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Image: Topcon RTK collecting survey points

High precision GPS systems are getting more known and used and RTK is already becoming a familiar acronym. This post is about the use of RTK for collecting ground control points to be used in high quality survey results, not about RTK for navigation use (that's a lot harder). What I was looking for is a method to collect about 5 control points for surveys in a way that is cost effective, but also doesn't cost me a lot of time. I'm working in the tropics, so standing 30 minutes in the sun and dust is a real chore and not very comfortable. Here I describe how I do this using low-cost L1 GPS modules that cost ~$50 each with an additional ~$150 for a recording device and a good antenna.

If I were to use the above RTK station, I would get really good accuracy (L1+L2), but I would also spend > $20k that I'd have to recover from business income and I'd have to spend 5-10 minutes per point to get an accurate fix, since I only had one station. With many modules you also depend on a proprietary radio link, so you'd eventually have to buy two if you need to set up your own reference station somewhere.

First, let's consider there are two kinds of accuracy:

1 Absolute accuracy; the difference between measured and actual coordinates of the point in world space.
2 Relative accuracy; the difference between measured and actual offset of one point and a reference point in the survey area.

The second characteristic by itself is already extremely useful for high precision volume and distance measurements. All professional surveys have this characteristic, which is essentially the characteristic of correct scale. Absolute precision is only really needed when you want to exchange this data with other systems and/or use different data sets together (now and in the future). The absolute accuracy of a data set can always be improved after the fact by simply applying a unique global offset to all the data or control points in 3D coordinates (meters, inch, whatever).

In this approach, we're going to store the raw observations in a file to be processed together at a later point in time. RTK GPS has a plethora of online discussions and examples where you require some kind of radio or wifi link between modules, but this is not actually a prerequisite of the technology. Having files is much better, because you can work out the exact configuration and settings in the office and so you won't make any configuration mistakes in the field. Also, a radio/wifi link is extra interference for your uav and not to forget the GPS module itself and if those links drop out for a reason, you lose all the data and have to come back again.

What you record is raw GPS output data, straight from the module, so there's little that can go wrong. You get the best results if the frequency for all modules is equal, because sometimes the tools get confused if there's an output frequency mismatch. Since modules are set up as static, I just set this frequency to 1Hz for all modules. I myself purchased some ~$50 modules (NEO-6T from emlid), put them in the field with any device that can read the modules (uart/usb) and record the data to an HD, SD, etc. If all modules were recording in the same time period, you can post-process the raw GPS logs from the comfort of your office.

I use RTKLIB to do all that: http://www.rtklib.com/

The process is:
1. select the dataset to be used as reference, call it REF
2. convert that to RINEX. Make sure to use the correct data source format.

optional:
To get high accuracy absolute positions from the field if you do not have an accurate reference yet, you can use PPP to derive it's position. This will give a higher absolute accuracy than a cellphone:
3. download the "SP3" file from the IGS data repository (available 3hrs-24hrs after the survey): https://igscb.jpl.nasa.gov/components/prods_cb.html ; (available 3-24hrs later).
4. post-process the reference point REF using "PPP static", "Precise" solution, "Iono estimate" and "ZTD estimate" and select the SP3 file together with the NAV and SBS file using rtkpost.exe. You can also use the CLK file, but in my case it actually hurt results. Some more research there is needed.
5. if you have ~30 mins of data, the processed solution should be accurate to about 70cm (absolute) and probably is better than a meter. If you need better accuracy, the only way to get this better with PPP is to keep the module longer in the field (6-24 hours). After that, it's very much a black art which gives best results, but some people report you should expect ~10cm. That's not 1-2cm for the relative accuracy.

6. Use either the calculated position or the one from your cellphone. Let's call that REF.
7. select the dataset (one of the other modules) to be used as "rover", let's call this ROVER here.
8. convert ROVER dataset to "RINEX" data format using the rtkconv.exe program. Make sure to set the right source data format (of the source file).
9. then use rtkpost.exe to post-process both files, applying the settings you'd use in rtknavi.exe and process away. You should insert the position from REF in the "antenna position" now in the settings dialog and use "static" as the mechanism for post-processing, probably with "continuous" instead of "fix and hold".
10. Eventually there should be a fix for a long period of time. You can then average that out or see how much the variation is and not bother further. It should be 1-2cm when modules were not far apart.
11. Repeat from 2 for the other modules.

This method is cost- and time-effective. With 5 modules, you'd spend $1000 (raspi+GPS+good antenna ~= $200) to be able to measure 5 control points at the same time without waiting 5-10 minutes for each one. You can drop them in the field prior to a mission, turn them on, walk back and execute the uav flight. If that took about 30 minutes to complete, you can already retrieve the modules and go back to the office for post-processing.

Here's a nice tutorial about PPP with rtklib, but some options only make sense when you have L1+L2 data:

http://blog.latitude51.ca/rtklib-part-3-precise-point-positioning-with-igs-products/

I tested the above processing using a couple of well known reference positions of the local university, so the numbers of 1-2cm precision for RTK, 70cm PPP static are from such tests, however all tests were executed in one afternoon, so this doesn't give a lot of variance given the weather, ionosphere and solar activity. Actual results in the field will therefore vary, and vary per day too.