# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**The rapid-fire fix**

**From:**Frank Reed

**Date:**2009 Apr 25, 19:16 -0700

A statistical fix from a series of sights around noon is a special case of, what I have called in the absence of another name, a "rapid-fire fix". Consider the general case... At any time of day or night, I take a series of sextant sights of a single body in relatively rapid succession. From the perspective of a traditional LOP plot, each of these yields a line of position and when plotted on a chart and advanced/retarded for the motion of the vessel, they should cross at some single point, and that is the fix. Of course, the body's azimuth will not change much in a short period of time, so the individual LOPs cross at shallow angles with respect to each other, and careful plotting is essential. This is generally considered a "bad thing". In the real world, the sights are "noisy" --each has some small error arising from various causes (just random error --presumably systematic errors have been eliminated). So if we attempt to plot the LOPs, they no longer cross in a single point but instead in some tangled spider web of crossing lines. This tangle obscures the fact that numerous sights produces a better fix than fewer sights. The error in the fix resulting from the noise in the sights is the usual reason that navigators are advised to take sights of different objects at widely separated azimuths, or with a single body you should wait a few hours so that a later sight of that same body would be taken at a significantly different azimuth. This necessity for a wide separation in azimuth is easy to see from simple geometric arguments when we look at a single pair of LOPs. If they cross at a shallow angle, and we shift either of them slightly in the direction perpendicular to the LOP (corresponding to an error in the sight as taken), the crossing point moves by a distance that is approximately inversely proportional to the angle between the two LOPs, for small angles. What happens when we take multiple sights? If we shoot a dozen or even a hundred sights in rapid succession, what happens to the error in the fix? Clearly the error in the fix is minimized in the direction towards the celestial body. The error is largest in the direction perpendicular to the body. We get an "error ellipse". But as we increase the number of sights taken, the error in both directions decreases rapidly. We get a good position fix in short order. For a practical case, imagine spending forty-eight hours under stormy skies. You don't get any sights for two days. Then in the morning of the next day, the clouds begin to break. Let's say that you are able to shoot the Sun once every five to ten minutes as the clouds break up. How long does it take to get a usable fix? From the very first sight, you can plot an LOP, though it will not in general be a pure latitude or a pure longitude. Every navigator knows this, and indeed it's the basis of Sumner's original explanation of his discovery of the Sumner line of position. But what happens as you continue taking sights every few minutes? Most practitioners of standardized 20th century celestial navigation would see little merit in this and might simply average the altitudes to get a refined (single) LOP. A modern celestial navigator with sight reduction software can enter each sight as taken and the software can quickly generate a least squares position combining all of the data in a way that is nearly impossible to see in a traditional plot of LOPs. This is an aspect of sight reduction that is generally known to navigators --most people are aware that this feature exists in software-- but the fact that it produces a good fix in latitude and longitude quickly is not widely appreciated. And by the way, "better" software will also plot an error ellipse which directly shows how the fix is improving despite the increasing area of the "tangle" made by the crossing LOPs (in fact one of the earliest binary attachments for the group, just over SEVEN years ago, demonstrated this: http://www.fer3.com/arc/m2.aspx?i=006308). So how good is a "rapid-fire fix"? If I shoot N altitudes of a single body over a time interval T, how good is the fix in the position? If each altitude has a standard deviation error S_i, then what is the standard deviation of the fix in the direction perpendicular to the mean azimuth of the body, and what is the standard deviation of the fix in the direction towards the mean azimuth? Anyone care to work it out? What you may find is that the fix is suprisingly accurate, within a couple of miles, even when the total time interval is just ten or fifteen minutes (see last paragraph). Also, the result may be easier to express by changing variables and using the total range of azimuth A from the beginning to the end of the time interval T. It's certainly true in celestial navigation that there is rarely anything "new under the Sun". It may be worth mentioning that we can understand in a very general sense what's going on by mapping a rapid-fire fix onto some of the earliest methods for determining a vessel's position: a time sight for longitude and a "double altitude" sight for latitude. For example, if I take twelve equally-spaced sights of the Sun over the hour from 0900 to 1000 in the morning, I could group them into sets of four and average each set. Each average would presumably be better than any of the individual altitudes taken alone. We would then have three averaged altitudes at effective times of 0910, 0930, and 0950. The middle sight can be treated as a time sight. We calculate local apparent time from it using standard methods, compare that with the chronometer time, and we have the vessel's longitude. Then we take the averaged sights from 0910 and 0950 and, using the rather complicated methods found in most 19th century navigation manuals, we would get latitude from the two altitudes and the time interval between them. So there's the fix. Similarly, for a 20th century LOP plot, we could average the sights just the same way in sets of four, and then cross the three resulting LOPs and place the fix somewhere inside the small triangle formed where they cross. By averaging sets of four sights, we have achieved some of the statistical least squares fitting in a modern rapid-fire fix. In fact, in some cases, the results should be identical. You can advance/retard the LOPs in a statistical fix just the same way you would with a Noon Sun fix. That is, rather than attempting to slide each LOP around on the chart, you can adjust the altitudes before you plot. You find the component of the vessel's true velocity that is along the mean azimuth to the celestial body and add that in proportion to the time from the middle sight (or any sight you choose). So for example, if the Sun is in the southeast at azimuth 135 in the morning and I am shooting a series of sights in rapid succession while travelling due south at 8 knots, then the component of my velocity towards the southwest is 5.7 knots so for every ten minutes, I should subtract just about 1 minute of arc from the altitudes of the later sights (I'm moving towards the GP of the celestial body at that rate). The correction for the changing position of the object (changing SHA and Dec) is more complicated in this case, so let's just assume it's small enough to be ignored. Finally, I hope it's clear that taking a series of sights around noon for latitude and longitude is simply a special case of this "rapid-fire fix". It's unique for a couple of reasons. First, the corrections for motion are a little easier since it's relatively easy to understand the north-south component of a vessel's motion as opposed to the components relative to some other bearing. Second, the correction for the changing position of the celestial body (doesn't have to be the Sun, by the way) are simpler and effectively limited to changing declination. That correction just adds or subtracts from the north-south component of the vessel's motion. And last, but most importantly, there are simple graphical techniques which can do an excellent job replacing the math of a statistical fix (like the "paper folding" method that I've described). This means that we get all the advantages of using all of the available sight data to get our vessel's position without requiring some calculating device. Perhaps someone can discover a graphical (or other) technique to get a statistical fix in other special cases, like sights on the Prime Vertical, or perhaps even in the general case... Almost a year ago, when I mentioned this idea of a rapid-fire fix, NavList member Jeremy Allen did what any good navigator should do: he got out his sextant and tried it out (no spreadsheet simulations required!). He shot eleven altitudes of the Moon over a period of just eight minutes. The sight data was in message 5416: http://www.fer3.com/arc/m2.aspx?i=105416. Most other NavList members either didn't know what to do with this set of sights or simply assumed that the altitudes were meant to be averaged to get a single line of position. Instead, using software, this set of sights yielded a true (statistical) fix in both latitude and longitude with very good accuracy. Jeremy's solution was posted in NavList 6066: http://www.fer3.com/arc/m2.aspx?i=106066. -FER --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To unsubscribe, email NavList-unsubscribe@fer3.com -~----------~----~----~----~------~----~------~--~---