[{"data":1,"prerenderedAt":384},["ShallowReactive",2],{"learn-\u002Flearn\u002Fmode-s-and-ads-b-explained":3,"learn-nav-\u002Flearn\u002Fmode-s-and-ads-b-explained":368},{"id":4,"title":5,"body":6,"date":294,"description":295,"draft":296,"extension":297,"faqs":298,"howTo":308,"keyTakeaways":316,"meta":321,"navigation":322,"path":323,"quiz":324,"seo":351,"series":352,"seriesOrder":352,"sources":353,"stem":365,"topic":366,"__hash__":367},"learn\u002Flearn\u002Fmode-s-and-ads-b-explained.md","Mode S and ADS-B explained",{"type":7,"value":8,"toc":284},"minimark",[9,32,38,43,51,69,99,103,106,128,132,141,171,187,191,194,233,249,253,273,277],[10,11,12,13,20,21,26,27,31],"p",{},"The transponder that answers a controller's radar has quietly become one of the most capable radios on the aircraft. ",[14,15,19],"a",{"href":16,"className":17},"\u002Flearn\u002Fglossary#gt-mode-s",[18],"glossary-link","Mode S"," and ",[14,22,25],{"href":23,"className":24},"\u002Flearn\u002Fglossary#gt-ads-b",[18],"ADS-B"," are why a modern airframe can be tracked precisely, selectively, and even where there is no radar at all. Here is how they build on the ",[14,28,30],{"href":29},"\u002Flearn\u002Fsquawk-codes-and-transponders","basic transponder",".",[33,34,35],"blockquote",{},[10,36,37],{},"This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.",[39,40,42],"h2",{"id":41},"from-mode-a-and-c-to-mode-s","From Mode A and C to Mode S",[10,44,45,46,50],{},"A classic transponder replies to ",[47,48,49],"strong",{},"Secondary Surveillance Radar"," with the four-digit identity code in Mode A and, with Mode C, its pressure altitude. The weakness is that the ground station has to interrogate everyone in the beam and sort out the overlapping replies, which becomes congested in busy airspace.",[10,52,53,55,56,62,63,68],{},[47,54,19],{}," (the S is for Select) fixes that with three additions, as ",[14,57,61],{"href":58,"rel":59},"https:\u002F\u002Fskybrary.aero\u002Farticles\u002Fmode-s",[60],"nofollow","SKYbrary"," and the ",[14,64,67],{"href":65,"rel":66},"https:\u002F\u002Fstore.icao.int\u002Fen\u002Ftechnical-provisions-for-mode-s-services-and-extended-squitter-doc-9871",[60],"ICAO technical provisions"," set out:",[70,71,72,86,92],"ul",{},[73,74,75,76,79,80,85],"li",{},"A permanent ",[47,77,78],{},"24-bit aircraft address"," unique to the airframe, separate from the ",[14,81,84],{"href":82,"className":83},"\u002Flearn\u002Fglossary#gt-squawk-code",[18],"squawk code"," the pilot sets.",[73,87,88,91],{},[47,89,90],{},"Selective interrogation",": the radar can address one specific aircraft by its address, rather than triggering every transponder in range, which cuts radio congestion.",[73,93,94,95,98],{},"A two-way ",[47,96,97],{},"data link",", which lets the aircraft downlink extra information and underpins ADS-B.",[39,100,102],{"id":101},"elementary-and-enhanced-surveillance","Elementary and enhanced surveillance",[10,104,105],{},"Mode S surveillance comes in two service levels:",[70,107,108,118],{},[73,109,110,113,114,117],{},[47,111,112],{},"Elementary surveillance (ELS)"," adds the ",[47,115,116],{},"flight identification"," (the callsign you filed) and basic status, so the controller's label reads the flight, not just a code.",[73,119,120,123,124,127],{},[47,121,122],{},"Enhanced surveillance (EHS)"," goes further and ",[47,125,126],{},"downlinks aircraft parameters",", such as the selected altitude set in the autopilot, the magnetic heading, indicated airspeed or Mach, and the vertical rate. A controller seeing the selected altitude can spot a level bust being set up before it happens.",[39,129,131],{"id":130},"what-ads-b-actually-is","What ADS-B actually is",[10,133,134,136,137,140],{},[47,135,25],{}," stands for ",[47,138,139],{},"Automatic Dependent Surveillance, Broadcast",", and each word matters:",[70,142,143,149,159,165],{},[73,144,145,148],{},[47,146,147],{},"Automatic:"," it transmits on its own, with no interrogation and no pilot action.",[73,150,151,154,155,158],{},[47,152,153],{},"Dependent:"," it depends on the aircraft's own navigation, typically ",[47,156,157],{},"GPS",", for the position it sends.",[73,160,161,164],{},[47,162,163],{},"Surveillance:"," it serves the same purpose as radar, letting others know where the aircraft is.",[73,166,167,170],{},[47,168,169],{},"Broadcast:"," it is sent out for any suitably equipped receiver, on the ground or in the air.",[10,172,173,176,177,180,181,186],{},[47,174,175],{},"ADS-B Out"," is the aircraft broadcasting its position, velocity, altitude and identity. ",[47,178,179],{},"ADS-B In"," is the aircraft receiving those broadcasts, which can drive a traffic display and, on some systems, weather and flight information. Because the position comes from GPS rather