LV_Forestry
 First Officer
Flight distance : 4726654 ft
Latvia
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Hi,
There are a few things to consider before buying a GNSS receiver.
The budget and the precision are the main ones.
1st Solution, the zero cost:
The method consists in creating a support that can receive your drone equipped with RTK. You drill a hole under the support which must be in the axis of the camera of the drone. You put the center of the GCP in the middle of the hole. You take a photo with your drone and the RTK FIX. Then you just have to extract the EXIF coordinates (Agisoft have a dedicated function for that), remove the distance between the camera and the support, and voila! accuracy +/-5cm guaranteed. I made one for my P4R in case my receiver runs out of battery. It's more of a workaround than anything else (Receiver' battery failure...), but it gets the job done.
2nd Solution, the low cost:
SparkFun GPS-RTK-SMA Breakout - ZED-F9P (Qwiic) - GPS-16481 - SparkFun Electronics
If you have some knowledge to assemble development boards, there are plenty of tutorials on the internet. I make custom GNSS receivers from this board. Accuracy +/-1.5cm Horizontal and +/-2cm Vertical. You can also use it as a base station to cast NTRIP or save OBS files for the RINEX/PPK.
3rd Solution, the low cost +
SparkFun RTK Facet - GPS-19984 - SparkFun Electronics
It's the same board as the previous one but it comes pre-assembled and configured. All you have to do is download SWMap on your phone (Android yes, APPLE not sure and never tested). To take GCPs without breaking your bank account, this is the ideal tool. The precision is the same as the previous one.
4th Solution, For low budget surveyors
S580 GNSS Receiver - Stonex
We go upmarket, there are more features. Calculation of distance, area... The precision is the same as the two previous ones.
5th Solution, For Surveyor
Trimble R12i | Trimble Geospatial
The functionalities are very advanced, and the precision is according to the manufacturer a bit better.
All these solutions require a stable connection, via GSM/4G, WiFi, Radio... on a base station located at a reasonable distance. Beyond 30-35km the quality begins to seriously deteriorate.
My best advice, if you don't intend to open a surveyor's office, just take GCP points, invest in the low cost + solution.
Also, if you have to take GCPs under the canopy, go directly to the Stonex receiver, it works pretty well in the forest. I have the Trimble R8s (discontinued), honestly I'm constantly looking for open spaces to get a FIX. Even my low cost DIY has less trouble fixing RTK. Test carried out with identical settings. Of course if you set +/-5m instead of +/-5cm, all the receivers will fix.
If you decide to invest in high-end equipment, ask to try first. Because Trimble is good, but it's also quickly complicated for everything. Once I broke the screen, it was a nightmare to get the license transferred to another device. It's more a reseller's problem than Trimble's, but what I mean is that with a low cost Sparkfun antenna, there's no such hassle.
And finally you need to know what coordinate systems you are going to use. You can configure your receiver in UTM, that is to deal with coordinates in meters instead of degrees, it is more readable. This is only my opinion, I have a certain colleague who prefers WGS84, I don't judge! You will need cadastral markers to calibrate your instruments, but I understand that the geography agency in the US is quite efficient, this should not be a problem for you.
Below is the support of the P4R. You have to put the landing gear in the slots and against the stops. The center of the GCP and put in the middle of the viewfinder in the center of the support.
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