[Closed] A tip to improve Alpha’s positioning accuracy when doing RTK/PPK with a VRS/CORS source

Ths is the updated version to this topic.

Some users may find their Alpha has better positioning accuracy when they use it to work with a local (Alpha) base, comparing to working with a public VRS/CORS base.

The reason is that the Glonass signals provided by some VRS’s/CORS’s may not include a specific field needed by Alpha to calculate coordinates. In this case, omitting bad Glonass signals and using only GPS signals in RTK/PPK helps accuracy improvement.

Here is a tip for those who want to try to improve this situation: use GPS+BeiDou firmware. When we are using GPS+BeiDou firmware, the Glonass GNSS data will not be used in RTK/PPK, and BeiDou signals help to gain better accuracy of single-point positioning.

First, we check the current firmware. The processes are:

  • Switch to position A (left-hand side) when you are going to config Alpha.
  • Connect Alpha with PC and start RTK Viewer

If you are already using GPS+BeiDou firmware (as shown ), then do nothing.

Otherwise, we re-program Alpha to use GPS+BeiDou firmware. The processes are:

I tried this today after your recommendation, Jim. I did a test in the afternoon, with GPS + BEIDOU and I had very good accuracy, real submetric once it was stable, with 9 to 11 satellites, HDOP 1.3, and standard deviation across several minutes of 0,6 meters.

But a few hours later, I try the same setup and I after half an hour ON, I get 5 to 7 satellites, NO RTK or FloatRTK solution at all. Is the Beidou constellation reliable? Can I have coverage gaps across days?

Here attached is my NMEA log for that condition. https://drive.google.com/file/d/1-hwAns5QDLVOFmyDYH2LyVr4oHhgmYRH/view?usp=sharing

The BDGSV messages report that there are 2 BD sats in view. The BDGSA messages report a 3D fix using either 2, 3 or 4 sats.

The GPGSV message report 2, 3 and 4 GPS sats in view across the test.

So, why can’t I have an RTK solution at all? I tried power-cycliong the receiver. Now I will COLD-start it and see what happens.

Update 1: After plugging the receiver onto rtk-viewer ans issuing a cold start and retasting, I got a brief period of FloatRTK of around 45 seconds, then GPS status again.

While plugged to rtk viewer I checked the constellation availability at the test site, and it was:

Hi Luis,
Thank you so much for your update. It shows GPS+BeiDou helps in your VRS receiving scenario.

In the “a few hours later” story, you are having too few visible satellites. (in that case RTK units can do nothing) Can you try at better environments/timing?

Thank you again.


Yes, actually, just by moving the antenna a couple of meters farther from the walls, I got RTK signal. As a practical rule of thumb I am requiring our customer to consider:

  • FloatRTK or RTK state.
  • At least 9 satellites in view.
  • Look at the elevation indication, check that after a few seconds it gets stable, not moving more than 1 meter.

and then the position can be considered sub-meter…

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I do not know if the recommendations for using only GPS + Beidou is still the recommendation for a VRS connection. Yesterday and today on the top of a 4 floor parking garage I tested the Alpha with the following setup:

  • Alpha on
  • Connection to state VRS with Lefebure client (using secure with reflection)
  • VRS mountpiont using corrections with GPS + Glonass
  • Topcon PG-A1 antenna (ground plane and NGS calibration)
  • Antenna on 2-meter pole
  • from 12 - 13 satellite visible 95% of time
  • Run for about 10 minutes until I see a Fix. (aka RTK)
  • Mobile Topographer

While I can keep a FloatRTK for maybe 90% of the time the FIX positions are very rare and intermittent (longest time about 10 seconds but typically about 1-2 second… 10 minutes between one Fix and the next)

Our state VRS broadcasts correction data for GPS, GPS + Glonass, or GPS + Glonass + Galileo. Beidou is not in any of the combinations.

What is your recommendations to reach a Fix position more often?

Can you try the antenna in UPM-Std kit? Some antenna has too large gain and it may cause saturation in RTK calculation.

