Traffic Avoidance Systems


Traffic Alert and Collision Avoidance System (TCAS I & II):

  • Developed by the FAA that operates independently from the ground-based ATC system
  • "Last line of defense" for the prevention of mid-air collisions
  • Only responds to transponder equipped aircraft
  • TCAS I:

    • Developed for GA and regional airlines
    • Provides proximity warning only, to assist the pilot in the visual acquisition of intruder aircraft
    • No recommended avoidance maneuvers are provided nor authorized as a direct result of a TCAS I warning
    • Issues TAs of potentially conflicting aircraft
    • It is intended for general aviation or smaller commuter aircraft which have up to 30 passenger seats
    • Provides approximate bearing and relative altitude with a selectable range
  • TCAS II:

    • More sophisticated than TCAS I, providing traffic advisories (TAs) and resolution advisories (RAs)
    • Analyzes projected flight path and issues RA in addition to what TCAS I provides
    • Designed for larger aircraft holding 31 passengers or more
    • RAs provide recommended maneuvers in the vertical direction (climbs and descents) only
      • Each pilot who deviates from an ATC clearance in response to a TCAS II RA must notify ATC of that deviation as soon as practicable and expeditiously return to the current ATC clearance when the traffic conflict is resolved
      • Deviations from rules, policies, or clearances should be kept to the minimum necessary to satisfy a TCAS II RA
      • The serving IFR air traffic facility is not responsible to provide approved standard IFR separation to an aircraft after a TCAS II RA maneuver until one of the following conditions exists:
        1. The aircraft has returned to its assigned altitude and course
        2. Alternate ATC instructions have been issued
        3. TCAS does not alter or diminish the pilot's basic authority and responsibility to ensure safe flight. Since TCAS does not respond to aircraft which are not transponder equipped or aircraft with a transponder failure, TCAS alone does not ensure safe separation in every case
        4. At this time, no air traffic service nor handling is predicated on the availability of TCAS equipment in the aircraft
  • TCAS Alerts:

    • Traffic Advisory (TA):
      • Caution level alert
      • Aircraft will reach closest point of approach in about 45 seconds
    • Resolution Advisory (RA):
      • Warning level alert
      • Aircraft will reach closest point of approach in about 25 seconds
    • Proximate Traffic:
      • Another aircraft not classified as TA or RA and:
        • < 6 NM and < 1,200' vertically
        • < 6 NM and not providing altitude information
    • Other Traffic:
      • Another aircraft not classified as TA, RA or proximate and > 6 NM or > 1,200' vertically from your airplane
    • TCAS Alerts
      TCAS Alerts
  • TCAS Inhibits:

    • All TCAS alerts inhibited by:
      • GPWS alerts
      • Windshear alert
    • "INCREASE DESCENT" RAs inhibited below 1,400' radio altitude
    • "DESCEND" RAs inhibit below 1,000' radio altitude when descending; below 1,200' radio altitude when climbing
    • All RAs inhibited below 1,100' radio altitude when climbing; below 900' radio altitude when descending
    • All TCAS voice alerts inhibited below 1,100' radio altitude when climbing; below 900' radio altitude when descending
    • For some aircraft, TCAS voice alerts are inhibited below 500' radio altitude
    • TCAS Inhibits
      TCAS Inhibits

Traffic Alerting Systems:

  • Ground Proximity Warning System (GPWS):

    • Used as early as the 1970s
    • Uses radio altimeter, speed, and barometric altitude to determine position relative to the ground
    • Provides limited predictability based on algorithms
    • Unable to provide predictive information in mountainous areas
    • Can determine gear and flap status and glide slope deviation to detect unsafe conditions
    • Provides advisory callouts
    • Usually tied into the hot bus bar to prevent inadvertent switch off
  • Terrain Awareness and Warning System (TAWS):

    • Uses GPS positioning and a database of terrain and obstructions to predict conflicts
    • Aural and visual warnings instructing the pilot what action to take
    • Relies on GPS
    • Land based and compensates for performance and speed
    • A turbine powered registered airplane with 6 or more passengers must have a TAWS
    • The Airplane Flight Manual must have procedures for:
      • The use of the terrain awareness and warning system; and
      • Proper flight crew reaction in response to the terrain awareness and warning system audio and visual warnings
      • This does not apply to parachute operations within 50nm of the local airport, firefighting operations, flight operations when incident to the aerial application of chemicals and other substances
  • Heads-Up Display (HUD):

