For the past four hours, I have been using my Magellan GPS 3000 XL and Fugawi 3.0 software on the laptop to monitor the location of a chair which is supporting my GPS antenna on the patio. It is moving all over the neighborhood! For a few hours it was quite happy in the back yard, shuffling back and forth as one would expect, but lately it has moved a couple blocks south and back, then a couple blocks west and back. Yesterday it took a 150 meter trip to the southeast, but returned fairly promptly. The GPS is currently locked on five satellites.
The total area of its excursions today is about 150 meters east-west and 250 meters north-south, a good deal outside the published accuracy of the system. I would not like to be using this to find my way back to the marina at night and would be extremely wary if I were driving an autopilot from the GPS.
Is anyone else monitoring the GPS system? Does anyone have a DGPS they can fire up to monitor the system? Any clues would be appreciated.
Keep on sailing,
Ken Coit, ND7N
CD/14 #538
CD/36 #84 Parfait
Hailing Port: Raleigh, NC
Sailing from: Beaufort, NC
Currently rocking in my chair somewhere in Raleigh, NC
parfaitNOSPAM@nc.rr.com
GPS Accuracy??
Moderator: Jim Walsh
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Don Carr
Re: GPS Accuracy??
Ken; I have found that the Magellan units go into 'averaging mode' when you are not underway. Apparently the units seem to require some motion. I'll bet you would find accuracy to with 3 meters if you walk around your house. Just my observation with the Magellan GPS.
Fair winds.
Don Carr
s/v Lolita - Noank Ct
dc1c@aol.com
Fair winds.
Don Carr
s/v Lolita - Noank Ct
dc1c@aol.com
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Ken Coit
Well, maybe I'll take it for a drive.
Don,
Thanks for the information.
I have an inquiry into Magellan to see what they say. I am wondering if there isn't something, like changing satellites, that causes the unit to drift away and back. I am recording the postion every 15 seconds. Thus far, the greatest distance "moved" in 15 seconds is 168 meters or over 550 feet. Interestingly, no speed was recorded for this move.
More to come...
Ken
parfaitNOSPAM@nc.rr.com
Thanks for the information.
I have an inquiry into Magellan to see what they say. I am wondering if there isn't something, like changing satellites, that causes the unit to drift away and back. I am recording the postion every 15 seconds. Thus far, the greatest distance "moved" in 15 seconds is 168 meters or over 550 feet. Interestingly, no speed was recorded for this move.
More to come...
Ken
Don Carr wrote: Ken; I have found that the Magellan units go into 'averaging mode' when you are not underway. Apparently the units seem to require some motion. I'll bet you would find accuracy to with 3 meters if you walk around your house. Just my observation with the Magellan GPS.
Fair winds.
Don Carr
s/v Lolita - Noank Ct
parfaitNOSPAM@nc.rr.com
-
Ken Coit
-
Richard Hill
Re: ROFL, but why do you ask?
OK, I'll bite. What does ROFL stand for? I have guesses for some of the letters, but not all (especially the L).Ken Coit wrote: Yes, I am finally my own boss save one.
Ken
jack wrote: ????
captrahill@comcast.net
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Ken Coit
Rolling On the Floor Laughing (ROFL)
Sorry, I had second thoughts about that after it was posted.
Here's another:
ROFLASTC = ROFL And Scaring The Cat
The link below will provide you with a lot more and many you won't care about. I haven't found a short list of strictly non-technical web acronyms, but I will post a link if I do.
Keep on sailing,
Ken
parfaitNOSPAM@nc.rr.com
Here's another:
ROFLASTC = ROFL And Scaring The Cat
The link below will provide you with a lot more and many you won't care about. I haven't found a short list of strictly non-technical web acronyms, but I will post a link if I do.
Keep on sailing,
Ken
Richard Hill wrote:OK, I'll bite. What does ROFL stand for? I have guesses for some of the letters, but not all (especially the L).Ken Coit wrote: Yes, I am finally my own boss save one.
Ken
jack wrote: ????
parfaitNOSPAM@nc.rr.com
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Richard Hill
Re: Rolling On the Floor Laughing (ROFL)
And I thought it was something dirty! Where has my mind been? Thanks.
Ken Coit wrote: Sorry, I had second thoughts about that after it was posted.
Here's another:
ROFLASTC = ROFL And Scaring The Cat
The link below will provide you with a lot more and many you won't care about. I haven't found a short list of strictly non-technical web acronyms, but I will post a link if I do.
