Re: [Xastir] GPS question, off topic.
On 14 Feb 2008, at 21:45, Gerry Creager wrote: As a rule of thumb, one gets an improvement of ~one order of magnitude with augmentation. Not quite so good with WAAS as with truly local DGPS, and nowhere nearly as good as RTK. We were able to get, in the old days, with SA on, <10m accuracy with DGPS, and 15cm accuracies with RTK, assuming a good correlator and stable oscillators. This was when we could document >90m induced error with SA. [bigsnip] Thanks for the replies folks, much appreciated. John -- John Ronan <[EMAIL PROTECTED]>, +353-51-302938 Telecommunications Software & Systems Group, http://www.tssg.org ___ Xastir mailing list Xastir@xastir.org http://lists.xastir.org/cgi-bin/mailman/listinfo/xastir
Re: [Xastir] GPS question, off topic.
As a rule of thumb, one gets an improvement of ~one order of magnitude with augmentation. Not quite so good with WAAS as with truly local DGPS, and nowhere nearly as good as RTK. We were able to get, in the old days, with SA on, <10m accuracy with DGPS, and 15cm accuracies with RTK, assuming a good correlator and stable oscillators. This was when we could document >90m induced error with SA. After SA was turned off, we've started seeing ~6m accuracy most of the time without augmentation for a fixed antenna. I've seen somewhere near that (but not verified it rigorously) with a fully exposed patch antenna on a moving vehicle: I can keep it inside the perimeter of a 2 lane road and generally on the right side of the line, with no augmentation. With WAAS, it's always inside the perimeter of the roadway and always on the same side. Assuming I'm driving where I'm supposed to be, but that's another story (If you don't like my driving, say off the sidewalk...). Let's assume, then, that the 6m number with cheap receivers (and they're getting better, too) is valid, oh, say, 90% of the time, with the outliers caused by local distortion of iono and tropo refraction. If one were to then augment with WAAS, and see a 2x instead of 10x improvement, one's accuracy should be in the 3m range, and a 4x improvement would be ~1.5m, or pretty close to 1e-05 degrees. So, we're not too far from being below a meter at this point. Add in some info on tropo, which can be modeled and observed, and doesn't move very fast, and you've reduced your error budget toward the submeter range. If one can then calculate iono, you're almost certain to get below the magic number, and the ionosphere, while constantly changing, is stochastic and predictable. And relatively slow. While most of the iono effect is caused by scinitllation, which is fast, the net effect of action on such a large target is effectively a slow system. And, John, just driving around in a field shouldn't make the locals think too much... Try having the locals see cows in a pasture... gerry Richard Polivka wrote: A tidbit... In decimal degrees, N43.0 W88.0 to N43.1 W88.0 is equal to about 1.1 meters. If they are looking for sub-meter accuracy, they will need at least six decimals out, unless they are saying that their accuracy is to five decimals +/- 0.05. That's specsmanship in my book. Moving accuracy is not going to be too good as to get some honest accuracy, you can't be moving. The ionosphere is always moving and models will not really be able to figure it out accurately. I wonder if their system is some form of a two station RTK system using a stationary system to supply correction to the rover. If it sounds too good to be true 73 form 807, Richard, N6NKO John Ronan <[EMAIL PROTECTED]> wrote: > Evening, What would the GPS be used for? APRS Tracking? Farming? Farming actually. The Phoenix GPS claims sub-meter accuracy. Barring marketing lies, this is not possible with only WAAS. WAAS would likely get you to 3 meter accuracy at best (Gerry N5JXS certainly knows more about this). Ok thats what I was reckoning. The eDif feature mentioned for th Phoenix appears to attempt to model ionosphere delays by analyzing multiple satellite signals over time. Normally ionospheric delay (signal bending) correction is done using a 2nd GPS frequency, on which the actual data is encrypted, but the clock information can be extracted. This 2nd frequency is what is normally referred to as "Military grade" GPS. The eDif claims to be able to get the same type of correction data out of a single frequency receiver. It also sounds like a costly add-on. ok The other differential positioning technologies mentioned are likely DGPS, where you have a separate GPS receiver at a precisely known position which radios out the offsets (within some "local" radius) between the GPS-derived position and its precisely known position. DGPS receivers recieve and incorporate these offsets into their calculations. ok The question back to you, I guess, would be: do your friend need sub-meter accuracy and 10Hz position updates? (Most NMEA-out GPSs output positions only at 1Hz or less, depending on the NMEA sentences enabled.) Ok. Short answer is I'm not really sure I'll try and address Curt and Gerry's questions as well. Firstly.. WAAS signals aren't a problem, the only mountains are North, and going south, next stop would be Spain. Most of the land is pretty flat. The system is being marketed to do several things (apologies, I'm not at my desk, so the piece of paper I wrote the URL on is not available to me.. scratch that.. found it. http://www.farmworks.co.uk/gps.php click on GPS Swath Guidance ), one of which is to allow him to spread fertiliser with as little overlap as possible. This is because over the course of a season, the cost of the overlap is quite h
Re: [Xastir] GPS question, off topic.