    • Provides a projection on a transparent screen
    • Diminishes the shift between looking inside and outside
    • Variety of information can be displayed

Traffic Information Service (TIS):

  • TIS Proximity Coverage Volume
    TIS Proximity Coverage Volume
  • Terminal Mode S Radar Sites
    Terminal Mode S Radar Sites
  • Traffic Information Service Avionics Block Diagram
    Traffic Information Service Avionics Block Diagram
  • The Traffic Information Service (TIS) provides information to the cockpit via data link, that is similar to VFR radar traffic advisories normally received over voice radio
  • TIS is intended to improve the safety and efficiency of "see and avoid" flight through an automatic display that informs the pilot of nearby traffic and potential conflict situations
  • This traffic display is intended to assist the pilot in visual acquisition of these aircraft
    • No recommended avoidance maneuvers are provided nor authorized as a direct result of a TIS intruder display or TIS alert
    • It is intended for use by aircraft in which TCAS is not required
  • TIS does not alter or diminish the pilot's basic authority and responsibility to ensure safe flight. Since TIS does not respond to aircraft which are not transponder equipped, aircraft with a transponder failure, or aircraft out of radar coverage, TIS alone does not ensure safe separation in every case
  • At this time, no air traffic service nor handling is predicated on the availability of TIS equipment in the aircraft
  • Presently, no air traffic services or handling is predicated on the availability of an ADS-B cockpit display. A "traffic-in-sight" reply to ATC must be based on seeing an aircraft out-the-window, NOT on the cockpit display
  • TIS employs an enhanced capability of the terminal Mode S radar system, which contains the surveillance data, as well as the data link required to "uplink" this information to suitably-equipped aircraft (known as a TIS "client")
  • TIS provides estimated position, altitude, altitude trend, and ground track information for up to 8 intruder aircraft within 7 NM horizontally, +3,500 and -3,000 feet vertically of the client aircraft [Figure 3]
  • The range of a target reported at a distance greater than 7 NM only indicates that this target will be a threat within 34 seconds and does not display a precise distance
  • TIS will alert the pilot to aircraft (under surveillance of the Mode S radar) that are estimated to be within 34 seconds of potential collision, regardless of distance of altitude
  • TIS surveillance data is derived from the same radar used by ATC; this data is uplinked to the client aircraft on each radar scan (nominally every 5 seconds)
  • TIS Proximity Coverage Volume
    TIS Proximity Coverage Volume
  • Terminal Mode S Radar Sites
    Terminal Mode S Radar Sites
  • Traffic Information Service Avionics Block Diagram
    Traffic Information Service Avionics Block Diagram
  • TIS Requirements:

    • In order to use TIS, the client and any intruder aircraft must be equipped with the appropriate cockpit equipment and fly within the radar coverage of a Mode S radar capable of providing TIS
      • Typically, this will be within 55 NM of Terminal Mode S Radar Sites [Figure 4]
      • ATC communication is not a requirement to receive TIS, although it may be required by the particular airspace or flight operations in which TIS is being used
    • The cockpit equipment functionality required by a TIS client aircraft to receive the service consists of the following [Figure 5]:
      • Mode S data link transponder with altitude encoder
      • Data link applications processor with TIS software installed
      • Control-display unit
      • Optional equipment includes a digital heading source to correct display errors caused by "crab angle" and turning maneuvers
      • NOTE: Some of the above functions will likely be combined into single pieces of avionics, such as the transponder and processor
    • To be visible to the TIS client, the intruder aircraft must, at a minimum, have an operating transponder (Mode A, C or S)
      • All altitude information provided by TIS from intruder aircraft is derived from Mode C reports, if appropriately equipped
    • TIS will initially be provided by the terminal Mode S systems that are paired with ASR-9 digital primary radars
      • These systems are in locations with the greatest traffic densities, thus will provide the greatest initial benefit
      • The remaining terminal Mode S sensors, which are paired with ASR-7 or ASR-8 analog primary radars, will provide TIS pending modification or relocation of these sites [Figure 4]
      • There is no mechanism in place, such as NOTAMs, to provide status update on individual radar sites since TIS is a nonessential, supplemental information service
    • The FAA also operates en route Mode S radars (not illustrated) that rotate once every 12 seconds
      • These sites will require additional development of TIS before any possible implementation
      • There are no plans to implement TIS in the en route Mode S radars at the present time
  • Capabilities:

