A Novice's Quick Guide To Avionics


© 2013 by Richard Harris
Rev.2A, Jan. 16, 2014



"Avionics" (the trade jargon for "aviation electronics") are the radios and related electronic gadgets that make it possible for a pilot to communicate with the ground (or other aircraft aloft), and to successfully navigate an airplane -- or have it fly itself -- from one place to another, while providing the pilot with electronic information about the ground over which he's flying, and the about the condition of his aircraft and its environment.


(not comprehensive, but covering most major types in wide civilian use;
 illustrated version coming soon):







Like the rest of the electronics world, avionics has undergone a string of basic technology changes --
•  from battery power to generated current,
•  from vacuum tubes to transistors to integrated circuits,
•  from low frequencies to increasingly higher frequencies,
•  from mechanical displays to video displays,
•  from analog to digital,
•  from standalone units to integrated systems,
•  from simple-and-basic to complex-and-sophisticated.
  Avionics prices, too, have fallen like the rest of the electronics world, but not at the same rate. Avonics remain among the costliest of all the world's electronics.





Originally, in the very early days (World War I), aircraft radios were simple "aerial telegraphs," allowing the pilot or other crewman to communicate with the ground by sending a stream of beeps -- "dots" and "dashes" in "Morse code" -- to the person monitoring a ground receiver. 


Eventually the aircraft radio worked both ways. By the late 1920s, two-way voice communications ("radio-telephone") became common for military and commercial aircraft.


Pilots listed to the radio by speaker or (preferably) headphones, and transmitted voice through a hand-held microphone connected to the radio transmitter. The radio receiver was a simple AM-type analog-dial radio, while the transmitter was typically tuned by a complex of tuning coils, sometimes an array of them fixed at specific frequencies, which were selected by a rotaing switch. By the 1940s, some radios were "crystal-controlled," using a small quartz crystal tuned to an exact frequency for precise tuning.


For light aircraft, though, transmitters were often prohibitively expensive, so communications was largely receive-only. Receiving, alone, though, was a huge advance over no communications at all. Pilots could receive routinely-broadcast weather reports, and when in the airport traffic area, hear reports of traffic conditions, and even receive take-off and landing instructions from the tower.








Many aircraft had simple straight-wire, or fixed-loop, antennas that were directional in their sensitivity, but rigidly mounted in a fixed positon on the aircraft.


Turning the aircraft until the radio station signal faded out due to the antenna's angle (a narrow heading range, known as the "null zone" — one of two equal and opposite null-zones of the antenna) -- could point the way directly to-or-from the station.


(NOTE: the pilot couldn't distinguish which of the two opposite indicated directions would take him directly TO the station, and which would take him directly away FROM it — just that the station was on that particular line of position.)


For instance, on airplanes with an antenna aligned along the aircraft's longintudinal axis (nose-to-tail), the two null zones were straight ahead and straight behind — so a when a pilot heard the station signal suddenly fade during a turn, he was — at that moment — on a heading directly to or from that station.


(This technique can be easily demonstrated by taking a battery-powered portable AM radio outdoors, tuning it to a radio station whose transmitter tower's location is known to you, and walking in a circle, holding the radio rigidly in front of you. At two opposite points on your walking circle, the station's transmitter antenna should be sharply weaker in the "null zone" of your receiver's antenna. Once you know how your radio points, you can use it to determine the line of position between you and other radio stations, as well.)


Knowing a line of position was helpful, and greatly improved pilot's idea of his location. But a cross-reference was needed to get an exact "fix" on his precisse location. That could often be easily obtained by simply tuning to another radio station, and finding its line of position. The two lines, when plotted on a map (drawn through the radio stations at the indicated bearing angles), intersected at the pilot's exact location.




Turning the aircraft around in the sky was a cumbersome way to get your radio bearings. A better way was to simply put the antenna on a swivel, and rotate it, instead.


By the 1930s, the development of the radio direction-finder (DF or RDF) allowed pilots to know their location aloft, even in the clouds, by rotating an external "loop" antenna (several loops of wire mounted in a tubular ring, usually on the outside of the plane) until a radio station signal was as strong (or as weak) as possible. From this, the pilot knew that the radio station was in one of two directions (forwards or backwards), relative to the position of the antenna.


Two sets of radio bearings — directions to/from two different radio stations, and represented by a pair of intersecting lines drawn by the pilot or navigator on a map -- identified ("fixed") the plane's exact location relative to the two stations: the navigational "fix".




    Later DF versions included the automatic direction finder (ADF) which included a compass-like instrument, with one or more indicator needles pointing the way to radio stations, and eliminating the to/from ambiguity.