Yes I can try the antenna but I will need to wait until May. I left the antenna in Italy (it works well with another L1/L2 receiver I have).
I did try the Alpha a few days ago using a Ublox antenna (ANN-MB-00) and I could only obtain a DGPS fix. (see screenshot) Using the same antenna, battery power, and a different L1/L2 antenna and I could obtain a RTK Fix location.

One thought, when I purchased the 1-year usage plan you sent me two different authorizations… each for about 6 months or maybe one ended on 12/31/2020. I didnt’ think about it until just now but would I need to reauthorize with the 2nd code you sent me? Could that explain why I did’t get a fix with the Ublox antenna?

Here is the screenshot I mentioned in the previous message.

Ok. I added the authorization code for this 6 months. Using the U-blox antenna (ANN-MB-00) I have I was able to obtain a FIX soution. The receiver bounced between Float and Fix (indicated as RTK in Lefebure) for almost 4 minutes before it became stable. :slight_smile:

It is very important to know if a recorded point used a Float or Fix solution. Is there a good phone app that will use only Fix solutions when averaging or saving a point? The Mobile Topographer (MT) app indicates the predicted accuracy for a single point but when averaging the MT app appears to use all observations. The ideal solution would be an app that records the DOP values and DGPS/Float/Fix status for each point.

You can try inserting an attenuator between a third-party antenna and Alpha.
You need to input the second code for using in 2020.

Lefebure itself can do the NMEA recording task for you.

You can know whether it is of Fix RTK or Float RTK quality from each NMEA GPGGA sentence.


you could try the Mapit Spatial app which allows you to set some parameter like minimum accuracy and averaging time.
Best regards!

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First of all thanks to Polaris staff, I ordered 1y usage plan on Saturday morning (spanish time ) and on the same morning I received the code.

My question: is still necessary to deactivate Glonass constellation? I did it following old topic because rtkviewer2 doesn’t have this option.

Now I have just GPS+Beidou in rover mode.

I can’t get a Fix RTK. My Alpha+ UPM-lite kit achieves just a permanent FloatRtk only when it receives corrections from a CORS located 6.5 km from my home.
I don’t get corrections using VRS technology, I tried with VRS3, MAC3, CERCANA3, (some times “cercana3” works may be cause it uses the same cors 6.5 km far from home).


I also manually entered the coordinates in Ntrip Lefebure but I do not receive corrections from in Vrs mount point.
Any suggestions?

If your Alpha+ is working with some VRS/CORS and the RTK Fix rate is not good enough, you can try this fix.

Now I have just GPS+Beidou in rover mode.

Please update both your RTK Viewer and Alpha+ firmware (from RTK Viewer UI). If you disabled the GLONASS signal, you should be still using GPS+BeiDou+Galileo signals.

I also manually entered the coordinates in Ntrip Lefebure but I do not receive corrections from in Vrs mount point.

Try to use the PC software STRSVR from RTKLib first, if it works, then go to Bluetooth interface.

Maybe it depends on the antenna which is not L1/L2?
“This High Precision Antenna covers L1 GPS/GLONASS, E1 Galileo, B1 Beidou frequency bands…”
I activated Glonass again, without improving or worsening the performance. Last night I got some Rtk fixes that lasted around 20 seconds…
I regret not having bought survey standard kit…
The experiment continues

The antenna included in your UPM-Lite+ kit is a multiple frequency antenna. However, the survey antenna has better GNSS satellite signal reception.
If you need to purchase a survey antenna for your field use, please let me know.

In another test this evening, with all constellations actived, a long session of fix RTK. Unfortunately not VRS but with direct connection to a geodesic station.

I might be interested in buying an antenna, what options do I have? Do I have to write to the support email?

I have sent you a message about antenna purchase.

I dont know if this can help to find a possible bug, when I change constellation configuration I get this message:
Configure GNSS constellation type successfully
Unknown message:
Detect NMEA checksum error:
Detect NMEA checksum error: