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#11
posted to rec.boats.cruising
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RDF (radio direction finding) ... do you ?
On Sun, 05 Mar 2006 14:15:42 GMT, chuck wrote:
While opinions about accuracy vary, no numbers have been posted. Come Spring, it would be great to see a couple of sailors with RDF's take them out of hiding and check their accuracy against their GPS. I used to rely on an RDF back in the 70s and have a good feel for both accuracy and the source of errors. I'm not talking about aircraft type VHF/VOR, but rather the traditional marine HF radio type with rotatating antenna sensor. RDF works by providing a bearing to a known object, in this case the radio tower which is plotted on a chart. The bearing provided by the RDF was typically relative to the heading of the boat except for the hand held RDFs with built in bearing compass. Either way, the accuracy of the bearing was only as accurate as the compass used, typically plus or minus 2 or 3 degrees. A single bearing to a radio tower provides only a single line of position, i.e., your boat is located somewhere along that line but you don't know exactly where without a second LOP from another radio station. Ideally the second bearing should cross the first at 90 degrees but in real life that hardly ever happens. The narrower the crossing angle, the more that any bearing error is magnified, and the more uncertainty that is introduced in your position plot. Back to accuracy. An error of plus/minus 3 degrees at 10 nautical miles translates to an uncertainty of plus/minus 3/10ths of a mile or about 3600 feet in total. Taken with a 90 degree crossing angle of the same error magnitude, you now have an uncertainty box about 6/10ths of a mile on each side. With less than ideal crossing angles the uncertainty box elongates to as much as one or two miles in real world situations. Compare that to LORAN-Cs 100 yard accuracy, or GPS/WAAS at 10 feet and make your own decision regarding RDF accuracy. |
#12
posted to rec.boats.cruising
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RDF (radio direction finding) ... do you ?
On 5 Mar 2006 08:11:31 -0800, "Da Kine"
wrote: If someone goes off shore with the hopes that GPS is going to do it all for them then sooner or later, they will lose their boat and maybe their life. That is nonsense. There is no viable alternative offshore other than celestial, and even the Navy has stopped teaching it. A couple of spare GPS handhelds with some extra batteries and you are as secure as it gets these days. You DO need to know how to use them of course. |
#13
posted to rec.boats.cruising
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RDF (radio direction finding) ... do you ?
The USCG requires it - today - for higher licensing. It's not
nonsense. A sextant is a great tool used to measure angles and a tool I don't go off shore without. You can get an extremely accurate fix with a sextant and a compass with only 1 known point on the horizon. Just because you don't see a need for something doesn't mean anything other then you don't know how to use it. Offshore, I can think of more then a dozen ways to navigate using nothing more then cloud formation, wind direction and change thereof, wave height and shape and crossing, Wave interval between crests and OH so many more. GPS is great right up until you get a lighting strike and everything blows out. Those that say "that'll never happen to me" have never been sailing in the tropics. My boat was hit along with 2 other boats with the same lighting strike. My boat was spared because I was navigating at the time with NO POWER ON and everything disconnected at the breakers and I had no water in my bilge. Another boat with me lost about 20K worth of electronics and was done for the season but suffered no damage other then electronic and the third boat was unplugged but somehow got a smoldering spark that later that night burned the boat to the water line. There is not much you can do about fire but the lazy ass friend of mine that wouldn't hand steer and thought GPS was the only answer found himself on a boat that he couldn't do a thing with after one little strike! Those that advocate that GPS is the second coming are fooling themselves. I hope they stay close to some of the not so lazy mariners so that WHEN they step in it they'll have someone to pull them out. |
#14
posted to rec.boats.cruising
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RDF (radio direction finding) ... do you ?