    • TIS provides ground-based surveillance information over the Mode S data link to properly equipped client aircraft to aid in visual acquisition of proximate air traffic. The actual avionics capability of each installation will vary and the supplemental handbook material must be consulted prior to using TIS. A maximum of eight (8) intruder aircraft may be displayed; if more than eight aircraft match intruder parameters, the eight "most significant" intruders are uplinked. These "most significant" intruders are usually the ones in closest proximity and/or the greatest threat to the TIS client
    • TIS, through the Mode S ground sensor, provides the following data on each intruder aircraft:
      • Relative bearing information in 6° increments
      • Relative range information in 1/8 NM to 1 NM increments (depending on range)
      • Relative altitude in 100-foot increments (within 1,000 feet) or 500-foot increments (from 1,000-3,500 feet) if the intruder aircraft has operating altitude reporting capability
      • Estimated intruder ground track in 45° increments
      • Altitude trend data (level within 500 fpm or climbing/descending >500 fpm) if the intruder aircraft has operating altitude reporting capability
      • Intruder priority as either an "traffic advisory" or "proximate" intruder
    • When flying from surveillance coverage of one Mode S sensor to another, the transfer of TIS is an automatic function of the avionics system and requires no action from the pilot
    • There are a variety of status messages that are provided by either the airborne system or ground equipment to alert the pilot of high priority intruders and data link system status. These messages include the following:
      • Alert:

        • Identifies a potential collision hazard within 34 seconds. This alert may be visual and/or audible, such as a flashing display symbol or a headset tone. A target is a threat if the time to the closest approach in vertical and horizontal coordinates is less than 30 seconds and the closest approach is expected to be within 500 feet vertically and 0.5 nautical miles laterally
      • TIS Traffic:

        • TIS traffic data is displayed
      • Coasting:

        • The TIS display is more than 6 seconds old. This indicates a missing uplink from the ground system. When the TIS display information is more than 12 seconds old, the "No Traffic" status will be indicated
      • No Traffic:

        • No intruders meet proximate or alert criteria. This condition may exist when the TIS system is fully functional or may indicate "coasting" between 12 and 59 seconds old (see coasting, above)
      • TIS Unavailable::

        • The pilot has requested TIS, but no ground system is available. This condition will also be displayed when TIS uplinks are missing for 60 seconds or more
      • TIS Disabled:

        • The pilot has not requested TIS or has disconnected from TIS
      • Good-bye:

        • The client aircraft has flown outside of TIS coverage
      • NOTE: Depending on the avionics manufacturer implementation, it is possible that some of these messages will not be directly available to the pilot
    • Depending on avionics system design, TIS may be presented to the pilot in a variety of different displays, including text and/or graphics. Voice annunciation may also be used, either alone or in combination with a visual display. FIG 4-5-6, Traffic Information Service (TIS), Avionics Block Diagram, shows an example of a TIS display using symbology similar to the Traffic Alert and Collision Avoidance System (TCAS) installed on most passenger air carrier/commuter aircraft in the U.S. The small symbol in the center represents the client aircraft and the display is oriented "track up," with the 12 o'clock position at the top. The range rings indicate 2 and 5 NM. Each intruder is depicted by a symbol positioned at the approximate relative bearing and range from the client aircraft. The circular symbol near the center indicates an "alert" intruder and the diamond symbols indicate "proximate" intruders
    • The inset in the lower right corner of FIG 4-5-6, Traffic Information Service (TIS), Avionics Block Diagram, shows a possible TIS data block display. The following information is contained in this data block:
      • The intruder, located approximately four o'clock, three miles, is a "proximate" aircraft and currently not a collision threat to the client aircraft. This is indicated by the diamond symbol used in this example
      • The intruder ground track diverges to the right of the client aircraft, indicated by the small arrow
      • The intruder altitude is 700 feet less than or below the client aircraft, indicated by the "-07" located under the symbol
      • The intruder is descending >500 fpm, indicated by the downward arrow next to the "-07" relative altitude information. The absence of this arrow when an altitude tag is present indicates level flight or a climb/descent rate less than 500 fpm
        • Note: If the intruder did not have an operating altitude encoder (Mode C), the altitude and altitude trend "tags" would have been omitted
  • Limitations:

