
                 Valid Output 
               
               
                 11 
                 (ARINC 429 IN A) 
               
               
                 12 
                 (ARINC 429 IN B) 
               
               
                 13 
                 Hijack Mode Discrete (OUT) 
               
               
                 14 
                 Not Connected 
               
               
                 15 
                 Air/Ground Switch 
               
               
                 16 
                 Altitude Air Select 
               
               
                 17 
                 Altitude Compare ON/OFF 
               
               
                 18 
                 Monitor Light Power 
               
               
                 19 
                 Altitude Compare Fail #2 
               
               
                 20 
                 Transponder Fail #2 
               
               
                 21 
                 Display Test 
               
               
                 22 
                 429 2 A (OUT) 
               
               
                 23 
                 429 2 B (OUT) 
               
               
                 24 
                 Air/Ground 
               
               
                   
               
             
          
         
       
     
     The wiring harness connector  204  may, for example, have a pin assignment shown below in Table 5. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
               
                 Pin # 
                 Function 
                 Notes 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 DC Ground 
                   
               
               
                 2 
                 Chassis Ground 
               
               
                 3 
                 Secured Power 
               
               
                   
                 115VAC 
               
               
                 4 
                 115 V AC Low 
               
               
                 5 
                 Antenna Transfer 2 (IN) 
               
               
                 6 
                 Antenna Transfer 2 (OUT) 
               
               
                 7 
                 Standby/On 2 (IN) 
               
               
                 8 
                 Standby/On 2 (OUT) 
               
               
                 9 
                 Antenna strap 
               
               
                 10 
                 Transponder 2 Enable 
               
               
                 11 
                 429 2 A (IN) 
               
               
                 12 
                 429 2 B (IN) 
               
               
                 13 
                 429 2 A (OUT) 
               
               
                 14 
                 429 2 B (OUT) 
               
               
                 16 
                 Hijack Switch 1 
                 Captain&#39;s switch 
               
               
                   
                 Hijack Switch 2 
                 First Officer&#39;s switch 
               
               
                 18 
                 Hijack Mode Discrete 
               
               
                   
                 (OUT) 
               
               
                   
               
             
          
         
       
     
