Re: handheld Loran
HeeeHeeeeHeeeee<br /><br />This is going to be a long post but if you don't know anything much about how to find places on the globe or how our navigation gear works maybe it will be helpful.<br /><br />There's an old Jimmy Buffett song that has a line in it that goes "changes in latitudes, changes in attitudes." The line means to say that as you move south to the islands your outlook on life becomes more relaxed. Well, that's what a poet might say, but for us hard realists there's another message - it is that as you move north or south you move through latitude.<br /><br />Now, let's see how you could find where I live.<br /><br />Think about the earth
for a minute and picture the poles at top and bottom and and line around the equator, right in the middle of where it spins. The equator is a line of lattitude and it is numbered zero. We divide each half of the globe into degrees and the poles are at 90 degrees so if you like you can immagine 88 more lines between the equator and the poles, each one of them precisely parallel to the others, and you could do this going in both directions. As I said, the equator itself is numbered zero, and each succedding parallel line (which are often simply called parallels by the old salts) has a higher number until you get to the pole. The pole itself is numbered 90 but it is just a point. For whatever its worth I am at a place that is about 70 miles south of Pittsburgh, Pa and I am located very close to where the 40th parallel to the north is located. So to find my house you might begin by going to 40 degrees North Latitude.<br /><br />That information alone wouldn't do you any good if you were looking for me though because that parallel at 40 degrees above the equator stretches all the way around the earth and you don't know where I am on the line.<br /><br />Enter Longitude. If you could perch yourself above one of the poles and looked down on the earth you would see a circle. Like any circle it contains 360 degrees and so you could take a protractor and draw lines from the point which is the pole to connect to the equator at equal intervals and give a second dimension to defining a place on earth. That is becasue by having parallels of latitude and lines of longitude we divide the earth into a grid of roughly rectangular blocks, with a rational numbering system. The only question is where to start. Well, the truth is that you could start anywhere on the globe you liked but for the method to be useful everyone has to agree on where to begin marking lines. <br /><br />When this system was being devised a number of different starting places were used, Roam, Cairo, Paris just to name a few, but England ruled the seas in the end and so in time it came to be agreed that the first line, line zero, would pass through the Royal Observatory in Greenwich, England, just to the west of London as I understand it. A parallel line passing through that spot will cross the equator in west africa and then continue on to the south pole. These lines are called meridians and the one that passes through Greenwich is called the Prime Meridian, or number zero. Lines of longitude are equally spaced at the equator, at one degree intervals, and meet at the poles, so as you progress to the north or south of the equator the lines get closer and closer together.<br /><br />Now a bit about degrees of either latitude or longitude. A degree can further be divided in to what are called minutes. There are 60 degrees in a minute of distance and each of those degrees can be further divided into seconds. These further divisions give us the ability to further define a spot, but in fact we could have simply used decimal places of a degree to get accuracy. <br /><br />So we could define a spot by saying it is some number of paralles north or south of the equator and some number of meridians east or west of the Prime Meridian. For instance, at the moment I am at 39 degrees and 38 minutes North by 79 degrees and 32 minutes (DDD MM) west. I could further divide that into seconds (DDD MM SS) if I wanted to and even add decimal points to the seconds (DDD MM SS.S) if I wanted to in order to give great precision to my location. The thing to keep in mind is that the same location could be described more than one way. Here is an example of what I mean. The very northwest corner of the state of Maryland could be described in these ways:<br /><br />N. 39.71283 x W. 079.47455 (DDD.ddddd<br />N. 39 42.770 x W.079 28.473 (DDD MM.mmm)<br />N. 39 42 46.2 x W 079 28 28.4 (DDD MM SS.s)<br /><br />All three sets of numbers refer to exactly the same post, just slightly differenct ways of describing it.<br /><br />So that's how the grid system works, pretty simple actually.<br /><br />Now, how do we nagigate using our electronics? Well, this stuff is pretty easy to understand too and suprisingly LORAN is the best one to start with because its very easy to understand.