Larger antenna for RTK Express

GPS/GNSS Development Boards
1

Good day all,

I’m looking into getting two SparkFun RTK Express kits: one as a rover and one as a base station. They will require an RTCM correction via radio over a distance between 3 to 5 km, and I cannot find a suitable radio antenna on SparkFun’s store. I hope someone could suggest a good option.

The SparkFun RTK Express Hookup Guide mentions that it was possible to get larger distances with better antennas. However, the 915MHz 500mW antennas (WRL-17255) from the Guide are no longer sold, and a larger antenna (WRL-14868) is also no longer sold. The 915 MHz 100mW antennas (WRL-19032) that are currently offered seem less powerful.

I found a larger antenna (WRL-15597) - https://www.sparkfun.com/products/15597 that I could mount on the rover and the base station but I cannot figure out what radio modules to connect to these antennas that could be compatible with the RTK Express.

What would be a good radio module for this application?

Could 915MHz 100mW radio module provide enough power to these WRL-15597 antennas in order to communicate over 5 km?

Many thanks!

2

I have a pair of RTK Facets and the Holybro 100mW radios (WRL-19032) for my base rover RTK setup.

I use the 5.8dBi fiberglass antenna (WRL-15597) with my base Facet. I use an antenna that I already had from another project for my rover Facet; it looks identical to WRL-14876. Using these two antennas instead of the antennas supplied with the Holybro radios made a significant improvement in my range. The 5.8dBi antenna was too large for my rover work, I have taken 100s of shots in an afternoon with my rover. If you can use the 5.8dBi antenna with your rover also, I suspect it will be another big improvement in range.

It’s hard to say what type of range you’ll get, as there are so many variables. If you have a clear line of sight and do everything right you could get a great range. Terrain, vegetation, trees, buildings, antenna height, loose connections, etc. will limit range.

If you look up what professional surveyors spend $1000s on and use every day for reliable RTK surveying, you’ll see their radios generally start at 1W and go up to 35W or so. Many use cellular and internet connections when they can. Their full RTK kits can run $25K and more. And they accept that even after they spend all that money, it might not work well in dense forest or challenging terrain. One project I’m working on with a professional surveyor is on-hold until the leaves drop in the fall.

The Sparkfun gear definitely works and fits my budget.

3

toeknee,

Thank you for the information. This is really helpful.

I think I could start with 100mW radio modules (WRL-19032) with larger antennas (WRL-15597) and see how this setup performs.

If there is a need to install a more powerful radio module later on, could RTK Facet / RTK Express work with SparkFun MicroMod Lora 1W 915MHz transreceiver (WRL-18573), which also requires a MicroMod processor board and a MicroMod main board?

https://learn.sparkfun.com/tutorials/1w … okup-guide

Thanks!

4

I have the same question/situation. Please let me know how this works out. Thanks

5

I was looking into using a pair of the RFD900 1w radios (along with various variants) to extend my RTK range for my pair of Facets. They appear to be very popular, but I have no personal experience. I was working out how to enclose and power the RFD900s. Then I got pulled in a different direction as I need to do a fair bit of static PPP with my Facets first. We are working in an area with no cellular service, so we decided to increase our control network (setting survey marks for our base) before we do more RTK mapping. We hope to get a few more control points set and will then be doing both RTK and static PPP as we survey more of our project area.

6

I’ve never used the RTK Express. So - in Rover mode, it receives satellite GNSS data through an antenna attached directly, like the TOP106; but assesses it’s own position more-accurately if it also receives RTK correction data. And the RTK data can be transmitted via an attached radio, like the WRL-19032, from a base station, which could be a second RTK Express, as the product page says. But it also says the RTK data can come from a smartphone. What’s the benefit of using a couple of telemetry radios, and a second unit as a base station, to get the RTK data?

7

In short, whenever you can’t or don’t want to use the internet for the comm link between the base and rover. It’s just simpler to not use the internet, fewer moving parts. And you might not have internet access. Or you might not want to bother getting the internet comm working. But an internet comm link does have advantages - no radio interference, communication over longer distances, etc. I do most of my RTK work where there is no cell service or internet access, so I use telemetry radios.

There are many ways to achieve RTK.

They all involve a rover GPS/GNSS receiver measuring coordinates of points of interest while receiving raw GPS observation data in RTCM format or similar from a base. (Sometimes people call this data stream ‘correction data’ but it technically is not, despite what the SparkFun documents call it.)

They all involve some type of base GPS/GNSS receiver at a known or assumed position, transmitting its raw GPS observations in RTCM format or similar.

They all involve some type of comm link between the base and rover.

The rover uses its own GPS/GNSS observation data and the stream of received observation data from the base to calculate a very accurate relative vector between the base and the rover. And with the coordinates of the base and that vector, the rover can calculate the coordinates of its location.

There are many options for the comm link.

One is to connect both the base and rover to the internet, and transmit the data from the base to the rover over the internet. But of course you need internet access at both locations, which isn’t always possible or practical. And of course when you connect two devices to the internet, they need some way of connecting to each other, which generally involves the base having a fixed ip address, or using a third server (or service) to allow them to make a connection.

Another is to use radio modems to transmit the data from the base to the rover. Telemetry radios are radio modems. This is useful if you’re working out in the woods and there is no WiFi or cell service. It is also conceptually simpler - setup the base, turn it on. Turn on the rover. Go. No need to connect to the internet, etc.

You could use even use a cable, though you couldn’t move very far.

You can even log both the rover and base observations to a file, and later post process the data to find the location of the rover. But that’s not Real Time Kinematic, it’s Post Processed Kinematic.

8

toeknee:
I was looking into using a pair of the RFD900 1w radios (along with various variants) to extend my RTK range for my pair of Facets.

By the way, I did get a pair of RFD900x (forgot the x the first time) working with my RTK Facets. The range is definitely better than the Holybros, perhaps 2x but definitely not 10x better. They have diversity antenna capability, so I run two big antennas up high on a 12’ surveyor’s pole at my base. One of the antennas I got from Sparkfun, the other one from a helium mining vendor.

My rover has two 1/2 wave antennas. (As an aside, toss those 1/4 wave stubbies that come with the Holybros! Even with the Holybros, 1/2 wave antennas work much better.) Getting the base antenna up high helps a lot, I work in hilly terrain.

The power available on the Facet’s radio port isn’t enough to run the RFD900x’s at full power (by my calculations, not experimentation), so I might setup a separate power source for the RFD900x’s. It’s mostly an exercise in making cables.

I’ve just ordered a third RFD900x and am going to attempt to setup all three in a mesh so I can use one as a repeater.

There are many drone equipment vendors that sell the RFD900x’s, better antennas, cables, etc. And sometimes I just need to order cables from Mouser.