Finding Accurate Base Position

Hello,

I have poor SNRs in my base device although it is well placed in an open area with a clear sky, is there any thing i am missing ?!

I am trying to set the base position,
I used only GPS ( based on recommendations especially that GPS SNRs are better than any other system in my location ).
I used average single method for 30 min. twice, and saved coordinates to manual.

The results were different:

1st Time:
30.0709468428,31.0206792686
309216.0,3328298.5 UTM

2nd Time:
30.0709625669,31.0206701668
309215.2,3328300.3 UTM

What should i do to get an accurate position ?

With the following settings:



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Your accurate measurements with Reach are made between the base and rover. First you have to supply Reach with an accurate base location yourself.

The best way is to start at a place of known accurate coordinates. For example, a survey marker with published coordinates.

Otherwise you could use corrections from a 3rd party. For example an NTRIP connection or by downloading historical CORS data. Hopefully there is a station within 10km of your location.

The next best is to log data for a long time and submit it to a PPP service.

The last resort is to average base coordinates, like you did above. Let us assume that you have no other option than to use average base coordinates. So:

  1. You average base coordinates for, say 30min to get a good average of single-mode coordinates. Now you record that average coordinate permanently. You have just declared that the spot your base station sits on is the exact coordinate you recorded. You never do an average on that spot again. You just use that same coordinate for your base every time you place your base on that spot.

Now, in the future, maybe you get access to a better method of achieving a base coordinate (like a NTRIP correction stream from a station less than 10km away). Well, you can take your base back to that spot and use NTRIP to get a new, more accurate coordinate. You update your old coordinate, and update all your previously surveyed points by shifting them by the difference between the old and new coordinate.

To make a long story short, your only mistake was averaging a coordinate two times. You should only do it once, because you’ll never get the same result twice! Single-mode is not that accurate!

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I would also turn on SBAS and see if you can pick up any stronger signals there.

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Some other good questions to ask yourself:

  1. Is there a need for absolute accuracy?
  2. In terms of accuracy, who or what are you needing to be absolutely accurate in relation to?
  3. Is the relative +/-2cm accuracy of RTK between base and rover good enough?
  4. Why is averaging a single-mode base position for 30min not good enough for your project?
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Thanks bide for your answer.

Is there a need for absolute accuracy?
In terms of accuracy, who or what are you needing to be absolutely accurate in relation to?
Is the relative +/-2cm accuracy of RTK between base and rover good enough?
Why is averaging a single-mode base position for 30min not good enough for your project?

I’m not sure if a fixed inaccurate base position affects the consistency of rover positions while the rover is in same place or not . If no , I don’t care about absolute accuracy.
I need the rover to be consistently accurate in relation to the base.
My application is localization of a car in a google map in a 1Km range.
Unfortunately Google map isn’t good enough (streets aren’t accurate enough).
So I want to create a map with accurate street edges using reach GPS and then localize the car within the map.

An accuracy of 5 cm of the rover is good enough for my application. Only I wanna be sure that every time I’m in a specific position in reality , The gps give the same latitude , longitude with an error less than 5 cm.
also I don’t need a very frequent gps message , 0.5 Hz Frequency is OK to me.


Here are some of my results , It’s not consistent at all . Although the status is fix , Being in the same position twice gives a different lat & long with a distance more than 2 meters between them.

Also , I’m confused , Why the maximum antenna height of base in base mode settings is only 6.5 meters ?
I’m placing the antenna on the roof of a building which is 7.15 m.

OK, good. You determine a base coordinate and use it consistently with all your surveying/rover work. Your results should all be within 2cm. This, of course, assumes everything is alright with your rover satellite view, etc.

Is this two different surveys with the same base postition/coordinate or is it different. If different base coordinate, then you can’t compare. You must use the same base coordinate for each survey. Or, if you move the base, then you must determine the RTK result from the original base position to the new base position. Then you will have 2cm accuracy to your new base and 4cm accuracy to your new rover. I am speaking in general terms though. Your results may vary.

I think is is because of a limitation in the RTCM3 standard. If you set 6.5m for your base and you know it is .65m too low, then just add .65m to your actual rover antenna height and it will work out the same. :slight_smile:

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If you are averaging base position your antenna height is zero. Non-zero antenna height should be used if base is set up on a known point. Antenna height is elevation from known point (which is usually on the ground) to antenna level (antenna is usually on a tripod). In your case you are directly measuring the location of your antenna using GPS, so base antenna height is zero.

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:bowing_man: I defer to Igor on this. Sorry, I did not pick up on that.

Is this two different surveys with the same base postition/coordinate or is it different. If different base coordinate, then you can’t compare. You must use the same base coordinate for each survey. Or, if you move the base, then you must determine the RTK result from the original base position to the new base position. Then you will have 2cm accuracy to your new base and 4cm accuracy to your new rover. I am speaking in general terms though. Your results may vary.

These two surveys were in the same base position with the settings i screened in the post ( used almost all satellites and used 5Hz frequency and 0 antenna height and gps ar mode : fix and hold)
The results were too bad , 4 meter different in some positions.

I think is is because of a limitation in the RTCM3 standard. If you set 6.5m for your base and you know it is .65m too low, then just add .65m to your actual rover antenna height and it will work out the same. :slight_smile:

Do you say that antenna height should be the vertical distance between the rover antenna and the base antenna ?
Rover antenna : 1.9 - 2.1 m to ground
Base antenna : 7.15m to ground

So Antenna height in base mode settings should be 5.15m( 7.15-2.0) ??

If you have restarted your base that would have triggered reaveraging and as a result change of base position. That would also translate to rover position shift.

Also, I see that you have disabled all GNSS apart from GPS. Using GPS+Glonass+Galileo you will get much better result.

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If you have restarted your base that would have triggered reaveraging and as a result change of base position. That would also translate to rover position shift.

Also, I see that you have disabled all GNSS apart from GPS. Using GPS+Glonass+Galileo you will get much better result.

No , After averaging setting I pressed (save position to manual button . so it turned to manual " No re -averaging"

Also the setting in the surveys were in the presence of almost all satellites.(Please see the pictures in the last topic i screened all setting (Different Fix Lat&Lng in the same Position!)

When I saw it was very bad and some users recommended that we reduce the rate and the number of systems to enhance the output , I limited it to GPS and SPAS .

Also . I don’t know if my current base SNR good enough ??
These are screenshots of 2 samples in different time ( 5 minutes difference):

Your base sees only GPS satellites and SNR is not good. How is the antenna installed?

Here are some photos …

Reduce messages not needed. You need as many satellites as you can get, but no point enabling a system if it is never in range for region. As Igor mentioned a lot, you need as many satellites as possible… which I assume he meant also that are in your regional range.

I think you have your antenna height incorrectly inputted? I.e. as Igor states, if averaging leave at zero. Since you saved your average position to manual, I think you have your antenna height calculations incorrect for that part though? Maybe too short?

If RS rover unit, antenna height from ground point being measured to BOTTOM of unit PLUS 65mm.

The base might be screwing you up?

Just two things.
-The plate under the antenna is of metal right?
-Either way, you should try have more distance from cables like the ones you have plave the antenna on. Try a different location with less interference.

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In regard to the 2 layers of black tape directly over the antenna element:

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