     Turning next to FIG. 5, that figure illustrates a detailed circuitry  500  for implementing the control, detection, and signaling functionality of the transponder lock  100  present in the control circuitry  208 . The control circuitry  208  includes a microcontroller  502  (in this implementation, an Amtel AT90S8515) executing instructions out of an on-board Flash and EEPROM memory. The microcontroller  502  and the instructions in memory implement transponder code detector functionality and transponder signal generator functionality described above. 
     Although the operation of the transponder lock  100  will be described below with reference to the microcontroller implementation shown in FIG. 5, it is noted that in other implementations, the control circuitry  208  may be replaced by discrete logic, multiple microcontrollers, and the like. Thus, for example, while the microcontroller  502  implements both the transponder code detector functions and the transponder signal generator functions, those functions may be replaced by individual discrete logic circuits or by individual microcontroller circuits. 
     As shown in FIG. 5, the receiver  246  and transmitter  248  are implemented using a HI-8588 and a HI-8586, respectively, available from Holt Integrated Circuits Inc. The microcontroller  502  monitors the operational voltages used in the transponder lock  100  for over-voltage conditions. To that end, as shown in FIG. 5, the 5V (VCC), 26V, 13.5V and −13.5V supplies are routed through conditioning circuitry to the microcontroller  502  for monitoring purposes. 
     The primary power supply  210  provides DC power for the control circuitry from the AC power supplied on the primary power input  236 . Typically, the primary power input  236  supplies a 115 VAC 400 Hz signal that the primary power supply  210  converts to the DC voltage appropriate for running the control circuitry  208  (e.g., 5 Volts). 
     The control circuitry  208  monitors the voltage provided by the primary power supply  210  and the secondary power supply  212  by reading a digital value on the primary power supply monitor input  504  and the secondary power supply monitor input  506 . In other implementations, a microcontroller may use an internal A/D converter to provide a direct representation of the voltage, and periodically compare the voltage to a minimum reference point. If the primary power supply voltage falls below the minimum reference point, or if the digital value reads zero instead of one, then the microcontroller may assert the backup power supply enable  244  to connect the secondary power input  238  to the rest of the aircraft as a backup power supply. To that end, the backup power supply selector  214  may be implemented using a relay (or, alternatively, solid state switches) activated by the backup power supply enable  244 . Thus, a loss of primary power, whether deliberate or accidental, will not disable the transponder lock  100  when a secondary power source is connected to the secondary power input  238 . 
     A portion of the instructions in the memory instruct the microcontroller  502  to act as a transponder code detector. To that end, the microcontroller  502  monitors the digital communications present on the side- 1  transponder code input  224  by periodically reading the contents of the data present on the transponder code input  224 . In the absence of a preselected transponder lock code, the transponder lock  100  leaves the transponder override output  242  unasserted. As a result, normal transponder operation occurs, and transponder signals from the control panel pass to the side- 1  and side- 2  transponders (although, as noted above, only one of the transponders is active at a given time). 
     However, when the control circuitry  208  detects the transponder lock code (e.g., code 7500 input at the control panel), then the transponder lock  100  switches into an override mode. The override mode is thus generally responsive to the presence of an emergency condition (e.g., a hijacking). In override mode, the transponder lock  100  asserts the transponder override output  242 . 
     The selectors  216 ,  218 ,  220 , and  222  may be implemented using one or more relays (or, alternatively, solid state switches) activated by the transponder override output  242 . As shown in FIG. 2, when the transponder override output  242  is active, the transponder lock  100  delivers the transponder alerting output  240  to either the side- 1  transponder or side- 2  transponder. In particular, the transponder lock  100  communicates with one transponder while placing the other in standby mode. The microcontroller  502  may place a transponder in standby mode by connecting the side- 1  standby input  228  or side- 2  standby input  234  to a standby level (e.g., grounded as opposed to left open) through the selectors  220  or  222 , while the remaining standby input is held at an active level (e.g., left open). The choice of side- 1  or side- 2  transponder may be made, for example, depending on a discrete input signal (e.g., the side- 2  antenna transfer input  232 ), or may be made by default to the side- 1  or side- 2  transponder in every case. 
     At the same time, the instructions in the memory instruct the microcontroller  502  to operate as a transponder signal generator. When operating as a transponder signal generator, the control circuitry  208  outputs a preselected transponder alerting signal. The transponder alerting signal is output through the ARINC-429 transmitter  248  and comprises a selected transponder code (e.g., code 7500), or any other preselected code to indicate an emergency situation. Thus, in override mode, the transponder lock  100  provides the code to the transponder. 
     As a result, subsequent tampering or manipulation of the transponder control panel after the transponder lock code is detected will have no effect on the operation of the transponders. Rather, one of the transponders will continue to transmit the selected transponder alerting signal (e.g., code 7500) provided by the transponder lock  100 . 
     In some implementations, one or more emergency switches may be connected to the emergency switch inputs  250  and  252 . Thus, in addition to monitoring for the transponder lock code, the control circuitry  208  may monitor for activation of the emergency switches. When the control circuitry  208  detects that one or more of the emergency switches has been depressed (e.g., by the detecting the presence of an emergency switch signal asserted on an emergence switch input), the control circuitry  208  immediately switches into override mode. 
