NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: accuracy of automatic celestial navigation
From: Robert Eno
Date: 2002 Dec 8, 12:52 -0500
From: Robert Eno
Date: 2002 Dec 8, 12:52 -0500
Hello Mr. Geers, Have you an address, internet or snail mail, to where one can direct a request for reprints? This sounds like very interesting stuff. Robert Eno ----- Original Message ----- From: Glenn GeersTo: Sent: Saturday, December 07, 2002 5:08 PM Subject: Re: accuracy of automatic celestial navigation > On Sat, 7 Dec 2002 05:18 pm, you wrote: > > The algorithms used in STELLA have been published. The author is George Kaplan > former head of the Astronomical Applications Department at USNO. I have the > full set available (have to dig them out). The USNO will provide reprints of > the papers that aren't available on the web. It's pretty neat stuff. The > algorithms take the motion of the vessel into account and can provide very > high (30m) accuracy. > > Also of interest: Polaris subs of the 60's carried an automatic astro-nav unit > that they used to reset their inertial systems from time to time. Most auto > systems use infrared optics so they can track stars during the day. > > Regards, > Glenn > > > > On Friday, December 6, 2002, at 04:05 PM, George Huxtable wrote: > > > For those of us that sail our small craft out at sea, my opinion is > > > that if we can achieve a precision of 2 min, we are doing pretty well. > > > What do others think? > > > > I would agree. 2 nmi at sea is very decent; on land we should be able > > to get to 1/2 mile or better. > > > > On land I recently took a string of shots using a Tamaya Jupiter > > sextant with a 7X scope. I shot the sun mid afternoon over a period of > > 20 minutes. (This was on the coast of Oregon at a beachhouse on the > > water.) I used a Garmin GPS in averaging mode to determine my actual > > position to 15 feet or so. The resulting intercepts in nmi are > > interesting to analyze. Because my assumed position is known to within > > 15 feet, these can be taken as errors in my sights. Here they are in > > order: > > > > 0.91 > > -0.06 > > 0.59 > > -0.05 > > 0.22 > > 0.55 > > 0.22 > > -0.28 > > -0.53 > > -0.57 > > -0.77 > > -0.03 > > -0.19 > > -0.11 > > -0.43 > > -0.62 > > 0.27 > > 0.24 > > 0.44 > > 0.25 > > > > The mean of the intercepts is a mere 0.0025 nmi -- very good -- but the > > individual readings varied over a 1.68 nmi range -- not so good. > > > > When we are "warming up" and we haven't used our sextants in a while I > > find that the first shots are often in greater error than later sights. > > This is in fact the case with this set of shots shown above: the worst > > error came from the very first sight. Therefore I have found that it > > is wise throw out the the first reading or two, or at least to discount > > them. > > > > Other experiments I have done: > > > > I took five shots in my backyard using the same sextant and an > > artificial horizon and got a 1.27 nmi range of errors and a mean error > > of 0.644 nmi. > > > > I took fourteen shots in my backyard using a Tamaya Venus (not nearly > > as powerful of scope) and an artificial horizon and got a 4.3 nmi range > > of errors but a mean error of -0.45 nmi because of the greater number > > of sights taken. > > > > And for my finale, one afternoon I used six different sextants in my > > backyard and I took a total of 46 sights using an artificial horizon. > > I got an 11.2 nmi range of errors with a mean error of > > -1.23 nmi. > > > > From this the statistical nature is shown to be very useful: although > > any one shot could be in error up to almost a mile, a series of shots > > averaged does indeed improve accuracy to very good levels; STELLA > > appears to do this, and thus an automated system is very appealing. > > > > Dan > >