EGGERT Demonstration of -Status Lights at Logdn

feet or less. Runway incursions represent a larger class ton. A much more detailed description of this system of events than accidents and high-hazard incidents. A and its components is given in a separate report [2]. is defined as any occurrence at an airport involving an , vehicle, person, or object System Design on the ground that creates a collision hazard or results A complete airport surface-traffic safety system in loss of separation with an aircraft taking off, in- should include three products that together can ad- tending to take off, landing, or intending to land. dress all the major airport surface-conflict scenarios. Clearly, preventing runway incursions is an effective These three products are runway-status lights, con- way to prevent a certain set of airport surface acci- troller alerts, and enhanced controller displays. dents, and a good airport surface-traffic safety system A runway-status light system (RSLS) will provide cur- must be effective at reducing runway incursions. rent runway-status information to pilots and vehicle Many of the fundamental concepts of such a sur- operators, indicating when the runway is unsafe to face-traffic automation system have been discussed enter or unsafe for takeoff (Figure 1). The informa- previously [I]. This article concentrates on the real- tion provided by these lights will prevent many run- ization of a real-time but off-line surface-traffic auto- way incursions before they happen. Controller alerts mation system at Logan International Airport in Bos- will be used to direct controllers' attention to existing

FIGURE 1. Runway-status light system (RSLS) concept. The runway-status lights indicate to aircraft pilots and surface-vehicle operators when the runway is unsafe to enter or unsafe for takeoff. The system is oper- ated automatically, based on surveillance provided by an Airport Surface Detection Equipment (ASDE) sur- face radar, an Airport Surveillance Radar (ASR-9) approach radar, and future surveillance systems such as GPS-Squitter or Mode S multilateration.

170 THE LINCOLN LABORATORY JOURNAL VOLUME 7, NUMBER 2, 1994 EGGERT Demonstration of Runway-Status Lights at Logan Airport conflicts between aircraft on or near the runways. Be- sions and accidents. The runway-status lights func- cause runway-status lights do not address some of the tion fully automatically in response to real-time sur- top accident and incident scenarios, and because con- veillance. The off-line RSLS Logan Demonstration troller alerts do not always provide sufficient time for does not in fact incorporate an actual field-lighting controllers and pilots to correct a situation once it has system, but simulates the runway-status lights by the developed, only a combination of runway-status use of an illuminated model board and computer- lights and controller alerts will address all the most driven displays. common scenarios. (The FAA has contracted for the The runway-status lights have two states: on (red) development of an operational controller-alerting sys- and off. These lights indicate runway status only; they tem known as the Airport Movement Area Safety Sys- do not indicate clearance. A green state was specifical- tem, or AMASS.) Enhanced ASDE controller dis- ly avoided to prevent any false impression of clear- plays will present symbology to describe aircraft ance. Clearance is to remain the sole responsibility of position, size, altitude, flight number, equipment the air traffic controller, and is not to be provided or type, and direction and speed of motion. In addition implied by the RSLS. An amber state was also avoid- to airport surface trafEc, aircraft on approach to run- ed because in the case of runway-entrance lights it ways will also be depicted on the ASDE displays. could tend to be confused with the amber color of the The off-line proof-of-concept RSLS Logan Dem- International Civil Aviation Organization (ICAO) onstration incorporates simulated runway-status standard taxi hold-position (wig-wag) lights. The lights and enhanced controller displays, but does not runway-status lights are designed to be as conspicu- include a complete controller-alerting system. Run- ous as possible while minimizing the possibility of way-status lights provide the greatest part of the pro- confusion with other light systems. tection afforded by the safety system for three impor- Runway-status lights are designed to be generally tant reasons. First, in any time-critical conflict invisible to pilots of aircraft at high speed. This design scenario, the most effective safety-system product is decision was made so that red lights, especially lights one that is directly accessible by the pilots. That direct that suddenly turn red, will not be shown to pilots access is allowed by runway-status lights but not by whose aircraft speed precludes them from making controller alerts. Second, runway-status lights act to sudden maneuvers. Runway-entrance lights are hood- prevent runway incursions before they happen, ed so as not to be visible to pilots of aircraft on the whereas controller alerts occur only after a conflict runway, and they are generally not active at runway- has been identified. Third, runway-status lights are runway intersections. Takeoff-hold lights are also effective in a greater fraction of the accident and inci- hooded, and they require that an aircraft be in posi- dent scenarios than are controller alerts. Therefore, tion for takeoff for the lights to be illuminated. The for a combination of reasons, including maximizing design of the fixtures and light logic thus generally system effectiveness in the face of developmental prevents pilots of aircraft at high speed from seeing schedule constraints and reducing the duplication of red runway-status lights. research efforts, the RSLS Logan Demonstration does A proposed fixture for the runway-status lights not include controller alerts except for limited dem- would be the standard fixture used for ICAO wig-wag onstratlon purposes. lights, with the amber lenses replaced by red lens& and the lamps upgraded to brighter bulbs (Figure 2). Runway-Status Lights These fixtures use redundant light bulbs and other There are two types of runway-status lights: nmway- electrical components to minimize the impact of sin- entrance lights, which indicate when the runway is un- gle-component failures on the operation of the sys- safe to enter, and takeofibold lights, which indicate tem. They are also in current production, allowing when the runway is unsafe for takeoff. The two types off-the-shelf delivery. of lights are driven in concert by a single safety logic, Runway-entrance lights will be located at the taxi- and they operate together to prevent runway incur- way entrances to runways and will be positioned on

