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sailaway21 05-24-2007 02:16 AM

Lines of Position
I had an interesting experience with the GPS receiver in my water well drilling rig today. For some reason my receiver, while picking up 6 of 7 available satellites, was giving me only a "good" to occassionally "fair" position as indicated by the receiver. It did not seem to be getting the WAAS signal. I was driving, in an area I knew, and did not have time to putz around with it. What was interesting was that I was a tenth of a mile or better off from where I knew myself to be. That got me thinking about LOPs.

Originally, Loran sets, both A & B, required the operator to identify each signal and match the leading edge of the signals sine wave against the leading edge of the master signal's sine wave. If you haven't had the pleasure of operating one of those suitcase size babies, not to worry, you didn't miss much. You could just about shoot stars in the time it sometimes took to get a Loran fix. But it's what we had in the day.

The advantage to such a Loran set was that you could actually see the different signals. You saw the "ground wave" and the "sky wave" next to it. Depending on which was the more stable you matched one with the "master" and then consulted either your Loran tables or your Loran overprinted chart. You knew what you had, even if it wasn't the quickest or most convenient set from an operational standpoint. You were able to make a determination of reliability of your LOP. You might have one weak wave and you discounted it in favor of the two stronger waves. So, with a three LOP fix, forming a triangle where the three LOPs crossed you would normally deduce your position to be in the center of the triangle. With practise, and experience, though you came to the point where you "weighted" each LOP and would find that your actual position was where the two stronger LOPs crossed, discarding the weak LOP as unreliable.

This was an easy practise to acquire because it is the same procedure the experienced celestial navigator uses. The novice sextant user shoots five stars, plots them, and proudly announces his position to be at the center of the resulting pentagon. Every once in a while he gets a perfect pinwheel and is mocked for having greased his parallel rules. (g) With experience, the celestial navigator, as he's noting altitude and time for each star in his workbook, assigns a rating to each observation. Glare from the moon shine to port may make his observation of Arcturus only a "fair" shot due to an unreliable horizon in that area of observation. There are numerous other factors that could result in an observation being less than good, and other's, including the body observed that make a particular observation excellent. Each of these is noted in the work book. Once each LOP is plotted, the evaluation begins. One might have three stars that intersect perfectly with each other and, yet, discard that position in favor of the other two stars that were excellent observations. More often, one could only snag three observations, one of which was considered weak, and thence decide that the vessel's position was at the intersection of the two stronger LOPs. All LOPs were left on the chart, subject to future re-evaluation. You cannot master the art of navigation, or develop the intuition, if you start discarding data with the eraser-no matter how pretty the result may look to the Captain in the morning!

This brings me around to the day when I first encountered the "automatic" Loran set. These were almost invariably Loran C sets. I, for one, lamented the phase out of Loran A due to it's greater range, even though Loran C was touted as more accurate. It was indeed more accurate. Every time I looked out the window. passing Ambrose Light abeam, the Loran C said I was abeam of Ambrose Light! Further out to sea, where I could actually benefit from a Loran fix the damn thing was mute. I wasn't greedy-all I wanted was one good old Loran A LOP per hour, and I could run all of them ahead based on speed made good for a fix at the end of the watch. I kinda missed that old suitcase. The thing I missed most was the ability to actually see the signals on the cathode ray tube. I then had confidence in the LOP I was deriving. The various techno wizards assured me that the new Loran C automatic sets could match up and resolve signals that I could not even detect on the CRT. I was, and remain, sceptical on that point. I never did develop a great deal of confidence in those units with their "you are here" capabilities, although like every other navigational tool, they worked great alongside the dock which, by the way, is where I am firmly convinced most of them were evaluated by their makers. Subsequent celestial observations confirmed my inate distrust of the "you are here" systems at least to the point where i treated them with the proverbial grain of salt.