     Note further that the transponder lock  100  may indicate switching into override mode by giving feedback to the personnel in the cockpit. As one example, the control circuitry  208  may assert the lock indicator output  254  for a predetermined amount of time (e.g., 3 seconds). Asserting the lock indicator output  254  (which couples to the transponder control display test signal) activates all the display elements on the transponder control panel to provide visual feedback that the transponder lock  100  has entered override mode. In addition, as shown in FIG. 2, the transponder override output  242  may be routed to a discrete emergency output  256  for connection to any additional downstream circuitry desired. 
     Note that the microcontroller  502  may also perform self test diagnostics. If a failure is detected, the microcontroller  502  may then assert the lock indicator output  254  for a longer period of time (e.g., 30 seconds) to indicate the failure. 
     The control circuitry  208  remains in override mode until a preselected transponder unlock code (e.g., code 3785) is input. In other words, the control circuitry  208 , even in override mode, continues to monitor the transponder code input  224  searching for the transponder unlock code. Optionally, the control circuitry  208  may further require that the air/ground input  258  indicate that the aircraft is on the ground before leaving override mode. 
     Once out of override mode, the transponder lock  100  deasserts the transponder override output  242 . Normal transponder operation under control of the control panels then resumes. 
     With respect next to FIG. 6, that figure shows a flow diagram  600  of the operation of the control circuitry  208 . The control circuitry is initially passive, but monitors the transponder code input  224  for an emergency code, and monitors the emergency switch inputs  250  and  252  for activation of a hijack switch  404 ,  406  (steps  602 ,  604 ). If detected, the control circuitry  208  asserts the transponder override output  242  and disables the standby for the transponder (step  606 ). In other words, the control circuitry  208  enters override mode. 
     The transponder lock  100  subsequently provides a selected emergency transponder code to the transponder (step  608 ). In the meantime, the transponder lock  100  monitors the transponder code input  224  for a predetermined unlock code, and optionally checks to see that ground status is indicated for the aircraft (steps  608 ,  610 ). If the unlock code is detected, then control circuitry  208  deasserts the transponder override output  242 , and enables the standby for the transponder (step  614 ). In other words, the transponder lock  100  leaves override mode. 
     FIG. 7 shows a detailed circuit diagram for one implementation of the power supply section  700  of the transponder lock. The power supply section  700  includes a primary power input  236 , a secondary power input  702 , a primary power monitor output  704 , and a secondary power monitor output  706 . A primary bridge  708  and a secondary bridge  710  independently provide full wave rectification of input AC voltages. The rectified output  718  carries an approximately 26 volt DC signal. 
     FIG. 7 also shows a first DC to DC converter  712 , a second DC to DC converter  714 , and a third DC to DC converter  716 . The first DC to DC converter  712  (e.g., an LT 1956-5 available from Linear Technology) accepts the voltage from the rectified output  718  and produces a 5 Volt VCC output  720  that powers the digital circuitry of the transponder lock  100 . The VCC output  720  is also supplied to the second DC to DC converter  714  (e.g., an LT 1930) to generate a 13.5 Volt DC auxiliary output  722 . In addition, the VCC output  720  is supplied to the third DC to DC converter  716  (e.g., an LT 1931) to generate a negative 13.5 volt DC auxiliary output  724 . 
     FIG. 8 shows a detailed connection diagram  800  for placing the transponder lock  100  in-circuit with the control panels and transponders. In particular, the connection diagram  800  shows the interconnection between the control and signaling section  500  of the transponder lock  100  and the power supply section  700 . The connection diagram  800  also shows the interconnection between the side- 1  control panel connector  202 , wiring harness connector  204 , and aircraft wiring connector  206  with the signaling section  500  and the power supply section  700 . 
     The connection diagram  800  further illustrates the connections for the side- 1  transponder selector  216 , side- 2  transponder selector  218 , side- 1  auxiliary selector  220 , and side- 2  auxiliary selector  222 . The selectors  216 - 222  accept the 26 Volt DC output and are collectively controlled by the transponder override output  242 . As shown in FIG. 8, the backup power supply selector  214  is separately controlled by the backup power supply enable  244 . 
     Generally, the means for generating a transponder signal and the means for detecting the transponder lock code may be regarded as the transponder lock  100  as a whole, the control circuitry  208 , the microcontroller  502 , or the microcontroller  502  in concert with the program instructions executed by the microcontroller  502 . Similarly, the means for coupling the transponder alerting output to the transponder code output may be regarded as the transponder lock  100  as a whole, the control circuitry  208  (or microcontroller  502 ) in concert with the switch  216  or the switch  218  itself. 
     Thus, the transponder lock allows a transponder to continue transmitting an alerting code for the duration of an emergency. The transponder lock connects in-line with existing aircraft wiring, thereby minimizing aircraft downtime arising from installation of the transponder lock. In particular, the control panel connectors  202 ,  302  and the aircraft wiring connectors  206 ,  306  are chosen to mate with connectors already present for existing aircraft wiring. The reduction in installation complexity permits, for example, installation of the transponder lock at the airport gate, rather than requiring an airline to incur substantial time and expense penalties from flying each aircraft to a remote retrofitting hangar. 
     Note also that the transponder lock is fail-safe in that if the transponder lock circuitry should fail, the aircraft transponder system remains functional. In other words, if the transponder lock fails to function, the relays that switch the transponder over to transponder alerting output generated by the transponder lock are not energized. As a result, the aircraft transponder continues to receive code information from the cockpit control. Thus, even in the unlikely event that the transponder lock fails, it does not deprive the aircraft of transponder functionality. 
     The foregoing description of an implementation of the invention has been presented for purposes of illustration and description. It is not exhaustive and does not limit the invention to the precise form disclosed. Modifications and variations are possible in light of the above explanation or may be acquired from practicing of the invention. The claims and their equivalents define the scope of the invention.