VOLUME 7, NUMBER 2, 1994 THE LINCOLN LABORATORY JOURNAL EGGERT Demonstration of Runway-Status Lights at Logan Airport

either side of the taxiway, near the runway edge and well beyond the hold line (Figure 3). Runway-en- trance lights will also be located at runway-runway intersections, although they will not always be imple- mented or actuated there. Runway-entrance lights will be illuminated to indicate to aircrafi pilots and surface-vehicle operators that the runway is hot (i.e., it is being used for a high-speed operation like takeoff or landing), and that the runway is currently unsafe to enter at that intersection. Runway-entrance lights will be extinguished when the runway is no longer unsafe to enter at that intersection. Takeoff-hold lights will be located at takeoff-hold positions and placed on either side of the runway near FIGURE 2. A proposed runway-status light fixture. This the runway edge (Figure 4). These lights will indicate fixture is based on the standard International Civil Avia- to aircrafi pilots that the runway is unsafe for takeoff tion Organization taxi hold-position (wig-wag) light, with (i.e., the runway is currently occupied or is about to the amber lenses replaced by red lenses and the lamps upgraded to brighter bulbs. An addressable light con- be occupied); they will be extinguished when the run- troller would be mounted in or near the base of this fix- way is safe for takeoff, or if the aircrafi in position for ture to allow individual control over each lamp. takeoff vacates the runway.

Taxi hold-position (wig-wag) lights Runway-entrance lie hts f akeoff-hold lights

-

FIGURE 3. Runway-entrance lights in operation for an aircraft landing on a runway. The runway-entrance lights in front of the landing aircraft are illuminated red, indicating to the taxiing aircraft that the runway is unsafe to enter. The runway- entrance lights behind the landing aircraft are extinguished, indicating that the runway is safe to enter there.

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FlGURE4. Takeoff-hold lights in operation for an aircraft in position for departure with crossing traffic. The takeoff-hold lights indicate that the runway is currently unsafe for departure because the runway is about to be occupied by a cross- ing aircraft.

Tracks, or indicators of stationary and moving air- RSLS Enhancements to ASDE Display craft or other surface traffic, are displayed as icons on The RSLS provides several display enhancements to the enhanced ASDE display (Figure 5). Each icon an ASDE display. An ASDE display without en- represents the position and direction of motion of the hancements typically contains radar video with track and, for tracks with ASDE image information, blanking to reduce visible clutter, and line graphics to is drawn with a size proportional to the area of the depict runway and taxiway edges and building out- ASDE image. Each displayed track has a data tag con- lines. The RSLS provides an iconic depiction of nected to the icon with a leader line. The ASDE dis- tracked traffic, with symbolic tags for each icon, ap- play sohare selects the leader-line direction to elimi- proach bars for aircraft inside the outer marker, depic- nate possible overlapping tags and crossing leader tion of runway-status-light states, and special mark- lines. The data tag can be displayed in two formats. ings for aircraft identified as being in conflict. For The primary tag format shows aircraft altitude in demonstration and development purposes, additional hundreds of feet and track velocity in knots. For ex- internal surface-monitor information can also be dis- ample, the data tag 001 122 in Figure 5 indicates an played. The RSLS Logan Demonstration supports aircraft at 100 feet traveling at 122 knots. The prima- both monochrome and color ASDE displays. The off- ry tag format also shows aircraft flight code and line RSLS Logan Demonstration does not include ra- equipment type when this information is available, dar video on its display. This temporary omission was the latter alternating with the velocity field. The sec- chosen to reduce development time and equipment ondary tag format is meant primarily for system de- expenses, and is not envisioned for a complete RSLS. velopment, and shows internal track numbers, track

VOLUME 7, NUMBER 2, 1994 THE LINCOLN LABORATORY JOURNAL EGGERT Demonstration of Runway-Status Lights at Logan Airport