Enter satellite navigation. The original Navy Navigation Satellite System, known as Transit, was pretty neat. The satellites rose and fell just like stars. As usual, for me, the first set I was exposed to was rather crude. One had to program the set each time one wished to operate it. I'll bet there's one still sitting in the electonics lab at Kings Point today. They've probably relegated it to a coffee pot stand. It would support a very large coffee pot. In any event, in the programming, one had a row of approx 32 levers one had to position and each lever was a "bit" and you positioned them and then hit the "enter" lever and could then go on to programming the next line. Believe me, it seemed like ships had crossed the Atlantic in less time than it took to program that "suitcase". But when you were done it worked great and even had a speaker that you could listen to the doppler shift of each satellite on. Neat. The Transit system was problematic in that you were dependant on the satellites being above the horizon to get a fix. If you were at a high latitude you'd get a fix every twenty minutes or so. If in equatorial waters, you might go a whole day with no more than a dumb look from that box. Any significant error in the speed of the vessel programmed in to the receiver would result in wildly inaccurate fixes and there's the rub. You didn't know the fix was inaccurate other than by other navigational means. If it had been cloudy for days, with no celestial observations possible, and the black box spit out a fix showing you ten miles north of your DR track you could not assume that you had encountered a substantial current, or myopic helmsman, nor could you discount it. This will go a long way towards explaining my reliance on my Tamaya "hambone" and why I took stars and sun observations frequently, even though equipped to the most "modern" level. Through experience, I had confidence in celestial navigation and my ability to perform it. It will come as no surprise to my detractors that, even in those days, I was noted for discounting a probably perfectly good Loran fix until my own star fix confirmed it. I was probably too young to actually be a curmudgeon, but, hey-fake it 'til you make it.

Back to my truck mounted GPS. I have noted that every couple of months this phenomena occurs, and I never have my "blue signal light" on indicating WAAS reception. It's no big deal in the truck, and probably no big deal on a boat-unless you are relying on that box to tell you exactly where you are at. Again, the rub is that the GPS set does little to disabuse one of the notion that they are anywhere else other than where it says. After all, it's computing a fix. It couldn't compute a fix if the LOPs didn't match up, could it? Well, patient reader, yeah-it could. And you don't have any way of knowing you're being led down the garden path other than you might have an indication of "weak" or "fair", or even "good" fixing, but not "excellent". Given the fact that the market, or at least the manufacturer's marketing department, indicates we all want "automatic" sets I see little occuring that will change our units design. I would offer the computer user's axiom, "garbage in-garbage out" as a caution. In any effort at navigation, electronic or visual, the absence of an ability to evaluate the strength of an LOP is a detriment. "You are here" or, in the immortal words of "Mr. Kimball" of Green Acres fame, "you might be here, on the other hand, you might be..."
Those navigating in confined waters by electronic means alone might take this as a caution.

sailingdog 05-24-2007 07:42 AM

On many GPS units, you can see which satellites it is currently monitoring, and one some, you can get an idea of what their position in the sky relative to you is. While this doesn't show you exactly what information the GPS unit is using to calculate the position, it does allow you to discount the position's accuracy a bit if the satellites are relatively close to each other, rather than covering a major portion of the visible sky. You can also often get a readout of the "estimated radius of error" for the GPS on that screen—which also gives you an idea of what kind of information the GPS is receiving at any moment.

Idiens 05-24-2007 08:24 AM

The snag with satellite navigation is that it is mostly very good indeed, occasionaly not bad, but on rare events - wrong. I just saw some measurement statistics from a receiver in London for 2006 - 99.5% of the time, the horizontal error was under 4.9m but 99.9% of the time it was under 3.6 km.
Some receivers do give an indication of Dilution of Precision (DOP) which is caused mainly by a poor combination of satellite positions. Some provide an indication of signal (strictly signal to noise ratio) strength per satellite, as weak signals correlate badly and cause measurement errors. Some (expensive ones) provide an estimate of multipath effects, which is the most likely cause of large errors in a ground vehicle installations. Then there are those nasty ionospheric induced errors, which WAAS reception should help to remove, or you need a dual frequency receiver, and then your local troposphere, which injects a bit more error.
One saving grace is that most of the errors are transient.

EpicAdventure 05-24-2007 01:54 PM

These errors probably don't make difference for typical boat navigation. Although, it is interesting to think about them as far as precision landing approach aids for aircraft.

