Frames Of Reference

When people first started to fly, an important question emerged.  The question was “which way was up?” That seems like and easy question to answer, but when you are flying around in an airplane in a fog, it becomes more complicated. Here’s what aircraft in the early days of flight looked like.

Here’s an early instrument panel.  At this time, the only way of knowing which way was up was a spirit level that is in the middle of the panel. Which is fine if you can see the ground or don’t roll too far.  The problem with a spirt level as a roll indicator was that force other than gravity could affect it.  Since an aircraft in a turn encounters centripetal forces this was a large issue as aircraft performance increased during WW1. Also while a level could show roll, it couldn’t show pitch witch could be a bad thing when you are forced to fly by the seat of your pants and can’t see the ground.



A bit later instruments started toward a true artificial horizon. This was possible due to Lawrence Sperry’s realization that a gyroscope could be used to maintain a straight line regardless of the motion of the aircraft around it. This that if you attached a indicator to the gyro it could be used to represent the horizon.  This amazingly simple concept was one of the keys to allowing aircraft to fly in all weathers and safely, even at night.  The Artificial horizon hasn’t changed very much in function since it was invented.

Here’s an early artificial horizon from the Smithsonian.  Surprisingly it’s instantly recognizable and could probably be fitted and used as intended.

Here’s a bunch more links that I found about artificial horizons.

Click to access ADISept05.pdf

In the late 1950’s a new problem emerged.  There was a big move to get off the planets surface and into space.  The thing is that when you go into space there is no horizon at all.  For a ballistic missile this isn’t a problem because an inertial guidance system can tell the missile where it is by measuring the changes in velocity and attitude.

For spacecraft that orbit the earth or move beyond Earth’s orbit the frame of reference question reasserts itself.  solving that issue was key to getting men on the moon and beyond .


Of course once you get into space “up” becomes a frame of reference question. If everything is moving, how do choose the frame of reference. At first the idea was to have a globe repeat where you were over the earth and telescope to look down.  That proved to be useless for a variety of reasons and after Glenn’s Mercury flight the telescope and “earth path indicator” were removed and replaced with an attitude indicator that more or less behaved the same way as an artificial horizon.  Still the earth path indicator is an elegant little machine. Here’s a link to the Smithsonian’s Earth Path Indicator.

This NASA report show the Mercury Attitude Indicator that was later replaced.

Here’s some pictures of various spacecraft dashboards.

And some more stuff on the attitude indicators and gyroscopes.

Click to access hrg.pdf

The definition of a frame of reference is an important part of knowing where you are going. So while the “up” is arbitrary you need an “up.” The gyroscope gives vehicles that have no fixed direction that “up.” Which is why it’s one of most important technologies of aerospace.



  1. Old NFO · August 27, 2016

    And ring laser gyros are STILL being used today to provide AHRS in a number of aircraft… Good ideas never go out of style, they just get updated by technology!


  2. Ron A. Tully · September 1, 2016

    The Arts and Crafts movement of the nineteenth century challenged an emerging culture of mass produced objects that lacked originality and quality.


  3. Ron A. Tully · September 11, 2016

    The problem with a spirt level as a roll indicator was that force other than gravity could affect it.


  4. Ron A. Tully · September 12, 2016

    solving that issue was key to getting men on the moon and beyond .


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