FlGURE5. RSLS enhancements to the ASDE controller display. Arrows indicate position, direction of motion, and size of the radar tracks. Stationary tracks are also optionally depicted by circles. Data tags present the radar track's altitude, velocity, flight number, and equipment type. The approach bar depicts an aircraft on approach to a particular runway; its two endpoints represent approximately five miles of airspace from the outer marker to the runway threshold. surveillance source or sources, altitude in feet, veloci- be represented as a bar across the runway. Alternative- ty in knots, and aircraft flight code, when available. ly, runway-entrance lights and takeoff-hold lights can Aircraft on approach to runways and inside the be drawn as acute triangles on either side of the taxi- outer marker are displayed on approach bars. The way or runway and oriented to depict the directional- outer marker, which is part of the Instrument Land- ity of the actual lights. ing System (ILS), is a radio navigational aid located If the RSLS safety logic identifies targets as being on the runway centerline at the point where an ILS in conflict, this information can be drawn to the standard approach begins its final descent. An ap- ASDE display. The targets are circled in white and re- proach bar is a short line segment drawn near the ap- main highlighted until the conflict is resolved. Addi- proach end of the runway. It is drawn at a different tional RSLS internal information can also be shown scale and represents the approximately five-nautical- on the ASDE display. This information includes the mile distance from the outer marker to the runway target state identification (taxi, stopped, arrival, de- threshold. Aircraft identified as being on approach to parture, departure abort, or unknown), the range of a runway are shown as diamonds on the approach bar. predicted target positions produced by the surface When the aircraft is near enough to the runway to monitor, and artificial target (sprite) positions and appear on the scale of the ASDE display, it disappears control information. from the approach bar and appears as a normally dis- The ASDE display enhancements also allow for played target. the possibility that future tower displays could be in Runway-status-light state information is also ren- color. The color displays show the runway, taxiway, dered to the enhanced ASDE display. It can be drawn building outlines, and approach bars in green; target in two different symbologies. An illuminated run- icons and tags in yellow; illuminated runway-status way-entrance light can be represented by a bar across lights in red; and conflict alert circles in white. Color the intersecting taxiway, and takeoff-hold lights can in an ASDE display is extremely useful in enhancing

1 THE LINCOLN LABORATORY JOURNAL VOLUME 7, NUMBER 2, 1994 EGGERT Demonstration of Runway-Status Lights at Logan Airport the visibility of the display to controllers, which thus runway-status lights and controller alerts. improves the rate and efficiency of information com- 7. A display system to allow basic evaluation and prehension by controllers. demonstration of the entire system. 8. A performance-analysis suite to allow a detailed Controller Alerts evaluation of the operation of the RSLS. Although the RSLS Logan Demonstration does not Figure G shows an overview of the system architec- supply a complete controller-alerting system, it does ture. The analog signal from the ASDE surface radar have an architecture that supports such an alerting is digitized and processed in the radar surveillance system. To demonstrate this capability we included a processing system. Its tracks, along with those derived single type of alert in the system. The conflict that can from Airport Surveillance Radar (ASR-9) radar sur- be detected is between an arriving or landing aircraft veillance using the ARTS tap, are passed on to sensor and a stopped target on the arrival runway. When a fusion. The output of sensor fusion is a single set of conflict is detected, the RSLS circles the conflicting tracks presenting a coherent view of the airport sur- targets on the ASDE display and generates a synthe- face and approach traffic to the surface monitor, sized voice alert. The voice alert gives a warning sig- which identifies aircraft states, predicts future target nal, and then it gives the location and type of the con- positions, determines runway-status-light states, and flict. A complete controller-alerting system would generates alert commands. The system output is include the capability of detecting perhaps a dozen shown on several displays. The various stages of pro- general conflict types. cessing are described in more detail below. Several system requirements resulted in basic engi- RSLS Logan Demonstration Methodology neering design choices. These requirements included The main objective of the RSLS Logan Demonstra- the following: (1) December 1992 demonstration, tion is to develop a surface-traffic safety system that (2) off-line noninterfering demonstration, (3) real- can prevent most runway incursions and identiFy im- time response to live traffic, (4) minimal system re- pending surface conflicts. This objective required the sponse time, (5) minimal hardware design time, and development of several significant capabilities: (6) adequate design flexibility. The RSLS Logan 1. An ASDE surface radar to provide radar images Demonstration was required to be functional in the with sufficient resolution and scan frequency December 1992 time frame, which precluded the use for tracking surface traffic. of the ASDE-3 surface radar at Logan Airport because 2. A radar interface board to digitize, time-stamp, that radar was not expected to be operational in time. and limit the radar coverage defined by a down- Thus another surface-radar system-a Raytheon loadable censor map. Pathfinder X-band marine radar-was installed on 3. A surface-radar processing system to process in- the roof of the old control-tower building at Logan formation from the ASDE radar automatically, Airport for use in the development and demonstra- performing clutter rejection, target morphologi- tion of the system. This radar is called the ASDE-X. cal processing, and scan-to-scan association. The RSLS Logan Demonstration was required to 4. An interface to the Automated Radar Terminal have no operational impact. Thus there are no actual System (ARTS) computer to provide radar sur- runway-status lights and no RSLS presence in the veillance data for aircraft on approach to the control-tower cab, and the RSLS does not interfere runways. with normal FAA or aircraft operations. All demon- 5. A sensor-fusion process to merge tracks from stration displays and system control screens are locat- the ASDE processing system automatically with ed in a demonstration room on the sixteenth floor of tracks from the ARTS computer, and perform the Logan Airport tower, or in other noninterfering multipath rejection. areas. All demonstration equipment operates on a 6. A surface monitor to classiFy and predict aircraft noninterfering basis; a failure in any demonstration behavior, identify surface conflicts, and drive subsystem cannot result in operational interference.