than a radar return, ADS-B can be more accurate and update faster than older radar, and it works where ground radar is impractical, which is why it extends surveillance over oceans and remote terrain. The ",[14,182,185],{"href":183,"rel":184},"https:\u002F\u002Fwww.faa.gov\u002Fair_traffic\u002Fpublications\u002Fatpubs\u002Faim_html\u002Fchap4_section_5.html",[60],"FAA AIM"," describes the system in detail.",[39,188,190],{"id":189},"two-data-links-1090es-and-uat","Two data links: 1090ES and UAT",[10,192,193],{},"ADS-B is carried on two different links:",[70,195,196,210],{},[73,197,198,201,202,205,206,209],{},[47,199,200],{},"1090 MHz Extended Squitter (1090ES)"," is broadcast by ",[47,203,204],{},"Mode S transponders"," and is used ",[47,207,208],{},"worldwide",", including by airline aircraft.",[73,211,212,213,216,217,220,221,224,225,228,229,232],{},"The ",[47,214,215],{},"978 MHz Universal Access Transceiver (UAT)"," is used in the ",[47,218,219],{},"United States"," for lower airspace, and it carries ",[47,222,223],{},"FIS-B"," (flight information, such as weather and notices), which is available only on UAT. ",[47,226,227],{},"TIS-B"," (a ground rebroadcast of traffic) is uplinked on ",[47,230,231],{},"both"," UAT and 1090ES.",[10,234,235,236,238,239,244,245,248],{},"Equipping ",[47,237,175],{}," is now mandatory in much of the world's controlled airspace. In the United States it has been required in the airspace listed in ",[14,240,243],{"href":241,"rel":242},"https:\u002F\u002Fwww.ecfr.gov\u002Fcurrent\u002Ftitle-14\u002Fchapter-I\u002Fsubchapter-F\u002Fpart-91\u002Fsubpart-C\u002Fsection-91.225",[60],"14 CFR 91.225"," since ",[47,246,247],{},"1 January 2020",", and other regions, including Europe, have their own mandates, so check the rule for where you fly.",[39,250,252],{"id":251},"common-pitfalls","Common pitfalls",[70,254,255,261,267],{},[73,256,257,260],{},[47,258,259],{},"The 24-bit address is not the squawk."," The address is fixed to the airframe; the four-digit code is still set by the pilot and can change on every flight.",[73,262,263,266],{},[47,264,265],{},"ADS-B is only as good as its position source."," Because it is dependent on on-board navigation, a degraded GPS input degrades what it broadcasts.",[73,268,269,272],{},[47,270,271],{},"FIS-B weather is a US, UAT feature."," Do not expect the weather uplink on a 1090ES-only installation, though TIS-B traffic is carried on both links. Treat any uplinked weather as situational, not a substitute for the official briefing.",[39,274,276],{"id":275},"in-pilot-efb","In Pilot EFB",[10,278,279,280,283],{},"Pilot EFB keeps a quick ",[47,281,282],{},"squawk-code reference"," for the transponder codes you set by hand, and understanding what Mode S and ADS-B add explains why that four-digit code is only part of what your aircraft is telling the world. It is a personal reference held on your device; Pilot EFB is offline-first and is not a certified Electronic Flight Bag, so your transponder and avionics manuals and ATC remain the authority on what your equipment transmits.",{"title":285,"searchDepth":286,"depth":286,"links":287},"",2,[288,289,290,291,292,293],{"id":41,"depth":286,"text":42},{"id":101,"depth":286,"text":102},{"id":130,"depth":286,"text":131},{"id":189,"depth":286,"text":190},{"id":251,"depth":286,"text":252},{"id":275,"depth":286,"text":276},"2026-06-21","How Mode S adds a 24-bit address, selective interrogation and a data link to the transponder, the difference between elementary and enhanced surveillance, and how ADS-B broadcasts your GPS position.",false,"md",[299,302,305],{"q":300,"a":301},"What does Mode S add over Mode A and Mode C?","Mode S adds a permanent 24-bit aircraft address unique to the airframe, lets a ground radar interrogate one specific aircraft instead of everyone in the beam, and carries a two-way data link. That selective interrogation cuts the radio congestion that older transponders create, and the data link is what allows the aircraft to downlink extra parameters and to support ADS-B.",{"q":303,"a":304},"What is the difference between ADS-B Out and ADS-B In?","ADS-B Out is the aircraft broadcasting its own position, velocity, altitude and identity, derived from on-board navigation such as GPS, without being interrogated. ADS-B In is the aircraft receiving those broadcasts from other aircraft and from ground stations, which can show nearby traffic and, on some systems, weather and flight information.",{"q":306,"a":307},"What is the difference between 1090ES and UAT?","They are two data links for ADS-B. 1090 MHz Extended Squitter (1090ES) is broadcast by Mode S transponders and is used worldwide, including by airline aircraft. The 978 MHz Universal Access Transceiver (UAT) is used in the United