[time-nuts] Precise positions for GPSDOs
A GPSDO typically makes the assumption that the position of its antenna is fixed and well-known. That removes position uncertainty from the navigation equations, and allows all the information from the satellite measurements to be used to improve the time estimate. Errors in this position create errors in timing, with a magnitude scaled by the speed of light (one ns per foot, three ns per meter). Most GPSDOs do some sort of position averaging when they are first turned on, to come up with a good-enough estimate of antenna position. For a true time-nut, that might not be good enough. GPS surveying equipment can easily determine the position of your antenna to within a few centimeters (~20 ps). Unfortunately, such equipment is expensive and difficult to borrow. A high-end GPSDO designed today should have the ability to record phase data into RINEX files, which could be sent to a service like OPUS to find the antenna position. http://www.ngs.noaa.gov/opus/ But few do, so far. The next best idea is to locate your antenna on Google Maps. Type in the self-surveyed position to the Google search box, either as decimal degrees or as DMS, formatted like this but without the quote marks: 37.384542, -122.005526 37 23 4.35, -122 0 19.89 Click on the map and zoom in. Click on the Map box in the upper right and uncheck the 45 degree view icon. Then right-click on the spot on the picture where your antenna is actually located, and select What's here? from the pop-up menu. A green arrow marker will appear, pointing to your antenna. Left-click on the arrow, and read your latitude and longitude in both formats. Enter one of them into your GPSDO, replacing the self-survey, and enjoy increased accuracy. A true time-nut will take one more step to improve accuracy. (Sorry, but the rest of this is specific to North America. Similar details apply to other parts of the world, but I only know the recipe for the place I live.) Google Maps photos are registered (quite accurately) to the North American Datum NAD83. Unfortunately, your GPSDO operates in a different datum known variously as WGS84, ITRF, or IGS (these are all essentially the same). The difference between these two datums can be a couple of meters, easily visible on the map photos and worth 5 ns or more of time error. Fortunately, you can convert NAD83 to ITRF2008 at this website: http://www.geod.nrcan.gc.ca/apps/tmobs/tmobs_e.php For ITRF epoch, just enter today's date. For ellipsoidal height, use the value from your self-survey if you don't have a better one. You might be able to get a better one from Google Earth, or by finding a nearby benchmark from this site (US only) and extrapolating to your antenna location. http://www.ngs.noaa.gov/cgi-bin/ds_radius.prl Note that the WGS84 ellipsoid is tens of meters higher than sea level through most of North America, so if you live near the ocean, your ellipsoidal height will probably be negative. Hope someone find this useful. Cheers! --Stu ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Precise positions for GPSDOs
In message CAPXiX5ricf=Ea0B=c2yr8ix+70srtfj9jeutkguqehh5izb...@mail.gmail.com, Stewart Cobb writes: The next best idea is to locate your antenna on Google Maps. [...] If your GPSDO's self-survey isn't better than the registration of Google Maps, you have different problems. In particular, be aware that the GPSDO does not need to know the antennas _actual_ position, it needs the _apperant_ position, which takes the reflection environment into account. (GW: fresnel zone) This is a much better strategy: http://phk.freebsd.dk/raga/sneak/ -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 p...@freebsd.org | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Precise positions for GPSDOs
On Thu, May 2, 2013 at 5:29 PM, Stewart Cobb stewart.c...@gmail.com wrote: GPS surveying equipment can easily determine the position of your antenna to within a few centimeters (~20 ps). Unfortunately, such equipment is expensive and difficult to borrow. A high-end GPSDO designed today should have the ability to record phase data into RINEX files, which could be sent to a service like OPUS to find the antenna position. http://www.ngs.noaa.gov/opus/ But few do, so far. There are relatively cheap single frequency GPS receivers that output raw (code and carrier phase) measurements. If you are near a base station (e.g., [1]) that provides similar measurements, you can use RTKLIB to post process both measurements and obtain a position within a few cm. A sample plot of the position of the patch antenna outside my window is attached. The receiver is a u-blox LEA-6T, the RINEX of the base station is from an IGS station 7.2 km away. [1] http://igscb.jpl.nasa.gov/network/netindex.html attachment: rtkplot.png___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Precise positions for GPSDOs
