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RTK GPS vs Static Post Processed GPS

Discussion in 'General Land Surveying' started by Nate The Surveyor, Jun 30, 2013.

  1. Nate The Surveyor

    Nate The Surveyor 5-Year Member

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    I have played with it a bit, and believe that Post Processed Static GPS is usually more accurate and robust than RTK GPS. The reason for this, as I gather it, is related to the fact that RTK GPS only sends CORRECTIONS, whereas PPS actually allows it to compare the data.


    IF this is true, and I have an older GPS set as base. Say, my Legacy E unit, which is L1 L2, and Glonass, and my rover is L1-L2, Glonass, and say 4 Galileo (EU Constellation), then this would hurt the solution for PPS, but would it hurt the solution for RTK?.

    The FCC has mandated that we have to change our radios, to a higher rate. Some have complained that it resulted in reduced RTK radio range. Will this higher rate allow MORE data to be sent, from base to rover?


    As more sats go up, as in Galileo, and China, and Korea, and Japan etc, then there becomes MORE data up there. Does this increase that data quantity, to be sent from base to rover, in an RTK situation? Does this demand more from the radio system? Or is in ONLY sending corrections?

    Periodic maintenance for our survey equipment USED to mean, cleaning the 200' tape, or the 300' tape. Dis-assembly of the old Transit, and cleaning the spindles, and adding petroleum jelly to them. Cleaning the lenses. Cleaning the scales. Airing up the truck tires, and changing it's oil. Even cleaning the gears in the Monroe hand crank calculator with kerosene.

    The idea of maintaining equipment now includes cleaning all data connections, terminals, checking battery capacity, and learning all we can about GPS, and signals, and interference. Defraging a hard drive. Getting up to date drivers.

    It's a wild world. One that includes many more disciplines than it used to. All I wanted to know was the CORRECT brg and dist between these 2 points! :)

    Somebody help me out a bit here. There is more unknown in this new world of 'lectric surveying, than there is known!

    Thank you

    Nate
     
  2. Cee Gee

    Cee Gee 5-Year Member

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    Topcon came and did a big equipment demo seminar hereabouts a few weeks ago. The Topcon guy asked how many of us were still doing static GPS with post-processing. My hand went up along with several others. Topcon guy said "So there are still a few of you -- from the old school." As though static is now totally passe and RTK is the standard.
     
  3. Paul in PA

    Paul in PA 5-Year Member

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    4, Even 8, Galileo Are Not Worth A Dime Extra

    You need 4 minimum to get a position, 5 for a useful position and 6 for a precise position. To have 6 visible in the sky you would will need 18+ in orbit. That is a few years away. Possibly about the time L2C and/or L5 become active.

    Nate you did not participate in the early GPS work with a limited constellation. You had to observe when the satellites were visible and with so few you observed for hours on end while they changed their positions significantly. With the precise result you can get from a 20 minutes OPUS-RS why clutter up the files with a few Galilleos. What they could add is not worth the time to download a precise? orbit for them.

    With a 1,000 plus CORS stations and their data so easily available automatically in today's post processing software you can confirm your control points in minutes. In my opinion any who uses RTK and does not post processing to confirm the base position and readjust it all is a statistical fool.

    The biggest improvement in GPS work in the USA is not going to come from more satellites in other contellations. The majority of those 1,000 plus CORS are owned by governemental agencies or colleges. They will not get replaced until they wear out, which will be a long time. A GPS receiver with a solid power supply, indoors with A/C is in an ideal envoronment for longevity. Without control data on those other systems they are less useful. The greater power of L2C and L5 will get you resolved signals faster and observation time is going to be the major improvement.

    Gallileo and other constellations will be very useful in other parts of the world away from a CORS like network, where postioning will still rely on vaeraging of a lot of less precise data. You, however, are her not there.

    I foresee OPUS-IS {Instant static) solutions. Data with L1/L1C, L2/L2C and L5/L5C will be accepted for 5 minutes or less post processing. Hopefully NGS includes L1/L5 ranging in the equation. Out in the field L1/L5 ranging corrections will allow RTK to actually be as accurate as it is now claimed.

