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GPS and Avionics: How We Got Here Depends on How We Used to Get Here

When Martha and I first started flying together, over 40 years ago, GPS for General Aviation wasn’t even on the drawing board and glass cockpits were just planes with lots of windows…

We just finished a major upgrade to our course on the Garmin G1000, version 8 (Flying the Garmin G1000). We’re excited about how the course turned out, especially the integrated procedures trainer (no need to sit in your plane to learn your G1000). But while we were teaching mastery of the 125 knobs and buttons on the G1000, we found ourselves appreciating the wonderful tools available to today’s pilots.

Recently, we have been flying the C162 Skycatcher, making video for a new Cessna course (YouTube: First Skycatcher flies to King Schools).  Even that wonderful light sport aircraft has a glass cockpit, and we love flying with the G1000 in our Cessna 172SP (the sweepstakes plane we’re giving away in January!).  Even our super-old Falcon 10 has glass in it, since it was updated to have EFIS and an MFD some time back.

Occasionally, when we take a “round dial” airplane up, we are reminded just how much things have changed. So we thought we’d take a moment and fly through the waypoints of civilian aviation navigation history. Let’s see how this technology came together to enable GPS, the foundations of avionics, and the G1000 in particular:

Departure: ca 1761

Some say the best place to start is at the beginning. So, to see where it all began, we’ll jump back to 1761 when the chronometer combined with the compass and the sextant to give mariners complete open-sea navigation. This was about as low-tech as you could get, yet using a number of stable reference points provided by nature (stars and time), travelers could triangulate their location across the expanse of the seas. As much as technology has evolved, we find that even the most sophisticated navigation system shares traits with this earliest approach.

The earliest set of navigation instruments used star position and time to determine position
The earliest set of navigation instruments used star position and time to determine position

Beacons Beckon – Keeping the Home Fires Lit

Fast forward to 1919, when pilots decide to make their own “transmitting” navigational aids—bonfires. Pilots found this simple approach could be used for visual reference flight at night.  Later, these were replaced with light beacons.  Still no real tools or electronics required; just good pilotage and dead-reckoning.

By 1929, the first four-course radio range went into service.  This was a real electronic breakthrough for flight.  Twin towers emitted alternating signals that converged to a steady tone when you were “on the beam”.    The technology was widely used during WWII.  It was a great navaid if you wanted to be “on the beam”.  Anywhere else and you were “on your own” with your gyroscope and altimeter.

Sometimes Delays are a Good Thing!

LORAN Station from the Aleutian Island Chain, ca 1943
LORAN Station from the Aleutian Island Chain, ca 1943

It took 10 years before the next historical waypoint to GPS navigation would come along.  It was the war-time development of LOng RAnge Navigation, or LORAN.  This 1940’s system introduced the concept of measuring the time-delay of a signal traveling to a receiver to calculate position—still used in today’s GPS.  Whereas the four-course radio range told you where you should be, LORAN told you where you were.  Knowing where you were as well as where you wanted to go was a big step.

Around the same time that LORAN started, the first Ultra-High Frequency (UHF) radio was used for scheduled airline navigation.

By the 1950’s VHF Omnidirectional Radio Range (VOR) was widely available for US navigation.  Like the four-course radio range, VOR used two transmitters.  But in VOR, the first signal broadcasts an omnidirectional reference signal and the second signal rotates a line-of-sight beam, much like a lighthouse.  This second signal goes 180 degrees out of phase when it rotates 180 degrees from magnetic North.  

When this second signal sweeps the plane, the receiver calculates the phase difference of the two signals and uses this to determine the plane’s current radial from the VOR station. With two VORs, the aircraft’s position can be fixed over the ground.

Alternatively, Distance Measuring Equipment (DME) was added to many VOR stations.  It was based on WWII Identification Friend or Foe (IFF) systems and used relayed pulse signals and measurement of time delay (remember 1940’s LORAN?) to determine the distance (slant range) to the ground station. Eventually, enough of these VOR stations were linked together to form over 45,000 miles of air highways, or “Victor airways”.

In the 1960’s, we entered the era of solid state and the VOR’s were upgraded to this new, more reliable circuitry.  At the same time, these ground stations took over the duties of their older four-course radio range ancestors and extended “Victor airways” and “jet routes” worldwide.

Navigation Takes it Up a Notch—Way Up

Up ‘til now, much of the groundwork for electronic navigation was already in place.  But the key word here is “groundwork”. That’s because no satellites were used yet and beacons were stationary.  That changed in 1964, when the military navigation satellite constellation known as TRANSIT or the Navy Navigation Satellite System (NAVSAT), became operational. 

Satellite for the TRANSIT system, also known as NAVSTAR
Satellite for the TRANSIT system, also known as NAVSTAR

This first Global Navigation Satellite System (GNSS) exploited a discovery made after the Sputnik launch in 1957, when folks realized that the Doppler shift of a moving spacecraft’s (satellite’s) radio signal could be used to determine the location of a receiver on Earth. Satellite navigation was an amazing feat.  Still, for us pilots, there was a serious trade-off.  Although its range covered the entire Earth with 200m accuracy, hourly updates were the most it could deliver–a win for ships, but not too useful for planes.  And since it was military use only, even that point was moot.  At least satellites were now on the stage.