Google maps is NOT that good, it can be off by a lot, tens of meters. I had to have my property line surveyed some years ago to get a city building permit. So now I have two brass markers at know position. The survey crew used traditional transits from a brass benchmark. Google Earth thinks these brass markers are a few meters from here the survey crew said. (Yes I know about WGS84, we are all working in that system) I think the problem is that the lland is not flat here. If I lived in Kanas the Google system might work. But I don't think Google warps the images to account for hills and even slopes. I don't know the source of Google's error. The 1 Sigma on the self survey is about .5 meters more or less. I think the best why to measure is to let the self survey run for a full 24 hours so you get two full orbital periods of each satellite. And also to make sure you have 360 degree view of the sky.I think a view in only one direction might be biased. But yu can check Google. Find a few brass government benchmarks near your house and have Google locate them and if you got a match go with Google On Thu, May 2, 2013 at 2:29 AM, Stewart Cobb stewart.c...@gmail.com wrote: A GPSDO typically makes the assumption that the position of its antenna is fixed and well-known. That removes position uncertainty from the navigation equations, and allows all the information from the satellite measurements to be used to improve the time estimate. Errors in this position create errors in timing, with a magnitude scaled by the speed of light (one ns per foot, three ns per meter). Most GPSDOs do some sort of position averaging when they are first turned on, to come up with a good-enough estimate of antenna position. For a true time-nut, that might not be good enough. GPS surveying equipment can easily determine the position of your antenna to within a few centimeters (~20 ps). Unfortunately, such equipment is expensive and difficult to borrow. A high-end GPSDO designed today should have the ability to record phase data into RINEX files, which could be sent to a service like OPUS to find the antenna position. http://www.ngs.noaa.gov/opus/ But few do, so far. The next best idea is to locate your antenna on Google Maps. Type in the self-surveyed position to the Google search box, either as decimal degrees or as DMS, formatted like this but without the quote marks: 37.384542, -122.005526 37 23 4.35, -122 0 19.89 Click on the map and zoom in. Click on the Map box in the upper right and uncheck the 45 degree view icon. Then right-click on the spot on the picture where your antenna is actually located, and select What's here? from the pop-up menu. A green arrow marker will appear, pointing to your antenna. Left-click on the arrow, and read your latitude and longitude in both formats. Enter one of them into your GPSDO, replacing the self-survey, and enjoy increased accuracy. A true time-nut will take one more step to improve accuracy. (Sorry, but the rest of this is specific to North America. Similar details apply to other parts of the world, but I only know the recipe for the place I live.) Google Maps photos are registered (quite accurately) to the North American Datum NAD83. Unfortunately, your GPSDO operates in a different datum known variously as WGS84, ITRF, or IGS (these are all essentially the same). The difference between these two datums can be a couple of meters, easily visible on the map photos and worth 5 ns or more of time error. Fortunately, you can convert NAD83 to ITRF2008 at this website: http://www.geod.nrcan.gc.ca/apps/tmobs/tmobs_e.php For ITRF epoch, just enter today's date. For ellipsoidal height, use the value from your self-survey if you don't have a better one. You might be able to get a better one from Google Earth, or by finding a nearby benchmark from this site (US only) and extrapolating to your antenna location. http://www.ngs.noaa.gov/cgi-bin/ds_radius.prl Note that the WGS84 ellipsoid is tens of meters higher than sea level through most of North America, so if you live near the ocean, your ellipsoidal height will probably be negative. Hope someone find this useful. Cheers! --Stu ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. -- Chris Albertson Redondo Beach, California ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Precise positions for GPSDOs
I have a worse than optimal antenna location for my t-bolt and that just choked on being fed the google earth location which is 7.5 meters away. Le 2 mai 2013 à 14:22, Poul-Henning Kamp a écrit : In message CAPXiX5ricf=Ea0B=c2yr8ix+70srtfj9jeutkguqehh5izb...@mail.gmail.com, Stewart Cobb writes: The next best idea is to locate your antenna on Google Maps. [...] If your GPSDO's self-survey isn't better than the registration of Google Maps, you have different problems. In particular, be aware that the GPSDO does not need to know the antennas _actual_ position, it needs the _apperant_ position, which takes the reflection environment into account. (GW: fresnel zone) This is a much better strategy: http://phk.freebsd.dk/raga/sneak/ -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 p...@freebsd.org | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Precise positions for GPSDOs