A tidbit... In decimal degrees, N43.0 W88.0 to N43.1 W88.0 is equal to about 1.1 meters. If they are looking for sub-meter accuracy, they will need at least six decimals out, unless they are saying that their accuracy is to five decimals +/- 0.05. That's specsmanship in my book. Moving accuracy is not going to be too good as to get some honest accuracy, you can't be moving. The ionosphere is always moving and models will not really be able to figure it out accurately. I wonder if their system is some form of a two station RTK system using a stationary system to supply correction to the rover. If it sounds too good to be true 73 form 807, Richard, N6NKO John Ronan <[EMAIL PROTECTED]> wrote: > Evening, > > What would the GPS be used for? APRS Tracking? Farming? > Farming actually. > The Phoenix GPS claims sub-meter accuracy. Barring marketing lies, > this is not possible with only WAAS. WAAS would likely get you to 3 > meter accuracy at best (Gerry N5JXS certainly knows more about this). > Ok thats what I was reckoning. > The eDif feature mentioned for th Phoenix appears to attempt to > model ionosphere delays by analyzing multiple satellite signals > over time. Normally ionospheric delay (signal bending) correction > is done using a 2nd GPS frequency, on which the actual data is > encrypted, but the clock information can be extracted. This 2nd > frequency is what is normally referred to as "Military grade" GPS. > The eDif claims to be able to get the same type of correction data > out of a single frequency receiver. It also sounds like a costly > add-on. > ok > The other differential positioning technologies mentioned are > likely DGPS, where you have a separate GPS receiver at a precisely > known position which radios out the offsets (within some "local" > radius) between the GPS-derived position and its precisely known > position. DGPS receivers recieve and incorporate these offsets into > their calculations. > ok > The question back to you, I guess, would be: do your friend need > sub-meter accuracy and 10Hz position updates? (Most NMEA-out GPSs > output positions only at 1Hz or less, depending on the NMEA > sentences enabled.) > Ok. Short answer is I'm not really sure I'll try and address Curt and Gerry's questions as well. Firstly.. WAAS signals aren't a problem, the only mountains are North, and going south, next stop would be Spain. Most of the land is pretty flat. The system is being marketed to do several things (apologies, I'm not at my desk, so the piece of paper I wrote the URL on is not available to me.. scratch that.. found it. http://www.farmworks.co.uk/gps.php click on GPS Swath Guidance ), one of which is to allow him to spread fertiliser with as little overlap as possible. This is because over the course of a season, the cost of the overlap is quite high apparently (in monetary terms). It must be appreciable as he is looking for a method to reduce that cost, but is sceptical of the solutions presented. Purely as an exercise, I was going to try the GPS-18 that I have already, put GPSman running (or something) and drive one or two of his fields.. just to see what he thinks...all the locals looking at their APRS displays will think I've gone barmy... Regards John -- John Ronan , +353-51-302938 Telecommunications Software & Systems Group, http://www.tssg.org ___ Xastir mailing list Xastir@xastir.org http://lists.xastir.org/cgi-bin/mailman/listinfo/xastir ___ Xastir mailing list Xastir@xastir.org http://lists.xastir.org/cgi-bin/mailman/listinfo/xastir
Re: [Xastir] GPS question, off topic.
