Reach RS Drone Survey Help

Hi Guys,

I have spent the past couple of days trying to get my head around the Reach RS system and how to use it to create Ground Control Points to increase accuracy on my 3D drone surveying, but am still struggling to grasp it. I plan to build 3D models for architecture companies instead of them using ground surveyors, they need the models at the CM level of accuracy.

I am in the UK and have the following:

1x Reach RS Unit
1x Reach M+ Unit (The older kind without any casing)
DJI Inspire 1 Pro

From my reading I think I understand that PPK is the best way to go using GCP’s using a DJI drone. I plan to use tiles as GCP’s and understand that I need to keep the RS as a base unit and then take the rover unit around to each GCP to measure them.

Where I am confused is the following:

  1. How do the two devices talk to each other. I can connect to each one’s bluetooth and see they are receiving signals but how does the Reach RS unit know where the M+ rover is capturing the GCP’s.

  2. Will using the M+ as a rover to collect GCP’s Work or do I need a second RS unit.

  3. What is the best workflow to collect the GCP’s.

Really appreciate any help with this as I have these great bits of tech but struggling how to use them!



To start you off:

There’s RTK and there’s PPK.

RTK is “Real-time kinematic” as in getting instant results
PPK is “Post-processed kinematic” as in getting results later after a processing job is done.

For you, the easiest thing will be to do PPK. The devices don’t talk to each other. They just record log files. Then later you download them and process them with the post-processing software (usually RTKLIB - see the tutorial in the docs)

There are also some 3rd party software apps that sort of do the post-processing for you.


One more start-up question, does your GCP’s need be referenced to the real-world, or is relative precision ok ?


Hi Bide thanks for your really helpful reply, it is much appreciated.
That makes a lot more sense now! Do you also happen to know if I can use the M+ as my rover unit or do I need to invest in a second RS unit.

Kind regards,


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Hi Wizprod,

From my understanding relative position is fine. I am creating the 3d models so the architect can pull them into a CAD program so as long as the measurements between the building itself are correct I don’t think it matters on absolute.

Thanks in advance

Any Emlid Reach device can be used as either base or rover (original Reach module, M+, RS, RS+). Actually they can even act as base and rover at the same time (which is not normally done, but possible).

Also, and I hope it is not confusing, but when collecting PPK data, technically there is no base or rover, only data (as log files). It is during the post-processing job that you must decide which log file will be used as the ‘base’ and which will be used as the ‘rover’.

Of course you will put one unit over a known point and you will call it ‘the base’ and the other unit will be called ‘the rover’. It is just that the data doesn’t know that yet. And this info is regarding PPK only. It is different for RTK.


You have seriously been such a great help thank you! I think I was overthinking things a bit.

Does this mean that essentially I can just use the RS device and put it over a point and make a note that it is my base. Then move the same RS device around on top of each of my GCP’s and then when I come to post processing as long as I know which point was my base I do not need a second device? If so do I need to change the base settings to accomodate for it not being static?

Assuming that is correct then my GCP’s will be relative to the base and should provide my 3D drone model relative accuracy.

Thanks again Bide!

Oops, and well that would be nice, but not quite!

Two GNSS receivers must log data simultaneously in order to do PPK (or RTK) processing.

The theory is that one receiver is over a stationary ‘known’ point. That would be your ‘base’ or a third-party’s base e.g. CORS/VRS/etc.). By one method or another, coordinates are pre- or post-determined for that base receiver’s location. The other receiver is the rover and it is maybe stationary over an ‘unknown’ point or maybe moving along an ‘unknown’ track.

In the most simplistic description, the processing software is told about the ‘known’ coordinates where the base was located. Then it looks at the apparent position as calculated from the satellites. The apparent position is almost never ‘on the spot’ and is moving around in the general area. The difference between the apparent position and the known position is the error.

The error at that specific moment in time is then subtracted from the rover’s apparent position. Since the error is eliminated, now the rover position has become accurate. And that must be done separately for every epoch (every second at 1Hz or 5 times a second at 5Hz)

Behind the scenes, this correction mechanism within the processing software is:

  1. reading the data from each satellite signal and coarsely aligning itself
  2. monitoring the phase of the sinusoidal carrier wave of each satellite signal and fine-tuning the alignment
  3. then when the data of all these signals meets a certain criteria, the position locks in and becomes ‘fixed’

So, in a nutshell you need 2 Reach units to record GCPs. That way your data collection is self-contained within your equipment. Otherwise you need access to an NTRIP server that feeds you corrections live so that you can do RTK - or - you need access to historical data from a CORS so that you can do PPK.

p.s. to others reading - if I misspoke somewhere, feel free to correct or clarify, as I wrote this hurriedly.

If you are close enough to a CORS station, you could also use the log from that station as the “base” log and only need to use 1 Reach device. (correct bide?)

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You are correct, but since he already has a Reach RS and an original Reach module, his localized accuracy will be better with his own base and rover.

CORS data would be useful to process into an accurate/aligned base coordinate though.

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