wait...WHAT?
It's true. There is a methodology called PPP, Precise Point Positioning, that only uses float solutions. It should only be used when the rover is on and in no danger of losing satellites for several hours, such as when collecting LiDAR data from a helicopter. The precision eventually attains better than 1 cm.
Stephen
Yeah. That's the way I had it but our most excellant "spelchk" said that was wrong and I didn't have a Goombah handy to check it for me.
Besides, REAL rednecks, don't need spell check! 🙂
wait...WHAT?
Stephen
I utilize PPP quite often for processing flight path information. While the PPP can result in precision near that of normal post processed kinematic, we are talking post processed. That does not do you a lot of good when your staking line or do not have a way to post process.
As a side note, with additional frequencies a type of real-time PPP is possible.
Why go away from the convention that users have been have accustomed to for 20 years?
Is it more accurate?
If there's not a significant technical advantage, then it smells like the marketing people are trying to create something out of nothing.
> Correct, it is a "New Realization" of the receiver's true position. It is supposed to be a much more accurate representation of the point position. You are either RTK Initialized or RTK Not Initialized. No more fixed/float. It is a little confusing for a lot of guys. It's good to know the DC software (Access) has set of tolerances that keep you from storing a position that has too high of precision estimates...you can be initialized at 0.50'.
wait...WHAT?
If that was true, then how come Omnistar accuracy only achieves 10cm no matter how long you sit on it? RTK will give you 1cm in a couple of minutes.
> It's true. There is a methodology called PPP, Precise Point Positioning, that only uses float solutions. It should only be used when the rover is on and in no danger of losing satellites for several hours, such as when collecting LiDAR data from a helicopter. The precision eventually attains better than 1 cm.
>
> Stephen
Technical Explanation
Carrier phase measurements are very precise and in a GPS receiver we can measure them with mm-cm level measurement accuracy. However, we can only measure these quantities in fractions of one cycle. One cycle being 19 cm for the L1 GPS frequency. We do not know the number of cycles that exist between the satellite and the receiver. This unknown number of cycles is called an ambiguity parameter.
Theoretically, this ambiguity value must be an integer number (ie. ...-infinity,...-1,0,1,... infinity). However, in reality there are biases that exist in the satellites and receivers that cause this to not be the case. To remove these fractional biases, we can perform relative positioning (double difference between receivers and satellites) which cancels out these non-integer biases and which leaves only the unknown integer value remaining. When we estimate this value, if the remaining error sources are properly modelled, the estimated value will be very close to an integer value. Because we know that in theory it should be an integer number, we can choose to remove the remaining fractional part and "fix" the ambiguity to the nearest integer value. If we are able to do this for all satellites in view, and we fix them to the correct integer value, we will get a very precise and accurate solution because we now have an unambiguous cm level measurement.
The big "if" here is whether we fix to the correct value or not. Because of multipath and other unmodelled errors, it can be very difficult to fix the ambiguity to the nearest value. For example, if estimated ambiguity is 1.5, should I round to 1 or 2? In this case we would not try to fix it to an integer value. Instead we would let it "float" or keep the "real" number value for the ambiguity. This is a float solution. Over time this value will tend to converge towards its final value but this will take time. For long baselines or for PPP, this may take more than an hour! However, given enough time to fully converge the float solution will become ALMOST as good as the fixed solution, but not quite.
A fixed solution will always be more precise than a float solution because the ambiguity (number of cycles between the satellite and receiver) is constant. However, if I fix my solution to the wrong integer number , maybe because there was multipath or other unmodelled error sources, my result will not be accurate. It will have a bias but may still be precise (very small variation) which can mislead a user into thinking that they have a good solution.
Companies spend a lot of time and money on ensuring that they properly validate their solution so that they minimize the number of incorrect fixes. A wrong fix is basically a disaster! Therefore if you can achieve a "fixed" solution this is an indicator that you usually also have an accurate solution. However, it is not a guarantee. This is why you never trust a single occupation for short RTK observations.
Practical Explanation
Imagine you have a straight line painted on the ground. After several beer, you try to walk that line. You can do a pretty good job of staying on the line but overall you tend to wobble a little from side to side. If you looked back at your track, you would notice that you wandered off the line in some cases but generally kept it near that line. This would be similar to a float solution. Accurate but not that stable, but may be good enough for the police officer to allow you to pass!
For a fixed solution imagine the same setup, but with a handrail placed along the line. While you walk the line you hold the rail which constrains your movement very precisely to the line so that you no longer have these wiggles or wandering. When you look back at your track you see that you almost perfectly stayed on the line.
For GIS applications 0.5+ meter accuracy no need to worry, float or fixed will be fine. That being said, waiting for a fixed solution can be a means of validation. Generally, if your measurement quality is poor, a fixed solution won't be achieved.
If the Trimble rep. says that R10 doesn't do "Fixed" or "float" this would be a simplification of what is actually occurring. Internally, they are still doing a fixed or a float solution. They just have a new method of specifying whether the solution is accurate or not and this is what the user sees rather than "Fixed" or "float"
In PPP we can only estimate float ambiguities because the fractional biases do not cancel out and the ambiguity is no longer an integer value. This causes the PPP solution to be a little less stable than a fixed RTK solution. The leading edge of research right now is to account for these biases in PPP to allow us to take advantage of the integer nature of the ambiguities and get the same performance as RTK.
wait...WHAT?
My experience with the Trimble PPP style is that it generally was USELESS. If there were problems with RTK, it was better to go to static/conventional surveying or come back at a later time if RTK was really the only way to make the project work.
as always, your results may vary.
Andy
wait...WHAT?
> Stephen
> ... While the PPP can result in precision near that of normal post processed kinematic, we are talking post processed. That does not do you a lot of good when your staking line or do not have a way to post process.
>
>
I agree, it is certainly not suitable for that. It is excellent for post processing a flight path.
Stephen
What if I drink 12 ounces of my favorite Irish Whiskey and pass out halfway down the line?
PS: That is one of the best explanations I have seen on this.
That would be considered a hadware failure 😉
As Dave K. said, excellent explanation.
Yep
Excellent!
Thanks for the great post trah.
Loyal
Thanks Trah. I had a feeling that somebody could do a better job than I could on this.
Nate
How long does it take your equipment to fix the solution on a 162km baseline?
I haven't done those types of baselines since the '90's so I'm just wondering how many epochs it takes these days. Ten, twenty?
Hi Carl,
The time for fixing is mainly a function of ionosphere activity and measurement quality (multipath, pseudorange noise etc). To be honest,the method for fixing long baselines has probably not changed much since the mid 90's, although receivers have improved somewhat. Also things may have gotten a bit easier since now we have services like the International GNSS Service (IGS) which freely provide very accurate orbit and clock products on a routine basis.
To ensure an accurate solution with high confidence, I would recommend collecting data for at least an hour, observing with dual frequency receivers. If the survey is performed at night when ionosphere activity is low, it could be claimed to fix in less than 30 seconds but I wouldn't recommend relying on an assumption that the ionosphere activity will be low because it could lead to very bad results.
Once the new L5 frequency is available on a sufficient number of satellites (or when Galileo becomes operational) this is when we will start to see some big gains in ambiguity resolution speed. With the L5 frequency we can form a combination of the measurements that creates a wavelength of almost 6 meters (~30x larger than the L1 wavelength) which can be resolved very quickly even under typical ionosphere activity and for baselines over 5000 km.