Good news for all of you who are using a GNSS RTK without the fancy tilt meter and IMU onboard ! 

You can turn your today GNSS receiver to do the "free bubble style" by mixing up 2 technologies that popped up last year.

The first one is about software receiver as we learnt we can feed the digital signal processing part of a GNSS receiver into a smartphone and the second one is that we don't need to take the GNSS survey pole vertical !

My approach of this problem has been developed and tested already in 1998 when surveyors were looking for a hidden bar based approach using a total station. By fixing 2 prisms on a pole and kept it steady, you can determine the coordinates of a hidden point.

At that time we suggested using a dynamic approach (with a 360° prism) where the operator had to move around the hidden point the pole and having all the 3D positions of the prism to solve the problem considering a spherical fit approach. You can get the centre of the sphere and a check on the pole length.

We can use the same approach with a GNSS pole. Instead of using a tilt meter and a digital compass (as you do need to align the tilt meter) or a smart combination of a GNSS board and a IMU, why not to move your pole keeping the bottom fixed on the point ?

Practically you engage an automatic collection of points using a spacing of 10 or 25 cm and you keep moving ... All the coordinates will fit a sphere around the point ! You can use a direct linear approach to solve the equations.

A nice variant would be to measure the same way, but by using different pole height. In that case you need to have at least 3 positions with your GNSS pole (1,6 m 1,8 m and 2,0 m). You are just replicating the fundamental navigation problem of GNSS as your receiver will look like "satellites" orbiting around your point and the pseudo range will be your pole length.

That's the "Software Free Bubble" method.

Is it working well ? Well, we did a collection of points using that method and the results are pretty good and accurate when comparing to a vertical fix.

As you can see, either by direct or Least Squares Adjustment approach (the functional model is different) except for one set, we are pretty much on the same accuracy.

I left the wrong set to show you that the standard deviation of the weight unit is a good indicator !

You can also see the different pole height sets (1,8 m 1,6 m and 2,0 m) and the mixed solution ... you have also the number of points collected per set ( 38, 33, 84, 155 and 16 ).

So what you need is just an app. that can drive the recording of points at a given space interval to create some geometry diversity and to add the processing.

That's the software free bubble GNSS new style !

Enjoy and be happy ! 


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