+EraSeek Posted November 8, 2010 Share Posted November 8, 2010 38 microseconds a day: http://www.astronomy.ohio-state.edu/~pogge.../Unit5/gps.html Quote Link to comment
savant9 Posted November 8, 2010 Share Posted November 8, 2010 Great little writeup, thanks for sharing. Quote Link to comment
+embra Posted November 8, 2010 Share Posted November 8, 2010 The more I learn about how GPS works, the more I am amazed. Quote Link to comment
MtnHermit Posted November 8, 2010 Share Posted November 8, 2010 with differential techiques that compare two nearby receivers, precisions of order centimeters or millimeters in relative position are often obtained in under an hour or so So I wonder, is this how the survey units work, have two receivers inside one box? The time scale is a bit long, but the accuracy is compelling. Less time - less accuracy, but how much? Quote Link to comment
+bcblues Posted November 9, 2010 Share Posted November 9, 2010 Better antenna for one thing. I don't think my Trimble has more than one receiver (in fact I am certain of it). Quote Link to comment
+scuba_2 Posted November 10, 2010 Share Posted November 10, 2010 wow I will never shout at mine again! Quote Link to comment
+miamiyogi Posted November 10, 2010 Share Posted November 10, 2010 So I wonder, is this how the survey units work, have two receivers inside one box? The time scale is a bit long, but the accuracy is compelling. Less time - less accuracy, but how much? I used to work in telecommunications and got to play with a survey Trimble (sorry don't remember the model). We would use it to map out network elements (thing like manhole locations, splice points, pole locations, etc.) for use with our internal GIS. To the best of my knowledge, the unit only had one receiver, but was able to track many more satellites than a handheld unit (up to 45). It would frequently lock up on 15-20 sats. But it did take quite a while (1-2 hours). I was told the accuracy was within 5mm. Quote Link to comment
MtnHermit Posted November 10, 2010 Share Posted November 10, 2010 So I wonder, is this how the survey units work, have two receivers inside one box? The time scale is a bit long, but the accuracy is compelling. Less time - less accuracy, but how much? I used to work in telecommunications and got to play with a survey Trimble (sorry don't remember the model). We would use it to map out network elements (thing like manhole locations, splice points, pole locations, etc.) for use with our internal GIS. To the best of my knowledge, the unit only had one receiver, but was able to track many more satellites than a handheld unit (up to 45). It would frequently lock up on 15-20 sats. But it did take quite a while (1-2 hours). I was told the accuracy was within 5mm. Since two people say the Trimble has only one receiver and given the article cared about nano-seconds. Then the obvious answer must be the internal clock. Consumer units all have an internal thermometer for the clock crystal, you can't change the temperature of the crystal but you could easily have a lookup table for a mathematical offset. On the Trimble it isn't inconceivable that there would be a small "oven" to precisely control the temperature of the crystal. More accurate time, more accurate measurement. Quote Link to comment
seldom_sn Posted November 10, 2010 Share Posted November 10, 2010 I read someplace that some Trimble gear required a ground station within 100 miles. Quote Link to comment
+GeePa Posted November 11, 2010 Share Posted November 11, 2010 Since two people say the Trimble has only one receiver and given the article cared about nano-seconds. Then the obvious answer must be the internal clock. Consumer units all have an internal thermometer for the clock crystal, you can't change the temperature of the crystal but you could easily have a lookup table for a mathematical offset. On the Trimble it isn't inconceivable that there would be a small "oven" to precisely control the temperature of the crystal. More accurate time, more accurate measurement. As I understand it, the accuracy of the clock in the GPSr is not important. On Trimble's own site (here) they describe how the GPSr syncs with the satellites such that it does not require an accurate clock on the GPSr. The fact that the system works as well as it does amazes me. Incredible technology it is. Quote Link to comment
+Chrysalides Posted November 11, 2010 Share Posted November 11, 2010 As I understand it, the accuracy of the clock in the GPSr is not important. On Trimble's own site (here) they describe how the GPSr syncs with the satellites such that it does not require an accurate clock on the GPSr. The fact that the system works as well as it does amazes me. Incredible technology it is. While the unit itself does not need to know the absolute correct time, wouldn't the quality of its clock still matter, in that it can more accurately determine the difference in the clock of the satellites? Quote Link to comment
