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38 microseconds a day


EraSeek

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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?

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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.

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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.

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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.

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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?

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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.
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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.

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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.

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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.

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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?

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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.

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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/

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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

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