A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: The cocked hat
From: Joel Jacobs
Date: 2004 Apr 4, 09:37 -0400
From: Joel Jacobs
Date: 2004 Apr 4, 09:37 -0400
This is in response to Robert Enro. I have a few problems with Robert's recommending RCAF Navigator's Keith Greenway's two sight, two average fix method for use at sea. IMO, it is subject to the following deficiencies. Greenway is taking sights from an aircraft using a bubble sextant so that the instrument provides the same (bubble) horizon no matter in which direction he is shooting. You don't have that at sea when using a traditional sextant. As everyone knows, the horizon at sea's definition varies with the direction you are looking due to differences in light, sea conditions and cloud cover. Therefore, you have variables with which Greenway is not confronted since his horizon is a constant. The other issue is taking only two sights. I think that's a bad idea because an error in one can only be halved when combined with the other. In most things, where you are trying to have a definitive outcome it is always wiser to use an odd number so that you don't have an even split where one group can cancel the other out. I don't understand how Greenway concludes that an error in a two sight fix is obvious. IMO, that it is impossible to judge which of the two parallel sights is in error. So what do you do? He suggests that you go back and shoot it over again. That is not always possible, an if it is, because of the significant time lapse, you have a running fix, not a fix. All something to think about. Robert, with that nifty and very unusual star finder we discussed off group, I suggest you predetermine the alt and az of the bodies beforehand. Set you sextant's to same (adjust for IC, REF, HE) accordingly, then adjust for variation and deviation, and note the adjusted compass bearing to the body for each object, and just aim along that line. You should have the target in view. Some minor adjustment's and you have your sight, I preferred taking a minimum of three near simultaneous sights of the same body, and then moved on to the next. The planning took into account diminishing or increasing light conditions, and where I had to position myself for steadiness etc., as I moved from one target to the next. I based this on where I would have the best horizon. If possible, I always used at least three bodies. Two sight fixes would more often be shooting an early or late rising planet or moon and crossing it with the sun. Averaging was done with a calculator that had the ability to manage time using Sexagsimals, base 60 numbering system. Let me add, this was more a matter of self-entertainment, because as I've said before, when you're out in the middle of the ocean, with no hazards at hand, all that precision is unnecessary. But it was fun. Joel Jacobs ----- Original Message ----- From: "Robert Eno"
To: Sent: Saturday, April 03, 2004 11:07 PM Subject: The cocked hat > I'll throw my two bits' worth in. I agree with Joel and in fact what he > describes is more or less the method that I have employed for quite a few > years. > > I'll take it a step further: I generally examine my intercepts before > plotting them out, discarding the obviously wonky ones. I do, however, > average out sights taken on land with a bubble attachment because bubble > attachments are inherently inaccurate and in my experience, it is better to > take multiple observations of a few stars and average them out. > > I was persuaded, some years ago, that the 3 star fix is overrated. Better to > take two observations of two stars. This is not an an original idea on my > part: my inspiration comes from a little-known book -- Arctic Air > Navigation -- by a former RCAF Air Navigator, Keith Greenaway:. In his book > Greenaway writes: > > "Navigators who use asto continually prefer 2-star fixes with each star > sighted twice, rather than 3 star fixes with each star sighted only once. > Although an extra sight is required, time is actually saved, because only > two stars have to be located, the course setting of the sextant changed only > once, and the tables entered for two stars only. It is also easier to detect > inaccurate computation or observation as shown in Fig. 27". > > Figure 27 shows two parallel LOP's spaced very closely and two parallel LOPs > at right angles to the former, spaced far apart. The figure beside it shows > a classic "cocked hat". The caption to the figure reads: "Comparison of a > 2-star and a 3-star fix. In the case of the 3-star fix, it is not obvious > which sight may be in error, while in the case of the 2-star fix it is > immediately apparent which star should be re-sighted or sight computations > checked". > > While it is true that I have quoted from a book pertaining to Air > Navigation, I have used this technique, successfully both on land and at > sea. > > As for MPPs, I cannot argue with George and Herbert about the efficacy of > performing the necessary statistical computations to arrive at this figure. > I have read only a little on MPPs but I certainly agree that it is a very > real and very valid concept. Nevertheless, for purposes of practical > navigation, who is really going to perform this exercise while at sea? In > reality, navigators will simply take the centre of the cocked hat or the > intersection of two lines of position. > > I believe that the two times two star technique described by Greenaway is a > reasonable compromise. > > Robert > > > > ----- Original Message ----- > From: "Joel Jacobs" > To: > Sent: Saturday, April 03, 2004 7:58 AM > Subject: Re: The cocked hat, again. Was: "100 Problems in Celestial > Navigation" > > > > In an off group exchange with Herbert Prinz, to which I have added a few > > words to for clarity, I said this: > > > > "It is really a matter of personal judgment. The navigator's selection of > an > > MPP will vary as to how reliable or accurate the navigator thinks the > sights > > were based in the conditions he encountered when taking the sights. A > > mathematical solution may not be appropriate." > > > > I submit that this is what George is correctly introducing. > > > > Let me add this thought. When I took the USCG Celestial Exam, year ago, > one > > of the questions had a series of sights that when plotted, had one sight's > > LOP distant from the others. If you answered the question including that > > sight in your calculations, even by averaging, you were wrong. If you > > rejected it as a bad sight, you were right. As a matter of pride, I got > > 100%. Of course I was a lot younger and sharper. > > > > Joel Jacobs > > > >