Abstract:
The surveillance system provides a means to augment Automatic Dependent Surveillance—Broadcast (ADS-B) with “look alike ADS-B” or “pseudo ADS-B” surveillance transmissions for aircraft which may not be ADS-B equipped. The system uses ground based surveillance to determine the position of aircraft not equipped with ADS-B, then broadcasts the identification/positional information over the ADS-B data link. ADS-B equipped aircraft broadcast their own position over the ADS-B data link. The system enables aircraft equipped with ADS-B and Cockpit Display of Traffic Information (CDTI) to obtain surveillance information on all aircraft whether or not the proximate aircraft are equipped with ADS-B.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a Divisional Application of U.S. patent application Ser. No. 09/516,215, filed on Feb. 29, 2000, entitled “METHOD AND APPARATUS FOR IMPROVING THE UTILITY OF AUTOMATIC DEPENDENT SURVEILLANCE”, incorporated herein by reference. 
     This application claims priority from Provisional U.S. patent application Ser. No. 60/123,170, filed Mar. 5, 1999, and incorporated herein by reference in its entirety. 
    
    
     The subject matter of the present application is related to that in the following copending U.S. Patent Applications: 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Ser. No. 
                 Filing Date 
                 Inventor 
               
               
                   
                   
               
             
             
               
                   
                 08/891,227 
                 July 10, 1997 
                 Rudel et al. 
               
               
                   
                 09/114,921 
                 July 14, 1998 
                 Evers et al. 
               
               
                   
                 09/209,008 
                 December 11, 1998 
                 Smith et al. 
               
               
                   
                 60/113,169 
                 December 21, 1998 
                 Smith et al. 
               
               
                   
                   
               
             
          
         
       
