u-Blox ZED-F9P

As Roby over at Deep South Robotics can tell you, there is a lot of snake oil being peddled in the RTK GPS space. I’ve been a quiet spectator in this area for as long as I’ve been working on the mower project, and I’ve seen a lot of systems come and go.

Two modules that I had my eye on for a long time were SwiftNav’s Piksi V1 and Emlid’s Reach. Both were L1 only systems. I religiously visited their forums and poured over commenter’s experiences with both modules. In a nutshell, here’s what I learned:

  1. L1 only systems can take a long time to get an RTK fix, and they can lose them in a heartbeat
  2. If you’re in the air with a quadcopter or fixed wing plane, an L1 system generally works well because there is no multipath
  3. If you have any kind of multipath that you are contending with, you are absolutely screwed with an L1 system

On top of that, integrating these systems require that master’s degree in computer engineering I talked about earlier (remember, I am a lowly mechanical engineer, wires aren’t really my thing). Neither system is truly plug and play, from what I can tell. A lot of the integration has to do with injecting GPS corrections through telemetry over MAVLink, as I understand it.

Then last year I discovered that u-Blox was coming out with their own L1 system, the NEO-M8P. I watched a very thorough introduction to the module and was almost going to buy one when I noticed that u-Blox was coming out with a multi-band, multi-constellation RTK GNSS receiver, the ZED-F9P.

I contacted the folks out at u-Blox and they said that an evaluation board would be available in Q1 of 2019, but in my research I stumbled across the folks at Ardusimple. These guys are making a similar evaluation board for the ZED-F9P module that includes a Pixhawk GPS port. Talk about simple.

Is this the holy grail of RTK GNSS? From my experience with this board so far, the answer is yes.

A Simple Test

I conducted a test similar to the one in the video I linked to above. I took reported baseline measurements from u-Blox’s u-Center software, converted the NED components to a net distance and then compared the value with a tape measure.

The test setup. The receiver on the left is the base, the on the right near the laptop is the rover. Yes, that is my neighbor’s two story houses in the background.

I intentionally placed the receivers in a really crappy area of my yard to see what these modules are capable of. My neighborhood has lots of two story houses, and many fully grown trees, although they are without leaves this time of year which certainly helps. Here’s a panoramic shot of the backyard, with my house to the right:

My backyard. The deck where the test was conducted is on the right.

Bottom line is that this was a pretty challenging multipath environment. I would estimate 50% of the horizon was occluded by my and my neighbor’s house. The remaining areas were pretty well covered up to about 20° if you exclude bare trees. How well did the modules perform?

It’s tough to see in the picture but the NED measures in meters are:

N = -2.1990    E = 0.1063    D = -0.0006

Using the 3D distance formula on these components and converting to inches gives a baseline distance of 86.6759in. You can see in the picture above that the rover antenna is sitting right at 87in. Holy geography Batman indeed!

Some Other Observations

A few other things I noticed in my brief time working with these modules:

  1. The Ardusimple boards have the LED status “backward” from what you’d expect. No corrections means solid blue light, receiving corrections but no fix is blinking blue, and RTK fix is no light. Counter intuitive, but their documentation does mention this.
  2. Just for fun I covered the antenna and waited for the light to turn on, indicating the RTK fix was lost. When I removed my hand, the light would turn back off consistently after less than a second. It appears that if you lose your fix with these modules, you can get it back very quickly, even in a challenging environment.
  3. I kept moving the boards toward the alley between my house and my neighbor’s house, just to see what it would take to force the modules to lose the RTK fix and I was able to get as far as 227in from the base point in the picture above, which was about 3 feet away from my covered patio area before I lost my RTK fix.
  4. Even after I lost the RTK fix, the baseline error was only 12in in 3D float mode. Still really good. Keep in mind during most of this test, I am standing at the laptop taking pictures and fiddling with u-Center. So my body is blocking some signals, too, I’d imagine.

So far these boards are looking awesome! Next step is to hook things up to the Pixhawk.

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