Dual Reach RS - Moving baseline

I’m wondering if with two Emlid Reach RS I will be able to get a dual RTK correction considering a moving baseline. Both modules will be set up in an Autonomous Surface Vehicle which can not access NTRIP corrections.

Moving baseline? In RTK mode the baseline will change all the time. Do you mean moving base? or double input correction?
I am not sure i understand you question, could rephrase it?

I you have 2 units on a vehicle, then that is moving base as @TB_RTK mentioned.

With that type of setup, you are typically looking for accurate heading information in real time.

If you can post-process later against data from a fixed base station, then you can produce an accurate track for each of the two moving receivers.

What is your intended use case? Moving base is mostly used to calculate direction and is not officially supported in ReachView. We might however, help propose a way to pass corrections onto your vehicle.

The intended use case (I am co-working with Eric) is to mount two RS units on a boat (which won’t always have access to the internet) and we’d like to get good positioning (heading is not important, we have a magnetometer for that) using the constraint that both units will always keep their relative distance.

We know that RTKLIB is prepared for moving baseline, but we are unsure whether this feature is working and if it is, is it feasible to get it in ReachView?

For good positioning you need a base station providing corrections from a known point (which is presumably stationary).

Moving base might be good for the instance where you have an aircraft carrier floating in the middle of the sea with a base station on it. Your aircraft and watercraft running around could all have rovers and be getting precise RTK positioning relative to the aircraft carrier.

Moving base might also be food for a large fish processing vessel with an operating base station where there is a fleet of fishing boats and nets operating in the vicinity as rovers.

In contrast, having two receivers (base and rover) mounted to one boat will let you calculate precise distance and bearing between the two, and it will give you rough positioning in the world unless you can provide another source of corrections coming from some fixed base station.

We can try to play with that “fixed groundstation”. How relaxed the term “fixed” can be? Can we use a mooring buoy as base station (it will be slowly moving, drifting), and correct the Rover (the boat)?

In that situation, you give the base a fixed coordinate. Then the rover position is calculated relative to that coordinate.

So after the survey starts if your boat and your buoy both drift west 10 meters. Then you will show no position change because the relative distance is the same.

If your boat and your buoy drifts 10 meters, and you sailed the boat west 10 meters also, then you would show a position change of 10 meters west.

I think it would be hard to utilize the accuracy of RTK positioning in this type of environment. What you could do is use RTK for the accurate heading, and use single-mode SBAS-corrected positioning. To do processing of those two different solutions at the same time with a Reach device, you need to do some command line hacking. And to be clear, this scenario is back to using two Reach devices on the boat (no buoy).

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I think I should explain a little further, so that we know why the behaviour is the way it is. This will be an oversimplification and I’ll just use one direction (East-West) for the example:

You let your stationary base on land sit and average it’s coordinate for a few minutes. We’ll think in meters and use 10m West as the rough place of the base station. This is what Reach would be receiving:
10m, 9.5m, 9m, 11m, 13m, 10.5m, 8m, 9m, 10.5m, 9.5m
The average of that is: 10m West
So that is what you enter as a base coordinate.

Now you start your base and enable the base corrections (RTCM3 messages). Reach receives a similar sequence of coordinates, and forwards them to the rover, along with the base coordinate of 10m West. It might look like this:
Base coord: 10m West
Base observations: 10m, 9.5m, 9m, 11m, 13m, 10.5m, 8m, 9m, 10.5m, 9.5m

The rover would look at that and essentially make subtractions to come up with corrections:
0m, 0.5m, 1m, -1m, -3m, -0.5m, 2m, 1m, -0.5m, 0.5m

The rover is sitting about 20m West and receives these positions:
20m, 19.5m, 21m, 23m, 20.5m, 18m, 19m, 20.5m, 19.5m

Then it adds the corrections to get:
20m, 20m, 20m, 20m, 20m, 20m, 20m, 20m, 20m, 20m

So that is how RTK works in a nutshell. It is actually not true because it is much more complicated than that, but you get the idea. And now you should understand why two moving receivers will not give accurate RTK positioning relative to some fixed point on earth. They will only give accurate positioning relative to one another. This is why RTK base stations are almost exclusively on a very solid fixed point.

I hope that helps!

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