Maybe I am misreading - GPS is not the only tool you need and it seems
that you are saying it is. Also, that little lightning detector that's being sold is nothing more then an RDF with no other features. I'm not tryiing to start a brawl here, I just see people thinking to highly of GPS. The funniest thing is that most of the charts for other parts of the world are not nearly accurate enough to even think GPS. There are island groups on Caribbean charts with notes like "last survey by captain so and so of the English admiralty 1841 - No chart datum can fix that chart. Este sud Este is about 4 miles off of the chart position and none of the little islands are in the same place or the same shape as on the chart. Half moon key (cayo media luna) isn't even there anymore, there is an atoll where it use to be (but it is still on the charts. The point is, if you don't KNOW how to navigate you're likely to trust the wrong thing or not trust the right thing. If you know how to really navigate - read waves, understand currents, read clouds, know weather, do dead reckoning, keep a log, triangulate, use a sextant - You'll need the old tools to do time honored navigation. GPS is a great tool as long as it works and as long as the charts are accurate (which they aren't and which get worse as you get further away from places people frequent - like where most people like to cruise). GPS is great if it doesn't run you aground on the reef that was suppose to be 4 miles away! As for RDF - I think anyone sailing in a tropical area should have one for the one time they need to see the direction of a lightning storm. I also think everyone there needs a sextant and knowledge of how to do triangulation so they can keep track of a distant storms movement with bass height (which requires weather knowledge). I think that anyone stuck in a problem concerning those things that has not learned what they should know and has left sight of land without the right tools is asking for trouble. |
#15
posted to rec.boats.cruising
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RDF (radio direction finding) ... do you ?
waynes said ...
There is no viable alternative offshore other than celestial, and even the Navy has stopped teaching it. waynes i'm guessing that you mean that the navy doesn't teach RDF, not that they don't teach celestial. only reason i say that is because it seems like the military would need backups like celestial because i understood that the EMP from a nuclear weapons blast could take out electronics such as GPS systems. is that wrong ? i'm surprised that someone in the navy who was going to navigate such important ships would be able to get out of their training without knowing absolutely every way there was in the world to navigate. it seems like a navy sailor would have to know everything there is to know about navigation before they could make decisions that affect the ship. da kine said ... GPS is great right up until you get a lighting strike and everything blows out. thanks da kine and everyone for the great posts. i want to know as much about navigation as i can and really enjoy learning about all these alternatives. you mentioned some others including cloud formation, wind direction, wave intervals, etc, and i hope things like that get mentioned in the two fat navigation books i'm currently reading. i wonder what other situations could lead to GPS not working ? i'm with everyone else who thinks that GPS with some spares is really important. i agree relying on just a GPS wired into the boat is a bad idea because what happens if it stops working, or if your boat batteries die, or you just forget to bring enough fuses or something, or your power generation methods fail. i can't really think of much that would stop a handheld unit from working except for losing/destroying them, not having batteries, etc. i wonder what situations outside of a sailors control could make GPS fail ? i mean, i assume because so many people rely on GPS that the military wouldn't just shut it off unless there was some really drastic reason for doing so. i've read that they might make it more inaccurate or shut it off in the event of war, would they ? and i assume too that they take really good care of it since they and everyone relies on it so much ... has it ever failed ? |
#16
posted to rec.boats.cruising
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RDF (radio direction finding) ... do you ?
As I understand it, they actually run off of a gyro system that is
better then GPS. Also, I have a good friend that captains a navy sub that told me celestial navigation was required. In your reading, pick up a red/orange colored paperback called “Emergency Navigation�. Its available all over and you should be able to find it at http://www.amazon.com for cheap. You won’t find much of the good stuff in most books. As for something else making GPS not work, there are a few things I have come across and here they are. On 9/11 GPS was scrabbled in many areas. For anyone in a critical position in one of the scrabbled areas, OOPS! In Mexico (and I love this one) I ran across and “electrical engineer� that had a broken GPS and his spare didn’t work either. This was a while back but … Turned out to be a bad antenna that I fixed for him – the engineer☺ That’s the only way gps failures other then lighting that I know of. As for a spare GPS, the rumor is that if you wrap a handheld in tinfoil and get hit by lightning, your spare will still work. I don’t know anyone that has put it to the test but I keep my spare wrapped just the same. GPS signals use to be scrabbled all the time. In about 1991 or so the military started allowing public access of the gps signal. In times of heightened defense alerts, the gps signal does get less accurate but then there is also a differential signal that I don’t know much about but is used to help the gps become more accurate again. All in all, I’m more worried about lightning, having been through enough of it, then anything else. |
#17
posted to rec.boats.cruising
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RDF (radio direction finding) ... do you ?
