NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Lat/Lon by "Noon Sun" & The Noon Fix PROVE IT
From: Brad Morris
Date: 2009 May 12, 17:44 -0400
From: Brad Morris
Date: 2009 May 12, 17:44 -0400
Celestial Navigation provides a certain level of positional information. Not accurate enough to slew munitions through a window, but certainly good enough to navigate around the planet safely, as evidenced by the many sailings through history. CelNav "works". The GPS (& Galileo & GLONASS) provide two other levels, one for the civilian market and the other for the military market. Even when using the military GPS, the accuracy is not a nanometer. For those that don't know just how big that is, it is one billionth of a meter. Another way to think of this is 1/25 of a millionth of an inch. Okay, I recognize hyperbole :-) No need to beat me up. We are on the same side of the fence here. I don't expect mathematical purity using closed form equations, yielding 19 decimal places with ideal repeatability. I just want to know where I am, plus or minus. Best Regards Brad -----Original Message----- From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of Gary LaPook Sent: Tuesday, May 12, 2009 5:07 PM To: NavList@fer3.com Subject: [NavList 8239] Re: Lat/Lon by "Noon Sun" & The Noon Fix PROVE IT You get us back to the drum that I am constantly beating, "just how accurate do you need your fix to be at the moment"? First, there is never going to be a perfect celestial fix, accurate to a nanometer. Now that we have GPS we can compare our celestial fixes to a highly accurate benchmark, we couldn't do that in the past. Many on this list try to squeeze almost perfection out of their celestial fixes but I think it is useful to ask how much accuracy is actually needed and see if the technique yields that level of accuracy. The traditional noon fix was achieved by crossing an advanced morning sun line with the noon latitude sight. How accurate could we expect this type of fix to be? Pick your own value for the probable accuracy of each sextant shot by let's use one half minute for this discussion. So the morning sun line has an error band one NM wide. Lets say we advance this for one hour at six knots westbound and add one tenth of the distance covered as the degradation of the position due to to possible errors in the DR. (This is a commonly used estimate in flight navigation but a smaller number may be appropriate on a boat.) This adds another six-tenths of a NM to the error band making it 2.2 NM wide. (Since the morning sun line will not have an azimuth of straight east or west not all of the error will be in the longitude but we have bounded it worst case, plus and minus 1.1 NM in longitude.) Assuming the same level of accuracy with the noon sight, you have determined your position plus or minus one-half NM in latitude and 2.2 NM in longitude. Let's call this the accuracy level of a traditional noon fix. We have been taught that you cannot determine longitude from the noon observation due to the difficulty in determining the exact instant of LAN. What this actually means is that you cannot determine your longitude to the "accuracy level of a traditional noon fix." But if you can determine the time of LAN within one minute then the uncertainty in longitude is only plus and minus 15 minutes of longitude, plus or minus 15 NM at the most. Although this is not to the "accuracy level of a traditional noon fix" it is probably accurate enough for the purpose of a backup navigation system. There are techniques to use in an attempt to identify LAN more accurately (equal altitude sights) and the technique discussed on this thread, producing about five minutes of longitude uncertainty and also accurate enough for backup navigation. (I am impressed by the level of accuracy achieved by this method.) Soooo, why go to the extra complication of the method discussed here, just mark down the time of LAN, place an error band +/- 15 NM on the longitude, and plan your landfall based on the most critical longitude determined? One of the arguments made for this method is that it is easy to relearn, but it is not nearly as easy to relearn as just taking the time of LAN. gl Brad Morris wrote: > Report of the Secretary of the Navy > > "We recognize in navigation two classes of astronomical problems; one possessing perfect accuracy of computation; the other uncertain, and approximate only in its results. The first class is, of course, always employed when the weather and other circumstances are favorable. The second class of problems, that which gives only approximate deductions, is never resorted to except from necessity. The latter, however, serves a useful purpose by enabling the navigator to know his position more nearly than he could otherwise do, and by often affording perfect security by informing him of the limit of safety, beyond which he should not venture." > > -------------------------- > > > I would like to assign the latitude and longitude by the noon sun fix to the second class of problems. It can provide us with a useful figure of merit, albeit without perfect accuracy. Certainly, with a least squares curve fit, the method affords us a very reasonable fix. > > So if Frank wants to "trot out his hobby horse" one more time, I for one will applaud it. > > Proven?? A resounding YES! > > Best Regards > Brad > > > "Confidentiality and Privilege Notice > The information transmitted by this electronic mail (and any attachments) is being sent by or on behalf of Tactronics; it is intended for the exclusive use of the addressee named above and may constitute information that is privileged or confidential or otherwise legally exempt from disclosure. If you are not the addressee or an employee or agent responsible for delivering this message to same, you are not authorized to retain, read, copy or disseminate this electronic mail (or any attachments) or any part thereof. If you have received this electronic mail (and any attachments) in error, please call us immediately and send written confirmation that same has been deleted from your system. Thank you." > > > > > "Confidentiality and Privilege Notice The information transmitted by this electronic mail (and any attachments) is being sent by or on behalf of Tactronics; it is intended for the exclusive use of the addressee named above and may constitute information that is privileged or confidential or otherwise legally exempt from disclosure. If you are not the addressee or an employee or agent responsible for delivering this message to same, you are not authorized to retain, read, copy or disseminate this electronic mail (or any attachments) or any part thereof. If you have received this electronic mail (and any attachments) in error, please call us immediately and send written confirmation that same has been deleted from your system. Thank you." --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---