VOLUME 7, NUMBER 2, 1994 THE LINCOLN LABORATORY JOURNAL 175 EGGERT Demonstration of Runway-Status Lights at Logan Airport

Surveillance processing surface radar

Sensor , fusion

ASR-9 terminal mdar ARTS lllA term i nal-radar computer

FIGURE 6. Overview of the RSLS Logan Demonstration architecture. Surveillance provided by a surface radar and a terminal radar is processed separately and then fused to provide aircraft tracks on the airport surface and in the approach space. The surface monitor assesses the traffic picture and drives the run- way-status lights. The different displays show the traffic and runway-status light information on a map of the airport and its vicinity.

The real-time nature of the RSLS mandated that nition of this fact lead to the decision that the use of the system should have sufficient processing through- custom hardware would be avoided wherever possi- put to keep up with peak data loads. For the case of ble, and much of the system functionality would be ASDE-X surface-radar processing, this processing re- performed in sofnvare by using commercial off-the- quirement demanded the use of a fairly powerful shelf equipment. This decision proved to be of great computer. Most of the subsystems operate on sepa- benefit throughout the system design, and was made rate computer platforms to distribute the computa- possible by the explosion in computer system perfor- tional load and reduce the system impact of a single- mance in the past few years. In the case of the ASDE point failure. radar interface and certain required improvements to A real-time surface-traffic safety system must take the ASDE-X marine radar, however, custom hard- into account the fact that time-critical situations can ware was required. occur, making large processing delays intolerable. Several design choices, most notably the order of op- RSLS Logan Demonstration Description eration in the ASDE clutter-rejection process, and the The off-line RSLS Logan Demonstration is installed design of a dual tap to the ARTS computer, were a at 's Logan International Airport. Figure 7 is a result of this consideration. pilot's diagram of Logan Airport showing the run- Because the RSLS Logan Demonstration develop- ways, taxiways, runway designations, runway dimen- ment overlapped design and implementation, design sions, hold positions, and terminal areas [3]. The changes along the way were clearly inevitable. Recog- demonstration room, which is shown in Figure 8, is

176 THE LINCOLN LABORATORY JOURNAL VOLUME 7, NUMBER 2, 1994 EGGERT Demonstration of Runway-Status Lights at Logan Aivport on the sixteenth floor of the Logan Airport control uid-crystal color technology to demonstrate how a tower, in the Massport conference room. This room color display could be usable in a high-ambient-light provides a clear view of most of the airport's runways environment. and taxiways, allowing good visual verification of the Figure 9 shows a Logan Airport model board that operation of the system. The demonstration room has includes architectural models of the terminal build- several displays showing various aspects of the system ings, depictions of the runways and taxiways, and a operation. A Raytheon Pathfinder radar display variety of actively controlled field-lighting systems. shows an image of the raw ASDE-X surface-radar sur- The field-lighting systems are simulated by using fi- veillance. Two monochrome high-brightness displays ber optics, and they include the RSLS runway-status (manufactured by Orwin Associates) simulate an en- lights, runway-centerline and edge lights, taxiway- hanced ASDE display and a DBRITE (Digital Bright edge lights, approach lights, taxi hold-position lights, Radar Indicator Tower Equipment) display. A third and stopbars. These systems are driven actively by an high-brightness display uses backlit active-matrix liq- integrated lighting-control system, which is inter- faced to the rest of the RSLS Logan Demonstration by using an RS-232 interface. Transition from an off- line demonstration of the runway-status lights using the model board to a real field-lighting system can in principle be performed by unplugging the model board and plugging in the field-lighting controller. A DECTalk digital voice-synthesizer system gener- ates audible voice alerts in response to the alert com- mands from safety logic. The DECTalk voice quality is insufficient for a real controller-alerting system, but it is adequate for a demonstration system. The RSLS Logan Demonstration also has two con- trol displays located in the demonstration room. These are the control displays for the surveillance processing computer and for the sensor-hsion and surface-monitor workstation. The former display can also be used to show real-time radar images either be- fore or after clutter rejection. The latter display hnc- tions as an additional color ASDE display (although it is not a high-brightness display), and it is used to generate and control artificial targets. The other components of the RSLS Logan Dem- onstration are located outside the demonstration room itself. The ASDE-X radar is located on the roof of the old control-tower building (the building la- beled "control tower" in Figure 7),and its associated I Y, A'RPoRT i BOSlON,MASSACHUSETTS BOSTON GENERAL EDWARD LAWRENCE LOGAN INTL tl3OS) electronics are located nearby and on the fifteenth floor of the new control tower behind the old control FIGURE 7. Boston Logan International Airport runway tower. The ARTS interface hardware is located in the and taxiway map. The runways and buildings are shown ARTS equipment rooms on the sixth and seventh in black and the taxiways are shown in gray. The taxiway configuration is shown as of 1992; some new taxiway floors of the old control-tower building. The comput- construction and change in nomenclature has occurred ers used to drive the two high-brightness mono- since this map was produced. chrome displays and to receive the information from

VOLUME 7, NUMBER 2, 1994 THE LINCOLN LABORATORY JOURNRL 17 EGGERT Demonstration of Runway-Status Lights at Logan Airport