<br /><br />LORAN is a system that uses relativly low frequency radio waves, and the time it takes them to move from a transmitter to a receiver, for locating places. Like all navigation systems it requires very accurate clocks that are syncronized precisely. Here's why. Radio waves travel across the earth at a more or less constant speed and so if a radio wave is emitted you can tell how long it will take to get to a receiver at some distance away. If you have an extremely accurate clock you can actually measure how long it take a radio wave to travel from one point to another with extreme accuracy. Its all a matter of the clock. <br /><br />Now, remember that you can design a radio's antenna to be directional or you can have it just radiate in a pattern that is shaped like a globe, with all power going out in all directions equally, but of course the part pointing at the earth isn't going anywhere. Also, because the speed is constant you could measure how far away from the antenna by measuring the Time Delay between transmission and reception. Time Delay. Time Delay. Have you ever seen LORAN numbers called TD's? Well, that's what it stand for, the time delay between when a pulse of energy was emitted by a transmitter somewhere and when it was received where you are standing right at the moment. Unfortunately just haveing one TD won't do you any more good than having just my lattidute with no longitude. Well LORAN had more than one station, a lot more than one to tell the truth.<br /><br />The LORAN system is divided into what are called Chains and each chain has a number. The number is actually what is called the pulse repitition rate, which is a measure in milliseconds of the spacing between pulses of energy emitted by the radio transmitter. Think of it as the little clicking of a depth finder, its sort of the same thing. So, the chain is named for its repition rate in ms. and as an example if you are in one of the mid atlantic states in the east you'd be using the chanin that is called the 9960, because the pulses from its several transimtters are fired at intervals of 9960 milliseconds. <br /><br />So, there are many LORAN stations, all around the world actually, and they have a low frequency they operate on. The receiving loran has internal software to look for several stations in each chain and then it uses the time delays of the signals to triangulate a location. The other stations that are used are called "Secondary" and they have letters to define them, such as W, X, Y, or Z. The actual LORAN location will use the Time Delay (TD) from two of those stations and will include the Chain number. So, for instacne, that upper left corner of the state of Maryland that I mentioned earlier would be described like this: <br /><br />TD 9960<br />X,Y 28379.8 x 43028.8<br /><br />Notice that it was at exactly the same place that I indicated in Lat/Lon above, just expressed in a different way. Also, all lines from all transmitting stations are in great circles, with none of the at all straight. Just like the ringlet waves from tossing a stone into a pond.<br /><br />So, what else do you need to know about LORAN? Well, a lot actually. Radio Waves don't always travel at the same speed. Also, land based radio statons can be disrupted without warning. Stations went down from time to time, equipment had to be recalibrated, particularly the clocks, and the signals might be slowed down by things like the annual greening of forests or when passing over large lakes, even storms to a very small extent. That is what caused the inherent inaccuracy of the system, because the rings were not perfect circles. The best accuracy you could ever expect from a LORAN was a quarter mile, but that was only in transfering a theoretical point, calculated by the distance from the transmitters, into a machine and expecting for it to match the actual location. The waves might have been off by a bit but the amount they were off didn't change - unless someone drained Lake Erie or the Chesepeake, or the forrests of the north east just got up and moved away. So what LORAN was great at was repeatability, you could go back to a point saved with extreme accuracy, but its absolute accuracy, matching a point on a map to an actual location, wasn't very good.<br /><br />Now, you want the very simplest explaination possible of GPS? Here goes - it works the same way except the radio stations are up in the sky.<br /><br />Really.<br /><br />Well, there are some differences other than the statons are up in the sky. For one thing there are two dozen stations up there rather than the hundreds of LORAN transmitter world-wide. The second thing is that there aren't a bunch of different chains. Another thing is that GPS signals can actually self diagnose any problems they are experiencing and cause corrections to be made. But overall its still a matter of measuring time between transmission and receipt of a signal to determine the distance from several stations at one time and then calculate the only positon on the face of the earth where the calculated points come together. Its really that simple.