     
     All of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to the field of aircraft surveillance and monitoring, particularly toward a technique known as Automatic Dependent Surveillance. 
     BACKGROUND OF THE INVENTION 
     The Automatic Dependent Surveillance—Broadcast (ADS-B) concept has been introduced as a means to enhance future ground and avionics based surveillance of aircraft. This concept is defined in Minimum Aviation System Performance Standards for Automatic Dependent Surveillance Broadcast (ADS-B), RTCA/DO-186, February 1998, which is later referred to as ADS-B MASPS and is incorporated herein by reference. The ADS-B concept provides for aircraft and ground vehicles to periodically broadcast their state vector (horizontal and vertical position, horizontal and vertical velocity) and other information. 
     A specific implementation of a 1090 MHz based ADS-B system is described in Drouilhet et al., U.S. Pat. No. 5,570,095, issued Oct. 29, 1996 and incorporated herein by reference. The broadcast ADS-B message provides surveillance information to other users, principally Air Traffic Control (ATC) and aircraft/vehicle operators. 
     Applications for ADS-B include ATC display of traffic, runway incursion detection and alerting, and Cockpit Display of Traffic Information (CDTI). One example of CDTI is a map-like display centered on a pilot&#39;s aircraft showing relative positions and intentions of other proximate aircraft. Another example of CDTI is provided in Buchanan et al., U.S. Pat. No. 4,196,474, issued Apr. 1, 1980, also incorporated herein by reference. 
     The Federal Aviation Administration (FAA) and the National Aeronautics and Space Administration (NASA) have investigated the suitability of this technology to support these applications in the airport surface environment. NASA recently tested ADS-B using 1090 MHz data transmission in an airport surface environment as part of the Low Visibility Landing and Surface Operations (LVLASO) program. Tests have been performed to assess how well 1090 MHz ADS-B performs with respect to surveillance system requirements established by the International Civil Aviation Organization (ICAO) and RTCA. 
     Two issues were identified during ADS-B system implementation and testing at Atlanta Hartsfield International Airport (ATL) as described in “Application of ADS-B for Airport Surface Surveillance”, Dan Hicok, Derrick Lee, 17 th  Digital Avionics System Conference, November, 1998: 
     1. A method may be required for CDTI equipped aircraft to obtain surveillance information on aircraft and ground vehicles which are not equipped with ADS-B. 
     2. Loss of ADS-B surveillance may occur due to multipath, blockage, and antenna pattern nulls. 
     Aircraft equipped with ADS-B and CDTI receive surveillance information directly from ADS-B transmissions. ADS-B implementation may require installation of new avionics equipment. There may inevitably be a transition period when some aircraft are ADS-B equipped and other aircraft are not. ADS-B MASPS has defined a means to augment ADS-B with a Traffic Information Services (TIS) data link, whereby ground based surveillance information for all aircraft is transmitted to CDTI capable aircraft. 
     An example of a TIS implementation may be found in Crow, U.S. Pat. No. 5,627,546, issued May 6, 1997, and incorporated herein by reference. Two sources of TIS traffic information are secondary surveillance radar and multilateration, as described in Schwab, U.S. Pat. No. 5,528,244, issued Jun. 18, 1996, and Alsup et al., U.S. Pat. No. 5,191,342, issued Mar. 3, 1993, both of which are incorporated herein by reference. A TIS data link was implemented at ATL for testing. A major limitation of TIS is the implementation may require aircraft owners to purchase a second data link in addition to the ADS-B link. 
     ATL testing also showed obstructions and multipath from structures may result in degradation or total loss of direct aircraft-to-aircraft ADS-B surveillance. The airport surface environment may be particularly challenging due to the presence of large structures, such as concourses and hangars. Loss of surveillance and degraded surveillance negatively impacts the ability of a pilot to maintain situational awareness of arrivals, departures and runway occupancy. 
     Accordingly, what is needed is a new method to augment the ADS-B concept using the ADS-B data link to provide surveillance information for aircraft and ground vehicles which are not equipped with ADS-B. This new method needs to provide a means to reinforce ADS-B transmissions which are adversely impacted by the environment. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved apparatus and method for use with Automatic Dependent Surveillance—Broadcast (ADS-B). In a method and apparatus of the present invention, each ADS-B equipped aircraft may periodically broadcast its position as derived from its navigation system. 
     Aircraft with ADS-B receivers may then be able to receive these broadcasts to obtain the location of proximate ADS-B equipped aircraft. The invention provides a means to augment ADS-B transmissions with position and identification information of aircraft which may not be ADS-B equipped. 
     In addition, the present invention reinforces ADS-B transmissions in areas where line-of-sight or multipath issues prevent reliable ADS-B communications between two aircraft. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram depicting “pseudo” ADS-B augmentation ADS-B concept using SSR Surveillance. 
     FIG. 2 is a diagram depicting “pseudo” ADS-B augmentation ADS-B concept using Multilateration Surveillance. 
     FIG. 3 is a diagram depicting a 1090 MHz ADS-B format. 
     FIG. 4 is a diagram depicting “pseudo” ADS-B reinforcement of ADS-B transmissions. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is now described with reference to the accompanying Figures where like reference numbers denote like element or steps. 
     In the preferred embodiment, a “pseudo” ADS-B ground system comprising one or more 1090 MHz remote receiver/transmitters  100  and a central workstation  170  may be used to provide a source for 1090 MHz “pseudo” ADS-B transmissions  110 , as illustrated in FIG. 1. 1090 MHz “pseudo” ADS-B transmissions  110  serve to augment 1090 MHz ADS-B transmission  140  thus providing ADS-B/CDTI equipped aircraft  120  with a complete picture of all proximate aircraft  130 ,  160 . 
     As defined by the ADS-B concept [RTCA ADS-B MASPS], aircraft equipped with ADS-B  130 ,  120  periodically broadcast their own position information via ADS-B transmission. In the preferred embodiment these ADS-B transmissions may be performed using an aircraft transponder frequency of 1090 MHz. ADS-B/CDTI equipped aircraft  120  receive and decode these 1090 MHz ADS-B transmissions  140  to obtain position information on proximate ADS-B equipped aircraft  130 . 
     The 1090 MHz “pseudo” ADS-B ground system provides ADS-B/CDTI equipped aircraft  120  with periodic 1090 MHz ADS-B like broadcast transmissions or 1090 MHz “pseudo” ADS-B transmissions  110  representing position data for aircraft not equipped with ADS-B  160 . While FIG. 1 illustrates only one 1090 MHz remote receiver/transmitter  100  performing 1090 MHz “pseudo” ADS-B transmissions  110 , any or all 1090 MHz remote receiver/transmitters  100  may broadcast 1090 MHz “pseudo” ADS-B transmissions  110 . 