Evening, What would the GPS be used for? APRS Tracking? Farming? Farming actually. The Phoenix GPS claims sub-meter accuracy. Barring marketing lies, this is not possible with only WAAS. WAAS would likely get you to 3 meter accuracy at best (Gerry N5JXS certainly knows more about this). Ok thats what I was reckoning. The eDif feature mentioned for th Phoenix appears to attempt to model ionosphere delays by analyzing multiple satellite signals over time. Normally ionospheric delay (signal bending) correction is done using a 2nd GPS frequency, on which the actual data is encrypted, but the clock information can be extracted. This 2nd frequency is what is normally referred to as "Military grade" GPS. The eDif claims to be able to get the same type of correction data out of a single frequency receiver. It also sounds like a costly add-on. ok The other differential positioning technologies mentioned are likely DGPS, where you have a separate GPS receiver at a precisely known position which radios out the offsets (within some "local" radius) between the GPS-derived position and its precisely known position. DGPS receivers recieve and incorporate these offsets into their calculations. ok The question back to you, I guess, would be: do your friend need sub-meter accuracy and 10Hz position updates? (Most NMEA-out GPSs output positions only at 1Hz or less, depending on the NMEA sentences enabled.) Ok. Short answer is I'm not really sure I'll try and address Curt and Gerry's questions as well. Firstly.. WAAS signals aren't a problem, the only mountains are North, and going south, next stop would be Spain. Most of the land is pretty flat. The system is being marketed to do several things (apologies, I'm not at my desk, so the piece of paper I wrote the URL on is not available to me.. scratch that.. found it. http://www.farmworks.co.uk/gps.php click on GPS Swath Guidance ), one of which is to allow him to spread fertiliser with as little overlap as possible. This is because over the course of a season, the cost of the overlap is quite high apparently (in monetary terms). It must be appreciable as he is looking for a method to reduce that cost, but is sceptical of the solutions presented. Purely as an exercise, I was going to try the GPS-18 that I have already, put GPSman running (or something) and drive one or two of his fields.. just to see what he thinks...all the locals looking at their APRS displays will think I've gone barmy... Regards John -- John Ronan <[EMAIL PROTECTED]>, +353-51-302938 Telecommunications Software & Systems Group, http://www.tssg.org ___ Xastir mailing list Xastir@xastir.org http://lists.xastir.org/cgi-bin/mailman/listinfo/xastir
Re: [Xastir] GPS question, off topic.
On Thu, 14 Feb 2008, Gerry Creager wrote: > A lot of precision agriculture applications do need meter or better > accuracies, especially if the GPS is driving the tractor. I'm not sure > the Phoenix is the right answer for this, though, unless I got more > information and had the opportunity to play with a pair of them for > evaluation. Also: Is the area flat, with a good view to the south of the geostationary satellites carrying the WAAS signals? Last I recall (my info might be dated), the WAAS signals were sent from satellites over the equator, not from the GPS satellites themselves. If you have any blockage to the south, say bye-bye to your WAAS corrections. -- Curt, WE7U: XASTIR: "Lotto: A tax on people who are bad at math." -- unknown "Windows: Microsoft's tax on computer illiterates." -- WE7U The world DOES revolve around me: I picked the coordinate system! ___ Xastir mailing list Xastir@xastir.org http://lists.xastir.org/cgi-bin/mailman/listinfo/xastir
Re: [Xastir] GPS question, off topic.
Couple of minor points, although Lance's pretty well on-track. Lance Cotton wrote: John Ronan wrote: Assuming that WAAS signals are quite strong, what would the advantage be between a Phoenix 200 Smart Antenna http://www.ravenprecision.com/ca/Products/description.jsp?partNum=117-0171-071&Category=1&Type=1 over a garmin GPS-18. It's for an agricultural guidance solution they are the two GPS options available. I suggested to him to borrow my GPS-18 (which I use with my OpenTracker 2) to test it before deciding on which to use, as I know WAAS signals are quite strong here. What would the GPS be used for? APRS Tracking? Farming? The Phoenix GPS claims sub-meter accuracy. Barring marketing lies, this is not possible with only WAAS. WAAS would likely get you to 3 meter accuracy at best (Gerry N5JXS certainly knows more about this). The Phoenix also claims to use a helical antenna. There's little benefit of a quadrifilar helix over a dielectric patch antenna for low-angle satellites, and low angle satellites tend to detract from accurate positioning because of the interplay of more atmospheric traversal than higher-elevation constellation selection. I think they're letting marketing invade technical space here. The eDif feature mentioned for th Phoenix appears to attempt to model ionosphere delays by analyzing multiple satellite signals over time. Normally ionospheric delay (signal bending) correction is done using a 2nd GPS frequency, on which the actual