The good thing is that the FAA is quite the stickler about making sure the probability of these types of errors affect the safe operation of a aircraft is very low.

I have heard that a two frequency version will be made available for commercial purposes just because the FAA is so concerned. (Currently, two frequency GPS is restricted to military use.)

sailingdog 05-24-2007 02:14 PM

Of course, those errors being transient, can be easily corrected for or avoided. If you're using GPS to navigate while near shore, you're making a grave mistake. Coastal pilotage skills should take priority over an icon on a screen if you are within Mark I eyeball range of land.

camaraderie 05-24-2007 08:36 PM

Dawg....let's keep chartplotters and GPS separate. Getting a lat/long from a GPS in coastal situations is an accurate means of establishing a position within a few hundred feet at worst. Not in anyway dismissing the "eyeball" but in VERY many coastal conditions (i.e. it is dark outside and the eyeball has no way of establishing a true position or fog or haze etc.) the GPS is by far the best nav tool available.

sailaway21 05-25-2007 01:06 AM

Not surprisingly, my friend Camaraderie, has missed the main point of my post.

That point being, you have no way of knowing whether your position is accurate to within the few hundred feet you mention, or the six hundred feet I mentioned, or worse. For the GPS "fixed" this is not an attack upon GPS, it's actually an attack upon the receivers, and the dumbing down of those receivers for convenience sake. I own a GPS and find it to be a fine navigational tool. IMHO, relying on GPS alone, in a coastal navigation situation, ie..."piloting" is foolish. Darkness alone should be no barrier to navigating coastal, with the plethora of lights along the US coast. Fog, and perhaps haze, would behoove one to haul further offshore, but then you'd not be piloting anymore, and would be using GPS at it's strongest suit-offshore navigation. If I have somehow misconstrued cam's post I am sure illumination, if not conflagration, will soon follow.(g)

sailingdog 05-25-2007 02:49 AM


Originally Posted by camaraderie
Dawg....let's keep chartplotters and GPS separate. Getting a lat/long from a GPS in coastal situations is an accurate means of establishing a position within a few hundred feet at worst. Not in anyway dismissing the "eyeball" but in VERY many coastal conditions (i.e. it is dark outside and the eyeball has no way of establishing a true position or fog or haze etc.) the GPS is by far the best nav tool available.

Actually, in that situation, RADAR would be far better IMHO, since it works with what is actually there, not what might or might not be there, depending on how good the signals from the satellites are and whether the SA feature is turned on or off.

camaraderie 05-25-2007 11:56 AM

Aww...c'mon guys...all this "in theory" is nice...but my little GPS tells me exactly where I am and tells me what my margin of error might be.
Would I navigate without it...NO...Would I like additional input from radar...sure. In my recent trip up the Bay...I used both and NEEDED both but my eyeballs would have been absolutely useless. In ALL the last 10 years or so of using a GPS I have never once had cause to doubt the accuracy of my GPS within a few hundred feet. Most often I am within 10-30 feet of my waypoints.
As far as Loran goes...I used that too a long time ago and it was a good tool in its' day (and still excellent for REPLICATION of previous waypoints...i.e. fishermen!) but then the abacus was a good tool in its' day as well. (G)

Valiente 05-25-2007 12:02 PM

In most pilotage situations, I don't bother with the GPS. I use a pelorus and "60 D Street". I use it to compare my DR to my lat/lon a couple of times a day, but I find that when I can see the coast, it's most useful for ETAs, compass headings and cross-track estimates.

I don't assume it correct, but do assume it's "correct enough". Offshore, I have the sextant, after all, and truly gross errors are going to become apparent pretty quickly, particularly if you've been charting your course on paper. Besides, hourly fixes gets me off the deck to reapply sunscreen and to rehydrate!

I also agree that radar makes a nice rangefinder and that a few classes in how to use it can make a big difference in how it's used (self-evident, one might think, but I've seen it used incorrectly on more than one occasion). You can, after all, avoid another boat easier than you can avoid a penisula with a strong current wrapping around it.

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