Keep on sailing,
Ken
Richard Hill wrote:OK, I'll bite. What does ROFL stand for? I have guesses for some of the letters, but not all (especially the L).Ken Coit wrote: Yes, I am finally my own boss save one.
Ken
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Murray Glue
Re: GPS Accuracy??
Ken,
The error is due to a deliberate system degrading known as SA which stands for " Sofa Adjustment ". This is currently switched on to reduce the accuracy of GPS guided weapons. The solution is to take back the chair and move the sofa onto the patio.
Seriously though...5 satellites is few enough to obtain a reasonable standard of positioning. The geometry ( relative position in the heavens ) of the 5 satellites is what produces the fix accuracy. More sophisticated ( should that be sofa-isticated ) receivers provide a GDOP figure which is a combined error indication for a number of other DOP ( Dilution of Precision ) values, an indication of the received position accuracy...normally better than 1.5 for good data.
DGPS is currently able to produce better than 2.0 metres repeatability continuously in its more developed forms. The correction data from one DGPS reference station alone is usually not much better then 5 to 8 metres depending on how far away it is from the user. If a network of DGPS sources is used then processed for the numerous variables which can be relevant, the 2.0 metre figure or better is reached.
In my offshore work we now apply a number of correction types from a wide area network of reference stations, to a 12 channel receiver, and the solution is number crunched in a fast computer to provide this kind of accuracy.
Generally a small cheap GPS such as the Magellan should be viewed with a very critical eye if attempting to use it for tight navigation.
Chairs, or " sofa so good "
Murray
CD30 Dayspring
Nelson
New Zealand
murray@offshoretechnical.com
The error is due to a deliberate system degrading known as SA which stands for " Sofa Adjustment ". This is currently switched on to reduce the accuracy of GPS guided weapons. The solution is to take back the chair and move the sofa onto the patio.
Seriously though...5 satellites is few enough to obtain a reasonable standard of positioning. The geometry ( relative position in the heavens ) of the 5 satellites is what produces the fix accuracy. More sophisticated ( should that be sofa-isticated ) receivers provide a GDOP figure which is a combined error indication for a number of other DOP ( Dilution of Precision ) values, an indication of the received position accuracy...normally better than 1.5 for good data.
DGPS is currently able to produce better than 2.0 metres repeatability continuously in its more developed forms. The correction data from one DGPS reference station alone is usually not much better then 5 to 8 metres depending on how far away it is from the user. If a network of DGPS sources is used then processed for the numerous variables which can be relevant, the 2.0 metre figure or better is reached.
In my offshore work we now apply a number of correction types from a wide area network of reference stations, to a 12 channel receiver, and the solution is number crunched in a fast computer to provide this kind of accuracy.
Generally a small cheap GPS such as the Magellan should be viewed with a very critical eye if attempting to use it for tight navigation.
Chairs, or " sofa so good "
Murray
CD30 Dayspring
Nelson
New Zealand
murray@offshoretechnical.com
-
Ken Coit
Re: GPS Accuracy??
Murray,
Thanks for the information. I am running the same experiment today and the same chair has settled down considerably; it is staying pretty close to home. The wild trip it took the other day seemed rather deliberate, like someone was trying to guide a specific missle into the Potomac River rather than the White House or the Washington Monument. It was only out of the yard for a few minutes, but it sure took a long trip.
I am trying to avoid the purchase of a new-fangled gadget, but I may just have to do it. Maybe the LORAN can feed the computer, I'll have to check it out.
More to come, I am sure.
Keep on sailing,
Ken
parfaitNOSPAM@nc.rr.com
Thanks for the information. I am running the same experiment today and the same chair has settled down considerably; it is staying pretty close to home. The wild trip it took the other day seemed rather deliberate, like someone was trying to guide a specific missle into the Potomac River rather than the White House or the Washington Monument. It was only out of the yard for a few minutes, but it sure took a long trip.
I am trying to avoid the purchase of a new-fangled gadget, but I may just have to do it. Maybe the LORAN can feed the computer, I'll have to check it out.
More to come, I am sure.
Keep on sailing,
Ken
Murray Glue wrote: Ken,
The error is due to a deliberate system degrading known as SA which stands for " Sofa Adjustment ". This is currently switched on to reduce the accuracy of GPS guided weapons. The solution is to take back the chair and move the sofa onto the patio.