    • TIS is NOT intended to be used as a collision avoidance system and does not relieve the pilot's responsibility to "see and avoid" other aircraft (see See and Avoid). TIS must not be for avoidance maneuvers during IMC or other times when there is no visual contact with the intruder aircraft. TIS is intended only to assist in visual acquisition of other aircraft in VMC. Avoidance maneuvers are neither provided nor authorized as a direct result of a TIS intruder display or TIS alert
    • While TIS is a useful aid to visual traffic avoidance, it has some system limitations that must be fully understood to ensure proper use. Many of these limitations are inherent in secondary radar surveillance. In other words, the information provided by TIS will be no better than that provided to ATC. Other limitations and anomalies are associated with the TIS predictive algorithm
      • Intruder Display Limitations:

        • TIS will only display aircraft with operating transponders installed. TIS relies on surveillance of the Mode S radar, which is a "secondary surveillance" radar similar to the ATCRBS
      • TIS Client Altitude Reporting Requirement:

        • Altitude reporting is required by the TIS client aircraft in order to receive TIS. If the altitude encoder is inoperative or disabled, TIS will be unavailable, as TIS requests will not be honored by the ground system. As such, TIS requires altitude reporting to determine the Proximity Coverage Volume as indicated in FIG 4-5-4. TIS users must be alert to altitude encoder malfunctions, as TIS has no mechanism to determine if client altitude reporting is correct. A failure of this nature will cause erroneous and possibly unpredictable TIS operation. If this malfunction is suspected, confirmation of altitude reporting with ATC is suggested
      • Intruder Altitude Reporting:

        • Intruders without altitude reporting capability will be displayed without the accompanying altitude tag. Additionally, nonaltitude reporting intruders are assumed to be at the same altitude as the TIS client for alert computations. This helps to ensure that the pilot will be alerted to all traffic under radar coverage, but the actual altitude difference may be substantial. Therefore, visual acquisition may be difficult in this instance
      • Coverage Limitations:

        • Since TIS is provided by ground-based, secondary surveillance radar, it is subject to all limitations of that radar. If an aircraft is not detected by the radar, it cannot be displayed on TIS. Examples of these limitations are as follows:
          • TIS will typically be provided within 55 NM of Terminal Mode S Radar Sites [Figure 4]. This maximum range can vary by radar site and is always subject to "line of sight" limitations; the radar and data link signals will be blocked by obstructions, terrain, and curvature of the earth
          • TIS will be unavailable at low altitudes in many areas of the country, particularly in mountainous regions. Also, when flying near the "floor" of radar coverage in a particular area, intruders below the client aircraft may not be detected by TIS
          • TIS will be temporarily disrupted when flying directly over the radar site providing coverage if no adjacent site assumes the service. A ground-based radar, similar to a VOR or NDB, has a zenith cone, sometimes referred to as the cone of confusion or cone of silence. This is the area of ambiguity directly above the station where bearing information is unreliable. The zenith cone setting for TIS is 34 degrees: Any aircraft above that angle with respect to the radar horizon will lose TIS coverage from that radar until it is below this 34 degree angle. The aircraft may not actually lose service in areas of multiple radar coverage since an adjacent radar will provide TIS. If no other TIS-capable radar is available, the "Good-bye" message will be received and TIS terminated until coverage is resumed
      • Intermittent Operations:

        • TIS operation may be intermittent during turns or other maneuvering, particularly if the transponder system does not include antenna diversity (antenna mounted on the top and bottom of the aircraft). As in coverage limits noted above, TIS is dependent on two-way, "line of sight" communications between the aircraft and the Mode S radar. Whenever the structure of the client aircraft comes between the transponder antenna (usually located on the underside of the aircraft) and the ground-based radar antenna, the signal may be temporarily interrupted
      • TIS Predictive Algorithm:

