Ron Johnson wrote: The Pres. signed the edict to remove dithering from the signal which makes regular GPS more equivalent to dGPS. SA(selective availability) is still alive and well. (also reported in latest Cruising World). Questions: How accurate is GPS now? From observations, it would appear that altitude on a G48 varies by only 30 feet rather than 300. And location varies by even less. But does anyone have an accurate answer <grin>. Also, what sea level is it referencing? It appears to be 20 to 30 feet above local charts. And finally, is GPS time exactly the same time as GMT time, or is there a variation between the two? Jim
- Status
- Not open for further replies.
J
Jim Ewing
It depends
GPS is supposed to be +/- 10 meters or so. Since they turned off SA I haven't seen an EPE greater than 6'. The reference elevation (and position)depends on the reference datum you selected (should be WGS 84). If you see some differences in position make sure the reference datum (which defines the "shape" of the earth) is the same on the GPS as it is on the chart. GPS time is Coordinated Universal Time which uses Greenwich as the prime meridian but has some other (I can't remember what) difference with GMT. Jim "Prospect"
GPS is supposed to be +/- 10 meters or so. Since they turned off SA I haven't seen an EPE greater than 6'. The reference elevation (and position)depends on the reference datum you selected (should be WGS 84). If you see some differences in position make sure the reference datum (which defines the "shape" of the earth) is the same on the GPS as it is on the chart. GPS time is Coordinated Universal Time which uses Greenwich as the prime meridian but has some other (I can't remember what) difference with GMT. Jim "Prospect"
R
Ron Doescher
GPS S/A
Regarding your questions: 1. With S/A "removed" (what the Gov't gives, the Gov't can takeith away!) the 10 meter (95% confidence) accuracy is probaply the BEST horizontal position accuracy you'll get. Since that equates to about one boatlength, that isn't all bad. However, you can get worse depending on the number and positions of satellites in-view. Most GPS receivers will give an estimated position error (EPA) reading based on these parameters. 2. DGPS will still be functioning along coastal & inland waterways & could give even better horizontal position accuracy. (See the USCG Navcenter web site). 3. Vertical position error also depends on the number of satellites captured & their postions in the constellation. This shouldn't be a problem for sailors. 4. All GPS calculations are based on the "universal" World Geodetic System (WGS-84) datum. It represents the best available compromise geoidal model of the entire earth. (NASA & DOD have a multi-year program starting to re-map the entire earth's surface with a space-based radar system. Once done, I believe those data will be used to upgrade the WGS geoidal model.) Other localized datums can be more accurate, but you'll need to translate (I believe the last North American Datum (NAD) is generally considered equivalent to WGS-84). Many commercially available GPS receivers have prestored datum converters -- just make sure that you know which datum underlies what your GPS is telling you on-screen, and what datum is used for the paper/digital chart on your nav station. Hope that helps --Ron
Regarding your questions: 1. With S/A "removed" (what the Gov't gives, the Gov't can takeith away!) the 10 meter (95% confidence) accuracy is probaply the BEST horizontal position accuracy you'll get. Since that equates to about one boatlength, that isn't all bad. However, you can get worse depending on the number and positions of satellites in-view. Most GPS receivers will give an estimated position error (EPA) reading based on these parameters. 2. DGPS will still be functioning along coastal & inland waterways & could give even better horizontal position accuracy. (See the USCG Navcenter web site). 3. Vertical position error also depends on the number of satellites captured & their postions in the constellation. This shouldn't be a problem for sailors. 4. All GPS calculations are based on the "universal" World Geodetic System (WGS-84) datum. It represents the best available compromise geoidal model of the entire earth. (NASA & DOD have a multi-year program starting to re-map the entire earth's surface with a space-based radar system. Once done, I believe those data will be used to upgrade the WGS geoidal model.) Other localized datums can be more accurate, but you'll need to translate (I believe the last North American Datum (NAD) is generally considered equivalent to WGS-84). Many commercially available GPS receivers have prestored datum converters -- just make sure that you know which datum underlies what your GPS is telling you on-screen, and what datum is used for the paper/digital chart on your nav station. Hope that helps --Ron
R
Ron Doescher
GPS Time
The differences between GMT & GPS time are so minor as to not be noticable. There are periodic GPS "epoch adjustments" made to split those hairs. Again, be careful about the setup time differentials used in your GPS receiver. It won't display the right time (local std, local DST, GMT, etc) unless that differential is properly set.
The differences between GMT & GPS time are so minor as to not be noticable. There are periodic GPS "epoch adjustments" made to split those hairs. Again, be careful about the setup time differentials used in your GPS receiver. It won't display the right time (local std, local DST, GMT, etc) unless that differential is properly set.