    The DF receivers worked on Low Frequency / Medium Frequency (LF/MF) channels, including regular commercial AM broadcast radio stations, and particularly on aviation-only radio beacons which simply transmitted a Morse code identification, repeated continuously. This system worked well during mild weather.  However, during stormy weather, lightning would play havoc with the system, triggering the directional indicator's wild needle swings towards the sudden jolts of electricity.





By the mid-1930s, the U.S. government established a network of aerial navigation radio towers across the nation, along well-traveled air routes -- "airways" -- with the transmitter and their antennas so arranged as to project a "beam" with a constant tone, audible on the radio when the pilot was on course -- "on the beam."  When off to one side or another, different sounds would be heard.

   This system -- the "four-course radio range" (so named because each station radiated four beams) -- worked well for aircraft on specific narrow air routes, during mild weather, and more or less tolerably during lightning storms (though, being an LF/MF range, the lightning static noise was a common nuisance, often painful to pilots' ears). The system suffered other problems as well -- including a seriously disorienting vagueness and ambiguity in signals, which required some careful thinking and interpretation while using the system -- not too difficult in fair weather, but challenging when flying at night, or on instruments, especially in rough weather.


(For more on on the four-course radio range, click here for a brief illustrated tutorial.)





In the 1940s, World War II caused a rapid growth in avionics development.  The first widely-used "automatic pilot" ("autopilot") was a system that monitored certain instruments and adjusted the position of the pilot's flight controls to keep the airplane upright and flying on a specific heading, and/or at a specified altitude; more advanced autopilots were developed to allow the plane to automatically track a radio beam, remaining pointed towards the radio signal.





During World War II, British and American forces invented a system of high-powered, narrow-beam radio transmissions that could travel dozens or hundreds of miles, and reflect off of metal objects (including buildings, ships and planes), with the return signal being detected by a special antenna, and the distance measured by an electronic instrument that indicated how long the signal took to return to the station, when transmitted in a given direction.  The concept was called "Radio Detection And Ranging" ("RADAR")

  Radar was quickly developed into a sophisticated instrument that could portray the relative direction and distance to an object by television-like display.  Powerful aircraft-detecting radar was useful for both mililtary aircraft and ground-based air-traffic controllers (and air-defense systems).

An additional value for radar was in detecting the presence of rain or other precipitation, surface water, and obstacles, at a distance, allowing safe navigation around (or through) bad weather, even in darkness.  The ability of the radar to detect and display the outlines of bodies of water and obstacle-filled cities improved navigation, as well. This worked even on low-powered radar carried in civilian aircraft, which, however, could not detect other aircraft.





During wartime, it was important to know if the aircraft you were seeing on radar were friend or foe.  The solution was to develop a radio ("transponder") that detected the signal from a radar, and responded by transmitting a coded signal that could trigger a different kind of display on radar.  The pilot could "dial in" a secret code number, and the transponder would send that signal in reply whenever "interrogated" by a radar signal.

   The device became known as "IFF" -- "Identification: Friend or Foe."  In peacetime, the same transponder technology proved useful in assisting air traffic controllers in differentiating between multiple targets on a radar scope, and transponders became required equipment -- first on airliners, then, increasingly, on smaller and smaller craft.






Applying the lessons from radar, the aviation industry developed a very short-range, low-powered radar, pointed down from the aircraft towards the ground, to precisely measure the plane's altitude above the ground.  This "radar altimeter," along with regular radar, provided the first "terrain avoidance system" for blind flying.  Radar altimeters also provided precision guidance during takeoff and landing.





By the 1950s, frustrations with LF/MF radios -- both communications and navigation -- led to a shift towards static-free VHF (very high frequency) radios (just above the frequency range of commercial FM broadcast radio), which were far less susceptible to lightning and other interference.


At this time, early VHF radios served as both communication and navigation radios, with both communications transmitter/reciever ("COM transciever") and navigation reciever ("NAV" reciever) contained in the same unit. This combination radio unit became known as a "NAV/COM," and the term -- and concept -- remains common to the present.


Early low-cost VHF COM transcievers often were very limited in the number of individual frequencies ("channels") on which they could transmit. While simple continuous analog tuning (similar to a cheap portable AM/FM radio) was adequate for the COM receiver, the COM transmitter was legally required to operate with extreme precision, on exactly the right frequency, to prevent interference with others on adjacent channels; this required the use of crystals to provide exact frequency tuning for the transmitter.