<br /><br />Notice another difference between LORAN and GPS. The location system that came out of the mechanical workings of the LORAN system led to its own grid system being used. Although most of the later model LORAN units that were sold had software built in that could convert the TD's to good old Latitude and Longitude for the most part folks didn't use it. That is why so many guys still have LORAN TD's saved even though its been years since they owned an LORAN. Notice that with the GPS system that Lat/Lon is used as the default. Because the locaton is really calculated by reference to satelite signals they could be converted for display into any conceivable system, it really makes no difference as far as the accuracy goes. The only thing that effects accuracy of a reciever is the accuracy of its own internal clock.<br /><br />Now one last thing then I'll go. WAAS. What one earth is WAAS? Well, it stands for Wide Area Augmentation System. It is a method of improving the accuracy of the GPS signals sent down. <br /><br />In the early days of recreational GPS an artificial inaccuracy was put into the system by its owner - the US Department of Defense. President Clinton signed an Executive Order that did away with that and vastly improved the safety of navigation for both commercial and recreational boaters while reducing the system's operating cost. Pretty smart move. Prior to that order we used a system remarkable similar to the LORAN system along with a second receiver (called a differential receiver) to make corrections to the GPS signals comming down. WAAS killed that correction system in the USA because its so much less expensive from a user perspective.<br /><br />Basically WAAS works like this. Very accurate GPS recievers are placed at exactly known spots and then they just listen to the satellites. They listen to see where the birds tell them they are located and they compare that to their known postion. Corrections are calculated for any error (which might be casued by a divergence in orbit, a clock error, high altitude storms, whatever) that might exist and those corrections are broadcast back to the satellites. The satellites then send that information back down to recievers that are equiped to use the information (WAAS capablie machines) and improved accuracy is the result. Notice that with the WAAS system that the actual improvements in the reciever end don't come from having a separate radio receiver, they are just software applicaitons. That is why it is so inexpensive to make a GPS WAAS capable.<br /><br />So, that's that.<br /><br />Now to answer your question. The error in converting TD's to Lat/Lon actually is not at all great. What is great is the inaccuracy of the old LORAN system. The more obtuse the angle of intersecting radio signals from the Master, X, Y, or Z stations the worse the error would be. Now that should tell you something. If you were at a place where the signals from two of the stations were intersecting at almost a 90 degree angle then in fact you could expect a very good correlation in calculating the conversion. So if you have a Nautical Chart for your area, which is coastal Virginia if I remember correctly, you could look at the very light purple (or is that some shade of red?) TD lines that overlay the chart. And if you inspected the grid they make you could determine if they were sort of diamond shaped, meaning an unfavorable angle of intersecting lines you'd know the conversion wouldn't go very well. But if the intersecting lines looked like neat little squares with nice 90-degree corners you could expect the conversion to go very well and be quite accurate.<br /><br />Are you getting ready to run to the closet to see how your lines intersect? I can save you some troble. You are in the 9960 chain and if you were to drop just a little south of Rudee's Inlet , down to just about the VA/NC state line, you'd find almost perfect 90-degree rectangles. You are in one of the best spots in the country for getting accurate conversions. Want to see a place where they don't work out well at all? Go to the Keys.<br /><br />Oh, and with what you know now one thing should be pretty obvious to you. A hand held LORAN will be just as accurate as a fixed mount one. That's because if you can recieve a usable signal at all its going to work, it doesn't matter if the signal is strong or weak, it doesn't matter if your antenna is long or short, high or low, all that matters if it can receive a usalble signal - if it can it will have all the accuray it is capable of. Only the clock made any real difference and the clocks in the hand held units were absolutly identical to the fixed mount ones. Now the recievers may not have been as good so it may not have worked as often, and the battery life may have been pretty crappy, but if it worked at all a hand held was just as good as a fixed mount.<br /><br />There, that help any? If you've got question's I can try again.<br /><br />Thom