John E Cache Posted November 11, 2010 Share Posted November 11, 2010 Since two people say the Trimble has only one receiver and given the article cared about nano-seconds. Then the obvious answer must be the internal clock. Consumer units all have an internal thermometer for the clock crystal, you can't change the temperature of the crystal but you could easily have a lookup table for a mathematical offset. On the Trimble it isn't inconceivable that there would be a small "oven" to precisely control the temperature of the crystal. More accurate time, more accurate measurement. As I understand it, the accuracy of the clock in the GPSr is not important. On Trimble's own site (here) they describe how the GPSr syncs with the satellites such that it does not require an accurate clock on the GPSr. The fact that the system works as well as it does amazes me. Incredible technology it is. The clock frequency may not need to be accurate, but the time of day has needed to be accurate for longitude calculation since the the days of ships navigating by the stars. The earth is spinning under the satellites and the stars. Quote Link to comment
+GeePa Posted November 11, 2010 Share Posted November 11, 2010 While the unit itself does not need to know the absolute correct time, wouldn't the quality of its clock still matter, in that it can more accurately determine the difference in the clock of the satellites? Not as I understand it from the web site. This part: Since any offset from universal time will affect all of our measurements, the receiver looks for a single correction factor that it can subtract from all its timing measurements that would cause them all to intersect at a single point. That correction brings the receiver's clock back into sync with universal time, and bingo! - you've got atomic accuracy time right in the palm of your hand. Indicates to me that any GPSr essential obtains atomic clock accuracy by taking the extra satellite measurement. I don't understand fully how this is working even though they attempt to explain it, but the text seems to indicate that the accuracy of the GPSr clock is not important because it gets super accurate time measurements from the satellite signals. Quote Link to comment
+Chrysalides Posted November 11, 2010 Share Posted November 11, 2010 While the unit itself does not need to know the absolute correct time, wouldn't the quality of its clock still matter, in that it can more accurately determine the difference in the clock of the satellites? Not as I understand it from the web site. This part: Since any offset from universal time will affect all of our measurements, the receiver looks for a single correction factor that it can subtract from all its timing measurements that would cause them all to intersect at a single point. That correction brings the receiver's clock back into sync with universal time, and bingo! - you've got atomic accuracy time right in the palm of your hand. Indicates to me that any GPSr essential obtains atomic clock accuracy by taking the extra satellite measurement. I don't understand fully how this is working even though they attempt to explain it, but the text seems to indicate that the accuracy of the GPSr clock is not important because it gets super accurate time measurements from the satellite signals. That's still talking about how accurate the time is. I was referring to how accurate the internal clock signal is. If it is meant to produce, say, a 10 MHz clock, how close to 10 MHz is it? The unit needs a standard to measure what time satellite A's signal was received, relative to satellite B's and the others. If it determines it received A's signal at T, and B's signal at T + 2.7 milliseconds, how much error will there be if B's signal was actually received at T + 2.695 millisecond? A more accurate clock will enable more accurate measurement. Quote Link to comment
jholly Posted November 11, 2010 Share Posted November 11, 2010 While the unit itself does not need to know the absolute correct time, wouldn't the quality of its clock still matter, in that it can more accurately determine the difference in the clock of the satellites? Not as I understand it from the web site. This part: Since any offset from universal time will affect all of our measurements, the receiver looks for a single correction factor that it can subtract from all its timing measurements that would cause them all to intersect at a single point. That correction brings the receiver's clock back into sync with universal time, and bingo! - you've got atomic accuracy time right in the palm of your hand. Indicates to me that any GPSr essential obtains atomic clock accuracy by taking the extra satellite measurement. I don't understand fully how this is working even though they attempt to explain it, but the text seems to indicate that the accuracy of the GPSr clock is not important because it gets super accurate time measurements from the satellite signals. That's still talking about how accurate the time is. I was referring to how accurate the internal clock signal is. If it is meant to produce, say, a 10 MHz clock, how close to 10 MHz is it? The unit needs a standard to measure what time satellite A's signal was received, relative