NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: gyroscopic compasses
From: Nicol�s de Hilster
Date: 2007 Jun 24, 11:39 +0200
From: Nicol�s de Hilster
Date: 2007 Jun 24, 11:39 +0200
George Huxtable wrote: > ... the plane of > the Earth's surface, at the position of the navigator, is changing, all the > time, for two reasons; because of the rotation of the Earth, and because of > the motion of the observer with respect to the surface of the Earth. So, any > such laser "gyro" would need to have some sort of knowledge of these > quantities. In a similar way, a ship's gyroscopic compass needs to have some > sensor of the local horizontal, and have correction factors applied which > allow for the vessel's motion. > > I imagine that in the case of a laser "gyro", this information would be > provided by an interface with some form of GPS receiver, with a sensor of > the direction of gravity. The Octans not only measures the heading, pitch and roll, or rather the rotations of the X, Y and Z axis, but also the acceleration along these same axis. This makes the apparatus ideal for inertial navigation. In the past I did tests with the Octans to find out the stability of the pitch and roll measurements. Most motion sensors have problems when exposed to lateral accelerations. These accelerations would normally induce an amount of pitch and/or roll. The Octans however also measures the rotations of the pitch and roll axis and therefore knows the difference between the earth gravity and applied acceleration. The test was quite simple: the Octans was laid on a flat surface (a table) and then swung fore and backwards very quickly, as fast as one could move these 4.8kgs by human power. There was no change in pitch and roll at all, very impressive (the representative of the manufacturer turned quite pale when I did that ;-)). For the speed of the Octans one could rely on inertial calculation, but that is not the way the Octans handles it. If not corrected for the speed will induce an heading error equal to V/5PI x secants(latitude), where V is the North component of the speed in knots. There are two options: either a fixed value for the speed is entered into the on-board software, or the speed, calculated by a GPS-receiver, is sent to the device. The same applies for the latitude. The Octans also needs latitude for optimum performance (The heading error varies with 0.045 degrees at 0 degrees latitude to -0.02 degrees at 90 degrees latitude). This value can also be entered manually or by sending a NMEA message from a GPS-receiver to the device. As far as I know the Octans is a downgraded device that originates from military use, so I wonder how well the original device performs... --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---