data is encrypted, but the clock information can be extracted. This 2nd frequency is what is normally referred to as "Military grade" GPS. The eDif claims to be able to get the same type of correction data out of a single frequency receiver. It also sounds like a costly add-on. Aside from the fact that I've got and use dual-frequency receivers and have no current connections to the military other than having being married to a retired Army Nurse, dual-frequency is usually expensive to obtain. Relatively few are purchased compared to Garmin and other OEM and consumer units, so there's no economy of scale. Although I've theorized about how to determine iono and tropo delays over the years using single-frequency techniques, it's virtually always required two or more receivers to make the theory work, over known baselines. The other differential positioning technologies mentioned are likely DGPS, where you have a separate GPS receiver at a precisely known position which radios out the offsets (within some "local" radius) between the GPS-derived position and its precisely known position. DGPS receivers recieve and incorporate these offsets into their calculations. Some experiments concerning "local radius" conducted here several years ago suggest that said radius was a tool designed by Trimble to sell more DGPS base stations... or more precisely, that "local radius" was pretty general, and on the order of hundreds to thousands of kilometers. The question back to you, I guess, would be: do your friend need sub-meter accuracy and 10Hz position updates? (Most NMEA-out GPSs output positions only at 1Hz or less, depending on the NMEA sentences enabled.) A lot of precision agriculture applications do need meter or better accuracies, especially if the GPS is driving the tractor. I'm not sure the Phoenix is the right answer for this, though, unless I got more information and had the opportunity to play with a pair of them for evaluation. gerry -- Gerry Creager -- [EMAIL PROTECTED] Texas Mesonet -- AATLT, Texas A&M University Cell: 979.229.5301 Office: 979.458.4020 FAX: 979.862.3983 Office: 1700 Research Parkway Ste 160, TAMU, College Station, TX 77843 ___ Xastir mailing list Xastir@xastir.org http://lists.xastir.org/cgi-bin/mailman/listinfo/xastir
Re: [Xastir] GPS question, off topic.
John Ronan wrote: Assuming that WAAS signals are quite strong, what would the advantage be between a Phoenix 200 Smart Antenna http://www.ravenprecision.com/ca/Products/description.jsp?partNum=117-0171-071&Category=1&Type=1 over a garmin GPS-18. It's for an agricultural guidance solution they are the two GPS options available. I suggested to him to borrow my GPS-18 (which I use with my OpenTracker 2) to test it before deciding on which to use, as I know WAAS signals are quite strong here. What would the GPS be used for? APRS Tracking? Farming? The Phoenix GPS claims sub-meter accuracy. Barring marketing lies, this is not possible with only WAAS. WAAS would likely get you to 3 meter accuracy at best (Gerry N5JXS certainly knows more about this). The eDif feature mentioned for th Phoenix appears to attempt to model ionosphere delays by analyzing multiple satellite signals over time. Normally ionospheric delay (signal bending) correction is done using a 2nd GPS frequency, on which the actual data is encrypted, but the clock information can be extracted. This 2nd frequency is what is normally referred to as "Military grade" GPS. The eDif claims to be able to get the same type of correction data out of a single frequency receiver. It also sounds like a costly add-on. The other differential positioning technologies mentioned are likely DGPS, where you have a separate GPS receiver at a precisely known position which radios out the offsets (within some "local" radius) between the GPS-derived position and its precisely known position. DGPS receivers recieve and incorporate these offsets into their calculations. The question back to you, I guess, would be: do your friend need sub-meter accuracy and 10Hz position updates? (Most NMEA-out GPSs output positions only at 1Hz or less, depending on the NMEA sentences enabled.) -Lance KJ5O -- J. Lance Cotton, KJ5O [EMAIL PROTECTED] http://kj5o.lightningflash.net Three Step Plan: 1. Take over the world. 2. Get a lot of cookies. 3. Eat the cookies. ___ Xastir mailing list Xastir@xastir.org http://lists.xastir.org/cgi-bin/mailman/listinfo/xastir
[Xastir] GPS question, off topic.
Hi, A friend of mine asked me this question today, Assuming that WAAS signals are quite strong, what would the advantage be between a Phoenix 200 Smart Antenna http://www.ravenprecision.com/ca/Products/description.jsp? partNum=117-0171-071&Category=1&Type=1 over a garmin GPS-18. It's for an agricultural guidance solution they are the two GPS options available. I suggested to him to borrow my GPS-18 (which I use with my OpenTracker 2) to test it before deciding on which to use, as I know WAAS signals are quite strong here. Regards John -- John Ronan <[EMAIL PROTECTED]>, +353-51-302938 Telecommunications Software & Systems Group, http://www.tssg.org ___ Xastir mailing list Xastir@xastir.org http://lists.xastir.org/cgi-bin/mailman/listinfo/xastir
