This is a response to the post from Harold regarding GPS errors. Harold, A friend of mine by the name of Sarah Ohmann told me a few years ago about a user group that went to test GPS systems. A group of three testers stood right next to each other and turned on the expensive GPS systems to see how much the lat. long. readings would be between the units. They were surprised at how different the results were from each other. Then one of the testers thought that the units might be interfering with each other. They tested the units at the same spot one at a time. They found that the readings all of the units gave were very similar. That could be the reason why your GPS error occurred in your group. Good Paddling and GPS reading, Don Dimond *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
Recently there have been several threads going on this list about choosing a GPS and I thank everyone who contributed. I learned a lot. I don't mean to reopen those discussions, but a question came up the other night which I felt y'all could help me with. A friend of mine went sailing as part of a class, and there were 6 handheld GPS units on board, plus the large console GPS installed in the boat. Throughout the trip, the console unit consistently indicated positions which were 3 miles off of the positions shown by the six handheld units and the compass-bearing fixes taken by the students. Now, naive Harold always assumed that a GPS either worked accurately or not at all. Apparently not. Would this discrepancy be due to improper factory calibration of the unit, or can a unit slip out of calibration? Or might the discrepancies be due to the installation of the ship-mounted unit, or what? Inquiring (armchair) navigators want to know. Oh, one other thing. I've heard of cases where people have thought their GPS was off several miles when cruising or paddling in Baja, only to find that the Mexican maps were the source of the inaccuracies. But this trip was right here in So. Cal., using NOAA charts, and all but the console unit agreed with the charts. Any ideas? Harold So. Cal. *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
> -----Original Message----- > From: owner-paddlewise_at_lists.intelenet.net > [mailto:owner-paddlewise_at_lists.intelenet.net]On Behalf Of > HTERVORT_at_aol.com > Sent: Saturday, March 13, 1999 1:40 AM > To: paddlewise_at_lists.intelenet.net > Subject: Re: [Paddlewise] GPS errors > > > Recently there have been several threads going on this list about > choosing a > GPS and I thank everyone who contributed. I learned a lot. > > I don't mean to reopen those discussions, but a question came up the other > night which I felt y'all could help me with. > > A friend of mine went sailing as part of a class, and there were > 6 handheld > GPS units on board, plus the large console GPS installed in the boat. > Throughout the trip, the console unit consistently indicated > positions which > were 3 miles off of the positions shown by the six handheld units and the > compass-bearing fixes taken by the students. Now, naive Harold > always assumed > that a GPS either worked accurately or not at all. Apparently not. > > Would this discrepancy be due to improper factory calibration of > the unit, or > can a unit slip out of calibration? Or might the discrepancies > be due to the > installation of the ship-mounted unit, or what? Inquiring (armchair) > navigators want to know. > > Oh, one other thing. I've heard of cases where people have > thought their GPS > was off several miles when cruising or paddling in Baja, only to > find that the > Mexican maps were the source of the inaccuracies. But this trip was right > here in So. Cal., using NOAA charts, and all but the console unit > agreed with > the charts. > > Any ideas? > > Harold > So. Cal. > ****************************************************************** > ********* > PaddleWise Paddling Mailing List > Submissions: paddlewise_at_lists.intelenet.net > Subscriptions: paddlewise-request_at_lists.intelenet.net > Website: http://www.gasp-seakayak.net/paddlewise/ > ****************************************************************** > ********* > *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
Well, its hard to diagnose why a particular unit was failing but I do know that *every* time I have recorded a track and downloaded it onto the computer, and overlayed it graphically on to a map it has been accurate to well within the stated tolerances of GPS with Selective Availability. I have a Garmin 12XL. Richard Walker Houston, TX http://www.neosoft.com/~rww/kayak_log.html *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
> Would this discrepancy be due to improper factory calibration of > the unit, or can a unit slip out of calibration? Or might the > discrepancies be due to the installation of the ship-mounted > unit, or what? Inquiring (armchair) navigators want to know. Could the ship mounted unit have been set up to read nautical miles and the hand helds statute miles? Possibly using a different datum reference? Woody *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