Seriously though...5 satellites is few enough to obtain a reasonable standard of positioning. The geometry ( relative position in the heavens ) of the 5 satellites is what produces the fix accuracy. More sophisticated ( should that be sofa-isticated ) receivers provide a GDOP figure which is a combined error indication for a number of other DOP ( Dilution of Precision ) values, an indication of the received position accuracy...normally better than 1.5 for good data.
DGPS is currently able to produce better than 2.0 metres repeatability continuously in its more developed forms. The correction data from one DGPS reference station alone is usually not much better then 5 to 8 metres depending on how far away it is from the user. If a network of DGPS sources is used then processed for the numerous variables which can be relevant, the 2.0 metre figure or better is reached.
In my offshore work we now apply a number of correction types from a wide area network of reference stations, to a 12 channel receiver, and the solution is number crunched in a fast computer to provide this kind of accuracy.
Generally a small cheap GPS such as the Magellan should be viewed with a very critical eye if attempting to use it for tight navigation.
Chairs, or " sofa so good "
Murray
CD30 Dayspring
Nelson
New Zealand
parfaitNOSPAM@nc.rr.com
-
will parker
Re: GPS Accuracy??
I have had very satisfactory performance from my GARMIN 126, including getting me through Indian Cay Pass at West End, Bahamas, and getting me to Bermuda and back. Mine is 'off the shelf' and about three years old.
From my observation of anecdotal evidence, I would not trust a Magellan.
Will
Jambalaya
CD30
; I have found that the Magellan units go into 'averaging mode' when you are not underway. Apparently the units seem to require some motion. I'll bet you would find accuracy to with 3 meters if you walk around your house. Just my observation with the Magellan GPS.
whildenp@earthlink.net
From my observation of anecdotal evidence, I would not trust a Magellan.
Will
Jambalaya
CD30
; I have found that the Magellan units go into 'averaging mode' when you are not underway. Apparently the units seem to require some motion. I'll bet you would find accuracy to with 3 meters if you walk around your house. Just my observation with the Magellan GPS.
Don Carr wrote: Fair winds.
Don Carr
s/v Lolita - Noank Ct
whildenp@earthlink.net
-
Ken Coit
Let's Just Say That I Am Wary
Will,
Thanks for your input. I am running my fifth experiment at the moment and it is going better than the others, but it is only 3 hours old. All the other experiments, over 10 or more hours each, have shown very significant errors for a few minutes at a time. You wouldn't want to enter that narrow cut during one of these excursions, but it would be pretty safe just after one.
How would you tell it had passed? I guess you'd have to anchor out until you knew that the GPS had made up its mind about where you were and diplayed the nominal position as a small blob of tracks.
A Garmin may be in my future.
Keep on sailing,
Ken Coit, ND7N
CD/14 #538
CD/36 #84 Parfait
Hailing Port: Raleigh, NC
Sailing from: Beaufort, NC
parfaitNOSPAM@nc.rr.com
Thanks for your input. I am running my fifth experiment at the moment and it is going better than the others, but it is only 3 hours old. All the other experiments, over 10 or more hours each, have shown very significant errors for a few minutes at a time. You wouldn't want to enter that narrow cut during one of these excursions, but it would be pretty safe just after one.
How would you tell it had passed? I guess you'd have to anchor out until you knew that the GPS had made up its mind about where you were and diplayed the nominal position as a small blob of tracks.
A Garmin may be in my future.
Keep on sailing,
Ken Coit, ND7N
CD/14 #538
CD/36 #84 Parfait
Hailing Port: Raleigh, NC
Sailing from: Beaufort, NC
will parker wrote:
I have had very satisfactory performance from my GARMIN 126, including getting me through Indian Cay Pass at West End, Bahamas, and getting me to Bermuda and back. Mine is 'off the shelf' and about three years old.
From my observation of anecdotal evidence, I would not trust a Magellan.
Will
Jambalaya
CD30
; I have found that the Magellan units go into 'averaging mode' when you are not underway. Apparently the units seem to require some motion. I'll bet you would find accuracy to with 3 meters if you walk around your house. Just my observation with the Magellan GPS.
Don Carr wrote: Fair winds.
Don Carr
s/v Lolita - Noank Ct
parfaitNOSPAM@nc.rr.com
-
Larry DeMers
Re: GPS Accuracy??
