NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Paul Dolkas
Date: 2016 Jul 21, 22:07 -0700
David-
I have to admit that the instrument I was designing for my class was actually more of a theodolite that could double a an emergency sextant. It was for use from a stationary platform - specifically the surface of the moon or Mars – and not for a pitching & rolling ship. That being said, since the inclinometer can instantaneously determine which way is up, you really don’t need to steady it much. You just push the button whenever the target star crosses the target center, and the electronics will average out the multiple sightings. It would not be much harder to digitize the whole thing, and have the electronics take a rapid series of “shots” whenever the star passes within a target circle - the star’s exact position within the circle being tracked in real time.
However, you do have a very valid point in that the whole thing becomes much more difficult whenever there is any acceleration to tilt the direction of the “up” vector. I once tried to take shots with a bubble sextant on a long road trip (much to the acute embarrassment of my wife, who was driving). All I learned was a tremendous respect for the folks who did this for real in an actual aircraft.
Paul Dolkas
From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of David Pike
Sent: Thursday, July 21, 2016 11:54 AM
To: paul@dolkas.net
Subject: [NavList] Re: Sextant accuracy and possibility of future improvement
Paul Dolkas you wrote: I always thought that the biggest problem with sextants (especially the aircraft bubble type) was the problem of figuring out where horizontal is. So why not replace the liquid bubble with an electronic inclinometer? They are now readily available reading down to sub-second accuracy. It could either be attached to the index mirror for a direct readout of elevation, or attached to the horizontal “base” with a rotary encoder to figure the elevation from there. You sight the star, push the button, and you’re done.
Paul. I’m not quite sure what you mean when you talk about an electronic sextant or inclinometer. In the case of the sextant, do you mean electronic measurement of the angle between the index mirror and the horizon mirror, or do you mean electronic sensing of the vertical, or ideally both? Digital angular measurement would be great. (I’ve recently bought a Vernier calliper gauge with a digital read-out for engineering use and it has turned what was once a chore into a pleasure). However, you would still need to know where the horizontal or vertical was or to level the inclinometer. I’m told that this is possible electronically with MEMS technology and is used in smart phones. This would be fine on land, but I fail to see how such a system could separate the acceleration due to gravity from the accelerations of a small vessel or an aircraft. I suspect that to do that your instrument would need to be connected to an even more complex arrangement such as an inertial platform. I suppose if you could lock on to four or more bodies at the same time, you might be able to make the device self-levelling by making the jigsaw puzzle of angles fit a bit like GPS does with ranges to slew the receiver clock.
With respect to bubble horizons, the thing that allowed them to catch on was the introduction of the concave roofed bubble chamber proposed by Fave in 1906 and patented by Booth and Smith from RAE in 1919. This obviated the need to hold the bubble between fiduciary marks in the fore and aft direction and made it much easier to hold the sextant vertical laterally. I attach a short paper I put together on the subject for Navigation News. DaveP
Attached File:
The-Evolution-of-the-Aircraft-Sextant.doc