FIGURE 8. RSLS Logan Demonstration room. The windows offer a sixteenth-floor view of Logan Airport and Boston harbor. The RSLS model board is on the left, while the computer monitors are on the right. The two displays above the model board are high- brightness monochrome displays used to show the traffic in and near the airport.

the ARTS interface are located on the fifteenth floor data can be played back through parts or all of the of the new control tower. Normal operation of the RSLS sohare to review interesting scenarios, evalu- demonstration system includes a startup procedure ate performance, and help refine the various process- that takes approximately five minutes. Thereafter, the ing algorithms. system is completely functional and normally oper- ates without requiring user input. Surface-Radar SurveiZZance Processing The first task in the development of the RSLS was the Subsystem Descriptions creation of a high-quality surface-radar tracking sys- The RSLS sohare has three major modules: ASDE tem. This system is, in fact, separately fieldable from surface-radar surveillance processing, sensor fusion, the runway-status lights themselves, and represents a and surface monitor. These three modules are de- major advance in surveillance capability. Because it scribed in more detail here, along with the radar in- produces good surface surveillance by using an inex- terfaces. Additional modules are used to accomplish pensive radar and advanced image-processing and the various required ARTS interface, display play- tracking techniques, this surface-radar system can be back, and analysis functions. These sohare modules used at where a more expensive radar is not communicate with one another on the same or differ- justified. The use of this enhanced surface-radar sys- ent computer platforms by using efficient communi- tem can make the benefits of surface radar available to cations protocols. The system can record all relevant more and smaller airports on a cost-effective basis. ASDE, ARTS, sensor-fusion, and surface-monitor To develop the RSLS surface-radar tracking sys- data simultaneously and in real time. These recorded tem, we had to overcome several major problems with

178 THE LINCOLN LABORATORY JOURNAL VOLUME 7, NUMBER 2, 1994 EGGERT Demonstration of Runway-Status Lights at Logan Airport

FIGURE 9. RSLS Logan Demonstration model board. The model board is an architectural model of Logan Airport with computer-controlled fiber optic lights simulating the runway-status lights; runway approach, threshold, centerline, and edge lights; taxiway centerline and edge lights; and taxi hold-posi- tion lights. All of the runway lights are illuminated in this photograph, which is for illustration only and does not represent any real runway configuration. surface-radar systems-namely, clutter, target splits, morphological processing, and merge and scan-to- shadowing, and multipath. Clutter occurs because scan tracking. A dynamic clutter map is used to esti- the radar transmits energy down toward the airport mate and remove clutter from the radar images. This surface and receives returns from many surface ob- clutter map contains the mean and mean square for jects in addition to the aircraft and surface vehicles every pixel in the map and is updated every scan for that are of primary interest. Target splits occur be- all clutter pixels. This processing allows the clutter cause the surface radar has fairly high resolution, and map to accommodate changing conditions such as there are portions of an aircraft that reflect essentially rain and snow. At Logan Airport the clutter map con- no energy back to the radar. Shadowing occurs when tains approximately 1.2 million pixels. Morphologi- one aircraft obscures another aircraft from the view- cal, or shape, processing is used to reconnect split tar- point of the radar. Multipath occurs because the radar gets to avoid multiple-tracking and centroiding signal can bounce off several objects in turn and still errors, and to eliminate small objects that are not tar- return to the radar with enough intensity to be de- get-like in appearance. To decrease computational la- tected, thereby producing phantom outrange targets. tency, the surveillance area is split into wedges, and These effects make tracking primary surface-target ra- both the clutter rejection and the morphological pro- dar returns difficult. cessing are performed in parallel on these wedges in a Several techniques were developed to solve these multiprocessor computer. Targets output by the mor- surveillance problems, including clutter rejection, phological processing are pasted together at the