Da Kine wrote:
[snip] In your reading, pick up a red/orange colored paperback called "Emergency Navigation". Its available all over and you should be able to find it at http://www.amazon.com for cheap. You won't find much of the good stuff in most books. good stuff, i will check into it ... As for something else making GPS not work, there are a few things I have come across and here they are. i spent a little while reading about this since my last post and found some interesting things i didn't know, i'll post some of them here since you are interested too! from some of the things i read it looks like GPS does/has failed before, i didn't know that before now. there's a quote in one of these documents that i found interest, the document says the quote is from "Interagency GPS Executive Board. GPS policy, applications, modernization, international cooperation. February 01". here's what it says ... "GPS provides many benefits to civilian users. It is vulnerable, however, to interference and other disruptions that can have harmful consequences. GPS users must ensure that adequate independent backup systems or procedures can be used when needed." then later in that same document it is talking about situations where GPS doesn't work, and apparently it's pretty easy to make it not work! you can build GPS jammers using plans on the net to jam GPS signals, and there are even jammers sold for doing the same thing. one paper i read said that these jammers are sold to militaries that want to be able to selectively jam areas for security reasons, and to defend themselves from attack, and that some of these jammers are huge like radio stations. this one paper i read had these real world situations where GPS had failed ... "Jamming in Mesa, AZ 13 - 18 Dec 01, GPS jammer caused GPS failures within 180nm of Mesa, AZ Boeing was preparing for upcoming test Accidentally left Jammer on L1 frequency radiating at .8mW Jammer operated continuously for 4.5 days Impact to ATC operations A/C lost GPS 45nm from PHX, performed 35° turn toward traffic NOTAM was not issued until 2nd day Numerous pilots reported loss of GPS NavAid Reports of hand-held GPS receivers not working" "Jamming in Moss Landing Harbor, CA 15 Apr 01 - 22 May 01, VHF/UHF television antenna with pre-amplifier caused GPS failures to all of Moss Landing Harbor Boat owner purchased TV antenna, which was equipped with pre-amp From interior location Amp's emitter jammed all of Moss Harbor and 1km out to sea No GPS in entire area = 37 days Impact to Moss Harbor Research vessels relied heavily on timing from GPS Extreme difficulty going through harbor in foggy conditions Notification to all vessels in area that GPS was down Switched back to radar control for harbor entrances" the source for those was a document from a search engine with the title "Civilian GPS Systems and Potential Vulnerabilities" i also read online that geomagnetic storms can cause GPS to fail, especially in higher latitudes and during periods of intense solar activity. it said that storms can introduce errors of 1km and more depending on location and how active the ionosphere is. and then there are other kinds of errors too that i read about, ones that seem well known to people who are really into that kind of thing. thinking back on it, i also remember another reference to GPS failure, somewhere in the red sea or east coast of africa there was a harbor where GPS doesn't work, but now i can't remember where i read it. i think it may have been in jimmy cornell's "world cruising handbook" but i glanced through it just now and couldn't find the reference. apologies to the author if i am wrong about that! i guess like anything else, GPS is just another aide to navigation. |
#18
posted to rec.boats.cruising
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RDF (radio direction finding) ... do you ?
These are great examples of why I keep up on my other nav skills and
why I make a point to bust the chops of the guys that try to preach gps like they do. As well intentioned as people are, they are blindly falling into a trap and leading others with them when they set aside the old world skills of navigation. Good for you for being smart! |
#19
posted to rec.boats.cruising
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RDF (radio direction finding) ... do you ?
I meant that I really might have misread your post. The way I read it
said to me that you were saying gps is the way, and only way. Sorry that you thought I was being a jerk - I didn't want to come off like that. |
#20
posted to rec.boats.cruising
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RDF (radio direction finding) ... do you ?
chuck wrote:
A couple of comments. While opinions about accuracy vary, no numbers have been posted. Come Spring, it would be great to see a couple of sailors with RDF's take them out of hiding and check their accuracy against their GPS. No RDF is going to provide a position within a few meters except by accident. but most of us work really hard to avoid needing that kind of accuracy. i'm going to try it. i'm just going to get some of this RDF equipment and try it out on land while using my handheld GPS and some maps to check against. or maybe in the truck, i have an icom radio in the truck i can use with an antenna, it has a strength meter built into it. i'll have to find a good directional antenna to use, or make one. i'm going to check around for some of the actual RDF equipment used in marine navigation too though, i bet there is tons of this stuff in people's garages just waiting for someone to find. when i was reading it looks like you can also find old RDF/ADF equipment out of old airplanes too to play with, and these have needles that turn to whatever transmitter is using the frequency you tune too! fun stuff. |
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