    You have about 2 years to make your mind up on new GPS, so address your radio/comminication requirements with that in mind.

    Paul in PA
     
  4. Lance Andre

    Lance Andre 3-Year Member

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    IMO, Network RTK is the way to go, but I now see that Static GPS still has a huge market and use. Look at CHC, they have a high-end Network RTK unit(s) but just released the X90-OPUS in conjunction with iGage; which is a unit designed specifically for static OPUS work. The unit is only $2450, you simply turn it on and it works; plug it into the computer, it appears as a drive letter to automatically upload static to OPUS or other RINEX software programs. Best part is that it is more accurate than nearly all modern RTK rovers due to its large ground plane antenna. I think the Topcon sales guy is out of touch with the market. Over half of the surveyors in the USA still do not have any GPS. A low cost Static unit that is easy to use offers these surveyors a great way to tie their surveys into the National Datum(s) without breaking the bank on a $10K Network RTK rover is a no braining for surveyors who still do not have GPS.

    Mark at iGage has convinced me, and I now see time in the near future where all conventional surveys will use a unit like the X90-OPUS on the top of their back sight and fore sight targets as a way to not only to strengthen network adjustment but tie the survey geodetically to our national datum. I think this is the revolution that is being missed by the rest survey hardware manufacturers, possibly because they think we have all progressed to a point of instant satisfaction of RTK.
     
  5. Mark Mayer

    Mark Mayer 5-Year Member

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    > I have played with it a bit, and believe that Post Processed Static GPS is usually more accurate and robust than RTK GPS. The reason for this, as I gather it, is related to the fact that RTK GPS only sends CORRECTIONS, whereas PPS actually allows it to compare the data.
    I believe that static vectors can be higher quality than RTK vectors if the user takes advantage of the opportunity to refine the quality by selecting intervals for best GDOP, eliminating noisy data, loading precise emphemerii, etc. If you just take what comes with static, there isn't much inherent difference between a static and an RTK vector.
     
  6. gschrock

    gschrock 4-Year Member

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  7. Joe the Surveyor

    Joe the Surveyor 5-Year Member

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    I use GPS RTK network with good results. I've checked on several monuments in my area and found them to be with in 0.10'. Now for me that is all I need. I mainly use GPS to establish a datum and run conventional off the GPS points. We use the datum to bring in GIS info, soil type info and wetlands info.
     
  8. LRDay

    LRDay 4-Year Member

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    I've done quite a bit of static work. One thing I've also done along with the static is to also do a 3 minute RTK shot either before of after the static session (same tripod setup, same occupation if RTK corrections are available). Usually the results compare at about 1 cm. Makes you wonder if all the time spent doing the static is worth it BUT, it provides a redundant check and builds confidence in the work!

    My sense of it and experience is that if you really want good static results you need long sessions (6 hours or more). If you are doing short static sessions (15 minutes to an hour) RTK is probably just as good (3 minutes or so) if RTK conditions are good and you have solid RTK base coordinates.
     
  9. Paul in PA

    Paul in PA 5-Year Member

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    Maybe In Utah

    But in most of the USA there are sufficient CORS stations to make 1/2 hour OPUS-RS extremely reliable. Only the control points need such occupation. You can RTK with redundant shots and/or use low cost L1 receivers to tie in the rest.

    Were I in the market to expand my GPS inventory, I would defineite look at these new low cost OPUS focused receivers.

    But I would still keep my ProMark 2s because they can easily and cheaply tie into quite a traverse.

    Paul in PA
     
  10. Big Al

    Big Al 4-Year Member

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    Interesting unit, the X90-OPUS. I agree the price is attractive. Very good for dual frequency static work.
     