VOLUME 7, NUMBER 2, 1994 THE LINCOLN LABORATORY JOURNAL EGGERT Demonstration of Runway-Status Lights at Logan Airport wedge boundaries and tracked from scan to scan by radial runs. Runs shorter than three pixels are discard- merge processing. In merge processing, tracks that are ed, as are runs that are not adjacent to any other runs. dropped because of shadowing and other problems Discarding these runs produces a radial and an azi- are reacquired rapidly (usually within five seconds) by muthal prefiltering that does not affect the eventual using special reacquisition logic. Multipath is rejected results, but greatly reduces the amount of informa- by sensor fusion on the basis of track length and tion passed on to the morphological processing. ARTS information. An analysis of the tracking per- Morphological Processing. Morphological process- formance of this system, which is presented in a sepa- ing is used to coalesce the lists of target-pixel runs rate report [2],indicates that the probability of track- produced by the clutter-rejection algorithm into tar- ing an aircraft is approximately 98.6%. The gets representing the outlines of airplanes or surface conclusion is that surface traffic can be detected and vehicles. This processing is done in three steps, as tracked with high reliability. shown in Figure 10. First, a morphological opening is Clutter Rejection. The main purpose of clutter re- performed on the clutter-rejected pixel data, as shown jection is to estimate and eliminate constant or slowly in Figure 10(a). An opening is composed of an ero- varying clutter from the radar images, detect target sion followed by a dilation. An erosion has the effect pixels that stand out from the clutter, and transfer the of peeling off one layer of pixels from the outside of target-pixel information in an efficient fashion for lat- every clump of target pixels; this process destroys er processing. The clutter is estimated by using a lin- small clumps and shrinks larger ones. A dilation ac- ear recursive estimator for the mean (x) and mean cretes one layer of pixels onto the outside of every square (x2) for each pixel log-intensity measurement clump of target pixels. (The actual implementation of x in the surveillance map for each scan i, by using the the erosion and dilation operations does not require formulas that clumps of pixels be identified explicitly.) The net result of the opening is the elimination of salt-and- pepper noise in the detected image and the smooth- ing of the outlines of the larger images, as shown in Figure 10 (b). The second step in morphological processing is to where z is the time constant for the two estimators. group the remaining target-pixel runs into connected From the mean and mean square, a threshold tiis cal- components. This step is accomplished by using a pe- culated by the equations rimeter-tracking algorithm that steps around the boundary of a component until it returns to its start- ing point, as shown in Figure 10(c). In this algorithm, every target-pixel run must appear at least once on the boundary of a component. This algorithm fails only for bizarre cases with components inside of compo- where the function H(y, I, u) given by nents, a pixel run configuration that is essentially nev- er seen in real radar images. The process of grouping runs into connected components produces a repre- sentation of all the distinct components visible in the radar image. The third step in morphological processing is to limits the excursion of the thresholds from the mean. group components that belong to the same aircraft or Pixels whose log-intensity exceeds the threshold cal- surface vehicle, as shown in Figure 10 (d). Because the culated in the previous radar scan are identified as tar- ASDE radar is an imaging radar and because aircraft get pixels. Target pixels are not used to update the tend to self-shadow, aircraft images are often broken clutter statistics. Instead, they are grouped together in into completely separate components. A distance cri-

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FIGURE 10. Morphological processing example. (a) In the input radar image, the black squares denote clutter-rejected target pixels. (b) Erosion deletes the pixels marked in gray. (c) Subsequent dilation adds the pixels marked in light blue. (d) The two green components are close enough to be grouped as a single object. The blue component is a separate object.

terion is used to identify which components should es. This division of the image reduces latency prob- be grouped together to form one object. The algo- lems and allows the computation to be distributed rithm identifies component pairs that are fairly close over several computer processors. An associated com- and performs a test dilation on them to see if they plication is that the detected objects on the wedge merge into one component. If they do, then the com- boundaries must be pasted back together by a single ponent pairs before the test dilation are grouped to- process. This reconstruction of object images is done gether into a single object. The result of morphologi- by a technique called mergeprocessing, which carefully cal processing is a list of objects detected in the ASDE merges component segments back together and then radar surveillance space, where each object is de- groups components into objects correctly across the scribed by the target-pixel runs grouped into one or wedge boundaries. (Another potential difficulty more connected components. posed at the wedge boundaries by the azimuthal fil- Merge and Scan-to-Scan Tracking. A necessary com- tering performed by the clutter-rejection process is plication in ASDE radar image processing is the azi- circumvented by simply not doing azimuthal filtering muthal division of the surveillance region into wedg- at the wedge boundaries.)

VOLUME 7, NUMBER 2, 1994 THE LINCOLN LABORATORY JOURNAL EGGERT Demonstration of Runway-Status Lights at Logan Airport

After merge processing has correctly pasted the ob- jects together across wedge boundaries, the centroid Sensor Fusion and area are computed for each object. Each object is The tracks from the ASDE radar are fused with tracks then compared with tracks of objects computed in from the ARTS system in the sensor-fusion process. previous scans to look for matches. Potential matches These two sets of tracks are paired by sensor fusion to are identified by computing a simple two-point pro- form a combined track containing all the available in- jection of the track to the present scan, and accepting formation on aircraft in the surveillance area. The targets or objects that are within a certain association ASDE tracks are fused with the ARTS tracks by com- radius of the projected position. We must be careful paring their positions and velocities. If the position with this process, however, because sometimes more and velocity difference of two tracks from different than one target matches a given track, and sometimes sensors falls within an error ellipse in phase space, more than one track matches a given target. Thus we then the two tracks are considered to correspond to use a best-available-match algorithm, in which the the same aircraft, and the tracks are fused. The flight best match among all target-track pairs is taken first. number and equipment type are transferred to the This matching must be done in real time, even fused tracks and maintained even when the ARTS though all the targets may not yet be available for coverage is lost. When the pairing cannot be per- the present scan. The algorithm allows for corrected formed unambiguously, then fusion is not performed. updates-if a better match is found later, it is used in- This ambiguity avoidance prevents an ARTS flight stead, and the previously used match is withdrawn number from being applied to the wrong ASDE from the track and made available to other tracks. track. The tracks are divided into four classes, in order of The sensor-fusion process maintains filtered posi- priority for access to new targets; these four classes are tion, velocity, acceleration, and altitude estimates for high-confidence tracks, bad-drop tracks, established all tracks. It also estimates and corrects surveillance tracks, and new tracks. High-confidence tracks are clock offsets, and it maintains knowledge of the cur- those which have passed a lead-in filter, which is a rent barometric pressure for use in correcting the travel-distance requirement used to discriminate be- pressure altitudes provided by the ARTS tap. Sensor tween real aircraft or surface-vehicle tracks and those fusion includes a capability to filter tracks on the basis tracks which correspond to false detections or multi- of position, velocity, altitude, area, track length, track path. Bad-drop tracks are former high-confidence reliability, and surveillance source. This capability is tracks that were dropped in regions and at velocities used to suppress multipath and residual clutter, de- where a track drop is not expected. The algorithm fine overlapping radar coverage areas, and reject unin- uses special reacquisition logic based on matching tar- teresting tracks such as boats and overflights. Sensor get area as a function of range and aspect angle to fusion can also coast tracks to allow for following air- compare these tracks to targets not matched to high- craft through surveillance gaps or glitches. The out- confidence tracks. Established tracks are those which put of sensor fusion is one coherent picture of the air- have not yet passed the lead-in filter. New tracks are port surface and approach space, with reliable tracks those which have been seen only once, and thus have that include the information required by both the no associated velocity estimate. New tracks are al- tower controllers and the surface monitor. lowed a much larger association radius to allow air- borne (and hence quickly moving) aircraft as well as Surjace Monitor surface traffic to be tracked when they are first ac- The fused tracks created in the sensor-fusion process quired. A target that does not match to a track in any are passed to the surface monitor, which forms an op- of the four groups will start its own new track for the erational view of the airport trafic. The surface mon- next scan. Thus the result of the scan-to-scan process- itor first locates the tracks with respect to the network ing is a series of track reports for all the detected ob- of runways, taxiways, and approach areas at the air- jects in the surveillance area. port. These areas are defined by bounding polygons,