        • TIS information is collected one radar scan prior to the scan during which the uplink occurs. Therefore, the surveillance information is approximately 5 seconds old. In order to present the intruders in a "real time" position, TIS uses a "predictive algorithm" in its tracking software. This algorithm uses track history data to extrapolate intruders to their expected positions consistent with the time of display in the cockpit. Occasionally, aircraft maneuvering will cause this algorithm to induce errors in the TIS display. These errors primarily affect relative bearing information; intruder distance and altitude will remain relatively accurate and may be used to assist in "see and avoid." Some of the more common examples of these errors are as follows:
          • When client or intruder aircraft maneuver excessively or abruptly, the tracking algorithm will report incorrect horizontal position until the maneuvering aircraft stabilizes
          • When a rapidly closing intruder is on a course that crosses the client at a shallow angle (either overtaking or head on) and either aircraft abruptly changes course within 1/4 NM, TIS will display the intruder on the opposite side of the client than it actually is
            • These are relatively rare occurrences and will be corrected in a few radar scans once the course has stabilized
      • Heading/Course Reference:

        • Not all TIS aircraft installations will have onboard heading reference information. In these installations, aircraft course reference to the TIS display is provided by the Mode S radar. The radar only determines ground track information and has no indication of the client aircraft heading. In these installations, all intruder bearing information is referenced to ground track and does not account for wind correction. Additionally, since ground-based radar will require several scans to determine aircraft course following a course change, a lag in TIS display orientation (intruder aircraft bearing) will occur. As in the TIS predictive algorithm above, intruder distance and altitude are still usable
      • Closely-Spaced Intruder Errors:

        • When operating more than 30 NM from the Mode S sensor, TIS forces any intruder within 3/8 NM of the TIS client to appear at the same horizontal position as the client aircraft. Without this feature, TIS could display intruders in critical situations (e.g., a closely-spaced intruder that is actually to the right of the client may appear on the TIS display to the left). At longer distances from the radar, TIS cannot accurately determine relative bearing/distance information on intruder aircraft that are in close proximity to the client
          • Because TIS uses a ground-based, rotating radar for surveillance information, the accuracy of TIS data is dependent on the distance from the sensor (radar) providing the service. This is much the same phenomenon as experienced with ground-based navigational aids, such as VOR. As distance from the radar increases, the accuracy of surveillance decreases. Since TIS does not inform the pilot of distance from the Mode S radar, the pilot must assume that any intruder appearing at the same position as the client aircraft may actually be up to 3/8 NM away in any direction. Consistent with the operation of TIS, an alert on the display (regardless of distance from the radar) should stimulate an outside visual scan, intruder acquisition, and traffic avoidance based on outside reference
  • Reports of TIS Malfunctions:

    • Users of TIS can render valuable assistance in the early correction of malfunctions by reporting their observations of undesirable performance. Reporters should identify the time of observation, location, type and identity of aircraft, and describe the condition observed; the type of transponder processor, and software in use can also be useful information. Since TIS performance is monitored by maintenance personnel rather than ATC, it is suggested that malfunctions be reported by radio or telephone to the nearest Flight Service Station (FSS) facility

Traffic Information Service-Broadcast (TIS-B):

  • Traffic Information Service-Broadcast (TIS-B) is the broadcast of ATC derived traffic information to ADS-B equipped (1090ES or UAT) aircraft from ground radio stations. The source of this traffic information is derived from ground-based air traffic surveillance sensors. TIS-B service will be available throughout the NAS where there are both adequate surveillance coverage from ground sensors and adequate broadcast coverage from ADS-B ground radio stations. The quality level of traffic information provided by TIS-B is dependent upon the number and type of ground sensors available as TIS-B sources and the timeliness of the reported data [Figure 2/3]
  • TIS-B Requirements:

    • In order to receive TIS-B service, the following conditions must exist:
      • Aircraft must be equipped with an ADS-B transmitter/receiver or transceiver, and a cockpit display of traffic information (CDTI)
      • Aircraft must fly within the coverage volume of a compatible ground radio station that is configured for TIS-B uplinks (Not all ground radio stations provide TIS-B due to a lack of radar coverage or because a radar feed is not available)
      • Aircraft must be within the coverage of and detected by at least one ATC radar serving the ground radio station in use
  • TIS-B Capabilities:

    • TIS-B is intended to provide ADS-B equipped aircraft with a more complete traffic picture in situations where not all nearby aircraft are equipped with ADS-B Out. This advisory-only application is intended to enhance a pilot's visual acquisition of other traffic
    • Only transponder-equipped targets (i.e., Mode A/C or Mode S transponders) are transmitted through the ATC ground system architecture. Current radar siting may result in limited radar surveillance coverage at lower altitudes near some airports, with subsequently limited TIS-B service volume coverage. If there is no radar coverage in a given area, then there will be no TIS-B coverage in that area
  • TIS-B Limitations:

