FIELD OF THE INVENTION 
     The present invention generally relates to aviation electronics, or avionics, and more particularly relates to global navigation, and even more particularly relates to a system and method for integrating LNAV and VNAV features of a global navigation system with an autopilot on an existing aircraft. 
     BACKGROUND OF THE INVENTION 
     In the past, designers of avionics systems have endeavored to provide systems with improved reliability and improved functionality. One example of such improvements is the incorporation of global navigation capabilities using the global positioning system (GPS). The GPS system has gained widespread acceptance across various fields of endeavor, including aviation. Global Navigation and Landing Units, GNLUs are becoming very common on new commercial air transport aircraft. These GNLUs can be used to provide enhanced lateral navigation (LNAV) and vertical navigation (VNAV) capabilities. However, it often is difficult to integrate this equipment with the many other interrelated avionics equipment on a typical existing commercial air transport aircraft. One particular example of integration is with an autopilot system. Often, complex interconnect hardware is required to couple a GNLU system with an autopilot on an existing aircraft. While these complex interconnect schemes have been used extensively in the past, for example on DC 9 aircraft manufactured by McDonnel Douglas, they also have significant drawbacks. 
     First of all, the system often requires t e addition of new interconnect wiring with complex logic controlled switching circuitry between the GNLU and the autopilot. 
     Secondly, in the DC 9 applications, as well as others, it also required expensive and time consuming changes to be made to the autopilot system itself. 
     Consequently, there exists a need for improvement in systems and methods for retrofitting GNLU systems with autopilots on existing aircraft. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a simplified interconnect system. 
     It is a feature of the present invention to use electronic flight instruments in the integration of a GNLU with an autopilot. 
     It is an advantage of the present invention to reduce the undesired complexity of interconnect wiring dedicated for interconnecting a GNLU with an autopilot. 
     It is another advantage of the present invention to reduce the weight of the interconnect wiring dedicated to interconnecting a GNLU with an autopilot. 
     It is yet another advantage to reduce required modifications to the autopilot system while retrofitting an aircraft with a GNLU. 
     It is still another advantage of the present invention to increase the reliability of the GNLU by avoiding the use of often unreliable interconnect relays between the GNLU and the autopilot. 
     The present invention is an apparatus and method for retrofitting a GNLU with an autopilot, which are designed to satisfy the aforementioned needs, provide the previously stated objects, include the above-listed features, and achieve the already articulated advantages. The present invention is carried out with a “relay-less interconnect” in a sense that the complexity and weight of an interconnect relay system has been eliminated. 
     Accordingly, the present invention is system and method for retrofitting a GNLU with an autopilot which includes the use of electronic flight instruments to perform coupling of an LNAV and/or VNAV capabilities with an existing autopilot. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be more fully under stood by reading the following description of the preferred embodiments of the invention, in conjunction with the appended drawings wherein: 
     FIG. 1 is a simplified block diagram of a retrofitting scheme of the prior art, which is focused upon the LNAV mode of operation. 
     FIG. 2 is a simplified block diagram of a GNLU retrofitting scheme of the present invention, which is focused on the LNAV mode of operation. 
     FIG. 3 is a simplified block diagram of a retrofitting scheme of the prior art focused upon the VNAV mode operation. 
     FIG. 4 is a simplified block diagram of a GNLU retrofitting scheme of the present invention which is focused upon the LNAV mode of operation. 
    
    
     DETAILED DESCRIPTION 
     Now referring to the drawings, wherein like numerals refer to like matter throughout, and more particularly to FIG. 1, there is shown a system of the prior art, generally designated  100 , which includes a global navigation and landing system  102 , which may be a GPS based navigation system or other wide area navigation system. FIG. 1 focuses primarily upon the LNAV mode while FIG. 3 primarily focuses on the VNAV mode. Global navigation and landing system  102  provides information pertaining to LNAV roll steering and valid information, as well as VNAV pitch steering and valid information, in a manner which is well known in the art. An auto pilot roll and pitch axes  104  represent a portion of a prior art autopilot system relating to roll and pitch axes, which has been adapted from conventional prior art autopilots to accommodate these LNAV and/or VNAV signals. Electronic attitude director indicator  106  is a typical EADI of the prior art. Electronic horizontal situation indicator  108  is a typical EHSI of the prior art. A digital data bus  116  provides navigation data from global navigation and landing system  102  for use by the display&#39;s electronic attitude director indicator  106  and electronic horizontal situation indicator  108 . Coupled to electronic attitude director indicator  106  and electronic horizontal situation indicator  108  is desired heading selector  110 , which is a well-known apparatus for inputting desired heading information into the electronic attitude director indicator  106  and electronic horizontal situation indicator  108 . A heading error/datum signal representing a difference between the selected heading and actual heading information can either be provided by other navigation sources on the aircraft such a inertial reference systems, radio navigation systems, and compasses etc, or computed by the EHSI  108  and/or EADI  106 . This heading error/datum is provided as an input to the auto pilot roll and pitch axes  104  when interconnect relay and logic system  114  is configured to be in the Heading Select Mode. When LNAV mode selector and annunciator control  112  provides a signal representing the pilot&#39;s desire to use LNAV as a source of input for the autopilot, then information is routed from global navigation and landing system  102  through interconnect relay and logic system  114  to a to pilot roll and pitch axes  104 . The above system is representative of various prior art systems, including those installed and operating on many DC 9 aircraft, as well as various other aircraft. 