182 THE LINCOLN LABORATORY JOURNAL VOLUME 7, NUMBER 2, 1994 EGGERT Demonstration of Runway-Status Lights at Logan Aitport so a linear search through the list of polygons is used be multibranched to allow possible turns at every in- with a point-in-polygon algorithm to identify the tersection. Impossible turns, in which the track could correct region. The surface monitor then identifies not make the turn even if it decelerated just for the the present operational state of the tracks, which is turn, are not allowed. These projection trees form the one of the following: stopped, taxiing, arriving, land- basis for the action of the safety logic. ing, departing, landing abort, or departure abort. A The safety logic determines which runway-status state machine with hysteresis in the transitions is used lights or controller alerts need to be illuminated or to provide accurate and stable state identifications. sounded. The projection trees are used to determine Figure 11 illustrates these different track states and which runway-status lights or abstraction thereof their associated transitions. need to be notified of the behavior of a particular The surface monitor then projects the future be- track. Once notified, the control logic for that partic- havior of the tracks. Two projections are made: the ular light determines the behavior of the light. The first is how far the track must move in a certain time projection trees are also used in the demonstration horizon even if it tries to stop, and the second is how alert logic to identify runway conflicts and sound an far the track might move in the same time horizon if audible alert. Using a single surface monitor to gener- it tries to accelerate. State-dependent assumptions are ate both light and alert events enables the system to made for the acceleration and deceleration profiles. maintain logical consistency for lights and alerts and The likely future position of the track lies between to avoid contradictory information being sent to the these two projections. Each projection is allowed to pilots and controllers.

FIGURE 11. Surface-monitor target state diagram. Allowed state transitions are indicated by arrows. The None state is a pseudostate, representing the source and sink for target states. UNK is the unknown state used for an initial indeterminate state or for a don't-care state. ARR is arrival, LDG is landing, TAX is taxiing, STP is stopped, DEP is departure, LBT is landing abort, and DBT is departure abort. The LBT state and the DBT state represent abnormal but not necessarily unsafe aircraft states.

VOLUME 7, NUMBER 2, 1994 THE LINCOLN LABORATORY JOURNAL EGGERT Demonstration of Runway-Status Lights at Logan Airport