     1090 MHz remote receiver/transmitters:  100  receives and decodes 1090 MHz ADS transmissions  140  to identify and locate ADS-B equipped aircraft  120 ,  130 . 1090 MHz remote receiver/transmitters  100  send ADS-B surveillance data  200  to central workstation  170 . Central workstation  170  receives target information  190  from Secondary Surveillance Radar (SSR)  150  and/or a 1090 MHz multilateration system to obtain surveillance information for all transponder equipped aircraft  120 ,  130 , including non-ADS-B aircraft  160 . 
     All aircraft which are ADS-B equipped may have a transponder. Non ADS-B aircraft may also be provided with a transponder. The transponder generates a radio signal identifying the aircraft (and optionally providing altitude or other data) either periodically, in response to a radar signal, or when “squawked” by the pilot or other operator of the aircraft. 
     Central workstation  170  correlates 1090 MHz ADS-B aircraft targets to transponder equipped targets. Central workstation  170  identifies transponder equipped targets which do not have a corresponding ADS-B position, thus may not be ADS-B equipped (e.g., aircraft  160  in FIG.  1 ). Transponder identification and position information for non-ADS-B equipped aircraft  210  may be sent to 1090 MHz remote receiver/transmitters  100  where it may be broadcasted via 1090 MHz “pseudo” ADS-B transmissions  110 . 
     One version of 1090 MHz ADS-B position report  140  format may be defined in FIG.  3 . Aircraft may be equipped with either a Mode S or an ATCRBS transponder as defined in Minimum Operational Performance Standards for Air Traffic Control Radar Beacon System/Mode Select (ATCRBS/MODE S) Airborne Equipment, RTCA/DO-181A, January 1992. The ADS-B message address may be identical to the Mode S transponder address. Accordingly, the ADS-B address may be obtained directly from the Mode S address. 
     Some aircraft may be equipped with ATCRBS transponders, instead of Mode S. The ATCRBS message contains a Mode A address, which may be used to generate a ADS-B address. One method to convert Mode A address to ADS-B message address may be to apply an algorithm which converts an aircraft tail number (e.g., registration number or N-number) to a 24 bit address. Mode A address may be converted from the tail number obtained by accessing flight plan information. In turn, the tail number may be converted to a ADS-B address. This algorithm is presently used by the Federal Aviation Administration to assign newly installed Mode S transponders with an address. 
     1090 MHz remote receiver/transmitters  100  may generate “pseudo” ADS-B transmissions  110  for ground vehicles operating on an airport movement area. Ground vehicle surface surveillance may be obtained from a primary radar or other surveillance means. The “pseudo” ADS-B transmissions mimic the format and style of “real” ADS-B transmissions, and thus are indistinguishable to ADS-B equipment provided in an aircraft. The “pseudo” ADS-B transmission is created from secondary aircraft location data (e.g., radar, multilateration, or the like) for non-ADS-B equipped aircraft. An airplane receiving “pseudo” ADS-B data processes such data in the same manner as “real” ADS-B data, and thus can locate, using ADS-B equipment, non-ADS-B equipped aircraft (e.g., aircraft  160 ). 
     As may be readily appreciated by one of ordinary skill in the art, the use of such “pseudo” ADS-B transmissions allows the ADS-B system to be used even in situations where not all aircraft are ADS-B equipped. Of course, non-ADS-B aircraft will still not be detected in areas where “pseudo” ADS-B transmission equipment is not located. However, the risk of collision and situations of heavy traffic usually occur in major metropolitan and airport areas which can be readily served by such a “pseudo” ADS-B system. 
     An ADS-B augmentation using 1090 MHz multilateration as a surveillance source is illustrated in FIG.  2 . Multilateration systems receive aircraft transponder transmissions  245  and apply Time Difference of Arrival techniques to determine an aircraft position. A basic requirement of a multilateration system may be to provide a Time of Arrival (TOA) measurement capability. An example of such a multilateration system is discussed in co-pending U.S. patent application Ser. No. 09/209,008, entitled “Passive Multilateration Auto-Calibration and Position Error Correction”, incorporated herein by reference. 
     A plurality of 1090 MHz remote receiver/transmitters  210  with TOA measurement hardware provide a means to perform multilateration to determine a position of aircraft not equipped with ADS-B 240. 1090 MHz remote receiver/transmitters  210  provide traffic information to ADS-B/CDTI equipped aircraft  260  via “pseudo” ADS-B transmissions  250 . 
     The “pseudo” ADS-B ground system provides a means to reinforce 1090 MHz ADS-B transmissions  340  with 1090 MHz “pseudo” ADS-B transmissions  310 , as illustrated in FIG. 4. A system comprising a central workstation  370  and a plurality of 1090 MHz receiver/transmitters  300  and  305  provides diversity for both receiving ADS-B messages  340  and transmitting “pseudo” ADS-B messages  310 . FIG. 4 illustrates a case where line-of-sight may be obstructed by a building  350  between two ADS-B/CDTI equipped aircraft  330  and  320  operating on intersecting runways. 
     One aircraft may be designated as the source aircraft  330  and the other aircraft may be designated as the destination aircraft  320 . When source aircraft  330  transmits a 1090 MHz ADS-B transmission  340 , it may be received by a 1090 MHz remote receiver/transmitter  300 . Decoded ADS-B transmission  380  may be sent to a central workstation  370 . Central workstation  370  determines when another 1090 MHz ADS-B/CDTI equipped aircraft  320  may require traffic information reinforcement with decoded ADS-B transmission  380 . 
     Central workstation  370  routes message  330  to a remote receiver/transmitter  305 , which has line-of-sight with destination aircraft  320 . 1090 MHz remote receiver/transmitter  305  transmits a reinforcing 1090 MHz “pseudo” ADS-B transmission  310 . 1090 MHz “pseudo” ADS-B transmission  310  may be identical in content to 1090 MHz ADS-B transmission  340  originating from source aircraft  330 . 
     The system selects 1090 MHz remote receiver/transmitter  300 ,  305 , which has line-of-sight and the highest probability of being received at the destination aircraft. A test or multipath simulation may be performed when the system may be first installed to determine, which remote receiver/transmitter has the highest probability of transmission reception success for each location of the movement area or airspace. 
     The main implementation of ADS-B may be through Mode S or 1090 MHz datalink technology. Mode S message formats have been allocated for ADS-B use. Note that, also, some implementations of ADS-B may use other datalinks (such as digital VHF, or TDMA-like formats). However, the same technology may be applied in the present invention regardless of the datalink selected for use. 
     While the preferred embodiment and various alternative embodiments of the invention have been disclosed and described in detail herein, it may be apparent to those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope thereof.