Actually, wouldn't you need a satellite visible mark to use google earth? Not every marker can be seen on google earth. Then often these markers are in places you can't use safely, such as in the middle of a road. Note that google earth does orthorectification on the imagery. If you knew where the imagery had the least correction, that might be a place where the position data is accurate. If a tall structure looks tilted, then you know the image has had a lot of post processing. -Original Message- From: Chris Albertson albertson.ch...@gmail.com Sender: time-nuts-boun...@febo.com Date: Thu, 2 May 2013 08:19:17 To: Discussion of precise time and frequency measurementtime-nuts@febo.com Reply-To: Discussion of precise time and frequency measurement time-nuts@febo.com Subject: Re: [time-nuts] Precise positions for GPSDOs Google maps is NOT that good, it can be off by a lot, tens of meters. I had to have my property line surveyed some years ago to get a city building permit. So now I have two brass markers at know position. The survey crew used traditional transits from a brass benchmark. Google Earth thinks these brass markers are a few meters from here the survey crew said. (Yes I know about WGS84, we are all working in that system) I think the problem is that the lland is not flat here. If I lived in Kanas the Google system might work. But I don't think Google warps the images to account for hills and even slopes. I don't know the source of Google's error. The 1 Sigma on the self survey is about .5 meters more or less. I think the best why to measure is to let the self survey run for a full 24 hours so you get two full orbital periods of each satellite. And also to make sure you have 360 degree view of the sky.I think a view in only one direction might be biased. But yu can check Google. Find a few brass government benchmarks near your house and have Google locate them and if you got a match go with Google On Thu, May 2, 2013 at 2:29 AM, Stewart Cobb stewart.c...@gmail.com wrote: A GPSDO typically makes the assumption that the position of its antenna is fixed and well-known. That removes position uncertainty from the navigation equations, and allows all the information from the satellite measurements to be used to improve the time estimate. Errors in this position create errors in timing, with a magnitude scaled by the speed of light (one ns per foot, three ns per meter). Most GPSDOs do some sort of position averaging when they are first turned on, to come up with a good-enough estimate of antenna position. For a true time-nut, that might not be good enough. GPS surveying equipment can easily determine the position of your antenna to within a few centimeters (~20 ps). Unfortunately, such equipment is expensive and difficult to borrow. A high-end GPSDO designed today should have the ability to record phase data into RINEX files, which could be sent to a service like OPUS to find the antenna position. http://www.ngs.noaa.gov/opus/ But few do, so far. The next best idea is to locate your antenna on Google Maps. Type in the self-surveyed position to the Google search box, either as decimal degrees or as DMS, formatted like this but without the quote marks: 37.384542, -122.005526 37 23 4.35, -122 0 19.89 Click on the map and zoom in. Click on the Map box in the upper right and uncheck the 45 degree view icon. Then right-click on the spot on the picture where your antenna is actually located, and select What's here? from the pop-up menu. A green arrow marker will appear, pointing to your antenna. Left-click on the arrow, and read your latitude and longitude in both formats. Enter one of them into your GPSDO, replacing the self-survey, and enjoy increased accuracy. A true time-nut will take one more step to improve accuracy. (Sorry, but the rest of this is specific to North America. Similar details apply to other parts of the world, but I only know the recipe for the place I live.) Google Maps photos are registered (quite accurately) to the North American Datum NAD83. Unfortunately, your GPSDO operates in a different datum known variously as WGS84, ITRF, or IGS (these are all essentially the same). The difference between these two datums can be a couple of meters, easily visible on the map photos and worth 5 ns or more of time error. Fortunately, you can convert NAD83 to ITRF2008 at this website: http://www.geod.nrcan.gc.ca/apps/tmobs/tmobs_e.php For ITRF epoch, just enter today's date. For ellipsoidal height, use the value from your self-survey if you don't have a better one. You might be able to get a better one from Google Earth, or by finding a nearby benchmark from this site (US only) and extrapolating to your antenna location. http://www.ngs.noaa.gov/cgi-bin/ds_radius.prl Note that the WGS84 ellipsoid is tens of meters higher than sea level through most of North America, so if you live near the ocean
Re: [time-nuts] Precise positions for GPSDOs
PHK, the big pdf link in your sneak page is broken (gives 404). Can you fix that for us? P.S., while you are there you could change goory' to gory. On 5/2/2013 5:22 AM, Poul-Henning Kamp wrote: In message CAPXiX5ricf=Ea0B=c2yr8ix+70srtfj9jeutkguqehh5izb...@mail.gmail.com, Stewart Cobb writes: The next best idea is to locate your antenna on Google Maps. [...] If your GPSDO's self-survey isn't better than the registration of Google Maps, you have different problems. In particular, be aware that the GPSDO does not need to know the antennas _actual_ position, it needs the _apperant_ position, which takes the reflection environment into account. (GW: fresnel zone) This is a much better strategy: http://phk.freebsd.dk/raga/sneak/ ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Precise positions for GPSDOs