ration information to the system. Third, system pro- ASDE Radar Inte face cessing performance will have to be improved. A custom radar interface was designed at Lincoln The performance of the RSLS Logan Demonstra- Laboratory to digitize the analog ASDE-X radar sig- tion can be improved by modifying both the system nal and send digitized samples of interest to the com- architecture and the various components of the sys- puter for processing. The radar output is digitized at tem. Certain system capabilities that might improve 42 MHz with an 8-bit AID converter. The resulting reliability were considered out of scope for the re- data are subjected to a censoring map that determines search system developed here. These capabilities, which regions are of interest and are to be sent on for which include redundant hardware and software, au- further processing, and discards data outside those re- tomatic built-in test procedures, and real-time perfor- gions. The censoring map is downloaded to the radar mance logging, should ultimately be included in a interface at system startup and must be designed sep- fully operational field system. arately for each airport. The use of the censoring map Other system-level improvements that should be at Logan Airport results in a reduction of the data rate considered for future incorporation into the RSLS to approximately 660 kBytelsec. The censored data Logan Demonstration concern greater information can also be recorded on tape for later playback and sharing between the various system components. For analysis. example, the ASDE processing component can better initiate new tracks for arriving aircraft if it is given in- ARTS Tap formation about these aircrafi derived from the ARTS Commercial off-the-shelf hardware was purchased to interface. Similarly, sensor fusion can better fuse tap the ARTS computer with minimal delay and tracks through surveillance gaps if it is given the maximal coverage. The ARTS tap has two parts: a Se- arrival runway predictions computed by the surface rial Communications Interface Processor (SCIP) tap monitor. that looks at the surveillance input to the ARTS com- Another way of improving the performance of the puter, and a Multiple Display Buffer Memory RSLS is through improved surveillance. On the sys- (MDBM) tap that looks at the display information tem level this improved surveillance would be accom- written by the ARTS computer to up to four control- plished by incorporating new surveillance technolo- ler displays. Each part can filter the information to a gies, such as GPS-Squitter, Mode S multilateration, particular geographical region and type of informa- the ASDE-3 radar, or multiple ASDE radars. Surveil- tion desired. The SCIP tap provides position, alti- lance can also be enhanced by improving the perfor- tude, and transponder Mode A code for each aircraft mance of surveillance processing, chiefly in the cases in the approach space. The MDBM tap provides po- of target location in shadows and merged images, and sition, flight identification, and equipment type for improved tracking. the same aircraft. The RSLS performance can also be enhanced by improving sensor fusion's treatment of ambiguous or Future Improvements conflicting surveillance information. Further im- Certain modifications are necessary before the RSLS provements can be made in the capability of the sur- Logan Demonstration can be turned into an opera- face monitor to estimate the time of future events and tional field demonstration. First, an actual field-light- to use such estimates to drive lights. A major im- ing system will need to be installed. This system provement of the system is also possible by carefully should include redundant electrical cabling and elec- tuning all the available parameters. Some of this sys- trical controllers to maintain high reliability, and a tem tuning has already been done, although some- maintenance monitoring facility to shorten down times the parameters used are a compromise between time. Second, a tower-controller interface will need to correct results and processing time, and sometimes ef- be implemented. The tower controllers or the con- fective tuning was impossible because of the lack of troller supervisors will need to input runway configu- adequate assessment tools.

184 THE LINCOLN LABORATORY JOURNAL VOLUME 7, NUMBER 2, 1994 EGGERT Demonstration of Runway-Status Lights at Logan Airport

Summary REFERENCES The off-line proof-of-concept RSLS Logan Demon- stration showed that the system can detect and track ! . E.F. Lyon, "Airport Surface Traffic Automation," Linc. Lab. ]. 4, 151 (1991). aircraft and surface vehicles on an airport surface by 2. J.R. Eggert, R. J. Sasiela, M.P. Kastner, W.H. Harman, H. Wil- using a primary radar, combine surface primary and helmsen, T.J. Morin, H.B. Schultz, J.L. Sturdy, D. Wyscho- approach secondary radar information into one view grod, and P.M. Daly, "Runway Status Light System Demon- stration at Logan Airport," Project Report ATC-206, Lincoln of the airport and its environs, determine what each Laboratory, 7 Jan. 1994. aircraft or surface vehicle is doing, predict the possible 3. U.S. Department of Commerce, U.S. Terminal Procedures, Northeast (NE) KVol. 1 of3, effective 30 Apr. 1992 to 25 June future positions of each track, and use those predic- 1992. tions to drive runway-status lights. The logical con- tinuation of the development of the RSLS should be to incorporate the discussed design improvements, test the system performance over a wide variety of trafic and weather conditions, and install a set of runway-status lights on the field for an operational suitability test.

Acknowledgments The RSLS Demonstration at Logan International Airport in Boston was developed by Lincoln Labora- tory under an interagency agreement between the Federal Aviation Administration and the U.S. Air Force. The RSLS program was previously called Air- port Surface Traffic Automation with (ASTA- I). The development of the RSLS Lo- gan Demonstration required the efforts of many ded- icated individuals at Lincoln Laboratory including Walter Brown, Richard Bush, Steve Bussolari, Chuck Crone, Peter Daly James Flavin, Teresa Hall, William Harman, Walter Heath, Marcia Kastner, Ervin Lyon, Doug Marquis, Ted Morin, Tom Petrillo, A1 Reich, Wallace Reid, Richard Sasiela, Kenneth Saunders, Hayden Schultz, Jim Sturdy, Steve Thompson, Jerry Welch, Harald Wilhelmsen, and Dan Wyschogrod.

VOLUME 7, NUMBER 2, 1994 THE LINCOLN LABORATORY JOURNAL EGGERT Demonstration of Runway-Status Lights at Logan Airport

JAMES R. EGGERT is a staff member in the Air Traffic Automation group. His research specialty is in airport surhce-tr&c surveillance and automation. He received a BA. degree in physics and linguistics from Rice Universi- ty in 1979 and a Ph.D. degree in physics from Harvard University in 1986. He has also pursued graduate studies in linguistics at the University of Vienna. As a graduate student at Harvard University, he was an I.B.M. Predoctoral Fellow and he received the John Tynclall Prize. Jim is married, with two children, and his hobbies include zymurgy and genealogy.

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