Woody. Good thought. Actually, I didn't state nautical miles in my post because I think of nothing else on the water. Besides, I _should_ have stated that the units disagreed by 3 degrees in the lat/lon fixes, but I equated those 3 degrees to approximate (at our latitude) 3 nM. Any other thoughts? Harold So. Cal., so needing a cup of coffee *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
Ooops! I did it again. In my answer to Woody I said: >I _should_ have stated that the units disagreed by 3 degrees in the lat/lon fixes<. Of course, I _SHOULD_ have stated that the units disagreed by 3 _minutes_ in the lat/lon fixes. Harold So. Cal. - see, the coffee helped *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
>Would this discrepancy be due to improper factory calibration of the unit, or >can a unit slip out of calibration? Or might the discrepancies be due to the >installation of the ship-mounted unit, or what? Inquiring (armchair) >navigators want to know. Almost always, this sort of discrepancy is due to setting the wrong reference datum into the receiver. Most topo sheets use NAD27 (North American Datum, 1927) as their baseline for measurement. There are many, many others. Chances are the discrepancies were all almost the same distance, and always the same distance and direction of displacement as well. In effect, GPS receivers don't go out of calibration because they constantly update their references which are continually downloaded from the satellites. *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
Joe Pylka wrote (regarding 3-minute discrepancies between GPS receivers): > Almost always, this sort of discrepancy is due to setting the wrong > reference datum into the receiver. ... Actually, this can't possibly be the explanation. There are no areas in North America for which the discrepancy between reference datums--such as NAD27 and WGS84--is as large as 3 minutes (or 3 nautical miles). I know of only two possible explanations for such large discrepancies: (1) multipath error, or (2) a serious problem with the GPS receiver. Multipath error could be the cause if the placement of the remote antenna for the console-mounted receiver is such that it is subject to reflected signals. This can lead to very large errors (as much as the 3 nautical miles that were observed). The way to check to see if this is the problem is to hook up a handheld receiver (with an external antenna connector) into the antenna that the console-mounted unit is using, and see if the problem is repeated for the new receiver. I think that this explanation, while possible, is unlikely based on your description, since multipath errors are variable and somewhat random (as the ship changes position). The second possibility--a faulty receiver--is more likely to be the problem. Again, hooking up a different receiver to the antenna should help to identify if the problem lies with the antenna placement or with the receiver. Dan Hagen *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
I suspect that the GPS may have been set up on a different Datum, perhaps from an offshore cruise in foreign waters. cu *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
Bob Denton wrote (regarding GPS discrepancies of 3 minutes of lat/long): > > I suspect that the GPS may have been set up on a different Datum, perhaps > from an offshore cruise in foreign waters. Again, this can't explain errors of 3 minutes of latitude and longitude, even if the offshore cruise was to very distant lands using a highly aberrant datum. Consider the following example. The original post was from southern California, so let's use as our example the Santa Monica pier, and set a waypoint at this position. The pier lies at N 34deg 00min, W 118deg 30min using the WGS84 datum. (Actually this is a spot near the pier.) Changing the datum changes the reported position of the waypoint, but not by very much (as we will see). First let's examine the "usual suspects", such as NAD27 (CONUS and Mexico). These cause a difference in reported latitude of about .001 minutes (33d 59.999min and 33d 00.001min, respectively) and a difference of less than .06 minutes of longitude (118d 29.946min and 118d 29.953min). We can get a larger error if we choose an aberrant datum from a far-away location (we are assuming here a very distant foreign cruise, and a failure to reset the GPS upon returning home). Checking a variety of such datums--from Afgooye to Zanderij--shows that the larger deviations for the position of our Santa Monica waypoint are in the range of .1 to .3 minutes, with somewhat larger discrepancies (still only a fraction of a minute) occurring in the case of a few exceptions. The largest discrepancies that I was able to find are for the Hu-Tzu-Shan datum and for the Tristan Astr. '68 datum. (Wouldn't a cruise to Tristan be fun? It is one of the most remote places on earth.) The Hu-Tzu-Shan datum yields a reported waypoint position of 34d 00.373min, 118d 29.808min. The Tristan datum yields a reported position of 34d 00.294min, 118d 29.504min. In terms of total distance, the champion is therefore Tristan Ast. '68. However this is still only a fraction of the reporting discrepancy of 3 minutes lat and long that was the focus of the original question. In short, I think that we can rule out an incorrectly set datum as the cause of such large errors. So that brings us back to other possible explanations, such as multipath error, a "broken" receiver, or (as someone else mentioned) interference from other electronic devices. Dan Hagen *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