States for lower airspace, and it carries the FIS-B flight information service, which is available only on UAT; the TIS-B traffic service is uplinked on both links.",{"name":309,"steps":310},"Trace what your transponder broadcasts",[311,312,313,314,315],"Mode A returns the four-digit identity code you set; Mode C adds pressure altitude.","Mode S adds a unique 24-bit aircraft address and a data link, and answers selective interrogations addressed to your airframe.","Elementary surveillance adds your flight identification and basic status; enhanced surveillance downlinks parameters such as selected altitude, heading and speeds.","ADS-B Out broadcasts your GPS-derived position, velocity and identity automatically, with no interrogation needed.","ADS-B In receives those broadcasts from other aircraft and ground stations, where the equipment is fitted.",[317,318,319,320],"Mode S adds a permanent 24-bit aircraft address, selective interrogation and a data link to the transponder.","Elementary surveillance adds flight identification and status; enhanced surveillance downlinks parameters such as selected altitude, heading and speeds.","ADS-B is automatic and dependent: it broadcasts position from on-board navigation such as GPS with no interrogation.","1090ES is the worldwide ADS-B link from Mode S transponders; UAT on 978 MHz is used in US lower airspace and carries FIS-B weather, while TIS-B traffic is uplinked on both links.",{},true,"\u002Flearn\u002Fmode-s-and-ads-b-explained",[325,334,343],{"q":326,"options":327,"answer":332,"explanation":333},"What makes a Mode S address unique?",[328,329,330,331],"It is the squawk code the pilot sets","It is a permanent 24-bit address tied to the airframe","It changes on every flight","It is the aircraft's registration in plain text",1,"Mode S adds a permanent 24-bit aircraft address unique to the airframe, separate from the four-digit code the pilot sets.",{"q":335,"options":336,"answer":341,"explanation":342},"What does the first word in ADS-B, 'automatic', mean?",[337,338,339,340],"The system needs no pilot action or interrogation to transmit","It automatically avoids other traffic","It selects the squawk code for you","It only works on autopilot",0,"ADS-B is automatic because it broadcasts on its own, with no interrogation and no pilot action; it is dependent because it relies on on-board navigation such as GPS.",{"q":344,"options":345,"answer":332,"explanation":350},"Which service is carried only on the 978 MHz UAT data link in the United States?",[346,347,348,349],"TIS-B traffic","FIS-B weather and flight information","ADS-B Out position","Mode C altitude","FIS-B (weather and flight information) is carried only on the 978 MHz UAT. TIS-B traffic, by contrast, is uplinked on both UAT and 1090ES.",{"title":5,"description":295},null,[354,356,358,360,363],{"label":355,"url":183},"FAA Aeronautical Information Manual, Chapter 4 Section 5 (Surveillance Systems)",{"label":357,"url":241},"14 CFR 91.225: ADS-B Out equipment and use",{"label":359,"url":65},"ICAO Doc 9871: Technical Provisions for Mode S Services and Extended Squitter",{"label":361,"url":362},"SKYbrary: Automatic Dependent Surveillance Broadcast (ADS-B)","https:\u002F\u002Fskybrary.aero\u002Farticles\u002Fautomatic-dependent-surveillance-broadcast-ads-b",{"label":364,"url":58},"SKYbrary: Mode S","learn\u002Fmode-s-and-ads-b-explained","Operations","Yw1ceU_efAGpRzydOqt43-J2Q0RPNqjnD8iDQLWIIzA",{"related":369,"newer":370,"older":375,"series":352},[370,375,380],{"path":371,"title":372,"description":373,"date":294,"topic":366,"draft":296,"minutes":374,"series":352,"seriesOrder":352},"\u002Flearn\u002Fcold-temperature-altimeter-corrections","Cold-temperature altimeter corrections","Why a pressure altimeter over-reads in cold air, leaving you lower than indicated, and how to correct minimum altitudes for temperature on an approach in mountainous or freezing conditions.",4,{"path":376,"title":377,"description":378,"date":294,"topic":366,"draft":296,"minutes":379,"series":352,"seriesOrder":352},"\u002Flearn\u002Fthe-global-reporting-format-for-runway-conditions","The Global Reporting Format for runway conditions","How the Global Reporting Format (GRF) describes a contaminated runway, including the runway condition code (RWYCC) from 6 to 0, the assessment matrix (RCAM), and how the report is split into thirds.",5,{"path":381,"title":382,"description":383,"date":294,"topic":366,"draft":296,"minutes":374,"series":352,"seriesOrder":352},"\u002Flearn\u002Ftop-of-descent-and-the-3-to-1-rule","Top of descent and the 3:1 rule","How to work out your top of descent with the 3:1 rule, the 60-to-1 relationship behind it, and the rate of descent that holds a roughly 3 degree path, with a worked example.",1781989190485]