  11. Shawn Billings

    Shawn Billings 3-Year Member

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    Just yesterday I post processed a trouble some rtk campaign. Most of the sites were great but a couple were pretty noisy. PP did great though. I don't always pp my rtk sessions but I wouldn't want to be without the capability. Besides there are still limits on radio range and cell coverage.
     
  12. Loyal

    Loyal 5-Year Member

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    Paul...

    Most of the US???... by area or population?

    Get out of town, and maybe get a clue!

    B-)
    Loyal
     
  13. Dave Karoly

    Dave Karoly 5-Year Member

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    Static is way too useful to be declared dead.

    RTK is a topo method.

    Then there is OPUS.

    If I was to own GPS again I would get those low cost OPUS units and some processing software.
     
  14. Dave Karoly

    Dave Karoly 5-Year Member

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    I'm just a redneck GPS surveyor but...

    I think RTK and Static are in essence the same thing, a processed vector between two receivers.

    RTK does it in real time and uses a kinematic processor usually in the head of the receiver.

    With Static you store files and process using a static or rapid static processor later which yields better results, allows you to change things like the processing mask and drop bad satellites.
     
  15. LRDay

    LRDay 4-Year Member

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    Maybe In Utah

    Utah is not inundated with CORS stations. Maybe OPUS-RS will work but I haven't gone there. The Utah RTN saved money on their stations. The antenna installs won't qualify for the CORS network. So that's about 30 or more stations with no CORS data. You can get the files, the coordinates are just not maintained by the CORS network.

    I don't know but isn't OPUS-RS just a post processed version of RTK? What's the range, 30 miles to a station or so? RTK works most of the time for me.
     
  16. trah

    trah 2-Year Member

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    Hey Nate,

    I think Norman summed it up pretty well.

    "IF this is true, and I have an older GPS set as base. Say, my Legacy E unit, which is L1 L2, and Glonass, and my rover is L1-L2, Glonass, and say 4 Galileo (EU Constellation), then this would hurt the solution for PPS, but would it hurt the solution for RTK?."

    This shouldn't hurt either solution. Most software will probably exclude the satellites that aren't observed at by both the base and rover, or if they try to use them they will place less weight ( or trust) on the satellites that don't have the proper corrections.
     
  17. Paul in PA

    Paul in PA 5-Year Member

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    Most, By Population, Definitely, Area, Some

    I read an article last night, don't have the link, that only a few western states were without OPUS-RS possibilities. OPUS-RS only has to find 3 CORS within range
    (250Mi/K?) and the observation no farther than 50(Mi/K?) off the polygon.

    I'll search it out tonight.

    Were I in Utah I would split an OPUS file into smaller parts and submit to OPUS-RS to see what I would learn. After all there is no additional charge.

    Paul in PA
     
  18. Paul in PA

    Paul in PA 5-Year Member

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    OPUS-RS Is More Vigorous Than OPUS

    OPUS-RS requires L1, C1 or P1, L2 and P2 to resolve a position, i.e it uses more observations. The C/P observations must be clean and some older receivers just don't cut it. By editing the poor C/P data out of the beginning of a file, one can get better results than with a longer file. OPUS-RS looks at your first few observations, if they do not look good it bails out. It is up to you to know if you have ggod data.

    When I was working with the beta version of RSGPS I could get good results (100% ambiguities resolved) at time with only 3 epochs of data, or bad results even with 15 minutes. NGS upped the minimum observation from 5 minutes to 15 minutes plus other changes and I now see reliable results with 15 minutes. My minimum control OPUS-RS is 30-45 minutes with 2 observations but I am looking for very good vertical solutions.

    Paul in PA
     
  19. foggyidea

    foggyidea 5-Year Member

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    OPUS-RS Is More Vigorous Than OPUS

    Now that's funny Paul, because here on the sandbar I can't get a solution since I am well outside the network. OPUS Static works well, though.
     
  20. Tom Wilson

    Tom Wilson 5-Year Member

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    OPUS-RS Is More Vigorous Than OPUS

    Don:

    You'll have to get a CORS site set up on one of the new windmills for you guys that live down on the beach.
     

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