Ken,
Last year we bought the Raynav 300 SDGPS Rcvr., and this tool is by far the neatest nav tool I have ever worked with. These wild position reports that you are getting would have been corrected for using SDGPS (If indeed these are a systemic error, rather than intentional jamming or S.A. implementation. Since the satellite incorporates the differential signal correction in the position report signal, your receiver processes it simultaneously, spewing forth with position reports that are repeatably accurate to what an estimated 3ft. for us, using the old "Drive me to the waypoint that is midpoint between the marina entrance buoys" test. Perhaps 50 times this summer, we have used SDGPS to steer us thru the entrance..not because it was necessary, but for exercise and testing of various routes that I have setup in the islands, to take us home in fog and at night (used this past week for real)). Each and ever single time, it drove us to the slightly left of channel position that I originally set the waypoint at. Since the channell is only 25 ft. wide there, it is easy to detect about how far off we are each time. This is with the Autopilot linked to SDGPS and steering by the way.
You are right about unlimbering the LORAN-C Rcvr though. There are too many variables as far as GPS availability for the civilian mode is concerned, and it is certainly possible that it could be unavailable, jammed or spoofed at some times.
Cheers,
Larry DeMers
s/v DeLaMer
Cape Dory 30 Lake Superior
demers@sgi.com
Last year we bought the Raynav 300 SDGPS Rcvr., and this tool is by far the neatest nav tool I have ever worked with. These wild position reports that you are getting would have been corrected for using SDGPS (If indeed these are a systemic error, rather than intentional jamming or S.A. implementation. Since the satellite incorporates the differential signal correction in the position report signal, your receiver processes it simultaneously, spewing forth with position reports that are repeatably accurate to what an estimated 3ft. for us, using the old "Drive me to the waypoint that is midpoint between the marina entrance buoys" test. Perhaps 50 times this summer, we have used SDGPS to steer us thru the entrance..not because it was necessary, but for exercise and testing of various routes that I have setup in the islands, to take us home in fog and at night (used this past week for real)). Each and ever single time, it drove us to the slightly left of channel position that I originally set the waypoint at. Since the channell is only 25 ft. wide there, it is easy to detect about how far off we are each time. This is with the Autopilot linked to SDGPS and steering by the way.
You are right about unlimbering the LORAN-C Rcvr though. There are too many variables as far as GPS availability for the civilian mode is concerned, and it is certainly possible that it could be unavailable, jammed or spoofed at some times.
Cheers,
Larry DeMers
s/v DeLaMer
Cape Dory 30 Lake Superior
Ken Coit wrote: For the past four hours, I have been using my Magellan GPS 3000 XL and Fugawi 3.0 software on the laptop to monitor the location of a chair which is supporting my GPS antenna on the patio. It is moving all over the neighborhood! For a few hours it was quite happy in the back yard, shuffling back and forth as one would expect, but lately it has moved a couple blocks south and back, then a couple blocks west and back. Yesterday it took a 150 meter trip to the southeast, but returned fairly promptly. The GPS is currently locked on five satellites.
The total area of its excursions today is about 150 meters east-west and 250 meters north-south, a good deal outside the published accuracy of the system. I would not like to be using this to find my way back to the marina at night and would be extremely wary if I were driving an autopilot from the GPS.
Is anyone else monitoring the GPS system? Does anyone have a DGPS they can fire up to monitor the system? Any clues would be appreciated.
Keep on sailing,
Ken Coit, ND7N
CD/14 #538
CD/36 #84 Parfait
Hailing Port: Raleigh, NC
Sailing from: Beaufort, NC
Currently rocking in my chair somewhere in Raleigh, NC
demers@sgi.com
-
Ken Coit
More Experiments to Come
Larry,
There seems to be a very consistent inaccuracy introduced every few hours. The whole excursion occurs in a matter of minutes. I plan to run some additional experiments from different locations away from tall pines and fairly regular commercial air traffic, to see what effect that may have. I will probably borrow a Garmin and check it out as well.
I am certainly not ready to bet the boat on what I am seeing so far, but it would probably work well enough for finding the outer buoy coming in from the Atlantic. I'm not threading any channels with what I have, so a further investment may be necessary.
Keep on sailing,
Ken
parfaitNOSPAM@nc.rr.com
There seems to be a very consistent inaccuracy introduced every few hours. The whole excursion occurs in a matter of minutes. I plan to run some additional experiments from different locations away from tall pines and fairly regular commercial air traffic, to see what effect that may have. I will probably borrow a Garmin and check it out as well.
I am certainly not ready to bet the boat on what I am seeing so far, but it would probably work well enough for finding the outer buoy coming in from the Atlantic. I'm not threading any channels with what I have, so a further investment may be necessary.