I have been following this but I'm not sure I digested all of the details so I apologize if I am repeating. Datum was my first suspect but you are right. Datums show up usually as a fraction of a NM difference (assuming the common 100 or so datums). However, some GPS units allow you to enter a custom or USER datum which could have any amount of error. So, you still want to double check this. Multipath usually shows up as a jump. For those that don't know what multipath is, it is a reflected signal to your GPS rather than a direct signal from the satellite to your GPS. Since the basis of your position is a calculation of distance from the satellite to your GPS antenna, a reflection is an additional distance traveled, hence an induced error. However, at the frequency of the GPS, there is rarely a sustained reflection path, especially on the water. Therefore you will see a temporary jump of position for a few seconds. A high quality GPS can do a lot to discriminate between direct and incident signals. Less expensive ones cant. Question, were you docked when you did these tests? Was there a lot of stuff around like buildings, bridges etc? Jamming. GPS by its very nature is very resistant to jamming and spoofing. Remember this system was designed for military use first and jamming is an A#1 tactic. The code and frequency, even in the civilian C/A section is pretty stable. Interference from other things will shut most GPS units down rather than give false readings. Again, I'm not up on the really inexpensive architectures of systems out there. Possibly low end units use analog circuits that are susceptable to outside noise. This seems unlikely though. The incident out here of the grounding of the "Royal Majesty" was due to a GPS error on the order of a couple of miles. It turned out that the GPS was not locked into the satellite system. The antenna system had failed and the unit was dead reakoning. The officer on watch failed to pick up this subtle problem. Everything "seemed" to be working. Are you SURE this unit was actually LOCKED onto the constellation? Finally, how was altitude being determined? Was it entered manually and you were navigating in 2D mode or were you resolving lat/long and altitude? If a false antenna height were entered ...say of 30,000 feet, and the unit were forced into 2 dimensional resolution, you will see an error because it is trying to use 30K feet as a part of an assumed position in its calculation. Worth checking. Finally, finally, some units have an oscillator offset check in the service screen. There is a tolerance for offset on some units....maybe this unit is .....broken. *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
Great input everyone. Especially Dan and ?WILAX? (who is that masked poster who posts knowledgeably and drives too fast?). I have little more input to help with this conundrum, but I'll try to at least guess. <Multipath usually shows up as a jump. Question, were you docked when you did these tests?> No, the ones involved (a friend and others in a class) were actively sailing about, practicing navigational and sailing skills. <Was there a lot of stuff around like buildings, bridges etc?> I assume no closer than 1/2 to 1 nM. But even if, they would be moving away and toward. <The antenna system had failed and the unit was dead reckoning.> Wouldn't the officer on watch (or previous watch) have needed to manually initialize the unit to start DR? I never heard of a GPS doing DR, so I assume you are referring to some fancy nav system which integrates a GPS receiver for obtaining fixes? <Are you SURE this unit was actually LOCKED onto the constellation?> Nope. <Finally, how was altitude being determined? Was it entered manually and you were navigating in 2D mode or were you resolving lat/long and altitude?> Huh? Sorry, I'm the dummy who asked the question. Actually, the handhelds were confirmed (supposedly) as being accurate. Since the console unit was mounted in a sailboat rather than an RV or helicopter, I would be surprised if anyone ever dinked with the altitude setting after initial installation. Of course, its precisely because it should always be set for sea level + antenna height that nobody would ever think to check it, and since no instrument is idiot proof. . . <Finally, finally, some units have an oscillator offset check in the service screen. There is a tolerance for offset on some units....maybe this unit is .....broken.> Could be, but your analytical skills are functioning. Thanks Harold So. Cal., so impressed - thanks guys. *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
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