J
Justin - O'day Owner's Web
Short Answer -
Your GPS is more accurate than you need it to be. At any range where the accuracy becomes a factor, you ought to be looking around, not at your instruments. (assuming offset, time, datum, etc are set up correctly). Flame suit on. Justin - O'day Owner's Web
Your GPS is more accurate than you need it to be. At any range where the accuracy becomes a factor, you ought to be looking around, not at your instruments. (assuming offset, time, datum, etc are set up correctly). Flame suit on. Justin - O'day Owner's Web
J
Jim Russell
Thanks for All
Your info is what I needed. Actually have been using G45 before the Garmin 48 so had most the stuff set correctly. Might want to look at my datum to make sure it is set to WGS-84, but suspect it is. Also saw a nice discussion in a Boat US sales flier today. And you are right, it is more accurate than I needed to be. At anchorage with a steady wind last year I had a nice pattern of travel. This year the spot doesn't move. So will Garmin and others be building GPS models that have a greater definition than .2 miles <grin>. Okay, I suppose that they are already out there if one choses to buy. Thanks again for the discussion. Jim
Your info is what I needed. Actually have been using G45 before the Garmin 48 so had most the stuff set correctly. Might want to look at my datum to make sure it is set to WGS-84, but suspect it is. Also saw a nice discussion in a Boat US sales flier today. And you are right, it is more accurate than I needed to be. At anchorage with a steady wind last year I had a nice pattern of travel. This year the spot doesn't move. So will Garmin and others be building GPS models that have a greater definition than .2 miles <grin>. Okay, I suppose that they are already out there if one choses to buy. Thanks again for the discussion. Jim
T
Tom Senator
Don't trust your GPS !!! You can get into trouble!
I know that with SA (Selective Availability) turned off it can be VERY accurate. And as of late I notice it can be dead on. BUT...the government can turn SA on or off at will. If you get too used to having it dead on and are not vigilant, when they turn on SA again you might be duped into thinking you are in a channel when you are not !! Case in point --- I went down to New York Harbour for the Tall ships Parade on July 4th. And the GPS was WAY off (I think close to a half mile at some times --) I'm sure they turned on SA because of the President and all the Naval Vessels....but you should very careful, you never know when they are goung to "flick the switch" in your area.
I know that with SA (Selective Availability) turned off it can be VERY accurate. And as of late I notice it can be dead on. BUT...the government can turn SA on or off at will. If you get too used to having it dead on and are not vigilant, when they turn on SA again you might be duped into thinking you are in a channel when you are not !! Case in point --- I went down to New York Harbour for the Tall ships Parade on July 4th. And the GPS was WAY off (I think close to a half mile at some times --) I'm sure they turned on SA because of the President and all the Naval Vessels....but you should very careful, you never know when they are goung to "flick the switch" in your area.
K
Ken Shubert
Check the E.P.E.
Check the Estimated Position Error occasionally and also check to see how many satellites are being used. This also performs a basic system check to see if the antenna is still working properly. Most GPS units have some form of alarm to inform us when these drop to unsafe levels but...... read your own. Understand your GPS and trust it, but keep your eyes open too. Ken S/V Wouff Hong
Check the Estimated Position Error occasionally and also check to see how many satellites are being used. This also performs a basic system check to see if the antenna is still working properly. Most GPS units have some form of alarm to inform us when these drop to unsafe levels but...... read your own. Understand your GPS and trust it, but keep your eyes open too. Ken S/V Wouff Hong
B
Bob England
Strange GPS readings
I sailed across Lake Ontario last Monday, and at one point, far away from any likely interference, my GPS went crazy! This is a Magellan 2000 XL that I have been using for three years with no previous trouble. Monday, it showed me going 12 to 13 knots on a course about 90 degrees off my actual course. I immediately checked the satellite status, and it showed about 8 satellites firmly locked in. I had to power down the unit, and when it reacquired the signals, all was well again. I was glad of this later, when I got back to my yacht club well after dark. The north shore of Lake Ontario around Toronto looks like a horizontal Christmas tree at night with all the lights, including car tail lights and red and green traffic signals that are visible from a surprising distance, and the navigation lights are very hard to pick out. It turned out that the lighted tower at the entrance to my club's harbour was out! So, without my GPS (working) I might have wandered around out there for hours trying to find home port! And, coast guard weather radio was issuing severe thunderstorm warnings and there were lightning strikes all around in the distance. So, don't trust the GPS unless you have to, and always apply the "test of reasonableness" to anything you get from an electronic device.
I sailed across Lake Ontario last Monday, and at one point, far away from any likely interference, my GPS went crazy! This is a Magellan 2000 XL that I have been using for three years with no previous trouble. Monday, it showed me going 12 to 13 knots on a course about 90 degrees off my actual course. I immediately checked the satellite status, and it showed about 8 satellites firmly locked in. I had to power down the unit, and when it reacquired the signals, all was well again. I was glad of this later, when I got back to my yacht club well after dark. The north shore of Lake Ontario around Toronto looks like a horizontal Christmas tree at night with all the lights, including car tail lights and red and green traffic signals that are visible from a surprising distance, and the navigation lights are very hard to pick out. It turned out that the lighted tower at the entrance to my club's harbour was out! So, without my GPS (working) I might have wandered around out there for hours trying to find home port! And, coast guard weather radio was issuing severe thunderstorm warnings and there were lightning strikes all around in the distance. So, don't trust the GPS unless you have to, and always apply the "test of reasonableness" to anything you get from an electronic device.
J
John J
GPS
This may sound stupid, but..... Earlier this week there wast tremendous solar activity that was affecting satellite and micro wave transmissions. You may have been a victim of it.
This may sound stupid, but..... Earlier this week there wast tremendous solar activity that was affecting satellite and micro wave transmissions. You may have been a victim of it.
- Status
- Not open for further replies.