I fully agree with Chris, do not trust Google Earth for any serious technical use, I found errors in 100-200 m range. You only need to check where two images are stitched. Google Earth images are not produced by Google, they get them from other companies or government bodies involved in making geographical information, I can't speak about it in a whole but I actually know cases in what Google tried to get the info for free. The metric quality (or QUALITY) is not controlled by Google as far as I know. Think of Google Earth as a means of providing geographical information for the layman, for finding places, advertising and so but you don't know how accurate it is, even the date of the images can be erroneous, you can verify this yourself. I had professionally advised many customers to not rely on this info for any serious use, giving them actual examples. It is a very good and amazing product but its goal is not to make any precise measurement, and the GPS antenna position determination is in fact a surveying task. Regards, Ignacio EB4APL On 02/05/2013 17:19, Chris Albertson wrote: Google maps is NOT that good, it can be off by a lot, tens of meters. I had to have my property line surveyed some years ago to get a city building permit. So now I have two brass markers at know position. The survey crew used traditional transits from a brass benchmark. Google Earth thinks these brass markers are a few meters from here the survey crew said. (Yes I know about WGS84, we are all working in that system) I think the problem is that the lland is not flat here. If I lived in Kanas the Google system might work. But I don't think Google warps the images to account for hills and even slopes. I don't know the source of Google's error. The 1 Sigma on the self survey is about .5 meters more or less. I think the best why to measure is to let the self survey run for a full 24 hours so you get two full orbital periods of each satellite. And also to make sure you have 360 degree view of the sky.I think a view in only one direction might be biased. But yu can check Google. Find a few brass government benchmarks near your house and have Google locate them and if you got a match go with Google On Thu, May 2, 2013 at 2:29 AM, Stewart Cobb stewart.c...@gmail.com wrote: A GPSDO typically makes the assumption that the position of its antenna is fixed and well-known. That removes position uncertainty from the navigation equations, and allows all the information from the satellite measurements to be used to improve the time estimate. Errors in this position create errors in timing, with a magnitude scaled by the speed of light (one ns per foot, three ns per meter). Most GPSDOs do some sort of position averaging when they are first turned on, to come up with a good-enough estimate of antenna position. For a true time-nut, that might not be good enough. GPS surveying equipment can easily determine the position of your antenna to within a few centimeters (~20 ps). Unfortunately, such equipment is expensive and difficult to borrow. A high-end GPSDO designed today should have the ability to record phase data into RINEX files, which could be sent to a service like OPUS to find the antenna position. http://www.ngs.noaa.gov/opus/ But few do, so far. The next best idea is to locate your antenna on Google Maps. Type in the self-surveyed position to the Google search box, either as decimal degrees or as DMS, formatted like this but without the quote marks: 37.384542, -122.005526 37 23 4.35, -122 0 19.89 Click on the map and zoom in. Click on the Map box in the upper right and uncheck the 45 degree view icon. Then right-click on the spot on the picture where your antenna is actually located, and select What's here? from the pop-up menu. A green arrow marker will appear, pointing to your antenna. Left-click on the arrow, and read your latitude and longitude in both formats. Enter one of them into your GPSDO, replacing the self-survey, and enjoy increased accuracy. A true time-nut will take one more step to improve accuracy. (Sorry, but the rest of this is specific to North America. Similar details apply to other parts of the world, but I only know the recipe for the place I live.) Google Maps photos are registered (quite accurately) to the North American Datum NAD83. Unfortunately, your GPSDO operates in a different datum known variously as WGS84, ITRF, or IGS (these are all essentially the same). The difference between these two datums can be a couple of meters, easily visible on the map photos and worth 5 ns or more of time error. Fortunately, you can convert NAD83 to ITRF2008 at this website: http://www.geod.nrcan.gc.ca/apps/tmobs/tmobs_e.php For ITRF epoch, just enter today's date. For ellipsoidal height, use the value from your self-survey if you don't have a better one. You might be able to get a better one from Google Earth, or by finding a nearby benchmark from this site (US only) and