With crystal-controlled transmitting, a pilot could tune his continuous analog tuner to the same frequency as his transmitter by keying his microphone (or setting a button on the radio) while tuning the receiver. When he heard a whistling sound, his receiver was tuned to the same exact frequency as his transmitter. This technique was known as "whistle-stop" tuning.


More sophisticated NAV/COM units contained crystals for both the transmitter and reciever, and used a frequency selector knob that tuned both simultaneously. By the 1960s, nearly all VHF NAV/COM radios operated this way, with some accommodating up to 360 channels.


By the late 1970s, a new electronic technology -- "phase-locked loop" -- became available, allowing the VHF COM radios to be tuned to any of hundreds of frequencies with only a handful of crystals, or none at all, sharply reducing the cost of multi-channel NAV/COM radios, and making it practical to further divide the aviation radio band into more channels -- 720 channels in all -- reducing confusion in densely populated areas by reducing the number of airports using the same radio channels.





While the old four-course radio ranges worked well for navigating aircraft on specific narrow air routes, during mild weather, and more or less tolerably during lightning storms, the switch of navigation radios to static-free VHF frequencies was also accompanied by a more sophisticated and versatile direction-finding system:  the VHF Omni-directional (Radio) Range ("VOR").

   VOR transmitters transmitted signals that allowed the VOR receiver in the airplane to know the precise directions to (and from) the station, in any weather.  A very slightly different signal was sent out on each of 360 different "radials" (one for each degree of the compass). The VOR receiver could determine which radial the plane was on -- what direction the plane was from the station -- by the precise signal it received from the VOR.

  The new VOR indicators in the cockpit presented a slightly different appearance from the traditional ADF compass dial.  By comparison, the VOR indicator allowed the pilot to "dial in" a specific desired direction to (or from) the station, and a needle (if centered) would tell him he was on that "radial", and (if not centered) how many degrees the airplane was right or left of the desired radial.

  With this system, a single station could transmit "radial" beams for more than just four airways -- as many as a dozen airways could easily converge on a single station (and, theoretically, up to 360 -- the number of degrees in a full circle around the station).  This made possible more airways, connecting more airports and communities with safe instrument-flight guidance.





 Marker Beacons,

 LOC- Localizer, &

 GS - Glide Slope)


While the directional guidance available from radio ranges and beacons was useful enroute, a miniature (low-powered) version of these ranges was needed for precise local flight guidance at an airport, particularly to guide pilots precisely to the end of a runway in "blind flying" conditions, and enable the pilot to line the aircraft up with the runway.

  Early "blind landing" systems were mostly just simple radio beacons -- "Marker Beacons" -- placed along a route towards the end of a runway. Typically, there were two or three beacons (Outer Marker, Middle Marker, Inner Marker).

   Typically, using a DF or ADF, the pilot flew to the Outer marker, then steered his plane to a course matching the runway alignment.  As the pilot slowed down and descended for landing, he listened in his headphones for the beeping of the Middle Marker and Inner Marker, to gauge his progress and position on final approach.

   Getting the pilot "lined up" exactly on the runway, though, required something more. For precise local guidance, the airports installed UHF (ultra high frequency) "Localizer" ("LOC") beacons projecting a beam precisely along the centerline of a runway.  An indicator on the pilot's panel (typically the VOR indicator, doing double-duty), would show whether the pilot was lined up correctly, or how many degrees left or right of the runway centerline the airplane was.

   However, owing to obstacles and runway characteristics, some airport approach paths required more than horizontal guidance by the localizer -- vertical guidance on the exact angle of descent (glide slope) was needed. Accordingly, "glide slope" ("GS") system emerged, using horizontal UHF beams to enable the pilot's VOR indicator (when equipped with an extra horizontal needle) to let the pilot know if he was "on the glideslope" or above or below (and by how many degrees).

  Today, most ILS systems incorporate one or more of these features: Outer Marker beacon, Middle Marker beacon, Localizer, and/or Glideslope. In modern times, MLS ("microwave landing system") and other ILS techniques have been developed, including variations on GPS systems (see "GPS" below), but VOR-related ILS systems remain widespread.



DME - Distance Measuring Equipment
& TACAN - Tactical Air Navigation


One frustration with radio ranges and beacons was that direction-finding avionics could tell you which DIRECTION they were, but not how far.

   The military led the way to solving this issue with the "Tactical Air Navigation" system ("TACAN") -- an enhanced version of the VOR, but operating at UHF (ultra-high frequency).

   Other than the higher (military) frequency, the key difference of TACAN was the addition of a special kind of transponder, in which the aircraft transmitter sent a momentary pulse out (in all directions), and waited to hear a corresponding reply signal from the TACAN.  By precisely measuring the delay (just a microscopic fraction of a second), the airplane's TACAN receiver could calculate the distance to the station.

