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GeeOddMike
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Given that the thread "Static GNSS Survey question" has lost focus, I create a new topic.

Those wishing to explore the result of session lengths (how much time) on the accuracy (or precision) of GPS processing can access observation files and associated data made during the 2011 Texas geoid-slope validation survey  here: https://geodesy.noaa.gov/GEOID/GSVS11/data/long-session-gps.shtml

This is a response in that thread that I upload the data file I used to generate OPUS (static) solutions using a range of lengths to CORS. I choose no to do so.  

The RINEX observation files I've examined had their headers edited to include receiver and antenna make and model as well as ARP values.

To re-irritate the issue, I find the formal specifications for receivers to be meaningless. The reduction of GPS observations to vectors do not show the length-relative error these specifications use.  The Mark Eckl paper here:

is a more accessible version of his paper published by the J of G here:

I don't know who and how these specifications e.g. Trimble R12 is said to have a vertical performance of 5mm + 0.5ppm are implemented by users? It seems to me that some are using this type specification to mean that they do not need to adjust their data. 

 

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Bill93
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I note that their plots start at 25 km and 1 hour.  I'd sure like to see similar data for times in minutes and distances 0-25 kilometers.

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GeeOddMike
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@bill93

As I recollect, the Eckl study used CORS data therefore baselines less than 25 km were unavailable. At the time of the study, CORS required two hours (but could be used with one hour).

Of course height determinations over short distances are best performed with traditional terrestrial leveling techniques. Over short distances my experience is that the best result is via an L1-only solution due to common errors and reduction of the noise introduced by frequency combinations.

I do not recollect any studies using short baselines and short session lengths along the lines you find of interest. Perhaps there is something regarding OPUS-RS? Such research might/probably exist/s.

Do a Google Scholar search or check out some of the research posted on university or GNSS-oriented web pages (e.g. FIG conference proceedings., The Ohio State University, UNB, etc).

I recollect a discussion in the early days of the NGS CORS network where an eminent scientist stated that only one site was needed for the entire country. His point being that such a system was possible. Consider the small set of sites used create and update the ITRF. The NGS plans for a FOUNDATION CORS (?) show how relatively few sites are needed to deliver a reference frame for the U.S..

The decision to pursue densification was to serve users whose desire for short session times required more sites. Dense networks for purposes like geophysical research/monitoring are unique applications.  There remain challenges to using GPS for heighting, mainly the wet tropo. Using closely located sites increase the likelihood of common impacts; long sessions allow the impacts to be randomized.  

Hope this is responsive,

DMM 

 

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GeeOddMike
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jt1950
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GeeOddMike
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@jt50

I like the Leica discussion of real-time RTK. I note the proviso that “...among other things the distance...” is important. I did not address my comments to techniques other than static GPS. 

As the Leica document states, ambiguity resolution is critical. Obtaining enough data for the determination of these unknowns is a function of time. With more signals and good SV geometry less time is required.

I invite correction as I have limited experience with RTK. Does not RTK rely on correctors being supplied by a base station with the base observing longer sessions. Issues like latency, radio interference are also factors.

 

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jt1950
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I recently used OPUS with some data I had handy. I chose the CORS for three submissions of the same data. I chose sites to explore how solutions changed with increasing average lengths to CORS. The average lengths were:  136 km, 1,670km and 2,054km. The shortest length was 31km, the longest was 2,227km. My ellipsoid heights for the three solutions were: -25.087, -25.091 and -25.092 (all meters and NAD83(2011). 

I am interested in what you posted in the other 'Static' thread. The entire length W-->E of continental USA is approximately 4000 km. If what you are saying about GPS accuracy is correct, then why are there so many CORS established across the USA? 

Based on your observation data sent to OPUS, observation length was around 6 hours. If you could get a fix for a 2000 km baseline in just 6 hours of static data then shouldn't the NGS just established maybe 1 CORS somewhere in Kansas or Missouri? Why the need to establish 2000+ CORS?  

 

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GeeOddMike
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@jt50

See my reply to Bill93.

I nonetheless make one last try to clarify my objection to the adoption of these receiver performance specifications as meaningful guides to field operations (planning and conduct of field surveys).

Perhaps I am being pedantic but stating that an individual receiver can obtain a vertical precision of say 3mm + 1 ppm is invalid in the sense that it is only one element in positioning. It should always be remembered that this is also with respect the determination of an ellipsoid height.

As the specifications add reference to their specs being contingent upon good SV geometry, no multipath, no unusual tropo, no abnormal iono, no setup errors, no SV errors, no site obstructions, no electronic interference, etc. they should be taken as a “rule of thumb” and not tabulated as a rule dictating field operations. I would also add that large errors result when different antennas are used without correction for their different performance. 

Does the spec of 3mm + 1 ppm for a 10km baseline (3mm+10km/1ppm = 0.013 m) mean that if I need to obtain a vertical accuracy of one cm (0.01m) that I must add an additional setup?  (I neglect confidence levels here).

Blindly using this specification, I would expect a vertical precision of 0.003 m on a 250m baseline and a 0.253m on a 250 km baseline. On the shorter baseline it is too optimistic; on the longer baseline too pessimistic with respect to expected results using appropriate session durations.

 

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Rover83
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Posted by: @jt50

Based on your observation data sent to OPUS, observation length was around 6 hours. If you could get a fix for a 2000 km baseline in just 6 hours of static data then shouldn't the NGS just established maybe 1 CORS somewhere in Kansas or Missouri? Why the need to establish 2000+ CORS? 

If you have to ask those questions, especially after the patient, lengthy and informative replies by other posters to your previous, almost-troll-level line of questioning, you should not be performing professional-level work with satellite positioning equipment.

Your insistence on distance error plus PPM as the end-all, be-all of satellite positioning is akin to reasoning that a total station is always, under any and all conditions, good to 2mm + 2 PPM, because the spec sheet says that's what the EDM can do.

By that logic, we should be able to drop a total station on top of the control point without a tripod or tribrach, prop up the prism with a rock near the backsight, perform a station setup routine, and expect to check to another control point within 2mm.

Do you disregard centering error, measure-up error, pointing error, levelling error, target error, atmospheric corrections, circle reading error, and collimation error when performing total station work?

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