Keep on sailing,
Ken
Larry DeMers wrote: Ken,
Last year we bought the Raynav 300 SDGPS Rcvr., and this tool is by far the neatest nav tool I have ever worked with. These wild position reports that you are getting would have been corrected for using SDGPS (If indeed these are a systemic error, rather than intentional jamming or S.A. implementation. Since the satellite incorporates the differential signal correction in the position report signal, your receiver processes it simultaneously, spewing forth with position reports that are repeatably accurate to what an estimated 3ft. for us, using the old "Drive me to the waypoint that is midpoint between the marina entrance buoys" test. Perhaps 50 times this summer, we have used SDGPS to steer us thru the entrance..not because it was necessary, but for exercise and testing of various routes that I have setup in the islands, to take us home in fog and at night (used this past week for real)). Each and ever single time, it drove us to the slightly left of channel position that I originally set the waypoint at. Since the channell is only 25 ft. wide there, it is easy to detect about how far off we are each time. This is with the Autopilot linked to SDGPS and steering by the way.
You are right about unlimbering the LORAN-C Rcvr though. There are too many variables as far as GPS availability for the civilian mode is concerned, and it is certainly possible that it could be unavailable, jammed or spoofed at some times.
Cheers,
Larry DeMers
s/v DeLaMer
Cape Dory 30 Lake Superior
Ken Coit wrote: For the past four hours, I have been using my Magellan GPS 3000 XL and Fugawi 3.0 software on the laptop to monitor the location of a chair which is supporting my GPS antenna on the patio. It is moving all over the neighborhood! For a few hours it was quite happy in the back yard, shuffling back and forth as one would expect, but lately it has moved a couple blocks south and back, then a couple blocks west and back. Yesterday it took a 150 meter trip to the southeast, but returned fairly promptly. The GPS is currently locked on five satellites.
The total area of its excursions today is about 150 meters east-west and 250 meters north-south, a good deal outside the published accuracy of the system. I would not like to be using this to find my way back to the marina at night and would be extremely wary if I were driving an autopilot from the GPS.
Is anyone else monitoring the GPS system? Does anyone have a DGPS they can fire up to monitor the system? Any clues would be appreciated.
Keep on sailing,
Ken Coit, ND7N
CD/14 #538
CD/36 #84 Parfait
Hailing Port: Raleigh, NC
Sailing from: Beaufort, NC
Currently rocking in my chair somewhere in Raleigh, NC
parfaitNOSPAM@nc.rr.com
-
JimL
Re: GPS Accuracy--Garmin 162 & 176C
I'm using the 162 chartplotting on Odyssey III, and my sons use the 176C color mapping in their cars. Both types seem about the same, with a slight nod to the 176C for showing you "on the highway" so to speak. My 162 will occasionally show me smoothly motoring across the roof of the Harbor Patrol building, when I'm actually about 40 feet off the rocks. That means about 80-100 foot error. Usually, it shows me running on the rocks (about 40 foot error). Interestingly, the error seems more North-South, then East-West.
I believe some of the problem is the charting software, because I can motor out to the Red Nun, just outside the harbor, and (scaling in to maximum detail) watch myself motor around the bouy with about 5 foot accuracy!
Luckily for us Southern California sailors, we have no rocks, no shoals, no high wind, no high seas, no fog, and plenty of beautiful mermaids on jet skis to lead us back to the harbor! (At the end of the day, they leave a rainbow colored wake of 2-stroke oil, converging on the entrance).
Additionally, to get to Hawaii, we follow the white lane markings in the sky. They form a continuous overhead arrow, pointing to the islands, generated by the commercial jets! What a life!!
Regards, JimL
leinfam@earthlink.net
I believe some of the problem is the charting software, because I can motor out to the Red Nun, just outside the harbor, and (scaling in to maximum detail) watch myself motor around the bouy with about 5 foot accuracy!
Luckily for us Southern California sailors, we have no rocks, no shoals, no high wind, no high seas, no fog, and plenty of beautiful mermaids on jet skis to lead us back to the harbor! (At the end of the day, they leave a rainbow colored wake of 2-stroke oil, converging on the entrance).
Additionally, to get to Hawaii, we follow the white lane markings in the sky. They form a continuous overhead arrow, pointing to the islands, generated by the commercial jets! What a life!!
Regards, JimL
leinfam@earthlink.net