The Birth of Avionics

For avionics, 1968 was a banner year.  Components in the aircraft of the day each had their own dedicated wiring and connections.   With the advent of many new devices, things under the hood were getting pretty complicated.  This is when the military’s idea of a “multiplexed avionics data bus” first surfaced, an idea not too different from today’s USB port. The thought was that if you could take the data from each component, tag it with an “address”, then send all the data down a single wire to be sorted on the other end, you could eliminate lots of wires, connections and weight.  That year, an F-15 was tested with such a platform and out of this came Mil-Std-1533B.  This was the interface standard that would shape the future avionics, making them more reliable, lighter and cheaper.

It’s the 70’s.  And while folks were starting to try on clothes made of  new “synthetic fabric”, the military was working to synthesize flight information.  To date, most flight systems were individual mechanical, electric or magnetic components, with radio being the most sophisticated thing on board.  Wanting more “intel”, the military set out to integrate various types of new sensors into aircraft, with the goal of tying this information together using “aviation electronics” or “avionics”, a  buzz word we still use today.

The military was also beginning to build out GPS.  The first of dozens of satellites to follow launched in 1976.

This NASA 737 cockpit shows the co-pilot's position upgraded to electronic flight displays.  The pilot side was not yet modified.  ca 1974
This NASA 737 cockpit shows the co-pilot's position upgraded to monochromatic electronic flight displays. The pilot side was not yet modified. ca 1974

NASA was busy, too.  Recognizing how complex transport aircraft had become—with more than a hundred cockpit instruments and controls—NASA sought to develop a way for pilots to display “situational awareness”.  The result?  The first full glass cockpit demonstrator.  It was a rousing success and the commercial industry quickly adopted the concept, with the MD-80 first to roll out “glass” in 1979.  Other commercial planes were soon to follow.

Around that same time, the ground-based Omega very low frequency worldwide navaid was being used by the airlines.  It could be considered a very low frequency version of LORAN.  Although it had 1-2 mile accuracy, those who could afford it could supplement it with expensive self-contained Inertial Navigation Systems (INS)—think big gyroscope coupled with an ability to track every change in position from a pilot-entered starting point.

So, now, we have a proven satellite navigation system (still military) and a glass cockpit (still commercial).  We just need to put the two together and make it available and cheap enough for General Aviation!

The ball would be set rolling by a tragic event in 1983.  Due to the inaccuracies of current commercial navigation systems, Korean Air Lines Flight 007 was shot down when it unknowingly wandered into USSR airspace.  President Reagan responded by issuing a directive to open up GPS development for civilian purposes.

GPS Goes Up, Civil Avionics Speeds Up

In 1987, the first glass cockpit went into non-airline service.  Gulfstream had taken a big chance and bet on a cockpit whose dials were “drawn” by cathode-ray tubes.  The bet paid off and the Gulfstream IV business jet set a new standard in civil aviation.

The Gulfstream delivers the first General Aviation craft delivered with a glass cockpit
Gulfstream delivers the first General Aviation craft delivered with a glass cockpit

At this time, GPS was not yet ready for use, yet several parallel developments were based in it.  Among these was the successful project in 1991 that interfaced the first portable/panel mounted GPS with autopilot.

In the beginning of 1994, the 24th GPS satellite was placed into in orbit, completing the constellation.  By early 1995, the new GPS system, called NAVSTAR was declared fully operational. Now, just like the earliest mariners, travelers could look to the “stars” for guidance.

GA Gets a Constellation Prize

In 1996, President Clinton, recognizing the importance of GPS to civilian users, declared it a dual-use national asset.  At this time, in the interest of security, only a Selective Availability (SA) signal was made available to civilians.  SA effectively increased positional error, but GPS receivers were now approved for IFR!  The military also made GPS their primary system and decommissioned their TRANSIT system. 

In 1998, Vice President Gore commissioned the upgrade of GPS to provide two additional civilian signals enhancing accuracy and reliability for aviation use.  Two years later, Selective Availability was removed, instantly improving civilian GPS precision.  Within 10 years, 31 GPS satellites would be in orbit, providing redundancy and precision to 15 meters.  Why not better than 15 meters?  We’ll get to that in a moment.

Crystal Glass

Let’s catch up on the glass cockpit.  By the end of the decade and with the help of mass-marketed PC’s and TV’s to drive costs down, the vibrant color, reliability and low power requirements of Liquid Crystal Display (LCD) screens had replaced cathode-ray tube displays.  Today’s familiar full-color screens became available at a cost we GA pilots could finally affordafford, GPS and all!