    • TIS-B is NOT intended to be used as a collision avoidance system and does not relieve the pilot's responsibility to "see and avoid" other aircraft, in accordance with 14 CFR 91.113b. TIS-B must not be used for avoidance maneuvers during times when there is no visual contact with the intruder aircraft. TIS-B is intended only to assist in the visual acquisition of other aircraft
      • No aircraft avoidance maneuvers are authorized as a direct result of a TIS-B target being displayed in the cockpit
    • While TIS-B is a useful aid to visual traffic avoidance, its inherent system limitations must be understood to ensure proper use
      • A pilot may receive an intermittent TIS-B target of themselves, typically when maneuvering (e.g., climbing turns) due to the radar not tracking the aircraft as quickly as ADS-B
      • The ADS-B-to-radar association process within the ground system may at times have difficulty correlating an ADS-B report with corresponding radar returns from the same aircraft. When this happens the pilot may see duplicate traffic symbols (i.e., "TIS-B shadows") on the cockpit display
      • Updates of TIS-B traffic reports will occur less often than ADS-B traffic updates. TIS-B position updates will occur approximately once every 3-13 seconds depending on the type of radar system in use within the coverage area. In comparison, the update rate for ADS-B is nominally once per second
      • The TIS-B system only uplinks data pertaining to transponder-equipped aircraft. Aircraft without a transponder will not be displayed as TIS-B traffic
      • There is no indication provided when any aircraft is operating inside or outside the TIS-B service volume, therefore it is difficult to know if one is receiving uplinked TIS-B traffic information
    • Pilots and operators are reminded that the airborne equipment that displays TIS-B targets is for pilot situational awareness only and is not approved as a collision avoidance tool. Unless there is an imminent emergency requiring immediate action, any deviation from an air traffic control clearance in response to perceived converging traffic appearing on a TIS-B display must be approved by the controlling ATC facility before commencing the maneuver, except as permitted under certain conditions in 14CFR 91.123. Uncoordinated deviations may place an aircraft in close proximity to other aircraft under ATC control not seen on the airborne equipment and may result in a pilot deviation or other incident
  • Reports of TIS-B Malfunctions:

    • Users of TIS-B can provide valuable assistance in the correction of malfunctions by reporting instances of undesirable system performance. Since TIS-B performance is monitored by maintenance personnel rather than ATC, report malfunctions to the nearest Flight Service Station (FSS) facility by radio or telephone, or by sending an email to the ADS-B help desk at
    • Reporters should identify:
      1. Condition observed
      2. Date and time of observation
      3. Altitude and location of observation
      4. Type and call sign of the aircraft
      5. Type and software version of avionics system

Traffic Alert Systems (TA):

  • Receive transponder information to determine relative position to equipped aircraft
  • Provide three-dimensional location of other aircraft and are cost effective alternatives to TCAS equipage for smaller aircraft

Safety Alert Responsibilities:

  • Pilot Responsibilities:

    • Initiates appropriate action if a safety alert is received from ATC
    • Be aware that this service is not always available and that many factors affect the ability of the controller to be aware of a situation in which unsafe proximity to terrain, obstructions, or another aircraft may be developing
  • Controller Responsibilities:

    • Issues a safety alert if aware an aircraft under their control is at an altitude which, in the controller's judgment, places the aircraft in unsafe proximity to terrain, obstructions or another aircraft
    • Types of safety alerts are:
      • Terrain or Obstruction Alert:

        • Immediately issued to an aircraft under their control if aware the aircraft is at an altitude believed to place the aircraft in unsafe proximity to terrain or obstructions
      • Aircraft Conflict Alert:

        • Immediately issued to an aircraft under their control if aware of an aircraft not under their control at an altitude believed to place the aircraft in unsafe proximity to each other
        • With the alert, they offer the pilot an alternative, if feasible
    • Discontinue further alerts if informed by the pilot action is being taken to correct the situation or that the other aircraft is in sight


  • TIS operates at only those terminal Mode S radar sites depicted in AIM FIG 4-5-5
    • Though similar in some ways, TIS is not related to TIS-B (Traffic Information Service-Broadcast)
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