CORS vs Personal RTK Base Station

I’m new to the RTK game and currently have an account to use the CORS network to get my feet wet. It’s free and I figured it’d get me started at least.

I have a base station that’s 20-30 miles away from me and have a couple of questions:

  1. Distance from base station effects accuracy, does it also effect the ability to get a fix (speed, reliability, ect)

  2. Is CORS update frequency fast enough or what other advantages is there for going the DIY way?

For this reason the RTCM corrections are only valid up to a distance of 35km from the Base Station (some people say 25km, some people 50km, the exact number will depend on the antenna and receiver quality)
From Ardusimple website

If it works ok I would keep using it. My area doesn’t have CORS, so I don’t know a lot about it, have my own base.

Update frequency from CORS is likely the same as most of us use for a private base.

Fix time and position accuracy degrade when the distance to the base increases but 20 miles should be OK.

Thanks guys!

Distance aside, do RTK corrections have an update frequency? I was reading that John Deere was 1 second or less which I assume can make it more accurate? I would expect this with either a paid service or DIY but not ‘free’ like CORS.

I can image if I had my own base station I could send/receive data as fast as I want? The CORS network I currently have access to says 5 seconds I believe. That is if I was actually looking at the right thing.

Pretty much every RTK source is using 1 second update rate (for the delay sensitive messages). A higher frequency would not improve accuracy. This is different from the rover position measurement frequency which should be 10 Hz and higher would be at least a bit better if supported.

Are you sure the CORS network sends at 0.2 Hz? If it was 5 s latency, there would still be something wrong. If it is just some messages like RTCM 1005 that are sent every 5 seconds, that would be fine and not causing any accuracy degradation.

when my base went offline i used the next nearest which is 60mile away and didnt notice much if any loss in performance while out mowing

I just confirmed the data rate is 1 sec, good deal.

The reason I ask about distance is that my RTK fix isn’t very stable. I’m new to it so maybe it’s normal? As I understand, the rover and base need to see the same satellites (minimum 3 or 4?) to get a RTK fix. Is there a way to see where the ball is being dropped or why there’s no RTK fix? To be clear it goes between Float and RTK with seemingly no rhyme or reason yet.

For my mount point, I chose RTCM3.2 from the source table. I’ve tried others but it doesn’t seem to make much difference. I was under the impression that I should be picking the mount point that was the station closest to me, they all seem to be combined and simply listed by protocol…?

Usually with a correction network, you are required to send a GGA message to them at least once per second. This is used by the network to get your GPS position and then the server will pick the base station closest to you and send you its corrections. In this situation the mount point is used to allow you to select the type of messages you want, but the actual base station you are connected to is chosen by the NTRIP caster server.

If you are having a hard time maintaining fix, it could be that the nearest base station is a bit too far away from you and the satellites the base station can see is differing from the satellites you are seeing (less overlap anyway). But I’ve also noticed that moving from the little magnetic ublox antenna to a survey-style antenna globe will improve things when fix drops to float frequently. Also with the ublox antenna make sure you have a big enough metal ground plane underneath of it. And of course a good view of the sky.

If you want to make your own base station, you can align your base station with the CORS network by using CORS to get the RTK position of your bases station’s antenna, and then in your base station receiver set the coordinates to be that which you got from CORS RTK. Then you can use your own base station and be able to swap back to CORS at any time and still be in the same exact lat/lon. Otherwise you’ll see a shift in your lines if you switch between your own base station and CORS.

Awesome info, thank you all! Hopefully all this info helps others as well.

3 satellites are enough for autonomous positioning if the receiver had an absolutely accurate time base. No practical receiver has any sufficiently accurate clock and need a minimum of 4 satellites to be able to calculate any position estimate. RTK fix needs one more satellite theoretically. In practise more than 5 is needed but it should not be any problem under open sky. Using all 4 constellations you would have of the order of 40 satellites totally.

@samduster, do you know what constellations the CORS mount point is using? The more constellations it has, the more robust the RTK fix is, in my experience. Like I said before, you might want to try a different antenna. Like a survey antenna such as a BT-300S or BT-300D. Or something similar that is multiband.

As for your questions, if I’m not mistaken, the answer to #1 is yes it can affect that, yes. But you should still be able to get a fix from 30 miles. It just might not be better than say 1" accuracy. As for #2, CORS update frequency is definitely fast enough.

If CORS was working for you, there’s little advantage to running your own base station in my opinion.

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I use a state run cors system. It is free and not far from me. It works well and is up 99.9 percent of the time. They send out an email when they expect a maintenance down time. They called me once and said I needed to fix my system because my references was 0,0, off the coast of Africa.

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At the moment I just have this antenna to get my feet wet. It says it’s multiband and supports all constellations but I’m certain there’s upgrades to be had here…

Also, I chose the ‘AGRTCM32’ for my CORS mount point which lists all except BeiDou (none of the listed mount points show more than 3.)

The ann mb is perfectly good, so you must have something else wrong in settings, or parked right under a very big tree.

Inside of the broad term GPS, Galileo, etc. there are many satellites. GPS is US based. Galileo is Russian based. So here in the US I can easily just use GPS and do very well. It is usually picking up 12 to 20 satellites if I don’t filter them. The main goal is to choose the closest Cors to you. It will automatically do the calculations based on the same satellites. The corrections do not come in as just an x,y,z adjustment. Each of your satellite signals are corrected based on what the cors is getting. Remember that the cors is getting the same signals you are. The cors just says hey everyone, something is wrong with the signals. Here is my list of corrections to make each signal right. Your GPS uses how many it needs to get an agreement on your fix. If it cannot agree it will not get to “fix”. It will say float.

GLONASS → Russia
BEIDOU → China
GALILEO → European

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Ahh, yes. I had it backwards. Thanks.

You cannot have 20 GPS satellites in the US. That would be about 2/3 of all satellites. At the equator at sea level the maximum seems to be 14 if you accept all satellites down to 0 degree elevation (commercial rovers usually ignore satellites below 10 degrees making it 12).

The minimum number of GPS satellites starts to fluctuate a lot when you go towards the poles (Alaska is part of the US) and apply any elevation cut-off. Add foliage and any single GNSS system starts to suffer, even fail. Of course all combined fail at some point on someone’s field but the likelihood goes down a lot.

All great info!

As long as the information train is rolling, does foliage / obstructions make the fix inaccurate or just less likely to obtain one? I assume if you can get a RTK lock, the position is the position.

less likely to obtain one,
I have this input to get a picture of it:
If your rover can see 10 satellites, and base also see 10 satellites you could think rtk was possible. But if all those 10, are different satellites then rtk is not possible.
As stated somewhere above practical minimum is 4 satellites that are the same.
That is why a base 100 km away sometimes can give you a fix (i have tested with 4 constellations enabled on f9p), but off course accuracy is bad, due to the distance.
Edit: And if one of the same 4 sats (that were possible at that moment) are blocked by foliage then rtk will be lost.