     Now referring to FIG. 2, there is shown an avionics system of the present invention, generally designated  200 , which is primarily focused upon the LNAV mode of operation, including a global navigation and landing system  102  and a standard autopilot roll/pitch axes  204 , which is similar to autopilots installed on many aircraft and is different from global navigation and landing system  102  (FIG. 1) in that it does not include numerous changes to the autopilot to accommodate the use of LNAV and/or VNAV capabilities provided by global navigation and landing system  102 . The present invention also includes improved electronic attitude director indicator  206  and improved electronic horizontal situation indicator  208 , which are similar to electronic attitude director indicator  106  and electronic horizontal situation indicator  108  (FIG. 1) except that they are modified to perform LNAV/VNAV and Heading select mode switching and associated logic, which is similar to that which is performed by interconnect relay and logic system  114  (FIG.  1 ). Improved electronic attitude director indicator  206  and improved electronic horizontal situation indicator  208  are preferably capable of extracting LNAV and/or VNAV guidance signals from navigation data provided on digital data bus  16  from global navigation and landing system  102 . Improved electronic attitude director indicator  206  and improved electronic horizontal situation indicator  208  also would preferably implement logic and interlocks required for LNAV and/or VNAV engagement and disengagement, thereby simplifying the interconnect lines  214 , which now can be merely a standard wiring harness. Improved electronic attitude director indicator  206  and improved electronic horizontal situation indicator  208  may also perform additional signal conditioning functions to improve the coupling performance between the standard autopilot roll/pitch axes  204 , the global navigation and landing system  102  (LNAV and/or VNAV) and the improved electronic attitude director indicator  206  and improved electronic horizontal situation indicator  208 . This signal conditioning may include, but need not be limited to, the following: phase advance (to overcome any sluggishnes in the response) and/or gain variation (to address non-linearity or varying aircraft configuration or flight regimes). Improved electronic attitude director indicator  206  and improved electronic horizontal situation indicator  208  receive desired heading information from desired heading selector  110  in a well-known manner, but now also receive from LNAV/VNAV mode selector and annunciator control  112  the LNAV and/or VNAV mode select information. 
     Now referring to FIG. 3, there is shown a system of the prior art, generally designated  300 , which is primarily for used upon the VNAV mode of operation. As a VNAV implementation, there is also shown an input  302  for Manual pitch command and/or altitude hold error signals. 
     Now referring to FIG. 4, there is shown a system of the present invention, generally designated  400 , which is primarily focused upon the VNAV mode of operation. The EADI  206  and the EHSI  208  are the same as those in FIG. 2; however, here they are shown as electronically performing glide slope and VNAV operations as opposed to the LNAV/VOR/LOC functions which are highlighted in FIG.  2 . 
     In operation, the present invention could operate in a typical manner as follows: during a portion of the flight over North America, the pilot can select a desired heading as is customary, the improved electronic attitude director indicator  206  and improved electronic horizontal situation indicator  208  provide heading error/datum signals to the standard autopilot roll/pitch axes  204  in a well-known manner. However, if the flight extends over an ocean or remote regions, the pilot can now operate LNAV/VNAV mode selector and annunciator control  112  to select LNAV and/or VNAV, thereby causing improved electronic attitude director indicator  206  and improved electronic horizontal situation indicator  208  to substitute LNAV and/or VNAV guidance signals s to standard autopilot roll/pitch axes  204 . This switching to LNAV and/or VNAV can be accomplished without any need for changes to the existing standard autopilot roll/pitch axes  204 , and the interconnect lines  214 , which have already been certified by the FAA or other airworthiness authorities. 
     It is thought that the method and apparatus of the present invention will be understood from the foregoing description and that it will be apparent that various changes may be made in the form, construct steps, and arrangement of the parts and steps thereof, without departing from the spirit and scope of the invention or sacrificing all of their material advantages. The form herein described is merely a preferred exemplary embodiment thereof.
An avionics system for retrofitting a GPS system to work with an existing autopilot by using signal switching and conditioning in an electronic flight display to accomplish the switching without the need for logic controlled relays in the GPS to autopilot interconnect wiring.