   Two or more distance measurements, compared to an internal clock in the TACAN receiver, indicated the speed of the aircraft towards (or away from) the VORTAC or VOR-DME.  A simple switch on the TACAN receiver enabled an indication of speed rather than distance.

   TACAN's distance-and-speed measuring features were so attractive that a version ("DME" -- distance-measuring equipment) was integrated into many civilian VOR stations on the ground, which then became known as VORTAC or VOR-DME stations.



RNAV - Area Navigation


While the VOR was a solid advance over the LF/MF beacons and radio ranges, it still limited pilots to navigating along specific radials between stations.  To achieve "free flight", a new system called "RNAV" --  Area Navigation -- was developed and promoted in the 1970s.

   RNAV is essentially a navigation computer, in the airplane, that automatically calculates the precise position of the airplane based upon direction and distance to a VORTAC or VOR-DME, using information from the plane's VOR and DME receivers.  Once the position of the plane is calculated, the pilot can program the RNAV computer to create a "waypoint" anywhere within the reach of the station and direct the RNAV computer to treat that waypoint as an imaginary VOR station, triggering the VOR indicator to operate as if it was receiving a signal from a station at that waypoint.

  With this pilot-programmable "waypoint" system, RNAV computers allow pilots to create their own "airways", and fly on them, without being confined to official airways, flying to-and-from the locations of real VOR stations.  This concept was promoted as "free flight" (though not the only meaning of that term).



INS - Inertial Navigation System


   Aircraft operating over vast stretches of ocean, and military aircraft operating over large areas of hostile territory, were often deprived of radio navigation aids.  An exotic system of instruments -- using gyroscopes and accelerometers that sensed the movement of the plane by effects on their inertia, and tracking changes over time with a precise clock -- physically measured an airplane's position and movement relative to a fixed point of origin. The INS indicated precise latitude and longitude coordinates to the pilot throughout the flight.

  INS systems, at their most precise, were used to guide bombers to distant targets around the globe, without any reference to external radio signals.  Due to the exotic nature of their high-precision electronic and mechanical instrumentation, INS systems were extremely expensive, and normally limited to military and commercial aircraft, and high-end business jets.



LORAN - Long-Range Navigation


   LORAN -- "Long-Range Navigation" -- was originally developed to guide ships and submarines at sea.  Using VLF (very low frequency) radio signals, which have extremely long range even with relatively little power, LORAN stations transmitted a complex varying range of signals that overlapped signals from other LORAN stations.  By knowing the geographic locations (latitude and longitude coordinates) of two or more LORAN stations, a LORAN reciever tuned to them could compute the (fairly precise) location of the aircraft.

  While not as useful over land as the other forms of navigation, LORAN's extraordinary range made it useful over water, and other vast, unpopulated areas.  With the development of more modern LORAN instruments, the system became popular even for light aircraft use over land.  Around the turn of the century, with the rise of GPS (see below) LORAN became largely obsolete, and was generally discontinued.





The ideal cockpit instrument for navigation has always been a "moving map" that would always display the pilot's exact position on a map that moved as the aircraft moved.  In practice, this has been impractical until fairly recently, for a number of reasons.  But as early as the 1950s, moving map systems became a concept that grew into limited practical use by the 1980s.

   The first "moving maps" were just that:  small paper maps that physically moved on a set of rollers, on a panel in the cockpit.  Timed to the speed of the plane, and pre-oriented by the pilot along the route of flight, these primitive moving maps were, at first, somewhat unreliable in presenting the pilot's exact position. 

   Eventually, the technology evolved to TV screens showing recorded map images, which adjusted their display based on the aircraft's exact position and speed, as indicated by navigation signals from the other avionics.  The development of DME, RNAV, INS and LORAN made true precision moving-map systems possible and reliable. The rise of digital electronics with the "computer revolution" of the 1970s and 1980s made them far more practical and versatile.








·        Automatic & Precision Navigation:

o   HSI

o   Flight Director

o   FMS

o   GPS & WAAS


·        Aircraft/Ground Collision Avoidance:




·        Position & Identity Reporting

o   Mode C

o   Mode S

o   ADS-B


·        Integration & Display:

o   "glass cockpits"



Author Richard Harris is an FAA-certified aeronautics instructor who has developed and edited pilot training manuals and official operating manuals for a wide range of aircraft, from single-engine propeller aircraft to multi-engine business jets.  He is an FCC-certified radio technician and certified computer instructor, with occupational experience as an aircraft instrument technician.