to satellite B's and the others. If it determines it received A's signal at T, and B's signal at T + 2.7 milliseconds, how much error will there be if B's signal was actually received at T + 2.695 millisecond? A more accurate clock will enable more accurate measurement. With a three satellite fix the GPSr is able to compute a position based on the intersection of three circles. Using a fourth satellite this circle should also intersect the previous intersection point. If it does not we can assume the internal clock is in error and adjust the clock. The calculation is done iteratively until the clock is adjusted and we arrive at a four satellite intersection solution. There are as few other "details", but the answer boils down to the internal clock is adjusted until it is correct. SMOP. Quote Link to comment
+PDOP's Posted November 11, 2010 Share Posted November 11, 2010 I read someplace that some Trimble gear required a ground station within 100 miles. How Differential GPS works Quote Link to comment
+ecanderson Posted November 11, 2010 Share Posted November 11, 2010 I don't see any difference between the Trimble DGPS methodology and being within the "within a few hundred kilometers" of a WAAS ground point. Same idea. Ground station knows where it is, detects error, sends error to GPS receiver to be used as correction data. Only difference is that WAAS uses satellite transmissions to deliver the data. Trimble uses ground based transmission. So how is Trimble claiming accuracy far beyond WAAS? Tighter control of the correction data? Quote Link to comment
+PDOP's Posted November 11, 2010 Share Posted November 11, 2010 ... Only difference is that WAAS uses satellite transmissions to deliver the data. Trimble uses ground based transmission. So how is Trimble claiming accuracy far beyond WAAS? Tighter control of the correction data? Surveyors Do It Differently Quote Link to comment
+ecanderson Posted November 11, 2010 Share Posted November 11, 2010 But when we're talking about the interferometry method, we're back to the "They use multiple receivers like the differential systems we've been discussing..." per Trimble. Are there two entirely different Trimble systems in use based upon need for accuracy? One single receiver system and another system using multiple receivers? What is meant by "multiple receivers"? One fixed and one mobile, or ??? That question was raised quite a few posts up. I get the bit about dealing with phase within cycle as more accurate than cycle alone. That should be able to be incorporated into any unit with sufficient processor power and timing resolution. Quote Link to comment
+bcblues Posted November 12, 2010 Share Posted November 12, 2010 I use post-processing at work to get sub meter accuracy with my Trimble GeoXT. It basically operates like a local base-station, but it downloads the base-station data from permanent, always running stations (hopefully one near where I am working). It ios called post-processing because you cannot achieve a sub-meter solution in the field (at least without a local base), but rather later, in the office when you post-process the data. I do this with Trimble's "Pathfinder Office" "Differential Correction" utility. With my last survey grade Trimble, it had an OmniStar beacon to achieve sub-meter accuracy real-time. Now, with the CORS stations, it is just easier (and cheaper) to post-process. http://www.ngs.noaa.gov/CORS/ Quote Link to comment
+PDOP's Posted November 12, 2010 Share Posted November 12, 2010 I'm not totally up on the finer details of the technology but will try to answer your questions. Are there two entirely different Trimble systems in use based upon need for accuracy? Yes. Survey systems will have millimetre accuracy while GIS/Mapping systems have sub metre accuracy. The survey systems use more complicated electronics and are much more expensive. One single receiver system and another system using multiple receivers? What is meant by "multiple receivers"? One fixed and one mobile, or ??? All differential systems (including recreational grade with WAAS) use multiple receivers in some respect. WAAS ground stations use receivers on known surveyed points to collect error data and a network of these stations is required to cover the service area. The error/corrections data is relayed to your receiver from geostationary satellites by being piggy-backed on the GPS frequency. Trimble systems use base station receivers (again set on known points) with either a real-time relay of correction data or saved data to use in 'post processing' as described by bcblues post. The survey systems I worked with (10 years ago) had a single base station that relayed the correction signals using a single radio transmitter to multiple roving GPS receivers paired with separate radio receivers. This allowed for real-time layout and pickup of points in the field. The corrections are for each individual GPS satellite so the general rule is that the base station and the rovers must be able to see the same satellites Quote Link to comment
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