Final Approach…

That brings us back to where we started, which is today’s Glass Cockpit.  So, here’s a review of the events or “waypoints” that got us here:

Historic events that enabled GPS for GA

  • The earlier mariners looked to space for their navigation
  • The 1919 bonfires showed us that we can “transmit” waypoints
  • In 1929, we learned that radio signals could establish navigation paths
  • In 1940’s LORAN, we learned that signal transit times could be used for triangulating position
  • In 1957 Sputnik showed us that satellites could be used for navigation
  • In 1994 the 24th GPS satellite completed the constellation
  • In 1996 President Clinton made GPS available to General Aviation

Historic events that enabled the Glass Cockpit for GA

  • In 1968, the idea of a “digital data bus” made way for lighter, cheaper and more reliable avionics systems
  • In the 1970’s the military adds new sensors to aircraft and integrates the results
  • At the same time, NASA demonstrates the first all-glass cathode-ray cockpit
  • In 1987, the first GA Glass Cockpit goes into service
  • In the late 90’s, LCD screen production for PC’s and TV’s lowers the production cost for LCD-based cockpit screen.

GPS in the Nutshell

Since we’re here, let’s take a closer look at how GPS works for us pilots.  Then we’ll circle back to the finish what we started on—the G1000. 

GPS receivers use the constantly emitting GPS satellite signals that all the satellites send in unison.  Based on the time it takes to receive three satellite’s signals and the Doppler Effect of each signal, a series of computer calculations can narrow the position of your receiver to one of two places—a point close to Earth and a point far into space.  It’s a reasonable assumption (we hope) that you are not in space, so the computer can always pick the point closest to Earth.  From this, your position relative to ground can then be determined.

How GPS Locates you

But believe it or not, the speed of light causes some trouble for us here.  It is used in the calculations and since the satellite signals travel for only an extremely short duration, (micro-seconds) GPS is very sensitive to the accuracy of your GPS receiver’s clock.  To address this, you simply need to have a really, really expensive atomic clock on board, right?

We know that the cost of GPS has come way down, so we couldn’t possibly have an atomic clock in our receiver.  The receiver manufacturers must have done something, right?  What did the manufacturers do?  Actually, some very cleaver folks realized that if they used a fourth satellite, they could check where the first three satellites say you are, then compare this to the distance to the 4th satellite—four equations (4 satellites) for four unknowns (x,y,z,time).  Since the satellites all have atomic clocks, any discrepancy would be due to error in your clock.  The good news is that this can be used to compensate your system and provide you with accurate (and inexpensive) position results down to 15 meters!  Amazing!

Why not better than 15 meters?  Because GPS uses signal timing to determine position and although we figured out the expensive clock issue, there was another troublemaker lurking in our atmosphere.  Apparently, when the ionosphere “billows”, it slows down the GPS signals, throwing off our readings.  Once again, cleaver folks stepped in to figure this out and with a few more ground stations and satellites, came up with the Wide Area Augmentation System or WAAS.

WAAS first became available to General Aviation in 2003.  Using what’s called Differential GPS, its sole job is to tell your receiver how to compensate for changes in the atmosphere.  If you have equipment that supports WAAS, then you can count on accuracy down to 3 meters.  Sounds almost good enough to use for automated landings, but that’s LAAS and that’s another story!

It’s been a long journey through history.  We hope you’ve enjoyed your flight.

We started this blog by talking about our updated G1000 version 8 course, which lead to looking at the historical breakthroughs that made GPS and avionics possible.  The exciting part is that this is just the start.  Within the GPS and avionics framework, so many other navaids became possible.  For the G1000, it is things like:

  • Moving map display
  • Flight Director and Autopilot
  • Vertical Navigation
  • Terrain display and warning
  • Real-time weather overlays
  • 3D Virtual Reality landscape
  • Traffic Information Services (TIS) alerts
  • Wide Area Augmentation Service (WAAS)

And these merely scratch the surface of what the Garmin G1000 can do.  Like most avionics today, it only helps if you know how to use it!  We think you’ll agree that GA cockpits have come a long way and we like where they’ve landed.


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  2. Tony Harrison

    Thanks Martha and John for a very comprehensive article.

    I’ve only been flying since 1994 and recently started flying a G1000 equipped C182. Amazing technology, and finally puts to bed my NDB inspired nightmares.

    I believe the biggest issue faced by pilots today, certainly pilots who rent aircraft from one or more schools, is similar to the one I was warned about by my wise instructor back in 94. Each aircraft has different switches, switch locations, and position of instruments, so spend the first few minutes of every flight sitting in the cockpit ensuring you know where *this* aircraft’s switches, gauges, etc are located.

    Switching between round and glass is a massive mental shift for a pilot not engaged in daily flying. The advantage of glass would be the similarity in presentation of information and similar (if not the same) functionality of switches.

    Thanks again for a great article. Looking forward to more soon.

  3. Neil Archer

    I learned to fly during the time that radio ranges were common and the VOR was rare. The ranges required the pilot to have a very good imagination (situational awareness) the VOR not as much and it has continued to the point with the current GPS systems and their displays require very little. The risk today is pushing the right button at the right time. I own and fly a Garmin equiped Bonanza it is much more of a learning curve for the button pushing than it ever was with situational awareness. Staying current with the operation of the airplane is much easier than with the modern nav equipment.

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